Optimal design of buried pipelines considering failure by mechanical damage

Mechanical damage by external interference can represent a serious threat to the structural integrity of onshore pipelines. International databases indicate that, nowadays, this failure mode is the main cause of spillages in pipelines. The present paper addresses the optimal design of buried pipelines considering failure by mechanical damage. Historical incident data relating failure rates with problem parameters are used to guide the optimisation process. The design objective is to minimise total costs over the lifecycle, which include costs of construction, inspection, and expected costs of failure and repair. The optimal wall thickness, depth of cover and surveillance interval are considered as design variables. Results of numerical examples are presented and discussed. Results show that operators should favour deeper depths-of-cover. In some cases, it is shown that the higher initial costs associated to larger wall thicknesses are compensated by reduction in expected costs of failure. Results also show that, for higher costs of failure, the optimal surveillance interval is significantly smaller than the commonly-practiced 15 days.     

Effects of gross errors on structural design

In this paper, an attempt is made to take into account the effects of gross errors at the design stage, by using either two dimensional or fuzzified measures. The first measure uses the two-dimensional concept proposed by Ditlevsen, while the ‘fuzzified’ probability of failure is used as the second measure. In both instances, the design problem is described as a model of a decision-making process. Some consideration is given to the criterion of minimizing total expected cost.     

Study on seismic design method of high-rise structures resistant to rare earthquakes on basis of progressive seismic failure mode

针对高层结构罕遇地震作用下局部集中损伤破坏易导致整体倒塌的问题，及超静定次数消耗集中且不充分的结构地震失效本质，确定了高层结构耗能构件分批次充分消耗超静定次数的多阶段塑性发展路径，提出了高层结构塑性发展充分、失效方向可控、失效路径延长的渐进地震失效模式。同时，考虑结构多阶段塑性发展过程中地震作用的往复特性、高阶振型影响和非线性内力重分布特点，建立基于多阶段振型组合的标准地震作用，给出了多阶段塑性发展的抗震性能量化需求，建立了基于渐进地震失效模式的高层结构抗罕遇地震设计方法。该方法将高层结构渐进地震失效全过程的复杂设计简化为有限塑性阶段的分段抗罕遇地震设计，解决了高层结构难于抗罕遇地震量化设计的难题。为验证该设计方法的有效性，对一幢10层高层结构进行了抗罕遇地震设计，通过非线性时程分析方法验证了按所提方法设计的结构能够实现结构渐进地震失效模式以及结构抗震性能目标，并对比分析了按所提方法设计的结构与按现行规范方法设计的结构的抗震性能，得到按所提方法设计的结构在地震动峰值加速度为0.22g作用下耗能提高了57%，在非线性静力推覆作用下非线性抗力（基底剪力）最大值提高了15.59%。     

基于模糊综合评判的结构寿命周期总费用评估

结构寿命周期总费用评估是实施基于性能的抗震设计的关键问题,其中失效损失期望评估中结构失效概率的计算往往需要多次结构重分析,计算量很大,成为结构寿命周期总费用评估中的一个主要问题。本文利用模糊综合评判理论进行结构寿命周期损失期望的评估,以失效概率方法评估结果为参照,研究了隶属度函数的不同形式、结构不同重要性对模糊评判方法结果的影响,并计算了钢筋混凝土框架算例。结果表明,利用模糊综合评判进行结构损失期望评估,可以在保证一定精度的前提下,大大简化了失效损失期望评估的计算量。     

基于渐进地震失效模式的高层结构抗罕遇地震设计方法研究

针对高层结构罕遇地震作用下局部集中损伤破坏易导致整体倒塌的问题,及超静定次数消耗集中且不充分的结构地震失效本质,确定了高层结构耗能构件分批次充分消耗超静定次数的多阶段塑性发展路径,提出了高层结构塑性发展充分、失效方向可控、失效路径延长的渐进地震失效模式。同时,考虑结构多阶段塑性发展过程中地震作用的往复特性、高阶振型影响和非线性内力重分布特点,建立基于多阶段振型组合的标准地震作用,给出了多阶段塑性发展的抗震性能量化需求,建立了基于渐进地震失效模式的高层结构抗罕遇地震设计方法。该方法将高层结构渐进地震失效全过程的复杂设计简化为有限塑性阶段的分段抗罕遇地震设计,解决了高层结构难于抗罕遇地震量化设计的难题。为验证该设计方法的有效性,对一幢10层高层结构进行了抗罕遇地震设计,通过非线性时程分析方法验证了按所提方法设计的结构能够实现结构渐进地震失效模式以及结构抗震性能目标,并对比分析了按所提方法设计的结构与按现行规范方法设计的结构的抗震性能,得到按所提方法设计的结构在地震动峰值加速度为0.22g作用下耗能提高了57%,在非线性静力推覆作用下非线性抗力（基底剪力）最大值提高了15.59%。     

单层网壳结构的模糊决策分析

本文提出了单层网壳结构的模糊决策方法 ,其中决策模型的目标函数中不仅考虑了单层网壳结构的杆件费用和节点费用 ,而且还考虑了结构的维护费用以及遇到自然灾害时损失的期望值 ,另外在约束条件中考虑了强度、刚度、稳定等因素中存在的模糊信息 ,从而构成了单层网壳结构的模糊决策问题。在其模糊决策问题的求解过程中 ,分两个阶段进行求解。在第一阶段 ,以结构造价为目标函数 ,求解其模糊最小设计 ;而在第二阶段是在第一阶段中求出的模糊最小设计的基础上 ,进一步考虑结构的维护费用和损失期望值 ,求出单层网壳结构的最终模糊优化解。     

Reliability analysis of a glulam beam

The present case study is an example of the use of reliability analysis to asses the failure probability of a tapered glulam beam. This beam is part of a true structure built for a super market in the town of Kokemäki in Finland. The reliability analysis is carried out using the snow load statistics available from the site and on material strength information available from previous experiments. The Eurocode 5 and the Finnish building code are used as the deterministic methods to which the probabilistic method is compared to. The calculations show that the effect of the strength variation is not significant, when the coefficient of variation of the strength is around 15% as usually assumed for glulam. The probability of failure resulting from a deterministic design based on Eurocode 5 is low compared to the target values and lower sections are possible if applying a probabilistic design method. In fire design, if a 60 min resistance is required, this is not the case according to Eurocode 5 design procedures, a higher section would be required. However, a probabilistic based fire analysis results in bounds for the yearly probability of failure which are comparable to the target value and to the values obtained from the normal probabilistic based design.     

硬岩中的可重复使用抗爆洞室结构设计技术

在原岩中直接进行化爆模拟试验是研究地震耦合效应的重要方法,但存在爆心附近原岩介质状态改变或地震传播路径变化的问题,从而导致地震信号质量差、工程代价高。为此,提出了在地下洞室中构建可重复使用钢纤维混凝土衬砌结构的抗爆设计思路。采用数值模拟方法,对结构损伤破坏的高风险区域及破坏形式进行了预测;根据数值模拟结果,制定了针对性结构设计方案。在设计建设的密闭设施内重复开展了数百公斤级TNT当量化爆试验,通过爆后壁面破损情况对比、钻孔取样与钻孔摄像观测,证实多次爆炸未对衬砌与原岩界面产生明显影响;相似事件在同一台站的地震信号表现出了较高稳定性和重复性,证明该抗爆结构达到了预期设计目标。该工作可为类似抗爆设施建设提供工程实例与技术参考。     

超高层建筑结构生命周期多维度经济成本评估及优化方法

超高层建筑结构具有结构体系复杂、结构功能多样、生命周期长、投资量大、持有人固定不变等特点,其全生命周期成本费用包含初始费用、维护费用、灾害失效费用以及拆除费用等多项费用。为了实现超高层结构全生命周期经济利益最大化而采用的生命周期经济评估方法通过方案对比、构件对比、费用项目等多维度对超高层结构成本费用进行管理。使用该方法能够在项目初期就对结构项目进行全面的评估与优化,同时,一个超高层建筑的实例将被引用来说明其有效性与适用性。     

Design and sensitivity analysis using the probability-safety-factor method. An application to retaining walls

This paper presents a new method for designing engineering works that makes the classical approach, based on safety factors, and the modern, probability-based, approach compatible, and includes a sensitivity analysis. The method consists of a sequence of classical designs, based on given safety factors, that (a) minimize cost or optimize an alternative objective function, (b) calculate the different failure mode probabilities or their upper bounds, and (c) update the safety factors to satisfy both the safety factors and the failure probability requirements. The process is repeated until convergence. As a result, an automatic design of the engineering work, the safety factors and the corresponding probabilities of failure for all failure modes are obtained. A double safety check is used and the correspondence between safety factors and probabilities of failure for the different modes are easily understood. An advantage of this approach is that the optimization procedure and the reliability calculations are decoupled. In addition, a sensitivity analysis is performed using a method that consists of transforming the data parameters into artificial variables and using the dual associated problem. The method is illustrated by its application to a retaining wall design.     

Risk assessments of long-span bridges considering life-cycle cost concept and near-fault ground motion effect

The influence of the near-fault ground motion on the response of long-span bridges must be considered as a critical factor for seismic design because the response indicates different aspects from existing earthquake characteristics. Also, it is important to note that the safety index for the risk assessment of long-span bridges is determined based on the minimum expected life-cycle cost E(LCC). In this study, earthquake characteristics are analyzed by creating elastic and inelastic response spectrums with actual measurement records (Chi-Chi earthquake records) and then the numerical analysis of the long-span bridge in Namhae, Korea is performed according to the increase and reduction of the member stiffness based on the standard design., the reliability evaluation of the long-span bridge considering aleatory uncertainties is performed on the basis of the combined results of static analysis and seismic response analysis. Also, the minimum LCC is estimated based on failure probabilities by the different alternative design. Because of epistemic uncertainties, the results of reliability evaluation and the LCC of optimal design are selected as random variables; the safety index, failure probability and expected minimum LCC are re-evaluated with regard to critical percentage values for a risk-averse design of the long-span bridge, and are presented graphically using cumulative percentages. It is, therefore, expected that this study will provide the basic information for the risk assessment and optimal design method in performing seismic design of the long-span bridge considering earthquake characteristics.     

抗震结构全寿命预期总费用最小优化设计

提出的优化方法采用了最新国际标准(ISO 2394,1998)所提出的优化策略作为目标函数,使结构全寿命预期总费用最小,其中包括结构造价、预期失效损失和预期维修和拆除费用;首先决策最优设防烈度而后进行结构参数优化设计的二阶段优化方法,使其可以与任何设计规范相结合;结构受到各级破坏的概率可根据场地基本烈度I_0和结构设防烈度I_d直接查表求出,从而极大地简化了优化设计的方法和计算过程;推荐了结构各级破坏损失值的一些经验参数;优化方法极其实用和简便。     

隔震技术在某住宅中的应用

本文对高烈度区的某栋住宅楼进行了隔震设计。时程分析表明：在8度（0.3g）多遇地震作用下,隔震结构与非隔震结构上部结构层间剪力最大比值为0.30,减震效果显著,可以实现上部结构降一度设计的要求,并减小了上部结构的构件的尺寸,改善了住宅的建筑功能。同时,造价对比分析表明,该楼按照隔震结构设计,与抗震设计相比,总的土建造价...     

Mitigation of vortex-induced vibrations in bridges under conflicting objectives

Aerodynamic performance of long-span bridges can be improved by retrofitting cross-sectional shapes or by adding tuned mass dampers. When both the approaches are used simultaneously, the design problem may become too complicated to solve by engineering judgment. The complexity becomes even greater in a real world problem in which both the performance and the cost have to be considered. We propose a multi-objective based approach to obtain optimal solution of the design problem. The approach effectively deals with highly nonlinear design space and the conflicting objectives between the performance and the cost. Due to the conflict of the two objectives, multiple optimal solutions are obtained, which are called Pareto-optimal solutions. The final design is obtained considering the cost and the performance criteria. The proposed method is applied to the mitigation of vortex-induced vibrations, but its general framework is applicable to any other aerodynamic problems.     

A quadratic complementarity problem related to the optimal design of a pipeline freely resting on a rough sea bottom

A submarine pipeline resting on a rigid, frictionless seabed assumes an equilibrium configuration which can be determined by solving a unilateral contact problem, i.e. a quadratic program or a variational inequality.When the sea bed is irregular, its profile along the sealine can be modified, in present offshore technology, by means of costly trench excavations or artificial supports, in order to avoid excessive bending moments in the pipe. Thus, the problem arises of determining the minimum cost of the profile changes provided that a specified curvature is nowhere exceeded.This optimal design problem is solved by assuming a quadratic cost function and reducing it to a sequence of convex quadratic programming problems leading to the optimal design situation. Upper and lower bounds on the absolute minimum cost and optimality conditions are established. A suitable algorithm for solving the problem in a finite number of steps is developed. Its generalization to allow for ‘fixed charges’ due to the equipment mobilization cost is presented. Two numerical examples illustrate the optimum (minimum cost) proposed design method.     

体系能力设计法与基于性态/位移抗震设计

首先论述了基于性态 /位移抗震设计方法的发展及其需解决的问题 ,综合了结构抗震耗能原理、破坏机制控制概念、抗震体系和能力设计法等结构抗震原理 ,提出了体系能力设计法思想。体系能力设计法要求结构具有多重抗侧力抗震体系 ,并对不同子抗侧力结构采用不同的抗震能力 ,通过对抗侧力体系间能力差的控制 ,以实现主体抗侧力结构对整体结构在地震作用下的位移模式和破坏机制控制 ,进而使得整体结构的弹塑性地震响应和耗能分布的规律便于把握和确定 ,从而解决了基于性态 /位移抗震设计方法中的一些难以确定和计算的关键问题。最后通过一实例介绍了体系能力设计法的应用 ,并讨论了实现体系能力设计法基本概念和需进一步研究的问题     

多破环模式结构模糊可靠度的计算

给出一种计算多破坏模式结构模糊可靠度的实用方法，通过适当的数学变换将复杂的高维积分问题转化为常规的可靠度分析问题，从而使计算量大为减少．实例计算表明，本文方法具有较高的精度．     

A quadratic complementarity problem related to the optimal design of a pipeline freely resting on a rough sea bottom

A submarine pipeline resting on a rigid, frictionless seabed assumes an equilibrium configuration which can be determined by solving a unilateral contact problem, i.e. a quadratic program or a variational inequality.When the sea bed is irregular, its profile along the sealine can be modified, in present offshore technology, by means of costly trench excavations or artificial supports, in order to avoid excessive bending moments in the pipe. Thus, the problem arises of determining the minimum cost of the profile changes provided that a specified curvature is nowhere exceeded.This optimal design problem is solved by assuming a quadratic cost function and reducing it to a sequence of convex quadratic programming problems leading to the optimal design situation. Upper and lower bounds on the absolute minimum cost and optimality conditions are established. A suitable algorithm for solving the problem in a finite number of steps is developed. Its generalization to allow for ‘fixed charges’ due to the equipment mobilization cost is presented. Two numerical examples illustrate the optimum (minimum cost) proposed design method.     

Cost-benefit analysis for the optimal rehabilitation of deteriorating structures

A cost-benefit analysis for planning rehabilitation efforts of deteriorating structures is proposed, which takes into account all life-cycle costs, such as construction, failure, inspection and state-dependent rehabilitation costs, as well as state- or time-dependent benefit rates. Rehabilitations can take place anytime throughout the lifetime and are optimized by maximizing the expected net present benefit rate. This approach not only allows to determine optimal sequence of rehabilitation times and rehabilitation levels, but allows also to determine optimal lifetimes and acceptable failure rates. Numerical examples demonstrate that proper planning of rehabilitations allows to extend the lifetime of a structure – as long as the expected costs for such efforts outweigh the expected future benefit. If this is not the case, then, indeed, the structure is obsolete and alternatives have to be evaluated.     

劲性搅拌桩的优化设计

劲性搅拌桩是在水泥土搅拌桩内插入刚性芯桩形成的一种桩型,具有承载力高、造价低等优点,适用于软粘土地基。本文结合设计、试验和工程经验,讨论了劲性搅拌桩的破坏模式,并提出了劲性搅拌桩的优化设计方法,为设计和施工提供可靠依据。