Cyclic performance of concrete-filled steel CHS columns under flexural loading

Concrete-filled steel CHS (circular hollow section) columns are currently being increasingly used in the construction of buildings. Limited information is available on the models for the moment (M) versus curvature (?) response, and the lateral load (P) versus lateral displacement (Δ) relationship of these columns subjected to axial load and cyclically increasing flexural loading.Eight concrete-filled steel CHS specimens were tested under constant axial load and cyclically increasing flexural loading. The parameters in the study included the concrete strength (f_cu) and the axial load level (n). A mechanics model is developed in this paper for concrete-filled steel CHS columns subjected to constant axial load and cyclically increasing flexural loading. The predicted cyclic responses for the composite columns are in good agreement with test results.Based on the theoretical model, parametric analysis was performed on the behaviours of the moment (M) versus curvature (?) response, and the lateral load (P) versus lateral displacement (Δ) relationship, as well as the ductility coefficient (μ) for the composite columns. Finally, simplified models for the moment (M) versus curvature (?) response, and the lateral load (P) versus lateral displacement (Δ) relationship are suggested. A formula for the calculation of the ductility coefficient (μ) of the composite columns under constant axial load and cyclically increasing flexural loading is developed.     

Behaviour of concrete-filled steel columns under the seismic loading

This paper describes results of a dynamic analysis on partially concrete-filled steel columns, which are used for piers of the motorway viaduct. As a seismic loading the ground motion measured in the main shock and one of the post shocks of Chuetsu earthquake in 2004 are used. These two shocks have different vibration patterns and therefore the influence of the vibration pattern of the seismic load can be investigated. Through the analysis it is found that the main shock, which has smaller ground acceleration than the post shock, causes a larger displacement of the column because the frequency component of the main shock is similar to the natural frequency of the column.     

Cyclic performance of repaired concrete-filled steel tubular columns after exposure to fire

This paper provides new test data of cyclic behavior of repaired concrete-filled steel tubular (CFST) columns after exposure to fire, the fire-damaged CFST columns being strengthened by wrapping the original columns by concrete and a thin-walled steel tube. The test parameters included the cross-section type (circular, square and rectangular), and the axial load level (0, 0.3, 0.6). It was found that all the test specimens behaved in a ductile manner and testing proceeded in a smooth and controlled way. Based on the experiment measurements, the ultimate lateral strength, flexural stiffness, dissipated energy and ductility of the columns are analyzed and compared. The test results indicate that the ultimate lateral strength and flexural stiffness of concrete-filled hollow structural columns decrease after exposure to fire, however, the ductility of the columns was not adversely affected due to the fire exposure. The test results also indicate that the strength and stiffness of the fire-damaged columns can be restored over the original level of the specimens.     

Study on seismic performance of rectangular concrete-filled steel tube columns made with tailings and recycled aggregates

为研究矩形截面钢管尾矿砂再生混凝土柱的抗震性能，设计并制作了6个该类柱试件并对其进行拟静力试验，变化参数为再生粗骨料取代率(0%，50%，100%)和轴压比(0.3，0.5)。观察了矩形截面钢管尾矿砂再生混凝土柱变形和破坏形态，分析了其滞回曲线、骨架曲线、承载力退化、刚度退化、延性及耗能能力等。采用ABAQUS软件对矩形截面钢管尾矿砂再生混凝土柱抗震性能进行有限元分析，并与试验结果进行对比，验证有限元模型的合理性。试验研究及数值分析结果表明：矩形截面钢管尾矿砂再生混凝土柱变形过程及破坏形态与普通钢管混凝土柱相似，表现为柱脚的鼓曲变形；轴压比为0.3时，再生骨料取代率为50%和100%的柱峰值荷载较取代率为0%的柱分别降低8.3%和13.1%；轴压比为0.5时峰值荷载较轴压比为0.3时降低约9%；平均位移延性系数接近2.7，说明其具有良好的变形能力；矩形截面钢管尾矿砂再生混凝土柱具有良好的抗震性能。     

矩形截面钢管尾矿砂再生混凝土柱抗震性能研究

为研究矩形截面钢管尾矿砂再生混凝土柱的抗震性能,设计并制作了6个该类柱试件并对其进行拟静力试验,变化参数为再生粗骨料取代率(0%,50%,100%)和轴压比(0.3,0.5)。观察了矩形截面钢管尾矿砂再生混凝土柱变形和破坏形态,分析了其滞回曲线、骨架曲线、承载力退化、刚度退化、延性及耗能能力等。采用ABAQUS软件对矩形截面钢管尾矿砂再生混凝土柱抗震性能进行有限元分析,并与试验结果进行对比,验证有限元模型的合理性。试验研究及数值分析结果表明：矩形截面钢管尾矿砂再生混凝土柱变形过程及破坏形态与普通钢管混凝土柱相似,表现为柱脚的鼓曲变形;轴压比为0.3时,再生骨料取代率为50%和100%的柱峰值荷载较取代率为0%的柱分别降低8.3%和13.1%;轴压比为0.5时峰值荷载较轴压比为0.3时降低约9%;平均位移延性系数接近2.7,说明其具有良好的变形能力;矩形截面钢管尾矿砂再生混凝土柱具有良好的抗震性能。     

Cyclic performance of fire-damaged concrete-filled steel tubular beam-columns repaired with CFRP wraps

This paper presents the results of cyclic tests of fire-damaged concrete-filled steel tubular (CFST) beam-columns repaired by unidirectional carbon fibre reinforced polymer (CFRP) composites. The test parameters included the cross-section type (circular and square), the number of CFRP layers (1 and 2) and the axial load level (0-0.78). Based on the experimental measurements, the repair effects of CFRP wraps on the ultimate lateral strength, flexural stiffness, dissipated energy and ductility of the beam-columns are studied. The test results showed that the ultimate lateral strength, flexural stiffness and ductility can be enhanced to some extent due to the CFRP confinement effect. However, the strength and stiffness of all damaged specimens have not been fully restored in the current tests. Since there exist several possible solutions for the repair/strengthening of CFST members to restore/enhance their strength or stiffness, it seems that an appropriate evaluation method should be developed to make a suitable selection among them based on future comparative study.     

Improving seismic performance of concrete-filled tube to base connections

This study is focused on experimentally investigating the relationship between design details and column base performance, such as strength and rigidity. Results from specimens tested under combined axial and lateral loads were used to define the effectiveness of the base connections. It was observed from test comparisons that the rigidities of the base connections increased when the embedded depths were increased. It was also found that the base connections possessed higher stiffness and energy dissipating capability when stiffeners were added to the base connections. Finally, an empirical expression for base rigidity estimation is proposed for design references.     

沿翼缘加载的型钢混凝土T形柱抗震性能试验及有限元分析

型钢混凝土T形柱截面不规则,沿腹板和翼缘两个主轴方向的受力性能差异较大。为研究实腹式型钢混凝土T形柱沿翼缘方向的抗震性能,对10个缩尺比为1∶2不同参数下的T形柱进行了水平加载试验。考虑了加载制度、轴压比和配钢率的影响,观察了T形柱的受力过程,分析其破坏形态及水平荷载-位移曲线、承载力、刚度、侧移角、延性、耗能能力等抗震性能指标。在此基础上,采用OpenSees对T形柱进行了有限元模拟,计算结果与试验结果吻合较好,进而分析了型钢的应力变化规律和P-δ效应对滞回性能的影响。结果表明:地震作用下,剪跨比为2.5的实腹式型钢混凝土T形柱沿翼缘方向发生弯曲破坏;不同的加载制度下,T形柱呈现出不同的破坏形态;滞回曲线饱满、对称、呈梭形,承载力稳定,耗能能力强;位移延性系数均大于5,破坏时侧移角介于1/16～1/10,显示出良好的延性和抗倒塌能力;加载循环次数越多,T形柱的延性越差;轴压比越大,刚度退化越快,延性和耗能能力越差;配钢率越大,T形柱的刚度退化越缓慢,延性和耗能能力变化较小;P-δ效应使得试件的承载力降低,但耗能能力发挥更充分。     

FRP包裹钢筋混凝土柱抗震加固的耐久性设计

虽然FRP材料已经越来越多地被采用在土木建设方面,但依然缺乏长期的耐久性试验数据,以及在结构水平上的明确的、长期劣化设计理论方法。针对一组试验数据,在材料水平上评定了FRP护套抗震计算的长期劣化标准。共测试了三种不同的系统,快速试验结果被用于材料长期性能的预测,以此分析在四种特殊工况下抗震加固的效果。显而易见,劣化的影响可用预期寿命对应的包裹厚度的增长值来表达。将结果与ACI-440的推荐值以及随时间变化的材料劣化模型结果进行比较,表明ACI的失效推荐值在考虑50年周期时有些保守。通过采用前面所提到的材料耐久性预测方法和FRP防护层厚度设计分析工具,不仅能够更好地对防护层厚度进行估算,而且可以对选择防护层厚度的主要控制机理进行评定。其中,防护层厚度随着暴露时间的长短而改变。快速试验的结果与自然露天测试结果的关系可以用一系列相关的系数来表达。     

FRP对HSS轴压柱承载力影响的有限元分析

此文研究的是用FRP加固的空心方型钢截面(用HSS表示)细长柱在轴压作用下的受力性能。用有限元分析软件Ansys建立"壳—弹簧—壳"的模型,分析参照试验共包括5根相同截面、相同长细比、沿柱纵向粘贴不同层数的FRP的HSS长柱。将分析的结果与试验结果做对比,承载力与试验值吻合较好,得出该模型的有效性。     

Seismic performance improvement of circular steel columns using precompressed concrete-filled steel tube

This paper presents a new method aimed at improving seismic resisting characteristics of circular-shaped steel columns representing highway bridge piers and an experimental investigation carried out to validate it. In this method, a special compression member is placed in the middle of the pier in order to take the axial load from the superstructure. As a result, the influence of axial load on the inelastic buckling deformation of plates can be greatly controlled. The special compression member consists of a precompressed concrete-filled steel tube (PC-CFT). Six specimens were tested to check the seismic performances of the proposed column system. It was clear from the test results that the specimens with PC-CFT could deform even up to ten times of their yield displacements without significant load deterioration. They also showed improved ultimate strength, ductility and energy absorption capacities than the corresponding benchmark specimens.     

预应力FRP筋混凝土梁的抗震性能试验研究

进行了8根混凝土梁在低周反复荷载下的拟静力试验,对其受力过程、破坏特征、滞回特性、骨架曲线、延性、刚度退化、耗能能力等抗震性能进行分析比较,研究不同预应力筋种类和预应力度(PPR)对预应力梁抗震性能的影响。考虑材料的非线性,对试验梁进行有限元计算分析。有限元计算和试验结果表明,预应力度是影响预应力梁抗震性能的主要因素,抗震性能随着预应力度的增加而逐渐降低;在同等预应力度的条件下,无粘结预应力FRP筋混凝土梁的耗能能力最好;采用有粘结和无粘结预应力相结合后,试验梁的延性指标有所提高,延性比有粘结和无粘结预应力梁分别提高12%和26%,滞回曲线变得更加丰满,对结构抗震性能有利;有限元计算结果与试验结果较为吻合。     

高轴压比CFRP约束钢筋混凝土圆柱抗震性能试验与有限元分析

为研究高轴压比下FRP约束钢筋混凝土圆柱的抗震性能,对6根碳纤维约束钢筋混凝土圆柱及2根对比柱进行伪静力试验。结果表明:塑性铰区包裹碳纤维可显著改善高轴压比柱的抗震性能,轴压比较高时不应忽略箍筋对核心混凝土的约束贡献。基于OpenSees中的纤维模型,对柱水平力-位移滞回曲线进行有限元模拟。轴压比小于0.45时,数值模拟与试验结果吻合较好;轴压比大于0.45时,考虑核心混凝土受箍筋及FRP双重约束的计算结果与试验结果更为接近。分析轴压比、FRP包裹长度等参数对约束柱抗震性能的影响,轴压比大于0.6时,FRP约束混凝土圆柱的水平承载力开始降低;FRP在塑性铰区的包裹长度为1.2倍柱直径时,可达到与全柱包裹相当的效果。     

Enhancement of seismic performance of reinforced concrete columns with buckling-restrained reinforcement

Reinforced concrete (RC) columns with light confinement prevalent in developing countries exhibit low ductility with brittle shear failure, especially when buckling of longitudinal rebars takes place. This study applies the buckling restraining concept widely used in seismic resistant steel structures to reinforcing bars. Two RC columns 270 mm×300 mm in cross section with a height of 1200 mm and minimum (non-seismic) transverse reinforcement were tested under cyclic lateral loading. Buckling-restrained reinforcement was provided over the critical zone. The buckling-restraining casing effectively prevented buckling of slender vertical bars under a substantially high axial load level, resulting in a more ductile mode of failure with the evident formation of plastic hinge at the base of the column. Prior to gravity load collapse, the drift capacities and the degraded concrete shear capacities of the specimens were significantly increased compared to their counterparts without casings.     

Real-time dynamic substructuring testing of viscous seismic protective devices for bridge structures

This paper presents a real-time dynamic substructuring (RTDS) test program carried out on bridge structures equipped with two innovative viscous seismic protective devices: a seismic damping unit and a shock transmission unit. In the RTDS tests, the seismic protective units were physically tested in the laboratory using a high performance dynamic actuator imposing, in real time, the displacement time histories obtained from numerical simulations being run in parallel. The integration scheme used in the test program was the Rosenbrock-W variant, and the integration was performed using The MathWorks’ Simulink and XPC target computer environment. The numerical counterpart included the bridge columns and the additional energy dissipation properties. The nonlinear response of these components was accounted for in the numerical models. The tests were run under various ground motions, and the influence of modeling assumptions such as damping and initial stiffness was investigated. Finally, the test results are compared to the predictions from nonlinear dynamic time history analyses performed using commercially available computer programs. The results indicate that simple numerical modeling techniques can lead to accurate prediction of the displacement response of bridge structures equipped with the seismic protective systems studied.     

矩形钢管混凝土异形柱-钢梁框架节点抗震性能试验研究

为研究矩形钢管混凝土异形柱-钢梁框架节点的破坏特征和抗震性能, 进行了5个中节点、2个边节点和2个角节点的低周反复加载试验。观察了节点的受力过程及破坏形态, 分析了试件的荷载-位移滞回曲线、承载能力、强度和刚度退化、层间位移角和延性以及耗能能力等力学特性。结果表明: 矩形钢管混凝土异形柱-钢梁框架节点的典型破坏形态是节点域腹板的剪切破坏、节点核心区腹板与柱翼缘连接的竖向焊缝断裂; 试件滞回曲线饱满,层间位移延性系数介于1.44~2.74,弹塑性极限层间位移角约为1/43~1/21;等效黏滞阻尼系数介于0.227~0.316,表明节点域的变形和耗能能力较强。当柱截面肢高肢厚比为3、4时,破坏时节点核心区的剪切角约为0.01~0.03;当柱截面肢高肢厚比为2时,破坏时节点核心区的剪切角约为0.08~0.10。     

Analytical model for the torsional behaviour of reinforced concrete beams retrofitted with FRP materials

An analytical approach for the prediction of the torsional response of reinforced concrete (RC) beams strengthened with fibre-reinforced-polymer (FRP) materials is described. The analysis method employs the combination of two different theoretical models: a smeared crack model for plain concrete in torsion for the elastic till the first cracking response and a properly modified truss model for the post-cracking response. The proposed method addresses the contribution of the externally bonded FRP materials to the torsional capacity of RC beams using specially developed equations in the well-known truss theory and utilizes softened and FRP-confined concrete stress–strain relationships. Detailed verification of this methodology is achieved through extensive comparisons between analytically predicted behaviour curves and experimentally obtained ones. The experimental data comprise a series of 12 tests in pure torsion and an additional database of experimental information for 24 specimens compiled from works around the world. These comparisons demonstrated that the proposed model is capable of adequately describing the full torsional response and of predicting with satisfactory accuracy the torsional moment at cracking and the ultimate torque capacity of FRP strengthened RC beams. Applications of the developed method as an assessment tool to strengthened beams with U-shaped FRP configurations and some first design examples that demonstrate the contribution of the FRP materials on the torsional response are also included.     

Experimental study of hysteretic behaviour for concrete-filled square thin-walled steel tubular columns

In this paper, the hysteretic behaviour of concrete-filled thin-walled steel tubular (CFTST) columns was investigated experimentally. The parameters in the study included the axial load level and steel tube section type. Nine CFTST columns, including six columns with a longitudinal stiffener on each inner face of the steel tube and three columns with a longitudinal stiffener on the two opposite inner faces of the steel tube, were tested under a constant axial load in addition to a cyclic lateral load. The effect of axial load level on the hysteretic behaviour (stiffness, ductility and energy dissipation) was studied. Experimental results indicated that the CFTST columns under an axial load level below 0.5 exhibited plump hysteretic loops with a slight pinching effect, better ductility and energy dissipation capacity. The displacement ductility decreases significantly with an increase in the axial load level. Columns with two steel tube sections had almost the same load capacity, whilst the ductility and energy dissipation capacity of columns with a longitudinal stiffener on each inner face of the steel tube was better than that of columns with two opposite stiffeners.     

FRP布加固偏心受压钢筋混凝土矩形柱承载力的简化计算方法

在已有的FRP约束混凝土矩形截面短柱的三阶段应力—应变关系基础上,给出了等效矩形应力图形系数β和γ的简化计算公式。根据截面力的平衡条件使用系数β和γ建立了FRP布加固钢筋混凝土矩形截面偏心受压柱承载力的简化计算方法,并与相关文献中的试验结果进行了对比。计算结果表明,用本文提出的承载力简化计算方法计算的承载力数值与试验值吻合较好,可供设计和施工参考。     

Numerical simulation on seismic retrofitting performance of reinforced concrete columns strengthened with fibre reinforced polymer sheets

This paper evaluates the seismic performance of reinforced concrete columns retrofitted with fibre reinforced polymer (FRP) sheets through numerical simulations of the load–deformation response using two-dimensional finite element analysis (2D-FEA). The relatively rational mesh configuration is verified through comparison of analysis results obtained from the different mesh configurations. The seismic performance of three reinforced concrete (RC) columns strengthened with FRP sheets is assessed through a series of parametric studies, and the applicability of existing crack models and constitutive relationships on crack discontinuity and concrete compressive behaviour are validated. Comparisons of analysis results with tests shows that an equivalent uniaxial strain model and a failure criterion can be used to accurately simulate nonlinear behaviour and the failure of concrete under a biaxial stress state, respectively. Moreover, it is shown that a modified confinement model can be simply adopted to evaluate confined effects from hoop steel and FRP on concrete, which generally operate in three-dimensional confinement. Lastly, the seismic retrofitting performance of RC columns wrapped with FRP sheets is verified by analysing load–deformation responses and the progression of stress–strain at inflection points and bottoms of the columns.