Analytical behavior of eccentrically loaded concrete encased steel columns subjected to standard fire including cooling phase

A nonlinear 3-D finite element analysis (FEA) model was developed to predict the behavior of eccentrically loaded concrete encased steel (CES) columns subjected to ISO-834 standard fire including heating and cooling phases. The finite element model has been validated against published tests conducted at elevated temperatures. Comparisons between the predicted results and the test results show that this model can accurately predict the behavior of CES columns under fire. The FEA model was then used to investigate the typical temperature-time curve and mid-height lateral deformation-time curve of eccentric compression CES columns in a complete loading history including initial loading, heating and cooling. It is shown that the temperature delay is obvious at the inner layers of concrete. The fire resistance of a CES column should be checked for the full process of fire exposure until temperatures everywhere in the column start to decrease. The lateral deformation of the column still gradually increases during the cooling phase and the column may fail during that phase. There is a large residual deformation after the fire exposure. Furthermore, the variables that influence the behavior of the CES columns under fire were investigated in parametric studies. It is found that the main parameters which influence the lateral deformation-time curve of the column during the full process of fire exposure are load ratio, slenderness ratio, duration time, depth to width ratio and steel ratio, and the main parameters which influence the residual deformation ratio of the column after fire are load ratio, duration time, cross-sectional depth and steel ratio.     

Residual strength of concrete-filled RHS columns after exposure to the ISO-834 standard fire

The residual strength of a composite column may be used to assess the potential damage caused by fire and help to establish an approach to calculate the structural fire protection for minimum post-fire repair. The behavior of six rectangular hollow structural steel (RHS) columns filled with concrete, with or without fire protection, after exposure to the ISO-834 standard fire (ISO 834, 1975), subjected to axial or eccentric loads have been experimentally investigated and the results presented in this paper. Comparisons are made with predicted column strengths using the existing codes such as LRFD-AISC-1994, AIJ-1997, EC4-1996 and GJB4142-2000. It was found, in general, that the loss of the strength of the specimens without protections was significantly greater than that of columns with fire protection. A mechanics model is developed in this paper for concrete-filled RHS columns after exposure to the ISO-834 Standard Fire (ISO 834, 1975), and is a development of the analysis used for ambient condition (Han et al., 2001). The predicted load versus mid-span deflection relationship for the composite columns is in good agreement with test results. Based on the theoretical model, influence of the changing strength of the materials, fire duration time, sectional dimensions, steel ratio, load eccentricity ratio, depth-to-width ratio and slenderness ratio on the residual strength index (RSI) is discussed. It was found that, in general, the slenderness ratio, sectional dimensions and the fire duration time have a significant influence on the residual strength index (RSI). However, the steel ratio, the depth-to-width ratio, the load eccentricity ratio and the strength of the materials have a moderate influence on RSI. Finally, formulas suitable for incorporation into a building code, for the calculation of the residual strength of the concrete-filled RHS columns after exposure to ISO-834 standard fire is developed based on the parametric analysis results.     

考虑受火全过程的高温作用后型钢混凝土柱力学性能研究及有限元分析

进行了高温作用后型钢混凝土柱力学性能试验,在试验的基础上建立了高温作用后型钢混凝土柱力学性能的数值模拟方法。通过在ABAQUS平台上开发子程序建立了升降温作用下型钢混凝土柱温度场的计算模型,通过选择合适的材料本构关系以及在ABAQUS平台上开发自定义场变量子程序,在考虑升温、降温以及高温作用后不同阶段材料本构关系的基础上,提出了高温作用后型钢混凝土柱力学性能分析的计算模型,实现了高温作用后型钢混凝土柱力学性能的计算模拟。计算结果与试验结果的对比表明,二者吻合较好,提出的方法合理可行。     

标准火灾作用下钢管混凝土短柱落锤动态冲击试验研究

采用落锤冲击实验机进行ISO-834标准火灾作用下钢管混凝土短柱抗冲击性能试验研究,考察受火时间、冲击速度、冲击能量和含钢率对其抗冲击性能的影响。试验量测钢管表面温度、冲击力与压缩变形时程曲线。试验结果表明,受火时间、冲击速度、冲击能量和含钢率均对高温下钢管混凝土的动态力学性能有影响;受火时间对冲击极限承载力和残余变形的影响最为显著,其余参数对冲击承载力影响不大,而试件的残余变形随着受火时间和冲击能量的增大而增大,随着含钢率的增大而减小。常温和火灾下钢管混凝土在冲击荷载作用下产生显著的压缩变形,遭受不同程度的破坏,但仍能够保持很好的截面完整性,说明钢管混凝土在火灾(高温)下具有良好的抗冲击能力,适用于有火灾(高温)抗冲击、抗倒塌设计需求的结构。     

火灾作用后圆钢管混凝土柱荷载-位移滞回性能研究

火灾作用后,钢管混凝土柱的强度和刚度均会遭受损失,需进一步考察其抗震能力。本文介绍了7个火灾后圆钢管混凝土试件滞回性能试验研究,采用数值方法对荷载位移滞回关系曲线进行全过程分析,计算分析了轴压比、长细比、含钢率、钢材屈服极限、混凝土抗压强度、受火时间等参数对荷载位移关系骨架线的影响。研究结果表明,火灾作用后圆钢管混凝土柱滞回关系曲线形状饱满,无明显的捏缩现象,但试件的极限承载力和弹性刚度会有所降低。本文的研究成果可为火灾后该类结构的抗震修复加固提供参考。     

Nonlinear analysis of SRC columns subjected to fire

Both experimental and numerical methods were employed to investigate the behavior of steel reinforced concrete (SRC) columns subjected to fire. Twelve specimens were tested at ambient and elevated temperatures. A procedure was developed to predict the temperature distribution and ultimate strength of the SRC columns subjected to fire by following certain assumptions. In the procedure, a combined method of finite element and finite-difference was adopted to analyze the temperature distribution, and the responses of the SRC columns under loads were predicted by the procedure both at ambient temperature and elevated temperatures. To illustrate the problem and verify the accuracy of the predictions, we present a few comparisons between the test data and calculated results. At the end of the paper, equations for the ultimate strength of the SRC columns subjected to fire were obtained by a regression analysis.     

轴心受压SRC柱受火全过程数值分析

在确定升、降温段钢材和混凝土材料性能的基础上,利用通用有限元软件ANSYS对轴心受压型钢混凝土柱受火全过程进行计算分析。结果表明截面温度滞后现象比较明显,在降温后的较长时间内构件内部仍在升温,SRC柱性能进一步劣化,其抗火能力最不利阶段发生在火灾降温过程中。     

带约束拉杆L形钢管混凝土短柱偏压试验研究

在钢板中部设置具有约束钢板外凸变形作用的水平拉杆,是改善异形钢管混凝土柱力学性能的有效方法。通过8个带约束拉杆L形钢管混凝土偏压短柱试件的试验研究,分析了不同约束拉杆设置、偏心率以及荷载角下带约束拉杆L形钢管混凝土短柱的偏压性能。试验结果表明：约束拉杆延迟了钢管局部屈曲的发生,有助于L形钢管混凝土偏压短柱的承载力和延性的提高;临近或达到极限承载力后近力侧拉杆和钢管对核心混凝土的约束作用明显;偏心受压下该柱的截面应变符合平截面假定。基于带约束拉杆L形钢管内核心混凝土的等效单轴本构关系,利用纤维模型法对试件偏压极限承载力进行计算,计算结果与试验结果吻合良好。利用该数值方法对带约束拉杆L形钢管混凝土短柱的偏压性能进行参数研究。分析结果表明：钢材屈服强度、含钢率越大,N/N_u-M/M_u相关曲线向内收拢;混凝土强度和拉杆约束系数越大,N/N_u-M/M_u相关曲线向外凸出。图12表4参11     

Experimental behaviour of eccentrically loaded slender circular hollow steel columns in-filled with fibre reinforced concrete

This paper is based on the experimental study of twelve slender steel tubular columns of circular sections filled with both plain and fibre reinforced concrete. The specimens were tested under eccentric compression to investigate the effects of fibre reinforced concrete on the strength and behaviour of slender composite columns. The slenderness ratio was considered to be the main test parameter. Hollow steel sections of similar specimens were also tested as reference columns. The test results were illustrated by load-deflection and load-strain curves. Various characteristics such as strength, stiffness, ductility, energy absorption capacity and failure mode are discussed. Interpretation of the experimental results indicates that the use of fibre reinforced concrete as infill material has a considerable effect on the strength and behaviour of slender composite columns.     

Performance of CFST column to steel beam joints subjected to simulated fire including the cooling phase

This paper presents the experimental results of three concrete filled steel tube (CFST) column to steel beam joints with reinforced concrete (RC) slabs under combined loading and fire including the heating and cooling phases. The test parameters include heating time and thickness of the fire protection material. Temperatures and deformations of the joint specimens during the heating, cooling and post-fire phases were measured. A finite element analysis (FEA) model to simulate the action of a CFST column to steel beam joint under combined loading and fire is developed. The FEM model was verified by the experimental results. The FEA model is then used to analyse the temperature distribution in the heating and cooling phases. The moment versus relative rotation angle between the CFST column and the beam under combined loading and fire including the heating and cooling phases is also discussed.     

Thermal behaviour of a steel door frame subjected to the standard fire of ISO 834: Measurements,numerical simulation and parameter study

An adjustable steel frame of a steel door, built in a massive brick wall, was subjected to fire according to the international standard ISO 834. Additional thermocouples, with respect to the ones required by the standard, were positioned at different sites of the steel frame cross-section to validate the numerical analysis carried out to study its thermal insulation performance. The leaf/frame interaction was monitored during the fire test to verify the integrity criterion, but was neglected in the calculations by assuming adiabatic conditions at the door clearance. Based on these findings, a parametric study was conducted to investigate the influence of boundary conditions, empty or filled frame cavities, and adjustable versus wrap-around frame on the temperature distribution of the steel frame. Combining the results of fire test and numerical simulation, the possibility to mutually assert the thermal performance, i.e. the insulation criterion for the cases of opening into and opening away from furnace, is discussed.     

Analytical modeling of the standard fire behavior of loaded CFT columns

This paper presents a three-step sequentially coupled analytical approach for predicting the standard fire behavior of concrete filled steel tube (CFT) columns. Numerical models were developed for each step of the approach, namely, (1) fire dynamics analysis, (2) nonlinear heat transfer analysis, and (3) nonlinear stress analysis. 3D finite element models with identical meshes were used for steps 2 and 3. An analytical matrix consisting of fifteen CFT column specimens subjected to standard fire tests by different researchers was selected to verify the numerical models. Sensitivity analyses were conducted to finalize the values of the major model input parameters. This paper summarizes the development of the numerical models, the findings from the sensitivity analyses, and the comparisons of the numerical predictions with experimental results. The verified numerical models (and model input parameters) are recommended for predicting the behavior of CFT columns subjected to fire loading, and for conducting analytical parametric studies.     

钢管混凝土偏压格构柱偏心率影响试验研究

进行了5根钢管混凝土格构柱和2根空钢管格构柱偏心受压试验,试验参数为偏心率。试验研究结果表明,随着偏心率的增加,格构柱的极限承载力逐渐降低。所有试件的近载侧柱肢始终处于小偏压状态,远载侧柱肢随试件整体偏心率的增大而由小偏压发展为大偏压状态。与空钢管格构柱相比,钢管混凝土格构柱的承载力、刚度和变形能力均较大。进行了偏心率对钢管混凝土格构柱极限承载力的影响分析。最后,对钢管混凝土设计规程的计算方法进行了探讨。     

PVC套管约束混凝土短柱偏心受压力学性能试验研究

为了研究PVC套管约束混凝土短柱的偏压力学性能,进行了12个PVC套管约束混凝土短柱和12个无PVC套管约束的混凝土短柱对比试验。研究了影响PVC约束混凝土短柱偏压力学性能的主要因素,包括PVC套管内混凝土强度、荷载偏心距、有无PVC管约束等。试验结果表明:由于PVC套管的约束,提高了混凝土的承压强度,增大了核心混凝土的变形能力;随着混凝土强度的提高,PVC套管混凝土的承载力提高,但PVC套管对混凝土的约束效果有所降低;随着荷载偏心距的增大,PVC套管混凝土的承载力降低、挠度增大,PVC套管对核心混凝土的约束效果逐渐降低。     

四面受火木柱耐火极限的试验研究

通过6根四面受火木柱耐火极限的对比试验,研究不同持荷水平、是否采用石灰膏抹面对木柱耐火极限的影响。研究结果表明,四面受火木柱耐火极限随着荷载水平的增加而明显降低。采用石灰膏抹面后,其耐火极限有所增加。石灰膏抹面能有效降低四面受火木柱内的温升梯度,延缓木柱开始炭化的时间,降低炭化速度。     

碳纤维布加固震损型钢混凝土柱抗震性能试验研究

基于震后快速修复的需求,提出了碳纤维布加固震损型钢混凝土框架柱方法。基于现行设计规范,按缩比1:3设计并制作了4根型钢管混凝土框架柱模型,其中1根为原型对比柱,1根为未损加固柱,2根为模拟不同震损程度后采用碳纤维布加固修复柱,进行了低周往复加载破坏试验。试验结果表明：试件在压、弯、剪复合受力下均表现为弯剪破坏,满足“强剪弱弯”的抗震设防要求|与原型对比柱相比,受中度震损柱经加固修复,其极限荷载平均提高了10.41%,极限位移平均提高了35.40%|受重度震损柱经加固修复,其极限荷载平均提高了13.71%,极限位移平均提高了32.68%。总体上,采用碳纤维布加固震损型钢混凝土框架柱是一种有效的抗震加固方法。     

钢骨-钢管自密实高强混凝土偏压柱力学性能试验研究

为研究钢骨-钢管混凝土组合柱在偏心荷载下的力学性能,进行了13根组合柱的偏心受压试验,研究了钢骨-钢管自密实高强混凝土偏压柱的荷载-变形关系曲线、宏观变形特征和破坏形态,分析了偏心率、长细比、套箍系数和配骨指标对偏压组合柱力学性能的影响。研究结果表明:内置钢骨能延缓甚至阻止核心混凝土中剪切斜裂缝的开展,可有效提高偏压组合柱的极限承载力和延性;在整个加载过程中,钢骨-钢管自密实高强混凝土偏压柱中截面纵向应变沿截面高度的变化基本符合平截面假设,且偏压长柱的侧向挠曲线基本符合正弦半波曲线;钢骨-钢管自密实高强混凝土偏压柱的极限承载力随偏心率和长细比的增大而急剧下降,随着配骨指标的增大而提高。     

Measured and calculated temperature evolution on the room side of a butted steel door frame subjected to the standard fire of ISO 834

The numerical simulation of the temperature distribution of a steel door frame subjected to fire has been verified by means of fire resistance tests. For this purpose, a butted steel frame/steel door assembly, built in a massive brick wall, was subjected for at least 30 min to the standard fire of ISO 834 for both opening into as well as opening away from the furnace. The temperature measured on the room side of the frame was compared with the results obtained from numerical heat transfer analysis. The leaf/frame interaction was monitored during both fire tests to verify the fulfilment of the integrity criterion, but was neglected in the calculations by assuming adiabatic conditions at the door clearance. The possibility of mutual assertion of the insulation criterion (thermal performance) for the cases of opening into and opening away from the furnace has been discussed. Additionally, the influences of a thermally insulated frame and of a wall/frame anchorage were quantified by calculations, and comparisons with adjustable steel frames were made.     

Experimental and numerical investigation of high-strength steel circular columns subjected to fire

This paper presents an experimental–numerical study on the behaviour of High Strength Steel (HSS) columns at elevated temperature, both on Circular Hollow Sections (CHS) and on a Concrete Filled Tube (CFT). The measured yield strength of the circular sections was in the order of 820 MPa. In detail, three HSS CHS and a HSS CFT were tested under the standard ISO fire with constant eccentric compression applied load. The evolution of temperature and deformation patterns were measured by means of a comprehensive instrumentation made of thermocouples and displacement transducers. Numerical analyses were performed and compared with experimental data by employing stress–strain relationships of carbon steel at elevated temperatures provided by the Eurocodes associated to two different sets of reduction factors: i) those provided by the Eurocodes valid up to S460 steel grades; and ii) those proposed in the literature and based on tests on HSS.     

四面受火胶合木中长柱耐火极限试验研究

通过4组10根胶合木中长柱四面受火的耐火极限试验,研究截面尺寸、持荷水平、阻燃涂料等对胶合木中长柱耐火极限的影响规律。通过理论分析提出了胶合木中长柱基于炭化速度的耐火极限计算方法,并采用有限元软件建立了胶合木柱热力耦合数值分析模型。结果表明,随着持荷水平增加,四面受火胶合木柱耐火极限明显降低,当持荷比由30%增加至50%时,耐火极限平均降低24. 5 min;随着截面尺寸增加,四面受火胶合木柱耐火极限显著提高,当截面尺寸由200 mm×200 mm增加至300 mm×300 mm时,耐火极限平均增加28. 0 min;当胶合木柱表面采用阻燃涂料涂刷后,耐火极限平均增加4. 0 min。胶层、持荷水平和截面尺寸对试件内部距离边缘相同位置处的温度变化无明显影响,表面涂抹阻燃涂料可稍降低试件内部温度的上升速度。垂直胶层方向和平行胶层方向的炭化速度无明显差异,有阻燃涂料处理的木柱炭化速度略小于无阻燃涂料处理的木柱炭化速度。基于剩余截面法计算的四面受火胶合木中长柱耐火极限计算值与试验值的相对误差绝对值的平均值为6. 5%,基本满足工程精度要求。有限元模拟得到的耐火极限与试验值的平均相对误差为8....