Three‐dimensional modal pushover analysis of buildings subjected to two components of ground motion,including its evaluation for tall buildings

The modal pushover analysis (MPA) procedure, presently restricted to one horizontal component of ground motion, is extended to three‐dimensional analysis of buildings—symmetric or unsymmetric in plan—subjected to two horizontal components of ground motion, simultaneously. Also presented is a variant of this method, called the practical modal pushover analysis (PMPA) procedure, which estimates seismic demands directly from the earthquake response (or design) spectrum. Its accuracy in estimating seismic demands for very tall buildings is evaluated, demonstrating that for nonlinear systems this procedure is almost as accurate as the response spectrum analysis procedure is for linear systems. Thus, for practical applications, the PMPA procedure offers an attractive alternative whereby seismic demands can be estimated directly from the (elastic) design spectrum, thus avoiding the complications of selecting and scaling ground motions for nonlinear response history analysis. Copyright © 2010 John Wiley & Sons, Ltd.     

使用偏心支撑减小不规则高层建筑的扭转振动效应

文章对比分析了一个不规则高层钢筋混凝土框-剪结构和在原结构基础上增设偏心支撑后的新结构的抗震性能,比较了它们的自振特性以及层间位移等地震作用效应。针对不规则高层建筑在地震作用下不可避免的扭转振动问题,提出在结构中适当地增设偏心支撑来减小结构的扭转振动效应及其它结构地震反应这种简单可行、经济适用的方法。     

Damage distribution during the 1994 Northridge, California, earthquake relative to generalized categories of surficial geology

The spatial distributions of severely damaged buildings (red-tagged) and of breaks in the water distribution system following the 1994 Northridge, California, earthquake (M_L = 6·4) are investigated relative to the local characteristics of surficial geology. The pipe breaks are used as an indicator of nonlinear soil response, and the red-tagged buildings as indicator of severe shaking. The surficial geology is described by several generalized categories based on age, textural character and thickness of the near surface layer. Two regions are studied: the San Fernando Valley and Los Angeles-Santa Monica. The analysis shows that there is no simple correlation between damage patterns and surficial geology. Single family wood-frame buildings were damaged less when built on fine silt and clay (0–3 m thick) from the late Holocene.     

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.     

Torsional mechanisms in ductile building systems

It is postulated that in order to estimate torsional effects on the seismic response of ductile building structures, the associated plastic mechanism to be developed in the three-dimensional system should be identified. The proposed approach is very different from that embodied in building codes. Inelastic structures are classified as either torsionally unrestrained or restrained. It is shown that clearly defined mechanisms that are to be mobilized, enable the acceptable system ductility demand to be estimated. This should ensure that the corresponding demands imposed on critical translatory elements of the system do not exceed their established displacement ductility capacity. To this end familiar quantities, such as element yield displacement and stiffness, are redefined. Comparisons are made of the intents of existing codified design approaches and those emphasising the role of imposed inelastic displacements. A simple treatment of the consequences of earthquake-induced inelastic skew displacements is also addressed. The primary aim of the paper is to offer very simple concepts, based on easily identifiable plastic mechanisms, to be utilized in structural design rather than advancement in analyses. Detailed design applications of these concepts are described elsewhere. The approach is an extension of the deterministic philosophy of capacity design, now used in some countries. © 1998 John Wiley & Sons, Ltd.     

混凝土核心筒的水平双向循环加载试验研究

为了研究地震多维性对于混凝土核心筒抗震性能的影响,对大尺寸混凝土核心筒试件进行了水平双向加载下的拟静力试验.重点研究在双向加载下,核心筒的破坏形态与机理、承载能力、变形性能、刚度退化等方面的问题.结果表明,由于水平双向加载下简体一个方向的损伤影响到另一个方向的性能,因而对混凝土核心筒的抗震能力造成不利影响.因此,在实际工程中应合理地考虑这种影响,这对于确保混凝土核心筒的抗震安全具有重要意义.     

Response reduction factor of irregular RC buildings in Kathmandu valley

Most current seismic design includes the nonlinear response of a structure through a response reduction factor(R). This allows the designer to use a linear elastic force-based approach while accounting for nonlinear behavior and deformation limits. In fact, the response reduction factor is used in modern seismic codes to scale down the elastic response of a structure. This study focuses on estimating the actual ‘R' value for engineered design/construction of reinforced concrete(RC) buildings in Kathmandu valley. The ductility and overstrength of representative RC buildings in Kathmandu are investigated. Nonlinear pushover analysis was performed on structural models in order to evaluate the seismic performance of buildings. Twelve representative engineered irregular buildings with a variety of characteristics located in the Kathmandu valley were selected and studied. Furthermore, the effects of overstrength on the ductility factor, beam column capacity ratio on the building ductility, and load path on the response reduction factor, are examined. Finally, the results are further analyzed and compared with different structural parameters of the buildings.     

Experimental evidence for flexibility of a building foundation supported by concrete friction piles

This article explores the possibility to measure deformations of building foundations from measurements of ambient noise and strong motion recordings. The case under study is a seven-storey hotel building in Van Nuys, California. It has been instrumented by strong motion accelerographs, and has recorded several earthquakes, including the 1971 San Fernando (M_L=6.6, R=22 km), 1987 Whittier–Narrows (M_L=5.9, R=41 km), 1992 Landers (M_L=7.5, R=186 km), 1992 Big Bear (M_L=6.5, R=149 km), and 1994 Northridge (M_L=6.4, R=1.5 km) earthquake and its aftershocks (20 March: M_L=5.2, R=1.2 km; 6 December, 1994: M_L=4.3, R=11 km). It suffered minor structural damage in 1971 earthquake and extensive damage in 1994. Two detailed ambient vibration tests were performed following the Northridge earthquake, one before and the other one after the 20 March aftershock. These included measurements at a grid of points on the ground floor and in the parking lot surrounding the building, presented and analyzed in this article. The analysis shows that the foundation system, consisting of grade beams on friction piles, does not act as a “rigid body” but deforms during the passage of microtremor and therefore earthquake waves. For this geometrically and by design essentially symmetric building, the center of stiffness of the foundation system appears to have large eccentricity (this is seen both from the microtremor measurements and from the earthquake recordings). This eccentricity may have contributed to strong coupling of transverse and torsional responses, and to larger than expected torsional response, contributing to damage during the 1994 Northridge, earthquake.     

汶川大地震四川省青川县砖混结构村镇房屋震害分析

2008年汶川 8.0级大地震造成了大量的房屋破坏,尤其村镇房屋的破坏最为严重.本文基于对四川省青川县部分乡镇村镇房屋震害的实地调查,对砖混结构房屋的震害从地形、地基基础和结构三方面进行了震害总结,分析了典型震害的成因,指出了目前村镇房屋建设存在的问题,最后对提高村镇民房的地震安全提出了一些建议.