New Physical Measures of Seismic Intensity

The physical measures of macroscopic seismic intensity have been extensively studied based on the new understanding of seismic intensity and the new analytical method and new database of strong ground motion.New physical measures of seismic intensity have been proposed.     

深埋隧道围岩-支护结构稳定性研究

依据隧址区工程地质特征,建立通-渝深埋公路隧道1:1实体模型。通过有限元数值模拟,结合量测资料,分析了隧道按全断面法和台阶法动态施工过程围岩-支护结构屈服接近度、位移和应力特征,以及支护结构钢支撑受力、二次衬砌轴力和弯矩特征。结果表明,在相同的地质条件和支护条件下,台阶法较全断面法没有明显的优势,为深埋公路隧道优化设计和施工提供了科学依据。     

基于中美抗震设计规范的双向水平地震效应组合方法的有效性评估

在地震来临时,一般假设建筑结构同时受到两个正交水平方向分量与一个竖向分量的地震动作用。双向水平地震效应组合方法用于估计两个正交水平分量地震动同时作用时结构的内力效应。本文主要对我国与美国抗震设计规范中规定使用的平方和开平方根（SRSS）方法与百分比组合方法的有效性进行了评估。首先,对比了我国与美国规范在考虑双向水平地震效应时的适用情况及相关规定上的异同。以一4层中心支撑-框架结构为工程案例,考虑两国规范在适用情况上的规定,设置了三个结构布置方案。对三个结构布置方案建立有限元模型,选取22组地震动,开展了动力时程分析。提出了针对SRSS方法与百分比组合方法的评估指标,基于时程分析结果,发展了双向水平地震效应组合的概率性评估方法。评估结果表明:SRSS方法与百分比组合方法用于平面扭转不规则结构的设计较为保守。在简化组合规则的适用条件上,美国规范对平面扭转不规则结构不进行考虑有一定的合理性。建议我国规范对中心支撑-框架结构中含双向受压柱的设计要求考虑双向水平地震效应组合。     

The complete SRSS modal combination rule

The complete Square‐Root‐of‐Sum‐of‐Squares (c‐SRSS) modal combination rule is presented. It expresses the structural response in terms of uncoupled SDOF modal responses, yet accounting fully for modal response variances and cross‐covariances. Thus, it is an improvement over the classical SRSS rule which neglects contributions from modal cross‐covariances. In the c‐SRSS rule the spectral moments of the structural response are expressed rigorously in terms of the spectral moments of uncoupled modal responses and of some coefficients that can be computed straightforwardly as a function of modal frequencies and damping, without involving the computation of cross‐correlation coefficients between modal responses. An example shows an application of the c‐SRSS rule for structural systems with well separated and closely spaced modal frequencies, subjected to wide‐band and narrow‐band excitations. Comparisons with response calculations using the SRSS and the Complete Quadratic Combination rules are given and discussed in detail. Based on the c‐SRSS rule a response spectrum formulation is introduced to estimate the maximum structural response. An example considering a narrow‐band excitation from the great Mexico earthquake of September 19, 1985, is given and the accuracy of the response spectrum formulation is examined. Copyright © 2010 John Wiley & Sons, Ltd.     

New real‐value modal combination rules for non‐classically damped structures

A procedure is presented to determine new modal combination rules (both CQC and SRSS) for non‐classically damped structures. The procedure presented in this paper does not need the solution of any complex eigenvalue problem, in contrast to other methods found in the literature. Thus, the modal combination rules presented here are easily applicable, even by those engineers who are unaccustomed to using complex algebra. Moreover, these formulations show the further advantage of requiring the response spectra only for the target damping ratio value. So the use of approximated formulae, necessary for passing from the response spectrum with the target damping ratio value to other ones, is avoided. Copyright © 2004 John Wiley & Sons, Ltd.     

Evaluation of combination rules for maximum response calculation in multicomponent seismic analysis

The existing rules for combining peak response to individual components of ground motion are evaluated. The response values r_e to two horizontal components of ground motion estimated by four multicomponent combination rules—SRSS‐, 30%‐, 40%‐ and simplified‐SRSS‐rules—are compared with the critical response, r_cr, obtained by the CQC3‐rule, which takes into account the direction of the principal ground components with respect to the structural axes and provides the largest response over all possible seismic incident angles. The following results are obtained in the first part of the paper and are valid for any elastic structure and any earthquake design response spectrum: For realistic values of the ratio γ of the design spectra for the two principal components of ground motion the SRSS‐rule estimate lies between 0.79r_cr and 1.00r_cr, the Simplified‐SRSS‐rule estimate lies between 1.00r_cr and 1.26r_cr, the 40%‐rule estimate lies between 0.99r_cr and 1.25r_cr, and the 30%‐rule estimate lies between 0.92r_cr and 1.16r_cr. None of the multicomponent combination rules account for the increase in response of systems if the vibration periods of the two modes that contribute most to the response to the x‐ and y‐components of ground motion are close to each other. Evaluated in the second part of the paper is the accuracy of the multicomponent combination rules in estimating the response of a range of one‐storey systems with (a) symmetrical plan and (b) unsymmetrical plan, and of two multistorey buildings. The SRSS‐rule underestimates the response by up to 16% and the other three rules overestimate it by up to 18%. Although these errors appear to be smaller than the many approximations inherent in structural design, they can be eliminated with very little additional computation by using an explicit formula for the critical response based on the CQC3 rule. Copyright © 2001 John Wiley & Sons, Ltd.     

Errors caused by peak factor assumptions in response‐spectrum‐based analyses

The modal combination rules commonly used in response spectrum analyses implicitly assume that the peak factor associated with the response quantity of interest is equal to the peak factors of the contributing modal responses. In this paper, we examine the validity of this assumption and demonstrate that it causes the modal combination rules to over‐represent the contribution of the higher modes of vibration to the total response and under‐represent the contribution of the lower modes. Consequently, a response‐spectrum‐based analysis can yield a biased estimate for the peak value of a response quantity when two or more well‐separated modal frequencies make significant contributions to the total response. To correct this potential bias in response‐spectrum‐based estimates, we develop a procedure for estimating the peak factors that is suitable to the response spectrum analysis calculations commonly used in the current design practice. Examples are presented to demonstrate the proper use and potential impact of the proposed procedure. Copyright © 2015 John Wiley & Sons, Ltd.     

Response to orthogonal components of ground motion and assessment of percentage combination rules

The effects of horizontal components of ground motion on the linear response of torsionally stiff and torsionally flexible systems, on soft and firm soil conditions, are examined. A one‐story, two‐way asymmetric structural system is used, subjected to uncorrelated ground motion components along their principal directions. Spectral densities for ground accelerations in firm and soft soils are modeled based on recorded data from large intensity Mexican earthquakes. It is shown that for firm soils, in general, these effects are important in the case of torsionally flexible systems that are stiff under translation, or for torsionally stiff systems that are flexible in translation. The percentage combination rules usually specified in seismic design codes are assessed against the dynamic response. Such combination rules can result in overly conservative design forces or underestimated design forces, particularly for torsionally flexible structures. Given the relative magnitude of the response to each ground motion component, it was found that using different percentage values in the combination rules has no significant effect on improving the estimation of the total response. Copyright © 2003 John Wiley & Sons, Ltd.