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.     

钢骨混凝土柱的耐火性能和抗火设计方法(Ⅰ)

确定了高温下组成钢骨混凝土的钢材和混凝土的热工参数和力-热本构关系模型,利用有限元法计算了钢骨混凝土柱截面温度场,计算结果得到实验结果的验证。利用数值方法对钢骨混凝土柱耐火极限及火灾下荷载-变形关系曲线进行了计算分析,理论计算结果和实验结果吻合良好。在此基础上,分析了截面尺寸、构件长细比、截面含钢率、截面配筋率、荷载偏心率、钢骨和钢筋屈服强度、混凝土强度、截面高宽比等参数对耐火极限以及火灾下构件承载力的影响规律。最后,提出了钢骨混凝土柱耐火极限的实用计算公式。本文是第一部分。     

钢骨混凝土柱的耐火性能和抗火设计方法(II)

确定了高温下组成钢骨混凝土的钢材和混凝土的热工参数和力-热本构关系模型,利用有限元法计算了钢骨混凝土柱截面温度场,计算结果得到实验结果的验证。利用数值方法对钢骨混凝土柱耐火极限及火灾下荷载-变形关系曲线进行了计算分析,理论计算结果和实验结果吻合良好。在此基础上,分析了截面尺寸、构件长细比、截面含钢率、截面配筋率、荷载偏心率、钢骨和钢筋屈服强度、混凝土强度、截面高宽比等参数对耐火极限以及火灾下构件承载力的影响规律。最后,提出了钢骨混凝土柱耐火极限的实用计算公式。本文是第二部分。     

Performance of steel H columns loaded under uniform temperature

This paper presents the experimental results for a series of H steel columns under fire load. The width-to-thickness ratio of steel plates and the slenderness ratios of steel columns are two dominating factors linked to local buckling and global buckling of columns, respectively. To evaluate the influence of these two factors on the structural behavior of steel columns in fire conditions, a series of H steel columns were loaded to their limit states at specified temperature levels. The steady state method has been adopted in order to derive the structural behavior of steel columns at specified temperatures directly.On the basis of the experimental results, it has been found that steel columns with non-compact section are able to reach yield strength at elevated temperature. That is, the width-to-thickness ratio, designed in accordance with current ambient temperature specifications, is capable of preventing brittle failure of steel columns in fire conditions. Depending on the slenderness ratio, the failure of steel columns may change from global buckling at ambient temperature to local buckling at elevated temperature. For plastic section columns with a slenderness ratio greater than 50, column strength drops dramatically to 40% of its strength at ambient temperature. At temperature levels of 500 ^°C, the column retains more than 70% of its ambient temperature strength if the slenderness ratio of the column is less than 50. However, in the case of temperature levels exceeding 500 ^°C, or when the slenderness ratio is greater than 50, column strength drops significantly. On the basis of this study, it is tentatively suggested that 500 ^°C be adopted as the critical temperature for steel members subjected to compression in order to ensure that the column strength keeps higher than 2/3 of the ambient temperature yield strength. The slenderness ratio of steel columns should be limited to 50, so as to prevent brittle failure of steel columns under fire attack.     

Residual stress analysis of structural stainless steel sections

The magnitude and distribution of residual stresses in structural carbon steel sections have been thoroughly investigated. However, few residual stress measurements have been made on structural stainless steel sections. Stainless steel has differing material stress-strain characteristics and thermal properties to carbon steel, both of which influence the formation of residual stresses. This suggests that established carbon steel residual stress models may not be appropriate for stainless steel. With increased use of stainless steel in load bearing applications, it is important to establish the residual stresses that exist within structural members. An experimental program to quantify the residual stresses in stainless steel sections from three different production routes has therefore been carried out. Comprehensive residual stress distributions have been obtained for three hot rolled angles, eight press braked angles and seven cold rolled box sections, with a total of over 800 readings taken. This paper presents the experimental techniques implemented and the residual stress distributions obtained as well as discussing the assumptions commonly made regarding through thickness residual stress variations. In the hot rolled and press braked sections, residual stresses were typically found to be below 20% of the material 0.2% proof stress, though for the cold rolled box sections, whilst membrane residual stresses were relatively low, bending residual stresses were found to be between 40% and 70% of the material 0.2% proof stress.     

Behaviour and design of restrained steel column in fire: Part 2. Parameter study

This paper investigates behaviours of the restrained steel column in fire. For the restrained steel column under axial load only, investigated parameters include the axial load, the axial restraint stiffness, and the column slenderness; for the restrained steel column under combined axial load and bending moment, studied parameters included the axial load, the bending moment load, the axial restraint stiffness, the column slenderness and the end moment ratio.The results of parametric studies show that (1) the axial restraint causes a reduction in the failure temperature of the restrained column. The reduction increases with the increase in the axial restraint stiffness. However, when the axial restraint stiffness ratio is greater than a certain value, no further reduction occurs; (2) the difference between failure and buckling temperatures of a restrained column is great for columns with great axial restraint stiffness or great slenderness or small load ratio. This means that in this situation, the fire resistance of the restrained column can be increased from the column buckling temperature by considering the post-buckling behaviour; (3) an increase in the column axial load ratio or bending moment ratio causes both the column buckling and failure temperatures to decrease; (4) with an increase in the column end moment ratio, the failure temperature of restrained column decreases. The results of parametric studies will form the basis of a simplified calculation method to be presented in the companion paper.     

Experimental study of structural fire behaviour of steel beam to concrete filled tubular column assemblies with different types of joints

This paper presents experimental results of structural fire behaviour of steel beam to concrete filled tubular (CFT) column assemblies using different types of joints. The joint types include fin plate, end plate, reverse channel and T-stub. The structural assembly was in the form of a “rugby goalpost”. In each test, loads were applied to the beam and then the structural assembly was exposed to the standard fire condition in a furnace while maintaining the applied loads. In total, 10 tests were carried out. In eight of the 10 tests, fire exposure continued until termination of the fire test, which was mainly caused by structural failure in the joints under tension when the beam was clearly in substantial catenary action. In the other two tests (one using fin plates and one using reverse channels), fire exposure stopped and forced cooling started when the beam was near a state of pure bending and just about to enter into catenary action. The results of the experiments indicate that even the relatively simple joints used in this study were able to allow the beams to develop substantial catenary action so that the final failure times and beam temperatures of the assemblies were much higher than those obtained by assuming the beams in pure bending. At termination of the tests, the beams reached very high deflections (about span/5), even then failure of the assemblies always occurred in the joints. Therefore, to enable the beams to reach their full potential in catenary action, the joints should be made to be much stronger. The results also indicate that reverse channel connection has the potential to be developed into a robust connection characterised by high stiffness, strength, rotational capacity and ductility. The beams in the two cooling tests developed high tension forces, however there was no structural failure in the assemblies. The principal aim of this paper is to present experimental results of joint behaviour in fire (which until now is lacking) to enable development of better understanding and rational design methods for robust construction of joints to resist extreme fire attack.     

Fire resistance of concrete filled circular hollow columns with restrained thermal elongation

The filling of circular hollow sections (CHS) with concrete is a good solution for strengthening columns since such procedure increase their load bearing capacity at room and high temperatures. However, in the event of a fire, restraining to thermal elongation may change their mechanical behavior. This paper presents the results of a large series of fire resistance tests on CHS columns with restrained thermal elongation. Parameters such as the slenderness of the column, its load level, the stiffness of the surrounding structure, the percentage of steel reinforcement and the degree of concrete filling inside the column, were tested. The results obtained show that the critical time of the columns was less than 46 min. The use of a concrete ring around the internal surface of the column's wall is of no advantage in terms of its behavior under fire conditions because this concrete ring suffers extensive spalling and cracking due to overheating of the steel tube. The main failure mode of the columns was global buckling. However in several cases local buckling also occurred.     

考虑蠕变和残余应力释放的Q460钢柱抗火设计方法

承受荷载的钢结构在火灾下可发生明显的蠕变变形,钢结构中的焊接残余应力在火灾下也会一定程度地释放,因而高温蠕变变形和残余应力会对钢柱的耐火性能产生影响.为了准确地对高强度Q460钢柱进行抗火设计,有必要定量分析高温蠕变和残余应力释放对钢柱承载力的影响.采用电炉对2根焊接H形Q460钢柱进行耐火试验,得到无保护Q460钢柱...     

约束受热伸长中空柱填充混凝土的耐火性能

在圆形中空柱中填充混凝土是一种很好的柱子加固方案,因为它能够增加柱子在室内和高温环境下的承载能力。然而,发生火灾时,对受热伸长的约束可能会改变其力学性能。通过对约束受热伸长中空柱进行一系列耐火试验后,得出试验结果。试验对柱子长细比﹑承载等级﹑周围结构刚度以及所占钢筋的百分比和混凝土强度等参数进行了测定。结果显示,这些柱子的极限抗火时间都小于46min。在柱子内层墙面使用混凝土环,从其性能上来讲在火灾时并没有太大优势,因为混凝土环会受到钢管过热而带来的大面积碎裂。柱子最主要的破坏方式即是整体屈曲。然而,在一些试验样本上也出现了局部屈曲。     

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.     

Modified Critical Temperatures for Steel Design Based on Simple Calculation Models in Eurocode 3

Critical temperature is defined as the temperature at which failure is expected to occur in a structural steel member given a uniform temperature distribution and load level. Determination of the critical temperature is a simple but efficient way for structural fire design. This paper proposed a new model which incorporated the buckling, load levels and non-dimensional slenderness to calculate the critical temperature of steel member under fire based on the simple calculation models in Eurocode 3. To advance the application of this new model, design charts for determining the critical temperatures were developed. The design charts showed that the critical temperature decreases with increasing load level, and increases as the buckling curve varies from “a₀” to “d”. It is also recommended to use higher grade steel in both normal and fire situations. The accuracy of this model was ascertained by comparing with the test results in available literature. The new model gave an average prediction-to-test ratio of 0.980 with a standard deviation of 0.077, indicating conservative and less scattered predictions. The percentage of over-prediction (i.e., prediction-to-test ratio >1.0) was less than 5.8% when the nominal yield strength of steel rather than its test strength was used for predictions. In general, reasonable agreements were obtained between the test results and the predictions.     

The application of BS5950: Part 8 of fire limit state design to the performance of ‘old’ structural mild steels

An investigation has been carried out to extend the guidance given in the new British Standard BS5950: Part 8, on fire limit state design, to the refurbishment and fire damage reinstatement of old steel framed, buildings.

Structural mild steel produced to BS15 approximately 50 years ago was found to be generally weaker at elevated temperatures than its modern counterpart—BS4360: Grade 43A (BS EN 10025: Grade 430A). However, providing in design calculations due recognition is given to the lower yield stress of old mild steel at ambient temperature, its performance in fire will be as good as that being currently produced. For the present time, it is therefore appropriate to adopt the same relationships between strength, loading and temperature for structural members given in the new Code, with no additional penalties on fire protection thickness should this be necessary.

Fire simulation treatments on steel manufactured to BS15 demonstrated that the degradation in strength properties is in agreement with work reported earlier on ‘weak’ mild steel—BS4360: Grade 43A. The results of a similar evaluation on mild steel produced since the 1986 revision of BS4360: Grade 43A are also in line with previous work.     

Local buckling and slenderness limits for flange outstands at elevated temperatures

This paper presents a study of the inelastic local buckling of flange outstands in thin-walled steel beams at elevated temperatures. A spline finite strip method of buckling analysis is modified in order to model the properties of the steel section at elevated temperatures, so that slenderness limits can be proposed. These limits are needed in describing the behaviour of members under fire loading, since the development of benign catenary action in a restrained beam that has received widespread research attention in fire engineering design is reliant on the formation of plastic hinges, and of the influence of local buckling on the formation of these hinges. A sensitivity analysis is undertaken in order to propose plasticity and yield slenderness limits at elevated temperature, and it is shown that these limits depend on the parameters describing the uniaxial stress-strain relationship at the elevated temperature. The local buckling temperature is predicted for two different boundary conditions for a flange outstand, and it is shown that at high temperatures the local buckling of the beam is highly significant in accelerating the favourable development of catenary action.     

Critical temperatures of steel columns exposed to fire

This contribution deals with the interdependence between material properties of structural steel at elevated temperatures and critical temperatures of steel columns. A simple design method for estimating lower and upper bound values of critical temperatures is presented. Critical temperatures of steel columns can be determined by means of the related slenderness and the utilization factor. Both parameters can be calculated on the basis of design methods at normal temperatures.     

Coupled instability of thin-walled members under combined bending moment, axial and shear force

In this paper we analyse the behaviour of thin-walled steel members with an I-shaped cross-section, in the presence of phenomena of coupled instability as overall-local type and combined state of stress of bending moment, axial and shear force. The analysis has been made by the ‘column model method’ and it has been used for bending moment-curvature-axial force diagrams, modified by the presence of local instability for web buckling of steel beams.

The previous analysis allows one to define interaction diagrams M_Imax−λ−N, which can be used to check the steel members in relation to the overall slenderness and the local slenderness of the web panel.     

高强度Q690钢柱耐火性能试验研究

为了获得高强度Q690钢柱的耐火性能，使用电炉对无防护足尺焊接H形Q690钢柱进行模拟ISO 834升温条件下耐火试验。测量得到不同荷载比下Q690钢柱温度、轴向位移、侧向位移与受火时间的关系，基于试验数据得到钢柱的临界温度和耐火极限。采用ABAQUS有限元软件建立钢柱耐火性能分析模型，考虑钢材高温蠕变和焊接残余应力的影响，模拟得到了钢柱的受火响应，其与试验结果吻合良好。利用验证的有限元模型分析了荷载比、长细比和升温速率对钢柱受力性能的影响。研究表明，无防护的Q690钢柱在受火20min左右发生破坏，破坏模式为整体失稳破坏；荷载比对临界温度影响较大，长细比和升温速率影响较小；Q690钢柱的临界温度比GB 51249—2017《建筑钢结构防火技术规范》和欧洲规范EN 1993-1-2的计算结果低60℃左右。最后提出了高强Q690钢柱抗火设计的简化方法。     

改善钢梁/柱连接节点耐火性能的方法

在有限元分析软件中使用详细的有限元模型,对提升钢梁/柱端板连接部件的耐火性能进行一系列的数值参数研究。对实际使用节点构造的梁柱组件进行研究,重点在于调查如何改进连接件设计,从而使整个结构能在高温下安全。高温下,结构中的梁要经历悬链作用,此作用会造成连接件的破裂并在柱施加附加力。研究了不同的节点构造(螺栓直径、螺栓强度级别、端板厚度、使用防火螺栓和端板使用耐火钢)对其结构性能及高温下耐火性能的影响。结果表明,可通过提高连接件的变形能力特别是使用防火螺栓来改善在高温下的耐火性能。     

Methods of improving survivability of steel beam/column connections in fire

This paper reports the results of a series of numerical parametric studies on methods of improving survivability of steel beam/column endplate connections in fire using a detailed finite element model in ABAQUS. The parametric studies were performed on beam-column assemblies with realistic connection details and the main focus of this research is to investigate methods of improving connection design to enable the structure to survive very high temperatures. At very high temperatures, the beam of the structure experiences catenary action which may fracture the connections and exert additional forces on the columns. This paper investigates how different connection details (bolt diameter, bolt grade, endplate thickness, the use of fire resistance bolt and fire resistant steel for endplate) affect structural behavior and survivability at high temperatures. It is found that improving connection deformation capacity, in particular, using fire resistant bolts, will enable very high temperatures to be resisted.