Optimum design of steel truss arch bridges using a hybrid genetic algorithm

The design of steel truss arch bridges is formulated as an optimization problem. The objective function considered is the weight of the steel truss arch bridge. The objective function is minimized subjected to the design constraints of strength (stress) and serviceability (deflection). An efficient, accurate, and robust algorithm is proposed for optimal design of steel truss arch bridges. The proposed algorithm integrates the concepts of the genetic algorithm (GA) and the finite element method. A real-coded/integer-coded method is used to realistically represent the values of the design variables. Three GA operators consisting of constraint aggregate selection procedure, arithmetic crossover, and non-uniform mutation are proposed. Finite element method is used to compute values of implicit objective functions. A numerical example involving a detailed computational model of a long span steel truss arch bridge with a main span of 552 m is presented to demonstrate the applicability and merits of the proposed method.     

System reliability of suspension bridges

Provisions for the design of existing suspension bridges often rely on a deterministic basis. Consequently, the reliability of these bridges cannot be assessed if current provisions are applied. In order to develop cost-effective design and maintenance strategies for suspension bridges a system reliability-based approach has to be used. This is accomplished by a probabilistic finite element geometrically nonlinear analysis approach. This study forms part of an investigation into the system reliability evaluation of geometrically nonlinear large span bridges recently undertaken at the University of Colorado. A brief review of reliability analysis of geometrically nonlinear elastic structures allows for the determination of its relevance to the assessment of suspension bridges. A probabilistic finite element geometrically nonlinear elastic code is used for system reliability evaluation of suspension structures. The allowable stress design procedures used by the Honshu Shikoku Bridge Authority for the design of suspension bridges are presented along with their application to the design of an existing bridge. This bridge is studied from a system reliability viewpoint to evaluate its reliability under different loading and damage scenarios. Such information calls attention to the fact that the reliability of cables, hanger ropes and girders are very different. Therefore, optimal maintenance decisions for suspension bridges designed according to allowable stress method are not consistent with those based on equal component reliability values.     

Risk-based probabilistic thermal-stress analysis of concrete arch dams

The probabilistic risk of arch dam failure under thermal loading is studied. The incorporated uncertainties, which are defined as random variables, are associated with the most affecting structural (material) properties of concrete and thermal loading conditions. Karaj arch dam is selected as case study. The dam is numerically modeled along with its foundation in three-dimensional space; the temperature and thermal stress distribution is investigated during the operating phase. The deterministic thermal finite element analysis of the dam is combined with the structural reliability methods in order to obtain thermal response predictions, and estimate the probability of failure in the risk analysis context. The tensile overstressing failure mode is considered for the reliability analysis. The thermal loading includes ambient air and reservoir temperature variations. The effect of solar radiation is considered by an increase in the ambient temperatures. Three reliability methods are employed: the first-order second-moment method, the first-order reliability method, and the Monte-Carlo simulation with Latin Hypercube sampling. The estimated failure probabilities are discussed and the sensitivity of random variables is investigated. Although most of the studies in this line of research are used only for academic purposes, the results of this investigation can be used for both academic and engineering purposes.     

Reliability-based design optimization using convex linearization and sequential optimization and reliability assessment method

In this study, an effective method for reliability-based design optimization (RBDO) is proposed enhancing sequential optimization and reliability assessment (SORA) method by convex linearization. In SORA, reliability estimation and deterministic optimization are performed sequentially. And the sensitivity and function value of probabilistic constraint at the most probable point (MPP) are obtained in the process of finding reliability information. In this study, the convex linearization is constructed by utilizing the sensitivity and function value of the probabilistic constraint at the MPP. So no additional evaluation of the probabilistic constraint is required in the deterministic optimization in SORA. The proposed RBDO method is applied to numerical examples and compared to various RBDO methods. It is shown that the proposed method is very efficient with similar accuracy.     

Reliability analysis of long span steel arch bridges against wind-induced stability failure

An efficient method for reliability assessment of long span steel arch bridges against wind-induced stability failure is presented in this paper. The prediction of wind-induced stability of such bridge structures is performed by an eigenvalue method and the reliability estimates are determined by a generalized first-order reliability algorithm. A software strategy for interfacing the present method with ANSYS is developed via a freely available MATLAB software tool (FERUM). A numerical example involving a detailed computational model of a long span steel arch bridge with a main span of 550 m is presented to demonstrate the applicability and merits of the present method and the software strategy. Finally, the most influential random variables on the reliability of long span steel bridges against wind-induced stability failure are identified by a sensitivity analysis.     

Probabilistic capacity model and fragility estimates for composite floor systems subjected to column loss

This paper presents probabilistic capacity models for composite floor systems subjected to column loss. The probabilistic capacity models are formulated by adding explanatory terms to an existing deterministic model. The explanatory terms are selected to correct the bias in deterministic capacity model. After that, virtual experiment data generated from finite element simulations are used to calibrate the model parameters. The finite element models consider the effect of axial loading on steel connection and the slab membrane action. The calibration of model parameters is conducted using the Bayesian inference approach. As an application and validation of the probabilistic capacity models, fragility analyses of typical composite floor systems are carried out. The developed fragility curves are compared with those from finite element analysis cooperated with Monte Carlo simulation. The comparison results show that the proposed capacity models are considered reasonable in predicting the resistance capacity of the composite floor and seek a compromise between model accuracy and computational efficiency. The proposed capacity models can be used to carry out a rapid fragility analysis against progressive collapse.     

首都国际机场T3航站楼交通中心钢结构体系稳定分析

以首都国际机场交通运输中心(GTC)大跨箱型钢拱结构为工程背景,以单拱、组拱和整体屋盖钢拱结构为研究对象,采用非线性有限元法分析结构失稳问题。在多种不利荷载组合作用下,研究对应于GTC钢结构体系的理想结构整体稳定性能并得出结构特征屈曲系数的基础上,考虑几何非线性、材料非线性,按最不利原则施加初始几何缺陷,计算结构的非线性稳定屈曲系数,获得相应的折减系数,并讨论了双重非线性对结构整体稳定的影响。同时,数值分析了主拱拱脚处腹板的局部稳定问题,结果说明,在拱脚处需合理设置加劲肋板来避免腹板局部失稳。     

钢管混凝土拱桥结构非线性分析

指出了近年来大跨度桥梁的结构分析广泛地采用有限单元法,通过建立有限元模型,对钢管混凝土拱桥在施工及营运阶段非线性应力进行了分析,从而得到更接近真实状态的应力结果。     

大跨度悬索桥颤振可靠度分析的改进响应面法

介绍一种新的计算悬索桥颤振可靠度方法——改进响应面法。该方法利用传统响应面法将极限状态方程近似表达为简单的多项式形式,有效地解决FORM和SORM无法求解隐式极限状态方程的可靠度问题。另外,改进响应面法的使用还能有效地利用现有的确定性颤振分析软件。通过引入有限元方法,改进响应面法可应用于悬索桥颤振可靠度问题。通过使用重要抽样技术,提高改进响应面法的计算效率和计算精度,有效地解决传统响应面法在结构可靠度较大或失效概率较低时出现的迭代不收敛问题。数值算例验证该方法的效率和精度。最后,用该方法计算江阴长江大桥的颤振可靠度,结果表明基于经验公式的改进响应面法会过高地估计大跨度悬索桥的颤振可靠度。实际的悬索桥颤振可靠度应该采用基于有限元法的改进响应面法进行计算。     

Reliability-based design optimization of adhesive bonded steel-concrete composite beams with probabilistic and non-probabilistic uncertainties

It is meaningful to account for various uncertainties in the optimization design of the adhesive bonded steel-concrete composite beam. Based on the definition of the mixed reliability index for structural safety evaluation with probabilistic and non-probabilistic uncertainties, the reliability-based optimization incorporating such mixed reliability constraints are mathematically formulated as a nested problem. The performance measure approach is employed to improve the convergence and the stability in solving the inner-loop. Moreover, the double-loop optimization problem is transformed into a series of approximate deterministic problems by incorporating the sequential approximate programming and the iteration scheme, which greatly reduces the burdensome computation workloads in seeking the optimal design. The validity of the proposed formulation as well as the efficiency of the presented numerical techniques is demonstrated by a mathematical example. Finally, reliability-based optimization designs of a single span adhesive bonded steel-concrete composite beam with different loading cases are achieved through integrating the present systematic method, the finite element analysis and the optimization package.     

基于概率屈服准则的弹塑性随机有限元分析

基于偏微分技术，采用增量理论建立了弹塑性随机有限元增量初应力法的计算格工，考虑屈服准则的不确定性，对岩体Drucker-Prager屈服准则进行随机分析，以代替随机有限计算中屈服准则的定值分析，最后，编制了基于概率屈他准则的弹塑性随机有限元分析程序，并对两个算例进行了分析和讨论，得出了结论。     

Reliability analysis of a long span steel arch bridge against wind-induced stability failure during construction

An efficient and accurate algorithm is proposed to evaluate the reliability of long span steel arch bridges against wind-induced stability failure during construction. The algorithm is developed based on stochastic finite-element method. Uncertainties in static wind load-related parameters are incorporated in the algorithm. The proposed algorithm integrates the finite-element method and the first-order reliability method. A long span steel arch bridge with a main span length of 550 m built in China is considered as an illustrative example. Two different construction stages are chosen for reliability analysis. Construction stage I involves the construction process before closure of main arch ribs. At Construction stage II, all remaining parts of the bridge have been completed except the stiffening girder of the main span. Three components of wind loads (drag force, lift force and pitch moment) acting on both steel girder and arch ribs are considered in the study. Results of the study show that the steel arch bridge at construction stage II is more vulnerable to wind-induced stability failure than that at construction stage I. Further, a detailed parametric study show that the variations of wind speed with height, drag force of wind loads, design wind speed at the bridge site and static aerodynamic coefficients have significant effects on the probability of wind-induced stability failure during the construction stages for the steel arch bridge.     

System Reliability of Slopes by RFEM

In a probabilistic slope stability analysis, the failure probability associated with the most critical slip surface (the one with the minimum reliability index) is known to be smaller than that for the system that comprises all potential slip surfaces. The first order reliability method (FORM) targets the minimum reliability index related to the critical slip surface, and thus cannot be used to predict the system reliability of slopes, except when all possible slip surfaces are perfectly correlated. It is shown in this paper that the random finite element method (RFEM), which uses elastoplastic finite elements combined with random field theory in a Monte-Carlo framework can accurately predict the system probability of failure (p_f) of slopes.     

Flutter reliability analysis of suspension bridges

A reliability analysis method is proposed in this paper through a combination of the advantages of the response surface method (RSM), finite element method (FEM), first-order reliability method (FORM) and the importance sampling updating method. The method is especially applicable for the reliability evaluation of complex structures of which the limit state surfaces are not known explicitly. After the accuracy and efficiency of the method are demonstrated through numerical examples, the method is used to estimate the flutter reliability of a suspension bridge. The uncertainties such as material properties, geometric parameters, structural damping ratio, flutter derivatives and extreme wind velocity at the bridge site are considered. The example suspension bridge is the Jiang Yin Bridge with a main span length of 1385 m built in China. The results show that the proposed method based on an empirical formula in which the limit state function is explicitly represented as a function of variables overestimates the flutter reliability of suspension bridges. The actual flutter reliability should be more accurately analyzed using the proposed method based on the deterministic finite element method in which the limit state function is implicitly represented as a function of variables. Finally, the most influential random variables on flutter reliability of suspension bridges are identified by using a sensitivity analysis.     

基于非侵入式随机有限元法的地下洞室可靠度分析

提出了地下洞室变形可靠度分析的非侵入式随机有限元法。介绍了随机多项式展开的基本原理,采用 GEOSLOPE 的 SIGMA/W 模块进行确定性有限元分析。提出了随机多项式展开与 SIGMA/W 模块接口方法及其流程图,从而实现了确定性有限元分析和随机分析一体化。最后研究了非侵入式随机有限元法在地下洞室变形可靠度分析中的应用。结果表明：非侵入式随机有限元法使得随机分析与确定性有限元分析互不耦合,其计算效率是传统的蒙特卡罗模拟方法无可比拟的,它是地下洞室变形可靠度问题分析一种有效的方法。采用二次衬砌支护是提高地下洞室可靠度有效的方法。此外,岩体变形模量的变异性对地下洞室变形可靠度有非常明显的影响,而岩体重度的变异性对可靠度基本上没有影响。因此,在地质勘查中要尽可能准确地确定岩体的变形模量,从而有效地提高地下洞室变形可靠度。     

浅谈钢管混凝土系杆拱桥拱肋的数值模拟方法

对钢管混凝土系杆拱桥的拱肋在有限元分析中的模拟方法进行了探讨,并以一40 m跨的钢管混凝土系杆拱桥为算例,运用大型有限元软件M IDAS/C IVIL对该桥进行全桥计算,主要考察了钢管混凝土拱肋在施工过程中各种模拟方法的异同和管内混凝土收缩徐变对钢管混凝土系杆拱桥拱肋的影响,通过对计算结果的比较分析,得出一些有指导意义的结论。     

Study on the mechanical performance of Butterfly Arch Bridge

This paper presents issues in the design concept, analysis and test results of a single span special‐shaped arch bridge—the Butterfly Arch Bridge located in Zhongshan, Guangdong, China. The bridge is composed of ribs of tow slope steel arch, steel box girder of bridge deck curves and inclined boom. It is a special‐shape bridge of spatial girder and arch combination. This paper discusses the experimental study and the finite element simulation analysis for the main components of the bridge. Emphasis will be put on the following three aspects: First, model design adopts the similarity principle of the stress and stiffness. The stress similarity was considered first when the two principles are in conflict. Second, the finite element model of the bridge should adopt a composite element according to the different structural members of the bridge. Third, the weight of the two arch ribs and all dead loads of the main girder, in addition to part of the live loads, must be in a balanced condition. Furthermore, based on results from tests and simulation analysis, the following four key issues were studied: (a) theory of the bridge calculation; (b) behaviour of the bridge spatial mechanics; (c) the dynamic behaviour of the bridge; and (d) stiffness relation between the two arch ribs, steel box beam and end cross beam. Copyright © 2008 John Wiley & Sons, Ltd.     

钢管混凝土拱桥吊装施工拱肋控制点预抬高值计算

对钢管混凝土拱桥吊装施工拱肋控制点的预抬高值分两部分进行考虑:一部分由刚体位移引起,一部分为拱肋弹性变形产生,两部分之和为实际的预抬高值。计算结果与有限元结果进行了比较,结果表明拱肋弹性变形可以达到刚体位移的量级,随着钢管混凝土拱桥跨度的增加,仍将拱肋简单的假定为刚体会带来不可接受的误差。     

ABAQUS用于约束钢柱抗火性能计算的验证

采用通用有限元分析软件ABAQUS对某试验工况中的约束钢柱和钢梁进行模拟计算,将试验测得的热电偶温度数据作为初始荷载进行热传导分析,得到整个约束钢柱和约束梁的温度场分布,并以此为输入温度场进行热力分析.通过将有限元分析软件计算所得的位移随温度变化关系同试验数据比较,验证了利用ABAQUS进行钢构件热力耦合分析的工具可靠,方法合理.     

单桩承载力可靠度的非线性摄动随机有限元分析

针对岩土工程中的Duncan-Chang非线性本构模型，提出了二阶非线性摄动随机有限单元法的计算方法。编制了相应的程序。在分析计算中，将随机土性分布参数模拟为轴对称随机场，对随机场的选择方法，随机场的局部平均离散法技术等进行了研究。给出了计算和组集相关随机变量自协方差矩阵的方法。推导出切线弹性矩阵的一阶和二阶偏导数，给出了总体刚度矩阵的偏导矩阵和组集方法，提出了单桩承载力二阶矩的非线性随机有限单元分析方法，研究了土性参数空间变异性和荷载变异性对单桩承载力可靠度指标的影响，并进行了实例计算，研究表明，非线性随机有限元法是研究单桩承载力可靠度的有效方法。