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.     

Behaviour of tapered concrete-filled double skin steel tubular (CFDST) stub columns

Recently, tapered concrete-filled double skin steel tubular (CFDST) columns have been applied in electricity transmission towers in China. However, there is still lack of information on the behaviour of such kind of members. This paper thus carried out the investigation on axially loaded tapered CFDST stub columns. The parameters included the tapered angle and the sectional profile. A finite element analysis (FEA) model was developed to predict the behaviour of the tapered member as well. The results showed that the steel tubes and the sandwiched concrete could work together well despite the tapered angle. The failure mode of the tapered CFDST stub column was similar to that of the straight one, and the failure occurred near the smallest section. The confinement effect of the tapered member was discussed by using the stress analysis. Finally, formulas were suggested for the calculation on the ultimate strength of the tapered CFDST stub column.     

Behaviour of concrete-filled double skin rectangular steel tubular beam-columns

Double skin composite columns are formed from two steel skins filled with concrete in between. This new form of hybrid column has the potential to be used in many domains such as high-rise bridge piers and large diameter columns in high-rise buildings, etc. This paper describes a series of tests carried out on concrete-filled double skin steel tubular (CFDST) stub columns, beams and beam-columns. Both outer and inner tubes are cold-formed rectangular hollow sections (RHS). The failure modes, and load-deformation behaviour of CFDST specimens are compared with those of conventional concrete-filled steel tubular members and empty double skin tubular members. A theoretical model is developed in this paper for the CFDST stub columns, beams and beam-columns. Reasonably good agreement is observed between the predicted and tested curves. Simplified models are derived to predict the load-carrying capacities of the composite members.     

Concrete-filled double skin steel tubular (CFDST) columns subjected to long-term sustained loading

This paper carried out a series of tests on concrete-filled double skin steel tubes (CFDST) columns under long-term sustained loading condition. The test process included two stages, i.e. long-term service test and ultimate strength test. Numerical models on analysis of the CFDST column under long-term sustained loading were presented. A comparison of results calculated using these models showed generally good agreement with the test results. Additionally, a detailed analysis was performed to analyse the long-term behaviour of CFDST columns. Finally, simplified formula for calculating the ultimate strength of CFDST columns subjected to long-term sustained loading was proposed.     

大空心率圆锥形中空夹层钢管混凝土压弯构件滞回性能研究

为研究大空心率圆锥形中空夹层钢管混凝土压弯构件的滞回性能,对10个圆锥形中空夹层钢管混凝土压弯试件(8个往复加载和2个单调加载试件)进行试验研究。考察空心率(0.6和0.8)和轴压比(0、0.2、0.4和0.6)对试件的破坏模态、P-Δ关系曲线和滞回性能的影响,并建立数值模型对单调加载的压弯试件的受力机理进行分析,最后讨论了圆锥形中空夹层钢管混凝土压弯构件侧向承载力简化计算方法。研究结果表明,两种空心率试件的破坏形态基本一致,表现为试件下部不同程度的局部破坏;往复压弯试件的P-Δ曲线较为饱满,表现出较好的滞回性能。空心率从0.6增加到0.8对试件的侧向承载力和延性影响较小,两种空心率下的峰值荷载平均相差约3%。随着轴压比增加,试件的极限荷载降低,延性逐渐变差。往复加载试件的强度退化不明显,空心率为0.8的试件刚度退化更显著。试件的平均位移延性系数为3.56,平均黏滞阻尼系数为0.322,表现出较好的耗能能力。当空心率为从0.6增加到0.8,试件的黏滞阻尼系数平均提高15%。     

Concrete-filled double skin (SHS outer and CHS inner) steel tubular beam-columns

A series of tests on concrete-filled double skin steel tubular (CFDST) stub columns (14), beams (four) and beam-columns (12) were carried out. The specimens had square hollow section (SHS) as outer skin and circular hollow section (CHS) as inner skin. A mechanics model is developed in this paper for the CFDST stub columns, columns and beam-columns. A unified theory is described where a confinement factor (ξ) is introduced to describe the composite action between the steel tubes and the sandwiched concrete. The load versus axial strain relationship for CFDST stub columns is predicted. Simplified model is derived for section capacities of CFDST. The predicted beam-column strength is compared with that obtained in beam and beam-column tests. The load versus mid-span deflection relationship for CFDST beams and beam-columns is predicted. A simplified model is developed for calculating the member capacity of the CFDST beams. Simplified interaction curves are derived for CFDST beam-columns.     

钢管混凝土异形柱与U形钢-混凝土组合梁穿心式连接抗震性能试验研究

为研究异形钢管混凝土柱与U形钢-混凝土组合梁穿心式连接的破坏特征和抗震性能,进行了3个足尺边节点试件低周反复循环加载试验,试验参数为节点域构造和轴压比,考察了试件的受力过程及破坏特征,分析了节点的荷载-位移滞回曲线、承载力、刚度和强度退化性能、耗能能力以及延性等力学指标。研究表明：该类型连接的典型破坏形态为节点域钢管柱壁鼓屈、节点域连接槽钢拉裂和组合楼板压溃;试件的滞回曲线较饱满;3个试件的层间位移延性系数μ为1.43~2.02,弹性层间位移角θy为1/62~1/48,弹塑性层间位移角θu为1/39~1/30,等效黏滞阻尼系数ξeq为0.140~0.170,节点的变形能力较好,也具有一定耗能能力;梁底布置贯穿节点域的钢筋能提高节点正向的承载力和刚度;提高轴压比可以提高该类型节点的耗能能力。     

往复荷载作用下钢管混凝土叠合柱-钢梁连接节点力学性能研究

建立了钢管混凝土叠合柱-钢梁连接节点在往复荷载作用下力学性能分析的有限元计算模型。利用平面与空间连接节点试验结果,对建立的数值模型进行验证。采用有限元模型分析不同空间双向加载方式对钢管混凝土叠合柱-钢梁空间连接节点力学性能的影响规律,研究该类组合节点在往复荷载作用下荷载(P)-位移(Δ)关系全过程受力机理与核心区剪力分配机制,并从承载力及刚度等方面对比平面与空间连接节点的性能。研究结果表明,不同加载方式对节点的承载力有明显影响。相比平面节点,钢管混凝土叠合柱-钢梁空间连接节点在双向往复荷载作用下正、负向承载力分别降低约14%和18%。     

带肋冷弯薄壁方钢管混凝土柱滞回性能研究

对3根带肋冷弯薄壁方钢管混凝土柱进行滞回试验,主要参数为轴压比。试验结果表明：纵向加劲肋有效延缓了钢管壁局部屈曲的发生;其滞回曲线饱满,具有良好的耗能能力;随着轴压比的增大,柱承载力略有增大,而延性、耗能能力则明显减小;当横向位移大于6倍的屈服位移时,大轴压比的刚度退化速度最快。建立了该类试件的有限元模型,对比可得有限元模拟结果与试验结果吻合较好。基于有限元模型对该类构件开展机理分析和参数分析。结果表明：在带肋冷弯薄壁方钢管的约束下,核心混凝土的强度得到了较大提高;钢管局部屈曲发生在峰值荷载后,局部屈曲只发生在纵向加劲肋和钢管角部间;材料强度、轴压比、钢管宽厚比和长细比等参数对该类构件的承载力有较大影响;混凝土强度、轴压比和长细比对荷载-位移骨架曲线形状有较大影响。基于参数分析建议了该类构件的简化滞回模型,简化计算结果和有限元计算结果吻合较好。     

Experimental behaviour and strength of concrete-encased composite beam-columns with T-shaped steel section under cyclic loading

Experimental research was conducted to investigate the structural behaviour of concrete-encased composite beam-columns with T-shaped steel section. Specimens were tested under lateral cyclic loading and axial compression. The test parameters included the distribution of longitudinal reinforcement, the spacing of transverse reinforcement, the presence of cross ties, and the axial compressive load level. The test results indicate that the cyclic behaviour and failure modes of the beam-columns are greatly affected by the direction of the bending moment owing to the unsymmetrical cross section. The concrete-encased composite beam-columns can develop stable hysteretic response and large energy absorption capacity by providing cross ties and decreased spacing of transverse ties. The current ACI and AISC-LRFD design provisions were also evaluated by comparing predicted strengths with the test results.     

Local buckling of steel plates in concrete-filled thin-walled steel tubular beam-columns

The availability of high strength steels and concrete leads to the use of thin steel plates in concrete-filled steel tubular beam-columns. However, the use of thin steel plates in composite beam-columns gives a rise to local buckling that would appreciably reduce the strength and ductility performance of the members. This paper studies the critical local and post-local buckling behavior of steel plates in concrete-filled thin-walled steel tubular beam-columns by using the finite element analysis method. Geometric and material nonlinear analyses are performed to investigate the critical local and post-local buckling strengths of steel plates under compression and in-plane bending. Initial geometric imperfections and residual stresses presented in steel plates, material yielding and strain hardening are taken into account in the nonlinear analysis. Based on the results obtained from the nonlinear finite element analyses, a set of design formulas are proposed for determining the critical local buckling and ultimate strengths of steel plates in concrete-filled steel tubular beam-columns. In addition, effective width formulas are developed for the ultimate strength design of clamped steel plates under non-uniform compression. The accuracy of the proposed design formulas is established by comparisons with available solutions. The proposed design formulas can be used directly in the design of composite beam-columns and adopted in the advanced analysis of concrete-filled thin-walled steel tubular beam-columns to account for local buckling effects.     

Strength and ductility of high strength concrete-filled steel tubular beam-columns

The ultimate strength and ductility of high strength thin-walled concrete-filled steel tubular (CFST) beam-columns with local buckling effects, are investigated in this paper, using a performance-based analysis (PBA) technique. The PBA technique accounts for the effects of geometric imperfections, residual stresses, strain hardening, local buckling and concrete confinement on the behavior of high strength thin-walled CFST beam-columns. The accuracy of the PBA technique is further examined by comparisons with experimental results. The PBA program is employed to study the effects of depth-to-thickness ratio, concrete compressive strengths, steel yield strengths and axial load levels on the stiffness, strength and ductility of high strength thin-walled CFST beam-columns under combined axial load and biaxial bending. The results obtained indicate that increasing the depth-to-thickness ratio and axial load levels significantly reduces the stiffness, strength and ductility of CFST beam-columns. Increasing concrete compressive strengths increases the stiffness and strength, but reduces the axial ductility and section performance of CFST beam-columns. Moreover, the steel yield strength has a significant effect on the section and strength performance of CFST beam-columns but does not have a significant effect on their axial and curvature ductility.     

Behavior of concrete-filled double skin steel tubular columns with octagon section under axial compression

Based on some conclusions of two kinds of concrete-filled double skin steel tube (CFDSST) members with circular or square sections, a new kind of CFDSST with octagonal section, where the outer tube has an octagonal section and the inner tube has a circular section, is proposed in this paper. Behaviors of the CFDSST members with octagon section subjected to axial compression are investigated, and some curves of load-strain of steel tubes and confined concrete and the bearing capacity of members are obtained. It is indicated that the bearing capacity of the columns with octagonal section is larger than that with square section and is smaller than that with circular section, and the bearing capacity of members is related to the ratio of the straight side to the bevelled one. Based on the proper stress-strain relationship, a couple of numerical analyses are made using the finite element software named ANSYS. Finally, a simplified formula is proposed in the paper, and the numerical results agree well with the experimental results and the mathematical solutions. The results are valuable for engineers. __________ Translated from China Civil Engineering Journal, 2007, 40(2): 33–38 [译自: 土木工程学报]     

斜向受力十字形钢管混凝土柱抗震性能试验研究

为研究十字形钢管混凝土柱在斜向受力下的抗震性能,以加载角度(0°和45°)、混凝土强度等级(C50和C70)、轴压比(0、0.25和0.5)以及是否设置加劲肋为试验参数,进行了9根十字形钢管混凝土柱在往复荷载作用下的试验研究,获得了柱的破坏形态、水平荷载-位移滞回曲线、骨架曲线、延性、累积耗能、变形等特性,分析了不同参数对柱抗震性能的影响规律。并建立了十字形钢管混凝土柱的有限元模型,有限元分析结果与试验结果吻合良好。试验结果表明:十字形钢管混凝土柱具有较好的滞回性能,所有柱的位移延性系数均高于3.5;轴压比对十字形钢管混凝土柱的抗震性能影响较大,轴压比越大,柱承载力越低,刚度退化越快,延性和耗能能力也越差;随着混凝土强度的增加,柱承载力增加,轴压比较大时,混凝土强度越高,延性下降越明显;内部间断焊接加劲肋的柱比未设置加劲肋柱的承载力提高约8%,但延性和耗能能力提高不大;加载角度为45°柱的滞回性能稍优于0°的柱。     

Nonlinear analysis of short concrete-filled steel tubular beam-columns under axial load and biaxial bending

This paper presents a nonlinear fiber element analysis method for determining the axial load-moment strength interaction diagrams for short concrete-filled steel tubular (CFST) beam-columns under axial load and biaxial bending. Nonlinear constitutive models for confined concrete and structural steel are considered in the fiber element analysis. Efficient secant algorithms are developed to iterate the depth and orientation of the neutral axis in a composite section to satisfy equilibrium conditions. The accuracy of the fiber element analysis program is verified by comparisons of fiber analysis results with experimental data and existing solutions. The fiber element analysis program developed is employed to study the effects of steel ratios, concrete compressive strengths and steel yield strengths on axial load-moment interaction diagrams and the C-ratio of CFST beam-columns. The proposed fiber element analysis technique is shown to be efficient and accurate and can be used directly in the design of CFST beam-columns and implemented in advanced analysis programs for the nonlinear analysis of composite columns and frames.     

Behavior of concrete-filled double skin steel tubular columns with octagon section under axial compression

Based on some conclusions of two kinds of concrete-filled double skin steel tube (CFDSST) members with circular or square sections, a new kind of CFDSST with octagonal section, where the outer tube has an octagonal section and the inner tube has a circular section, is proposed in this paper. Behaviors of the CFDSST members with octagon section subjected to axial compression are investigated, and some curves of load-strain of steel tubes and confined concrete and the bearing capacity of members are obtained. It is indicated that the bearing capacity of the columns with octagonal section is larger than that with square section and is smaller than that with circular section, and the bearing capacity of members is related to the ratio of the straight side to the bevelled one. Based on the proper stress-strain relationship, a couple of numerical analyses are made using the finite element software named ANSYS. Finally, a simplified formula is proposed in the paper, and the numerical results agree well with the experimental results and the mathematical solutions. The results are valuable for engineers.     

Hysteretic behaviour of tubular joints under cyclic loading

This paper examines the cyclic performance of CHS joints used in steel tubular structures. Quasi-static experimental study into the response of eight T-joint specimens is described. Four of them are subjected to cyclic axial load, and the other four are subjected to cyclic in-plane bending. The general test arrangement, specimen details, and most relevant results (failure modes and load-relative deformation hysteretical curves) are presented. Some indexes to assess the seismic performance of tubular joints, including strength, ductility and energy dissipation, are synthetically analyzed and compared. Test results show that failure modes of axially loaded joints mainly contain weld cracking in tension and chord plastification in compression. But for joints under cyclic in-plane bending, both punching shear and chord plastification become regular failure modes accompanied by ductile fracture of the welds. Hysteretic curves take on a plump form in general. Ultimate strengths of joints are also compared with equation values for monotonic loading from various design codes. Results indicate the strength at a certain deformation limit can be regarded as the ultimate strength of a T-joint under cyclic loading and existing codes can be used to check it. It is also found that there is a significant distinction in the energy dissipation mechanism for tubular joints under different loading conditions. Finite element analyses are performed by taking into account weld geometry to facilitate the interpretation of the test results. It is identified that high tensile stress triaxiality can be one primary cause of weld cracking which happened under low cyclic load level.     

方钢管混凝土柱与钢梁框架节点的抗震性能试验研究

为验证方钢管混凝土柱与工形钢梁框架节点的破坏特征和抗震性能,本文进行了A、B两组共6个足尺节点试件的低周反复循环加载试验,其中A组三个试件梁端采用栓-焊连接;B组三个试件梁端为全对接焊接。试验结果表明:6个试件的层间位移延性系数μ≈2.28～3.86,弹性极限层间位移角y≈0.001～0.014,弹塑性极限层间位移角u≈0.0297～0.0424,节点梁柱的相对极限塑性转角θu≈0.0163～0.0343rad,能量耗散系数E≈2.029～2.494;A、B两类节点均有良好的抗震性能,在相同轴压比下A类节点的延性较B类略好一些,而耗能能力B类节点却略大一些,但二者相差不大,均满足结构抗震设计的要求。文章还分析了节点域的剪切变形,给出了改造节点设计的几点建议。