Design equation for predicting fire resistance of reinforced concrete beams

An approach for evaluating the fire resistance of reinforced concrete (RC) beams is presented in this paper. A macroscopic finite element model is applied to study the influence of various parameters on the fire resistance of RC beams. Data from parametric studies is utilized to develop a simplified expression for evaluating the fire resistance of an RC beam as a function of influencing parameters. The validity of the proposed approach is established by comparing the fire resistance predictions with those obtained from finite element studies as well as from fire resistance tests. Predictions from the proposed equation are also compared with fire resistance estimates from current codes of practice. The applicability of the approach to design situations is illustrated through a numerical example. The proposed rational approach expresses fire resistance in terms of conventional structural and material design parameters, and thus facilitates easy evaluation of fire resistance. The proposed approach provides better estimates than those from current codes of practice and thus can be used to evaluate the fire resistance of RC beams with an accuracy that is adequate for design purposes.     

Macroscopic FE model for tracing the fire response of reinforced concrete structures

A macroscopic finite element model for tracing the fire response of reinforced concrete (RC) structural members is presented. The model accounts for critical factors that are to be considered for performance-based fire resistance assessment of RC structural members. Fire induced spalling, various strain components, high temperature material properties, restraint effects, different fire scenarios and failure criteria are incorporated in the model. The validity of the numerical model is established by comparing the predictions from the computer program with results from full-scale fire resistance tests. Case studies are conducted to demonstrate the use of the computer program for tracing the response of RC members under standard and design fire exposures. Through the results of the case studies, it is shown that the fire scenario has a significant effect on the fire resistance of RC columns and beams. It is also shown that macroscopic finite element models are capable of predicting the fire response of RC structural members with an adequate accuracy for practical applications.     

Fire Induced Spalling in High Strength Concrete Beams

A macroscopic finite element model is extended to account for fire induced spalling in high strength concrete (HSC) beams. The model is based on the principles of mechanics and thermodynamics and utilizes pore pressure calculations to predict fire induced spalling in concrete. For validating the model, spalling measurements were made by conducting fire resistance experiments on four normal strength and high strength concrete beams. Spalling predictions from the model are compared with the measured values of spalling at various stages of fire exposure. The validated model is applied to investigate the influence of fire scenario, concrete strength (permeability) and axial restraint on the fire induced spalling and fire response of RC beams. Results from the analysis show that fire scenario, and concrete permeability largely influence the extent of fire induced spalling in concrete beams. Further, it is also shown that the extent of spalling has significant influence on the fire resistance of RC beams.     

Effect of Location of Restraint on Fire Response of Steel Beams

Restrained steel beams, when exposed to fire, develop significant restraint forces and often behave as beam-columns. The response of such restrained steel beams under fire depends on many factors including: fire scenario, beam slenderness ratio, location of axial restraint at the supports, and high-temperature properties of steel. A set of numerical studies, using finite element computer program ANSYS, is carried out to study the fire response of steel beam-columns under realistic fire and restraint scenarios. Results from the parametric studies indicate that fire scenario, beam slenderness, location of axial restraint and high-temperature creep have significant influence on the behavior of restrained beams under fire conditions. Severe fires produce high axial forces at early stages of fire exposure; whereas in moderate fires, significant axial force develops only at later stages of fire exposure. Axial restraint enhances the fire resistance due to the development of tensile catenary action in restrained beams. Furthermore, restrained beams with low slenderness ratio exhibit better fire performance when the axial restraint at the support is located at the bottom flange.     

Fire resistance of partially encased steel columns with restrained thermal elongation

The behaviour of composite columns made of partially encased steel sections subjected to fire has been studied numerically by several researchers. Experimental studies are scarce and there are still many phenomena to study. The influence of the axial and rotational restraint on the behaviour of these types of columns subjected to fire is still under research. This paper presents the results of a series of fire resistance tests on these types of columns with restrained thermal elongation. A new experimental set-up, specially conceived for fire resistance tests on building columns, was used for the tests. The experimental set-up was conceived so that the axial and rotational restraint of the columns would be similar to the conditions in a real building. The parameters studied were the load level, the axial and rotational restraint ratios and the slenderness of the column. The main conclusion of this work is that for low load levels the stiffness of the surrounding structure has a major influence on the behaviour of the column subjected to fire. Increasing the stiffness of the surrounding structure led to reductions in the critical times. The same behaviour was not observed for the high load levels.     

约束高强度Q460钢梁的抗火性能

为了研究约束高强度Q460钢梁的抗火性能,在已有约束钢梁分析理论的基础上,引入残余应力,提出了约束钢梁的抗火性能分析方法,并采用普通强度约束钢梁试验数据对分析方法进行了验证。考虑高强度Q460钢材高温下力学性能参数,利用所提出的方法分析了约束高强度Q460钢梁的抗火性能,并与普通强度Q345钢梁进行了对比。对影响约束高强度Q460钢梁的抗火性能参数进行了分析,包括荷载比、残余应力、轴向约束刚度、转动约束刚度和受火方式等。研究表明:所提出的分析方法准确可靠,高强度Q460钢梁抗火性能与普通强度钢梁具有较大的区别,高强度Q460约束钢梁的抗火性能明显优于普通强度约束钢梁。荷载比、轴向约束刚度、转动约束刚度、受火方式对高强度Q460约束钢梁有较大影响。     

Behavior of axially-and-rotationally restrained concrete columns with ‘+’-shaped cross section and subjected to fire

Most of the previous investigations studied the structural behavior of concrete columns with ‘+’-shaped cross section at room temperature, but the behavior of such columns during a fire and the effect of boundary conditions on the mechanical properties of the heated columns have seldom been examined. In this paper, the influence of axial-and-rotational restraint on the behavior of heated concrete columns with ‘+’-shaped cross section is investigated. A self-developed finite element program RCSSCF is applied in this study. The development of column internal forces as well as deflections at the mid-height of concrete columns are discussed. Simulation results show that: (1) axial restraint can induce significant additional axial forces in concrete columns with ‘+’-shaped cross section and subjected to fire, and the additional axial forces in strongly restrained concrete columns during a fire can reach approximately 65–70% of the axial forces in columns at room temperature; (2) the fire resistance of concrete columns without rotational restraint decreases significantly with an increase of load eccentricity ratio, but that with rotational restraint is influenced lightly by load eccentricity ratio; and (3) for columns with different non-zero rotational restraint ratios, the internal axial forces, and the internal moments and deflections at the mid-height of these columns appear to follow common trends.     

Response of steel beam–columns exposed to fire

Steel beams when exposed to fire develop significant restraint forces and often behave as beam–columns. The response of such restrained steel beams under fire depends on many factors including fire scenario, load level, degree of restraint at the supports, and high-temperature properties of steel. A set of numerical studies, using finite element computer program ANSYS, is carried out to study the fire response of steel beam–columns under realistic fire, load and restraint scenarios. The finite element model is validated against experimental data, and the importance of high-temperature creep on the fire response of steel beam–columns is illustrated. The validated model is used to carry out a set of parametric studies. Results from the parametric studies indicate that fire scenario, load level, degree of end-restraint and high-temperature creep have significant influence on the behavior of beams under fire conditions. The type of fire scenario plays a critical role in determining the fire response of the laterally-unrestrained steel beam within a space subframe. Increased load level leads to higher catenary forces resulting in lower fire resistance. Rotational restraint enhances the fire resistance of a laterally-unrestrained steel beam, while the axial restraint has detrimental effect on fire resistance.     

A Numerical Investigation on Restrained High Strength Q460 Steel Beams Including Creep Effect

Most of previous studies on fire resistance of restrained steel beams neglected creep effect due to lack of suitable creep model. This paper presents a finite element model (FEM) for accessing the fire resistance of restrained high strength Q460 steel beams by taking high temperature Norton creep model of steel into consideration. The validation of the established model is verified by comparing the axial force and deflection of restrained beams obtained by finite element analysis with test results. In order to explore the creep effect on fire response of restrained Q460 steel beams, the thermal axial force and deflection of the beams are also analyzed excluding creep effect. Results from comparison infer that creep plays a crucial role in fire response of restrained steel beam and neglecting the effect of creep may lead to unsafe design. A set of parametric studies are accomplished by using the calibrated FEM to evaluate the governed factors influencing fire response of restrained Q460 steel beams. The parametric studies indicate that load level, rotational restraint stiffness, span–depth ratio, heating rate and temperature distribution pattern are key factors in determining fire resistance of restrained Q460 steel beam. A simplified design approach to determine the moment capacity of restrained Q460 steel beams is proposed based on the parametric studies by considering creep effect.     

具有端部约束的碳纤维布加固混凝土梁耐火性能试验研究

在ISO834标准升温条件下,开展了具有端部约束的7根碳纤维布加固混凝土梁和1根未加固混凝土梁的耐火性能试验,考察了梁端轴向和转动约束、防火涂料厚度、荷载比等参数对约束梁高温变形及内力的影响。试验结果表明:约束梁的轴向变形最大值随轴向约束刚度比和防火涂料厚度增大而减小,附加轴力最大值随轴向约束刚度比增加或防火涂料厚度减小而增大,梁端附加弯矩最大值随防火涂料厚度减小而增大,但防火涂料厚度在10~20mm范围内变化时影响幅度有限;降温后约束梁的附加轴力仅少量恢复,梁端附加弯矩却较大幅度回落;与非加固梁相比,加固梁的梁端附加弯矩最大值不仅数值更小,而且出现时刻明显偏晚。     

限制热伸长的部分包裹钢柱的耐火性能

一些学者对火灾下部分包裹型钢组合柱性能进行了数值研究,但试验研究还比较缺乏,许多方面仍尚待研究。火灾下轴向和转动约束对这类柱性能的影响仍在研究中。介绍了限制热伸长时这类柱的一系列耐火试验结果。为进行耐火试验而专门设计了一种试验装置,使柱的轴向和转动约束与实际情况相近。研究参数为:荷载水平、轴向和转动约束率及柱长细比。结果表明:低荷载水平下,围护结构的刚度对火灾中柱子性能的影响很大。临界时间随围护结构刚度的增大而减小;高荷载水平下则不存在这种情况。     

Fire resistance of reinforced concrete columns with L-, T-, and +-shaped cross-sections

Four full-scale reinforced concrete (RC) columns with L-shaped cross-sections, four full-scale RC columns with T-shaped cross-sections, three full-scale RC columns with +-shaped cross-sections, and one full-scale RC column with a square cross-section were experimentally investigated for fire resistance following the ISO834 standard heating process. The effects of axial load ratio and fire exposure condition on failure mode, axial deformation and fire resistance of the columns were analyzed. The experimental results showed that: (a) when the axial load ratio is 0.55, the fire resistances of the columns with L-, T-, and +-shaped cross-sections subjected to fire on all sides were 60–73% that of the column with the square cross-section. (b) In the case of samples subjected to fire on all sides, the fire resistance of columns with differently-shaped cross-sections increased in the following order: L-shaped cross-section <T-shaped cross-section <+-shaped cross-section. A computer program RCSSCF was developed to calculate temperature, deformation, and fire resistance of the loaded columns with L-, T-, and +-shaped cross-sections. The results of the numerical simulation were compared with those of the full-scale fire resistance tests.     

Experimental analysis on cold-formed steel beams subjected to fire

The great majority of the studies in this area emphasise further the structural behaviour of cold-formed steel members by means of analytical approximation and purely numerical methods. In addition, they generally only take into account the structural behaviour of members with just one profile. On the contrary, this paper reports a series of flexural tests under fire conditions focused on cold-formed galvanised steel beams consisting on compound cold-formed steel profiles which are often used in floors and roofs of warehouses and industrial buildings. The main objective of this research was to assess the failure modes, the critical temperature and the critical time of the studied beams. Other important goals of this research work were also to investigate the influence of the cross-sections, the axial restraint to the thermal elongation of the beam and the rotational stiffness of the beam supports. Finally, the results showed above all that the critical temperature of a cold-formed steel beam might be strongly affected by the axial restraint to the thermal elongation of the beam.     

型钢混凝土框架结构耐火性能有限元分析

为了研究型钢混凝土框架整体结构的耐火性能,为其抗火设计提供参考,采用受火楼层建立精细化有限元计算模型、非受火楼层建立梁单元计算模型的方法,建立了型钢混凝土框架整体结构的耐火性能计算模型。考虑火灾位置、荷载分布形式、柱轴压比等参数的影响,对火灾下型钢混凝土框架整体结构的变形规律、承载机制、破坏形态以及耐火极限进行参数分析。分析结果表明：火灾下框架结构出现了整体破坏和局部破坏两种典型的破坏形态,受火构件受到的约束作用对其耐火性能有较大的影响;在局部破坏形态中,由于受热膨胀,火灾下框架梁首先出现了较大的轴压力,受火框架梁处于压弯受力状态;之后,框架梁出现了悬链线效应,轴力对框架梁的受力状态有较大影响;在整体破坏形态中,根据轴压比及荷载分布形式的不同,框架出现了中柱破坏和边柱破坏两种典型破坏形态,同时,随楼层受火位置的升高,柱的轴压比减小,框架结构的耐火极限增加。     

考虑轴向约束的钢筋混凝土梁高温下 竖向推覆试验

为研究轴向约束对钢筋混凝土梁高温下大变形力学性能的影响,进行了1根常温和2根高温下约束梁的竖向推覆试验。比较分析了不同升温时间对轴向约束钢筋混凝土梁测点温度、升温反应及延性和耗能等的影响; 重点研究了轴向约束钢筋混凝土梁的高温反应、破坏形态、承载能力和受力机制。结果表明:升温过程中,随着温度的升高,轴向约束钢筋混凝土梁不断向下挠曲,但炉温超过800 ℃之后,挠度有所恢复; 由于受热膨胀引起的轴压力影响,高温下约束梁塑性铰区域明显缩小,其延性和耗能能力随着温度的升高而降低; 高温作用产生的初始轴压力提高了梁的峰值承载力,但随着升温时间的延长,其提高幅度低于材料劣化对梁峰值承载力的降低幅度; 初始轴压力使得高温梁在破坏前始终处于压弯机制; 研究结果可为验证和校正火灾下约束混凝土梁力学行为的数值模拟以及理论分析提供可靠的试验数据,并为进一步探讨高温下钢筋混凝土约束梁大变形时的破坏准则提供依据。     

Factors governing the shear response of prestressed concrete hollowcore slabs under fire conditions

This paper presents results from numerical studies on the effect of critical factors governing the shear response on prestressed concrete (PC) hollowcore slabs exposed to fire. A validated three dimensional finite element model is applied for evaluating failure of fire exposed prestressed concrete (PC) hollowcore slabs under different limiting states, including through shear. This model accounts for temperature induced property degradation in concrete and prestressing strands, cracking in concrete, varying fire exposure, loading and restraint conditions. The factors varied in the parametric study include, slab depth, load level, loading pattern, axial restraint, level of prestressing, and fire scenario. Results from parametric studies show that slab depth, load level, loading pattern, axial restraint, level of prestressing and fire scenario have significant influence on the fire response of PC hollowcore slabs, and failure under these conditions can occur through shear limiting state prior to reaching flexural limiting state. Results from parametric studies are further utilized to propose a simplified approach for evaluating shear capacity PC hollowcore slabs under fire conditions.     

Enhancing the fire resistance of composite floor assemblies through the use of steel fiber reinforced concrete

This paper presents a strategy for achieving the required fire resistance in composite floor systems through the use of steel fiber reinforced concrete (SFRC). Both experimental and numerical studies were carried out to evaluate the fire performance of floor systems comprising unprotected steel beams and concrete/SFRC deck slabs. The results from these studies show that SFRC composite deck slabs develop significant tensile forces (through tensile membrane action) that transfer load from fire-weakened steel beams to other cooler parts of the structure. Preliminary results indicate that the combined effect of composite construction, tensile membrane action, and the improved properties of SFRC under realistic fire, loading, and restraint conditions can provide sufficient fire resistance in steel beam-concrete deck slabs without the need for external fire protection on the floor assembly.     

Experimental investigation of reinforced concrete and hybrid fibre reinforced concrete shield tunnel segments subjected to elevated temperature

This paper presents a comprehensive experimental study on the comparative behaviour of the reinforced concrete (RC) and the hybrid fibre reinforced concrete (HFRC) shield TBM (Tunnel Boring Machine) tunnel lining segments exposed to fire. The tests were conducted using a newly developed test facility, which is capable of accommodating different mechanical loading and boundary conditions under different fire scenarios. Six RC segments and six HFRC segments were tested to the standard Eurocode HC (Hydrocarbon) curve, while two reference specimens, one for each type, were tested in ambient environment to provide benchmark data. Apart from the spalling resistance, the fire effects on the structural behaviour were investigated under different boundary conditions at the segment ends, including free sliding (no horizontal constraint), total horizontal restraint and controlled horizontal reaction. The vertical load capacities were investigated for both under-fire and post-fire scenarios. The experimental results revealed excellent spalling resistance in the HFRC segments under thermo-mechanical loading, while the RC segments exhibited better structural performance. A combination of RC design (with flexural reinforcement) and the use of hybrid fibres is deemed to be effective in providing good spalling resistance while at the same time ensuring a robust structural behaviour.     

碳纤维布抗弯加固钢筋混凝土梁的耐火极限分析

编制了碳纤维布抗弯加固钢筋混凝土梁的温度场计算程序和耐火极限分析程序,通过数值分析结果与试验结果的对比,初步验证了程序的有效性。利用上述程序,针对高温下加固梁的受弯破坏形态,就不同构件跨高比、防火涂料厚度、受拉纵筋配筋率、碳纤维布加固量、混凝土保护层厚度、荷载比等共计1728种工况进行了大量计算分析,同时考察了各参数对加固梁耐火极限的影响规律。在此基础上,建立了加固梁的耐火极限简化预测方法。研究结果表明,加固梁的耐火极限随防火涂料厚度、混凝土保护层厚度和受拉纵筋配筋率的增加而逐渐增大,同时随荷载比、碳纤维布加固量和跨高比的增加而逐渐减小,其中荷载比的影响最为显著。