Glass Fibre Reinforced Polymer (GFRP) Reinforced Concrete Slabs with Low Cover in Fire
One of the main safety requirements in the design of buildings is ensuring safe and appropriate fire endurance of the structures. To fill the knowledge gap on the fire resistance of fibre reinforced polymer (FRP) reinforced concrete members, comprehensive experimental and analytical studies are underway at Queen’s University with collaboration from industry and the National Research Council of Canada (NRC). As a part of the program, two full-scale glass FRP (GFRP) reinforced concrete slabs were tested under exposure to the ASTM-E119 standard fire. The slabs were identical except that they were reinforced with different types of GFRP and tested to examine the adequacy of 40 mm of clear concrete cover. Currently, CSAS806 allows a simplified method to design concrete slabs with FRP reinforcement and this results in a requirement for clear concrete cover of approximately 60 mm to achieve a 2 hour fire resistance rating. The test results showed that the slabs endured beyond 3 hours of fire exposure while resisting a high level of sustained load that was higher than the expected service load.
The Effect of LITS on Punching Shear in Fire
Several recent sets of experimental results on the punching shear behavior of flat slabs in fire have produced apparently anomalous deflections results, where the slab deflections on heating are in the opposite direction to that expected. Using numerical analysis, this paper shows that the results are explained by load induced thermal strains (LITS). Using two independent modelling approaches, the profound effect of LITS on deflection behavior is demonstrated. Analysis of the strain state near column connections identifies significant differences when LITS is included in analyses. By contrast for the unrestrained slabs considered here, stress states do not change much. The potential implications of these findings for design are noted.
Concrete Edge Failure of Headed Stud Anchors under Fire and Post-Fire Conditions: Verification of a 3D FE Code
Fire tests were conducted at University of Stuttgart to investigate the capacity of headed stud anchors under shear load perpendicular to free edge of concrete members. The headed stud anchors were cast in concrete with various edge distances, embedment depths and diameters. The specimens were specially designed so that the anchors could be loaded in shear after 90 min (hot state) of fire according to ISO 834. Reference tests and post-fire tests on the corresponding anchors were also conducted to make comparative analysis. More importantly, numerical analyses based on a threedimensional thermo-mechanical constitutive model for concrete that was implemented into a three-dimensional finite element code were performed to simulate experimental tests. The main aim of the paper is to verify numerical model based on the performed experimental tests. It is shown that the numerical analysis is able to predict the thermal and structural response of headed stud anchors cast in concrete close to edge very well. Based on the experimental results and numerical verification, the modification of the current formula according to Eurocode 2 for concrete edge failure of headed stud anchors under 90 min of fire is proposed.
Rational Design Approach for Predicting Fire Resistance of Hollowcore Slabs
A rational approach is proposed for evaluating fire resistance of PC hollowcore slabs. The proposed approach comprises of two main steps; namely, evaluating crosssectional temperatures in the slab, and then evaluating degrading moment and shear capacity at any given duration of the fire exposure. Fire resistance is evaluated by comparing the sectional moment and shear capacity at any given fire exposure time against applied moment and shear force. A simplified approach is also proposed for predicting sectional temperatures in hollowcore slabs under fire exposure by utilizing data obtained from numerical studies performed on wide range of hollowcore slabs.
Shear and Torsion in R/C Structural Members in Fire
Seldom structural designers are required to design from scratch or to check existing R/C structures or members in fire, under pure shear or torsion, because the structural response is in most cases controlled by either bending (with/without an axial force) or by shear with some bending. Furthermore, there is hardly a single test under prevailing shear or torsion in fire. How to design or to check a shear- or torsion-sensitive R/C member in fire is, therefore, an open issue, which needs a reasonable answer based on the available design models and on the knowledge of the shear-resistant mechanisms active in reinforced concrete. In this rather general paper, the design models wellknown in the calculations at the ultimate limit state in ordinary environmental conditions are recalled and their use in fire is discussed, with reference to both solid and thin-walled open sections. Both the effective-section method and the zone method are treated, as well as the fire-sensitivity of the various shear-transfer mechanisms, active in both shear and torsion, related to concrete-struts bending, aggregate interlock and dowel action along the cracks, not to mention the stirrups. On the whole, what stands out clearly in fire is the increasing role played by the stirrups in shear and that played by the rather cold concrete core in torsion.
Fire Behavior of Two-Way Post-Tensioned Concrete Slabs Provided with Bonded Tendons: An Experimental Study
Fire tests of three reduced-scale two-way post-tensioned concrete slabs with bonded tendons were carried out. This study was intended to investigate the influence of the layout of prestressing tendons, prestressing force and concrete spalling on the structural response of two-way post-tensioned concrete slabs with bonded tendons in fire. The material used to fabricate the specimens, the phenomena observed during the tests, the temperature distributions, the deflections and occurrence of concrete spalling were investigated.
Insights into the Complexity of Structural Fire Response from Repeated Heating Tests on Post- Tensioned Concrete
This paper extends discussion of previous research presented by the authors on post-tensioned (PT) concrete flexural elements in fire. Tests on two monostrand, continuous PT concrete slabs (one with an unbonded tendon and the other bonded) exposed to constant incident heat fluxes while under sustained load are reviewed and discussed. During testing these slabs demonstrated a distinct time-deflection response in heating and cooling consisting of five phases. For the first time, this paper gives the results from these unique slab tests during a second thermal cycle. The novelty of this study is that it was performed in an attempt to observe and better understand the thermal straining effects that contributed to the observed five-phase deflection response under first heating – illustrating many inter-related mechanisms that contribute to the complexity of the observed deflection responses. The resulting discussion is provided to advance the overall understanding of the response of real concrete structures (as opposed to isolated elements) in fire, and will hopefully assist structural fire modelers to validate (or otherwise) their modelling capabilities.
Quantifying the Effect of Temperature Induced Bond Degradation on Fire Resistance of RC Beams
This paper presents the effect of temperature induced bond degradation on fire resistance of reinforced concrete (RC) beams. Results from experimental and numerical studies, both at material and structural level, on temperature induced bond degradation in RC beams are presented. As part of material level tests, high temperature double tension pull-out (DTP) tests on normal strength concrete (NSC) and high strength concrete (HSC) specimens were conducted in a customized setup to ensure presence of longitudinal tensile stress in concrete surrounding the ribs. Data from tests is utilized to propose a simplified temperature dependent bi-linear bond stress-slip relationship. Furthermore, a finite element based numerical model is developed in ABAQUS that accounts for temperature induced bond degradation in evaluating response of RC beams under fire conditions. The model is applied to quantify the effect of temperature induced bond degradation on fire resistance of RC beams.
Modelling of Bond-slip between Prestressed Strands and Concrete at Elevated Temperatures
In this paper a new analytical model has been developed to simulate the average bond stress-slip relationship between strand and concrete for prestressed concrete structural members. Two different bond-slip curves have been proposed to represent the bond-slip characteristics for the three-wire and seven-wire strands. The model is able to take into account the type of strand surface, smooth or indented. The degradation of materials and bond characteristic at elevated temperatures are also considered in the model. The proposed model is validated against previous experimental results at ambient and elevated temperatures.
Performance Comparison between Rectangular and Circular Cross-section Columns in Fire Situation, Using Tabulated Methods
It is presented, in this study, a performance comparison between rectangular and circular cross-section’ columns in fire situation, using tabulated methods. For this purpose it appealed to the statistical analysis of the fire behaviour of 13129 columns inserted in the structures of 63 buildings carefully selected so that it corresponded to a representative sample of the licensed buildings in Portugal, covering different types of occupation. From these columns, 12243 presented a rectangular cross-section and 886 a circular cross-section. The fire resistance required for these columns was analysed using the methods presented in the form of tables in EN 1992-1-2 [1]. It was found that these methods were not applicable to a large number of columns (up to 45,8% in rectangular columns and 45,6% in circular columns) and, when applicable, led to a reduced number of columns that checked the fire resistance (up to 24,6% in rectangular columns and 36,5% in circular columns). It also concludes that the application of these methods to the circular columns leads to substantially less conservative results than when applied to the rectangular columns.
Fire Resistance of Reinforced Concrete Columns Subjected to Standard Fire—Comparison of an Advanced and a Simplified Method
For designing concrete columns subjected to a standard fire exposure, the Eurocode permits the use of simplified or advanced calculation methods. For the designer, the question of the respective advantages of these two types of methods arises. Which situations demand the use of an advanced method? When does a simple method provide sufficient accuracy? In this paper, laboratory tests are recalculated using Finite Element Modeling (FEM) as an advanced and Extended Zone Model (EZM) as a simple method in order to investigate these questions. The recalculations indicate that the simple EZM is of sufficient accuracy for symmetric heated columns without restraints. In contrast, the mechanical behavior of columns heated on three sides demands an advanced method such as FEM to be properly described.
Buckling Strength of Slender Reinforced HSC and VHSC Columns in Fire
In fire, it is anticipated that slender RC columns would be more vulnerable to buckling failure than to cross sectional failure because of thermal degradation in the cover concrete. This paper presents the main results from an experimental and analytical study of this phenomenon in slender RC columns using high-strength concrete (HSC) and very high-strength concrete (VHSC) under fire conditions. The parameters in the fire resistance tests were concrete strength, cross-sectional shape, axial force and the ratio of column height to cross-sectional diameter, and we consequently obtained fire-resistance time and failure mode. For analysis, a finiteelement method with an assumption of the Bernoulli-Euler theory was applied. The analysis, using a method proposed by the authors based on tangent-modulus theory, evaluates the buckling strength under fire conditions. A comparison of the experimental and analytical results shows that the proposed analysis method offers a good approximation of both fire-resistance time and failure mode.
Effects of Localised or Non-Uniform Heating on Reinforced Concrete Columns
This paper describes a research project being conducted to shed light on the response of concrete columns to non-uniform heating, and on the ability to credibly model the response of realistic concrete structures during non-standard fires. A series of 48 one-third scale reinforced concrete (RC) columns are being tested under sustained eccentric axial loading, and exposed to a localised radiant heat source to observe their response, to provide validation data for full-frame structural fire modelling, and to determine the “damageâ€ caused by non-standard fire exposures. All 48 columns are being loaded eccentrically and exposed to a constant incident heat flux at their mid height by means of a propane-fired radiant panel. This paper describes the project, including; the objectives and expected outputs of the project, the experimental test series which is underway, and initial a priori modelling results obtained using the commercially available Finite Element Software ATENA, which will be compared against the results of the experiments once these data are available.
Modelling the Thermal and Structural Performance of a Concrete Column Exposed to a Travelling Fire – Tisova Fire Test
The Tisova Fire Test was a large real scale fire test conducted in the Czech Republic in January 2015 inside of a 4-storey concrete frame building, with concrete and composite deck floors. The ground floor test compartment had a total area of ca. 230m2 with a height of 4.4m. The fire compartment included four columns from the original 1958 concrete construction, one of which was instrumented retroactively for temperatures, chosen due to its higher likelihood of observable thermal and structural response. This paper presents selected results from the thermal environment around, and the thermal response of, the column showing the variability of temperatures through the compartment height. The paper proceeds to model the thermal response of the column from 1) the real fire scenario, and 2) equivalent areas under the standard fire curve. These thermal profiles are then used to assess the reduction of the columns cross-sectional capacity, and shows that using equivalent fire durations is not an appropriate method to calculate the thermal and structural response of concrete columns exposed to a travelling fire.
An Analytical Approach for Predicting the Deformed Configuration of High Rise Concrete Walls Subjected to Fire Loading Conditions
This contribution presents an analytical approach for predicting the deformed configuration of high rise concrete walls subjected to fire loading conditions. The approach is based on the thermo-elastic analysis of a 2D plate model taking the geometry change of the plate due to its deflection, into account. A first illustrative application to the particular configuration of infinitely wide panel is performed leading to first conclusive results regarding the thermal-induced out-of-plane change of geometry of the wall.
A Thermo Mechanical Experimental Investigation on 3 Loaded Concrete Walls Exposed to ISO 834-1 Fire
This paper describes the thermo mechanical experimental response of three fullscale ordinary concrete (fc 28 days = 32 MPa) wall specimens (3.0 x 1.2 x 0.2 m3) subjected to thermal and mechanical loading. The wall specimens were heated on one side over the full length and height of the wall with the ISO 834-1 fire curve, while simultaneously being subjected to a constant uniaxial compressive load at the bottom of the wall. Walls 1, 2 and 3 were loaded at 0, 10 and 110 tons, respectively. The aim of this research is to provide the experimental results on the structural behavior of load bearing concrete walls in a fire situation. No spalling was observed on any of the 3 walls. It has been found that the deflections of the walls decreased with the increased applied mechanical loading. A thermo mechanical modeling benchmark has been organized. Eight laboratories have participated. It is encouraged for whoever wishes to carry out a simulation of the 3 walls in order to compare the calculated deflections with the experimental measurements presented in this paper.
On the Effect of Design Parameters and Boundary Conditions on the Post-Earthquake Fire Performance of RC Structural Walls
The post-earthquake fire (PEF) behavior of reinforced concrete (RC) structural walls investigated using finite element analysis. Damage due to an earthquake consists of damage to the concrete and steel at the heavily reinforced end regions of the wall. The loss of cover increases the spread of thermal damage through the wall, impacting the thermal fire resistance of the wall and damaging the material for load-bearing fire resistance. This paper investigates the influence of key wall characteristics (geometry, reinforcement ratios, and axial load demand) on the fire resistance. Results indicate that, for damaged RC walls, i) at low axial loads, the insulation criterion controls the fire resistance, ii) higher reinforcement ratios contribute to increased load-bearing fire resistance, and iii) longer boundary regions can have accelerate the rate of heat transfer through a damaged wall.
Effects of Polypropylene and Steel Fibers on Permeability of Ultra-high Performance Concrete at Hot State
Permeability of concrete is identified as one of the key parameters controlling explosive spalling. Measurements of permeability were performed on ultra-high performance concretes (UHPC) at elevated temperature ranging from ambient temperature to 300 °C. The effects of polypropylene fibers and steel fibers were investigated. The results show that plain UHPC and UHPC with steel fibers exhibit steady increase in permeability with increasing temperature. UHPC with polypropylene fibers exhibits a sudden increase of permeability at 150 °C. The microstructure of UHPC before and after exposure to elevated temperature was investigated by conducting Field Emission Scanning Electron Microscope (FESEM) observation. The results confirm that both the channels caused by melted polypropylene fibers as well as the generated micro crack network are the major factors causing a significant increase in permeability
Numerical Study on Steel Fiber Reinforced Concrete Columns Subjected to Fire
This paper presents a finite element (FE) based numerical approach to predict the thermal and mechanical response of steel fiber reinforced concrete (SFRC) columns subjected to fire. A sequentially coupled nonlinear thermo-mechanical analysis is carried out using ABAQUS® to obtain the temperature distribution and deformation profile in reinforced concrete (RC) columns subjected to simultaneous mechanical and thermal loading. The material and geometric nonlinearities, along with the temperature dependent thermal and mechanical properties of the reinforcing steel and the SFRC, are duly incorporated in the FE model. The thermal and mechanical response quantities predicted by the FE model are compared with that measured from the fire tests on the SFRC columns. The comparison shows that the developed FE model is capable of predicting the response of the SFRC columns subjected to thermal and mechanical loading with adequate accuracy. The developed FE model can be used to undertake detailed parametric study for quantifying the influence of critical factors on fire resistance of the SFRC columns.
Experimental Study of Ultra-high Performance Fibre Reinforced Concrete under ISO 834 Fire
Ultra-high performance fibre reinforced concrete (UHPFRC) is a relatively new construction material that possesses favourable mechanical properties over ordinary concrete. Although enormous strides have been achieved in understanding the performance and behaviour of UHPFRC at ambient temperature, there’s little literature and experimental data on their behaviour when submitted to fire. This study presents experimental results of four, 2m long, singly reinforced UHPFRC beams subjected to an ISO 834 fire curve for 60 minutes under a constant load. Two beams contained steel fibres with 2% volumetric ratio while the other set had 4%. Two load ratios of 0.4 and 0.6 were imposed prior to heating and kept constant throughout the fire test. Spalling was observed in all beams, being more severe in beams with steel dosage of 2%. The failure modes, displacement patterns, temperature profiles and corresponding fire endurance for each beam have been presented in this study
Fire Tests on Fiber Sheet Strengthened RC Beams using Geopolymers as Adhesive Agent
This study presents experimental results of fire tests on one control beam and four FRP strengthened RC beams, using geopolymers and epoxy resin as adhesive agent, to investigate the influence of temperatures on effectiveness of fiber sheet-geopolymer strengthening systems. The thermal response and deflection evolution of these beams with time were monitored. The test results show that although geopolymer material possess better bond properties than epoxy resin at high temperatures, the beams strengthened by carbon fiber sheet-geopolymer systems did not exhibit good fire resistance as expected, due to the falling off of the thermal insulation. Appropriate measurements to avoid the falling off of the insulation are required.
Elevated Temperature Response of RC Beams Strengthened with NSM FRP Bars Bonded with Cementitious Grout
This paper presents the results of 12 tests on small-scale reinforced concrete beams strengthened in flexure with a single near surface mounted (NSM) carbon FRP bar. To improve the performance of the FRP strengthening system at elevated temperature, the specific FRP bar has high values of glass transition Tg and decomposition Td temperature. Tests are performed to define the resin behavior at elevated temperature. The FRP is bonded using a cementitious grout rather than an epoxy adhesive. Flexural tests have been performed at both ambient and elevated temperatures on both unstrengthened and strengthened beams. Tests at elevated temperature were performed using propane-fired radiant panels rather than a fire testing furnace. Two heating configurations were used: (1) localised heating near midspan only; and (2) global heating over the entire bonded length of the FRP systems. Thermo-structural response was investigated under loads that give typical (i.e. maximum permissible) FRP service loading strain conditions. Internal temperatures, beam displacements, and slip of the FRP strengthening were measured. Bonded foil strain gauges were used to measure the FRP bar strains prior to heating, and digital image correlation (DIC) was also used to study displacements. The tests demonstrated good performance of this novel FRP strengthening system, both under localized (non-bond critical) and global heating (bond critical), and also demonstrated the grout-bonded NSM CFRP strengthening system’s ability to maintain structural effectiveness at temperatures up to about 600°C with adequate anchorage. However, similar tests with an epoxy adhesive are needed before the novel system can be confidently stated as being vastly superior to epoxyadhered NSM systems.
Flexural Capacity of NSM FRP Strengthened RC Beams at Elevated Temperature
This paper presents the thermal and the mechanical analyses, simulating the experimental tests conducted in the recent past on reinforced concrete beams strengthened with the Near Surface Mounted (NSM) FRP system. The thermal analyses were conducted through the software ABAQUS, whereas the mechanical analyses were carried out through the iterative-incremental procedure Moment- Curvature. The aim of these analyses is, firstly, assess the ability of simulating the experimental results of non-standard fire tests (local and global heating using propanefired radiant panels) and, then, generalise the experimental results providing a standard time of fire exposure that might lead to the un-effectiveness of the strengthening system due to debonding failure. The results of the numerical analyses (2D) cross-sectional finite element model (FEM) and 3D FEM were compared with the temperature recorded by the thermocouples during the tests, in order to assess the reliability of the FEM simulating the experimental results. The results of the 3D FEM model enabled also the evaluation of the heat transfer from the exposed to the unexposed zone and, therefore, to assess the length of the effective cold end-anchorage. Finally, thermal analyses of a 3D FEM of the NSM FRP strengthened RC beam, subjected to the standard ISO834 curve, were conducted in order to define useful fire safety design criteria for this investigated structural typology. The effect of the passive protection of the end-anchorage was also investigated, analyzing the structural element protected at both ends through a fire rated plasterboard with thickness ranging between ten and thirty millimeters.
Fire Testing of Insulated RC Beams Strengthened with Near Surface Mounted FRP Reinforcement
Seventeen NSM FRP (Near Surface Mounted Fibre Reinforced Polymer) strengthened and insulated full-scale beams, and two unstrengthened and unprotected beams, were subjected to fire tests. Insulation material type and dimensions and the insulation configuration were examined in order to develop practical methods for protecting FRP during fire exposure. Structural testing up to failure at room temperature of the fire tested beams has been carried out in order to evaluate their residual strength after one and two hours of fire exposure. The test program included the design and fabrication of steel reinforced concrete beams with rectangular crosssection. The experimental data demonstrated that all the insulated beams obtained the fire endurance ratings of 2h and 1h by satisfying both thermal and load bearing criteria. The paper will discuss the obtained test results.
Thermal Boundary Conditions When Fire Testing Concrete
Experimental studies of concrete in fire or at elevated temperature have traditionally controlled the thermal boundary condition by controlling the gas temperature in a furnace.
Predictive Testing for Heat Induced Spalling of Concrete Tunnels—The Influence of Mechanical Loading
This paper describes Phase II of a project being undertaken to develop a predictive test method to investigate heat-induced explosive spalling of concrete, with a specific focus on concrete used in tunneling applications (but obviously applicable to other applications). The test method seeks to allow careful control of the thermal and mechanical transient conditions influencing the occurrence of heat-induced concrete spalling, thus enabling convenient, representative, repeatable, and comparable testing to be carried out on various concrete mixes under various potentially relevant conditions. Phase I of the project focused on establishing suitable thermal exposures to use for testing based on the thermal exposures which a sample would be exposed to during a standard furnace test (cellulosic or modified hydrocarbon) in the Promethee testing facility at CERIB in France. The work described in this paper deals with establishing suitable mechanical loading conditions for a spalling test, the focus in the current work is to enable provision of a representative test for precast segmental concrete tunnel linings (as opposed to sprayed or cut-and-cover tunnel linings). With small adaptations the spalling test method could be adjusted to suit other applications. This paper focuses on the motivation for developing the testing method and outlines the testing to be carried out. Tests are currently underway, and the full suite of results will be presented at the conference.
Explosive Concrete Spalling During Large-Scale Fire Resistance Test
This paper presents a comprehensive investigation of explosive heat-induced spalling observed during a set of large-scale fire resistance tests (or standard furnace tests) on prestressed concrete slabs. The study, based on data from large-scale tests, examines the influence of numerous design parameters in the occurrence of spalling (age of concrete, inclusion of polypropylene fibres, depth of the slab, and prestressing level). Furthermore, a careful thermal analysis of the tested slabs is presented; a comparison of in-depth temperature distributions inside concrete slabs shows that spalling occurred for slabs with more rapid in-depth temperature increase. The analysis presented herein shows that the scatter of in-depth temperature increase experienced by concrete slabs tested simultaneously has a substantial influence in the occurrence of heat-induced concrete spalling.
Effect of Biaxial Mechanical Loading and Cement Type on the Fire Spalling Behaviour of Concrete
Fire spalling of concrete is a complex phenomenon, which might occur due to pressure build-up in the pores, thermal- and load-induced stresses. In this context, eight mid-size ordinary concrete slabs (4of B40-II and 4 of B40-III concrete: fc28 ≈ 40 MPa) were heated at the bottom face according to Standard Fire curve (ISO 834-1), while a constant biaxial compressive load was applied. Four different levels of biaxial mechanical loading have been investigated on both concretes. The test results showed that the loaded specimens are more prone to spalling than unloaded specimens, with increasing amount of spalling for higher values of applied load. Concrete made with CEM III cement (B40-III: 43 % of slag) exhibited less spalling than CEM II cement concrete (B40-II: 3 % of slag).
Effects of Polypropylene Fibers on Preventing Concrete Spalling In Fire
The addition of polypropylene fibers (PP-fibers) has been proved effective to prevent explosive spalling of concrete elements in tests. With PP-fibers, the permeability of concrete will increase significantly above the melting of PP-fibers, as a results, high pore pressure can be prevented. To investigate the effects, the permeability of concrete with PP-fibers has been measured. The effect on explosive spalling of concrete is investigated by experimental and numerical methods. Apart from the amount, the fiber geometry must be taken into account. For the PP-fiber combination investigated by fire test, the recommended amount by Eurocode 2 is effective for these thin PP-fibers. The test result has been simulated by a spalling model. The spalling model considers the influences from the combination of PP-fiber with a simplified increasing factor, the application of other types and amounts of PP-fibers can be evaluated by this model.
Reuse of Waste Tyre Fibres in Concrete—Fire-Spalling Mitigation
This paper presents an experimental study investigating the effectiveness of reused tyre polymer fibres (RTPF) and reused tyre steel fibres (RTSF) in highstrength concrete to mitigate fire-induced spalling. Concrete is an inherently fire resistant material due to its low thermal conductivity and non-combustibility. Rapid advances have been made in the last few decades to improve the performance of concrete in large-scale structures (e.g. tunnels), which are driven mainly by factors (e.g. strength, cost and sustainability) other than fire safety. It has been found that modern high performance, high strength concrete is more vulnerable to fire-induced spalling. Many research have been carried out to improve the resistance of such concrete against fire-induced spalling, and the most common solution is the addition of polypropylene fibres (PPF). Each year in the EU, more than 3.5 million tonnes of tyres reach the end of their lives. These tyres comprise roughly 15% and 5% (by weight) steel and polymer fibre reinforcements, respectively. These fibres are then extracted during the tyre recycling process, but they are mostly landfilled or used as fuel since they are too agglomerated and contaminated by rubber to find alternative use. A series of concrete specimens with RTPF and RTSF were tested under thermo-mechanical loading. The results indicate the potential of replacing the manufactured fibres with these re-used ones to mitigate fire-induced spalling in concrete.
Constitutive Models of Concrete at Elevated Temperatures: Studying the Effect of Temperature Gradients
The outbreak of fire can have serious consequences in the structural performance of a load-bearing concrete structure. To assure adequate fire performance, detailed knowledge of fundamental mechanical properties of concrete at elevated temperatures is crucial. This paper first highlights limitations of existing knowledge regarding the mechanical response of concrete at elevated temperatures, including the inconsistent thermal boundary conditions and intentionally-minimised temperature gradients in “standardizedâ€ conventional concrete material testing. Accordingly, it is argued that the effect of temperature gradients within concrete on its fire performance has not been extensively or directly addressed. On this basis, the paper outlines key features of an ongoing research programme at The University of Queensland aimed at studying the performance of concrete in fire using a novel medium-scale testing method. By heating using radiant panels, welldefined and consistently-controlled heat flux boundary conditions on concrete cylinders (ï¦100mm x 200mm) have been achieved. The repeatability, consistency, and uniformity of thermal boundary conditions are demonstrated using measurements of heat flux, temperature profile, and compressive strength. Analysis of initial obtained data shows that the incident heat fluxes, and thus the associated temperature gradients, have potentially significant effects on concrete properties at elevated temperatures. Further research is thus ongoing to quantify such effects and also to develop models for their inclusion into effective performance-based fire design and analysis of concrete structures.
Post-cooling Stress-strain Model of Traditional and High-Strength Concrete
As concrete structures suffer from severe fire damage, but may retain a certain remaining loadbearing capacity, it is important to have material properties for assessment by calculation after fire. This paper proposes a full stress-strain model for post-cooling conditions of a traditional calcareous concrete (TCC) and a high strength siliceous concrete (HSSC).
Experimental Study on Mechanical Properties of Ultra-high Strength Concrete at Elevated Temperatures
This paper introduces experimental study on mechanical properties of an ultrahigh strength concrete at elevated temperatures. The compressive strength on cylinders and modulus of elasticity of the ultra-high strength concrete (fc = 166 N/mm2) were investigated up to 800°C. The temperature dependent mechanical properties were compared with those of normal/high strength concretes provided in Eurocode 2 and ANSI/AISC 360-10, and with those of concretes in literature. The comparisons showed that the compressive strength and elastic modulus of the ultrahigh strength concrete were generally reduced less than those of normal/high strength concretes at elevated temperatures, indicating higher fire resistance when it is used in structural load-bearing elements. The temperature-dependent mechanical properties are proposed to evaluate fire resistance of concrete filled tubes using the ultra-high strength concrete in high-rise buildings.
Assessing Tensile Properties of Fire-Damaged Engineered Cementitious Composites with Hybrid Fibres
Engineered Cementitious Composite (ECC) is one special type of fibre-reinforced cementitious composite which is well known for its strain hardening behaviour under direct tension. For practical applications of ECC in buildings and infrastructures, performance of ECC under different environments should be evaluated. The fire resistance of ECC is questionable, especially when non-fire-resistant polymer fibres are used. PVA fibres are most widely used polymer fibres in ECC, however, they will melt at temperature around 220 °C leading to disappearance of strain-hardening behaviour. In the study, 0.5% volume of PVA fibres was replaced by steel fibres in ECC so as to alleviate the negative impact of melting of PVA fibres. Tensile stress-strain relationships of this type of ECC with hybrid fibres after 100 °C to 600 °C at an increment of 100 °C were investigated. Digital Image Correlation (DIC) technique was used to capture cracking patterns of fire-damaged ECC. It was found that steel fibres improve tensile behaviour of ECC at 300 °C. After 600 °C, 79% of tensile strength of ECC remained. The results provided useful information of tensile behaviour of ECC with hybrid fibres and yielded promising direction for future improvement of tensile properties of ECC at elevated temperatures.
Experimental Validation of the Damage-Plasticity Modeling Concept for Normal Strength Concrete in Fire
The softening behavior of concrete and its elastic stiffness evolution with increasing plastic straining can be investigated experimentally with strain-rate controlled uniaxial cyclic compression tests. Such tests at ambient temperature show that concrete exhibits the phenomenon of elastic stiffness degradation, which can be captured by damage-plasticity models. However, temperature-dependent concrete models implementing this modeling concept are often used today in structural fire engineering, despite the lack of experiment-based validation data. This paper presents the results of a preliminary study on the behavior of normal strength concrete under cyclic compressive loading at elevated temperatures. The experimentally derived evolutions of the elastic stiffness with plastic strain confirm (1) the suitability of the damage-plasticity modeling concept for concrete in compression at elevated temperatures and (2) provide novel calibration data
Post-Fire Residual Mechanical Properties of High Strength Concrete (HSC) Made with Basalt Aggregate
This paper addresses an investigation of the residual mechanical properties of high strength concrete produced with coarse basalt, limestone and granite aggregate and exposed to temperatures of 200ºC, 400ºC and 600ºC. The paper objectives are: comparatively analyze test results with data from standards and results from other authors, evidencing the influence of coarse aggregate in the material performanceÍ¾ and provide reduction coefficients of mechanical properties of concrete containing basalt aggregate exposed to high temperatures, which is used in many countries and have notable thermal properties, but is not addressed by the structural fire design standards
Thermal Properties of Green High-Performance Fiber-Reinforced Cementitious Composites Subjected to High Temperature
To study the influences of mix proportions and high temperature on the thermal properties of green high-performance fiber reinforced cementitious composites (GHPFRCC), sixteen mix proportions were designed using the Taguchi orthogonal method. Five variables investigated were the water-to-binder ratio (0.24, 0.28, 0.32, and 0.36), sand-to-binder ratio (0.36, 0.46, 0.56, and 0.66), polyvinyl alcohol (PVA) fiber content (1.5%, 1.7%, 2.0%, and 2.2%) by volume, fly ash replacement (60%, 65%, 70%, and 75%) by weight, and the superplasticizer content (0.10%, 0.15%, 0.20%, and 0.25%). Each GHPFRCC specimen was subjected to a target temperature (200 °C, 400 °C, 600 °C, and 800 °C), and then measured for its thermal conductivity using a transient plane source method. The experimental results showed that the thermal conductivity of GHPFRCC decreased dramatically in the initial stage up to 400℃, then slowly decreased with temperature until 800℃. Compared with normal concrete, the GHPFRCC is relatively low in thermal conductivity and, therefore, can be used as a better thermal insulation material for infrastructures.
Influence of Water on Load Induced Thermal Strain of Concrete
In the present study, the influence of the curing conditions and water cement ratios on the load induced thermal strain (LITS) was considered from the results of the transient tests for measuring the total thermal strain and weight loss of the specimens. The specimens (water/cement ratios were 40-65%) were cured with 3 methods. The results of the tests indicate that the LITS due to dehydration is much larger than that due to water evaporation. Moreover, the numerical model for the modeling of the LITS is proposed as a function of the weight loss of the specimens.
Elevated Temperature Behavior of Impact-Induced Partially Damaged Concrete
Designing protective structures capable of withstanding the combined extreme actions of impact/blast and fire necessitates the accurate prediction of material properties under the coupled effects of high-strain-rate and subsequent elevated temperature loadings. An extensive experimental program was being carried out in the Civil Engineering Laboratories at Monash University to investigate the postimpact fire properties of plain self-compacting concrete (SCC) material and the results are presented in this paper. This will help in evaluating whether partially damaged concrete elements can further sustain additional stresses in case of a subsequent fire outbreak. Specimens have undergone interrupted high-strain-rate compressive loading, controlled locally at defined levels of axial displacement, to account for different deformation states. Results indicate that the mechanical behavior of concrete subject to post-impact fire scenarios is dependent on the rate of loading, the damage history and the fire temperature to which it is subsequently exposed.
Beam-columns with Thin Wall Cross-sections in Case of Fire
The fire behaviour of class 4 cross-sections beam-columns is investigated in this paper. The local buckling of the cross-sections as well as the global instabilities are evaluated. A numerical parametric study is conducted with the help of different finite element software to analyse the efficiency of current EN 1993-1-2 simple design rules. It is demonstrated that these current simple rules do not allow a cost-effective design of such steel elements. Following the new simple design rules developed in the scope of FIDESC4 project to calculate the effective properties of class 4 cross-sections and the lateral torsional buckling resistance/buckling resistance of corresponding members, a new interaction curve is proposed and the numerical results are now compared with the new obtained resistances. Statistical results are given to approve the new method and show its efficiency in terms of safety level and economic design.
Temperature Assessment of a Vertical Steel Member Subjected to Localised Fire: Experimental Tests
This paper describes the series of pool fire tests performed by the Universities of Liege and Ulster in order to assess the heat fluxes received by vertical members in case of localised fires. All these tests, performed in the scope of the RFCS project LOCAFI, have allowed gathering a large amount of experimental data for calibration and validation of subsequent numerical and analytical models. The influence of several parameters was investigated, such as the type of fuel (heptane, diesel, wood cribs), the dimension of the fire pool (diameter from 0.7 m to 2.2 m), the presence of a column (tubular or I-shape sections) and the presence of a ceiling. Although the wind speed remained limited as these tests have been performed indoor, the inclination of the flames was nevertheless significant and, in many cases, the measurements of gas temperature close to the column showed a quite significant asymmetry.
Component-Based Element of Beam Local Buckling Adjacent to Connections in Fire
An analytical model based on the yield line mechanism [1] has been proposed by the authors to predict the beam-web shear buckling and bottom-flange buckling in fire. This paper described the development of a component-based element considering both buckling phenomena at the beam-ends, based on this analytical model. The component-based buckling element consists of top springs and bottom springs, all of which are capable of dealing with loading-unloading-reloading cycles. This new component-based element has been implemented into the software Vulcan, adjacent to the existing component-based connection element. An example case using a singlespan beam has been analysed in Vulcan. The modelling results have been validated against finite element modelling using ABAQUS, indicating that the newly developed component-based buckling element is of good accuracy to reflect the behaviour of local buckling phenomena. Comparison of the structural responses of the beams with and without the buckling element in Vulcan has been carried out. This comparison shows that with and without the buckling element does make a difference in the modelling results. This indicates the potential effects of the local buckling at beam ends on the connected joints and columns and on the entire frames at elevated temperatures.
Lateral-torsional Buckling of Beams with Corrugated Webs Subjected to Fire
The use of beams with corrugated webs (WCB) has been increasing considerably during the last years due to the high load-carrying capacity in relation to the material usage. The major asset of this structural solution lies in taking advantage of the increase of rigidity provided by the web corrugation leading to better resistance against local buckling and improved shear capacity resulting in higher load bearing capacity combined with significant weight reduction when compared with beams with flat webs. Despite known benefits of WCB, their behaviour, especially in fire situation, still lacks simplified formulae to enable the designers to exploit better this structural system. Focusing on this aspect, this study aims to develop simplified methods to predict the load bearing capacity, based on numerical simulations with the finite element method, overcoming the limitations encountered in adapting existing design methods of Eurocode 3 to the fire situation. A proposal is developed in this work which presents a new design philosophy for the fire situation closer to the principles of other structural systems while, at the same time, deliver more accurate solutions for the engineers in practice to use.
Fire-induced Progressive Collapse of 3D Steel Frames with Concrete Floors
This paper investigates the behavior of three-dimensional steel frames with reinforced concrete slabs exposed to localized fire against progressive collapse. The prototype of the model is based on the eight-story building in Cardington tests. The scenario of heating an individual column on the ground floor is adopted. The collapse modes and load redistribution scheme of the frame subjected to different load ratios of columns and fire locations are investigated. The results show that the frame does not collapse in the case of single column heated for a fire design load (load ratio of 0.25), due to the load redistribution to adjacent columns through floors. By increasing the load ratio to 0.5 as for the ambient design, progressive collapse occurs. The collapse modes are dominated by the uneven load redistribution in the two horizontal directions, which cannot be simulated by a 2D model. The loads previously sustained by the buckled heated column are transferred more along the short span than the long span, leading to the advanced buckling of columns along the short span. The critical temperature of a column in a frame is significantly lower (about 200oC) than that given in EC3, due to the fact that the translational and rotational restraints increase its load ratio and reduce its effective length, respectively.
Evaluating Post-fire Mechanical Behavior of Ultra-high Strength (Grade1200) Steel Tubes
This paper presents an experimental study on the post-fire mechanical behavior of ultra-high strength steel (UHSS) tubes with nominal yield strength of 1200MPa. In order to understand the changes occurring during cooling phase of a fire, the in-fire mechanical behavior of UHSS is also studied experimentally. The temperatures characteristic of fire simulated in this study are within the range of 300ºC to 800ºC. In the experimental tests, standard dog-bone UHSS specimens are subjected to quasistatic tensile tests at fire temperatures and after being cooled to room temperature. The stress-strain curves, strength and ductility of the tested specimens are discussed. To investigate the effect of steel grade on the post-fire mechanical response, the strength deterioration of different grades of steels available in literature including grades of 460, 690, 800 and 960, are compared with those of UHSS (Grade 1200). Finally, the obtained results are explained based on the phase changes occurring in the steel material during fire by calculating the thermodynamic stability of the ferrite and cementite phases in the tested materials.
Effect of Initial Geometric Imperfection on the Performance of Single-Layer Reticulated Shells at Elevated Temperatures
Single-layer reticulated shells are belong to the structures sensitive to initial geometric imperfections, however in the study of the behavior of single-layer reticulated shells under fire conditions, the effect of initial geometric imperfections was rarely considered. This paper presents a numerical study investigating the effect of initial geometric imperfections on the performance of single-layer reticulated shells at elevated temperatures using the finite element software ANSYS. The results of the stability analysis at different temperature points show that the buckling load of the structure with initial geometrical imperfections is less than that of the ideal structure, and the results of the analysis under condition of increasing temperatures at different loads indicate that the critical temperature of the structure with initial geometrical imperfections is less than that of the ideal structure.
The Performance of Structural Steel Beams Subject to a Localized Fire
This paper presents the results from the open flame, localized fire tests conducted on 6.17 m long, simply supported W16×26 beam specimens. The cross sections at midspan (i.e., expected plastic hinge zone) of the beam specimen were directly exposed to the natural gas fire. Two different tests were conducted: (1) firethermal tests to evaluate the effects of the prescribed heat release rates (HRR), provided by the 1 m2 natural gas burner, on the thermal responses of the specimen and (2) structural-fire test to evaluate the fire effects on the overall behavior and the loadbearing capacity of the specimen. The test results indicated that the prescribed heat release rates from the burner affected the heating rate of the specimen. When the HRR-time relationship of the burner followed a step function, the fire-exposed region of the beam specimen was heated essentially linearly with increasing time of fire exposure. When the HRR was set to a target magnitude of 400 kW throughout the test, the fire-exposed region was heated nonlinearly until it reached a steady-state temperature condition. When the beam specimen was subjected to linearly increasing flexural loads at a maintained HRR of 700 kW, combined flexural and lateral torsional failure of the specimen was exhibited. The lateral deformations in the compression flange at the fire-exposed critical sections initiated at 124 ± 5 kN-m, which is 39% of the plastic moment capacity at room temperature. The peak moment capacity was 171 ± 9 kN-m (54 % of the plastic moment capacity at room temperature), while the maximum temperature was 642 ± 28 ËšC at the HRR of 700 kW. The test results from the present study can be used for developing or calibrating analytical models, which can be eventually used for evaluating the performance of structural members subjected to a localized fire
3D FEA of Load-bearing Cold-formed Steel Wall Systems with Web-perforated Studs Subjected to Standard Fire
Presented in this paper is a study using finite element (FE) analysis to investigate the thermal and structural performance of CFS load-bearing walls with C-shape studs with web perforations subjected to standard fire. Sequentially coupled 3D FE thermalstress analysis is carried out in which the heat transfer analysis is conducted to obtain the temperature distribution of an entire CFS wall. In the subsequent structural analysis, a CFS wall frame rather than a single stud is modeled to achieve system-level structural responses. The axial force at the bottom end of the stud is used to predict the failure of the wall. The finite element analysis (FEA) results are compared with that from two full-scale fire tests.
Experimental and Numerical Analysis of Cold-Formed Steel Columns at Both Ambient Temperature and Simulated Fire Conditions
Cold-formed steel profiles with a wide range of cross-section shapes are commonly used in building construction as structural elements. Also the use of builtup members has increased significantly, however, so far, its design is only briefly addressed in the current design codes, such as EN 1993-1-3 [1]. Regarding fire behavior of built-up structural members the research is still scarce, especially if the influence of restraining to thermal elongation provided by the surrounding structure is considered. Taking into account the current limitations it was intended to conduct a large experimental campaign in order to assess the behavior of single and built-up cold-formed steel columns at both ambient temperature and simulated fire conditions with restraint to thermal elongation. In this paper some of the most relevant results as well as the finite element models developed to accurately reproduce the behavior of cold-formed steel columns are presented.
Modeling Fire Induced Burnthrough Rupture of Marine Grade Aluminum Panels
Burnthrough of aluminum plates subjected fire was experimentally and numerically explored. Experimental burnthrough was observed to occur below the material melting temperature and during thermal steady state leading to the development of a new creep rupture based burnthrough model. To characterize this behavior, a series of uniaxial, isothermal creep experiments were used in combination with published data. The creep rupture based burnthrough model was implemented in finite element models of the burnthrough experiments. Simulated burnthrough times were generally within 50% of experimental values. Much of this difference is attributed to the repeatability of the diffusion flame exposures which resulted in variations in experimental and simulated burnthrough times of up to 50%.
Buckling Analysis of Rack Uprights Exposed to Localised Fires
This paper describes a numerical investigation into buckling behaviour of bare cold-formed steel uprights subjected to compression when exposed to a localised fire in a large open-plan compartment (i.e. storage warehouse). Two positions of fire sources, at the bottom and at the mid-span of uprights, are considered in the study. Heat transfer analysis for both fire sources are initially studied to consider the temperature gradients along the length of upright. According to EN 1991-1-2, a subroutine is compiled to simulate the non-uniform temperature distributions of member exposed to a localised fire and the subroutine is implanted into ABAQUS. Steel rack uprights with various boundary conditions are investigated to determine the critical buckling loads of uprights. The developed finite element model is carefully validated against experimental data. The results reveal that the variations of temperature along the length of member have some influences on the buckling performance. Moreover, the boundary restraint appears to be a significant influence on the buckling performance when the members experience different fire sources.
Creep Behavior of Flush Endplate Connections at Elevated Temperatures Due to Fire
This paper highlights some preliminary results of a computational study investigating the effect of thermal creep of structural steel on the behavior of steel beam-column connections subjected to elevated temperatures due to fire. Through a series of finite element simulations that were partly verified against experimental data, a practical methodology was developed to investigate the time-dependent nature of the behavior of flush endplate beam-column connections at elevated temperatures. In this methodology, time effects on the strength and rotational capacity of flush endplate connections are explicitly presented in the form of isochronous force-rotation curves. The isochronous representation provides a possible framework for including creep effects in predicting the response of structural steel connections to fire.
Experimental and Numerical Study on High Strength Steel Endplate Connections after Fire
In order to reveal more information and understanding on performance and failure mechanisms of high strength steel endplate connections after fire, an experimental and numerical study has been carried out. Firstly, full-scale tests on seven endplate connections were carried out after cooling down from fire temperature 550°C. The moment resistance, rotation capacity and failure mode of high strength steel endplate connections after fire were obtained and compared with those of mild steel endplate connections, as well as with their performance at ambient temperature without fire exposure. Moreover, the provisions of Eurocode 3, which were mainly obtained based on the mild steel structures, were validated with the test results of high strength steel endplate connections. Furthermore, a numerical study via ABAQUS is reported and validated against the experimental results. It has been demonstrated that this finite element analysis gives reasonable accuracy compared with the experimental results, providing an efficient, economical, and accurate tool to assess the post-fire performance of high strength steel endplate connections.
Experimental Study of High-Strength Bolts under Combined Tension and Shear During and After Fire
Experimental studies on high-strength bolts which included both tests under tension and shear show deviating reduction of strength with rising temperatures depending on whether the bolts were loaded by tension or shear. In this paper an experimental study on high-strength bolts of the property class 10.9 under a combination of both tension and shear will be presented. Tests were carried out both under fire conditions and at ambient temperature on bolts first heated to a specified temperature and then cooled slowly to establish the post-fire performance
Post-Fire Mechanical Properties of High Strength Grade 8.8 Steel Bolts
In order to understand the post-fire performance of bolted connections, it is essential to have a deep knowledge of the material behavior of all the components, including bolts, after fire conditions. Very limited research has been carried out on the behavior of high-strength steel bolts during and after fire conditions. This paper presents experimental investigations of the residual mechanical properties of Grade 8.8 bolts after exposed to elevated temperatures up to 900°C. The post-fire stressstrain curves, residual elastic modulus, yield and ultimate strengths as well as tensile failure modes of bolts were investigated. Also, the effect of factors including target temperature level, chemical composition of feedstock steels (SAE 10B21 and 10B38) and heat treatment characteristics in production process of Grade 8.8 bolts were studied. It was observed that, the bolts exhibited drastic reduction in strength after cooling down from temperatures beyond 400°C, as they lost about 50% of their ultimate strength after experiencing 800°C. Furthermore, a set of predictive equations are proposed for evaluating the residual mechanical properties of Grade 8.8 bolts after fire.
Analytical Study on Critical Temperature of Bolted Splice Connections
The resistance of bolted splice connections is calculated on the basis of tensile strength obtained from high temperature tensile tests (coupon tests) of steel plates and bolt materials. The strengths of both steel plates and bolt materials over 500℃ are greatly affected by the strain rate applied in the high temperature coupon test. This paper investigates the effect of strain rate applied in high temperature coupon tests on the ultimate tensile strength of steel plates and bolt materials by comparing the test and analysis results of 20 cases. These strain rates eventually have a remarkable relation to the behaviour of the connections in elevated temperatures. Numerical analysis results on the basis of a component-based model using the strengths from high temperature coupon tests under high-strain-rate approximately agreed with the critical temperatures of bolted splice connections obtained from elevated temperature tests.
Fire Performance of Hybrid Stainless-carbon Steel Composite Beam-column Joints with Blind-bolted Connections
This paper presents the fire resistance and post-fire test results of eight full-scale concrete filled stainless steel tubular (CFSST) column to steel beam joints with blindbolted connections. The investigated parameters included whether applying fire protection to the steel beam or not, beam load ratio (0.25 and 0.5), steel tube type (stainless and carbon steel tubes) and presence of binding bars or not. The test results show that the joint failure was mainly dominated by flexural failure of the steel beams near the panel zone despite the variation of parameters. But when the beam load ratio was 0.25, the joint specimen with a circular column failed due to axial crushing of the column. In general, the blind-bolted connections demonstrated good workability in fire, and no bolt shank fracture or bolt pull-out failure was observed in any joint test.
The Effects of Load Intensity and Restraint on the Fire Resistance of Steel and Composite Beams
The paper presents the results of sixteen fire resistance tests (in accordance with ASTM E119 and ANSI/UL 263 standards) on structural steel beams and composite steel/concrete beams conducted by the authors in 2015 at the Underwriters Laboratories facility in Northbrook, Illinois. The described experimental program was designed to investigate the effects of load intensity and restraint on the fire resistance of steel and composite beams in the context of contemporary construction materials and structural design standards. In terms of the scope and the range of investigated parameters, such experimental study was carried-out in North America for the first time. The test results confirmed the beneficial effects of reduced loads and restraint on the performance of beams in standard fire resistance tests and quantified these beneficial effects in terms of critical (failure) temperatures. The tests results also confirmed and quantified the long existing knowledge about non-composite structural steel beams performing much better than comparable composite steel/concrete beams in fire resistance tests. The generated experimental data will be used to validate numerical models, conduct parametric studies and develop simplified correlations for load intensity versus fire resistance time (and/or protection thickness). It will be of interest to certification laboratories, product developers and the broader structural fire protection design community.
Numerical Modeling of Heat Transfer in Steel-concrete Composite Slabs
This paper presents detailed and reduced-order numerical modeling of heat transfer in composite floor slabs with profiled steel decking for analysis and design of structures exposed to fire. The detailed modeling approach represents the concrete slab with solid elements and the steel decking with shell elements. The reduced-order modeling approach represents the thick and thin parts of a composite slab with alternating strips of layered shell elements. The detailed modeling approach was validated against experimental results available in the literature, and the reduced-order modeling approach was calibrated and verified against the detailed model results. A parametric study using the detailed modeling approach was conducted to investigate the influence of slab geometry on the temperature distribution within composite slabs. The results show that the rib height of the decking and the width at the top of the rib are key factors governing the temperature distribution in the rib. The paper also presents comparisons with Eurocode 4 calculations of fire resistance of composite slabs. The comparisons indicate that the Eurocode 4 overestimates the fire resistance compared to the numerical results by up to 12 %.
Evaluation of Composite Steel-Concrete Slab Performance Subjected to Travelling Fire: A Case Study
This paper presents a case study which aims to contribute a better understanding of the fire performance of composite slab panel structural under non-uniform fire. A travelling fire which simulates the procedure of fire spread over a large office compartment is introduced. The coupled CFD modelling and heat transfer model is used for the fire and thermal analysis. The elevated composite slab temperatures are applied to finite element model for structural fire analysis by using ABAQUS. The phenomenon of tensile membrane action in partially fire protected composite slab when subjected to travelling fire is presented. It has been observed that the predicted office compartment fire has slower temperature growth rate than that of the ISO 834 standard fire as well as the parametric fire. The study has found out that the traditional structural fire engineering design may lead to over-conservative prediction with respect to composite slabs structural fire performance under travelling fire.
Development and Modification of Yield Line Patterns in Thin Slabs Subjected to Tensile Membrane Action
It is widely recognized that composite floor slabs experiencing large displacement develop a central zone of hydrostatic membrane tension, surrounded and equilibrated by a ring of membrane compression around the periphery. This mechanism, known as tensile membrane action, can greatly enhance the load-bearing capacity of a slab compared with that defined by yield line analysis. This is a very useful effect in cases where large deflections can be accepted, particularly in fire-resistance design of composite slabs, since the strength enhancement permits some beams to be left unprotected. Studies of tensile membrane action in the 1960s led to the development of several methods to define slab load capacity under large displacement. The method due to Hayes [5] has become the most widely accepted, and was adopted by Bailey [1, 2] in developing the BRE method for fire-safe design of composite floors. Based on observations from the Cardington fire tests and on assumptions concerning yield line patterns and membrane stresses, it calculates the load-carrying enhancement of a slab as a function of its deflection [2]. It also postulates a deflection limit at which the maximum acceptable strain in the rebar is reached. On close examination, however, several hypotheses, such as the assumed failure mechanisms, seem illogical. The BRE method assumes that a common observation, of a through-depth crack forming across the central short-span of the slab, represents the limit state for such slabs. However, it has been observed that similar cracks can also appear at the intersections of the yield lines, or even not appear at all [3]. This paper proposes a simple way to define the deflection at which the through-depth crack forms, and where on the slab it appears. Based on consistent kinematic assumptions, it calculates the tensile stresses at key points of the slab [4], and predicts the position and displacement at which through-depth cracking occurs.
Advanced Analysis of the Membrane Action of Composite Slabs under Natural Fire Scenarios: A Case Study of the New JTI Headquarters
The paper details the advanced natural fire simulations that were carried out for the composite steel-reinforced concrete structure of the JTI Building in Geneva. Such analyses led to a significant reduction of the fireproofing of the steel floor framing. Several scenarios were studied considering different thermal behaviours of the peripheral cladding. Despite the small thickness of the resisting slabs, the analyses carried out with SAFIR showed that the typical storey bay (12 m x 15.86 m) could resist to the design fire temperatures without the protection of the main and secondary beams while the spandrels remain protected. For study completeness, the composite frame-membrane model was also simulated with Hasemi localized fire routines on SAFIR. The results are here exposed and commented.
Stability Behavior of Concrete-Filled Steel Tube Columns in Real Fire Conditions
Finite element based method is used to simulate the fire behavior of CFT columns at elevated temperatures. The proposed technique is calibrated to against experimental data to find the values of several input parameters such as the post-crack tension behavior of concrete and the normal and tangential behavior of the interface between the steel and the concrete components. The preliminary results indicate that the interface model and the tension properties of concrete play a very critical role in determining the load capacity of CFT columns at elevated temperatures.
Development of a Calculation Method for the Fire Design of Concrete-Filled Steel Tubular Columns
This paper presents a method for evaluating the fire resistance of concrete-filled steel tubular columns (CFST), developed in the framework of the European Project FRISCC. The aim of this project is to develop a full methodology that solves the shortcomings of the current design rules in EN 1994-1-2, proved unsafe for slender columns. For that purpose, two numerical models are developed and validated against the results of an extensive experimental programme. The numerical models are subsequently used for conducting a comprehensive parametric study. Based on the results of the parametric study, design rules for the evaluation of the fire resistance of CFST columns of different cross-section shape are developed, extending the applicability of the current code provisions. Innovative sections, such as elliptical hollow sections (EHS) are accounted for in the method. The design proposal is divided into two parts: a simplified cross-sectional temperature field assessment and a method for evaluating the ultimate buckling load in the fire situation, using the equivalent temperatures obtained from the previous step. The effect of eccentricity is evaluated through a modification in the method, which accounts for eccentricities on both minor and major axis, reaching large eccentricities of e/D = 1.
Experimental and Numerical Investigations on the Composite Behavior of Concrete-Filled Tubular Columns with Massive Steel Core at High Temperatures
This paper deals with experimental and numerical investigations on concrete-filled tubular columns with embedded massive steel core (CFTES columns) aiming at justified design recommendations on reliable shear stresses.. Push-out tests at room temperature and high temperatures are described in detail and analyzed in terms of ultimate shear strength, bond strength and shear strength-displacement-curve shape. The test data reveal a distinctive reduction in both ultimate shear and bond strength for high temperatures, whereas the strength-displacement-curve shape is in general similar for all investigated temperature levels. The numerical implementation of the performed tests reproduces the experimentally observed shear strength-displacement-behavior in very good accordance. Using a three-dimensional volume model in Abaqus and applying temperature-dependent joint stiffness, maximum shear stress criterion and damage evolution, the composite behavior is described in a realistic manner.
Numerical Analysis of the Fire Performance of Innovative Steel-Concrete Composite Columns
Previous investigations have revealed the limited fire resistance of concrete-filled steel tubular (CFST) columns in applications where the slenderness is important. Innovative solutions are therefore sought to help improving the performance of this typology of composite columns in the fire situation. This paper aims at designing strategies for maximizing the fire resistance of CFST columns using innovative solutions such as double-tube sections, embedded HEB profiles or embedded steel cores. Additionally, the fire performance of such columns may be enhanced by using high strength steels (HSS). In this paper, a numerical model is developed and, after being validated against experimental results available in the literature, it is used for conducting a parametric study. Different steel grades are considered for the inner profiles, in order to investigate the influence of HSS. Through the results of the parametric study, the fire performance of the different cross-sectional geometries is compared, and recommendations are given for enhancing the fire resistance of CFST columns.
Experimental Investigation on Axially and Rotationally Restrained Circular and Elliptical Concrete-Filled Hollow Columns Subjected to Fire
This paper presents the outcomes of an experimental investigation on the performance of axially and rotationally restrained concrete-filled hollow columns subjected to fire. The specimens were uniformly exposed to a standard fire curve and the critical time (fire resistance), the failure temperature and the respective failure modes were assessed. The paper also includes a comparison of the behavior of these columns under both axial and rotational restraint with identical columns under axial restraint only. The primary test parameters take into account in this research work were the column slenderness, the type of cross-section (circular and elliptical) and the restraint level imposed by a surrounding steel frame to the column in test. Finally, the results of this research study showed mainly that the fire resistance of identical columns under low load levels may be not significantly affected by the stiffness of the surrounding structure, except for low rotational restraint levels and high slenderness values.
Flexural Behaviours of Reinforced Concrete Columns Confined by Circular Steel Tubes after Fire Exposure
The post-fire flexural behaviours of steel tube confined reinforced concrete (STCRC) columns (also known as reinforced concrete columns confined by circular steel tubes), were investigated in this paper. A 3D finite element (FE) model was developed with the program ABAQUS, using the sequentially coupled thermal-stress analysis method. A set of test data was used to verify the FE model and good agreement was achieved. The moment versus curvature curves were analysed and discussed. After that, parametric studies were performed to investigate influences of some key parameters on the flexural capacity and flexural stiffness of STCRC columns, including heating time, cross-sectional dimension, material strength, steel tube to concrete area ratio and reinforcement ratio. Simplified design methods were proposed for predicting flexural capacity and flexural stiffness of STCRC columns after exposure to ISO-834 standard fire condition.
Experimental Study on Bond-slip between Shaped Steel and Concrete under High Temperature
This paper presents results from an experimental study on the effect of temperature on bond strength between steel and concrete. 11 steel reinforced concrete (SRC) specimens were tested by home-made fire test furnace to evaluate bond strength under different constant high temperatures (20-600°C). Results from these tests indicate that the ultimate bond strength and the residual load decrease significantly in 20-250°C temperature range, the ultimate bond strength is much lower at high temperature and just retain 20-30% of its original property at 250°C. It is well established that the quantity of extreme slip (the slip at the ultimate bond load) at evaluate temperature is larger than that at room temperature, and fluctuate with the temperature from 1mm to 8mm. Data from tests also proves that the variation trend of bond-slip curve in 20- 250°C temperature range is similar to that at room temperature, and the descending stages of bond-slip curves disappear from 300°C to 600°C. Data from the test is utilized to propose the empirical relations of bond strength and temperature.
S500M and Grade 70 Steel Grades—Material Properties Assessed through Full Scale Tests
This paper presents two experimental fire tests performed at the University of Liege in the scope of an internal project supported by ArcelorMittal Global R&D and focused on the fire resistance of S460M and S500M steel grades at high temperatures. Two similar beams have respectively been subjected to fast (standard ISO curve) and slow (fixed 5°C/min heating rate) elevations of temperature under a mechanical loading kept constant. The objective of these fire tests is to provide data on the resistance of S460M and S500M steel grades in the critical temperature range [500 °C; 700 °C] through full scale tests. A comparison between the reduction factors obtained from experimental tests and those defined in European and American standards [1,2] will allow confirming that reduction factors defined in these standards also apply to S500M and Grade 70 steel grades.
Development of True Stress-Strain Curves of Structural Steel for Fracture Simulations
True stress-strain curves are needed for fracture simulation of structural steel using the finite element analysis method. This paper summarizes recent research on development of true stress-strain curves for ASTM A992 structural steel at ambient temperature and at elevated temperatures up to 1000°C. Calibration of true stressstrain curves was based on measured engineering stress-strain curves of ASTM A992 steel obtained from isothermal tests for tension coupons at both ambient and elevated temperatures. Prior to necking, the true stress-strain curve was derived directly from the engineering stress-strain curves using well established equations. After the onset of necking, due to the non-uniform deformation and complex stress state in the necked region, the established equations are no longer valid. Thus, in this research, true stressstrain curves for the post-necking region were developed by calibrating detailed finite element models of the tested tension coupons to match measured engineering stressstrain curves. Comparisons are presented between measured engineering stress-strain curves and simulated results to examine the accuracy of the calibrated true stress-strain curves. Furthermore, generalized true stress-strain curves based on the derived curves are proposed for fracture simulations for ASTM A992 steel.
Constitutive Modelling of Ductile Metals at High Temperature
Constitutive behaviour of ductile metals subjected to thermo-mechanical loads associated with a fire condition as well as small-to-medium rate mechanical loads is discussed in this paper. Effective analysis and design of structures in fire require material models that are valid over a wide range of stresses, strain rates, loading and unloading cycles and temperatures. Thermal loading of metals results in a slow, creep-dominated behaviour, which can lead to local or global component failures and progressive collapse. The collapse mechanism for structures and their components, once initiated, occurs at much higher strain rates than creep. A comprehensive modelling framework that allows simulation of large-scale metal structures subjected to a wide range of stresses, strain rates and temperatures is presented. An advanced viscoplastic constitutive model is developed based on micromechanical material failure mechanisms and experimentally observed time- and temperaturedependent material behaviour. This model (WAifire) allows bridging the time-scale gap between creep-dominated behaviour and resulting thermo-mechanical stiffness and strength degradation on the one hand, and the high strain rate deformation and fracture caused by structural collapse on the other hand.
Constitutive Modeling for Fire Engineering Purposes—The Challenge of Modeling Creep in an Appropriate Way
The phenomenon of creep of structural steel in fire engineering is gaining more and more interest. Due to shortcomings of the EC3 constitutive model and other approaches based on classical creep theory an alternative approach mainly based on the ideas of Krempl [3] is proposed. Due to this so-called operator approach the material as well as the constitutive model is looked upon as an operator. The application of this view serves as a guide through the analysis of the relationship between experimental facts and constitutive theory especially in the context of transient creep
High-temperature Material Constitutive Models for Structural-Fire Analysis
The applicability of three steel constitutive models was evaluated using finiteelement analyses and various member capacity equations. Three different hightemperature stress-strain models were compared: the model recently developed by the National Institute of Standards and Technology (NIST) [1], the Eurocode 3 model [2] and the model developed by Lie [3]. The testbed used in the analyses included twenty steel column tests and two restrained steel beam tests reported in the technical literature. The selected column tests reported buckling temperatures ranging from 500 oC to 700 oC and applied axial load ranging from 20 % to 65 % of the axial-load capacity at ambient temperature. Each reported test was analyzed in two different ways: (1) finite-element model was developed to predict the buckling temperature of the steel columns and response of the restrained steel beams in fire condition. (2) member capacity equations prescribed in Eurocode 3 and ANSI/AISC-360-10 [4] were used to compute the buckling temperature of the steel columns. Overall, the results indicate that all investigated material models give acceptable prediction of the buckling temperature of the steel columns and the behavior of restrained beams. The finite-element model with the NIST and the Lie material models predict the buckling temperature more accurately than that with the EC 3 material model. When the Eurocode column capacity equations were used, the buckling temperatures calculated using the NIST and the EC 3 models are more comparable with test results than those using the Lie model. It was also found that the current ANSI/AISC 360-10 Appendix 4 equation conservatively estimate the buckling temperature of the tested column specimens with difference of 20% on average. When the standard column equation in the Chapter E of ANSI/AISC 360-10 was used, both the EC 3 and the NIST models accurately predict the buckling temperature of the tested column specimen with difference less than 5% on average.
Behaviour of High Strength Steel under Fire Conditions
This paper is concerned with the material characteristics of various commercial high strength structural steels (yield strengths between 460 and 700 N/mm2) at elevated temperatures. These steels vary in chemical composition and production route but have similar tensile properties at ambient temperature. Preliminary data of the following: proportional limit (fp,θ), elastic modulus (Ea,θ), effective yield strength (fy,θ) based on the total strain level at 2% (in accordance with the Eurocode approach) obtained from isothermal tests are presented as reduction factors and compared with literature and the Eurocode (EN 1993-1-2). The consequences for material selection and design are also discussed.
Development of Wood Structural Elements for Fire Resistant Buildings
As forestry contributes to the reduction of greenhouse gases by CO2 fixation, in recent years, use of wood in buildings has attracted all over the world more attention. However construction of large wood structures is almost inexistent within urban areas in Japan. This is due to the Japanese law on fire protection of wood buildings in cities, which is considered very strict with severe requirements. This paper presents a research work relative to the development of fire resistant wood structural elements for buildings in cities. These wood structural elements, made of glued laminated timber with self-charring-stop, have sufficient fire resistance during and after a fire, and comply with the strict Japanese standard for wood structural elements, which stipulates that such elements have to withstand the whole dead-load of concerned buildings after fire. To comply with such requirements, new elements of glued laminated timber with self-charring-stop layer were developed and their performance was confirmed. Several fire resistant tests conducted on columns, beams, column-beam joints, connections between beams and walls, and beams with holes were carried out. All tests proved that the elements have sufficient fire resistance. No damage was found out at the load-bearing part of the elements after testing. These wood structural elements have already been applied in six projects, where large-size wooden buildings were constructed in urban areas in Japan.
Changing Failure Modes of Cross-Laminated Timber . . . . . . . . . 643 RICHARD EMBERLEY, ALEXANDER NICOLAIDIS,
Cross-laminated timber relies on the adhesive layer between adjacent timber plies to provide composite action between the lamella for increased member strength and stiffness. Previous research has shown that adhesive loses normal and shear stiffness at elevated temperatures increasing the slip between adjacent timber plies. Slip in the bond layer results in reduced composite action increased deflections and a potential loss in ultimate strength in the CLT member. In order to study the effects of temperature on the flexural behavior of CLT, two series of tests were conducted. The first series focused on identifying the changing failure modes while the second series established conditions that led to those failure modes in large CLT beams. The results clearly showed the failure mode of CLT changes from timber failure to failure in the adhesive as a function of the in-depth temperatures. The adhesive failure yielded larger deflections and a loss in stiffness and ultimate strength.
External and Internal Factors Influencing the Charring of Timber—An Experimental Study with Respect to Natural Fires and Moisture Conditions
The following publication summarizes the experimental work of fire tests as well as the accompanied analytical studies conducted by the author in order to assess the influence of the initial moisture content as well as varying temperature time scenarios, including the heating- and cooling phase of a fire on the charring of timber and on the temperature profiles within timber cross sections. The outcome of this research is a simplified design approach for calculating the charring depth, and an improved material model that can be used in numerical simulations
Parameters Influencing the Behaviour of Timber Frame Assemblies Exposed to Fire, Development of New Design Criteria for Insulation Materials
Insulation materials may give a different contribution to the fire resistance of timber frame assemblies. At present, Eurocode 5 Part 1-2 [1] provides a model for fire design of the load bearing function of timber frame assemblies with cavities completely filled with stone wool. The extension of this model for glass wool for post protection phase has been published in the European technical guideline Fire Safety in Timber Buildings [2]. Very little is known about the protection of other insulation materials. This paper attempts to analyze the most sensitive parameters that describe in a universal way the protection against the charring given by different insulations not included in Eurocode 5 part 1-2. A series of model scale furnace tests of floor specimens for three different insulation materials was carried out. An analysis on the resistance of the residual cross sections was conducted by means of a resistograph device. The study offers some important insights into the concept of a new design model.
Deflection Behavior and Load-Bearing-Period of Structural Glued Laminated Timber Beams in Fire Including Cooling Phase
This paper reports on the behavior of structural glued laminated (glulam) timber beams under the constant load during and after fire conditions. Previous tests have indicated that the resistance of a beam exposed to standard fire for one hour declined by approximately 30% compared to that of a beam in ambient conditions. Additionally, it was reported that the strength of a beam exposed to heat in a furnace for one hour, followed by three hours of natural cooling, declined by approximately 14%. In this report, the fire performance of glulam timber beams is discussed based on their deflection behavior and load-bearing-period, which were obtained from load-bearing fire tests under constant load conditions.
Simulation of Charring Depth of Wooden-based Products when Exposed to Non-standardized Fire Curves
Fire resistance of wooden structures is based on the calculation of the charring depth. Analytical models for charring are given in regulations, e.g. the European Eurocodes, only for standard fire curves and for parametrical fires. This paper presents a numerical model for heat diffusion in wooden products and the induced charring. After calibration of the model it is used to analyze the fire resistance of a wooden beam under different fire models, including a non-standard fire curve.
Sectional Analysis of Cross-Laminated Timber as a Design for Fire Methodology
Current European structural fire design methodology for cross-laminated timber (CLT) elements predominantly uses a reduced cross-section method, which fails to realistically capture the variation of mechanical properties of timber at elevated temperature or with variable grain directions. A thermomechanical sectional analysis, compared against fire tests on CLT slab strips is presented herein, using two different sets of temperature-dependent mechanical properties available in the literature. This is investigated as an alternative, more rational design approach for CLT flexural elements exposed to fire. Initial results show reasonable correlation with experimentally observed failure times (i.e. fire resistances). Both failure time and deflection response are shown to depend on the temperature-dependent mechanical properties of timber used as modelling inputs, and the recommended properties presented in the Eurocode may not be suitable on the basis of the testing and analysis presented. Further research is needed to corroborate this conclusion
Timber under Real Fire Conditions – the Influence of Oxygen Content and Gas Velocity on the Charring Behavior
As for any building material, verification of fire resistance is mandatory for separating and loadbearing timber members. While non-standard fire design for steel members has long tradition, the corresponding possibilities for timber members are limited. Reasons for this can be found in the degree of complexity of the material and the limited research done in the field. This paper summarizes selected outcomes of tests investigating the influences on the charring behavior varying the oxygen content and the gas velocity. Besides the charring rate the char layer depth was the focus of this study to investigate char contraction (consumption of the char layer). In general, measurements are in line with previous results reported in literature. Results show that charring is predominantly depending on the fire compartment temperature. Results show further that for gas oxygen contents below 15 percent the gas velocity has no influence on the charring. However, at higher oxygen rates char contraction was observed affecting the protective function of the char layer. Thus, the charring and the temperature distribution was affected and the residual cross-section was decreased. In fully developed fires increased charring due to char contraction may not be observed due to the low oxygen contents. Contrary, in travelling fires or in the decay phase char contraction may be considered. This may have significant impact to Performance Based Design using non-standard temperature fire curves where the complete fire duration has to be taken into account. The work was performed within a FORMAS project “Natural fire design of timber structuresâ€ and the test series presented here were conducted within a Master Thesis in collaboration with the University of Trieste by A. Santomaso.
Full-scale Fire Test of a Laterally Loaded Light Timber-framed Compartment
The lateral stability of buildings during and after fire is of interest to fire-fighters, for protection of other property, and for post-fire investigators who may access buildings after a fire has occurred. A common approach for designing for lateral stability in New Zealand (NZ) is to provide sufficient resistance to ensure that a firerated structure does not collapse when subjected to a uniformly distributed horizontal face load and vertical loading. For residential buildings their external fire-rated walls are required to resist a lateral load of 0.5 kPa. This requirement results in the need for thicker walls, as these walls are assessed as isolated elements exposed to the standard fire. It is thought that by the consideration of the support the adjacent structure could provide to these walls, the requirement can be met with reductions in construction costs. This paper reports on the fire performance of a laterally loaded light timberframed wall by a full-scale furnace experiment as part of a research project that investigates the lateral stability of buildings during and after fire.
A Framework for Finite Element Modelling of Timber Connections in Fire
This paper presents a framework to model timber connections exposed to fire, using the Python [1] interface of Abaqus FEA [2] software suite for FE analysis, currently being developed at ETH Zurich. Models of three-member connections can be easily generated (with any arrangement of fasteners, member geometry, and material properties), submitted to the solver, and the results post-processed. The framework has been used to perform stress and uncoupled heat transfer analyses and is being extended to perform sequentially coupled thermal-stress analyses.
Comparative Behavior of Fire-Exposed Composite Girders Subjected to Flexural and Shear Loading
This paper presents results from experimental studies on the comparative behavior of fire exposed composite steel girders subjected to flexural and shear loading. Three composite girders, comprising of steel girders and concrete slab, were tested under simultaneous structural loading and fire exposure. The main test variables are type and magnitude of loading, as well as level of composite action. The composite girder, mainly subjected to flexural loading, failed through flexural yielding of steel girder without any signs of shear failure. On the other hand, girders with shear loading failed in shear through shear web buckling early into fire exposure. A comparative performance evaluation of response parameters clearly shows that shear limit state should be considered when evaluating behavior of fire exposed composite girders.
Factors Governing Response of Steel Bridge Girders under Fire Conditions
This paper presents results from numerical studies on the response of fire exposed steel bridge girders. A finite element model is applied to quantify critical factors influencing the fire resistance of steel girders in bridges. The varied parameters include: fire intensity, exposure scenario, web slenderness, and span length. Results from numerical studies indicate that fire resistance and failure mode in steel bridge girders is highly influenced by fire intensity, exposure scenario, and web slenderness. A typical steel girder can experience failure in less than 20 minutes under hydrocarbon fires. Failure through web shear buckling is the most dominant failure limit state, when web slenderness of the girder exceeds 50.
Postbuckling Shear Strength at Elevated Temperatures Using a Compression-based Approach
Steel plate girders are susceptible to web shear buckling at elevated temperatures; however, the underlying mechanics governing this buckling behavior have eluded understanding despite decades of research. Tension field theory, which assumes that postbuckling shear strength is derived from the formation of diagonal tensile stresses after elastic buckling, has traditionally served as the basis for web shear buckling models. While grounded in extensive experimental work conducted at ambient temperatures, tension field theory has been shown to become inaccurate over a range of plate geometries, steel yield strengths, and temperatures from 20oC to 1100oC. Given these limitations, the authors have previously proposed a compression-based approach that characterizes a web plate’s ability to develop postbuckling shear strength based on an interaction of diagonal compressive and tensile stresses. This paper presents the derivation of this compression-based approach for steel temperatures of 20oC up to 1100oC. It is shown that correlation to experimental results is within 13%.
Analysis of the Factors that Influence the Maximum Adiabatic Temperatures in I-girder Bridges
Bridges are a critical element of the transportation system of a country. To ensure bridge safety and functionality, bridge design is guided by bridge standards which prescribe the loads to be considered when designing a bridge or when assessing its structural performance. However, bridge standards do not make any reference to fire, despite the fact that bridge fires are a real threat and a major concern nowadays. This paper uses Computational Fluid Dynamics simulations to study the parameters influencing the temperatures in the gas surrounding I girder bridges during a fire, since this is a critical parameter governing the response of the bridge. More specifically, the study considers four geometric parameters (vertical clearance, bridge substructure configuration, span length and bridge width) and two fire scenario parameters (position of the fire load and type of fuel). Results show that vertical clearance, type of fuel and position of the fire load are the most influential parameters for the flange temperatures and that web temperatures are also influenced by the bridge substructure configuration.
Resilient Bridge Design Framework to Extreme Fire Loading
Several recent fire-induced bridge failures have highlighted the need for improved simplified tools to evaluate the response of bridges to fire. A streamlined design framework has been previously developed [1] for efficient calculation of a steelsupported bridge’s response to an open-air hydrocarbon pool fire resulting from a tanker truck crash and subsequent fuel spill. The framework consists of four steps: (1) calculate the fire’s characteristics; (2) calculate the heat transfer from the fire to the structural elements; (3) calculate the temperature increase of the structural elements; and (4) calculate the resulting material and mechanical response of the structural elements. The approach synthesizes calculation techniques based on both first principles and empirical data to quantify the extent of damage caused by the fire hazard. Due to its efficiency, this approach can be used to calculate an envelope of effects for a wide range of fire parameters. This paper proposes a new framework that uses the point source fire model for quantitative measure and mathematically reproducible definitions of structural resiliency as it pertains to a bridge's ability to minimize the potential for undesirable consequences. The resiliency assessment and design process follows logical progression of steps, starting with the characterization of fire hazards, continuing through analysis simulations. The outcomes of each process are articulated through a series of generalized variables, termed as topology, geometry, damage, and hazard intensity measures. A rigorous probabilistic framework permits consistent characterization of the inherent uncertainties through the process. The proposed framework is well suited for design process through stochastic characterization of assessment measures. Through stepwise approach, the framework facilitates a system wide approach to multi-fire hazard threats by establishing functional relationships between the development of appropriate models, design methods, damage acceptance criteria and tools necessary for implementation. The proposed methodology can be implemented directly for performance assessment, or can be used to as a basis for establishing simpler performance criteria and provisions to achieve resilient structural solutions.
Assessing the Fires on the Deck of Cable Stayed Bridges
Bridges are community critical infrastructure and, therefore, the consequences arising from extensive structural damage due to a fire on the bridge could be significant. This paper presents the process for the characterisation of the design fires on a bridge deck utilised to assess the thermal and mechanical response of a cablestayed bridge structure. Future research requirements are also identified.
New Design Fires for Performance Based Engineering of Highway Bridges
This paper presents new design fires resulting from vehicle accidents applied to a commonly found highway bridge in the U.K. The new design fires have been derived by modifying localized fires corresponding to the size of vehicles, in which smoke layer and decay with respect to the distance form fire origin is modelled. Four scenarios including four standard vehicle types are investigated associated with a different mean heat release rate (HRR). The measured mean HRR from burning vehicle tests are used to determine a distribution of heat flux along the bridge span. A performance-based approach proposed, in which the bridge fire and its resistance to fire are determined. This methodology can be used by authorities, which responsible for managing a bridge network to assess its vulnerability to a vehicle fire incident
Performance-Based Prioritization of Fire Mitigation for Highway Bridges
The authors have recently developed a 3D Matlab-based modeling tool that links the actual geometry of a bridge structure with the realistic exposure to an open-air hydrocarbon fire caused by a tanker truck accident. The streamlined framework at the core of this tool synthesizes numerous calculation techniques based on both first principles and empirical data to quantify the extent of damage caused by a specified fire hazard. Due to its computational efficiency, this framework can be used to develop an envelope of performance for a range of fire hazard scenarios. In this study, 2,500 analyses are performed using fire hazard parameters that are selected via weighted random sampling from fields of probable inputs. The results of these analyses provide hazard-based consequence maps which can then be used to determine fire protection prioritization among a collection of overpass bridges in an interchange
Fire Test of FRP Members Applied to Railway Bridge
The research reported in this paper is focused on the application of FRP (fibrereinforced polymers) for the equipment of railway bridges. After investigation of the FRP manufacturer possibilities, several systems were proposed to be applied on railway bridges. As the FRP is a fire sensitive material, systems mounted on a test bridge were exposed to in-situ fire tests in order to verify human safety and possible environmental impact. Based on results of the fire test, FRP solutions for floor on sleepers, floor on consoles and railing were proposed for railway bridge reconstructions.
Behavior of Structural Tunnel Element Exposed to Fire and Mechanical Loading
Incidents like the fire in the Channel Tunnel [1], where concrete spalling with a depth of 30 cm over a length of 500 m was determined, have led to requirements in limiting the spalling depth and involved zone to local and compatible magnitudes. To account for the high heating rate of tunnel fires, the fire resistance of railway tunnel structures was investigated using the German EBA tunnel fire curve [2], which was derived from large-scale fire tests on railway cars [3]. The behavior of structural tunnel elements exposed to fire and mechanical loads have been investigated in several research programs during the last years [4],[5], [6]. In this paper, the results of an extensive research program investigating the fire behavior of two concrete mixtures four small- and two large-scale tests are presented. The small-scale tests prior to the main tests were conducted to determine an appropriate concrete mixture for the large-scale tests. For those tests, real-scale tunnel segments representing a design-relevant sector of an existing railway tunnel were used. During the large-scale tests, the tunnel segment is loaded with horizontal and vertical loads derived from a calculation taking into account the ground pressure, water pressure and dead loads corresponding to the situation of the existing railway tunnel. Additionally, the resulting restraint forces were calculated using the soil stiffness and the EBA tunnel fire curve [2] as fire scenario and induced via hydraulic jacks (force controlled). In order to avoid additional restraint forces during the experiment, thermal strains due to fire exposure were allowed by the statically determined boundary conditions.
Temperature and Strain Measurements with Fiber Optic Sensors for Steel Beams Subjected to Fire
This paper presents measurements of high temperatures using a Brillouin scattering based fiber optic sensor and large strains using an extrinsic Fabry-Perot interferometric sensor for assessing the thermo-mechanical behaviors of simplysupported steel beams subjected to combined thermal and mechanical loading. The distributed fiber optic sensor captures detailed, non-uniform temperature distributions that are compared with thermocouple measurements resulting in an average relative difference of less than 5 % at 95 % confidence level. The extrinsic Fabry-Perot interferometric sensor captures large strains at temperatures above 1000 °C. The strain results measured from the distributed fiber optic sensors and extrinsic Fabry-Perot interferometric sensors were compared, and the average relative difference was less than 10 % at 95 % confidence level.
Thermographic Digital Image Correlation (TDIC) Measurements of Mechanically-Loaded Structures
This research examined the use of Thermographic Digital Image Correlation (TDIC) in fire testing with flames in the field of view. The error and uncertainty in the deformation measurements obtained via TDIC with fire in the field of view were reduced by ensuring the grayscale histogram of the CCD images was neither under-saturated nor oversaturated once fire was present. Quantitatively, this corresponded to the greyscale histogram having a mean value between 13-91 out of 255 and a kurtosis between 2.7-10.7 without the fire present. Results obtained from testing using this setup were found to reduce data loss due to overexposure when compared to results obtained using arbitrary light conditions. Thus, future investigators may reduce the error in the system for live fire testing before exposing the sample to fire by setting up lighting and camera settings to obtain histograms in this range.
Consolidated Fire Analysis—Coupled Thermo-mechanical Modelling for Global Structural Fire Analysis
Consolidated fire analysis (CFA) consolidates numerical simulation and physical element testing in order to analyze the global structural behavior of structures in fire. As the testing technique facilitates the investigation of the global behavior of structures in fire while only physically testing a substructure, it might become increasingly important in fire safety engineering. In order to develop an analysis procedure that meets the requirements arising from fire engineering, a benchmark problem has been elaborated and is presented in this paper. This methodological problem can be solved by both, consolidated fire analysis and pure physical testing. This approach allows verifying the results obtained using the consolidated fire analysis technique by pure physical testing. In this paper, the benchmark problem is explained in detail and results from two consolidated fire tests at high temperatures are presented.
A Static Partitioned Solver for Hybrid Fire Testing
The paper presents the development and the application of a thermomechanical static solver based on the Finite Element Interconnecting Method (FETI) to be employed in hybrid fire tests. Hybrid fire testing is an appealing methodology that intends to test in relatively small furnaces, compared to the whole building, single structural elements or limited parts of the building that exhibit a highly nonlinear fire behaviour whereas the remainder of the structure is modelled numerically with good accuracy. On these premises, the paper comprehensively describes the proposed static solver by highlighting its ability to guarantee compatibility and equilibrium at the interface between the physical substructure (PS) and the numerical substructure (NS), both for non-floating and floating subdomains as well as for nonlinear behaviour of the PS. The algorithm development has been focused on providing a procedure that in principle can be actually used experimentally. Thus, an error propagation analysis that takes into account errors and uncertainties, such delay and measurement noise, is also presented. The validation carried out in a fully numerical framework, i.e. the PS is also numerically modelled, on a steel frame shows promising outcomes for future experimental implementations.
Stability in Hybrid Fire Testing. . . . . . . . . . . . . . . . . . . . . . 836 ANA SAUCA, THOMAS GERNAY, FABIENNE ROBERT,
Hybrid testing is an appealing technique to observe the behavior of an element in an experimental test while taking into account the interaction with the rest of the structure which is modelled numerically. Being widely used in the seismic field, this technique has been recently proposed in the fire field. The purpose of this paper is to demonstrate that the loading control process may be unstable during the hybrid testing when using the methodology applied in former tests presented in the literature. The stability in the latter method depends on the stiffness ratio between the two substructures. For the purpose of discussion, a one degree-of-freedom elastic system is studied. To overcome the stability issues, a new method is presented, independent on the stiffness ratio. Finally, the hybrid testing of a 2D beam being part of a moment resisting frame is analyzed in a virtual environment (both parts being modeled numerically) using the “first generation methodâ€ and the new proposed method.
Assessment of Concrete Exposed to Elevated Temperatures Using Scanning Electron Microscopy and X-Ray Diffraction
The microstructure of concrete subjected to high temperature is studied. The aim of this research is to investigate the potentially use of scanning electron microscopy (SEM) and x-ray diffraction (XRD) in the assessment of thermal damaged concrete. A reinforced column was exposed to ISO 834 fire curve for 4 hours. After heating, core samples were obtained and analyzed. Visual observations showed a change in the color of concrete exposed to high temperatures. SEM micrographs presented distinct changes in morphology, like cracks and voids. XRD diagrams showed a reduction of portlandite and presence of larnite as depth increases. The potential use of these techniques in the estimation of damage in concrete exposed to elevated temperatures was confirmed.
Pulse-Echo Monitoring of Concrete Damage and Spalling During Fire
Monitoring concrete damage and spalling progression in structural members during fire tests (hot conditions) is a central but challenging task, since the high temperatures involved make difficult the implementation of most of the common Non- Destructive evaluation methods. Hence, an advanced ultrasonic technique – Ultrasonic Pulse-Echo (UPE) – was recently adapted for real time survey in fire test, in order to evaluate the material damage during heating. The UPE technique was implemented at the cold (upper) face of concrete slabs (800x800x100 mm) heated at the bottom face according to the Standard Fire and subjected to biaxial compressive membrane loading. Different concretes were tested, with grades ranging from 40 to 60 MPa, with and without different kinds of fibre (monofilament or fibrillated polypropylene, or steel fibres). Furthermore, different load levels were applied, from 0 to 25% of the original compressive strength. During tests, spalling was generally observed in loaded plain concrete (up to 50-60 mm depth), while only slight scaling was experienced on unloaded samples or if polypropylene fibre was added. The method proved to be very effective in recognizing the decay of the Ultrasonic Pulse Velocity (UPV) with temperature and the role played by external loading and fibre type
Towards a Standard Measure of the Ability of a Structure to Resist a Natural Fire
Fire brigades face a major threat when intervening in a building in fire: the possibility of structural collapse during the cooling phase of the fire, or soon thereafter. In the current approaches to structural fire engineering, the fire resistance rating (R) is generally the only measure taken into consideration to characterize the fire performance of structural elements, although this measure does not reflect the response in real fire conditions. In this work, a standard measure is proposed to characterize the ability of structural members to resist a natural fire including the decay phases. This measure yields information about the potential occurrence of delayed failure as a function of the duration of the fire before it started to decrease, whether by self-extinction or due to the action of the fire fighters. The paper presents the method to derive this new standard measure as well as results for different typologies of structural elements. Finally, the interpretation and practical consequences are discussed, in particular regarding the safety of fire fighters during an intervention.
Application of Concepts of Fire Resistance of Load Bearing Structural Glass
The use of glass as a load bearing element in structures is a growing phenomenon. Glass elements cover both beams and shear walls and its use is driven mostly by aesthetical and architectural reasons but brings other benefits considering environmental and financial aspects. Glass is a strong material but with a brittle nature which means a sudden fracture on failure with negative consequences for safety. However, with new techniques of increasing ductile behaviour and with the use of glass elements embedded in a holistic fire safety design strategy there are now several examples of load bearing glass elements in modern construction. This paper highlights some aspects of the use of load bearing glass with recent updates on the general knowledge base and the standardisation work
Performance-Based Fire Engineering: Sensitivity Analysis on Design Parameters
Performance based fire engineering has recently been the focus of several studies due to its well-known benefits and the short-comings of the traditional prescriptive design methods. However, the method is not often used extensively because of the large number of parameters and decisions involved in the process. This study presents a framework to conduct a number of sensitivity analyses to help the engineer determine which of several parameters used in a performance based design can affect the outcome significantly. A special computational tool is developed to efficiently build several models and conduct analyses by varying different parameters. A simple residential building is used as a case study to show the capability of the tool in conducting sensitivity analysis and identifying key parameters.
Shear Breaking of Columns Subjected to Localized Fire
Resistance to fire of concrete structures is specified with prescriptive rules such as minimal dimensions and concrete cover in Switzerland, in accordance with SIA 262:2003 [1] or Eurocode EN 1992-1 - 2: 2004 [2] standards. Following a fire in an underground car park, a column underwent a shear force breaking, even though it had the dimensions and concrete cover specified in SIA 262:2003 standard [1] for R60 resistance required by the Association of Public Insurance Companies (APIB) [3] in Switzerland for this type of structure. After this unusual breaking, that resembles a "short column failure", research was carried out to understand the causes that have led to this situation and develop a calculation method to check resistance of shear force of columns subject to a localized fire.
Conversion of Visual Post Fire Measurements into Fire Severity with the Aid of Thermo-Plastic Analysis for Retrofitting
At Koksijde in Belgium a severe fire took place in an apartment building in 2015, resulting in the death of a young man and visible structural damage to four balconies. Following the fire, experts were mandated to assess the damage and the need for structural repair. They estimated that the balconies had to be refurbished but that there were no other structural elements affected, in particular the slab inside the apartment could be left in place with only a surface treatment and new plaster finishing. However, the floor slab in the apartment located above the fire apartment exhibited several visual indications that the fire could have had a structural impact, such as residual deformations and cracks in the tiles. This paper presents a methodology to infer the fire severity based on post-fire measurements and non-linear thermo-plastic numerical simulations. Finally, knowing the fire severity, its effect on the structure is evaluated and a reliability-based assessment is made of the residual load bearing capacity of the slab.
Engineering an Icon or the Probabilistic-based Structural Fire Engineering of the Battersea Power Station
For large compartment floors the assumption of full flashover fire is very unlikely to hold true; instead fires are likely to be localised whilst moving around the floor plate. The paper will describe how a series travelling design fires are developed to assess steel framed structures, in which a probabilistic Monte-Carlo approach is adopted as means of developing design fires that are reflective of the risk associated with the use and height of the building. The Monte-Carlo approach involves a bespoke probabilistic method in which a set of parameters are randomised to produce a large number of possible fires occurring within the floor space; this is a step forward from the deterministic approach in which design fires are often being defined. This paper describes how the above principles are adopted to the structural fire design of the office floors in the refurbishment of the Battersea Power Station – one of the largest redevelopment projects in London in recent years.
Fire Fragility Functions for Community Resilience Assessment
This work provides a framework to evaluate the response of buildings in a community to fire following earthquake. As part of the framework, the paper discusses two methodologies: (1) how to develop fire fragility functions; (2) how the fire fragility functions can be used in conjunction with an original fire ignition model to estimate the potential losses in a community from fire following earthquake. The paper focuses in particular on the development of fire fragility functions for an entire building to measure the probability of reaching a damage state given a fire scenario. Next, the paper proposes an ignition model to evaluate the probability of fire ignition after an earthquake. The ignition model together with fragility functions measure the probability of damage from fire following earthquake given an earthquake scenario
Critical Parameters in Deriving Fire Fragility Functions for Steel Gravity Frames
Fire fragility functions can be used to characterize the probabilistic vulnerability of buildings to fire in the context of urban resilience assessment. A methodology has been proposed to develop such functions for multi-story steel buildings. However, a large number of parameters with uncertainties play a role in the process of constructing the fragility functions. The goal of this research is to identify the critical parameters that most affect the global fire safety by investigating the sensitivity of the fragility functions to different input parameters. Sensitivity in parameters affecting the fire model, the heat transfer process and the thermo-mechanical response is examined. The effects of different design assumptions at the system level are also studied. The presented approach is useful for selecting the prevailing parameters in a fire reliability analysis and it provides important information for modeling tools that can be used to evaluate resilience for fire scenarios.
Applying Uncertainty Quantification in Modelling of a Steel Beam Exposed to Fire
Modelling of structures exposed to fires is prone to be heavily influenced by uncertainties in geometrical parameters, thermal material data as well as uncertainties in the boundary conditions. Assessing the effects and influences in variations of all the uncertain parameters is often cumbersome and traditional methods are impractical thus modelling of the total uncertainty is needed. Uncertainty Quantification with deterministic sampling is one possible way ahead to accommodate and evaluating the effects of uncertainties with as few repeated simulations as possible. In this paper the uncertainties stemming from error in the input data and boundary conditions on one example of a steel beam under four point bending exposed to fire is evaluated
Intensity Measures for Probabilistic Structural Fire Engineering
A framework for probabilistic structural fire engineering has recently been proposed. It is based on the Pacific Earthquake Engineering Research Center Performance-Based Earthquake Engineering approach which performs a hazard analysis, structural analysis and loss analysis to assess a building’s performance as a result of ground motions. Similar to earthquake engineering, the assessment in fire requires the calculation of intensity measures, engineering demand parameters, damage measures and decision variables for a full assessment of the fire effects, the structural response, the extent of damage and the resulting loss. A number of fire intensity measures have been proposed by a few authors. However, these have been found to only partially reflect the severity of the fire, thereby not uniquely capturing the consequent structural response. For realistic assessments of damage and costs of reinstatement, more appropriate intensity measures are required. Using a two-span reinforced concrete beam as an example, the present research identifies ideal intensity measures by investigating several alternatives and selecting the one with the least dispersion of results with engineering demand parameters.
A Probabilistic Study on Concrete Columns Exposed to Biaxial Bending at Elevated Temperatures
Fire, as one of the most severe accidental load situations, has an important effect on concrete columns. Furthermore, this effect is more pronounced in case an eccentricity is associated with the axial load. Since this situation occurs very frequently, it is significant to investigate the fire performance of concrete columns subjected to biaxial bending. Additionally, differences due to initial imperfections on configurations, cover thickness, as well as the common variability in the concrete compressive strength might cause significant differences to interaction diagrams. Therefore, a probabilistic evaluation is required as a first step to evaluate the safety of concrete columns during fire exposure. In this paper a column subjected to a 4-sided fire exposure in case of a symmetric biaxial load is investigated and Monte Carlo simulations are applied to evaluate probabilistic interaction diagrams for columns in case of fire.
Reliability of Reinforced Concrete Walls Subjected to Standard Fire Using Advanced and Simplified Methods
The probability of failure of concrete walls subjected to a standard fire on both surfaces is examined. The samples are chosen to match the parameters recommended in the tabulated values of Eurocode 2, which allow a classification of the fire resistance. The probabilities of failure are calculated by means of a Monte Carlo simulation considering the Advanced Calculation Method according to Eurocode 2. To examine the accuracy of a simplified method, the Extended Zone Method is also taken into account. The calculated failure rates of both methods are close to each other, which confirms, that the Extended Zone Method is suitable to determine the reliability of reinforced concrete walls subjected to fire. The calculated probabilities of failure indicate that the tabulated values are mean values with small additional safety elements
Implementation of a New Design Travelling Fire Model for Global Structural Analysis
This paper presents a new conceptual framework for travelling fires in large compartments with fire resistant islands in order to ensure that the structure so designed is able to resist more realistic fire exposures expected in such compartments, commonly found in modern office buildings. In addition, this paper also presents the implementation of this new travelling fire model in the SIFBuilder framework [1], which is an OpenSees based integrated computational tool for performing automated thermo-mechanical analyses for large structures subjected to a broad range of idealised design fires.
Global Structural Behavior of Steel-Concrete Composite Floor Systems under Traveling Fires
This paper presents a computational investigation aimed at better understanding the global structural behavior of a steel-concrete composite (SCC) floor system subjected to single-story 1D traveling fires. Using the traveling fire methodology (TFM), a range of spatially and time-varying fire exposures are applied to a threedimensional (3D) finite element (FE) macro-model of a SCC floor system designed following US design standards and practices. The sequentially coupled thermalmechanical simulations were carried out using ABAQUS, where the structural modeling approach was verified using experimental test data from the Cardington fire test. Essential factors influencing the fire resistance of SCC floor systems, namely the passive fire protection scheme of the floor beams, and the traveling fire burning size, were varied to investigate and characterize the global structural response. Simulation results were analyzed and useful trends were observed, in particular the dependency of the slab vertical displacement rate and the maximum vertical displacement to both the distance from the fire origin and traveling fire burning size. Particularly interesting was the observation of high tensile forces generated in the beam-to-column connections during the heating phase of the traveling fire.
Structural Response of a Generic Steel Frame Exposed to Travelling Fires
A design tool called Travelling Fires Methodology (TFM) [1] has been developed in recent years to account for travelling fires in large open-plan compartments. However, there has been little research carried out to understand the structural response of buildings subjected to such fires. The aim of this study is to investigate the behavior of a 10-story steel frame under travelling fires and compare it to the behavior under standard design fires. Each floor of the frame is subjected to four travelling fire scenarios and two Eurocode parametric temperature-time curves (‘short-hot’ and ‘long-cool’) using finite element software LS-DYNA. In total 60 fire scenarios are considered. Results show oscillations of axial forces and bending moments for the smallest travelling fire sizes, which are not observed for the parametric fires. In general the development of stresses and displacements within heated beams is found to follow similar trends irrespective of the frame level at which a fire occurs. For the investigated frame the critical fire scenarios are found to occur on the upper floors of the frame where column sections reduce in size. It has been found that depending on the structural metric examined both travelling fires and parametric fires can result in the most severe scenario. Therefore, it is highlighted that in the structural design for fires, in addition to the standard tools it is important to consider more realistic fire scenarios associated with travelling fires as they might trigger previously unnoticed structural mechanisms.
Numerical Calibration of Hydrocarbon Localised Fire Tests by Means of Fire Dynamics Simulator
The paper describes the Computational Fluid Dynamics (CFD) numerical simulations performed with the aim to reproduce the experimental outcomes of hydrocarbon pool fire tests carried out at the University of Liege (B) and at the University of Ulster (UK) within the European RFCS LOCAFI project. In detail, several pool fire tests of different dimensions (0.7 m Ã· 2.2 m) and fuels (heptane and diesel) were simulated by exploiting the CFD software Fire Dynamics Simulator (FDS). Tests without steel column, with steel column inside/outside of the pool fire and without/with ceiling were analysed. Calibration of numerical models was mainly focused on providing a good match between experimental and numerical heat flux. Hence, useful information on the parameters that influence most the numerical simulations is given in the paper
FDS-FEM Simulation Method and Its Application to Model Localized Fire Tests on Steel Members
Fire Dynamics Simulator (FDS) is an open source CFD code, developed by NIST. It has been widely used in fire engineering for modeling the gas phase environment (temperature, heat flux, velocity, species concentrations, etc.) in fires. Recently, there has been increased research in the application of FDS for structural fire analysis. Firestructure interface tools for transferring data from FDS to particular FEM codes (such as ANSYS, ABAQUS, SAFIR) have been developed. Although both FDS and FEM codes have been separately validated, there needs to be a direct validation for the integrated FDS-FEM simulation methodology. This paper validates the integrated FDS-FEM simulation methodology against a number of localized fire tests reported in literature. Localized fire tests on steel columns and on ceiling steel beams were simulated using the FDS-FEM method. The concept of adiabatic surface temperature used for transferring data from FDS to FEM was presented and the limitation of the concept was discussed. The FEM program ANSYS was used to conduct the heat transfer analyses and thermo-mechanical analyses to get the temperature and structural responses of the tested steel beams and steel columns.
Results of a Post Test Round Robin of the Calculated Response of a Loaded Steel Beam to a Furnace Test
Calculations or simulations are often used as an alternative means of evaluating the fire resistance of elements and structures as opposed to fire resistance testing. Indeed, in Europe, calculations carry the same credibility as testing for the certification of certain building products. However, while for testing there are accreditation requirements of the test laboratory, this is not the case for calculations. In other words, when evaluating building products for certification based on testing there is a formal control system that must be followed and this is not the same when doing the same job based on calculations. This paper details the second stage of a round robin study of the calculated response of structures in fire. In this instance, the study is based on a fire test which was conducted on two steel beams at SP’s fire resistance laboratory in Sweden. We describe the different approaches taken to the prediction and how the additional information provided to the participants was taken into account. We also compare the predicted deflection histories with the deflection history during the test. Finally, we compare the calculation round robin with the results of a fire resistance testing round robin of the same object, which illustrates the relative certainties in testing results compared with calculation results
A Unified Rheological Model for Analysis of Steel Behaviour at High Temperature
This paper presents the theoretical background for development of a unified rheological model (URM) for low-carbon steel which is sensitive to the temperature and strain-rate effects during fire exposure. The proposed URM consists of a serial connection of two Kelvin-Voight (KV) rheological elements. Each KV element is used to simulate a particular material strain (stress-related and creep strain) in fireaffected steel. The first KV element is capable of taking into account strain-rategoverned change of yield strength and the second is sensitive to variation of heating rate. A verification of the proposed URM by using test data from transient coupon tests of steel Grade 275 is also presented.
Scaling of Reaction Forces in a Displacement Controlled Steel Beam, Transitioning From Elastic to Plastic Behaviour
Fire resistance testing of structural elements and components is limited by the scale of fire resistance furnaces. Certification of long span or large structural systems may not be verified based on the results of full scale furnace tests but instead computer simulations. The response of structures exposed to fire depends on a number of different mechanisms which scale differently over different mechanical regimes. Consider the simple case of a fire exposed steel beam with a fixed point load, pinned at both ends. During the early period of fire exposure, it is the elastic behaviour which governs the flexural response. As the fire exposure increases, the beam starts to behave plastic and the axial response will be dominant. Dimensional analysis is applied to accurately create two different scale models of a prototype beam. The prototype and the models are modelled using Abaqus. We have shown that the total reaction force at displacements where a tensile mechanism is mobilised scales to the ratio of cross sectional areas of model to prototype. The total reaction force at low displacements under flexural behaviour scales with the ratio of the length of the model to the length of the prototype. Using a scale model to predict the response of a prototype exhibiting flexural and catenary mechanisms is therefore possible. There is a transition region between the elastic and the plastic region which does not scale with either length or area. It is our long term intention to develop scaling laws which could be used to help to 1) validate numerical models of structures exposed to fire; and 2) to better support extended application of test results.
Factors Governing the Fire Response of Reactive Powder Concrete Simply-Supported Beams
Reactive powder concrete (RPC) is a kind of new cementitious composite material with super high performance and high toughness, which has extensive application prospect. A numerical model for tracing the fire response of RPC simply - supported beams is developed using finite element analysis software. The validity of the numerical model is established by comparing the predictions from the computer program with results from reinforced concrete (RC) beams under fire. Results show that as compared with normal strength concrete (NSC), the thermal conductivity of RPC is higher and its compressive strength decreases faster at elevated temperature. With same reinforcement ratio, concrete cover thickness and cross section, the moment bearing capacity and deflection of reinforced RPC beams decreased faster than that of RC beams at elevated temperature. The analysis variables included load level, reinforcement ratio, concrete cover thickness. Results show that reinforcement ratio, concrete cover thickness and load level have significant influence on the fire resistance performance of reinforced RPC beams. It should be mentioned here that fire resistance of RPC simply-supported beams increases at the beginning and then decreases with the increase of reinforcement ratio. This is mainly because that the stress of longitudinal steel bar increases at the beginning and then decreases with the increase of heating time.
Numerical Analyses for Precast RC Structures at Fire Condition in Underground Space
To create sustainable urban development, it is widely recognized that underground space development is a long-term solution to cater for future economic growth. In urban cities, many underground projects have been constructed to mitigate the increasing demand for land and infrastructure. For structures in very deep underground construction, prescriptive fire code require 4 hour-rating for structures. In this paper, a typical 2 hour-rating RC frame structure is designed for underground space. Standard fire curve (ISO-834) is applied to affected RC members under certain fire scenarios. Numerical results show that with the help of tensile membrane action, even a 2-hour fire rating RC beam-slab system can achieve satisfactory result
Numerical Modelling of Load-Induced-Thermal-Strain of Prestressed Concrete Pressure Vessels
This paper presents the implementation of a 3D Load-Induced-Thermal-Strain (LITS) model in an isotropic linear thermoelastic material law to be used for modelling concrete under transient thermal conditions. A new approach for extending to 3D uniaxial temperature-LITS curves has been proposed and adopted, capable of capturing the experimentally demonstrated dependency of LITS on the confinement of the stress state. A stress dependent confinement coefficient which amplifies uniaxial LITS curves in the case of multiaxial stress confinement was defined to capture such a dependency. Besides, a novel practice-oriented bilinear LITS model has been defined and proved to reproduce experimental curves better than the existing models in the case of triaxial stress states and temperatures up to 250°C. The model was verified, validated and calibrated by modelling experiments performed on concrete specimens. In the last part of the paper, the model has been employed to assess the effects of LITS on a Prestressed Concrete Pressure Vessel (PCPV) subjected to an accidental heatingcooling cycle. It was found that LITS plays a key role for this sort of structure, and that significant aspects of structural behavior are missed if no confinement dependency is modelled.
Structural Behavior under Localized Fire Action—Modelled Using an Integrated Computational Tool
An integrated computational tool ‘SIFBuilder’ was developed to model the global behaviour of structures subjected to various fire scenarios. This OpenSees based tool is currently capable of performing weakly coupled sequential analyses and able to automatically apply idealised fire actions to the structure including localised fires. A few illustrative cases are thereafter conducted to demonstrate the usage of SIFBuilder, presenting the global response of a generic 2×2×2 frame building subjected to different fire actions. Two types of idealised fire scenarios are considered for the building, which include an idealised uniform fire (one hour standard fire) occurring in one confined compartment, and an idealised non-uniform fire (0.5 hour Eurocode 1 localised fire) assumed to wrap symmetrically around the central column. Greater slab deflections are observed in the case of confined compartment fire, whereas collapse may occur in the central column due to the exposure to localized fire.
Optimal Tuning of Thermomechanical Hybrid Simulation Parameters
According to current engineering practice, the fire performance qualification of structural elements relies on single component tests, where specimens are exposed to standard fire curves. As a result, the interactions between elements within the overall structural assembly are lost. This situation has motivated researchers to extend the hybrid simulation method, which has been deeply investigated in the seismic domain, to structures-in-fire testing. By linking numerical and physical substructures, hybrid simulation offers a flexible, cost-effective approach. Purely mechanical hybrid simulation tests are often performed slower than real-time (extended time scale), dictated by actuator capacity limitations, quality of displacement tracking, and synchronization among computational and experimental drivers. This practice is generally not acceptable when a rate-dependent response of the specimen is expected, as is often the case in fire (e.g. creep strain occurs when steel is subjected to high temperatures). In this scenario, the proper selection of the testing time scale must balance the laboratory testing capacity requirements (operating too close to real-time compromises the test stability if the equipment is not suited for this) and the expected rate-dependency of the structural response (operating at an extended time scale could reduce the solution accuracy if the specimen undergoes excessive creep). From this perspective, it is crucial to accurately tune the testing time scale to minimize experimental approximation. Thus the optimal tuning of the time integration scale in thermomechanical hybrid simulation (TMHS) is presented herein. Furthermore, updates to the TMHS experimental element, newly implemented in the OpenFresco hybrid simulation middleware, provide the ability to fully simulate a TMHS test prior to experimentally substructuring the physical specimen in the laboratory
Experimental Study of the Behavior of Steel Structures Protected by Different Intumescent Coatings and Exposed to Various Fire Scenarios
Three different experimental setups corresponding to three different fire scenarios were used to investigate how different heating conditions and heating rates affect the behavior of two different thin intumescent coatings (solventbased and water-based paints, respectively). The results confirm that the current procedure for the design of intumescent coatings has shortcomings, as different paints have different performances according to the heating conditions and, in particular, according to the fire’s heating rate. The tested water-based paint had better performance for low heating rates, while the tested solvent-based paint had better performance for high heating rates. However, for really low heating rates the solvent-based paint did not activate or provide proper insulation
An Improved Method for Thermal Analysis of Intumescent Coatings
Intumescent coating is a popular fire protection material for steel structural elements with beneficial temperature dependent thermal and mechanical properties. The analysis of the heat flux through the intumescent coating appears to be complicated, due to expansion and chemical reactions in material. In the paper an improved numerical method for the thermal analysis of intumescent coatings is presented. Temperature dependency on both the thermal and the mechanical properties of intumescent coating, expansion of the material and the chemical reaction are taken into account. The method is validated by comparing numerical and experimental results for two different intumescent coatings.
Experimental Tests on Intumescent Coating for Protecting Steel Structures
Intumescent coatings (IC) are fireproof materials that provide the thermal insulation of steel structures. When exposed to fire, IC generally expand in thickness by 15 to 30 times in a standard test, forming a thick layer of foam that protects the steel by thermally insulating it. As part of the expansion process, many coatings generate an outer ash-like char layer. Moreover, during the process the IC change their chemical, physical and also thermal properties. This work shows an ongoing experimental program that could allow to characterize the thermal properties of the IC, based on the measurements of thickness variation during the fire and highlighting the parameters which influence the thermal behavior of IC-protected steel members. The ongoing research is aimed to allow analytical or numerical modelling of steel members protected by IC through a finite element thermal analysis. This modelling could be useful for the fire designers, since it allows to take into account the presence of the IC also when the Structural Fire Safety Engineering approach (SFSE) is applied, according to the modern fire regulations. The experimental results show that (i) small thickness of IC (500 μm) do not react homogeneously for all the section factors and (ii) the equivalent thermal conductivity depends on both the initial thickness and the section factor.
An Experimental Study of the Damage Modes of Fireproof Coatings Under Complex Loads
Fire proof coatings used as passive fire protection in steel structures may sustain damage when subjected to seismic, blast or impact action. Complex states of stress exist within the coating as well as at the interface between the coating and the surface of the steel member. This paper reports an experimental study on damage modes of different fireproof coatings when steel columns under complex loads. Behavior of cementitious coating and intumescent coating are compared. Different failure modes of these two coatings are observed.
Fire Tests of Reactive Fire Protection Systems Applied to Steel Tension Members with Solid Circular Section
In civil engineering, steel tension members are normally used for bracings, suspensions and underbracing systems. Typically, slim circular solid sections (CSS) are used for such tension members. However, sufficient knowledge about the performance of reactive fire protection systems (RFPS) applied to solid steel tension members has so far been missing. The application of RFPS on such members was not covered by national German approvals (abZ) as well as European technical assessments (ETA) and therefore only possible by approvals in individual case by the building authority. This paper describes the world’s first scientifically investigated fire tests of RFPS applied to steel tension members with CSS. The influence of various parameters such as profile geometry, dry film thickness of the RFPS, level of the load utilization as well as member orientation was tested. In addition, the foaming and cracking behaviour and thermal protection of the RFPS used are investigated and failure mechanisms are identified. To measure the steel temperature of the tension member without weakening the cross-section a special method for the application of thermocouples was developed. The fire tests show that an application of RFPS on steel tension members with solid section is generally possible. However, due to the slim cross-sections and the missing possibility of load distribution within the tension member, high requirements are placed particularly on the effectiveness and reliability of the RFPS. In particular, the testing of mechanically loaded tension members is essential, since the necessary three-dimensional foaming results in the highest stress level for the RFPS. The findings obtained from the performed fire tests are of general nature. Recommendations for the testing and assessment of RFPS applied to steel tension members with solid section are briefly described in this paper and explained in detail in [1] and [2]. Based on this research, the German building authority (DIBt) has defined national approval guidelines for the testing and the assessment of RFPS applied to tension members with solid section. Since November 2015, a general building approval for a RFPS is available in Germany [3].
Numerical Investigation of Plasterboard Partitions Subjected to Fire: Confrontation between Numerical and Experimental Results
Plasterboard partitions which are widely used in construction have to satisfy technical approval for fire safety requirements from regulations. These fire safety approvals are generally evaluated from fire resistance tests performed in accredited laboratory. The dimensions of fire resistance furnace are a limitation to test and to approve high rise plasterboard partitions. In this context, a research program was initiated by Efectis whose main objective is to develop a numerical model for simulating a fire resistance test on plasterboard partitions. Based on simulated temperature field and estimation of the global bending of the partition wall, the model can predict collapse of height extension plasterboard partition in order to validate the fire resistance of high partition walls. In this paper, a thermo-mechanical model applied to plasterboard partitions is first presented. Simulation results of fire resistance tests on two plasterboard partitions under standard fire curve ISO-834-1 are detailed. Then, application of the model to height extensions and evaluation of fire performance are presented.
Material Characterisation and Numerical Modelling of Gypsum Plasterboards in Fire
The fire resistance characteristic of LSF wall systems mainly depends on the protective linings in use, commonly gypsum plasterboards. However, unclassified boards with varying composition and more notably with ambiguous thermal properties are increasingly becoming available in the market. Therefore a study was undertaken with an aim to set minimum standards for fire protective boards used in LSF wall applications. This paper presents the details of this study based on material characterisation and finite element thermal modelling of the most commonly used fire protective board, gypsum plasterboards, to address these critical issues related to fire safety design. In the material characterisation phase of this study, thermal properties of three different gypsum plasterboards manufactured in Australia were measured, analysed and compared. Subsequently, it proposes a thermal property based “k-factorâ€ capable of giving an overall measure of the fire performance of boards, so that it can be used in appropriately classifying fire protective boards. As it is not known how this factor relates to the overall fire performance of LSF wall systems, numerical models were also developed and used to simulate the performance of LSF walls exposed to the standard fire. Finally, a correlation between time-temperature profiles from numerical analyses and calculated k-factors was established.
Performance of Gypsum-Protected CLT and NLT in Natural and Standard Fires
Two large-scale tests were conducted to evaluate the performance of CLT and NLT construction protected with two 16 mm sheets of type X gypsum board in the pre-flashover, post-flashover and cool-down phases of a severe living room fire. Since minimal or no charring was observed in the room test, additional small-scale furnace tests were performed on gypsum-protected CLT and NLT specimens using standard and simulated natural thermal exposures. The furnace tests indicate that the wood continues to char for some time at a decreasing rate during the cool-down-phase. The corresponding reduction in the load-bearing section needs to be accounted for in the assessment of the fire performance of CLT and NLT structures
Study on Residual Flexural Performance of Precast Prestressed Hollow Core Slabs After Exposure to Fire
The structural performance of precast prestressed hollow core slabs after exposure to fire was experimentally and numerically investigated in this paper. Four levels of fire duration were considered in the experiments. All specimens both loaded without heating and after exposure to fire exhibited flexural failure. The initial stiffness, cracking loads and ultimate loads were observed to degrade with increasing levels of fire duration. A numerical model was developed to predict the response of precast hollow core slabs after fire using ABAQUS. The user subroutine USDFLD available in ABAQUS was employed to consider the effect of the maximum temperature experienced by the concrete and cold worked wires on their material properties. The predicted temperature and load-deflection relationships from the numerical model were in good agreement with the experimental results. Calculation method was proposed for the precast prestressed hollow core slabs after exposure to fire based on the plastic analysis to predict their ultimate moment capacity. The predicted moment capacity compares well with the experimental results