The use of 3D modeling for the prediction of the seismic demands on the gravity dams

Seismic behavior of gravity dams has long been evaluated using a representative two‐dimensional (2D) system. Formulated for the gravity dams built in wide canyons, the assumption is nevertheless utilized extensively for almost all concrete dams due to the established procedures as well as the expected computational costs of a three‐dimensional model. However, a significant number of roller‐compacted concrete dams, characterized as such systems, do not conform to the basic assumptions of these methods by violating the conditions on canyon dimensions and joint‐spacing/details. Based on the premise that the 2D modeling assumption is overstretched for practical purposes in a variety of settings, the purpose of this study is to critically evaluate the use of 2D modeling for the prediction of the seismic demands on these systems. Using a rigorous soil–structure interaction approach, the difference between the two and three‐dimensional response for gravity dams was investigated first in the frequency domain for a range of canyon widths and foundation to dam moduli ratios. Then, the time domain differences between the crest displacements and the maximum principal stress were obtained using 70 different ground motions in order to show the possible bias introduced into the analysis results due to the modeling approach. The results of the study show that even for relatively wide canyons, the 2D analysis can lead to misleading predictions. Copyright © 2017 John Wiley & Sons, Ltd.     

Boundary frame contribution in coupled and uncoupled steel plate shear walls

The steel plate shear wall (SPSW) system is a robust option for earthquake resistance due to the strength, stiffness, ductility and energy dissipation that it provides. Although thin infill plates are efficient for resisting lateral loads, boundary frames that are proportioned based on capacity design requirements add significant structural weight that appears to be one of the factors limiting the use of the system in practice. An alternate configuration, the SPSW with coupling (SPSW‐WC), was explored recently as an option for increasing architectural flexibility while also improving overall system economy and seismic performance. The SPSW‐WC, which extensively employs flexural boundary frame contribution, has shown promise in analytical, numerical and experimental studies, but recent research on uncoupled SPSWs suggests that boundary frame contribution should not be considered for carrying seismic design shear. As a result, in the present study, boundary frame contribution in SPSWs was explored with detailed three‐dimensional finite element models, which were validated against large‐scale SPSW‐WC tests. Six‐story systems were considered, and the study matrix included single and double uncoupled SPSWs along with coupled SPSWs that had various degrees of coupling. Variations in design methodology were also explored. The modeling framework was employed to conduct static monotonic and cyclic pushover analyses and dynamic response history analysis. These analyses demonstrate the beneficial effect of coupling in SPSWs and illustrate the need to consider boundary frame contribution in design of coupled SPSWs. In addition, sharing design shear between the infill plate and the boundary frame is more generally shown to not be detrimental if this sharing is done in the design stage based on elastic analysis and the resulting boundary frame provides adequate secondary strength and stiffness following infill plate yielding. Copyright © 2017 John Wiley & Sons, Ltd.     

Rapid adjustment of shoreline behavior to changing seasonality of storms: observations and modelling at an open‐coast beach

An 8‐year time series of weekly shoreline data collected at the Gold Coast, Australia, is used to examine the temporal evolution of a beach, focusing on the frequency response of the shoreline to time‐varying wave height and period. Intriguingly, during 2005 the movement of the shoreline at this site changed from a seasonally‐dominated mode (annual cycle) to a storm‐dominated (~monthly) mode. This unexpected observation provides the opportunity to explore the drivers of the observed shoreline response. Utilizing the calibration of an equilibrium shoreline model to explore the time‐scales of underlying beach behavior, the best‐fit frequency response (days^?1) is shown to be an order of magnitude higher post‐2004, suggesting that a relatively subtle change in wave forcing can drive a significant change in shoreline response. Analysis of available wave data reveals a statistically significant change in the seasonality of storms, from predominantly occurring at the start of the year pre‐2005 to being relatively consistent throughout the year after this time. The observed change from one mode of shoreline variability to another suggests that beaches can adapt relatively quickly to subtle changes in the intra‐annual distribution of wave energy. Copyright © 2016 John Wiley & Sons, Ltd.     

单体土遗址地震动力响应分析——以山丹县刘富寨敌台为例

我国西北地区遗存有大量的单体土遗址,时刻遭受地震等各种灾害的威胁。因其种类繁多,形式各异,很难以一个确定性的模型或计算方式来获取较为准确的结论。为更好地研究及保护单体土遗址,以山丹县刘富寨敌台为例,采用FLAC3D软件对其进行地震动力响应分析,包括位移、应力以及地震波的加速度放大效应。研究结果表明:在实测双向地震荷载作用下,敌台产生了一定的永久位移,且以竖向位移为主(主要由水平地震荷载引起);敌台内部裂隙处产生了拉应力集中,且其接近抗拉强度;地震加速度放大效应随高度增加而强烈,以竖向加速度更为明显;裂隙对竖向位移的产生具有强烈的不利影响,同时裂隙处加速度放大系数最大,但会弱化相邻测点的放大效应。     

考虑液-固耦合的大型超高压换流变压器地震响应研究

针对大型超高压换流变压器的地震响应问题,以一超高压±800kV换流变压器为研究对象,考虑其本体中油与箱壁的相互作用建立三维有限元分析模型,采用LS-DYNA分析换流变压器的自振特性、地震作用下的动力响应规律以及换流变压器的薄弱部位。研究结果显示,对于大型超高压换流变压器,本体中油与箱壁的相互作用对结构动力响应有一定的放大作用,进行自振特性和地震响应分析时应考虑;套管体系的薄弱部位为套管的底部和中间部位,在这两个位置拉应力和压应力最大值交替出现;换流变套管体系在地震作用下的位移较大,套管顶部的导线应设置足够大的伸缩距离。     

高烈度区大跨刚构-连续梁桥地震响应分析

为研究大跨刚构-连续梁桥在高烈度地区的地震响应,选取羊记沟左线大桥为工程背景,利用Midas软件建立多个有限元模型,考虑纵、横桥向的水平地震波输入,比较反应谱与时程分析结果,获得结构动力响应特点,为大跨刚构-连续梁桥的抗震设计提供参考依据。结果表明:刚构-连续体系仅单墩刚构时不宜采用反应谱分析,高烈度地区桥梁进行地震响应时程分析时,选用与场地适应的地震波的同时,应考虑采用本地地震参数转化人工波作为地震激励源。桥墩不等高时,可通过改变墩截面形状及其与主梁连接方式调节内力分布,避免矮墩刚度大导致的内力集中。     

不滑移状态下深埋圆形隧道地震响应研究

地下结构的地震响应主要取决于由地震波传播产生的土体变形与土结相互作用。剪切波传播过程中将会引起隧道衬砌的椭圆化变形,进而降低衬砌有效承载力。剪切波作用下的深埋圆形隧道可认为处于均质的纯剪状态,基于相对刚度法的拟静力解析解可充分考虑土结相互作用对隧道结构内力的影响。基于此,本文将通过有限元数值分析获得的自由场地地基变形引入不滑移状态下深埋圆形隧道内力求解公式,并结合二维和三维数值模拟途径,将动力分析结果与解析解结果进行对比分析,以评价各种解析方法的适用性和数值途径的可靠性。     

天津地区土动力学参数变异性对地表地震动参数的影响

收集天津地区十多年来的83份地震安全性评价报告,统计202个钻孔的1 650组动三轴数据,分别给出不同种类土在不同区间深度下的统计代表值及其标准差。以某典型Ⅲ类场地为例,用等效线性化方法进行多种地震动强度及相位输入下的水平成层场地地震反应分析计算,详细研究该地区覆盖土层动剪模量比和阻尼比变异性对地表峰值加速度及其反应谱的影响。结果表明:场地地表峰值加速度和反应谱随土动剪切模量比增大或减小而增大或减小,随土动阻尼比增大或减小而减小或增大;在大震输入条件下,地表峰值加速度和地表反应谱的中、高频段随土动剪切模量比减小而减小的变化尤为明显,动阻尼比变化也有一定影响,但不如土动剪切模量比减小变化时影响明显;在中震、小震地震动输入条件下,场地土的动剪模量比和阻尼比变异性对地表峰值加速度和反应谱影响并不显著。     

Evaluation of ASCE 7 equations for designing acceleration‐sensitive nonstructural components using data from instrumented buildings

This study uses instrumented buildings and models of code‐based designed buildings to validate the results of previous studies that highlighted the need to revise the ASCE 7 F_p equation for designing nonstructural components (NSCs) through utilizing oversimplified linear and nonlinear models. The evaluation of floor response spectra of a large number of instrumented buildings illustrates that, unlike the ASCE 7 approach, the in‐structure and the component amplification factors are a function of the ratio of NSC period to the supporting building modal periods, the ground motion intensity, and the NSC location. It is also shown that the recorded ground motions at the base of instrumented buildings in most cases are significantly lower than design earthquake (DE) ground motions. Because ASCE 7 is meant to provide demands at a DE level, for a more reliable evaluation of the F_p equation, 2 representative archetype buildings are designed based on the ASCE 7‐16 seismic provisions and exposed to various ground motion intensity levels (including those consistent with the ones experienced by instrumented buildings and the DE). Simulation results of the archetype buildings, consistent with previous numerical studies, illustrate the tendency of the ASCE 7 in‐structure amplification factor, [1 + 2(z/h)] , to significantly overestimate demands at all floor levels and the ASCE 7 limit of to in many cases underestimate the calculated NSC amplification factors. Furthermore, the product of these 2 amplification factors (that represents the normalized peak NSC acceleration) in some cases exceeds the ASCE 7 equation by a factor up to 1.50.     

Gully head modelling: A Mediterranean badland case study

Predicting the location of gully heads in various environments is an important step towards predicting gully erosion rates. So far, field data collection and modelling of topographic thresholds for gully head development has mainly focused on gullies that formed in forested areas, rangelands, pastures and cropland. Such information for gullies in badlands however is very scarce. Therefore, this paper aims to extend the database on gully head topographical thresholds through data collection in a badland area and to improve the prediction of gully heads forming at sites with a very low erosion resistance value. For this, we chose a badland site located in central Italy that is characterized by biancana forms and both active and dormant gullies. The definition of the conditions under which present‐day gully heads developed allowed a better modelling of the gully head threshold equation, with modification of a previous model and the exemplification of how to use the updated model. The model shows that the resistance to gully head retreat depends on slope gradient and drainage area at gully heads, land use at the moment of gully development (as numerically expressed using parameters derived from the Runoff Curve Number method), surface rock fragment cover, presence of joints, pipes, and factors/processes affecting detachment rate. This study attempted to better understand environmental conditions that control the development of gully heads in badlands through a combination of field data collection of gully heads, an analysis of land use changes over 10 centuries, focusing on the period 1820–2005, and land use management through repeat photography and a critical examination of historical documents. © 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.