Nonlinear behaviour of unprotected composite slim floor steel beams exposed to different fire conditions

An efficient nonlinear 3D finite element model has been developed to investigate the structural performance of composite slim floor steel beams with deep profiled steel decking under fire conditions. The composite steel beams were unprotected simply supported with different cross-sectional dimensions, structural steel sections, load ratios during fire and were subjected to different fire scenarios. The nonlinear material properties of steel, composite slim concrete floor and reinforcement bars were incorporated in the model at ambient and elevated temperatures. The interface between the structural steel section and composite slim concrete floor was also considered, allowing the bond behaviour to be modelled and the different components to retain its profile during the deformation of the composite beam. Furthermore the thermal properties of the interface were included in the finite element analysis. The finite element model has been validated against published fire tests on unprotected composite slim floor steel beams. The time–temperature relationships, deformed shapes at failure, time–vertical displacement relationships, failure modes and fire resistances of the composite steel beams were evaluated by the finite element model. Comparisons between predicted behaviour and that recorded in fire tests have shown that the finite element model can accurately predict the behaviour of the composite steel beams under fire conditions. Furthermore, the variables that influence the fire resistance and behaviour of the unprotected composite slim floor steel beams, comprising different load ratios during fire, cross-section geometries, beam length and fire scenarios, were investigated in parametric studies. It is shown that the failure of the composite beams under fire conditions occurred for the standard fire curve, but did not occur for the natural fires. The use of high strength structural steel considerably limited the vertical displacements after fire exposure. It is also shown that presence of additional top reinforcement mesh is necessary for composite beams exposed to short hot natural fires. The fire resistances of the composite beams obtained from the finite element analyses were compared with the design values obtained from the Eurocode 4 for composite beams at elevated temperatures. It is shown that the EC4 predictions are generally conservative for the design of composite slim floor steel beams heated using different fire scenarios.     

Axial restraint effects on the fire resistance of composite columns encasing I-section steel

This paper presents an experimental study of the axial restraint effect on fire resistance of four unprotected encased I-section composite columns. Axial restraints were applied to simulate thermal restraints from adjoining cool structures onto a heated composite column in a compartment. These real-sized 3.54 m long columns were subjected to concentric axial force at a load ratio of 0.7 at normal ambient temperature. Different degrees of axial restraint are investigated. An electric furnace was used to apply four-face heating condition on the columns for approximating a realistic fire scenario. All columns failed in flexural buckling mode. In the later part of the paper, finite element simulations were conducted to compare with test results. Numerical predictions of both temperature distribution and structural response during heating agree reasonably well with experimental data. Both test results and numerical analyses show that axial restraint significantly reduces the column fire resistance. Moreover, it was also observed that during heating all specimens underwent concrete spalling at mid-height, which noticeably decreased the fire resistance. Column critical times are also predicted according to Eurocode 4 Part 1.2, which are consistently shorter than the numerical predictions.     

Energy distribution analysis in full-scale open floor plan enclosure fires

Within a fast evolving built environment, understanding fire behaviour and the thermal exposure upon structural elements and systems is key for the continued provision of fire safe designs and solutions. Concepts of fire behaviour derived from research in enclosure fires has traditionally had a significant impact in general building design. At present, open floor plan enclosures are increasingly common – building design has drastically drifted away from traditional compartmentalisation. Nevertheless, the understanding of fire behaviour in open floor plan enclosures has not developed concurrently. The compartment fire framework, first conceived for under-ventilated fires in cubic compartments, has remained as standard practice. Although energy conservation within the enclosure was the basis for the current compartment fire framework that defines under-ventilated enclosure fires, little effort has been carried towards understanding the distribution of energy in design frameworks conceived for open floor plan enclosure fires. The work presented herein describes an analysis of the energy distribution established within an experimental full-scale open floor plan enclosure subjected to different fire modes and ventilation conditions. The results aim to enable the designer to estimate the fraction of the total energy released during a fire noteworthy to structural performance.     

Experimental behaviour of a steel structure under natural fire

Current design codes for fire resistance of structures are based on isolated member tests subjected to standard fire conditions. Such tests do not reflect the behaviour of a complete building under either normal temperature or fire conditions. Many aspects of behaviour occur due to the interaction between members and cannot be predicted or observed in tests of isolated elements. Performance of real structures subject to real fires is often much better than that predicted from standard tests due to structural continuity and the provision of alternative load paths.This paper reports on the results of a collaborative research project (Tensile membrane action and robustness of structural steel joints under natural fire, European Community FP5 project HPRI—CV 5535) involving the following institutions: Czech Technical University (Czech Republic), University of Coimbra (Portugal), Slovak Technical University (Slovak Republic) and Building Research Establishment (United Kingdom). It consists of an experimental programme to investigate the global structural behaviour of a compartment on the 8-storey steel–concrete composite frame building at the Cardington laboratory during a BRE large-scale fire test, aimed at the examination of the temperature development within the various structural elements, the corresponding (dynamic) distribution of internal forces and the behaviour of the composite slab, beams, columns and connections.     

Composite steel-framed structures in fire with protected and unprotected edge beams

The structural behaviour of a steel-concrete composite frame subject to a natural fire is analysed using a numerical model. The behaviour is compared when fire protection is applied to only the external beams and when no beams are fire protected. The behaviour of the structure in the two cases is significantly different. When the edge beams are protected the floor slab tends to span in 2 directions because the edge beams provide sufficient support around the perimeter of the floor for tensile membrane action to develop. When the edge beams are unprotected the slab tends to span in only one direction in a manner similar to a beam in catenary action. Catenary action is a weaker load carrying mechanism than tensile membrane action. As a consequence of the weaker mechanism, when the edge beams are unprotected, the columns displace inwards towards the end of the fire indicating the possibility of imminent runaway collapse.The pattern of mechanical strains in the floor slab reinforcement depends on the load carrying mechanism and therefore on whether edge beam protection is included. Although the average mechanical tensile strains are higher when the edge beams are protected the highest strains occur when the beams are unprotected. Conversely, an instability in the primary beam occurs at much lower temperatures when the edge beams are protected.It is concluded that fire protecting the edge beams of the structural layout studied has a number of effects on the fire resistance of the structure, some beneficial, some detrimental, however, in general, fire protecting the edge beams provides an increased level of fire resistance.     

Practical need of scientific material models for structural fire design — General review

More and more countries are now permitting a classification of structural elements with respect to fire exposure to be formulated analytically as an alternative to the internationally prevalent method of classification, based on results of standard fire resistance tests. In some countries, the authorities also have taken the next step to approve a general practical application of a direct analytical design procedure, based on the natural compartment fire concept.The process of an analytical structural fire design comprises three main components — the determination of the fire exposure, the thermal analysis and the mechanical behaviour analysis. The components require access to well-defined input information on: (a) material properties for describing the characteristics of the fire load and the compartment fire; (b) material properties for determining the transient temperature state of the fire-exposed structure; and (c) material properties for determining the related mechanical behaviour and load-bearing capacity.With a summary presentation of the development and characteristics of the analytical structural fire design as a background, the paper is focusing on the mechanical material properties at elevated temperatures. A systematic scheme of classification of available tests is referred to and the importance is stressed of using such functionally well-defined tests which give material properties, stringently connected to material behaviour models being independent of the type of load-bearing structure.     

基于足尺抗火试验的三维框架结构消防安全分析

分析三维框架结构(如工业厂房)的消防安全。冷弯薄钢板作为结构构件使用,并对建筑进行了足尺抗火试验。利用有限元模型SAFIR进行盲预测分析,对火灾荷载密度为625MJ/m2、通风系数为0.009m1/2的标准火灾和自然火灾下结构构件的热响应进行预测。与预期一样,由于是薄钢板,冷弯截面的温度或多或少与防火分区内气体温度相关。力学响应的预测结果给出了27min(标准火灾)和54min(自然火灾)的抗火能力。利用足尺抗火试验结果对模型进行了验证,证明了62min的抗火能力。与一个简单的2.5D模型进行比较,证明了所提供的结果足够精确、适用于日常设计。     

Experimental behaviour of composite slabs during the heating and cooling fire stages

Most theoretical and experimental research investigating the effect of fire on structures has previously concentrated only on the structural behaviour during the heating stages of the fire, partly due to the fact that internationally accepted standard fire tests only consider this stage of the fire. Evidence from real fires in real buildings has highlighted that the cooling phase of a fire is equally important and it is possible for structures to fail during this stage of the fire even though they have survived the heating stage up to a maximum fire temperature. This paper provides an insight into the behaviour of composite slabs under different fire scenarios considering both the heating and cooling phase of the fire. Extensive test data is presented which shows the redistribution of moments and strains in the deck and steel mesh, together with displacements during the full duration of the fire. The results show that the behaviour of composite slabs is dependent on the heating rate, the maximum temperature reached and the cooling rate. In terms of overall performance, displacements and the temperature on the non-fire side of the slab are important. For the tests presented in this paper it was shown that one fire scenario resulted in the maximum displacement but another fire scenario resulted in the maximum temperature on the unexposed face. In addition the maximum temperature of the unexposed side of the slab and the mesh reinforcement within the slab occurring during the cooling stages of the fire. This highlights the fact that the performance of structures must be checked in design under a range of possible fire scenarios, which must include both the heating and cooling stages of a fire.     

Heat transfer model for unprotected steel members in a standard compartment fire with participating medium

A heat transfer model accounting for the radiative properties of combustion products in a compartment standard fire is presented. The model used is based on a conceptual scheme of a grey gas mixture exchanging radiative energy with a black enclosure. The proposed model, with a radiation heat transfer that accounts for the effect of combustion products on the rate of heat transfer from the fire to the structural elements, is simple to use and the predictions of the temperature response of unprotected I-columns heated from all sides are superior to those predicted by the classical method using the Stefan-Boltzmann radiation equation with constant emissivity.     

Local buckling and slenderness limits for steel webs under combined bending, compression and shear at elevated temperatures

At elevated temperatures induced by fire loading, the I-section beam and column elements in a steel framed building experience combinations of axial, bending and shearing actions that may precipitate local buckling of the steel element. Under this loading regime, the Young's modulus, shear modulus and uniaxial yield strength may vary through the section depth because of the temperature gradient, and as a result predicting the local buckling capacity and the critical geometrical slenderness for coincident elastic local buckling and yielding are not straightforward. This paper presents an analysis of the elastic local buckling of the web of an I-section beam, by modifying a spline finite strip method of local buckling analysis to include the variation of material properties through the web. Necessarily, the method must also include the variation of mechanical strains through the web depth, in order that the limiting depth to thickness ratio that delineates yielding and elastic buckling (and hence the cross-section classification) can be prescribed. Under the combined loading, it is shown how the elastic local buckling coefficient and the web slenderness limit that classifies a non-compact section are dependent on the thermal gradient, the depth of the compression zone in the representation of the mechanical strain, and on the values of the shear strain. Graphical results are presented for the elastic local buckling coefficients as a function of the temperature and of the temperature gradient in a web with idealised edge restraint conditions. Since the local buckling response is crucial in establishing the formation hinges in flexural elements in the initial stages of thermal loading prior to the subsequent development of catenary action, the results are valuable for undertaking a rational fire engineering design of the steel elements exposed to a compartment fire, and they lend themselves to a codified approach for the structural behaviour.     

加热和冷却过程中约束复合板的试验及数值研究

在曼彻斯特大学,对约束复合板进行一系列耐火试验。在不同火灾场景下,对6块不同荷载比下的复合板进行试验,观察加热和冷却过程中板内力的变化。设计试验方案,建立两种不同的非线性有限元模型,模拟复合板在加热和冷却过程中的热学和力学性能。在热分析模型中,采用平面单元模拟。在结构分析中,混凝土、钢板、锚钉分别采用实体单元、壳单元、桁架单元模拟。混凝土和钢板间的连接简化为弹簧单元。根据试验结果和有限元计算结果,详细分析了复合板的性能。最后,进行参数研究,分析了混凝土强度、钢板厚度和锚钉尺寸的影响。     

Experimental review of the homogeneous temperature assumption in post-flashover compartment fires

Traditional methods for quantifying and modelling compartment fires for structural engineering analysis assume spatially homogeneous temperature conditions. The accuracy and range of validity of this assumption is examined here using the previously conducted fire tests of Cardington (1999) and Dalmarnock (2006). Statistical analyses of the test measurements provide insights into the temperature field in the compartments. The temperature distributions are statistically examined in terms of dispersion from the spatial compartment average. The results clearly show that uniform temperature conditions are not present and variation from the compartment average exists. Peak local temperatures range from 23% to 75% higher than the compartment average, with a mean peak increase of 38%. Local minimum temperatures range from 29% to 99% below the spatial average, with a mean local minimum temperature of 49%. The experimental data are then applied to typical structural elements as a case study to examine the potential impact of the gas temperature dispersion above the compartment average on the element heating. Compared to calculations using the compartment average, this analysis results in increased element temperature rises of up to 25% and reductions of the time to attain a pre-defined critical temperature of up to 31% for the 80th percentile temperature increase. The results show that the homogeneous temperature assumption does not hold well in post-flashover compartment fires. Instead, a rational statistical approach to fire behaviour could be used in fire safety and structural engineering applications.     

Burning behavior of sedan passenger cars

Four full-scale fire experiments using 4-door sedan passenger cars were carried out. The cars were ignited either at the splashguard of the right rear wheel or at the left front seat in the passenger compartment with a gasoline spill. The temperature inside the burning car and the mass loss rate were measured. The burning of the 4-door sedan was composed of three compartmental fires: the engine compartment, the passenger compartment, and the rear part inclusive of the fuel. In the experiments where ignition was initiated at the splashguard, the flame spread in the following order: to the rear part of the car, to the passenger compartment, and to the engine compartment. Breakage of the window glass markedly affected the spread of fire into the passenger compartment. The quantity of gasoline in the fuel tank also affected the speed of spread of the fire, because the gasoline ignited at an early stage of the fire. In the experiment where ignition was initiated in the passenger compartment, the fire gained force after the windshield was broken entirely. The flame spread in the following order: to the passenger compartment, to the engine compartment, and to the rear part of the car. The temperature within the passenger compartment peaked at 1000 °C. The heat release rate (HRR) curves showed several peaks depending on the burning of the three compartments. The HRR increased markedly when the fire spread to several different parts of the car at the same time. The HHR peaked at 3 MW when the passenger compartment and fuel (gasoline) burned simultaneously. The measured HRR curves were characterized by superposition of a Boltzmann curve and a Gaussian curve in order to obtain a model, which allowed us to make a more precise prediction of the fire spread probability from a burning car to nearby structures. The HRRs of burning cars were described by the sum of HRR from each compartment.     

Temperature of connections during fire on steel framed building

To study the global structural and thermal behaviour of buildings in fire, a research project was conducted including a fullscale test on a three storey steel frame building at Mittal Steel Ostrava before demolition. The main goal of the experiment was to verify the method for predicting joint temperatures and to improve it for the cooling phase. The fire compartment extending over a floor area of 24 m² was built on the second floor. The fire load was 140 kg/m² of wood and the ventilation was limited to an opening of 1,400 × 1,970 mm. This paper presents the time-temperature curves showing the development of the fire in the compartment and in the primary and secondary beams and its header plate connections. Comparisons are made between the test results and the temperatures predicted by the structural Eurocodes. The sensitivity of the connection behaviour to the estimated temperatures and associated degradation in material properties during the fire is demonstrated.     

Experimental and numerical study on restrained composite slab during heating and cooling

In this paper, a series of fire tests on restrained composite slabs, carried out at the University of Manchester, is presented. A total of six composite slabs were tested under different fire scenarios, with different load ratios. The tests were particularly concerned with the variation of internal forces within the slabs during both heating and cooling phases. In addition to the testing programme, two separate nonlinear finite element models have been developed to simulate the thermal and mechanical behaviour of composite slabs during heating and cooling, which is introduced in detail in this paper. In the thermal analysis model, plane elements were adopted to obtain a detailed thermal behaviour. In the structural analysis model, the concrete, steel deck and mesh were simulated by solid elements, shell elements and truss elements respectively. The interaction between the concrete and steel sheet was simplified to spring elements. According to the experimental results and FE modelling, the behaviour of composite slabs was analysed in detail. At last, the parametric study was performed where the effect of concrete strength, steel deck thickness and mesh size was analysed.     

Optimising design decision-making for steel structures in fire using a hybrid analysis technique

Steel structures can be protected against the effects of fully-developed fires by the use of sprayed on materials, board systems and intumescent paints, etc. or by using sufficiently large unprotected elements. This paper presents how optimum decisions for the protection of steel structures in fires can be achieved in a performance-based design environment, given conflicting structural fire design decision criteria and multidisciplinary fire design stakeholder views. In particular, a novel hybrid analysis approach is proposed for combining stakeholder views on the different fire protection options and the numerical outcomes of structural fire analysis. As for the stakeholder views, reference is made to benefits and costs criteria priorities for assessing competing options resulting from a previous study from the same authors. The fire protection structural performance is numerically and probabilistically assessed according to a parametric study. The proposed approach is exemplified by making reference to a limit state structural fire design of single steel elements. A synthesis and ranking technique is then applied to integrate the qualitative results obtained in terms of benefits and costs priority scores; and the quantitative measures of failure probabilities and costs for the different fire protection options. The results show that the ranking technique accounts for multidimensionality in synthesising the structural fire design decision problem. The results also show that intumescent paints and board systems are the most cost-effective options in different stakeholder influence scenarios, given a general selection of steel structural fire protection. The hybrid technique is proposed to support an optimal and cost-effective structural fire design decision-making for buildings in a performance-based design environment.     

Thermal and structural behaviour of a full-scale composite building subject to a severe compartment fire

This paper presents a numerical investigation of the thermal and structural results from a compartment fire test, conducted in January 2003 on the full-scale multi-storey composite building constructed at Cardington, United Kingdom, in 1994 for an original series of six tests during 1995–1996. The fire compartment's overall dimensions were 11 m×7 m with one edge at the building's perimeter, using largely unprotected steel downstand beams, and including within the compartment four steel columns protected with cementitious spray. The compartment was subjected to a natural fire of fire load 40 kg/m² of timber, in common with the original test series, but the composite slab forming its ceiling was subjected to a uniform applied load of 3.19 kN/m², which is higher than the original.     

Temperatures in unprotected joints between steel beams and concrete-filled tubular columns in fire

This paper presents experimental, numerical and analytical results of temperatures in different components of unprotected joints between steel beams and concrete-filled tubular columns in fire, which forms part of a research project to investigate joint behaviour under fire conditions and robustness of steel structures under accidental fire attack. Accurate calculation of connection temperatures is an essential first step of this process. The joint types include fin plate, endplate, reverse channel and T-stub. The results of the experiments indicate that the different components that are in the same region of a joint may be considered to have the same temperature. This suggests that the lumped mass temperature calculation equation in EN 1993-1-2 for unprotected steelwork may be used to calculate joint temperatures, by using an equivalent section factor for all the joint components in the same region of the joint. Therefore, the main objective of this study is to derive suitable expressions to calculate the section factors for the different components of the joints tested in this study. The experimental results have also been used to assess the simple temperature calculation method in Annex D of EN 1993-1-2, which relates the joint temperatures to the temperature of the connected beam. It has been found that this method generates grossly inaccurate results.     

Effects of exposed cross laminated timber on compartment fire dynamics

A series of compartment fire experiments has been undertaken to evaluate the impact of combustible cross laminated timber linings on the compartment fire behaviour. Compartment heat release rates and temperatures are reported for three configuration of exposed timber surfaces. Auto-extinction of the compartment was observed in one case but this was not observed when the experiment was repeated under identical condition. This highlights the strong interaction between the exposed combustible material and the resulting fire dynamics. For large areas of exposed timber linings heat transfer within the compartment dominates and prevents auto-extinction. A framework is presented based on the relative durations of the thermal penetration time of a timber layer and compartment fire duration to account for the observed differences in fire dynamics. This analysis shows that fall-off of the charred timber layers is a key contributor to whether auto-extinction can be achieved.     

Influencing factors for fire performance of simply supported RC beams with implicit and explicit transient creep strain material models

Knowledge of the temperature dependent material properties of concrete and steel bars is important for understanding the fire-response of a reinforced concrete (RC) structure. At high temperatures, the total strain of concrete is for a large extent influenced by load dependent strains, including transient strain and creep strain, which is also called transient creep strain in literature. The transient creep strain is much larger than the instantaneous stress-related strain under elevated temperature, and can lead to large influences on the deformation of the RC structure. Computer simulations that study the behaviour of heated concrete structures should consider this transient creep strain parameter, otherwise the deformations will be slightly overestimated. This remark is especially necessary for columns, because of their large compression area. However most of the fire resistance simulations use the implicit material models of concrete, as presented by EN 1992-1-2, which is a viable tool in current design practice, but cannot be used whenever transient creep may have an effect on the behaviour of the structural members. So it is necessary to compare the difference of the fire performance of RC beams with implicit and explicit models and study the factors influencing the difference between the two models.To solve this problem, a simplified numerical model proposed by the authors is adopted in this paper, the difference between fire performance of simple supported members with implicit and explicit models are compared and validated by experimental results from previously executed fire tests. Three influencing factors are discussed by comparing the fire behaviour simulation results of RC rectangular beams. The parameters that may have an impact on the difference of fire behaviour of members with the two models studied in this paper are heating curves, reinforcement and the size effect of the cross-section. The conclusions of the parametric study may lead to a better use of material models and more precise simulations on fire behaviour of RC elements, which are required by performance-based fire-resistance design.