Prediction of extreme 3-sec. gusts accounting for seasonal effects

In conventional statistical analysis of extreme 3-sec. gusts, extremal distributions are often used to fit the annual maximum value obtained from available records, invoking the assumption of stationarity and statistical independence. This paper presents an alternative approach in the form of an exponential distribution for the magnitude of the gust above a pre-determined base level compounded with a nonhomogenous Poisson model for the occurrence of the exceedances. The base level of 13.5 m/s is found to be appropriate. The prediction of the extreme gusts using this method agrees well with a modified conventional extreme value analysis which accounts for the seasonal variations within a year through the monthly extremal distribution parameters. The compound distribution approach is preferred over the conventional method since it does not rely solely upon the maximum value but rather utilizes directly more of the available information.     

Approximate reliability analysis using normal polynomial and simulation results

Approximate solutions are presented to evaluate the probability of failure of a structural system using simulation results. The solutions are based on a family of normal polynomial models fitted to the simulated data points with fractile constraints. The normal polynomial approximates well to some of the well-known distribution types. A simple model with a third-order normal polynomial appears appropriate for practical reliability analysis. The approximation reduces the required number of simulation trials.     

Specification of the design wind load based on wind tunnel experiments

Once a high-quality wind tunnel experiment with no major flaws on the experimental side has been performed and the data have been processed/translated appropriately to the design-decisive variable, the final step is to estimate the appropriate load or load effect coefficient for the specification of the design wind load. Four questions arise: what is the appropriate length of a single run in the wind tunnel, what is the appropriate fractile of the observed extreme coefficients, how many independent experiments are required to estimate this fractile and what should be the target confidence interval. The paper tries to give some answers to these questions and discusses the findings in regard to some wind load codes.     

The r largest order statistics model for extreme wind speed estimation

The paper presents the statistical estimation of extreme wind speed using annually r largest order statistics (r-LOS) extracted from the time series of wind data. The method is based on a joint generalized extreme value distribution of r-LOS derived from the theory of Poisson process. The parameter estimation is based on the method of maximum likelihood. The hourly wind speed data collected at 30 stations in Ontario, Canada, are analyzed in the paper. The results of r-LOS method are compared with those obtained from the method of independent storms (MIS) and specifications of the Canadian National Building Code (CNBC-1995). The CNBC estimates are apparently conservative upper bound due to large sampling error associated with annual maxima analysis. Using the r-LOS method, the paper shows that the wind pressure data can be suitably modelled by the Gumbel distribution.     

Local buckling and inelastic cyclic behavior of tubular members

Closed form expressions are obtained for the load-deflection behavior of the locally buckled circular tubular braces under the reversed loading conditions. These expressions are obtained by supplementing the deflected shape of the brace with closed form expressions for moment-curvature relationship of locally buckled circular tubular section. The developed closed form expressions are used to study the effects of the slenderness ratio and diameter-to-thickness ratio on the behavior of braces under reversed loading conditions.     

基于改进Harris分布的岩石损伤统计本构模型研究

结合连续损伤理论和统计损伤理论,从岩石内部缺陷分布的随机性出发,提出了一种新的岩石微元强度概率密度分布函数—改进的Harris函数,建立了基于改进Harris分布的岩石损伤统计本构模型,并分别用曲线拟合法和多元函数求极值法确定了模型参数,最后用试验数据对模型进行了验证。结果表明:该模型理论曲线与试验曲线吻合良好,能够较好的反映岩石在三维应力状态下的应力—应变关系和破坏全过程,特别是岩石的应变软化特性和峰值强度随围压增加而增大的特性,从而说明了模型的合理性和可行性。     

基于随机场理论的地基稳定性模糊可靠度分析

根据Vanmarcke提出的随机场理论描述土性参数的空间变异性,建立了考虑土性参数空间变异性的地基失稳模糊概率公式,并通过实例分析了两种情况下地基的模糊失效概率值,得出用随机场理论空间均值方差计算的模糊失效概率值比不考虑土性参数的空间变异性计算模糊失效概率值偏低的结论.     

楼面活荷载标准值和设计值的小样本推断方法

工程实际中有时需在测试数据不足的条件下推断楼面活荷载的标准值和设计值,这时的推断结果受统计不定性的影响较大,目前方法的推断结果偏于冒进,需建立小样本的推断方法。首先根据极大值I型分布参数、分位值的最好线性无偏和不变估计,提出了楼面活荷载标准值和设计值的线性回归推断方法;其次,根据贝叶斯理论,提出了楼面活荷载标准值和设计值的贝叶斯推断方法,并且对比分析了上述两种小样本推断方法的优劣,解决了应用中的具体问题,为小样本条件下楼面活荷载标准值和设计值的推断提供了实用的方法。     

Frequency response functions of circular arches for support motions

A Green's function approach is used to investigate the response of circular arches to uniform and non-uniform base excitations. These responses are obtained in the frequency domain in closed form as frequency response functions. Displacement as well as stress resultant solutions are calculated from exact expressions. No accuracy loss is present for the stress resultants, which are functions of higher derivatives of the displacement.     

Seismic loads predicted by spatial variability models

Seismic loads that result from different expressions describing the spatial variability of the seismic ground motions at the supports of lifelines are evaluated and compared in this study. The analysis considers two spatial variability models, that are commonly used in lifeline earthquake engineering, namely the ones developed by Harichandran and Vanmarcke (J. Eng. Mech., 112 (1986)) and by Luco and Wong (Earthquake Eng. Struct. Dyn., 14 (1986)). It is shown that the models produce significantly different seismic loads for the analysis of buried and above-ground lifelines.     

回弹法在非批构件检测中的问题与建议

本文结合工程实例提出对材料强度为正态分布的非批构件按其分布5%分位数并考虑样本容量修正系数确定构件混凝土强度推定值的方法。     

低矮房屋风致结构响应极值不确定性分析

风致结构响应极值估算在结构抗风的可靠度设计中十分重要。在整个极值估算过程中,由于许多不定或随机的因素存在(如：极值自身、估算方法、样本采集、极值概率模型等),得到的极值通常存在不确定性。在各种影响因素中,该文将考虑结构响应极值变量本身的随机特性,对任意分位点处响应极值的不确定性进行分析。首先,利用有限元软件对低矮房屋模型进行框架结构设计并优化,加载风压荷载得到结构响应时程数据。然后,基于Hermite多项式模型(HPM)转换过程方法,估算得到响应的极值Ⅰ型分布(Gumbel);基于该极值估算方法,提出时程样本偏度、峰度、零超越次数与Gumbel分布两个参数之间的经验公式。接着,考虑前四阶矩的不确定性,利用经验公式以及多步概率分析,对任意分位点处响应极值的不确定性进行估计。最后,给出相关结论。     

响应面法在隧道衬砌结构可靠度分析中的应用

  在进行结构可靠性分析中,荷载效应统计特征分析是一个比较复杂的问题,特别是在地下结构的可靠度分析中。采用有限元响应面方法计算隧道衬砌结构荷载效应的统计特征,并结合可靠度计算的分位值法进行结构可靠指标的计算。     

Rapid prediction of deflections in multi-span continuous composite bridges using neural networks

This paper proposes closed form expressions for the rapid prediction of deflections in steel-concrete composite bridges of large number of spans subjected to service load. The proposed expressions take into account shear lag effect, flexibility of shear connectors and cracking in concrete slabs. Three separate neural networks have been developed for right exterior span, left exterior span and interior spans. The closed form expressions have been obtained from the neural networks developed in the study. The training, validating and testing data sets for the neural networks are generated using finite element software ABAQUS. The proposed expressions have been validated for number of bridges and the errors are found to be small for practical purposes. Sensitivity studies have been carried out using the proposed expressions to evaluate the suitability of input parameters. The use of the proposed expressions requires a computational effort that is fraction of that required for the finite element analysis, therefore, can be used for rapid prediction of deflection for everyday design.     

透水模板布孔径分布测试方法与理论研究

根据负压法测孔原理,采用土壤吸力平板仪测试了透水模板布的孔径分布累计曲线,同时运用基于平面随机分割(Poisson polyhedron)理论得到的非织造土工织物孔径分布计算模型,计算了透水模板布孔径分布累计曲线.结果表明,尽管透水模板布孔径分布累计曲线两端约10%的大孔和5%的小孔其实测结果与理论计算有偏差,但实测曲线与模型计算曲线孔径范围和主体分布相当一致,说明孔径分布计算模型能够有效反映透水模板布的孔径分布特征.     

Response of flexible structures in random seas

A general stochastic formulation which addresses the relative motion effects of flexible structures in a random seaway is presented. The formulation utilizes a least-squares cubic polynomial approximation to the nonlinear drag force contribution. This influences the process of developing the stochastic equations of motion in two ways. First, by introducing higher order moments in the covariance form of the equations. Through the use of Gaussian closure these moments are replaced by equivalent expressions involving only second order moments. Secondly, the cubic drag force approximation results in the appearance of a three-fold convolution of the velocity spectrum in the final form of the spectral equations. Although, an iterative solution technique is required to solve the resulting spectral equations, this formulation provides the engineer with a means to directly evaluate the structural displacement spectrum at selected elevations. Once the response spectra are known extremal response behaviour can be predicted. A single-degree-of-freedom model of a monotower platform and a multi-degree-of-freedom model of a marine riser are used to illustrate the influence of the various approximations on the response predictions. Issues addressed include the impact on the viscous damping in the structural model as a result of first and third order approximations to the viscous drag force and the need to include higher modes for very flexible structures.     

关于土坡稳定性分析中的分项系数取值的讨论

基于可靠度理论的分项系数法已在结构工程设计中广泛应用,并成为当前岩土工程设计方法发展的一种趋势。针对土质边坡抗剪强度参数(凝聚力c和摩擦系数f)的分项系数取值进行讨论:首先结合条分法、响应面法以及设计域的概念,在不同工况下标定分项系数的取值;然后评估,在采用不同分项系数组合进行边坡设计时,实际边坡的可靠度水平。研究结果表明:1变异系数对分项系数的标定有重要影响;2减小分位比能够部分抵消一定目标可靠度下由变异系数所引起的分项系数标定值变化以及一定分项系数下实际边坡可靠度水平的波动;3设计验算点是设计域中特殊的一点,它并不是最优的设计值取值点,使用标准化空间里设计域上特定角度一点作为设计值取值点更具优势;4当分位比为0.1,目标可靠度3.2时,经过优化分析,推荐土质边坡分项系数取值为c?=1.30,f?=1.15;5使用该方法,可以对现有安全系数设计方法的可靠度水平进行评估。     

大型袋式除尘器花板活荷载标准值取值研究

根据现行规范中荷载标准值由设计基准期内最大荷载概率分布的特定分位值确定的方法,详细讨论了大型袋式除尘器中花板活荷载标准值的计算步骤,并以某大型袋式除尘器花板结构为例,计算了不同保证率下的活荷载标准值,为大型袋式除尘器结构计算分析和优化设计的荷载取值提供了参考。     

Probabilistic analysis of nonlinear response to sustained load processes

Reliability study of structural members subjected to a stochastic load process has been extended to include material nonlinearity. Tractable expressions for the first moment statistics of the number of load exceedances and damage duration were derived previously, primarily for a Poisson square wave process and for material with a bilinear force-deformation relationship. A probability distribution for the number of load exceedances is derived herein. It is found that the derived distribution, when approximated by a Taylor series, is in good agreement with that obtained from the simulated sustained load processes if the coefficient of variation of load exceedances is under 50%. In addition, the relationship between linear load effect and non-linear structural response is examined for different elastic limits both in load exceedances and in damage duration. It appears that for high threshold levels, nonlinear response may not be as critical as linear response.     

Minimum‐weight design of high‐rise structures subjected to flexural vibration at a desired natural frequency

In this paper, a parametric approach for design of high‐rise structures subjected to flexural vibration is proposed. The optimization problem is formed based on a preselected value for the fundamental natural frequency, and it is formulated for minimum structural weight. In a two‐step approach, first, an alternative formulation aimed at maximizing structural stiffness that in turn maximizes structure's fundamental frequency is introduced. Then, optimized results are used in obtaining a closed‐form solution of the actual problem. Because the resulting equations are rather complicated, approximate forms are developed in order to simplify the design process. In all relations, contributions from shear forces to lateral displacement are assumed to be negligible; hence, bending resistance is the only design variable, and its optimal value is computable using simple relations. Two numerical examples are presented in order to illustrate the efficiency of this method in practice.