Experimental investigation of eccentrically loaded fibre reinforced concrete-filled stainless steel tubular columns

This paper presents an experimental investigation of axially and eccentrically loaded plain and fibre reinforced (FR) concrete-filled stainless steel circular tubular columns. The composite columns were pin-ended subjected to axial and eccentric loads. The stainless steel tubes were relatively slender having a diameter-to-plate thickness ratio of 50. The composite columns had different lengths varied from 3D to 12D. The column ultimate loads, load–axial shortening relationships, load–strain relationships, load–mid-height lateral deflection relationships and failure modes of the concrete-filled stainless steel circular tubular columns were measured from the tests. The study has shown that FR concrete-filled stainless steel tubular columns offer a considerable increase in column ductility compared with plain concrete-filled tubular columns. The test ultimate loads were compared with the design ultimate loads calculated using the Eurocode 4 for composite columns. Generally, it has been shown that the EC4 accurately predicted the ultimate loads of axially loaded concrete-filled stainless steel circular tubular columns, but were quite conservative for predicting the ultimate loads of the eccentrically loaded columns. It has also been shown that the conservatism of the EC4 predictions is increased as the eccentricity is increased. The test results provide useful information regarding the behaviour of FR concrete-filled stainless steel columns.     

Behaviour of eccentrically loaded high-strength rectangular concrete-filled steel tubular columns

This paper presents an experimental and analytical study of the behaviour of high-strength rectangular concrete-filled steel tubular (CFT) columns subjected to eccentric loading. Four slender and 16 stub CFT columns were tested to investigate their structural behaviour. The test parameters were material strengths (), cross-sectional aspect ratio (1.0-2.0), slenderness ratio (10 and 60) and load eccentricity ratio (e/H=0.10-0.42). Favourable ductility performance was observed for all specimens during the tests. Experimental failure loads are employed to calibrate the specifications in the design codes EC4, ACI and AISC. Results show that EC4 overestimates the failure loads of the specimens by 4%. ACI and AISC conservatively predict the failure loads by 14% and 24%, respectively. An analytical model is developed to predict the behaviour of high-strength rectangular CFT columns subjected to eccentric loading. Calibration of the model against the test results shows that it closely estimates the ultimate capacities of the columns by 3%.     

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     

Simulation and design recommendations of eccentrically loaded slender concrete-filled tubular columns

This paper proposes an efficient numerical model for the simulation of the behavior of slender circular concrete-filled tubular columns subjected to eccentric axial load with single curvature, for the cases of both normal and high strength concrete. The paper focuses on the study of the influence that the variables affecting beam-column behavior (length and relative slenderness) and the variables affecting section behavior (diameter/thickness ratio, mechanical capacity of steel) have on the overall buckling of this type of column. An extended parametric study is carried out to propose design recommendations, primarily to establish the importance of the use of high strength concrete compared with that of normal strength concrete. The results show that for slender elements the optimum design is reached when the mechanical capacity of the steel is slightly lower than that of the concrete contribution.     

Fire behavior of eccentrically loaded slender high strength concrete-filled tubular columns

This paper describes a series of 24 fire tests conducted on slender circular hollow section columns filled with normal and high strength concrete, subjected to eccentric axial load. It is a continuation of a previous research paper (Romero et al., 2011 [1]), where test results on centrally loaded columns were presented. The test parameters covered in this fire testing program were the nominal strength of concrete (30 and 90 MPa), the infilling type (plain, bar-reinforced and steel fiber reinforced concrete), the axial load level (20% and 40%) and the load eccentricity (20 and 50 mm). The columns were tested under fixed-pinned boundary conditions, with a relative slenderness at room temperature higher than 0.5 for all the specimens. The aim of this paper is to study the influence of eccentricity in combination with the type of concrete infill. The results show that the addition of steel fibers does not improve the fire resistance of slender columns under eccentric loads, as compared to columns filled with plain concrete. However, the addition of reinforcing bars increases the fire resistance of the columns in this situation. Filling the steel hollow section columns with concrete increases their fire resistance, the increase in load bearing capacity being more noticeable for columns filled with high strength concrete. A comparison with the current simple calculation model in Eurocode 4 Part 1.2 shows that, although the method is safe for eccentrically loaded columns, it produces a high error in the predictions for columns filled with plain or steel fiber reinforced concrete.     

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。     

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.     

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.     

Behaviour of short and slender concrete-filled stainless steel tubular columns

In this paper, a series of tests were carried out on short and slender concrete-filled stainless steel tubular columns to explore their performance under axial compression or combined actions of axial force and bending moment. Empty short steel hollow sections were also tested for comparison. The test results showed that the performance of the composite columns was quite good and have the potential to be used extensively as structural members. Comparisons of the test results were also made with several existing design methods for conventional concrete-filled carbon steel tubular columns as presented in Australian standard AS 5100 (2004), American code AISC (2005), Chinese code DBJ/T 13-51-2010 (2010), and Eurocode 4 (2004), which indicates that all the codes are somewhat conservative in predicting the load-carrying capacities of both short and slender columns.     

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.     

Nonlinear behavior of concrete-filled stainless steel stiffened slender tube columns

This paper investigates the nonlinear behavior of concrete-filled high strength stainless steel stiffened slender square and rectangular hollow section columns. The stiffened slender tubes had overall depth-to-plate thickness (D/t) ratios ranging 60–160. The concrete strengths covered normal and high-strength concrete. The investigation focused on short axially loaded columns. A nonlinear finite element (FE) model has been developed to study the behavior of the concrete-filled stiffened tube columns. A parametric study was conducted to investigate the effects of cross-section geometry and concrete strength on the behavior and strength of the columns. The results of the concrete-filled stiffened tube columns were compared with the results of the companion concrete-filled unstiffened tube columns. It is shown that the concrete-filled stiffened slender tube columns offer a considerable increase in the column strength and ductility than the concrete-filled unstiffened slender tube columns. The column strengths obtained from the FE analysis were compared with the design strengths calculated using the American specifications and Australian/New Zealand standards. A design equation was proposed for concrete-filled stainless steel stiffened slender tube columns. It is shown that the proposed modified equation provides more accurate design strengths compared to the American and Australian/New Zealand predictions.     

圆钢管高强混凝土轴压短柱受剪承载力分析

为了深入研究钢管高强混凝土轴压短柱破坏模式与破坏机理,提出适合钢管高强混凝土轴压短柱极限承载力计算方法,针对圆钢管高强混凝土轴压短柱大都发生剪切破坏这一典型现象,引入莫尔-库仑强度理论,从理论上分析其发生剪切破坏的原因和受力机理,并从剪切破坏的角度提出了钢管高强混凝土轴压短柱承载力计算方法。利用基于圆钢管高强混凝土轴压短柱试验研究和有限元分析回归得到的处于复杂应力场中的钢管纵向应力σv-纵向应变ε关系曲线和钢管横向应力σh-纵向应变ε关系曲线的数学表达式,得到了钢管高强混凝土轴压短柱承载力包络线的简化计算方法,简化计算曲线与试验曲线吻合良好,可用于分析钢管高强混凝土轴压短柱的受剪承载力。     

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.     

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.     

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。     

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.     

Nonlinear analysis of circular concrete-filled steel tubular short columns under eccentric loading

This paper presents an effective theoretical model for the nonlinear inelastic analysis of circular concrete-filled steel tubular (CFST) short columns under eccentric loading. Accurate material constitutive relationships for normal and high strength concrete confined by either normal or high strength circular steel tubes are incorporated in the theoretical model to account for confinement effects that increase both the strength and ductility of concrete. The predicted ultimate bending strengths and complete moment-curvature responses of circular CFST columns under eccentric loading are compared with existing experimental results to examine the accuracy of the theoretical model developed. The fundamental behavior of circular CFST beam-columns with various diameter-to-thickness ratios, concrete compressive strengths, steel yield strengths, axial load levels and sectional shapes is studied using the verified theoretical model. Based on extensive numerical studies, a new design model for determining the ultimate pure bending strengths of circular CFST beam-columns is proposed. The theoretical model and formulas developed are shown to be effective simulation and design tools for the nonlinear inelastic behavior of circular CFST beam-columns under eccentric loading.     

Nonlinear analysis of circular concrete-filled steel tubular short columns under axial loading

The confinement effect provided by the steel tube in a circular concrete-filled steel tubular (CFST) short column remarkably increases the strength and ductility of the concrete core. The reliable prediction using nonlinear analysis methods for circular CFST columns relies on the use of accurate models for confined concrete. In this paper, accurate constitutive models for normal and high strength concrete confined by either normal or high strength circular steel tubes are proposed. A generic fiber element model that incorporates the proposed constitutive models of confined concrete is created for simulating the nonlinear inelastic behavior of circular CFST short columns under axial loading. The generic fiber element model developed is verified by comparisons of computational results with existing experimental data. Extensive parametric studies are conducted to examine the accuracy of various confining pressure models and the effects of the tube diameter-to-thickness ratio, concrete compressive strengths and steel yield strengths on the fundamental behavior of circular CFST columns. A new design formula accounting for concrete confinement effects is also proposed for circular CFST columns. It is demonstrated that the generic fiber element model and design formula adequately predict the ultimate strength and behavior of axially loaded circular CFST columns and can be used in the design of normal and high strength circular CFST columns.