城市供水管网抗震可靠性评估的随机模拟方法

在进行管网抗震可靠性评估时,现有方法大多采用相同方式进行管道的渗漏与爆管水力模拟,这会导致管网水力模拟及可靠性评估结果的不准确。考虑到地震时管道破损具有很大的随机性,对城市供水管网抗震可靠性评估的随机模拟方法进行研究。应用蒙特卡洛模拟产生管网震损场景,用泊松随机数与均匀随机数判定管道工作状态,用正态随机数确定管道渗漏系数;利用EPANET软件中喷嘴及管道关闭功能实现管道渗漏及爆管等效模拟,提出将长管道分段并应用"分步迭代"法求解低压管网水力方程,提高震损管网水力模拟精度。以震损场景下节点流量统计平均值与正常时节点流量的比值作为可靠度指标,分别采用所提出算法及GIRAFFE软件对云南某古镇供水管网在VIII、IV烈度时抗震可靠性进行评估,评估结果证明了所提出算法的可行性。评估结果还表明,除地震烈度外,管网本身水力条件对供水可靠性影响较大,管网末端、支管服务区域及地势较高区域供水可靠性远低于干管服务区。     

Vulnerability of structural systems

A structure is vulnerable if relatively small damage leads to disproportionately large consequences. A structure which is unacceptably vulnerable in any one way is not acceptably robust. A theory of structural vulnerability has been previously reported by the authors. The purpose of the theory is to identify particular failure scenarios by analysing the connectivity of the structural form. In this paper vulnerability theory is applied to three-dimensional frames and, in a preliminary way, to structural dynamics which is crucially important in assessing impact damage in for example progressive collapse. The concepts are illustrated through examples. The vulnerable failure scenarios may then be examined by conventional response analysis and/or by systems reliability theory and risk assessment. The theory is also applicable to structural damage assessment or for assessing structures under unforeseen terrorist attack.     

多热源环状热网可靠性评估方法研

提出了一种基于故障影响分析的多热源环状热网可靠性评估方法,该方法将元件故障概率与热网故障工况水力分析相结合,采用可修复系统模型来评估热网的可靠性。给出了一系列热用户供热可靠性指标和热网系统可靠性指标。对一实际热网进行了可靠性评估和分析,找出了热网的薄弱环节,提高了其可靠性。     

Prediction of the combination of failure modes for an arch bridge system

An innovative prediction method for the combination of failure modes of an arch bridge is proposed for its probabilistic risk assessment and is compared with the conventional system reliability analysis method. The suggested method reveals various combinations of failure modes in significantly reduced time and efforts in comparison to the previous permutation method or the conventional system reliability analysis method. Additionally, the suggested method can be used for the verification of the system reliability with more specific predictions of the failure modes.Component reliabilities of girders have been evaluated by using response surfaces of the design variables at the selected critical sections, i.e. the maximum shear and negative moment locations. Response Surface Method (RSM) is successfully applied for the reliability analyses of complex structure with relatively small probability of failure. Previous methods such as Monte Carlo Simulations or First Order Second Moment method cannot easily calculate the derivative terms of implicit limit state functions. Hence, the target bridge system is modeled as a series-connection system for the analysis of its system reliability. The upper and lower bound of the probabilities of failure for the structural system have been evaluated and are compared with the suggested prediction method for all possible combination of failure modes.     

Reliability analysis for highway bridge deck assessment

An assessment procedure based on structural system reliability principles, has been developed for highway bridges with concrete or composite decks. It concentrates on the ultimate flexural limit state of slabs and takes account of the fact that the failure of the structure is dependent on the combination of the applied loads, their variability, both spatial and in amplitude, and on the variability of geometric and material properties. Recognising that each of a number of topologically distinct collapse mechanisms may contribute to the overall probability of system failure, an optimisation technique is used in conjunction with structural reliability analysis in order to establish for each mechanism the critical geometry and the associated probability of occurrence.The procedure can be implemented in the assessment of existing bridge structures, where the use of plastic analysis may reveal strength reserves not utilised in design and, hence, alleviate the need for strengthening. By modelling the bridge as a virtual series system and analysing a finite number of collapse mechanisms, bounds on system failure probability in flexure can be estimated. An example of its intended use is presented for a solid slab concrete bridge deck.     

On the assessment of robustness

A framework for assessing robustness is proposed, taking basis in decision analysis theory. Robustness is assessed by computing both direct risk, which is associated with the direct consequences of potential damages to the system, and indirect risk, which corresponds to the increased risk of a damaged system. Indirect risk can be interpreted as risk from consequences disproportionate to the cause of the damage, and so the robustness of a system is indicated by the contribution of these indirect risks to total risk. A framework is presented for measuring robustness in this way, and implications for system modelling and acceptable levels of robustness are discussed. Numerical studies of idealized structural systems are performed using this framework, to demonstrate the use of the proposed robustness index and provide insight into system properties affecting robustness. Considered exposures include the design live load and an extraordinary exposure representing a fire or explosion that causes the loss of one or more system components. The results indicate that properties affecting system reliability, such as number of components or the stochastic properties of the load, also affect robustness. Perhaps more interestingly, it is seen that properties such as failure consequences and time to repair a damaged system also affect this measure of robustness. The assessment framework is applied here to study damage tolerance, but the procedure can be applied as well to other aspects of robustness such as tolerance to human error in design or construction.     

2D-numerical analysis of hydraulic fracturing in heterogeneous geo-materials

A geo-material failure process analysis (F-RFPA2D), considering the coupling of stress distribution, fluid flow, and element damage evolution, is used to investigate the mechanisms of crack initiation and propagation around a 2-D cylindrical cavity in heterogeneous stiff soils during hydraulic fracturing. A large number of numerical analysis on hydraulic fracturing in stiff soil with pre-existing injection cavity have been carried out to study the mechanism of hydraulic fracturing in stiff soil. In addition, the characteristic of acoustic emission (AE) due to hydraulic fractures are studied by numerical simulations. The results provide a better understanding of the crack initiation and propagation mechanisms during hydrofracturing. The simulation software package can be a powerful tool for study of soil behavior during hydraulic fractures.     

Probabilistic approach for failure assessment of steel structures in fire by means of plastic limit analysis

The issue of fire scenarios approach to fire protection is often subject of ongoing discussions within the fire safety engineering community. The choice of an adequate number and type of fire scenarios is not always univocal and straightforward. The fire scenarios to be used in structural assessment are quite complicated to obtain. Their definition often involves significant large-scale fire tests and sophisticated numerical simulations, taking into account numerous factors. For this reason, it seems appropriate to introduce probabilistic tools to assess the most probable fire scenarios. This work proposes a probabilistic approach integrating the Monte Carlo simulation with plastic limit analysis in order to assess the probability of failure of a structure subjected to fire. The underlying assumptions related to spatial–temporal evolution of the fire action and the response of materials and structural members comply with Eurocode provisions. The procedure, mainly devoted to identify the most critical fire scenarios for the structural response, is illustrated by means of a case study represented by a steel braced structure, used as parking. Beyond the limitations related to the simplified assumptions, the outcomes of the analyses demonstrate the potential of the approach for choosing fire scenarios by means of a probabilistic procedure and for evaluating the probability of fire-induced progressive collapse.     

Application of system reliability‐based assessment for collapse fragility of braced moment‐resisting frames

This paper presents a system reliability‐based framework for collapse fragility assessment of steel braced moment‐resisting frames (BMRFs). The conditional failure of intermediate events is calculated, considering two important features in the design of BMRFs: (i) different failure scenarios (FSs) with multiple sequences of components failure formation and (ii) structural reliability analysis based on the failure propagation from components to system. The system collapse reliability‐based assessment of BMRFs is developed with an efficient algorithm using the Monte Carlo simulation procedure incorporated into a nonlinear finite element (FE) analysis program. An appropriate nonlinear FE model of such systems is demonstrated, and the probability of various predefined components' failure over the most likely FSs in the presence of both epistemic and inherent uncertainties is calculated. Then, a system‐simulated reliability index (SSRI) is computed by lower and upper bounds in the probability of BMRF system collapse. Finally, fragility curves based on the SSRI is compared with the ones from incremental dynamic analysis, and later, the outcomes from multiple FSs are compared with the codified main collapse criterion. For the BMRFs analyzed herein, it is shown that the existing allowable story drift for the collapse limit state is conservative, and a new criterion is appraised. Copyright © 2015 John Wiley & Sons, Ltd.     

Developments in the management of flood defences and hydraulic infrastructure in the Netherlands

This article highlights recent developments in flood risk management in the Netherlands and presents approaches for reliability analysis and asset management for flood defences and hydraulic infrastructure. The functioning of this infrastructure is of great importance for the country as large parts of it are prone to flooding. Based on a nationwide flood risk assessment, new safety standards for flood defences have been derived in the form of maximal acceptable failure probabilities. A framework for the reliability-based analysis of the performance of hydraulic infrastructure is introduced. Within this context, various challenges are discussed, such as the dynamic nature of loads, resistance and reliability requirements over time. Various case studies are presented to highlight advances and challenges in various application fields. The first case illustrates how structural health monitoring contributes to a better characterisation of the reliability of the defences and how innovative measures can enhance the reliability. The second case discusses how the river system can be managed in the context of the new safety standards. The third case shows how upgrades and reinforcements of hydraulic structures can be evaluated taking into account (uncertain) future developments, such as sea level rise.     

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.     

正态分布下边坡稳定性二元指标体系研究

安全系数是边坡稳定性评价中的常用方法,在长期的工程实践中积累了丰富的经验,其最大的缺点是不能考虑边坡岩土体中实际存在的不确定性。可靠性理论可有效地考虑边坡系统内实际存在的不确定性和相关性,但是缺乏丰富的工程实践经验,因此,将工程实践经验丰富的安全系数与先进的可靠性理论相结合,建立安全系数与可靠性相耦合的二元评价体系具有重要的理论意义和应用价值。取影响参数的平均值,计算得到最大可能安全系数(中值安全系数)。设安全系数服从正态分布,取系列的最大可能安全系数和变异系数,计算得到边坡在不同最大可能安全系数和变异系数下的破坏概率Pf。将最大可能安全系数与边坡可靠性相乘,对最大可能安全系数进行折减,得到边坡可靠的安全系数。从折减后安全系数的极限状态中,计算出不同最大可能安全系数的临界变异系数和临界破坏概率。根据不同的最大可能安全系数及它的临界破坏概率,绘制出边坡稳定性分区图。选择典型的安全系数及相对应的可靠性,建立传统安全系数与可靠性耦合的边坡稳定性二元指标体系,由安全系数和可靠性共同度量边坡系统的稳定性。提供了小样本条件下,利用矩估计原理计算参数方差的方法,研究实例表明,边坡稳定性判别的二元指标体系是可行的。     

TY220J型推土机工作装置液压系统可靠度及常见故障分析

介绍TY220J型高原履带式推土机工作装置液压系统的组成及工作原理,为找出工作装置中的可靠性薄弱环节,建立工作装置液压系统串-并联结构可靠性框图,给出各液压元件的基本失效率,计算各液压系统的基本故障率,建立了系统可靠度表达式。介绍可靠性较低的铲刀液压系统的常见故障原因及排除方法。     

从设计入手提高工程机械液压系统可靠性

液压系统的可靠性和液压元件的使用寿命，在很大程度上取决于液压元件的耐污染能力和系统油液的污染状况，本通过分析影响液压系统可靠性和液压元件使用寿命的因素之间关系，提出了在工程机械设计中提高液压系统可靠性的措施。     

事故状态下供水管网的拓扑分析

当供水管网发生类似于爆管或水质污染等事故时,通过拓扑分析快速找到并关闭相应阀门以孤立事故源进而开展事故抢修是非常必要的。结合事故发生时管网的拓扑结构特点,以图论的理论和方法为基础,提出了一种可用于事故状态下供水管网拓扑分析的方法。该方法首先利用深度优先搜索算法找到距离事故源最近的阀门,然后通过剔除冗余阀门而确定最优关阀方案及关阀后的事故影响区域,最后调整管网水力模型以适应关阀后管网需水量和拓扑结构的变化。该方法不仅可用于供水管网事故时的关阀调度,还可用于供水系统污染控制分析以及供水管网的性能评价。     

城市燃气输配管网可靠性的多视角分析

以可靠性为切入点,对城市燃气输配管网进行多视角分析,阐述管道结构可靠性、管网连通可靠性、管网水力工况和供气可靠性等相关问题的区别与联系,明确不同角度的可靠性分析是遵循了不同的途径进行的,应视不同类型的生命线工程系统与研究目的而有所侧重。管道结构可靠性及连通可靠性是从机械角度对管网系统及组成单元进行分析,侧重考察组成管网的各部分及管网系统在一定条件下具有结构完整性的概率,而未考虑水力工况的影响。管网水力工况分析是从流体力学角度分析管网各节点压力、流量的整体工作状况,其侧重点是在个别管道失效情况下,管网还能够在多大程度上满足各类用户的用气需求,水力工况分析未考虑结构失效概率的影响。管网供气可靠性是在考虑结构可靠性和水力工况的基础上采用的一个综合评价标准,是一种概率意义上的对管网整体供气能力的综合评价,根据这种评价结果,可以从管道结构和水力工况两个方面来引导改善设计或改进已建成系统,进而更好地促进管网供气功能的发挥。     

Bridge design with reserve and residual reliability constraints

System reliability is evolving as an important design method for selecting components and topology. It should be used to investigate the original intact structure behavior as well as providing sufficient reserve capacity under likely accident and damage scenarios. Highway bridges are an area of special concern since effort to optimize the cost of construction may not provide adequate redundant capacity. The paper describes an example in which a typical girder bridge structure is optimized (both size and girder spacing) to achieve target system performance. System reliability constraints are imposed on the behavior of the intact structure under highway loads. Also, residual system reliability constraints are imposed to control safety under specified accident scenarios corresponding to damage and “lost” members due to corrosion, fatigue, fire or accidental collisions. An optimal design framework for controlling safety under all these life cycle situations is thus provided.     

A Fuzzy Expert System for Prioritizing Rehabilitation of Sewer Networks

Abstract: Rehabilitation of sewer networks is a huge and very costly global problem that has often been treated on a crisis‐based approach. The development of a rehabilitation program requires models and tools for assessing the condition and performance of sewers. The original contribution of this study is the development of a ranking scheme for sewer rehabilitation priorities. A fuzzy expert system was applied with inputs from a combined assessment of hydraulic, structural performance and potential failure consequences. The fuzzy structural system computes the global structural performance index for each pipe using internal condition, surrounding condition, and site vulnerability (SV) as inputs. The fuzzy hydraulic system uses hydraulic performance index (HPI), hydraulic performance impact, and SV to compute the global HPI. Finally, the fuzzy global system uses all these factors to compute the global performance index for each pipe. This methodology was successfully applied to the sewer system of the City of Laval in Canada. The results show how the fuzzy inference system may be used to establish rehabilitation priorities for each pipe section. The fuzzy expert system provides more realistic results than the intuitive approaches that use structural and hydraulic performance maximum and mean.     

Reliability-based robustness analysis for a Croatian sports hall

This paper presents a probabilistic approach for structural robustness assessment for a timber structure built a few years ago. The robustness analysis is based on a structural reliability based framework for robustness and a simplified mechanical system modelling of a timber truss system. A complex timber structure with a large number of failure modes is modelled with only a few dominant failure modes. First, a component based robustness analysis is performed based on the reliability indices of the remaining elements after the removal of selected critical elements. The robustness is expressed and evaluated by a robustness index. Next, the robustness is assessed using system reliability indices where the probabilistic failure model is modelled by a series system of parallel systems.     

Rapid assessment of seismic demand in existing building structures

Seismic assessment of building structures can be a very involved process. This paper presents a simplified and rational manual procedure for rapid predictions of maximum inter‐storey drift demand in tall buildings, which does not require frame analysis nor finite element analysis to be carried out and can be expedited by the use of a spreadsheet program. The methodology comprises a series of stages, beginning with the development of an elastic design response spectrum (or a uniform hazard spectrum) for specifying the level of seismic hazard, followed by the inclusion of a site‐specific factor that represents the effects of soil amplification. The maximum seismic inter‐storey drift demand of the building is then predicted from the displacement response spectrum for a given height and the estimated lateral natural periods of the building. The proposed manual procedure can be further developed to cater for limited ductility demand behaviour in the building. The predicted drift demand is an important indicator of potential seismic damage (risk), and may be utilized for the rapid assessment of damage and loss (cost) for considered earthquake scenarios. Copyright © 2008 John Wiley & Sons, Ltd.