Free‐field rotations during earthquakes: Relevance on buildings

Soil rotations around horizontal axes, during an earthquake, are studied through records collected by closely spaced arrays of strong motion accelerometers. The cross power spectrum of accelerations at nearby stations has been generally utilized to describe the spatial distribution of the motion. A number of cross spectra have been obtained during the training of these arrays. To take profit of these elaborations, a mathematical relation is established between the cross power spectrum and the power spectrum of the rotation. Rotation data presented by Liu et al, concerning 52 earthquake records collected at a single station in Taiwan, are compared with rotation data computed according to our procedure. The two series of data are suitably normalized to the peak horizontal acceleration. The data are shown in function of the distance from epicentre. The same ratio, computed according to our procedure, is in good agreement with the average value of these data. Direct measurements and the present approach have lead to evaluations of rotation higher than those predicted by mathematical investigations on the basis of the wave propagation theory, for comparable circumstances. The relevance of this input motion for relatively tall structures is examined, with reference to the structural effects that the horizontal motion concurrently provides. Meaningful will be ranked those effects of the order of magnitude of 20% or higher than those implied by the horizontal excitation. For understanding the relevance on building structures, the procedure has two areas of concern: 1) the coherence implicit in the cross power spectra, which depends on the interpolation process of the original records, collected in the arrays of instruments, and 2) the relative importance of the vertical to the horizontal input motion. As to the second item, the relevance of the rotation component on structures largely depends on the relative importance of the vertical to the horizontal input motion. When the records in an area a few km from the epicenter are considered, the response spectrum of vertical motion is comparable and in some records even higher, than that of horizontal motion, over the entire range of frequencies. This has been observed as well for the 2009 earthquake event of L'Aquila, Italy, and that at the Christchurch (New Zealand) 2011. When the response spectrum of vertical motion is comparable to that of horizontal motion, the effects of rotational motions on most engineering structures can be meaningful. Copyright © 2011 John Wiley & Sons, Ltd.     

The cause of unproportionately large higher mode contributions in the inelastic seismic responses of high‐rise core‐wall buildings

In the conventional seismic design of high‐rise reinforced concrete core‐wall buildings, the design demands such as design shear and bending moment in the core wall are typically determined by the response spectrum analysis procedure, and a plastic hinge is allowed to form at the wall base to limit the seismic demands. In this study, it is demonstrated by using a 40‐story core‐wall building that this conventional approach could lead to an unsafe design where the true demands—the maximum inelastic seismic demands induced by the maximum considered earthquake—could be several times greater than the design demands and be unproportionately dominated by higher vibration modes. To identify the cause of this problem, the true demands are decomposed into individual modal contributions by using the uncoupled modal response history analysis procedure. The results show that the true demands contributed by the first mode are reasonably close to the first‐mode design demands, while those contributed by other higher modes are much higher than the corresponding modal design demands. The flexural yielding in the plastic hinge at the wall base can effectively suppress the seismic demands of the first mode. For other higher modes, however, a similar yielding mechanism is either not fully mobilized or not mobilized at all, resulting in unexpectedly large contributions from higher modes. This finding suggests several possible approaches to improve the seismic design and to suppress the seismic demands of high‐rise core‐wall buildings. Copyright © 2012 John Wiley & Sons, Ltd.     

Corrective eccentricities for assessment by the nonlinear static method of 3D structures subjected to bidirectional ground motions

Nonlinear static methods are reliable in the evaluation of the seismic response of planar structural schemes, but they are not very effective in the assessment of three-dimensional building structures. The authors of this paper have recently proposed a nonlinear static approach for asymmetric structures, which is an improvement on that stipulated by seismic codes. This method is based on the observation that the distribution of the maximum dynamic displacements of the deck can be enveloped by two pushover analyses performed by applying the lateral force with two eccentricities with respect to the center of mass of the deck. These eccentricities, named “corrective eccentricities”, are defined so that the two corresponding pushover analyses provide displacements that are equal to those evaluated by nonlinear dynamic analysis at the two sides of the deck. In this paper, the corrective eccentricities are determined for a wide set of single-story systems. The equations for their analytical evaluation are determined and their reliability is demonstrated. Finally, the analysis of a multi-story structure is conducted to show how the method can be applied to real buildings. Copyright © 2012 John Wiley & Sons, Ltd.     

三维地震勘探在城市活断层精确定位中的意义

回顾了城市活断层探测现状,指出了采用二维地震探测在活断层精确定位中存在中浅层小构造控制程度较差,在地层倾角较大地区断层归位不够准确,受建筑物和其它障碍物限制致使测线布设与断层走向斜交造成断层定位不准,以及由于测网密度限制造成的断层交汇处、断层分段点、端点位置控制程度较差等主要问题．从理论上分析了三维地震勘探的优势,认为三维地震勘探可以较好地解决由于二维地震勘探本身的技术缺陷和地表环境、地质条件影响所造成的断层定位不准的问题．通过实例剖析,从野外采集难点, 仪器选择, 观测系统定义, 优化施工设计, 野外质量监控, 特殊数据处理手段, 以及取得的良好地质效果等方面进行详细阐述,论证了利用三维地震探测在城市进行活断层精确定位中的可行性及其意义．      

基于情景模拟的上海中心城区建筑暴雨内涝暴露性评价

由于城市的快速发展和土地利用覆被变化,由暴雨或台风引起的内涝已成为全球研究的热点。潜在的内涝风险危及建筑及室内财产安全。文章应用简化内涝模型模拟上海市中心城区不同情境下的积水情况,并根据每幢建筑室内进水深度对其进行暴露性分级。然后分析、评价了上海市中心城区建筑不同情境下的暴雨内涝灾害暴露性。研究结果发现:仓储和旧式住宅是最易暴露在内涝灾害中的建筑类型;总体上,杨浦、普陀和徐汇区是政府最需要采取安全措施的区域,长宁和虹口区处于中等暴露性水平;同时,内涝对黄浦、静安、卢湾和闸北区影响很小。研究结果可为地方政府改善内涝风险管理提供参考依据。     

The structural behaviour of timber joints subjected to bi‐axial bending

This study investigates the structural performance of interior, exterior and corner traditional timber joints found in a traditional temple in Japan subjected to bi‐axial bending. A total of 36 half‐scale specimens were fabricated and tested under both uni‐axial and bi‐axial cyclic bending for comparison of their structural behaviour. The results show that the corner joints exhibit lower rotational stiffness than the interior and exterior joints. Further, connections subjected to only uni‐axial moments exhibit better performance with regard to moment resistance, energy dissipation and stiffness degradation as compared with those subjected to bi‐axial moments. Hence, this study suggests that if the timber joints are subjected to bi‐axial bending, this effect on timber joints should be considered during the design and evaluation process in the future. Copyright © 2009 John Wiley & Sons, Ltd.     

具有相邻建筑物的岩质基坑支护结构优化设计的讨论

传统岩质基坑支护设计方法通常没有考虑岩质基坑与支护结构相互作用以及相邻建筑物对支挡结构和岩质基坑稳定性的影响。结合实际工程,采用PLAXIS有限元软件,分别应用有限元强度折减法和有限元法对具有相邻建筑物条件下的岩质基坑的稳定性和变形、岩石侧压力、基坑开挖对相邻建筑物的影响以及支护结构的优化设计等方面进行了分析。结合《高层建筑结构设计规程》和《建筑地基基础设计规范》提出了相邻既有建筑基础变形的控制条件,并以此作为控制基坑变形的依据,并根据岩质基坑的变形限值计算岩石侧压力和进行岩质基坑支护设计,该方法能够有效地节约造价和缩短工期。     

Envelope‐based pushover analysis procedure for the approximate seismic response analysis of buildings

An envelope‐based pushover analysis procedure is presented that assumes that the seismic demand for each response parameter is controlled by a predominant system failure mode that may vary according to the ground motion. To be able to simulate the most important system failure modes, several pushover analyses need to be performed, as in a modal pushover analysis procedure, whereas the total seismic demand is determined by enveloping the results associated with each pushover analysis. The demand for the most common system failure mode resulting from the ‘first‐mode’ pushover analysis is obtained by response history analysis for the equivalent ‘modal‐based’ SDOF model, whereas demand for other failure modes is based on the ‘failure‐based’ SDOF models. This makes the envelope‐based pushover analysis procedure equivalent to the N2 method provided that it involves only ‘first‐mode’ pushover analysis and response history analysis of the corresponding ‘modal‐based’ SDOF model. It is shown that the accuracy of the approximate 16th, 50th and 84th percentile response expressed in terms of IDA curves does not decrease with the height of the building or with the intensity of ground motion. This is because the estimates of the roof displacement and the maximum storey drift due to individual ground motions were predicted with a sufficient degree of accuracy for almost all the ground motions from the analysed sets. Copyright © 2013 John Wiley & Sons, Ltd.     

Seismic response of 20‐story base‐isolated and fixed‐base reinforced concrete structural wall buildings at a near‐fault site

This paper investigates numerically the seismic response of six seismically base‐isolated (BI) 20‐story reinforced concrete buildings and compares their response to that of a fixed‐base (FB) building with a similar structural system above ground. Located in Berkeley, California, 2 km from the Hayward fault, the buildings are designed with a core wall that provides most of the lateral force resistance above ground. For the BI buildings, the following are investigated: two isolation systems (both implemented below a three‐story basement), isolation periods equal to 4, 5, and 6 s, and two levels of flexural strength of the wall. The first isolation system combines tension‐resistant friction pendulum bearings and nonlinear fluid viscous dampers (NFVDs); the second combines low‐friction tension‐resistant crosslinear bearings, lead‐rubber bearings, and NFVDs. The designs of all buildings satisfy ASCE 7‐10 requirements, except that one component of horizontal excitation, is used in the 2D nonlinear response history analysis. Analysis is performed for a set of ground motions scaled to the design earthquake and to the maximum considered earthquake (MCE). At both the design earthquake and the MCE, the FB building develops large inelastic deformations and shear forces in the wall and large floor accelerations. At the MCE, four of the BI buildings experience nominally elastic response of the wall, with floor accelerations and shear forces being 0.25 to 0.55 times those experienced by the FB building. The response of the FB and four of the BI buildings to four unscaled historical pulse‐like near‐fault ground motions is also studied. Copyright © 2013 John Wiley & Sons, Ltd.     

An efficient methodology for simulating earthquake‐induced 3D pounding of buildings

The current paper presents an efficient methodology for numerically simulating in three dimensions adjacent buildings that may experience pounding during strong earthquakes. In particular, a new approach to the numerical problem of spatial impact modeling that does not require the ‘a priori’ determination of the contact points is presented, taking also into account the geometry at the vicinity of an impact. In the current study, the buildings are simulated as linear multi‐degree‐of‐freedom‐systems, but the methodology can be easily extended to consider nonlinear behavior as well. A software application has been specifically developed to implement the proposed methodology, using modern object‐oriented design and programming. The developed software is utilized in a simple example, and the computed results are compared with the corresponding analysis results obtained from a commercial general‐purpose software application that uses typical contact elements for the simulation of impacts. A discussion follows on the advantages and capabilities of the proposed methodology and the developed software. Copyright © 2013 John Wiley & Sons, Ltd.