Seismic behavior and strength capacity of steel tube‐reinforced concrete composite columns

The steel tube‐reinforced concrete (ST‐RC) composite column is a novel type of composite column, which consists of a steel tube embedded in RC. In this paper, the seismic behavior of ST‐RC columns is examined through a series of experiments in which 10 one‐third scale column specimens were subjected to axial forces and lateral cyclic loading. The test variables include the axial force ratio applied to the columns and the amount of transverse reinforcement. All specimens failed in a flexural mode, showing stable hysteresis loops. Thanks to the steel tube and the high‐strength concrete it is filled with, the ST‐RC column specimens had approximately 30% lower axial force ratios and 22% higher maximum bending moments relative to the comparable RC columns when subjected to identical axial compressive loads. The amount of transverse reinforcement made only a small difference to the lateral load‐carrying capacity but significantly affected the deformation and energy dissipation capacity of the ST‐RC columns. The specimens that satisfied the requirements for transverse reinforcement adopted for medium ductile RC columns as specified by the Chinese Code for Seismic Design of Buildings (GB 50011‐2010) and EuroCode 8 achieved an ultimate drift ratio of around 0.03 and a displacement ductility ratio of approximately 5. The design formulas used to evaluate the strength capacity of the ST‐RC columns were developed on the basis of the superposition method. The predictions from the formulas showed good agreement with the test results, with errors no greater than 10%. Copyright © 2013 John Wiley & Sons, Ltd.     

钢筋混凝土圆形截面柱式桥墩抗震性能评价

对4座典型的钢筋混凝土圆形截面双柱式桥墩，利用Priestley等建议的钢筋混凝土桥墩抗剪强度计算方法和型态描述方程，结合Rush-over方法进行了延性抗震能力评价。一般说来，纵桥向能够满足延性抗震要求，而横桥向一些配箍率较低的桥墩在地震中会发生脆性的弯剪破坏，其底部塑性铰将形成在柱基之中，部分桩基可能会遭受损害。     

Seismic performance of steel reinforced ultra high-strength concrete composite frame joints

To investigate the seismic performance of a composite frame comprised of steel reinforced ultra high-strength concrete (SRUHSC) columns and steel reinforced concrete (SRC) beams, six interior frame joint specimens were designed and tested under low cyclically lateral load. The effects of the axial load ratio and volumetric stirrup ratio were studied on the characteristics of the frame joint performance including crack pattern, failure mode, ductility, energy dissipation capacity, strength degradation and rigidity degradation. It was found that all joint specimens behaved in a ductile manner with flexural-shear failure in the joint core region while plastic hinges appeared at the beam ends. The ductility and energy absorption capacity of joints increased as the axial load ratio decreased and the volumetric stirrup ratio increased. The displacement ductility coefficient and equivalent damping coefficient of the joints fell between the corresponding coefficients of the steel reinforced concrete (SRC) frame joint and RC frame joint. The axial load ratio and volumetric stirrup ratio have less influence on the strength degradation and more influence on the stiffness degradation. The stiffness of the joint degrades more significantly for a low volumetric stirrup ratio and high axial load ratio. The characteristics obtained from the SRUHSC composite frame joint specimens with better seismic performance may be a useful reference in future engineering applications.     

中欧规范RC桥墩抗震设计及验算方法对比研究

地震作用下钢筋混凝土（RC）桥墩容易损坏。为完善RC桥墩的抗震设计及验算方法,对比最新中国和欧洲规范中关于RC桥墩的延性抗震设计及验算方法的不同之处。基于Midas/Civil软件所建立的常规连续梁桥有限元模型,对比分析采用中欧规范开展的RC桥墩延性抗震设计及验算结果。结果表明:中欧规范中关于RC桥墩的延性抗震设计理念、抗剪和变形验算方法及延性构造细节均有区别。基于中欧规范设计的RC桥墩配筋情况存在差异。与中国规范相比,欧洲规范关于RC桥墩的横向钢筋配筋率和纵筋最小配筋率要求较高,有利于保证结构的抗剪强度和延性;箍筋最大间距要求较低,不利于防止纵筋压曲。     

Prediction of the force–drift envelope for RC columns in flexure by the CAE method

A non‐parametric empirical approach, called the conditional average estimator (CAE) method, has been applied for the prediction of the normalized lateral force–drift envelope of reinforced concrete (RC) rectangular columns, as well as their characteristic drifts (effective yield drift, capping drift and ultimate drift), and drift‐related parameters (the ratio between the effective yield drift and elastic drift, and two ductility measures). A subset of the PEER RC column database was used. Five input parameters were employed: axial load index, index related to confinement, shear span index, concrete compressive strength, and longitudinal reinforcement index. The results suggest that the relations between the input and output parameters are complex, and that it is difficult to isolate the influence of a single parameter. Nevertheless, some trends were observed. The axial load index is the most influential input parameter. All the results decrease with an increasing axial load index, whereas they increase with an increasing longitudinal reinforcement index. An increase in the index related to confinement results in increases in the ultimate drift and in ductility. The influence of the shear span index is the most complex. The influence of the concrete strength is small with the exception of two output parameters related to elastic drift, which substantially decrease with increasing strength. The dispersion of the results is relatively large. The results of the predictions can be used for mathematical modelling of moment–rotation backbone curves for plastic hinges, and for the estimation of the deformation capacity of columns in seismic performance assessments. Copyright © 2007 John Wiley & Sons, Ltd.     

异形柱框架节点抗震性能试验研究

通过6个2/3比例的T形柱框架节点的低周反复试验,考察了轴压比、柱翼缘宽度等因素对节点裂缝开展、节点的破坏机理的影响。提出了翼缘宽度对节点抗剪性能的影响与梁柱抗弯刚度比有关,指出了翼缘板对节点抗剪承载力影响的修正系数ξf须考虑梁柱抗弯刚度比的影响,而不能仅按《混凝土异形柱结构技术规程》(征求意见稿)中的bf-bc来反映。     

关于日本框架柱受剪承载力公式的讨论

通过对日本抗震设计规范中有关钢筋混凝土框架柱受剪承载力代表性公式的研究，凸现了由经验公式到基于桁架一拱模型建立的发展趋势。对各公式中所包括的影响框架柱受剪承载力的主要因素(如剪跨比、轴压比、配箍率、混凝土强度、纵筋率等)进行了深入讨论，并采用框架柱受剪承载力试验数据考察了各个公式的有效性。     

Cyclic Displacement Model for Reinforced Concrete Columns

Reinforced concrete (RC) structures in low to moderate seismic regions and many older RC structures in high seismic regions include columns with steel reinforcement details not meeting the requirements of modern seismic design codes. These columns typically fail in shear or in a brittle manner and their behavior must be accurately captured when RC structures are modeled and analyzed. The total lateral displacement of a low ductility or shear critical RC column can be represented as the sum of three displacement components: (1) flexural displacement, (2) displacement due to slippage of the reinforcing bars at column ends, and (3) shear displacement. In this study, these three displacement components are separately modeled and then combined together following a proposed procedure based on the expected overall behavior of the column and its failure mechanism. A simplified slip model is proposed. The main objective of this research is to develop an easy-to-apply method to model and capture the cyclic behavior of RC columns considering the shear failure mechanism. The proposed model is validated using the available data from RC column and frame experiments.     

Experimental study of hollow rectangular bridge column performance under vertical and cyclically bilateral loads

To investigate the seismic performance of hollow reinforced concrete(RC) bridge columns of rectangular cross section under constant axial load and cyclically biaxial bending,five specimens were tested.A parametric study is carried out for different axial load ratios,longitudinal reinforcement ratios and lateral reinforcement ratios.The experimental results showed that all tested specimens failed in the flexural failure mode and their ultimate performance was dominated by flexural capacity,which is represented by the rupture/buckling of tensile longitudinal rebars at the bottom of the bridge columns.Biaxial force and displacement hysteresis loops showed significant stiffness and strength degradations,and the pinching effect and coupling interaction effect of both directions severely decrease the structural seismic resistance.However,the measured ductility coefficient varying from 3.5 to 5.7 and the equivalent viscous damping ratio varying from 0.19 and 0.26 can meet the requirements of the seismic design.The hollow RC rectangular bridge columns with configurations of lateral reinforcement in this study have excellent performance under bidirectional earthquake excitations,and may be considered as a substitute for current hollow RC rectangular section configurations described in the Guideline for Seismic Design of Highway Bridges(JTG/T B02-01-2008).The length of the plastic hinge region was found to approach one sixth of the hollow RC rectangular bridge column height for all specimen columns,and it was much less than those specified in the current JTG/T.Thus,the length of the plastic hinge region is more concentrated for RC rectangular hollow bridge columns.     

高强钢筋高延性纤维增强混凝土框架结构的屈服机制和抗震性能

为了对混凝土框架结构的地震破坏机制和抗震性能进行控制,在框架柱中配置高强钢筋,并将纤维增强混凝土(FRC)用于框架结构的预期损伤部位。结构柱中的高强钢筋用来减小结构的残余变形,FRC材料用来增加结构的耗能能力和损伤容限。设计了三个框架,采用动力弹塑性时程分析方法进行分析。研究结果表明,采用高强钢筋提高了结构的整体承载能力,在层间侧移角达到3%之前避免了柱铰的出现(包括底层柱底),并且减小了结构的残余变形;预期损伤部位采用FRC材料能够提高结构的塑性耗能。     

钢纤维高强混凝土框架边节点梁的曲率延性

进行了9个钢纤维高强混凝土框架边节点的抗震试验.通过测试钢纤维高强混凝土框架边节点梁端的荷载-变形滞回曲线和梁相关截面的横向变形,研究了钢纤维体积率、掺加范围和轴压比等因素对高强混凝土框架边节点梁截面曲率延性和滞回曲线的影响.结果表明,钢纤维能改善高强混凝土框架边节点梁截面延性,显著提高高强混凝土框架节点的抗震延性和耗能能力,对解决节点箍筋密集、改善施工条件具有明显效果.     

Research on the influencing factors for residual displacements of RC bridge columns subjected to earthquake loading

Following the 1995 Kobe earthquake, many RC bridge columns were demolished due to a residual drift ratio of more than 1.75 % even though they did not collapse. The residual drift ratio is a quantitative index for the performance objective of reparability in the bridge seismic design. Numerical models of the columns are built to study the factors that influence the residual displacement of RC bridge columns. In these models, both column bending and bar pulling out deformation are considered using the fiber column-beam element and zero-length section element, respectively. Then, nonlinear time history analyses are performed. The factors that influence column residual displacement, such as the characteristics of ground motion, the structural responses (the maximum lateral drift ratio and the displacement ductility factor), and the structural characteristics (the aspect ratio and the longitudinal reinforcement ratio) are investigated. It is found that the near-fault ground motion induces a larger residual drift ratio than the far-fault ground motion. The residual drift ratio becomes larger due to the increase of the maximum lateral drift ratio, the displacement ductility factor, and the aspect ratio. Further, a larger longitudinal reinforcement ratio can induce a larger residual drift ratio due to the contribution of the bar pulling out deformation.     

Effect of seismic design details on hysteresis performance of SRC-RC transfer columns

Four types of seismic design details were tested using 11 transfer column specimens and one comparison specimen of RC under low cyclic reversed loading. Test results show that diagonal cracks control the failure pattern and damage occurs mainly in the RC section with weak shear capacity in the transfer columns. There is a large difference in the bearing capacity and ductility of the transfer columns according to the test results, which indicates that the strengthening effect of diverse structural measures is quite different. The section ratio of I-section-encased steel and the axial compression ratio also have a great influence on the bearing capacity and ductility. Although the bearing capacity of transfer columns with additional longitudinal bars and additional X bars is relatively large, they have poor deformation capacity. Setting more stirrups along the columns is the best structural measure to enhance the seismic performance. The studs on the I-sectionencased steel by welding can help to complete the stress transfer between the steel and concrete, and avoid performance degradation of the two materials due to bonding failure.     

设计因素对RC框架结构地震易损性的影响

考虑到结构抵抗地震作用的机制为结构和地震动的不确定性与非线性相互耦合的过程,采用增量动力分析(IDA)考虑地震动的不确定性,选取16条地震动记录,基于OpenSEES的有限元建模理论对13榀平面RC框架结构进行基于IDA方法的地震易损性分析,分别讨论轴压比、高宽比、混凝土强度以及纵筋强度等结构参数对RC框架结构抗震性能的影响。结果表明:柱轴压比对结构抗震性能的影响显著,而高宽比对结构抗震性能的影响不明显;在保证柱轴压比相近的前提下,提高柱混凝土强度能够提升结构的抗震性能;相同地震作用下梁柱配置纵筋强度较高的框架结构达到立即使用(IO)状态和生命安全(LS)状态的概率较配置纵筋强度较低的大,配置纵筋强度较高的框架结构较配置纵筋强度较低的表现出更好的抗倒塌能力。     

锈蚀箍筋约束混凝土加固柱抗震性能分析

为研究加固后锈蚀箍筋约束混凝土柱的抗震性能,设计16个钢筋混凝土矩形柱,对加速锈蚀后的试件采用外包钢及CFRP、GFRP材料进行加固处理,随后开展低周往复荷载试验,以研究不同加固材料、不同加固包裹方式、不同黏结材料等因素对锈蚀箍筋约束混凝土柱滞回曲线、骨架曲线、刚度衰减、延性性能及耗能能力等抗震性能指标的影响。研究结果表明:(1)与未锈蚀柱相比,加固后的箍筋锈蚀柱以弯剪破坏为主,CFRP加固试件较GFRP加固试件具有更高的承载力,但破坏时延性不如GFRP加固试件;(2)外包钢加固试件、CFRP加固试件、GFRP加固试件的耗能能力较未锈蚀试件分别增大222.3%、123.9%、98.5%,延性系数也相应增大45.3%、22.3%、25.6%。总之,外包钢加固对箍筋锈蚀柱抗震性能的提升最大,CFRP加固次之,但优于GFRP加固;整包加固优于环包加固;环氧树脂胶黏结略优于水泥基灌浆料黏结。     

混凝土框架柱抗剪承载力计算公式的可靠度分析探讨

通过试验研究，提出了基于能力设计法的框架柱塑性铰区抗剪承载力计算公式，并对此进行了可靠度分析。定量分析了该计算公式的可靠指标与剪跨比、配箍率等因素的关系。同时针对目前我国现行规范抗剪承载力计算的可靠度指标偏低较多的情况，该建议公式对不同抗震要求的构件提出了不同的可靠指标，既考虑了我国设计习惯，又体现了能力设计法的要求，充分考虑了安全性的要求，对工程应用具有较大的参考价值。论文的主要研究成果已被《混凝土结构设计规范》(GB50010-2002)吸收。     

Development and validation of a metallic haunch seismic retrofit solution for existing under‐designed RC frame buildings

The feasibility and efficiency of a seismic retrofit solution for existing reinforced concrete frame systems, designed before the introduction of modern seismic‐oriented design codes in the mid 1970s, is conceptually presented and experimentally investigated. A diagonal metallic haunch system is introduced at the beam–column connections to protect the joint panel zone from extensive damage and brittle shear mechanisms, while inverting the hierarchy of strength within the beam–column subassemblies and forming a plastic hinge in the beam. A complete step‐by‐step design procedure is suggested for the proposed retrofit strategy to achieve the desired reversal of strength hierarchy. Analytical formulations of the internal force flow at the beam–column‐joint level are derived for the retrofitted joints. The study is particularly focused on exterior beam–column joints, since it is recognized that they are the most vulnerable, due to their lack of a reliable joint shear transfer mechanism. Results from an experimental program carried out to validate the concept and the design procedure are also presented. The program consisted of quasi‐static cyclic tests on four exterior, ? scaled, beam–column joint subassemblies, typical of pre‐1970 construction practice using plain round bars with end‐hooks, with limited joint transverse reinforcement and detailed without capacity design considerations. The first (control specimen) emulated the as‐built connection while the three others incorporated the proposed retrofitted configurations. The experimental results demonstrated the effectiveness of the proposed solution for upgrading non‐seismically designed RC frames and also confirmed the applicability of the proposed design procedure and of the analytical derivations. Copyright © 2006 John Wiley & Sons, Ltd.     

Seismic strengthening of existing RC beam‐column joints by wing walls

Numerous non‐ductile reinforced concrete (RC) buildings with little or no shear reinforcement in beam‐column joints can be found in regions of moderate seismicity. To strengthen such substandard beam‐column joints, this study proposes a method in which RC wing walls are installed beside existing columns, which overcomes the lack of realistic strengthening methods for congested connections in RC buildings. The proposed strengthening mechanism improves the joint moment capacity by utilizing tension and compression acting on the beam–wing wall boundaries; thus, brittle joint hinging failure is prevented. Three 3/4‐scale RC exterior beam‐column joint specimens without shear reinforcement, two of which were strengthened by installing wing walls with different strengthening elements, were fabricated and tested. The test results verified the effectiveness of the proposed strengthening method and the applicability of this method to seismically substandard beam‐column joints. © 2017 The Authors. Earthquake Engineering & Structural Dynamics Published by John Wiley & Sons Ltd.     

锈蚀钢筋混凝土圆柱抗震性能的试验研究

对不同锈蚀程度的钢筋混凝土圆柱进行低周反复试验,研究了不同轴压比下的钢筋锈蚀率对钢筋混凝土圆柱滞回曲线、骨架曲线、刚度、延性及耗能能力的影响;给出了试件累积耗能、屈服荷载、极限荷载、荷载最大值和位移延性系数与钢筋锈蚀率和轴压比的关系。研究表明,随着钢筋锈蚀率和轴压比的增大,试件的滞回曲线趋于干瘪,骨架曲线下降段变陡,试件的刚度、延性和耗能能力减小。     

Experimental study on seismic behavior of circular RC columns strengthened with pre-stressed FRP strips

Bonding fiber reinforced polymer （FRP） has been commonly used to improve the seismic behavior of circular reinforced concrete （RC） columns in engineering practice. However, FRP jackets have a significant stress hysteresis effect in this strengthening method, and pre-tensioning the FRP can overcome this problem. This paper presents test results of 25 circular RC columns strengthened with pre-stressed FRP strips under low cyclic loading. The pre-stressing of the FRP strips, types of FRP strips and longitudinal reinforcement, axial load ratio, pre-damage degree and surface treatments of the specimens are considered as the primary factors in the tests. According to the failure modes and hysteresis curves of the specimens, these factors are analyzed to investigate their effect on bearing capacity, ductility, hysteretic behavior, energy dissipation capacity and other important seismic behaviors. The results show that the initial lateral confined stress provided by pre-stressed FRP strips can effectively inhibit the emergence and development of diagonal shear cracks, and change the failure modes of specimens from brittle shear failure to bending or bending-shear failure with better ductility. As a result, the bearing capacity, ductility, energy dissipation capacity and deformation capacity of the strengthened specimens are all significantly improved.