Behavior and strength of tubed RC stub columns under axial compression

This paper presents an experimental and analytical study on the behavior of axially compressed tubed RC stub columns. Forty specimens including twenty circular tubed RC (STRC) and twenty square tubed RC (STRC) stub columns were tested to investigate the failure mode and axial load strength of tubed RC columns subjected to axial compression. The effect of diameter/width to thickness ratio of the tubes and compressive strength of concrete were also studied. The effect of height to diameter/width ratio of the separated tube in tubed RC columns was studied to investigate the effect of bond and friction between tube and concrete on the behavior of tubed RC columns. Elastic–plastic analysis on the steel tube was employed to study the mechanism of tubed RC stub columns subjected to axial compression. Equations for the prediction of the ultimate axial load strength of tubed RC stub columns were proposed and the results from prediction were compared with the test results.     

Seismic behavior and strength of tubed steel reinforced concrete (SRC) short columns

The seismic behavior of tubed SRC short columns has been investigated by testing eight specimens subjected to combined constant axial compression and lateral cyclic load. Three circular tubed SRC columns (CTSRC) and three square tubed SRC (STSRC) columns were tested in this research with two common SRC columns for comparison. Different axial load ratios (n₀=0.3, 0.4 and 0.5) have been adopted for the constant axial load. The test results indicate that the shear strength, plastic deformation capacity, ductility index, and energy dissipating capacity of the tubed SRC short columns were much higher than those of the SRC columns with the same steel ratio and axial compressive load. The lateral load strength of CTSRC and STSRC short columns increased with an increment in axial load level, while the axial load ratio has no obvious effect on the plastic deformation capacity of CTSRC and STSRC short columns. The steel tubes prevented the shear failure of the concrete more effectively in the circular columns than in the square ones. Shear connector studs should be used in CTSRC and STSRC short columns to prevent bond failure between concrete and flanges of the steel section. A modified ACI design method was adopted to calculate the nominal shear strength of STSRC columns as well as CTSRC columns.     

钢管混凝土(SRC)短柱的轴向受压性能和强度分析

给出轴压下钢管混凝土短柱性能的试验和理论研究。共对14根圆钢管混凝土短柱和15根方钢管混凝土短柱进行试验,以研究钢管混凝土短柱的失效模式和轴压性能。分析了直径(或宽度)、相对厚度比对钢管性能的影响。试验结果表明:在相同配钢率的前提下,钢管混凝土短柱比普通混凝土短柱的受压承载力大。钢管的弹塑性分析可用于研究轴压下钢管混凝土短柱的破坏机理。基于已有的试验成果,提出求解钢管混凝土短柱的轴向受压强度的方程式。     

Seismic behavior and moment strength of tubed steel reinforced-concrete (SRC) beam-columns

The seismic behavior of tubed SRC beam-columns has been investigated by testing six specimens subjected to combined constant axial compression and lateral cyclic load. Two circular tubed SRC columns (CTSRC) and two square tubed SRC (STSRC) columns as well as two common SRC columns as comparison were tested in this paper. Different axial load ratio (n₀ = 0.3 and 0.5) have been adopted for the constant axial load. The test results indicated that the flexural strength of a CTSRC column was much higher than that of a common SRC column with the same steel ratio and axial compressive load, whereas there was little difference on the flexural strength of a STSRC columns and a common SRC column. The ductility, plastic deformation capacity and energy dissipation capacity of CTSRC and STSRC columns were much higher than those of common SRC columns with the same steel ratio and axial compressive load. The flexural strength increased as the axial load ratio increased for CTSRC and STSRC columns, while an opposite trend was observed for the plastic deformation capacity. The flexural strength and plastic deformation capacity of CTSRC columns were clearly higher than that of the STSRC beam-columns with the same steel ratio and axial load ratio. A modified EC4 code method has been proposed for the calculation of the moment strength for tubed SRC columns.     

Shear strength and behavior of tubed reinforced and steel reinforced concrete (TRC and TSRC) short columns

Tubed RC and SRC short columns are special kinds of concrete filled tubular columns but the steel tube does not pass through the beam–column connection and is shorter than concrete core. In areas that suffered earthquakes, the short columns are vulnerable to brittle shear failure. TRC and TSRC short columns are widely used in bridges, high-rise buildings and large factories. So it is important to investigate the behaviors and approaches to improve the ductility of these kind of columns. The aim of this study is to develop a nonlinear finite element model (FEM) for TRC and TSRC short columns and to compare the results with those experimentally captured. Depending on the FEM results, the elastic–plastic method was used to analyze the stress status of the steel tube. A modified ACI design method is adopted to calculate the nominal shear strength of TRC and TSRC short columns based on the FEM and analysis results.     

Seismic behavior and shear strength of tubed RC short columns

Reinforced concrete (RC) short columns are vulnerable to brittle shear failure during an earthquake. The objective of this research is to evaluate the performance enhancement of RC short columns tubed with circular or square tubes. Eight short columns were tested under combined constant axial load and cyclic lateral load. The tested specimens included three circular tubed RC (CTRC) columns and three square tubed RC (STRC) columns. Two common RC short columns including one circular RC column and one square RC column were also tested as control specimens. The test results indicated that common RC short columns suffered brittle shear failure with little ductility, while the ductility of tubed RC short columns was excellent due to the effective confinement of the outer thin tube to the core concrete. The lateral load strength of CTRC short columns increases with the increasing of axial load ratio, while the axial load ratio has little effect on the plastic deformation capacity of CTRC short columns. The shear strength increases with increasing of axial load ratio, while the plastic deformation capacity decreases with increasing of axial load ratio for STRC short columns. A circular tube prevents the core concrete from shear failure more effectively than a square tube for the tubed RC short columns. A modified ACI design method is adopted to calculate the nominal shear strength of STRC columns as well as CTRC columns based on the test and analysis results.     

轴力作用下钢管短柱的性能和强度分析

对轴力作用下钢管短柱的性能和强度进行了试验和数值分析。对包括20个圆形和方形钢管混凝土短柱在内的40个构件进行了试验,以分析其在轴压作用下的破坏模式和轴向承载强度。同时也对管的径(宽)厚比和混凝土的抗压强度的影响进行了分析。对钢管混凝土柱中管的高径(宽)比的影响进行分析,以探究钢管和混凝土之间的粘结和摩擦对钢管混凝土柱性能的影响。利用钢管的弹塑性分析研究轴压下短柱的受力机制。提出了公式用以预测钢管混凝土短柱的极限轴向承载强度,并将预测结果与试验结果进行了对比。     

Behavior and strength of circular tube confined reinforced-concrete (CTRC) columns

An experimental investigation was conducted on the behavior of circular tube confined reinforced-concrete (CTRC) columns. Eighteen CTRC stub columns were tested under cyclic or monotonic axial compression. The results of the elastic-plastic analysis on the steel tubes indicate that the steel tube yields at the peak load point in the stub columns under axial compression. In addition, a design equation to calculate the axial load strength of CTRC columns is proposed. A total of five columns including one circular reinforced-concrete (CRC) column and four CTRC columns have been studied under combined axial compression and lateral cyclic load. The test results indicate that CTRC columns exhibit much higher flexural strength, displacement ductility and greater energy dissipation ability than CRC columns confined with hoop ties. The flexural strength increases as the axial load ratio or concrete compressive strength increases for CTRC columns, while the ductility is barely affected by the increase in axial load or concrete compressive strength. It is proposed that the moment strength of the cross section of CTRC columns can be calculated using a modified ACI code method.     

Seismic behavior and strength of square tube confined reinforced-concrete (STRC) columns

A steel tube confined reinforced-concrete (STRC) column is an ordinary RC column where most of the lateral ties are in the form of a thin steel tube. Twenty-three square tube confined concrete stub columns were tested in this paper under cyclic or monotonic axial compression. A design equation to calculate the axial load strength of square tube confined concrete stub columns is proposed in this paper. A total of five beam-columns have been studied under combined axial compression and lateral cyclic loads. The test results indicate that the columns confined with square steel tubes exhibit much higher flexural strength, displacement ductility, and energy dissipation ability than common RC columns confined with lateral ties. Fiber models were also developed for STRC beam-columns in this paper.     

方钢管约束钢筋高强混凝土超短柱抗震性能试验研究

本文进行3个剪跨比为1.5的方钢管约束钢筋高强混凝土超短柱和1个钢筋混凝土对比试件的试验研究,试验中的主要参数为轴压比(0.35,0.45和0.55)。试验结果表明,由于钢管对核心高强混凝土的约束作用和钢管的抗剪作用,方钢管约束钢筋混凝土超短柱的抗剪承载力、延性、变形能力和耗能性能明显高于钢筋混凝土超短柱。轴压为0.35的方钢管约束钢筋混凝土超短柱的破坏模式为弯曲破坏,而轴压比为0.45和0.55的方钢管约束钢筋混凝土超短柱的破坏模式为剪切破坏。随轴压比的提高,方钢管约束钢筋混凝土超短柱的抗剪承载力提高,但延性系数和极限变形能力降低。对钢管的弹塑性应力分析结果表明,水平荷载施加过程中,发生弯曲破坏试件的钢管不屈服,而发生剪切破坏试件的钢管可能在下降段屈服。根据试验结果和钢管应力分析结果建立方钢管约束钢筋混凝土柱的抗剪承载力公式,提出设计建议,可为工程实践提供参考。     

Experimental behavior of circular concrete-filled steel tube stub columns

This paper presented an experimental study on the behavior of circular, concrete-filled, steel tube (CFT) stub columns with self-compacting concrete (SCC) and normal concrete (NC) concentrically loaded in compression to failure. Four measurement methods on the axial deformation of specimens were compared. Seventeen specimens were tested to investigate the effects of concrete strength, notched holes or slots, and different loading conditions on the ultimate capacity and the load-deformation behavior of the columns. The behavior of these stub columns in confinement was discussed.It is concluded that for the specimens with the entire section loaded, strain gauges with different dimensions could record the strains of the steel tubes, and electronic displacement transducers with certain gauge lengths could record the axial displacement. By using higher strength concrete, the specimens with the entire section loaded experienced a significant increase in the ultimate capacity, but their residual capacity after failure is almost constant. However, once the steel tube was notched, the axial compressive stiffness of specimens was reduced; in some cases the ultimate capacity was also reduced, and the steel tube acted more as a transverse confinement than an axial compression component. Eurocode 4 predicted a reasonable capacity for the unnotched CFT stub columns with both SCC and NC if the entire section of the specimen is loaded.     

Rational design analysis of stub columns fabricated using very high strength circular steel tubes

The mechanics of the compressional behaviour of stub columns fabricated from Grade 350 steel plates welded to very high strength (VHS) circular tubes are investigated in this paper. In the investigations comparisons of experimental load-axial shortening curves are made with the results of a postulated design analysis set up to examine the complete load-shortening behaviour of such members. Ramberg-Osgood type stress–strain equations are set up for the plate and tube materials. A procedure is described to evaluate the loads in individual plate and tube elements as the axial shortening is increased until the 0.2% proof strain for the VHS tube material is attained. Upon reaching this strain it is assumed that failure ensues as the result of a simple plastic mechanism occurring in the tubes. Comparisons of the load-axial shortening curves given by this analysis and experimental results indicate good agreement.     

Experimental behaviour of stiffened concrete-filled thin-walled hollow steel structural (HSS) stub columns

Test results on concrete-filled steel tubular stub columns with inner or outer welded longitudinal stiffeners under axial compression are presented in this paper. The research was mainly focused on square hollow section (SHS) columns; two rectangular hollow section (RHS) columns were also tested. A longitudinal stiffener was provided on each side of the stiffened SHS column, while only two stiffeners were welded to the longer sides of the stiffened RHS column. The main experimental parameters considered were the height-to-thickness ratio and stiffener rigidity. In addition, empty tubes with or without stiffeners, as well as unstiffened concrete-filled steel tubes were also tested for comparison. Requirements for stiffener rigidity are developed by modifying a formula presented in the literature. Existing theoretical model and design codes were used to predict the load versus axial strain relationships and load-carrying capacities of the adequately stiffened composite sections respectively; reasonable results were achieved.     

Axial load behavior of square CFT stub column with binding bars

This paper is concerned with the axial load behavior of square concrete filled steel tubular (S-CFT) stub columns with binding bars provided to improve the mechanical behavior of S-CFT columns. Ten specimens with binding bars and 5 specimens without binding bars were tested to examine the effects of width-to-thickness ratios and binding bars on ultimate strength, stiffness and ductility of S-CFT columns. A method for calculating ultimate strength is proposed based on a constitutive model of confined concrete and a simplification corresponding to this method is conducted. Finally, the method proposed in this paper is verified with experimental results in this test program and data from other experiments. Corresponding values of ultimate strength calculated by EC4(1996) and GJB(2000) are given respectively for comparison. For S-CFT stub columns with binding bars, the results predicted by the method proposed herein agree well with the experimental results while that predicted by the methods of EC4(1996) and GJB(2000) scatter from the experimental results; for S-CFT without binding bars, the results given by the three methods mentioned above are all reasonable.     

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.     

Behaviour of concrete filled steel tubular (CFST) stub columns under eccentric partial compression

This paper investigates the behaviour of concrete filled steel tubular (CFST) stub columns subjected to eccentric partial compression. Twenty-eight specimens were tested and presented. The main parameters in test program include: (1) section type: circular, square and rectangular; (2) load eccentricity ratio (including uniaxial and biaxial loading): from 0 to 0.4; and (3) shape of the loading bearing plate (BP): circular, square, strip and rectangular. The test results indicated that, similar to the corresponding fully loaded CFST stub columns under eccentric loading, CFST stub columns under eccentric partial compression have generally reasonable bearing capacity and favorable ductility. A finite element analysis (FEA) model for CFST stub columns under eccentric partial compression is developed and the predicted performances are validated through measured results. The FEA model is then used to investigate the mechanisms of such composite columns further.     

钢管再生混合短柱的轴压性能试验

为揭示钢管再生混合短柱的轴压性能,通过17根试件的轴压试验,考察钢管再生混合短柱与钢管混凝土短柱的7d和28d轴向受力行为,比较二者的刚度、强度和延性。根据国内外钢管混凝土结构设计标准,对试件的抗压承载力进行计算,基于计算结果与试验结果的对比,建议钢管再生混合短柱的抗压承载力计算公式。研究结果表明:①虽然钢管再生混合短柱采用了32%～35%的废弃混凝土,但其轴向受力性能却与全现浇钢管混凝土短柱相当。②根据我国标准JCJ01—89给出的钢管再生混合短柱的抗压承载力计算结果与试验结果吻合较好。     

Axial strength of circular concrete-filled steel tube columns — DOE approach

This paper presents the effect of changes in diameter of the steel tube (D), wall thickness of the steel tube (t), strength of in-fill concrete (f_cu), and length of the tube (L) on ultimate axial load (P_ue) and axial shortening at the ultimate point (δ_ue) of circular Concrete Filled steel Tubes (CFT). Taguchi’s approach with an L9 orthogonal array is used to reduce the number of experiments. With the help of initial experiments, linear regression models are developed to predict the axial load and the axial shortening at the ultimate point. A total of 243 circular CFT samples are tested to verify the accuracy of these models at three factors with three levels. The experimental results are analyzed using Analysis Of Variance to investigate the most influencing factor on strength and axial shortening of CFT samples. Comparisons are made with predicted column strengths using the existing design codes, AISC-LRFD-2005 and EC4-1994.     

Tests on stub stainless steel-concrete-carbon steel double-skin tubular (DST) columns

A new type of composite member, a stainless steel-concrete-carbon steel double-skin tubular (DST) column, is introduced in this paper. This composite member is expected to combine the advantages of all three types of materials, and have additional advantages of esthetics and resistance to corrosion that outer stainless steel offers. A series of tests are performed to investigate the performance of the composite column, and a total of 80 specimens were tested. The main parameters of the tests are the sectional type (circular, square, round-end rectangular and elliptical), the column type (straight, inclined and tapered), and the hollow ratio of the composite section (from 0.5 to 0.75). The results show that all types of columns behaved in a ductile manner, and the mechanical behavior is similar to those columns with double carbon steel tubes. A simplified model for the prediction of the stainless steel-concrete-carbon steel double-skin tubular (DST) columns cross-sectional strengths is suggested as well.     

基于统一强度理论的圆钢管钢骨混凝土柱受压承载力分析

采用双剪统一强度理论,考虑了箍筋和内圆钢管对核心混凝土的双层约束作用,对配有圆钢管的钢骨混凝土轴心受压柱的受力性能进行了理论分析。在厚壁圆筒统一强度理论的基础上,推导出了柱的受压承载力公式,并对影响因素进行了分析。将该公式的计算结果与文献资料的试验结果进行了比较,结果吻合较好,表明统一强度理论对配有圆钢管的钢骨混凝土的理论分析有很好的适用性。该结果为配有圆钢管的钢骨混凝土柱承载力的分析计算提供了一定的理论依据。