Damping coefficients for near-fault ground motion response spectra

Damping coefficients are frequently used in earthquake engineering as a simple way to adjust the pseudo-acceleration or displacement response spectra associated with a viscous damping ratio of 5% to the higher values of viscous damping needed for design of structures equipped with base isolation and/or supplemental energy dissipation devices. In this study, damping coefficients for the single-degree-of-freedom system subjected to near-fault ground motions are calculated for a large range of periods and damping levels. The results indicate that damping coefficients proposed in design codes and previous studies, based primarily on far-field ground motion records, tend to not be conservative for near-fault seismic excitations. A new approach is recommended for the derivation of damping coefficients appropriate for engineering analysis and design in the immediate vicinity of the earthquake fault. This includes the normalization of the period axis with respect to the duration of the ground velocity pulses recorded in the near-fault region. The pulse duration is controlled by the rise time on the fault plane and scales directly with earthquake magnitude.     

强震下斜交桥地震反应与减隔震性能分析

针对斜交桥在破坏性地震中发生破坏和损伤的突出问题,采用铅芯橡胶支座(LRB)进行隔震和滞回耗能。基于OpenSees平台建立了不同斜度的传统非隔震和全桥采用LRB隔震的4跨斜交连续梁桥动力分析模型,沿2个水平方向输入远场地震动和具有向前方向性效应、滑冲效应以及无速度脉冲效应的近断层地震动,并进行非线性时程计算,研究桥墩和挡块的损伤状态、主梁旋转度、碰撞力与斜交桥斜度的关系以及LRB对斜交桥抗震性能的影响。结果表明:向前方向性效应和滑冲效应的脉冲型地震动作用下的斜交桥地震反应和损伤明显大于无速度脉冲近断层和远场地震动作用; 采用LRB隔震后,明显降低了固定墩的地震损伤,桥墩位移减震率可达到50%以上; LRB隔震桥主梁与挡块的间隙宜结合桥梁的地震风险和设计位移进行确定。     

Damping coefficients for soil–structure systems and evaluation of FEMA 440 subjected to pulse-like near-fault earthquakes

In this study, attempts are made to investigate the effects of inertial soil–structure interaction (SSI) on damping coefficients subjected to pulse-like near-fault ground motions. To this end, a suit of 91 pulse-like near-fault ground motions is adopted. The soil and superstructure are idealized employing cone model and single-degree-of-freedom (SDOF) oscillator, respectively. The results demonstrate that soil flexibility reduces and amplifies the damping coefficients for structural viscous damping levels higher and lower than 5%, respectively. The coefficients reach one for both acceleration and displacement responses in cases of dominant SSI effects. The effect of structure dimensions on damping confidents are found insignificant. Moreover, damping coefficients of displacement responses are higher than those of acceleration responses for both fixed-base and flexible-base systems. Evaluation of damping correction factor introduced by FEMA 440 shows its inefficiency to predict acceleration response of soil–structure systems under pulse-like near-fault ground motions. Soil flexibility makes the damping correction factor of moderate earthquakes more pronounced and a distinctive peak value is reported for cases with dominant SSI effects.     

Experimental and analytical studies on the performance of hybrid isolation systems

This paper presents experimental and analytical results on the seismic response of a rigid structure supported on isolation systems that consist of either lead rubber or sliding bearings. Shake table tests are conducted with various levels of isolation damping that is provided from the bearings and supplemental viscous fluid dampers. The table motions originated from recorded strong ground motions that have been compressed to the extent that the mass of the model structure corresponds to the mass of a typical freeway overcrossing. Experimental data are used to validate mechanical idealizations and numerical procedures. The study concludes that supplemental damping is most effective in suppressing displacements of rigid structures with moderately long isolation periods (T_I≤3 sec) without affecting base shears. Friction damping is most effective in suppressing displacement amplifications triggered by long duration pulses—in particular, pulses that have duration close to the isolation period. Copyright © 2001 John Wiley & Sons, Ltd.     

Collapse risk and residual drift performance of steel buildings using post-tensioned MRFs and viscous dampers in near-fault regions

The potential of post-tensioned self-centering moment-resisting frames (SC-MRFs) and viscous dampers to reduce the collapse risk and improve the residual drift performance of steel buildings in near-fault regions is evaluated. For this purpose, a prototype steel building is designed using different seismic-resistant frames, i.e.: moment-resisting frames (MRFs); MRFs with viscous dampers; SC-MRFs; and SC-MRFs with viscous dampers. The frames are modeled in OpenSees where material and geometrical nonlinearities are taken into account as well as stiffness and strength deterioration. A database of 91 near-fault, pulse-like ground motions with varying pulse periods is used to conduct incremental dynamic analysis (IDA), in which each ground motion is scaled until collapse occurs. The probability of collapse and the probability of exceeding different residual story drift threshold values are calculated as a function of the ground motion intensity and the period of the velocity pulse. The results of IDA are then combined with probabilistic seismic hazard analysis models that account for near-fault directivity to assess and compare the collapse risk and the residual drift performance of the frames. The paper highlights the benefit of combining the post-tensioning and supplemental viscous damping technologies in the near-source. In particular, the SC-MRF with viscous dampers is found to achieve significant reductions in collapse risk and probability of exceedance of residual story drift threshold values compared to the MRF.     

近断层脉冲型地震动作用下高层摩擦摆基础隔震结构的减震性能研究

近断层地震动中长周期、短持时和高能量的加速度脉冲将对高层摩擦摆基础隔震结构的减震性能产生不利影响,考虑土-结构相互作用（SSI效应）后的隔震结构将产生动力耦合效应,可能进一步放大隔震结构地震响应。为此,通过一幢框架-核心筒高层摩擦摆基础隔震结构的非线性地震响应分析,考察近断层脉冲型地震动作用下框架-核心筒摩擦摆基础隔震结构的层间位移角、楼层加速度和隔震层变形等响应规律,揭示隔震体系的损伤机理。基于集总参数SR （sway-rocking）模型,分析不同场地类别与不同地震动类型对隔震体系动力响应影响规律。结果表明:高层摩擦摆基础隔震结构在近断层脉冲型地震动作用下的减震效果相比普通地震动减震效果变差,楼层剪力、层间位移角和隔震层变形等超越普通地震动作用下的1.5倍;对于Ⅲ和Ⅳ类场地类别,考虑SSI效应使隔震结构的地震响应进一步放大,弹塑性层间位移角随着土质变软增大尤为明显。     

Effects of supplemental viscous damping on seismic response of asymmetric-plan systems

Coupling between lateral and torsional motions may lead to much larger edge deformations in asymmetric-plan systems compared to systems with a symmetric plan. Supplemental viscous damping has been found to be effective in reducing deformations in the symmetric-plan system. This investigation examined how supplemental damping affects the edge deformations in asymmetric-plan systems. First, the parameters that characterize supplemental viscous damping and its plan-wise distribution were identified, and then the effects of these parameters on edge deformations were investigated. It was found that supplemental damping reduces edge deformations and that reductions by a factor of up three are feasible with proper selection of system parameters. Furthermore, viscous damping may be used to reduce edge deformations in asymmetric-plan systems to levels equal to or smaller than those in the corresponding symmetric-plan system. © 1998 John Wiley & Sons, Ltd.     

Effect of viscous damping devices on the response of seismically isolated structures

Viscous and other damping devices are often used as elements of seismic isolation systems. Despite the widespread application of nonlinear viscous systems particularly in Japan (with fewer applications in the USA and Taiwan), the application of viscous damping devices in isolation systems in the USA progressed intentionally toward the use of supplementary linear viscous devices due to the advantages offered by these devices. This paper presents experimental results on the behavior of seismically isolated structures with low damping elastomeric (LDE) and single friction pendulum (SFP) bearings with and without linear and nonlinear viscous dampers. The isolation systems are tested within a six‐story structure configured as moment frame and then again as braced frame. Emphasis is placed both on the acquisition of data related to the structural system (drifts, story shear forces, and isolator displacements) and on non‐structural systems (floor accelerations, floor spectral accelerations, and floor velocities). Moreover, the accuracy of analytical prediction of response is investigated based on the results of a total of 227 experiments, using 14 historic ground motions of far‐fault and near‐fault characteristics, on flexible moment frame and stiff braced frame structures isolated with LDE or SFP bearings and linear or nonlinear viscous dampers. It is concluded that when damping is needed to reduce displacement demands in the isolation system, linear viscous damping results in the least detrimental effect on the isolated structure. Moreover, the study concludes that the analytical prediction of peak floor accelerations and floor response spectra may contain errors that need to be considered when designing secondary systems. Copyright © 2014 John Wiley & Sons, Ltd.     

Quantification of ground-motion parameters and response spectra in the near-fault region

This study focuses on the characteristics of near-fault ground motions in the forward-direction and structural response associated with them. These ground motions are narrow-banded in nature and are characterized by a predominant period at which structures excited by them are severely affected. In this work, predominant period is defined as the undamped natural period of a single-degree-of-freedom (SDOF) oscillator at which its 5% damped linear elastic pseudo-spectral velocity (PSV) contains a clear and dominant peak. It is found that a linear relationship exists between predominant period and seismic moment. An empirical equation describing this relationship is presented by using a large set of accelerograms. Attenuation equations are developed to estimate peak ground velocity (PGV) as a function of earthquake magnitude and source-to-site distance. In addition, a predictive equation for spectral shapes of PSV (i.e., PSV normalized by PGV) is presented as a continuous function of the undamped natural period of SDOF oscillators. The model is independent of PGV, and can be used in conjunction with any available PGV attenuation relation applicable to near-fault ground motion exhibiting forward-directivity effects. Furthermore, viscous damping of the SDOF is included in the model as a continuous parameter, eliminating the use of so-called damping correction factors. Finally, simple equations relating force reduction factors and displacement ductility of elasto-plastic SDOF systems are presented.     

近断层双向水平地震作用下隔震结构地震反应

近断层地震动脉冲特性在2个水平分量上具有差异,采用平方和开方法分析了近断层脉冲地震动双向地震作用下基础隔震结构和组合隔震结构的隔震层位移,并与近断层脉冲单向地震作用进行了对比分析,结果表明:若仅地震动加速度峰值大的分量或2个方向分量均存在明显速度脉冲,则产生的隔震层位移大于单向地震动;若仅地震动加速度峰值小的分量存在明...     

Interstory drift ratio of building structures subjected to near-fault ground motions based on generalized drift spectral analysis

This paper focuses on the interstory drift ratio (IDR) demands of building structures subjected to near-fault ground motions having different impulsive characteristics based on generalized interstory drift spectral analysis. The near-fault ground motions considered include the idealized simple pulses and three groups of near-fault ground motions with forward directivity pulses, fling-step pulses and without velocity pulse. Meanwhile, the building systems are equivalently taken as shear-flexural beams with representative lateral stiffness ratios. The IDR distribution of continuous beams subjected to three groups of near-fault ground motions is acquired. It is illustrated that the maximum IDR shifts from the upper half to the lower half of buildings with an increase in lateral stiffness ratio. For long-period systems, the average IDR under impulsive ground motions is significantly greater than that under non-pulse motions. Finally, for moment-resisting frame buildings the forward directivity pulses amplify the drift response of higher modes, while the fling-step pulses excite primarily their contribution in the first mode and generate large deformation in the lower stories. The essential reason for this phenomenon is revealed according to the distinct property of near-fault impulsive ground motions and generalized drift spectral analysis.     

摩擦摆支座在单层球面网壳结构中的隔震分析

将摩擦摆支座（FPS）应用于单层球面网壳结构的隔震，给出了隔震网壳结构的运动方程。通过对比分析不同强度地震动输入条件下的结构动力响应特征，考察了FPS支座应用于网壳结构隔震的有效性和适用性。研究结果表明，在不同强度的地震动作用下，隔震结构的节点加速度峰值和杆件轴力峰值都得到了有效控制，且地震动强度越大，控制效果越好。     

近断层地震动下设置BRB的双向减隔震桥梁地震反应

在近断层地震动下桥梁结构将发生较大反应,减隔震设计是减轻地震损伤的重要手段。提出了在桥梁双柱墩横桥向设置防屈曲支撑(BRB),在纵桥向设置铅芯橡胶支座(LRB)的双向减隔震体系。利用Midas Civil软件建立3种不同减隔震方式的桥梁结构模型:LRB仅单向,LRB双向与LRB联合BRB,运用非线性时程分析方法计算了桥墩反应(墩顶侧移角、残余位移角和曲率延性)、LRB支座变形和BRB的耗能特性等。结果表明:在近断层地震动输入下联合设置LRB和BRB的双向减隔震桥梁减震效果明显,相比其它2种方式,能有效降低墩柱的塑性变形及起到保护桥墩的作用。在横桥向,桥墩最大侧移角、残余位移角和最大曲率延性系数都显著降低。     

近断层脉冲型地震动作用下隔震结构地震反应分析

隔震结构在远震场地减震效果良好，但是近断层地震动的明显的长周期速度和位移脉冲运动可能对隔震建筑等长周期结构的抗震性能和设计带来不利影响，需要深入探讨。本文首先讨论近断层地震动的长周期脉冲运动特征，然后以台湾集集地震8条典型近震记录和其它4条常用近震记录以及4条远震记录作为地震动输入，对两幢安装铅芯橡胶隔震支座的钢筋混凝土框架隔震结构进行非线性地震反应时程分析，通过比较探讨了算例计算结果，定量说明隔震结构的近震脉冲效应显著，是隔震设计不容忽视的问题。     

Discussion on “Damping coefficients for near-fault ground motion response spectra”

This discussion is based on the paper by Hubbard and Mavroeidis [1]. In this paper, the authors have presented an interesting study on the effect of near-fault ground motions on the damping coefficients examining single degree of freedom (SDOF) systems. This discussion presents some comments on the results and the conclusions of that paper, which imply that some aspects need further clarification and/or improvement.     

近断层地震动作用下隔震结构的优化设计

首先讨论了近断层脉冲型地震动的特点,并以台湾集集地震实际脉冲型近震记录为地震动输入,应用含潜在约束策略的序列二次规划算法,对安装铅芯橡胶隔震支座的钢筋混凝土框架隔震结构的隔震器参数和上部结构构件截面几何尺寸进行一体化优化设计,然后输入E l Centro(1940)、Taft(1952)地震波对优化后的隔震结构进行地震反应分析。计算结果表明,对考虑脉冲型近断层地震动作用的隔震结构进行参数优化设计后,该隔震结构能同时满足脉冲型和普通非脉冲型近震作用的结构设计需求。     

Effects of near-fault and far-fault ground motions on nonlinear dynamic response and seismic damage of concrete gravity dams

As the forward directivity and fling effect characteristics of the near-fault ground motions, seismic response of structures in the near field of a rupturing fault can be significantly different from those observed in the far field. The unique characteristics of the near-fault ground motions can cause considerable damage during an earthquake. This paper presents results of a study aimed at evaluating the near-fault and far-fault ground motion effects on nonlinear dynamic response and seismic damage of concrete gravity dams including dam-reservoir-foundation interaction. For this purpose, 10 as-recorded earthquake records which display ground motions with an apparent velocity pulse are selected to represent the near-fault ground motion characteristics. The earthquake ground motions recorded at the same site from other events that the epicenter far away from the site are employed as the far-fault ground motions. The Koyna gravity dam, which is selected as a numerical application, is subjected to a set of as-recorded near-fault and far-fault strong ground motion records. The Concrete Damaged Plasticity (CDP) model including the strain hardening or softening behavior is employed in nonlinear analysis. Nonlinear dynamic response and seismic damage analyses of the selected concrete dam subjected to both near-fault and far-fault ground motions are performed. Both local and global damage indices are established as the response parameters. The results obtained from the analyses of the dam subjected to each fault effect are compared with each other. It is seen from the analysis results that the near-fault ground motions, which have significant influence on the dynamic response of dam–reservoir–foundation systems, have the potential to cause more severe damage to the dam body than far-fault ground motions.     

Improving the performance of conventional base isolation systems by an external variable negative stiffness device under near-fault and long-period ground motions

Recent studies have shown that base-isolated objects with long fundamental natural periods are highly influenced by long-period earthquakes. These long-period waves result in large displacements for isolators, possibly leading to exceedance of the allowable displacement limits. Conventional isolation systems, in general, fail to resist such large displacements. This has prompted the need to modify conventional base isolation systems. The current work focuses on the development of an external device, comprising a unit of negative and positive springs, for improving the performance of conventional base isolation systems. This unit accelerates the change in the stiffness of the isolation system where the stiffness of the positive spring varies linearly in terms of the displacement response of the isolated objects. The target objects of the present study are small structures such as computer servers, sensitive instruments and machinery. Numerical studies show that the increase in the damping of the system and the slope of the linear function is effective in reducing the displacement response. An optimal range of damping values and slope, satisfying the stability condition and the allowable limits of both displacement and acceleration responses when the system is subjected to near-fault and long-period ground motions simultaneously, is proposed.

     

Optimized friction pendulum and precast-prestressed pile to base-isolate a Chilean masonry house

In this study friction pendulum system (FPS) bearings and precast-prestressed pile (PPP) isolators are considered as base isolation devices for a Chilean confined masonry house. The house is numerically modeled using a multiple degree-of-freedom approach that is calibrated with experimental data. Dynamic behavior of the FPS and PPP isolators is simulated using analytical formulations based on laboratory testing. Optimization of the isolators is performed using an earthquake that is generated to match the design spectrum for the house based on Chilean seismic code. A non-dominated sorting genetic algorithm (NSGA-II) is applied to carry out the optimization. Seismic response of the base-isolated structure subjected to a suite of ground motions is compared to the performance of the traditionally-constructed structure by means of several performance indices (PIs). Numerical simulations indicate that the PPP isolation system is more effective in reducing the base and structural shear, interstory drift, and floor acceleration of the structure than the FPS isolation system, although both systems result in substantial reductions of the response.     

Evaluation of the damping modification factor for structures subjected to near-fault ground motions

The damping modification factor (DMF) has been extensively used in earthquake engineering to describe the variation of structural responses due to varied damping ratios. It is known that DMFs are dependent not only on structural dynamic properties but also on characteristics of ground motions. DMFs regulated in current seismic codes are generally developed based on far-fault ground motions and are inappropriately used in structural design where pulse-like near-fault ground motions are involved. In this paper, statistical investigation of the DMF is performed based on 50 carefully selected pulse-like near-fault ground motions. It is observed that DMFs for pulse-like ground motions exhibit significant dependence on the pulse period T _p in a specific period range. If the period of the structure in response is close to the pulse period, the DMF attains the same level as that derived from far-fault ground motions; as the period of the structure is considerably larger or smaller than the pulse period T _p , the response reduction effect by the increased damping ratio is generally small, except for large earthquakes with long pulse periods, which exhibit significant reduction of response for structures with periods smaller than T _p . Based on the statistical results of DMFs, the empirical formulas for estimating DMFs for displacement, velocity and acceleration spectra are proposed, the effect of structural period, pulse period and damping ratio are considered in the formulas, and the formulas are designed to satisfy the specific reliability requirement in the period range of 0.1 < T/T _p  < 1, which is of engineering interest.