Peak displacement patterns for the performance-based seismic design of steel eccentrically braced frames

Performance-based seismic design(PBSD) aims to assess structures at different damage states. Since damage can be directly associated to displacements, seismic design with consideration of displacement seems to be logical. In this study, simple formulae to estimate the peak floor displacement patterns of eccentrically braced frames(EBFs) at different performance levels subjected to earthquake ground motions are proposed. These formulae are applicable in a PBSD and especially in direct displacement-based design(DDBD). Parametric study is conducted on a group of 30 EBFs under a set of 15 far field and near field accelerograms which they scaled to different amplitudes to adapt various performance levels. The results of thousands of nonlinear dynamic analyses of EBFs have been post-processed by nonlinear regression analysis in order to recognize the major parameters that influence the peak displacement pattern of these frames. Results show that suggested displacement patterns have relatively good agreement with those acquired by an exact nonlinear dynamic analysis.

Inelastic displacement ratio of low-to mid-rise BRBFs designed under variable levels of seismicity

Buckling-restrained braces(BRBs)have shown their capability to provide building structures with stiffness,strength,and ductility.Estimating the seismic drifts of buckling-restrained braced frames(BRBFs)is an important design step to control structural and non-structural damage.In current practice of seismic design,the estimation of seismic drifts of BRBFs is performed by using empirical calculations that are independent upon either the type of the structural system or the design level of seismicity.In these empirical calculations,the seismic drifts are estimated by amplifying the reduced elastic drifts obtained under design lateral loading with a displacement amplification factor(DAF).The value of DAF is considered equal to the product of the response modification factor R and the inelastic displacement ratioρ.The goal of the current research is to assess the value ofρfor low-to mid-rise BRBFs designed under low and high levels of seismicity.This goal has been achieved by conducting a series of elastic and inelastic time-history analyses pertaining to an ensemble of earthquake records on 3-,6-and 9-story BRBFs.The results indicate that theρ-ratio increases with an increase in design seismic intensity and an increase in experienced inelasticity.The range ofρfor low seismicity designs ranges from 0.63 to 0.9,while for high seismicity designs this range stretches from 0.83 to 1.29.It has been found that the consideration of a generalρ-ratio of 1.0 is a reasonable estimation for the design of the BRBFs considered in this study.

Application of consequence-based design criteria in regions of moderate seismicity

Current design criteria and prineiples of earthquake engineering design are reviewed,including safety factors, probabilistic approach,and two-level and muhi-level functional design ideas.The modern multi-functional idea is discussed in greater details.When designing a structure,its resistance to and the intensity of the earthquake action are considered. The consequence of failure of the structure is considered only through a rough and empirical factor of importance,ranging usually from 1.0 to 1.5.This paper suggests a method of'consequence-based design,'which considers the consequences of malfunctioning instead of simply an importance factor.The main argument for this method is that damage to a structure located in different types of societies may have very different consequences,which are depeudant on its value and usefulness to the society and the seismicity in the region.

Constant-ductility strength demand spectra for seismic design of structures

In displacement-based seismic design, constant-ductility strength demand spectra （CDSDS） are very useful for preliminary design of new structures where the global displacement ductility capacity is known. The CDSDS can provide the required inelastic lateral strength of new structures from the required elastic lateral strength. Based on a statistical study of nonlinear time-history for an SDOF system, the mean CDSDS corresponding to four site conditions are presented and approximate expressions of the inelastic spectra are proposed, which are functions of the structural period and ductility level. The effects of site conditions, structural period, level of ductility, damping and post-yield stiffness of structures on CDSDS are also investigated. It is concluded that site conditions, ductility level and structural period have important effects on the CDSDS and damping, post-yield stiffness effects are rather complex and of minor importance. The damping, post-yield stiffness effects depend on both the level of ductility and the natural period of structures.

基于现行规范的型钢混凝土柱侧力-位移曲线预测方法研究

型钢混凝土(steel reinforced concrete,SRC)柱因承载力高、刚度大和耐久性能好等优点而被广泛应用于高层结构中。然而,现阶段SRC结构设计普遍基于承载力设计法,基于性能的抗震设计法在SRC结构方面的应用仍待完善。作为基于性能(位移)的抗震设计方法中最重要的一环,现阶段SRC构件的侧力-位移关系评估缺乏规范指导。基于美国现行规范ASCE/SEI 41-17中针对钢筋混凝土构件的相关规定,结合SRC构件的受力与变形特征,提出了发生弯曲破坏的SRC柱在地震作用下的侧力-位移曲线建模方法。该曲线采用约束混凝土强度计算SRC构件的峰值承载力;使用刚度折减法考虑型钢与混凝土之间的黏结滑移;基于SRC构件的变形特征考虑了纵向受力钢材端部滑移对柱顶侧移的贡献。最后,通过与28根SRC柱试验结果的比较证明了曲线的适用性。计算结果表明:所提出的曲线能合理预测发生弯曲破坏的SRC柱的开裂荷载、开裂位移、峰值荷载、峰值位移和强度退化,可为SRC结构的性能化抗震设计提供参考。

直接基于位移的非规则减隔震桥梁抗震设计

根据性能抗震设计的思想,提出一种高低墩减隔震桥梁直接基于位移的抗震设计方法,该方法可设计各桥墩配筋率及支座参数,使纵桥向地震作用下各墩保持弹性、各支座达到相同的损伤状态,并实现地震剪力或墩底弯矩的均匀分配。以某工程背景桥梁为例进行直接基于位移的设计,并建立有限元模型进行非线性时程分析,比较两者结果,验证所提方法的有效性。此外,对比了剪力均匀和墩底弯矩均匀这两种地震力分配方式下桥梁的地震响应。对于各墩采用相同截面尺寸和配筋的减隔震桥梁,当按照该方法进行设计时,推荐采用墩底弯矩均匀的地震力分配方式,该分配方式能更好的发挥材料性能、节约工程造价,但同时应保证墩高较低的桥墩有足够的抗剪能力。

近断层强震作用下城市组合钢板梁桥RC桥墩震后可修复性研究

为研究近断层地震动作用下城市组合钢板梁桥钢筋混凝土(Reinforced Concrete,RC)桥墩的震后可修复性,以8度区、Ⅲ类场地的城市组合钢板梁桥为背景,分析在E2水平近断层地震动作用下RC桥墩的残余位移比。采用正交试验设计方法,按现行规范试设计25座典型三跨连续城市组合钢板梁桥样本,采用验证后的有限元建模方法建立全桥动力模型,分析跨径、桥宽和桥墩长细比对RC桥墩残余位移比的影响。并以样本中某3×35 m三跨连续组合钢板梁桥为研究对象,进行有限元模型参数分析,研究RC桥墩设计参数(轴压比、长细比、纵筋配筋率和体积配箍率)对残余位移比的影响。结果表明:组合钢板梁桥跨径、桥宽和桥墩长细比均对RC桥墩残余位移比有明显影响;RC桥墩残余位移比与轴压比和长细比成正比关系,而与纵筋配筋率和箍筋体积配箍率成反比关系;为提高城市组合钢板梁桥在近断层地震动作用下桥墩的可修复性,建议RC桥墩轴压比不超过10%,长细比不超过8,纵筋配筋率取1.5%~2.0%,箍筋体积配箍率取0.6%~1.2%。

自由场位移修正值R在地下结构抗震设计中的应用

地下结构抗震设计中,土和结构之间的相对位移普遍采用反应位移法和非线性动力时程分析法。但是反应位移法参数过多、求解过程繁琐,非线性动力时程分析法耗时较长,不利于设计工作者快速得到可靠设计参数。本文基于地铁结构抗震设计的案例,在传统设计方法之外,研究了一种简便计算方法,即通过自由场位移修正值R快速准确地计算出土和结构之间的相对位移,并将该方法与传统方法进行了对比,发现计算结果较接近,为地下结构的抗震设计提供了参考。

Improved design of special boundary elements for T-shaped reinforced concrete walls

This study examines the design provisions of the Chinese GB 50011-2010 code for seismic design of buildings for the special boundary elements of T-shaped reinforced concrete walls and proposes an improved design method. Comparison of the design provisions of the GB 50011-2010 code and those of the American code ACI 318-14 indicates a possible deficiency in the T-shaped wall design provisions in GB 50011-2010. A case study of a typical T-shaped wall designed in accordance with GB 50011-2010 also indicates the insufficient extent of the boundary element at the non-flange end and overly conservative design of the flange end boundary element. Improved designs for special boundary elements ofT-shaped walls are developed using a displacement-based method. The proposed design formulas produce a longer boundary element at the non-flange end and a shorter boundary element at the flange end, relative to those of the GB 50011-2010 provisions. Extensive numerical analysis indicates that T-shaped walls designed using the proposed formulas develop inelastic drift of 0.01 for both cases of the flange in compression and in tension.

Seismic analysis of cantilever earth retaining walls embedded in dry sand by simplified approaches and finite element method

In engineering practice simplified methods are essential to the seismic design of embedded earth retaining walls,as fullydynamic numerical analyses are costly,time-consuming and require specific expertise.Recently developed pseudostatic methods provide earth stresses and internal forces,even in those cases in which the strength of the soil surrounding the structure is not entirely mobilised.Semiempirical correlations or Newmark sliding block method provide an estimate of earthquake-induced permanent displacements.However,the use of these methods is hindered by uncertainties in the evaluation of a few input parameters,affecting the reliability of the methods.This study uses 1 D site response analyses and 2 D fully-dynamic finite element analyses to show that simplified methods can provide a reasonable estimate of the maximum bending moment and permanent displacements for stiff cantilever walls embedded in uniform sand,providing that a few input parameters are evaluated through semiempirical correlations and a simple 1 D site response analysis.

The Analysis of Accelerograms for the Earthquake Resistant Design of Structures

In this paper, the analysis of ground motions (displacements, velocities and accelerations) has been performed focused to the seismic design. The relationships between the peak ground acceleration (PGA), the peak ground velocity (PGV), the peak ground displacement (PGD) and the bracketed duration, with the earthquake magnitude, are presented and their validity and applicability for seismic design is discussed. Finally, the dominant periods of the ground motions (displacement, velocity and acceleration) are obtained from their Fourier Spectrum. Their validity and applicability for the seismic design is discussed also. The results presented in this paper show that the relationships that exist between the important parameters: PGA, PGV, PGD and duration;and the earthquake magnitude, allow the prediction of the values for these parameters, in terms of the magnitude for future strong motions. These predictions can be very useful for seismic design. Particularly, the prediction of the magnitude associated to the critical acceleration, because the earthquakes with magnitude greater than this critical magnitude can produce serious damages in a structure (even its collapsing). The application of the relationships obtained in this paper must be very careful, because these equations are dependent on the source area, location and type of structure. The dominant periods of the ground motions (displacement, velocity and acceleration) that are computed and presented in this paper, are also important parameters for the seismic design, because recent studies have shown that the earthquake shaking is more destructive on structures having a natural period around some of these dominant periods. These parameters must also be handled with caution, because they show dependence with the source area, location and type of structure.

考虑倾滑断层上覆土层影响的地表位移概率地震危险性分析

地铁隧道等长线性工程结构因功能需要,不可避免地跨越活动断层,一旦断层错动将可能产生严重后果。断层错动引发的地表位移可作为近地表长线性工程结构抗断设计的输入依据。其中,覆盖土层对地表位错量的影响不可忽略。鉴于此,采用有限元数值模拟方法,基于试验校准模型,研究了基岩断层错动下上覆土层厚度和土体性质等因素对地表位移的影响,提出了考虑倾滑断层上覆土层影响的地表位移概率地震危险性分析方法。研究结果可为跨断层近地表长线性工程结构抗断设计提供支撑。

附加支撑-粘滞阻尼器体系的消能减震结构直接基于位移的抗震设计方法

目前附加粘滞阻尼器的消能减震结构基于性能的抗震设计方法一方面为了确保实现性能目标需要进行复杂的迭代计算,另一方面没有具体给出阻尼器相应支撑刚度的计算方法。为此,本文在直接基于位移的抗震设计方法基础上,根据附加粘滞阻尼器所提供的阻尼力、层间剪力与附加等效粘滞阻尼比之间的关系以及利用支撑-阻尼器串联结构体系可以视为截断频率与支撑刚度和阻尼器阻尼系数相关的一阶滤波器这一思路,提出了一种针对附加粘滞阻尼器结构直接基于位移进行抗震设计的设计方法,并且进一步通过非线性时程分析计算验证了该设计方法的可行性。

自复位消能摇摆模块复合钢框架协同抗侧机理与抗震加固设计方法

该文采用自复位消能摇摆模块对既有抗弯钢框架进行加固,以减小其震后残余位移、提升其地震抗倒塌性能、避免其因层间变形集中而发生薄弱层倒塌破坏。基于分布参数连续体模型,揭示了自复位消能摇摆模块与钢框架结构的协同抗侧机理,分析了自复位消能摇摆模块抗弯刚度及抗侧刚度对结构层间变形模式、摇摆模块和钢框架内力需求的影响。提出了利用自复位消能摇摆模块加固既有抗弯钢框架基于位移的抗震设计方法,给出了具体设计流程。基于提出的设计方法对某三层抗弯钢框架进行加固设计。静力推覆分析和动力分析结果表明,自复位消能摇摆模块可有效减小既有抗弯钢框架结构的震后残余变形、使结构获得均匀的层间变形模式。加固后的结构在设计地震下可达到预期设计目标。

适用于我国抗震设计规范的位移设计谱方法

由于位移参数更能科学、直观地描述建筑结构的抗震水平,基于位移的抗震设计法已成为地震工程领域研究的热点。位移设计谱是基于位移的抗震设计方法中确定地震作用的重要参考依据,但我国现行抗震设计规范(后简称规范)中尚未有关位移设计谱的相关规定。为便于实际应用,该研究基于实际地震动位移反应谱的特征,提出一种能够适用于规范的位移设计谱法;该方法未对规范参数作任何调整,仅增加了控制点周期T_(D)和峰值地面位移(peak ground displacement,PGD)两个参数,且所增参数与规范相关参数具有一一对应的关系。鉴于基于力的抗震设计在工程实践中仍占据主导地位,该研究给出的位移设计谱可与加速度设计谱互相转化。为避免与规范矛盾,当周期小于5T_(g)(场地特征周期)时,加速度设计谱与规范加速度设计谱一致;在长周期段,位移设计谱能反映地震动平均位移反应谱的主要特征,且加速度设计谱与平均加速度反应谱吻合。试验表明,该研究工作能服务于基于位移的抗震设计方法的工程应用,为规范中设计谱相关内容的修订提供指导意见。

基于位移的模块式加筋土挡墙抗震设计方法研究

性态设计是支挡结构工程抗震设计的前沿科学问题。以模块式加筋土挡墙为试验对象,通过振动台试验,探究模块式加筋土挡墙的变形模式;收集归纳挡土墙位移计算方法,分析不同破坏模式下屈服加速度系数分布规律;对比不同计算方法计算值与实测值的一致性。研究结果表明:挡墙的位移模式为平移与转动耦合,且以转动为主;不同破坏模式下安全系数法求解的屈服加速度系数均随输入加速度幅值增大而减小,简便方法和能量法所得屈服加速度系数为常数;将屈服加速度系数代入不同位移计算方法对比,提出在不同峰值加速度时,可分别采用Richards and Elms上限法(0.4g以下)、Cai and Bathurst平均上限法(0.4g~0.6g)、Newmark上限法(0.6g~0.8g)、Whitman and Liao平均拟合法(0.8g~1.0g)进行位移计算。最后,对模块式加筋土挡墙的抗震设计流程进行归纳。

永久位移型地震动对建筑隔震设计影响初探

以自振周期大于2s的某实际钢结构建筑为研究对象,分别在设防地震和罕遇地震下,输入含与不含永久位移型地震动,分别计算得到隔震结构和非隔震结构的非线性地震反应。通过对比含与不含永久位移型地震动作用下的上部结构地震反应,进而分析永久位移对该类型建筑隔震设计的影响。结果表明:无论地震动输入是否含永久位移,与非隔震结构相比,隔震结构的层间剪力和倾覆力矩均显著降低,且降低幅度随着楼层的升高而增大;在含永久位移型地震动与不含永久位移型地震动分别作用下,各楼层层间剪力比值与倾覆力矩比值无明显差别,比值基本在1左右;在两类地震动作用下,计算得到的减震系数差别不超过10%;在两类地震动作用下,隔震层支座平均最大拉、压应力及位移相差不大,永久位移型地震动作用的放大效应不明显。

具有位移放大功能的摩擦自复位支撑及其在双柱式桥墩中的应用

针对传统自复位支撑在低强度地震作用下难以充分耗能的问题,基于桥式放大工作机理提出了一种具有位移放大功能的扭转摩擦自复位支撑(self-centering displacement amplified rotational friction brace,SC-DARFB)。介绍了其基本构造、工作原理,给出了其理论恢复力模型,通过数值模拟研究了不同初始放大角度下支撑的力学性能,并对附加SC-DARFB的双柱式桥墩结构进行地震响应分析。结果表明:理论恢复力模型能够有效描述SC-DARFB的滞回性能,与数值模拟结果吻合较好;随着初始放大角度的减小,SC-DARFB起滑荷载、最大荷载和耗能能力均有显著提高,且具有较强的自复位能力;合理的设计能够确保支撑的拉、压最大荷载比满足美国钢结构规范限值1-3,基于此原因建议SC-DARFB采用倒V型对称布置形式;将SC-DARFB应用于双柱式桥墩结构,能够有效减小双柱式桥墩结构的最大墩顶位移及震后桥墩残余位移角,通过减小SC-DARFB初始放大角度能够进一步提高双柱式桥墩的减震耗能能力。

液化场地钢筋混凝土桥墩残余位移分析

为探究场地液化对钢筋混凝土(Reinforced Concrete,RC)桥墩残余位移影响机理,首先基于OpenSees有限元计算平台阐述了液化场地-结构有限元数值建模方法,通过与离心机试验结果对比验证了建模方法的可靠性。其次以一座实际工程中的单桩单柱式桥墩为原型,发展了液化场地-RC桥墩建模方法,建立非液化场地-桩-RC桥墩数值模型(模型1)和液化场地-桩-RC桥墩数值模型(模型2),对两模型输入近断层地震动进行非线性动力时程分析。讨论了模型2场地液化情况,并对模型1与模型2的场地位移、桩身位移、桩身最大屈服曲率延性系数、墩顶残余位移及墩顶残余位移随地震动峰值加速度(PGA)变化情况进行对比。结果表明:地震过程中,模型2场地各深度出现不同程度液化现象;且土体中上部均达完全液化状态,土体水平极限抗力损失最大,对桩身侧向约束严重降低。液化导致场地震后残余位移显著增加,场地对桩身震后约束增强,导致桩顶残余位移增大。另外,由于地震过程中中上部液化土体对桩身侧向约束严重降低导致桩身塑性变形显著增大、桩身最大塑性变形位置明显下移以及桩身塑性变形区扩展,引起桩身更为严重的塑性损伤,进而增大桩顶残余位移。液化场地震后残余位移增大与地震过程中对桩身约束降低共同导致桩顶残余位移增加,经桥墩放大传递至墩顶,引起墩顶残余位移增大。液化场地墩顶残余位移随PGA增大速度明显大于非液化场地桥墩,主要原因可能是随PGA增加,场地液化程度加深。

SMA负刚度双曲面减震装置的优化及抗震性能研究

SMA负刚度双曲面隔震装置的试验结果表明:绕支座布置的SMA索在支座转弯处不能自由滑动,导致SMA索的受力与设计目标严重不符。为解决上述问题,拟提出一种优化升级的SMA负刚度双曲面减震装置。基于某一连续梁桥,比较研究了正刚度、零刚度与该负刚度装置的抗震性能。结果表明:该优化装置具有良好的耗能能力、自恢复性能以及强震下的限位能力。与正刚度装置及零刚度装置相比,该装置具有更好的抗震性能。