Models of the flexure-controlled strength,stiffness and cyclic deformation capacity of rectangular RC columns with smooth bars,including lap-splicing and FRP jackets

Cyclic tests of single concrete columns with smooth (plain) bars are not representative of building columns with lap splices at floor levels and story-long starter bars. Column specimens with fixity at top and bottom resemble building columns best, but few of those tested so far had smooth bars and even then without bar lap-splicing at floor level or FRP jackets at column ends. Empirical models based on single-column tests, especially the numerous ones with cantilever-type specimens, cannot be readily extended to columns with smooth bars in real-life buildings. Physical models of the Strut-and-Tie type are developed and are validated or calibrated through comparisons with laboratory tests. Their scope includes anchorage and splicing of bars with either 180° hooks or straight ends. Once validated, they are adapted to real-life multistory rectangular RC columns with smooth bars, in order to obtain the column properties of interest: the chord rotation at yielding and the cyclic ultimate chord rotation, with or without FRP jacketing. Different expressions apply to the top and bottom end of a column in a story, but a single one is used to estimate the column’s effective stiffness. Empirical alternatives fitted to the single-element test results have slightly less scatter than physical models, but caution is needed for their application to columns of real buildings. Simulations of the 3D seismic response of a plan wise asymmetric full size building, tested pseudo-dynamically before or after retrofitting all columns with FRPs or just two of them with RC jackets, provide certain confidence in the extension of the physical models for the estimation of the stiffness and ultimate deformation of columns with smooth bars in real-life buildings.     

The reinforcement contribution to the cyclic behaviour of reinforced concrete beam hinges

The cyclic behaviour of plastic hinges is an essential component in tracking the behaviour of RC frames to failure, not only for monotonically increasing force/pressure loads such as under extreme wind loads but also for dynamic displacement-driven loads such as under earthquake ground motions. To describe member deformations at ultimate loading, traditional moment–curvature techniques have required the use of an empirical hinge length to predict rotations, and despite much research a definitive generic expression for this empirical hinge length is yet to be defined. To overcome this problem, a discrete rotation approach, which directly quantifies the rotation between crack faces using mechanics, has been developed for beams and been shown to be accurate under monotonic loading. In this paper, the discrete rotation approach for monotonic loads is extended to cope with cyclic loads for dynamic analyses, and this has led to the development of a new partial interaction numerical simulation capable of allowing for reversals of slip of the reinforcing bars. This numerical tool should be very useful for the nonlinear analysis of reinforced concrete beams and reinforced concrete columns with small axial loads under severe dynamic loads. Copyright © 2011 John Wiley & Sons, Ltd.     

Hollow bridge-pier properties for response spectrum analysis

The present paper proposes equivalent stiffness and energy dissipation properties of reinforced concrete hollow bridge piers to be used in the context of response spectrum performance based assessment and design. The work is carried out by performing parametric numerical analysis using a 2D fibre model calibrated against experimental results and by varying the longitudinal steel reinforcement ratio, height over width ratio, normalised axial force, level of confinement and concrete class of a rectangular hollow section reinforced with Tempcore B500C steel. The results of the analysis are given in the form of charts and closed form expressions for the yield curvature and moment, ultimate ductility, post yielding stiffness ratio and energy dissipated of the section, and are translated to the member level through the plastic hinge length approach. Likewise, the parameters of a Takeda model derived from the parametric analysis are given for use in nonlinear time history analysis.     

Probabilistic seismic performance assessment of lap-spliced RC columns retrofitted by steel wrapping jackets

In this study, the seismic fragility curves of two reinforced concrete (RC) columns that were lap-spliced at the bottom and retrofitted with steel wrapping jackets were generated. Their seismic performance was probabilistically assessed in comparison to that of lap-spliced or continuous reinforcement RC columns. This study used two types of steel wrapping jackets, a full jacket and a split jacket. Analytical models of the four types of columns were developed based on the experimental results of the columns using OpenSEES, which is effective in conducting nonlinear time history analyses. A suite of ten artificial ground motions, modified from recorded ground motions, was used to perform nonlinear time history analyses of the analytical models with scaling of the peak ground acceleration from 0.1 g to 1.0 g in increments of 0.1 g. The steel wrapping jackets did not increase the medians for yield (slight damage state) of the lap-spiced column and did not exceed the corresponding median of the continuous reinforcement column. However, the two steel jackets increased the medians for failure by 1.872 and 2.017 times, respectively, and exceeded the corresponding median of the continuous reinforcement column by 11.8% and 20.5%, respectively.     

Pseudodynamic response of torsionally unbalanced two‐storey test structure

Two one‐way eccentric, two‐storey, one‐by‐one‐bay reinforced concrete (RC) structures are pseudodynamically tested under unidirectional ground motions. Theoretical considerations about the effect of torsional coupling on modal periods and shapes agree with modal results of the test structure, considering member stiffness is equal to the secant stiffness to yielding in skew‐symmetric bending. Modal periods of such an elastic structure are in fair agreement with effective periods inferred from the measured response at the beginning of a test of a thoroughly cracked structure and at the end of the test. A time‐varying stiffness matrix and a non‐proportional damping matrix fitted to the test results may be used to reproduce the measured response approximately by modal superposition and identify the role of the four time‐varying modes. Flexible side columns sustained very large drift demands simultaneously in the two transverse directions and suffered significant but not heavy, damage at lap‐splices. RC‐jacketing of the flexible side columns practically eliminated the static eccentricity between the floor centres of twist and mass as well as the torsional response. Inelastic time‐history analysis with point‐hinge member models, using as elastic stiffness the secant stiffness to yielding and neglecting post‐ultimate‐strength cyclic degradation of resistance in members with plain bars and poor detailing, predicted fairly well the response until the peak displacements and member deformations occurred. After that, it underestimated displacement peaks and the lengthening of the apparent period and missed the gradual drifting of the response towards a permanent offset. Copyright © 2007 John Wiley & Sons, Ltd.     

Design of transverse reinforcement to avoid premature buckling of main bars

This paper proposes an enhancement to the current strength and confinement‐based design of transverse reinforcement in rectangular and circular reinforced concrete members to ensure that the flexural strength of reinforced concrete sections does not degrade excessively due to buckling of longitudinal bars until the desired level of plastic deformation is achieved. Antibuckling design criteria are developed based on a popular bar buckling model that uses a bar buckling parameter (combining the bar diameter, yield strength, and buckling length) to solely describe the bar buckling behavior. The value of buckling parameter that limits the buckling‐induced stress loss to 15% in compression bars at the strain corresponding to the design ductility is determined. For a bar of known diameter and yield strength, the maximum allowable buckling length can then be determined, which serves as the maximum limit for the tie/stirrup/hoop spacing. Lateral stiffness required to restrain the buckling tendency of main bars at the locations of the ties/stirrups/hoops depends on the flexural rigidity of the main bars and the buckling length (equal to or multiple of tie/hoop/stirrup spacing), whereas the antibuckling stiffness (ie, resistance) provided by the ties/stirrups/hoops depends on their size, number, and arrangement. Using the above concept, design recommendations for the amount, arrangement, and spacing of rectangular and circular ties/stirrups/hoops are then established to ensure that the antibuckling stiffness of the provided transverse reinforcement is greater than the stiffness required to restrain the buckling‐prone main bars. Key aspects of the developed method are verified using experimental tests from literature.     

Analytical prediction of ultimate moment and curvature of RC rectangular sections in compression

This paper presents closed form expressions linking the ultimate bearing capacity to the ultimate curvature of rectangular RC sections subjected to axial load and bending moment acting in one of the two symmetry planes of the section. With respect to possible simplified formulations the following effects are also considered: confinement of the concrete, hardening of the longitudinal reinforcement, and presence of reinforcing bars distributed orthogonally to the neutral axis. The formulation is proposed in dimensional terms after a preliminary definition of the geometrical and mechanical parameters governing the structural response of the class of sections considered. The analytical expressions derived using the proposed approach also allow one to determine the compression level that makes the ultimate bending moment maximum as well as to evaluate the curvature corresponding to the first yielding of the principal reinforcement in tension. Comparing this value of curvature with the ultimate one, an approximate estimation of the available ductility of curvature of the section can be made. The analytical procedure is validated by comparing the results with those obtained using a typical numerical approach. Some experimental results are also considered.     

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.     

反复荷载作用下锈蚀钢筋混凝土柱力学性能研究

采用有限元软件ABAQUS,以锈蚀率（0%、5%、10%、15%和20%）为变量,对5根钢筋混凝土柱的力学性能进行了数值模拟,研究各试件的滞回性能、骨架曲线、延性及耗能能力,分析钢筋锈蚀率对承载力、延性、耗能和塑性铰转动能力的影响。研究结果表明:模拟分析得到的锈蚀钢筋混凝土柱的强度和变形与试验结果吻合较好,建立的有限元模型可用于锈蚀钢筋混凝土柱的力学性能分析;混凝土开裂前,锈蚀构件的力学性能基本与未锈蚀构件相同,混凝土开裂后,构件的承载力、屈服荷载、极限位移、延性等均随钢筋锈蚀率的增大而降低;轻度锈蚀构件的滞回性能和破坏形式与未锈蚀构件类似,随着钢筋锈蚀率逐渐增大,滞回环的饱满程度降低,“捏拢”现象严重,滞回曲线由“弓形”逐渐发展成“反S形”,耗能能力降低,破坏形式趋于脆性破坏,位移延性系数、平均耗能系数等指标逐渐下降。     

Experimental investigation of conditions of lateral shear reinforcements in RC columns accompanied by buckling of longitudinal bars

This study presents a cyclic load test of four RC columns to obtain data on stresses and strains on lateral shear‐reinforcing bars that contact buckled longitudinal reinforcing bars. The specimens include columns laterally jacketed with all‐elastic fibre‐reinforced polymer (FRP) sheets. The buckling lengths and modes in the longitudinal bars of the four column specimens stabilized at a buckling deflection (= lateral deformation of buckled longitudinal bar) from 0.3 to 0.6 mm. The yield portion ratio r_by of shear reinforcement around the buckled longitudinal bars was introduced as an index of the development of buckling conditions. Here, the yield portion ratio r_by was defined as the ratio of the length of the region where the shear reinforcements yield l_by, to the buckling length l_b. The r_by values of the tested columns ranged from 0.45 to 0.76. The test results show that the buckling stress and the specific compressive stress of the longitudinal bars in the columns were smaller than those of the bare bars. Copyright © 2007 John Wiley & Sons, Ltd.     

Effects of near‐fault ground motions on the nonlinear dynamic response of base‐isolated r.c. framed buildings

Near‐fault ground motions are characterized by long‐period horizontal pulses and high values of the ratio between the peak value of the vertical acceleration, PGA_V, and the analogous value of the horizontal acceleration, PGA_H, which can become critical for base‐isolated (BI) structures. The objective of the present work is to check the effectiveness of the base isolation of framed buildings when using High‐Damping‐Rubber Bearings (HDRBs), taking into consideration the combined effects of the horizontal and vertical components of near‐fault ground motions. To this end, a numerical investigation is carried out with reference to BI reinforced concrete buildings designed according to the European seismic code (Eurocode 8). The design of the test structures is carried out in a high‐risk region considering (besides the gravity loads) the horizontal seismic loads acting alone or in combination with the vertical ones and assuming different values of the ratio between the vertical and horizontal stiffnesses of the HDRBs. The nonlinear seismic analysis is performed using a step‐by‐step procedure based on a two‐parameter implicit integration scheme and an initial‐stress‐like iterative procedure. At each step of the analysis, plastic conditions are checked at the potential critical sections of the girders (i.e. end sections of the sub‐elements in which a girder is discretized) and columns (i.e. end sections), where a bilinear moment–curvature law is adopted; the effect of the axial load on the ultimate bending moment (M‐N interaction) of the columns is also taken into account. The response of an HDRB is simulated by a model with variable stiffness properties in the horizontal and vertical directions, depending on the axial force and lateral deformation, and linear viscous damping. Copyright © 2011 John Wiley & Sons, Ltd.     

强震下四边钢柱支承单层柱面网壳弹塑性地震响应研究

采用集中塑性铰理论和SAP2000结构分析软件,对某体育练习馆（钢柱周边支承单层柱面网壳）整体结构进行了强震下弹塑性地震响应分析。分析中考虑了几何和材料双重非线性影响,获得了节点位移响应、杆件塑性铰的分布特征和结构的整体变形与失效形态,并评定了整体结构在强震下的极限承载力与失效类型。结果表明：该结构在强震下的失效界限地震加速度峰值为1260gal,最大竖向变形为短向跨度的1/163,满足＂避难与救灾建筑结构＂的抗震性能设防要求;结构的失效类型为动力失稳破坏,临界失效时出现塑性铰的杆件较少,结构塑性发展程度不充分;由整体稳定控制的单层柱面网壳在满足稳定承载力的要求下具有较大的抗震潜能。     

Interior wide beam-column connections in existing RC frames subjected to lateral earthquake loading

The seismic performance of two RC interior wide beam-column connections representative of existing frames designed and detailed according to past construction practices in the moderate-seismicity Mediterranean area was investigated experimentally. The specimens were subjected to axial loads, moderate levels of gravity loading and cyclic displacements up to failure. The specimens exhibited a “strong column-weak beam” type of flexural yielding mechanism. The wide beams did not reach the expected capacities corresponding to the formation of a full-width plastic hinge. The wide-beam longitudinal bars exhibited significant slippage, and the transverse beams underwent severe torsion cracking and even failure; this caused severe pinching in the load versus displacement hysteretic loops and exacerbated the intrinsic flexibility of this type of connection. The average drift ratios at first yielding of the wide beam longitudinal reinforcement and at failure were 2.7 and 4.5%, respectively. The displacement ductility ratio was about 2.8. The ultimate energy dissipation capacity of each specimen—obtained by dividing the total plastic strain energy by the product of the yield load and yield displacement—was approximately 9, which is about one fourth of the value recommended for providing adequate seismic performance. Finally, a simple approach is suggested for prediction of the bending capacity of existing connections.     

AN ANALYSIS OF THE SEISMIC CHARACTERISTICS OF STEEL-JACKETED CIRCULAR BRIDGE COLUMNS

The current approach for seismic retrofit of deficient bridge columns in California involves extensive use of steel jacketing. In this paper, the influence of steel jacketing on the lateral response of circular bridge columns is studied; particularly, the enhancement of the ultimate compressive strain of concrete, the increase in curvature ductility capacity and the increase in lateral stiffness are investigated. The current steel jacket thickness used in California is shown to enhance the ultimate compressive strain of concrete by 4–9 times the spalling strain of unconfined concrete. For larger steel jacket thickness, the ultimate limit state of steel-jacketed columns may be governed by the low-cycle fatigue fracture of the longitudinal reinforcement instead of the ultimate compressive strain of concrete. Steel jacketing is also expected to increase significantly the lateral stiffness of columns if full-height steel jackets are used. The increase in lateral stiffness of flexural columns (3⩽L/D⩽9) is estimated to be 35–60 per cent using current jacket thickness. Inelastic dynamic analyses of steel-jacketed columns using ground motions recorded during the 1989 Loma Prieta earthquake indicated that the current steel jacket thickness provides adequate protection against the damage potential of the ground motions with comparable spectral acceleration as that specified in current design spectra, and the damage sustained by the steel-jacketed column is likely to be repairable.     

钢筋混凝土桥墩刚度和强度折减系数确定

目的是解析地预测钢筋混凝土桥墩在反复荷载作用下的非线性滞回特性。使用实验中得到的力一位移滞回曲线,对随轴压比,配筋率和配箍率的变化而变化的刚度和强度折减系数,进行了回归分析,并提出了其表达式。按照提出的理论力一位移滞回模型,能够预测现存钢筋混凝土桥墩的刚度和强度折减情况。     

FRP约束混凝土矩形短柱的延性分析

将已有混凝土柱延性的定义、纤维截面模型及等效塑性铰方法应用于纤维增强复合材料(FRP)约束矩形混凝土试件的延性分析,通过FRP约束矩形混凝土应力-应变关系考虑FRP约束的影响,计算结果与试验结果符合较好。同时,对FRP约束混凝土矩形柱延性的主要影响参数进行了分析,该方法可以进一步用于FRP约束混凝土试件延性的定量参数研究。     

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。     

Efficiency of employing fiber-based finite-length plastic hinges in simulating the cyclic and seismic behavior of steel hollow columns compared with other common modelling approaches

The accuracy and efficiency of the modelling techniques utilized to model the nonlinear behavior of structural components is a significant issue in earthquake engineering. In this study, the sufficiency of three different modelling techniques that can be employed to simulate the structural behavior of columns is investigated. A fiber-based finite length plastic hinge (FB-FLPH) model is calibrated in this study. In order to calibrate the FB-FLPH model, a novel database of the cyclic behavior of hollow steel columns under simultaneous axial and lateral loading cycles with varying amplitudes is used. By employing the FB-FLPH model calibrated in this study, the interaction of the axial force and the bending moment in columns is directly taken into account, and the deterioration in the cyclic behavior of these members is implicitly considered. The superiority of the calibrated FB-FLPH modelling approach is examined compared with the cases in which conventional fiber-based distributed plasticity and concentrated plasticity models are utilized. The efficiency of the enumerated modelling techniques is probed when they are implemented to model the columns of a typical special moment frame in order to prove the advantage of the FB-FLPH modelling approach.

     

Effects of panel zone yielding on seismic behavior of welded-flange-plate connections

In this study, effects of panel zone yielding on the seismic performance of welded-flange-plate (WFP) connections are investigated. In this work, four full-scale beam-to-column connections were used to run the experiments under cyclic loading. The obtained results can potentially lead to a better understanding of the influence of the panel zone inelastic shear deformation on the cyclic behavior of WFP connections for external joints in steel moment resisting frames (SMRFs). The main parameter in the testing program was the panel zone strength having a wide variation to gain the different levels of panel zone yielding. Results showed that all specimens had a high connection rotation capacity to satisfy the requirements of special moment frame connections. However, specimens with different panel zone strengths could provide the different amount of energy dissipation. Severe beam buckling was followed by tearing along the k-line region of the beam in the plastic hinge location, as well as tearing of the beam at the nose of the bottom flange plates which were both observed as a predominant failure mode in the specimens with a stronger panel zone. However, specimens with weak panel zone could develop a significant plastic rotation without causing any major problem to the beam-to-column connection groove welds. Based on mentioned observations and considering the effect of panel zone yielding because of different panel zone strengths on the hysteresis behavior of specimens, failure modes, plastic rotation capacity, and energy dissipation, some modifications were proposed for design requirements of the panel zone strength.