A practice‐oriented estimation of the failure probability of building structures

In the presented practice‐oriented probabilistic approach for the seismic performance assessment of building structures, the SAC‐FEMA method, which is a part of the broader PEER probabilistic framework and permits probability assessment in closed form, is combined with the pushover‐based N2 method. The most demanding part of the PEER probabilistic framework, that is incremental dynamic analysis, is replaced by the much simpler N2 method, which requires considerably less input data and much less computational time, but which can, nevertheless, often provide: acceptable estimates for the mean values of the structural response. Using some additional simplifying assumptions that are consistent with seismic code procedures, an explicit equation for a quick estimation of the annual probability of “failure” (i.e. the probability of exceeding the near collapse limit state) of a structure can be derived, which is appropriate for practical applications, provided that predetermined default values for the dispersion measures are available. In the paper, this simplified approach is summarized and applied to the estimation of the “failure” probability of reinforced concrete frame buildings representing both old structures, not designed for earthquake resistance, and new structures designed according to Eurocode 8. The results of the analyses indicate a high probability of the “failure” of buildings, which have not been designed for seismic loads. For a building designed according to a modern code, the conservatively determined probability of “failure” is about 30 times less but still significant (about 1% over the lifetime of the structure). Copyright © 2011 John Wiley & Sons, Ltd.     

Rocking isolation of low‐rise frame structures founded on isolated footings

This paper explores the effectiveness of a new approach to foundation seismic design. Instead of the present practice of over‐design, the foundations are intentionally under‐dimensioned so as to uplift and mobilize the strength of the supporting (stiff) soil, in the hope that they will thus act as a rocking–isolation mechanism, limiting the inertia transmitted to the superstructure, and guiding plastic ‘hinging’ into soil and the foundation–soil interface. An idealized simple but realistic one‐bay two‐story reinforced concrete moment resisting frame serves as an example to compare the two alternatives. The problem is analyzed employing the finite element method, taking account of material (soil and superstructure) and geometric (uplifting and P–Δ effects) nonlinearities. The response is first investigated through static pushover analysis. It is shown that the axial forces N acting on the footings and the moment to shear (M/Q) ratio fluctuate substantially during shaking, leading to significant changes in footing moment‐rotation response. The seismic performance is explored through dynamic time history analyses, using a wide range of unscaled seismic records as excitation. It is shown that although the performance of both alternatives is acceptable for moderate seismic shaking, for very strong seismic shaking exceeding the design, the performance of the rocking‐isolated system is advantageous: it survives with no damage to the columns, sustaining non‐negligible but repairable damage to its beams and non‐structural elements (infill walls, etc.). Copyright © 2011 John Wiley & Sons, Ltd.     

Pin‐supported walls for enhancing the seismic performance of building structures

An application of a pin‐supported wall‐frame system in retrofitting an eleven‐story steel reinforced concrete frame is introduced. The retrofit aims at enhancing integrity and avoiding weak story failure in an existing moment‐resisting frame. Seismic performance of the building before and after the retrofit is assessed through nonlinear dynamic analysis. The results show that the pin‐supported walls are effective in controlling the deformation pattern of the ductile frame and hence in avoiding weak story failure. With the well‐controlled deformation pattern, carefully arranged energy dissipating devices are able to concentrate energy dissipations so that damage to the rest of the structure can be significantly reduced. Copyright © 2012 John Wiley & Sons, Ltd.     

Pushbroom-Frame Imagery Co-Registration

More and more littoral surveys are conducted with aerial platform sensor suites that include a hyperspectral pushbroom sensor and a frame camera. However, in some cases, data from auxiliary sensors may contain errors or are not available. For many research groups, a high-accuracy registration of the multi-sensor data relative to each other is essential, while absolute geo-location of each individual measurement is not. A co-registration procedure was developed for pseudo ortho-rectification of the hyperspectral imagery and to remove spatial distortions caused by the aircraft's trajectory during the survey based on a flat-earth assumption. This image-processing approach utilizes the aerial frame imagery as a reference. Each hyperspectral scan line is co-registered into the frame imagery coordinate system. The performance of the procedure was evaluated using hyperspectral imagery collected over northern New Hampshire and southern Maine. The evaluation results showed that the procedure is robust enough to pseudo ortho-rectify hyperspectral imagery over coastal areas and to remove significant spatial distortions.     

近地空中爆炸作用下钢框架结构冲击响应

采用多物质欧拉与拉格朗日耦合算法,对框架结构和混凝土地面采用Lagrange单元、空气和炸药采用多物质ALE单元,建立流固耦合有限元模型,对一个五层的钢框架结构在近地爆炸作用下冲击压力波的传播过程、结构冲击响应和变形以及破坏过程进行了数值模拟.数值模拟分析结果表明,拉格朗日-欧拉耦合算法较好地模拟了爆炸冲击波在介质中的传播和作用在结构上的爆炸荷载效应.在爆炸近区,空气爆炸冲击波的衰减速率快;爆炸产生强烈的冲击波首先使钢框架结构正面的柱和梁柱节点区产生很大的塑性变形,钢框架柱在发生爆炸的瞬间失去承载能力,最终导致建筑物整体倒塌.考虑结构变形和流体荷载间的相互影响可以较真实地模拟结构在爆炸荷载作用下的连续倒塌过程,为研究结构在爆炸荷载作用下的连续倒塌提供了有效手段.     

三维无缝工程地质建模系统的研制

在边界表示法的基础上,提出了无缝工程地质模型的概念,实现了基于模型线框架的无缝工程地质建模方法,研制了无缝建模系统（ROCKModel）。该系统包括3个部分：原始数据转换工具、实体建模工具与辅助工具,涉及剖分、插值、曲面求交等8个基本功能模块。在基本功能模块的基础上,提出并实现了形态复杂曲面的创建、构建模型线框架、界面编辑与重构、块体搜索、岩体质量三维分级等关键技术。该系统能够实现复杂地质对象与工程对象的无缝建模与可视化,并为数值模拟软件直接提供几何模型。     

顾及板块运动、稳定性和系统偏差的高精度GPS监测基准研究与实现

利用GPS技术监测城市地面沉降,监测基准的合理选取对获得真实的形变结果至关重要.本文详细分析了城市沉降监测网基准的特点,针对传统基准模型的局限性提出了顾及板块运动、基准点稳定性以及其他系统误差影响的拟稳基准模型.该模型考虑了基准点自身的稳定性以及板块运动对高程形变的影响,同时采用系统参数统一各期基线框架以及区域拟稳基准,有效解决了因基准点不稳定性和基线框架不一致对形变结果造成的误差,确保正确形变信息的获取.最后通过六期西安市地面沉降监测网数据试验,验证了该方案的合理性和可靠性.     

Shaking table tests of steel frame with superelastic Cu–Al–Mn SMA tension braces

Shaking table tests are performed on a one‐bay one‐story steel frame with superelastic Cu–Al–Mn shape memory alloy (SMA) tension braces. The frame is subjected to a series of scaled ground motions recorded during the 1995 Kobe earthquake, Japan. The test results demonstrate that the SMA braces are effective to prevent residual deformations and pinching. It is also shown that the time history responses observed from the shaking table tests agree well with the numerical predictions using a rate‐independent piecewise‐linear constitutive model calibrated to the quasi‐static component tests of the SMA braces. This suggests that the loading rate dependence of Cu–Al–Mn SMAs as well as the modeling error due to the piecewise linear approximation can be neglected in capturing the global response of the steel frame. Numerical simulations under a suite of near‐fault ground motion records are further performed using the calibrated analytical models to demonstrate the effectiveness of the SMA braces when the variability of near‐fault ground motions is taken into account. A stopper, or a deformation restraining device, is also proposed to prevent premature fracture of SMA bars in unexpectedly large ground motions while keeping the self‐centering capability in moderate to large ground motions. The effectiveness of the stopper is also demonstrated in the quasi‐static component and shaking table tests. Copyright © 2015 John Wiley & Sons, Ltd.     

层间滑移隔震结构地震作用有限元分析

将二硫化钼作为摩擦材料设计出一种带限位器的滑移隔震支座。根据多层框架结构变形的特点,给出适用于滑移框架隔震结构的计算模型,推导出层间滑移隔震结构的运动方程。运用SAP2000有限元软件建立一层间滑移框架隔震结构的有限元模型,对比分析El Centro地震波下摩擦系数和隔震层位置不同时隔震结构的地震反应。结果表明,上部结构的动力反应随摩擦系数的增加而不断增大,滑移隔震结构的减震效果逐渐减弱,但隔震层的滑移量却在不断减小;摩擦系数的选取应综合考虑减震效果和隔震层滑移量两个因素。随着隔震层的增高,结构的加速度反应和层间位移反应整体上呈增大趋势,隔震效果不断减弱,且隔震层的加速度值下部层比上部层要大得多,一层隔震和三层隔震时的变形主要集中于隔震层,而五层隔震时结构层间位移并未出现突变,说明隔震层设置在较高位置处对结构体系的影响较小。     

基于时变地震损伤模型的多龄期钢框架结构易损性分析

基于时变地震损伤模型提出酸性大气环境作用下多龄期钢框架结构概率地震易损性分析的方法及步骤;考虑服役龄期对钢框架结构抗震性能的影响,分别建立时变概率地震需求模型、时变概率抗震能力模型及时变易损性模型;在概率地震需求分析及概率抗震能力分析的基础上,得到多龄期(20年、30年、40年、50年)钢框架结构的易损性模型及易损性曲线。