Seismic performance of recycled concrete-filled square steel tube columns

An experimental study on the seismic performance of recycled concrete-filled square steel tube （RCFST） columns is carried out. Six specimens were designed and tested under constant axial compression and cyclic lateral loading. Two parameters, replacement percentage of recycled coarse aggregate （RCA） and axial compression level, were considered in the test. Based on the experimental data, the hysteretic loops, skeleton curves, ductility, energy dissipation capacity and stiffness degradation of RCFST columns were analyzed. The test results indicate that the failure modes of RCFST columns are the local buckling of the steel tube at the bottom of the columns, and the hysteretic loops are full and their shapes are similar to normal CFST columns. Furthermore, the ductility coefficient of all specimens are close to 3.0, and the equivalent viscous damping coefficient corresponding to the ultimate lateral load ranges from 0.323 to 0.360, which demonstrates that RCFST columns exhibit remarkable seismic performance.

Behavior of High Strength Concrete Filled Square Steel Tube Stub Columns with Inner CFRP Tube Under Biaxial Eccentric Compression

This paper studies the contribution of CFRP(carbon fiber-reinforced polymer)to the mechanical behavior of high strength concrete-filled square steel tube(HCFST)under biaxial eccentric compression.The new type of composite member is composed of an inner CFRP tube and an outer steel tube with concrete filled in the two tubes.The finite element analysis was made by ABAQUS on the behavior of high strength concrete filled square steel tubular columns with inner CFRP circular tube subjected to bi-axial eccentric loading.The results obtained from the finite element analysis were verified with the experimental results.In addition,the load-deflection curves in the whole process were calculated and analyzed,which can be divided into three segments:Elastic phase,plastic phase,descending phase.Based on the load-deflection curves,the stresses analysis on the core concrete,CFRP tube and steel tube were conducted.The confinement effect of the CFRP tube improves the ductility of HCFST-CFRP stub column.CFRP ratio and eccentricity affect the ultimate bearing capacity of HCFST stub column.Finally,a calculation formula of ultimate bearing capacity was proposed in the paper.

Performance Investigation of Square Concrete-filled Steel Tube Columns

The behaviour of square concrete-filled steel tube columns under concentrical loading was studied. More than one hundred specimens were tested to investigate the effects of thickness of steel tube on the load carrying capacity of the concrete-filled tubular columns （CFTs）. The effect of the grade of concrete and content of expansive agent were also investigated. The effect of these parameters on the confinement of the concrete core was studied as well. From the experimental study it was found that for both CFTs with different strength grade concrete core, the ultimate load carrying capacity increases with the increase in percentage of expansive agent up to 20% but it again decreases at 25% of expansive agent content. It was also shown that the failure mode of CFTs depends on the strength grade of concrete core.

Seismic Behavior of Diaphragm-Through Connections of Concrete-Filled Square Steel Tubular Columns and H-Shaped Steel Beams

Based on the introductions of a type of diaphragm-through connection between concrete-filled square steel tubular columns (CFSSTCs) and H-shaped steel beams,a finite element model of the connection is developed and used to investigate the seismic behavior of the connection.The results of the finite element model are validated by a set of cyclic loading tests.The cyclic loading tests and the finite element analyses indicate that the failure mode of the suggested connections is plastic hinge at the beam with inelastic rotation angle exceeding 0.04 rad.The suggested connections have sufficient strength,plastic deformation and energy dissipation capacity to be used in composite moment frames as beam-to-column rigid connections.

Influence of Axial Load Ratio on Shear Behavior of Through-Diaphragm Connections of Concrete-Filled Square Steel Tubular Columns

Nonlinear finite element analysis and parametric studies were carried out to study the influence of axial load ratio on the shear behavior of the through-diaphragm connections of concrete-filled square steel tubular columns. The analysis reveals that smaller axial load ratio can improve the shear bearing capacity and ductility while larger axial load ratio will decrease the shear behavior of the through-diaphragm connections. The parametric studies indicate that the axial load ratio should be limited to less than 0.4 and its influence should be considered in the analysis and design of such connections.

Damage assessment for seismic response of recycled concrete filled steel tube columns

A model for evaluating structural damage of recycled aggregate concrete filled steel tube （RCFST） columns under seismic effects is proposed in this paper. The proposed model takes the lateral deformation and the effect of repeated cyclic loading into account. Available test results were collected and utilized to calibrate the parameters of the proposed model. A seismic test for six RCFST columns was also performed to validate the proposed damage assessment model. The main test parameters were the recycled coarse aggregate （RCA） replacement percentage and the bond-slip property. The test results indicated that the seismic performance of the RCFST member depends on the RCA contents and their damage index increases as the RCA replacement percentage increases. It is also indicated that the damage degree of RCFST changes with the variation of the RCA replacement percentage. Finally, comparisons between the RCA contents, lateral deformation ratio and damage degree were implemented. It is suggested that an improvement procedure should be implemented in order to compensate for the performance difference between the RCFST and normal concrete filled steel tubes （CFST）.

Bearing capacity of composite columns reinforced by concrete filled steel tube

In this study, nine simplified short composite columns consisting of core CFST (concrete filled steel tube) of different diameters and outer reinforced concrete were constructed to study their compressive performance under axial or eccentric compression. The failure mode is characterized by the crush of the outer concrete. The bearing capacity increases at first and then decreases with further increase of the position coefficient. It can be concluded that position coefficient is an important structural parameter that has considerable influences on the ultimate bearing capacity of the composite columns. The outer concrete, steel tubes and longitudinal reinforcement are found to work in a cooperative manner under axial or eccentric compression when the position coefficient is about 0.5. An improved bearing capacity algorithm that takes the position coefficient into account has been proposed based on the experimental and simulation results and current technical specification in China. It has been proven to be precise and safe.

Three-dimensional nonlinear analysis of creep in concrete filled steel tube columns

This paper proposes a based on 3D-VLE （three-dimensional nonlinear viscoelastic theory） three-parameters viscoelastic model for studying the time-dependent behaviour of concrete filled steel tube （CFT） columns. The method of 3D-VLE was developed to analyze the effects of concrete creep behavior on CFT structures. After the evaluation of the parameters in the proposed creep model, experimental measurements of two prestressed reinforced concrete beams were used to investigate the creep phenomenon of three CFT columns under long-term axial and eccentric load was investigated. The experimentally obtained time-dependent creep behaviour accorded well with the cu~＇es obtained from the proposed method. Many factors （such as ratio of long-term load to strength, slenderness ratio, steel ratio, and eccentricity ratio） were considered to obtain the regularity of influence of concrete creep on CFT structures. The analytical results can be consulted in the engineering practice and design.

Prediction of Axial Capacity of Concrete-Filled Square Steel Tubes Using Neural Networks

The application of artificial neural network to predict the ultimate bearing capacity of CFST （ concrete-filled square steel tubes） short columns under axial loading is explored. Input parameters consiste of concrete compressive strength, yield strength of steel tube, confinement index, sectional dimension and width-to-thickness ratio. The ultimate bearing capacity is the only output parameter. A multilayer feedforward neural network is used to describe the nonlinear relationships between the input and output variables. Fifty-five experimental data of CFST short columns under axial loading are used to train and test the neural network. A comparison between the neural network model and three parameter models shows that the neural network model possesses good accuracy and could be a practical method for predicting the ultimate strength of axially loaded CFST short columns.

Axial Bearing Capacity of Short FRP Confined Concrete-filled Steel Tubular Columns

The bearing capacity of FRP confined concrete-filled steel tubular （FRP-CFST） columns under axial compression was investigated. This new type of composite column is a concrete-filled steel tube （CFST） confined with fiber-reinforced polymer （FRP） wraps. Totally 11 short column specimens were tested to failure under axial compression. The influences of the type and quantity of FRP, the thickness of steel tube and the concrete strength were studied. It was found that the bearing capacity of short FRP-CFST column was much higher than that of comparable CFST column. Furthermore, the formulas for calculating the bearing capacity of the FRP-CFST columns are proposed. The analytical calculated results agree well with the experimental results.

Experimental study on the seismic behavior of high strength concrete fi lled double-tube columns

To study the seismic behavior of high strength concrete fi lled double-tube(CFDT) columns,each consisting of an external square steel tube and an internal circular steel tube,quasi-static tests on eight CFDT column specimens were conducted.The test variables included the width-to-thickness ratio(β1) and the area ratio(β2) of the square steel tube,the wall thickness of the circular steel tube,and the axial force(or the axial force ratio) applied to the CFDT columns.The test results indicate that for CFDT columns with a square steel tube with β1 of 50.1 and 24.5,local buckling of the specimen was found at a drift ratio of 1/150 and 1/50,respectively.The lateral force-displacement hysteretic loops of all specimens were plump and stable.Reducing the width-to-thickness ratio of the square steel tube,increasing its area ratio,or increasing the wall thickness of the internal circular steel tube,led to an increased fl exural strength and deformation capacity of the specimens.Increasing the design value of the axial force ratio from 0.8 to 1.0 may increase the fl exural strength of the specimens,while it may also decrease the ultimate deformation capacity of the specimen with β1 of 50.1.

An Experimental Study on the Behavior of Square Concrete-Filled High Strength Steel Tube Columns

The paper presents an experimental study on the behavior of square concrete-filled high strength steel tube columns until failure.Four specimens were tested to investigate the effect of high strength steel and plain strength concrete on the load carrying capacity of the square concrete-filled steel tube columns.The effect of the concrete which filled in the steel tube was also investigated.Also,the effect of high strength steel and the presence of concrete were also investigated.The main parameters in the tests were:(1) the ultimate strength of steel (800 Mpa),(2) the presence of concrete in steel tube (hollow steel tube and concrete-filled steel tube),and (3) slender ratio.The main objectives of these tests were three-step:(1) to describe a series of tests on composite columns,(2) to analyze the influence of several parameters,and (3) to compare the accuracy of the predictions by using the specifications in the code (AISC-LRFD,EC4 and AIJ etc.) for the design of high-strength composite columns.Experimental results indicate that the ultimate strength of steel and presence of concrete had significant influence on both the axial compressive load capacity and the ductile of the square steel tube.

Seismic behavior of thin-walled circular and stiffened square steel tubed-reinforced-concrete columns

Steel tubed-reinforced-concrete(TRC) columns have been gradually used in the construction of high-rise buildings recently because of their high axial load-carrying capacities and excellent seismic behavior. Existing studies about their seismic behavior were focused on columns with relatively thick tubes, i.e., diameter-to-thickness/width-to-thickness(D/t) ratios were below 100,while little is known about thin-walled TRC columns, especially for square TRC columns. Considering the infilled concrete of square TRC columns is non-uniformly and non-effectively confined, accordingly, stiffened square TRC columns are usually adopted in practice. Thus, two thin-walled circular TRC columns(D/t=120) and two stiffened square ones with diagonal stiffeners in plastic hinge regions(D/t=106) were tested under a constant axial compression combined with cyclic lateral loading.Both the circular and stiffened square TRC columns had the same cross sectional area, tube thickness, reinforcing bar ratio and column height. Flexural failure occurred for all the four specimens. Test results showed the strengths of the stiffened square TRC columns were a little higher in comparison to their circular counterparts; the ductility and energy dissipation capacities were excellent for both the stiffened and circular TRC columns, indicating very good confinement was gained from the yielded steel tubes of the plastic hinge regions at the peak loads. And shear stresses(35–90 MPa) in the sheared plates showed their moderate contribution of carrying lateral loads. Finally, cross sectional capacity analysis results demonstrated the method for TRC columns is acceptable for the stiffened square TRC columns.

Analysis and seismic tests of composite shear walls with CFST columns and steel plate deep beams

A composite shear wall concept based on concrete filled steel tube （CFST） columns and steel plate （SP） deep beams is proposed and examined in this study. The new wall is composed of three different energy dissipation elements： CFST columns; SP deep beams; and reinforced concrete （RC） strips. The RC strips are intended to allow the core structural elements - the CFST columns and SP deep beams - to work as a single structure to consume energy. Six specimens of different configurations were tested under cyclic loading. The resulting data are analyzed herein. In addition, numerical simulations of the stress and damage processes for each specimen were carried out, and simulations were completed for a range of location and span-height ratio variations for the SP beams. The simulations show good agreement with the test results. The core structure exhibits a ductile yielding mechanism characteristic of strong column-weak beam structures, hysteretic curves are plump and the composite shear wall exhibits several seismic defense lines. The deformation of the shear wall specimens with encased CFST column and SP deep beam design appears to be closer to that of entire shear walls. Establishing optimal design parameters for the configuration of SP deep beams is pivotal to the best seismic behavior of the wall. The new composite shear wall is therefore suitable for use in the seismic design of building structures.

Behaviors of recycled aggregate concrete-filled steel tubular columns under eccentric loadings

The paper investigates the behaviors of recycled aggregate concrete-filled steel tubular(RACFST)columns under eccentric loadings with the incorporation of expansive agents.A total of 16 RACFST columns were tested in this study.The main parameters varied in this study are recycled coarse aggregate replacement percentages(0%,30%,50%,70%,and 100%),expansive agent dosages(0%,8%,and 15%)and an eccentric distance of compressive load from the center of the column(0 and 40 mm).Experimental results showed that the ultimate stresses of RACFST columns decreased with increasing recycled coarse aggregate replacement percentages but appropriate expansive agent dosages can reduce the decrement;the incorporation of expansive agent decreased the ultimate stresses of RACFST columns but an appropriate dosage can increase the deformation ability.The recycled coarse aggregate replacement percentages have limited influence on the ultimate stresses of the RACFST columns and has more effect than that of the normal aggregate concretesteel tubular columns.

A New Method for Predicting the UL of Circular CFST Columns

Concrete filled steel tube structures have gained booming development in recent decades, especially in China. Simplified methods have been proposed in design codes, such as the Eurocode 4 (EC4) and the China engineering and construction specification (CECS). In EC4, the confinement effect is reasonably related to slenderness and load eccentricity. The CECS method is much straight forward in that the slenderness ratio and load eccentricity are treated as independent reduction factors. To make use of the advantages of both the CECS and the EC4 methods, the CECS method is modified to consider the confinement effect associated with slenderness and load eccentricity. It is shown that the proposed method can predict well the ultimate load capacity of circular section concrete filled steel tube columns.

方钢管混凝土柱穿入式组合节点耗能机制

开展了4个翼缘削弱的钢管混凝土柱穿入式组合节点抗震性能与三维实体有限元精细化模型分析。在混凝土三轴塑性损伤和钢材混合强化模型基础上,进一步引入钢材的韧性损伤模型,探讨了钢梁削弱程度对节点抗震性能与破坏形式的影响规律,结果显示引入钢材韧性损伤的有限元模型分析结果与试验结果更加吻合,且钢梁削弱程度降低了该节点的刚度、承载力和耗能。随后对不同钢梁尺寸和柱端拉筋强构造下的组合节点进行了承载力、刚度和塑性耗能分配与失效机制的影响规律分析,分析结果表明:穿入式节点分别在梁-柱抗弯承载力比介于1.66~2.11、梁-柱线刚度比介于1.90~1.96、梁-柱截面刚度比介于1.16~1.21时,将由梁耗能向柱耗能转变;柱端拉筋强构造技术后,由于拉筋直接约束混凝土,大幅度提升了柱端抗弯承载力和耗能能力,组合节点在高轴压比时仍维持梁端失效破坏模式;根据《建筑抗震设计规范》,梁-柱抗弯承载力比取值小于1时为强柱弱梁,这对穿入式组合节点强柱弱梁的定义较为保守。

L形冷弯薄壁型钢组合钢管混凝土柱轴压性能

为解决异形钢管混凝土柱阴角对其承载力的影响,提出了用1个冷弯薄壁方钢管和2个U形钢管焊接成L形钢管,内填充混凝土形成L形冷弯薄壁型钢组合钢管混凝土柱形式。设计了5组共10根试件的轴压试验,并开展了有限元参数分析,研究了钢管厚度、U形管外伸长度和钢材强度等参数对构件承载力和延性等力学性能的影响。结果表明:该类试件的主要破坏形态为中上部局部鼓曲破坏,适量增大U形管外伸长度可以提高承载力,但增大到一定程度之后易发生弯扭破坏;承载力和延性随着钢管厚度和钢材强度增大而增加;混凝土强度对构件的初始刚度和峰值荷载的影响都很小,但对曲线下降段的影响较大;试件端部和中部截面阴角处的混凝土应力值比各边中部更大,说明采用U形钢管与方钢管组合的方式改善了阴角处钢管对混凝土约束普遍较弱的问题。基于“统一理论”,给出了两种承载力建议计算公式,与试验结果吻合较好,两种计算方法在0.44~1.94的约束效应系数范围内具有良好的适用性。

L形方钢管再生混凝土组合异形柱偏压承载力计算方法

基于叠加原理,在《矩形钢管混凝土组合异形柱技术规程》提供的理论计算公式基础上,研究了L形方钢管再生混凝土组合异形柱偏压承载力计算方法。在考虑再生混凝土有效约束面积影响的基础上,提出了L形方钢管再生混凝土组合异形柱偏压承载力计算公式,绘制出轴力-弯矩关系曲线。结果表明:通过引入钢管对内填再生混凝土的约束效应系数,考虑钢管对不同粗骨料取代率再生混凝土的有效约束作用,并根据约束强弱将钢管对内填再生混凝土的约束区域进行划分,可得到再生混凝土的有效约束面积;通过与试验及数值模拟结果对比,验证了L形方钢管再生混凝土组合异形柱偏压承载力计算公式的有效性,该公式可用于预测L形方钢管再生混凝土组合异形柱的偏压承载力,轴力-弯矩关系曲线可较为准确且安全地预测试件的承载力。

RPC外包方钢管混凝土柱轴压受力性能研究

为了研究活性粉末混凝土(Reactive powder concrete,RPC)外包方钢管混凝土柱的轴压受力性能,以RPC厚度和长细比为参数,对6根构件开展轴压试验研究,探讨该类组合柱的破坏模式、受力机理、套箍效应和承载力。试验结果表明:外层RPC可为内部方钢管混凝土柱提供较强的约束力,在受力后期套箍效应凸显。随着RPC厚度的增加,组合柱的承载力逐渐提高;随着组合柱长细比的增大,虽然其弹性刚度和承载力逐渐降低,但延性改善。采用ABAQUS软件建立RPC外包方钢管混凝土柱的有限元模型,分析RPC强度、RPC厚度、钢管强度等级、含钢率、内部混凝土强度和长细比等参数对组合柱承载力的影响规律,可得方钢管混凝土柱发挥系数的变化幅值为0.78~1.01。在试验研究、有限元参数分析和理论探讨的基础上,借鉴钢管混凝土结构技术规范,提出了RPC外包方钢管混凝土轴压短柱和长柱的承载力简化计算方法,公式计算值与已有数据样本吻合良好。