钢筋混凝土保护钢管混凝土短柱火灾后力学性能研究

采用试验验证的有限元模型分析了主要影响因素。提出了钢筋混凝土保护钢管混凝土短柱火灾后轴压承载力和轴向刚度的计算方法。研究结果表明：（1）钢筋混凝土保护钢管混凝土短柱火灾后的轴压承载力和轴向刚度明显下降,随着受火时间的增加,轴向刚度的降低幅度更大;（2）截面核心面积比是火灾后轴压承载力降幅变化的主要影响因素。截面核心面积比和含管率是火灾后轴向刚度降幅变化的主要影响因素;（3）提出的简化计算方法可以预测钢筋混凝土保护钢管混凝土短柱火灾后的轴压承载力和轴向刚度。     

高温后钢筋混凝土短柱抗震性能试验研究

通过对7根高温后钢筋混凝土短柱试件和1根常温对比试件的拟静力试验,研究受火时间、轴压比和剪跨比对火灾后钢筋混凝土短柱的破坏形态、受剪承载力、骨架曲线、刚度退化、延性比、滞回耗能等的影响。利用有限元程序计算了高温后截面混凝土抗压强度的平均折减系数,提出了四周受火后钢筋混凝土柱受剪承载力的计算方法。研究结果表明：高温后短柱的受剪承载力随受火时间的增加、剪跨比的增大而减小;剪跨比对受火前后受剪承载力影响变化的程度相差较小;高温后短柱的受剪承载力先随轴压比的增大而增大,但当轴压比增大至界限轴压比后,受剪承载力反将降低,且高温后柱的界限轴压比较未受火时有减小趋势;提出的高温后钢筋混凝土柱受剪承载力计算式具有一定的安全保证率,可用于火灾后该类构件受剪性能的损伤评定。     

约束高强钢柱受火后轴压剩余承载力影响参数研究

利用经试验验证的ABAQUS有限元模型,对轴心压力作用下的轴向约束高强钢柱受火后的剩余承载性能进行了参数分析,考虑了过火温度、荷载比、轴向约束刚度比、长细比以及高强钢材强度等级等参数的影响。参数分析结果表明：当过火温度小于屈曲临界温度时,受火过程对轴向约束钢柱的轴压承载力没有影响;当过火温度大于屈曲临界温度时,钢柱的轴压剩余承载力会明显减小,且随着过火温度的升高,剩余承载力逐渐降低;轴向约束刚度比和长细比对约束钢柱的过火温度为屈曲临界温度时对应的轴压剩余承载力影响显著;荷载比对约束钢柱的过火温度为破坏临界温度时对应的轴压剩余承载力影响明显;高强钢材强度等级对约束钢柱受火后轴压承载力影响较小。根据参数分析所得数据及规律,提出了超500MPa高强钢轴向约束柱受火后轴压剩余承载力的简化计算方法,将计算结果与有限元分析结果对比,验证了该简化计算方法的可靠性。     

中空夹层方钢管再生混凝土柱火灾后剩余承载力

为研究中空夹层方钢管再生混凝土柱火灾后剩余力学性能,运用有限元分析软件ABAQUS建立了ISO 834标准火灾作用后中空夹层方钢管再生混凝土柱的有限元模型,分析了截面温度场和应力场的变化规律。在此基础上分析了混凝土强度、钢材强度、计算长度、受火时间、空心率、混凝土取代率、偏心率等参数对构件火灾后剩余力学性能的影响;结合大量计算结果给出了中空夹层方钢管再生混凝土柱火灾后剩余承载力简化计算公式。结果表明:构件的火灾后剩余承载力随钢材强度和混凝土强度的增加而增加;构件火灾后剩余承载力随计算长度和受火时间的增加,总体呈降低趋势,当受火时间超过60 min时,构件火灾后剩余承载力急剧降低;空心率和混凝土取代率对构件火灾后剩余承载力影响较小,构件火灾后剩余承载力随空心率的增加而降低,随混凝土取代率的增加先增加后降低;构件火灾后剩余承载力随偏心率的增加而降低;所提公式计算精度良好,可为中空夹层方钢管再生混凝土柱抗火设计提供参考。     

部分预制装配型钢混凝土柱受火后轴压性能试验研究

为研究部分预制装配型钢混凝土(PPSRC)柱在受火后的受压性能,开展了8个足尺PPSRC柱试件在历经ISO 834标准明火试验后的轴压试验和1个未受火PPSRC柱对比试件的轴压试验,研究核心部分混凝土强度、配箍间距及截面尺寸对标准受火条件下试件截面温度-时间曲线、轴压荷载-位移曲线和剩余承载力的影响。结果表明：RPC预制外壳能对内部型钢和核心混凝土起到良好保护作用;箍筋间距、现浇混凝土强度对截面温度场影响不大,型钢保护层越大,截面内部温度升高越慢且最高温度越低;PPSRC柱受火后轴压破坏形态与常温下类似;核心混凝土强度增加,试件承载力增大;箍筋间距减小对受火后试件承载力提高有利。     

温度和荷载共同作用下钢管混凝土短柱的性能分析

采用有限元模型研究温度与荷载共同作用下的钢管混凝土短柱的荷载-变形关系。该模型用来模拟一系列各种温度和力学加载条件下的钢管混凝土短柱试验,包括高温测试、常温下构件残余强度测试,以及无初始荷载的ISO-834标准温度下的构件测试。预测结果以及试验结果的对比表明:模型可以准确预测构件的荷载-变形关系。然后采用该有限元模型分析在具有初始荷载、加热和制冷条件下完全加载过程中的钢管混凝土短柱的性能,以观察构件横截面的应力分布和各个加载阶段的应力发展情况。所有构件在制冷过程之后都加载至极限强度,利用一系列参数对其残余应力进行了分析。结果发现:在温度和荷载共同作用下的构件极限强度,要比只有温度作用而无初始荷载的构件极限强度略低,但是在极限强度时,前者的最大应变显著增加。     

Tragwiderstand von Betonbauteilen nach dem Brand

Load carrying capacity of concrete structures after fire exposure. The mechanical and thermal properties of building materials change at elevated temperatures. This change of material properties has an important influence on the load carrying and deformation behaviour in case of fire. The rate of increase of temperature through the cross section in a concrete element is relatively slow and so internal zones are protected against heat. Only a small part of the cross‐section is affected by the temperature action (loss of strength and stiffness). For these reasons reinforced concrete structures with adequate structural detailing usually reach high fire resistance without any additional fire protection. However after the fire has been extinguished the heat penetration into the cross section continues for hours. Additionally calcium hydroxide reformats during the cooling phase widening up micro cracks. The combination of these two phenomena can lead to a significant reduction of the compressive strength of concrete after the fire. This paper analyses the influence of the loss of strength during and after the cooling on the load carrying capacity of concrete elements. The numerical calculations presented in this study showed that after the cooling phase concrete elements can exhibit a load carrying capacity lower than during the fire. The design of concrete structures based on equivalent time of standard fire exposure without cooling phase can lead to unsafe results. For this reason it may be important to take into account the cooling phase of the fire for the calculation of the load bearing capacity of concrete structures.     

约束高强钢柱受火后轴压剩余承载力试验研究

设计了3根截面尺寸、长度均相同的Q550高强度钢柱,其中两根受到轴向约束,并对其进行了恒载作用下升温、降温的受火全过程试验,以及自然降温至室温后的轴压剩余承载力试验,对未受火的钢柱进行了常温下的极限承载力试验。研究了高温试验中钢柱的轴向位移-温度和中点侧向挠度-温度关系、极限承载力试验中钢柱的轴力-轴向位移和轴力-柱中点侧向挠度关系,并进行了有限元模拟。试验以及有限元模拟分析显示,若约束钢柱在高温过程中发生屈曲,则降温后钢柱会有明显的残余弯曲变形,并且柱中截面会产生比初始残余应力更为显著的残余应力,从而显著降低钢柱的剩余承载力和轴向刚度。试验结果与有限元分析结果吻合较好,验证了有限元分析模型的有效性。     

冻融循环-酸雨锈蚀交替作用后圆钢管混凝土轴压短柱力学性能分析

基于材料强度折减及钢管壁厚折减的方法,对冻融循环-酸雨锈蚀交替作用后圆钢管混凝土轴压力学性能采用有限元法进行了研究。基于合理的有限元分析模型,对冻融循环-酸雨锈蚀交替作用后圆钢管混凝土柱的破坏模态、轴向荷载-位移关系、钢管与混凝土相互作用进行了分析,研究了含钢率、截面尺寸、钢管屈服强度、混凝土轴心抗压强度以及冻融循环-酸雨交替次数对试件轴压极限承载力的影响。结果表明:有限元模拟结果与试验结果吻合良好,验证了模型的有效性; 冻融循环-酸雨锈蚀交替作用后轴压圆钢管混凝土短柱的破坏模态与普通试件相似,轴向荷载-位移曲线变化趋势一致,试件均为塑性破坏; 圆钢管混凝土轴压短柱随冻融循环-酸雨锈蚀交替次数的增加,材料性能劣化严重,外钢管对核心混凝土约束作用减弱,试件极限承载力明显下降。     

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.     

Finite element analysis of concrete-filled circular steel tubular columns subjected to post-earthquake fire

为研究圆钢管混凝土柱经历地震损伤后的耐火性能,选取了合理的地震损伤指数及材料本构模型,采用有限元分析软件ABAQUS,对钢管混凝土柱在往复荷载和火灾等不同工况下的试验进行了数值模拟验证。在验证模型可靠性的基础上,利用ABAQUS中的数据传递功能,建立了震损后圆钢管混凝土柱耐火极限有限元计算模型。以圆钢管混凝土典型轴心受压柱为分析对象,对其先后经历地震和火灾作用下的破坏形态、损伤机理进行了分析,研究了损伤指数对圆钢管混凝土柱震后耐火极限的影响。结果表明：震损后圆钢管混凝土柱在高温下的破坏形态是在前期地震损伤基础上的发展和蔓延;地震损伤指数是影响圆钢管混凝土柱震后耐火极限的重要参数,随着地震损伤指数增大,圆钢管混凝土柱的耐火极限有所减小。     

Investigation of concrete encased steel composite columns at elevated temperatures

This paper presents a nonlinear 3-D finite element model investigating the behaviour of concrete encased steel composite columns at elevated temperatures. The composite columns were pin-ended axially loaded columns having different cross-sectional dimensions, different structural steel sections, different coarse aggregates and different load ratios during fire. The nonlinear material properties of steel, concrete, longitudinal and transverse reinforcement bars as well as the effect of concrete confinement at ambient and elevated temperatures were considered in the finite element models. The interface between the steel section and concrete, the longitudinal and transverse reinforcement bars, and the reinforcement bars and concrete were also considered allowing the bond behaviour to be modelled and the different components to retain its profile during the deformation of the column. The initial overall (out-of-straightness) geometric imperfection was carefully included in the model. The finite element model has been validated against published tests conducted at elevated temperatures. The time–temperature relationships, deformed shapes at failure, time–axial displacement relationships, failure modes and fire resistances of the columns were evaluated by the finite element model. It has been shown that the finite element model can accurately predict the behaviour of the columns at elevated temperatures. Furthermore, the variables that influence the fire resistance and behaviour of the composite columns comprising different load ratios during fire, different coarse aggregates and different slenderness ratios were investigated in parametric studies. It is shown that the fire resistance of the columns generally increases with the decrease in the column slenderness ratio as well as the increase in the structural steel ratio. It is also shown that the time–axial displacement relationship is considerably affected by the coarse aggregate. The fire resistances of the composite columns obtained from the finite element analyses were compared with the design values obtained from the Eurocode 4 for composite columns at elevated temperatures. It is shown that the EC4 is conservative for all the concrete encased steel composite columns, except for the columns having a load ratio of 0.5 as well as the columns having a slenderness ratio of 0.69 and a load ratio of 0.4.     

Fire behaviour of high strength self-consolidating concrete filled steel tubular stub columns

This paper reports an investigation into the behaviour of high strength SCC (self-consolidating concrete) filled steel tubular stub columns exposed to standard fire. A series of tests were carried out to obtain the temperature distribution, axial deformation, limiting temperature of steel and fire endurance of the SCC filled steel tubular stub columns. In addition, a finite element analysis (FEA) model was proposed and used to simulate the fire behaviour of the columns. In the FEA modeling, a sensitivity study was conducted to determine the concrete fracture energy and the contact property of the steel and concrete interface. The verified FEA model was used to analyse the structural behaviour of the columns under fire exposure, such as strain, stress, the load sharing between the steel tube and concrete and local buckling of the steel tube, to gain an insight into the failure mechanism of the columns.     

火灾后方中空钢管混凝土柱滞回性能有限元分析

为研究火灾后方中空夹层钢管混凝土柱的滞回性能,采用合理的本构关系,建立有限元模型,分析参数有受火时间、轴压比、混凝土强度、空心率,并与相应试验结果进行比较,承载能力、滞回曲线以及破坏模态吻合较好。同时对火灾后方中空夹层钢管混凝土柱滞回性能进行机理分析,结果表明：混凝土强度对滞回曲线影响不明显,分析滞回骨架曲线得知承载力随着构件受火时间增加、轴压比的增大,空心率减小而减小。随着受火时间的增加,空心率的增大,构件的滞回耗能能力更好。     

Performance of axially restrained concrete encased steel composite columns at elevated temperatures

The structural performance of axially restrained concrete encased steel composite columns at elevated temperatures is investigated in this study. An efficient nonlinear 3-D finite element model was presented for the analysis of the pin-ended axially loaded columns. The restraint ratios varied from 20% to 100% of the axial stiffness of the composite columns at ambient temperature. The finite element model was verified against published test results on axially restrained concrete encased steel composite columns at elevated temperatures. The columns investigated had different cross-sectional dimensions, different coarse aggregates and different load ratios during fire. The nonlinear material properties of steel, concrete, longitudinal and transverse reinforcement bars as well as the effect of concrete confinement at ambient and elevated temperatures were considered in the finite element model. The interface between the steel section and concrete, the longitudinal and transverse reinforcement bars, and the reinforcement bars and concrete were also considered allowing the bond behaviour to be modelled and the different components to retain its profile during the deformation of the column. The initial overall geometric imperfection was carefully included in the model. The time-temperature relationships, deformed shapes at failure, time-axial displacement relationships, failure modes and fire resistances of the columns were evaluated by the finite element model and compared well against test results. Furthermore, the variables that influence the fire resistance and behaviour of the axially restrained composite columns comprising different axial restraint ratios, different load ratios during fire, different coarse aggregates and different slenderness ratios were investigated in a parametric study. It is shown that axially restrained composite columns behave differently in fire compared to the unrestrained columns since the typical “runaway” failure was not predicted from the finite element analysis. The fire resistances of the composite columns obtained from the finite element analysis were compared with the design values obtained from the Eurocode 4 for composite columns at elevated temperatures. It is shown that the EC4 is generally conservative for all the axially restrained concrete encased steel composite columns, except for some columns with higher load and slenderness ratios.     

The effects of explosive mass ratio on residual compressive capacity of contact blast damaged composite columns

The risks associated with suitcase bombs are of serious concern because they can be easily handled and placed within close proximity of key structural components of building structures. The most common failure mode of structures subjected to blast loads from satchel and suitcase bombs is progressive collapse. The high-fidelity physics-based computer program, LS-DYNA is utilized in this study to provide numerical simulations of the dynamic response and residual axial capacity of composite columns subjected to blast loads. A field test using contact explosive was conducted on one composite column specimen. The test results were compared with the analytical results to validate the finite element model. An extensive parametric study was conducted to investigate the relationship between residual axial capacity and structural and loading parameters such as material strength, column detail and blast conditions. Two empirical equations were derived through a multivariable regression analysis in terms of the various parameters to predict the residual capacity index based on the explosive mass ratio (ω_TNT).     

An approach for evaluating the residual strength of fire exposed bridge girders

This paper presents an approach for evaluating residual strength of fire exposed steel bridge girders. The approach involves three stages of analysis that is to be carried out at ambient conditions, during exposure to fire, and after cooling of the fire exposed bridge girder. In the first stage, load carrying capacity of the girder is evaluated at room temperature. In the second stage of analysis, thermal and structural response of the bridge girder is traced under specified fire exposure and loading conditions. In the third stage (after the bridge girder has cooled down), residual capacity of the girder is evaluated by incrementing load on the girder till failure occurs. The proposed approach is applied to carry out a set of numerical studies on a typical steel girder using finite element computer program ANSYS. Results from numerical studies indicate that the maximum fire temperature (and associated temperature in steel) is the most critical factor that influences residual strength of a fire exposed bridge girder. A girder exposed to typical “external” fire conditions, with maximum fire temperatures reaching to 600–700 °C, retains about 70 to 80% of its strength on cooling. On the other hand, a steel bridge girder exposed to hydrocarbon fire, with a maximum temperature of about 1100 °C, looses most of its strength during heating phase of the fire and experiences failure.     

火灾后型钢混凝土柱受力性能试验研究

通过6个火灾后型钢混凝土柱的静力加载试验以及2个常温下型钢混凝土柱的对比试验,研究了火灾后型钢混凝土柱的破坏形态、变形特性、剩余承载力、刚度等。试验结果表明：火灾后型钢混凝土柱的破坏形态与常温下基本相同,但柱的轴压刚度、抗弯刚度、承载力均有不同程度降低;从加载到荷载达到80%极限荷载的不同阶段,火灾后型钢混凝土柱轴压刚度影响系数kz约为0.33~0.44,抗弯刚度影响系数kw约为0.30~0.59。利用规程JGJ 138-2001的计算方法,对所有试件常温下的承载力进行了计算,并将试验结果与计算结果进行了对比,经历火灾作用后轴压柱的剩余承载力约为常温下承载力的51%~81%,偏压柱的剩余承载力约为常温下承载力的69%~81%,火灾后型钢混凝土柱承载能力退化显著。     

钢-混凝土组合楼盖抗火性能的数值分析方法

为模拟高温下钢筋与混凝土、压型钢板与混凝土之间的粘结-滑移性能,本文分别建立了四节点粘结-滑移空间单元刚度方程和八节点粘结-滑移单元刚度方程,给出了高温下混凝土的破坏模型和非线性本构关系及结构钢在高温下的应力-应变关系和屈服模型;然后建立了用于模拟钢.混凝土组合楼盖在火灾下的结构性能的有限元格式,并编制了相应的分析程序。通过与常温下和高温下试验结果进行对比,初步验证了本文分析理论和程序的正确性和可靠性。