Generation of dominant modes and reliability analysis of frames

The paper presents a technique for reliability analysis of frames using the stiffness matrix method of linear elastic and piecewise linear elastic-plastic structural analysis and the first order second moment reliability method. It involves automatic generation of stochastically dominant mechanisms (modes) and their safety margin equations and calculation of bounds on the system reliability. Based on the computational experiments, logical strategies are proposed for selection of plastic hinges and branching of failure paths to identify stochastically dominant mechanisms. The proposed method is illustrated with numerical examples. The results show very good agreement with the results of other research workers. The proposed method of generating stochastically dominant modes is simple and fairly efficient and less computer-time consuming.     

Reliability analysis of frame structures with brittle components

A review is provided of the recent developments in the reliability analysis of ductile (elastic-plastic) systems and of the stable configuration approach of structural system reliability analysis. These are combined to obtain a methodology for evaluating the reliability of frames that have primarily ductile components but also a few brittle components. The methodology is applied to two frame structures.     

Practical random field discretization in stochastic finite element analysis

Stochastic finite element-based reliability analysis is applied to structures with distributed parameters that can be modeled as random fields. In this method, reliability is estimated through analytical computation of the sensitivity of stochastic response to the basic random variables. The random fields are discretized into sets of correlated random variables using two methods of discretization. The sensitivity measures are further used to selectively consider only a few of the distributed parameters as random fields, to ensure computational efficiency. The issue of choosing the appropriate mesh for the discretization of the random field is addressed through mesh refinement studies. With the help of three numerical examples, the paper examines the effects of the correlation characteristics of the random field on discretization and reliability analysis, and develops guidelines for efficient application of stochastic finite element analysis to structures with distributed parameters.     

Ductile structural system reliability analysis using adaptive importance sampling

This paper presents a hybrid technique for efficient system reliability estimation of large ductile framed structures. The proposed procedure starts with a simple enumeration scheme, but quickly changes to an adaptive importance sampling scheme to make the process more efficient and easier to implement. The method solves the problem of including the effect of multiple failure sequences in an importance sampling scheme, for the system reliability estimation of large structures. The enumeration method is used to identify the first complete failure sequence. This failure sequence defines the initial failure domain for starting the adaptive sampling process. A weighted multi-modal sampling density is used to account for the contribution of different regions in the sampling domain to the system failure probability. As the simulation progresses, the failure domain is gradually modified to include the effect of other significant failure sequences and arrive at an accurate estimate of the system failure probability.     

正交异性钢桥面板结构体系的疲劳破坏模式和抗力评估

正交异性钢桥面板的疲劳问题属于包含多疲劳破坏模式的结构体系疲劳问题。基于这一本质特性,以典型的正交异性钢桥面板结构体系为研究对象,由结构体系的主导疲劳破坏模式出发,提出正交异性钢桥面板结构体系疲劳抗力评估的新方法。以纵肋与顶板焊接细节和纵肋与横隔板交叉构造细节为主要研究对象,设计8个足尺节段模型,主要包括传统纵肋与顶板焊接细节、新型镦边纵肋与顶板焊接细节和纵肋与横隔板交叉构造细节,通过模型试验研究了两类重要构造细节的主导疲劳破坏模式和实际疲劳抗力,在此基础上结合切口应力评估方法探讨正交异性钢桥面板构造细节切口应力S-N曲线方程、结构体系的主导疲劳破坏模式等关键问题。研究结果表明：传统纵肋与顶板焊接细节和新型镦边纵肋与顶板焊接细节的主导疲劳破坏模式均为疲劳裂纹萌生于焊根并沿顶板厚度方向扩展,二者的实际疲劳抗力基本相同;纵肋与横隔板交叉构造细节的疲劳破坏模式为焊趾开裂沿纵肋腹板方向扩展;对于研究对象而言,萌生于纵肋与顶板焊接细节焊根并沿顶板厚度方向扩展的疲劳破坏模式为控制结构体系疲劳抗力的主导疲劳破坏模式。     

An assessment of high-order bounds for structural reliability

Second-order bounds are often used to estimate the reliability of series structural systems. However, the quality of second-order bounds is poor when the failure modes are highly correlated. This paper examines the merit of third and higher-order bounds. First, the general form of the nth-order upper and lower bounds is derived. Second, an efficient algorithm for computing the n-dimensional multinormal integral is presented. Finally, the performance of the third and fourth-order bounds is examined in three numerical examples. These bounds can provide significant improvement over the second-order bounds, particularly when the number of significant failure modes is small (less than 20) or when the correlations between modes are somewhat localized.     

Sensitivity and importance measures in structural reliability

In multivariate reliability problems, which depend on one or more parameters τ, a sensitivity factor α_E[τ] is defined as the derivative of the equivalent reliability index β_E(τ)=?φ^?1[P _f (τ)] with P _f (τ) the failure probability. α_E[τ] expresses the change of α_E(τ) due to small variations of τ. Since the numerical evaluation of α_E[τ] is usually impractical, an approximation α_E[τ]≈α[τ] is derived, which is asymptotically exact for extreme reliability levels. Simple formulae for α[τ] are given. The approximation α_E[τ]:a[τ] also provides the basis for a better understanding of the commonly used alpha values α_i=?1/βu i? as importance measures for stochastic variables.     

Equivalent linear limit states

The replacement of any limit state function by an equivalent linear function is discussed in detail and it is shown that there are several methods by which such an equivalence may be established. In the case of multiple failure mode problems the definition of fully equivalent linear functions requires the additional consideration of mode correlations. The manner in which this may be carried out is discussed and it is shown that, for most practical cases, a relatively simple analysis is likely to be sufficient to define the set of equivalent linear limit states, with the advantage that the established theory of such linear limit states may then be employed in the majority of reliability analyses.     

具有中介状态的工程系统的可靠性分析

提出了其单元具有"安全-中介-失效"工作模式的各种系统的可靠性分析方法.这时,系统及其单元均具有安全、中介、失效三种工作状态.可靠性分析的结果是求出系统的安全概率PR、中介概率PM和失效概率PF.它们联合起来构成可靠性向量[PR,PM,PF].在分析中还考虑了系统各单元间失效相关性问题.     

基于典型失效模式可靠度分析的柱端弯矩增大系数概率评定

提出一种基于典型失效模式可靠度分析的柱端弯矩增大系数（column overdesign factor）η_c的概率评定方法。该方法利用可靠度理论对结构典型失效模式的发生概率进行分析,获得能使结构最可能发生“强柱弱梁”破坏的最优η_c取值。以一栋五层钢筋混凝土框架结构为例,对提出方法进行应用研究,考虑屈服失效、极限变形失效和剪切失效3类失效机制,选取7种典型失效模式。研究结果表明：我国现行规范建议的η_c取值无法保证钢筋混凝土框架结构最大可能发生“强柱弱梁”破坏,而当η_c取值大于3.1时,钢筋混凝土框架结构发生“强柱弱梁”破坏的概率最大。     

A fast and efficient response surface approach for structural reliability problems

The reliability analysis of large and complex structural requires approximate techniques in order to reduce computational efforts to an acceptable level. Since it is, from an engineering point of view, desirable to make approximative assumptions at the level of the mechanical rather than the probabilistic modeling, simplifications should be carried out in the space of physically meaningful system- or loading variables.Within the context of this paper, a new adaptive interpolation scheme is suggested which enables fast and accurate representation of the system behavior by a response surface (RS). This response surface approach utilizes elementary statistical information on the basic variables (mean values and standard deviations) to increase the efficiency and accuracy. Thus the RS obtained is independent of the type of distribution or correlations among the basic variables which enables sensitivity studies with respect to these parameters without much computational effort.Subsequently, the response surface is utilized in conjunction with advanced Monte Carlo simulation techniques (importance sampling) to obtain the desired reliability estimates.Numerical examples are carried out in order to show the applicability of the suggested approach to structural systems reliability problems. The proposed method is shown to be superior both in efficiency and accuracy to existing approximate methods, i.e., the first order reliability methods.     

Probability-based criteria for structural design

Probability-based loading and resistance criteria are presented that are suitable for routine safety checking in design. The criteria are based on a comprehensive analysis of statistical data on structural loads and resistances and an examination of levels of reliability implied by the use of current design standards and specifications. The criteria are intended to be used in specifications that are oriented towards limit states design.     

Estimating the annual probability of failure using improved progressive incremental dynamic analysis of structural systems

A methodology based on the progressive incremental dynamic analysis has been introduced in this paper to estimate the structural response and the corresponding annual probability of failure. The proposed methodology employs the genetic algorithm optimisation technique and an equivalent single‐degree‐of‐freedom system corresponding to the first‐mode period of a considered structure. The proposed methodology can significantly reduce the number of ground motion records needed for estimating the annual probability of failure. The numerical results indicate that the proposed method can effectively reduce the computational effort needed for computation of probability of failure for the first‐mode dominated structures, which is advantageous as the structure becomes larger. A relatively huge set of single‐degree‐of‐freedom systems as well as three multi‐degree‐of‐freedom systems including 3, 8 and 12 storeyed reinforced concrete structures was taken into account to test the proposed methodology. It has been shown that the probability of failure can be estimated within ±15% error with 95% confidence. The proposed method can speed up the decision‐making process in the probability‐based seismic performance assessment of structures, and it also incorporates the randomness of strong ground motions explicitly. Copyright © 2012 John Wiley & Sons, Ltd.     

Risk analysis of bridge falsework cuplok systems

Bridge falsework systems are one of the most common temporary structures used to support the formwork during the construction of concrete bridges. In this paper, the development of a risk analysis of selected bridge falsework structures using the Cuplok^® system is presented by means of a risk-informed structural design methodology based on newly developed structural robustness and structural fragility indices. Illustrative examples are given detailing the steps and calculations needed, including consideration of model and statistical uncertainties, and the results obtained are discussed. Furthermore, strategies for decreasing risks are presented and risks are determined for a reference (baseline) scenario and one selected alternative (improved) scenario and later evaluated against proper risk criteria. For the cases analysed, it is concluded that if the cost of the permanent structure considerably exceeds the cost of the temporary structure, which is often the case, the extent of improvements in terms of structural and economical risks may justify the extra costs incurred by implementing simple quality management procedures.     

Analysis of structural failures in timber structures: Typical causes for failure and failure modes

Collapses of timber structures are negative for the competitiveness of timber on the construction market. The question is what can be done to reduce the risk for failure in timber structures in the future. For this purpose a comprehensive survey and analysis of failures in 127 timber structures have been made. The present paper summarises the results from this survey with an analysis of the underlying causes and associated conclusions and recommendations. The most common cause of failure is related to weaknesses in or lack of strength design (41.5%), followed by poor principles during erection (14.1%), on-site alterations (12.5%) and insufficient or lack of design with respect to environmental actions (11.4%). In total, about half of the failures are caused by the designer and about one fourth of the failures are caused by the personnel working at the building site. Wood quality, production methods and production principles only cause a small part (together about 11%) of the failures.     

结构全寿命的耐久性与安全性、适用性的关系

在结构可靠度分析和设计中,尚未考虑耐久性对结构安全性和适用性的影响,缺乏相应的耐久性失效准则,对耐久性概念的认识还存在一定的分歧,针对上述存在的问题,从结构全寿命的角度出发,分析了安全性、适用性和耐久性三者的基本概念,明确了三者的基本内涵。通过分析影响结构抗力变化的各种因素,结构性能随时间变化的过程,指出在结构全寿命期内,不同的耐久性水平对结构安全性和适用性将产生不同的影响,三者之间是相互制约、相互影响的交叉关系,重新定义了结构的耐久性。提出了基于性能设计的耐久性极限状态设计新理念,同时指出性能极限状态设计可根据不同的耐久性要求,定义不同的耐久性极限状态,对同一结构构件,若采用不同的性能极限状态,结构的失效概率或使用寿命会有较大的差别。     

关于结构理论在施工中的若干问题

阐述了施工中结构理论对于建筑工程质量的重要性，从施工荷载、施工阶段的强度问题、施工阶段钢筋位置变化问题等多方面，对施工过程中存在的许多结构理论问题作了探讨，以确保工程质量。