Effects of heating and loading histories on post-fire cooling behaviour of concrete-filled steel tubular columns

A finite element method (FEM) program is developed and used in this paper to analyse the behaviour of concrete-filled steel tubular (CFST) columns during the entire stage of fire exposure, including: loading at ambient temperature, heating, cooling to the ambient temperature and post-fire loading to failure. The emphasis of this paper is on CFST column behaviours during the cooling and post-fire stages because these behaviours are affected by the loading and heating histories, but they have not previously been studied. This paper will present the mechanical property models for these different loading and heating stages. To validate the FEM program, some experimental data, including fire resistance, axial deformation and ultimate strength of CFST columns are compared and it is found that the FEM program can predict the test results with good accuracy. Using the FEM program, a parametric study is then conducted to investigate the influences of ambient temperature loading and heating history on the cooling and post-fire behaviours of CFST columns. It is concluded that various parameters (such as load ratio and elevating temperature time ratio etc.) affect the residual strength of CFST columns severely. Finally, this paper proposes a set of formulas which can be used to predict the residual strength of CFST columns after going through the whole fire exposure process.     

方钢管混凝土短柱轴压极限承载力研究

采用统一强度理论,对方钢管混凝土短柱的核心混凝土有效约束区和非有效约束区进行受力分析,通过方钢管的宽厚比对方钢管与核心混凝土间的约束效应进行控制,提出了方钢管混凝土短柱轴压极限承载力的计算公式,并对其影响因素进行了分析。然后将其推广到不同截面形式的钢管混凝土短柱轴压极限承载力的计算中,将计算值与试验值进行对比,验证了公式的合理性。该研究为钢管混凝土短柱承载力的研究提供了一定的理论依据。     

Analytical behaviour of concrete-filled double skin steel tubular (CFDST) stub columns

This paper reports a finite element analysis of the compressive behaviour of CFDST stub columns with SHS (square hollow section) or CHS (circular hollow section) outer tube and CHS inner tube. A set of test data reported by different researchers were used to verify the FE modelling. Typical curves of average stress versus longitudinal strain, stress distributions of concrete, interaction of concrete and steel tubes, as well as effects of hollow ratio on the behaviour of CFDST stub columns, were presented. The influences of important parameters that determine sectional capacities of the composite columns were investigated.     

Experimental investigation on concrete-filled stainless steel stiffened tubular stub columns

This paper presents an experimental investigation on concrete-filled normal-strength stainless steel stiffened tubular stub columns using the austenitic stainless steel grade EN 1.4301 (304). The stiffened stainless steel tubes were fabricated by welding four lipped angles or two lipped channels at the lips. Therefore, the stiffeners were formed at the mid-depth of the sections. In total, five hollow columns and ten concrete-filled columns were tested. The longitudinal stiffener of the column plate was formed to avoid shrinkage of the concrete and to behave as a continuous connector between the concrete core and the stainless steel tube. The behavior of the columns was investigated using two different nominal concrete cubic strengths of 30 and 60 MPa. A series of tests was performed to investigate the effects of cross-section shape and concrete strength on the behavior and strength of concrete-filled stainless steel stiffened tubular stub columns. The measured average overall depth-to-width ratios (aspect ratio) varied from 1.0 to 1.8. The depth-to-plate thickness ratio of the tube sections varied from 60 to 90. Different lengths of columns were selected to fix the length-to-depth ratio to a constant value of 3. The concrete-filled stiffened stainless steel tubular columns were subjected to uniform axial compression over the concrete core and the stainless steel tube to force the entire section to undergo the same deformations by blocking action. The column strengths, load–axial strain relationships and failure modes of the columns are presented. Several comparisons were made to evaluate the test results. The results of the experimental study showed that the design rules, as specified in the European specifications and the ASCE, are highly conservative for square and rectangular cold-formed concrete-filled normal-strength stainless steel stiffened stub columns.     

轴压下带约束拉杆L形钢管混凝土短柱的试验研究

对7根轴压下L形钢管混凝土短柱(6根带约束拉杆,1根无约束拉杆)进行试验,介绍试件和试验方案的设计,分析钢板厚度、约束拉杆的直径和水平间距等主要参数对带约束拉杆L形钢管混凝土短柱极限承载力和延性的影响.试验表明,在L形钢管混凝土短柱上设置约束拉杆,可以有效地延缓钢管壁的局部屈曲,改善钢板对核心混凝土的约束效应,提高该类构件的极限承载力和延性.最后,将带约束拉杆L形钢管混凝土短柱划分成1个方形和2个带约束拉杆的矩形钢管混凝土短柱,并借鉴带约束拉杆方形钢管混凝土承载力的计算公式,对带约束拉杆L形钢管混凝土短柱的极限承载力计算方法作初步探讨.     

An experimental behaviour of concrete-filled steel tubular columns

In this paper results of tests conducted on 27 concrete-filled steel tubular columns are reported. The test parameters were the column slenderness, the load eccentricity covering axially and eccentrically loaded columns with single or double curvature bending and the compressive strength of the concrete core. The test results demonstrate the influence of these parameters on the strength and behaviour of concrete-filled steel tubular columns. A comparison of experimental failure loads with the predicted failure loads in accordance with the method described in Eurocode 4 Part 1.1 showed good agreement for axially and eccentrically loaded columns with single curvature bending whereas for columns with double curvature bending the Eurocode loads were higher and on the unsafe side. More tests are needed for the case of double curvature bending.     

Time-dependent behaviour of expansive concrete-filled steel tubular columns

Expansive concrete-filled steel tubes (ECFST) are commonly used in modern building and bridge applications. Despite their popularity, limited attention has been devoted to investigate the time-dependent behaviour of such elements. This paper intends to provide new experimental data for the benchmarking of numerical models. Particular attention is devoted to ECFST elements first loaded at quite early concrete ages, e.g. 5 days after concrete casting, to reflect the construction site practice. Eleven ECFST short columns were subjected to different levels of sustained axial loads applied at different concrete ages. Seven columns were then tested to failure to evaluate the long-term effects on their ultimate capacity. The accuracy of four currently available concrete models, EC2, MC90, AFREM and B3, in predicting the long-term response of ECFST elements was investigated based on the related experimental results. Investigation shows that the assumption of linear creep can apply to ECFST elements with initial concrete compressive stresses up to approximately 80% of the concrete strength, rather than the normally accepted upper limit of 40%-50%. During the service life, confinement does not affect the performance of ECFST elements. Model EC2 is adequate to predict the time-dependent response of ECFST elements.     

Confinement effect of stiffened and unstiffened concrete-filled stainless steel tubular stub columns

This paper presents a comparative study between stiffened and unstiffened concrete-filled stainless steel hollow tubular stub columns using the austenitic stainless steel grade EN 1.4301 (304). Finite element analysis of concrete-filled stainless steel unstiffened tubular stub columns is constructed herein based on the confined concrete model recently available in the literature. It is then compared with the experimental results of concrete-filled stainless steel stiffened tubular stub columns. The stiffened stainless steel tubular sections were fabricated by welding four lipped angles or two lipped channels at the lips. The longitudinal stiffener of the column plate was formed to avoid shrinkage of the concrete and to act as a continuous connector between the concrete core and the stainless steel tube. The behavior of the columns was investigated using two different nominal concrete cubic strengths of 30 and 60 MPa. The overall depth-to-width ratios (aspect ratio) varied from 1.0 to 1.8. The depth-to-plate thickness ratio of the tube sections varied from 60 to 90. The stiffened and unstiffened concrete-filled stainless steel tube specimens were subjected to uniform axial compression over the concrete and stainless steel tube to force the entire section to undergo the same deformations by blocking action. The ABAQUS 6.6 program, as a finite element package, is used in the current work. The results of the comparative study showed that the stainless steel tubes in stiffened concrete-filled columns offered a high average of increase in the confinement of the concrete core than that of the unstiffened concrete-filled columns.     

Nonlinear analysis of concrete-filled square stainless steel stub columns under axial compression

Concrete-filled stainless steel tubes (CFSST) can be considered as a new and innovative kind of composite construction technique, and have the potential to be used extensively in civil engineering. This paper employs a nonlinear analysis of square CFSST stub columns under axial compression. A three-dimensional nonlinear finite element (FE) model is developed using ABAQUS, where nonlinear material behaviour, enhanced strength corner properties of steel, and initial geometric imperfections are included. Close agreement is achieved between the test and FE results in terms of load-deformation response and ultimate strength. In light of the numerical results, the behaviour of stainless steel composite columns is compared with that of carbon steel composite columns. A simple model is proposed to calculate the ultimate strength of square CFSST stub columns.     

Numerical modelling of concrete-filled lean duplex slender stainless steel tubular stub columns

Major technological advances in materials processing have led to the development of duplex stainless steels with exceptional mechanical properties. Duplexes have great potential for expanding future structural design possibilities, enabling a reduction in section sizes leading to lighter structures. The duplex grades offer combination of higher strength than austenitics as well as a great majority of carbon steels with similar or superior corrosion resistance. However, high nickel prices have more recently led to a demand for lean duplexes with low nickel content, such as grade EN 1.4162. Extensive work is needed to include the lean duplex grade EN 1.4162, into design standards such as EN 1993-1-4 and ENV 1994-1-1. Accordingly, finite element modelling for concrete-filled lean duplex slender stainless steel tubular stub columns of Grade EN 1.4162 is presented in this paper. The paper is predominantly concerned with two parameters: cross-section shape and concrete compressive strength, which have not yet been investigated. The non-linear displacement analysis of the columns was constructed herein based on the confined concrete model provided by Hu et al. (2003) [15]. The behaviour of the columns was investigated using a range of concrete cylinder strengths (25–100 MPa). The overall depth-to-width ratios (aspect ratio) varied from 1.0 to 1.8. The depth-to-plate thickness ratio of the tube sections varied from 60 to 90. The concrete-filled lean duplex slender stainless steel tubular columns were subjected to uniform axial compression over the concrete and stainless steel tube to force the entire section to undergo the same deformations by blocking action. The ABAQUS 6.6 program, as a finite element package, is used in the current work. The results showed that the design rules specified in the ASCE are highly conservative for square and rectangular concrete-filled lean duplex slender stainless steel stub columns while they are conservative in the case of European specifications. A new design strength is, therefore, proposed that is accurately found to represent the behaviour of concrete-filled lean duplex stainless steel tubular stub columns.     

Behaviour of concrete-filled steel tubular stub columns subjected to axially local compression

The behaviour of concrete-filled steel tubular (CFST) stub columns subjected to axially local compression was experimentally investigated in this paper. A total of thirty-two specimens were tested. The main parameters varied in the tests are: (1) sectional types: circular and square; (2) local compression area ratio (concrete cross-sectional area to local compression area): 1.44 and 16; and (3) thickness of the endplate: from 2 to 12 mm. A finite element analysis modelling was used for the analysis of CFST stub columns subjected to axially local compression, and a comparison of results calculated using this modelling shows generally good agreement with the test results. The theoretical modelling was then used to investigate the mechanism of the composite columns subjected to axially local compression.     

Finite element modelling of concrete-filled lean duplex stainless steel tubular stub columns

This paper presents a finite element (FE) study on concrete-filled lean duplex slender stainless steel tubular (CFDSST) stub columns of square and L-, T-, and +-shape (Non-Rectangular Sections or NRSs) sections under pure axial compression. The effect of cross-sectional shape and concrete compressive strength, by considering equal steel consumption (i.e. equal cross-sectional area) for all the square and NRSs sections have been reported. In CFDSST stub columns, the axial deformation (δ _u ) at ultimate load (P _u ) decreases with increasing concrete strengths, but increases as the sections changes from Square→L→T→+-shape. For normal concrete strength (≤40 MPa), NRSs appear to have similar or slightly enhanced P _u , in comparison with the representaive square section. But in the case of a high strength concrete core (i.e. >40 MPa), NRSs are clearly at a disadvantage as far as the values of P _u is concerned, however as the NRSs are lighter by 37%, they still offer an attractive option for the designers. The FE strengths over predicts the EN 1994-1-1 (2004) specification by about an average of 21, 19, 14, and 4% for the square, L, T, and +-shape sections, respectively.     

多边形空心钢管混凝土轴压短柱非线性分析

在轴压试验结果的基础上,选择合理的材料本构关系模型,应用大型通用ANSYS有限元软件对四边形和八边形空心钢管混凝土短柱进行了有限元模拟,着重讨论了有限元模型的建立方法,并通过模拟结果与试验结果的对比,验证了有限元模型的合理性。     

Experimental behaviour of concrete-filled stiffened thin-walled steel tubular columns

An experimental study on the structural behaviour of concrete-filled stiffened thin-walled steel tubular columns is presented in this paper. The stiffening was achieved by welding longitudinal stiffeners on the inner surfaces of the steel tubes. Companion tests were also undertaken on 12 unstiffened concrete-filled steel tubular (CFST) columns, with or without steel fibres in the infill concrete. The test results showed that the local buckling of the tubes was effectively delayed by the stiffeners. The plate buckling initially occurred when the maximum load had almost reached for stiffened specimens, thus they had higher serviceability benefits compared to those of unstiffened ones. Some of the existing design codes were used to predict the load-carrying capacities of the tested composite columns.     

Experimental behaviour of high performance concrete-filled steel tubular columns

In recent years, the utilization of high performance concrete has been the interests of the structural engineers and researchers. As a high performance concrete, self-consolidating concrete (SCC) is a highly flowable concrete that can fill formwork without any mechanical vibration. SCC's unique property gives it significant economic, constructability and engineering advantages. The aim of this paper is thus an attempt to study the possibility of using thin-walled hollow structural steel (HSS) columns filled with very high strength SCC. Tests on 28 HSS columns filled with very high strength SCC were conducted, where the main parameters varied are: (1) section types, circular and square; (2) slenderness ratio, from 12 to 120; and (3) load eccentricity ratio, from 0 to 0.6. Comparisons are made with predicted column strengths using the existing codes such as AISC, EC4 and DBJ13-51-2003.     

多室式钢管混凝土T形中长柱轴压性能研究

为了实现对多室式钢管混凝土T形柱轴压稳定性能的研究,进行6个中长柱试件及4个短柱对照试件的轴压试验,重点考察前者的破坏形态、荷载-变形关系及截面尺寸对承载力的影响。试验结果表明,中长柱的力学行为与短柱有较大不同,其破坏属于整体弯曲破坏,增大腹板高度对承载能力会产生有利影响。同时,将改进Mander模型后所得的混凝土本构关系引入有限元软件ABAQUS中,对试验进行了有限元模拟,其结果与试验结果吻合较好。在试验及有限元分析的基础上确定了中长柱与短柱之间的界限长细比,并对这一新型结构柱稳定承载力的计算方法进行了探讨,给出可供实际应用参考的计算公式。     

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。     

Tests on inclined,tapered and STS concrete-filled steel tubular (CFST) stub columns

This paper is an attempt to study the behaviour of the inclined, tapered and straight-tapered-straight (STS) concrete-filled steel tubular (CFST) stub columns. A total of 34 specimens were tested. The main parameters were the column type (inclined, tapered and STS), inclined or tapered angle (from 0°to 9°) and the cross-sectional type (circular and square). Comparisons with predicted cross-sectional strengths were made using the existing codes: AIJ-2008, AISC-2005, BS5400-2005, DBJ-13-51-2003 and EC4-2004.     

方形薄壁钢管混凝土柱的耐火性能研究

为研究薄壁钢管混凝土柱的耐火性能,进行4个方形薄壁钢管混凝土柱在标准升温曲线下的耐火试验。试验参数为荷载偏心率、是否设置加劲肋以及是否设置防火保护。基于试验结果,研究方形薄壁钢管混凝土柱在高温下的破坏模态、温度场分布和耐火极限,并将其耐火性能和普通钢管混凝土柱进行比较。试验结果表明,方形薄壁钢管混凝土柱具有较好的耐火性能。