基于全寿命环境成本的工程结构维护方案优化

工程结构全寿命过程中存在有害气体、污水和固体废弃物排放等环境问题,但是由其造成的经济损失却很少在结构设计或工程决策过程中予以考虑。为此,将三类污染物排放的防治成本计入工程活动的环境成本中,得到了多种建筑材料、能源消耗、运输方式和建筑机械使用的环境成本。通过将环境成本与经典全寿命成本模型相结合,建立了包含环境成本的全寿命总成本模型。根据所提出的环境成本模型和全寿命总成本模型,以某一沿海高速公路桥梁结构为例,通过设置预防性维护措施和必要性维护措施,对其进行维护方案的多目标优化,分析环境成本对工程决策的影响。并对直接成本和环境成本的不确定性进行了分析,研究了环境成本折现率对环境成本影响。结果表明,在结构优化过程中考虑环境成本能够得到经济效益和环境综合性能更佳的工程方案。     

Life-cycle cost analysis as a tool in the developing process for new bridge edge beam solutions

Currently in Sweden, the life-cycle measures applied on bridge edge beams may take up to 60% of the total costs incurred along the road bridges’ life span. Moreover, significant disturbances for the road users are caused. Therefore, the Swedish Transport Administration has started a project to develop alternative edge beam design solutions that are better for society in terms of cost. The purpose of this article is to investigate whether these proposals can qualify for more detailed studies through an evaluation and comparison based on a comprehensive life-cycle cost analysis. The alternatives including the standard design are applied to typical Swedish bridges. The impact of the values of the parameters with the largest influence is investigated by sensitivity analyses. Results with different life-cycle strategies are shown. The positive influences in the total life-cycle cost of a stainless steel reinforced solution and of the enhanced construction technique are estimated. The concrete edge beam integrated with the deck seems to be favourable, which is in line with international experience observed. Different designs may be appropriate depending on the bridge case and the life-cycle strategy. The Swedish Transport Administration will carry out a demonstration project in a bridge with one of the proposals.     

Capacity loss evaluation of reinforced concrete bridges located in extreme chloride-laden environments

This article provides a comprehensive procedure for the structural performance evaluation and life-cycle cost (LCC) analysis of reinforced concrete highway bridges located in extreme chloride-laden environments. An integrated computational methodology is developed to simulate the chloride intrusion and to estimate the corrosion initiation time. The effects of various influential parameters on the chloride diffusion process are examined and the changes in geometry and material properties of structural members are calculated over the entire life of the bridge. In order to evaluate the global structural degradation due to the corrosion mechanisms, an inventory of bridges with different structural attributes are investigated. The extent of capacity loss is calculated using the moment-curvature and nonlinear static (pushover) analysis. Results of this study are then utilised to find the LCC of bridges. Different inspection and maintenance strategies are considered to minimise the total LCC, which includes the initial construction cost, inspection and maintenance costs and service failure costs. The proposed approach indicates the inspection and maintenance intervals in a way that the inspection and maintenance costs are optimised while the safety of the bridge is ensured.     

User costs in pavement construction and rehabilitation alternative evaluation

The decision-making processes used by many state transportation agencies in US and Canada for selecting a strategy for pavement rehabilitation primarily depend on initial construction and maintenance costs for identifying the most economically efficient pavement rehabilitation alternative. While this approach is appropriate for optimally allocating increasingly limited funds in the face of increasing investment backlog, it is not economically efficient because it does not account for user costs. Despite a significant body of research, the state transportation agencies are hesitant to include user costs in their pavement life-cycle cost analysis processes because of a lack of availability of standard method of calculating user costs. This article presents the results of a study conducted to review the pavement alternative selection processes used by the Ohio Department of Transportation and recommend ways to include user costs in the decision-making process. Incorporating user costs in the decision-making processes right from project-development phase will help transportation agencies stay focused on the customer and minimise overall impact of rehabilitation of the highway system.     

Bridge user cost estimation – a synthesis of existing methods and addressing the issues of multiple counting,workzones and traffic capacity limitation

Efforts to incorporate the concerns of bridge users in bridge investment evaluation are often stymied by lack of a comprehensive framework for assessing different user costs. There is a need to synthesise and update existing user cost estimation techniques so that the incorporation of user costs in bridge investment evaluation can be more consistent and streamlined. Secondly, a bridge detour may occur for more than one reason, thus there is a danger of multiple counting that could cause overestimation of user costs. Thirdly, user costs during bridge workzones have rarely been considered in the literature. To address these issues, this article presents a framework for comprehensive estimation of bridge user costs, an approach to address the multiple-counting problem, and a methodology for bridge workzone user cost estimation. Furthermore, the article develops a method to estimate bridge user delay cost due to traffic capacity limitation. The methodologies are demonstrated using a case study.     

Assessing the efficacy of stainless steel for bridge deck reinforcement under uncertainty using Monte Carlo simulation

Bridge engineers seek new materials, configurations and other design elements to minimise rehabilitation frequency and intensity over the facility life and hence, reduce life-cycle cost. This article discusses the potential of stainless steel as a bridge deck reinforcement material. The superiority of stainless steel over traditional steel was evaluated on the basis of long-term agency cost and user cost. Using data from a US Midwestern state, the analysis was carried out for a majority of analysis scenarios. For the deterministic situation, the use of stainless steel as bridge deck reinforcement material was found to yield a significantly higher initial cost but drastically reduced the overall life-cycle costs, and thus is more cost-effective over the long term. For the uncertainty situation, Monte Carlo simulation was used to generate the distributions of the life-cycle cost for the stainless and the traditional steel reinforcement materials. It was found that the life-cycle cost for the stainless steel option stochastically dominates the life-cycle cost for traditional steel. Thus, under an uncertainty situation, the use of stainless steel as bridge deck reinforcement material is also more cost-effective compared to traditional steel.     

谈钢桥设计的新型方法-寿命周期成本

主要针对钢桥有效的寿命周期成本(LCC)的优化设计中实际产生的LCC进行了简要的描述和规定。一般钢桥优化设计的LCC模型中,包括钢结构桥梁的初始成本和直接/间接的修复成本,维修/更换成本,材料损坏或者疲劳损失,道路使用者的成本,间接的经济损失。因此,论文着重介绍一般配方的模型和适合实际操作的钢桥有效的LCC系统模型设计。     

Spatial variability of damage and expected maintenance costsfor deteriorating RC structures

The paper develops a technique to predict life-cycle costs, using probabilistic information about the likelihood and extent of corrosion-induced cracking to reinforced concrete (RC) structures. The present paper focuses on the likelihood and extent of severe cracking as the criterion for the timing and cost of maintenance. The life-cycle cost and expected maintenance cost considers multiple repairs and various inspection intervals over the service life of the structural element. A repair cost function is also developed. Two common maintenance strategies are considered: repair and rehabilitation. It was found that for a 2% discount rate the benefits of delaying the timing of repairs outweigh the cost of increased extent of damage, for maintenance of a RC bridge deck.     

Comparative life-cycle analysis of steel-concrete composite bridges

Competitiveness of steel construction and, in particular, steel and composite bridges, requires a broader view that encompasses the concepts of sustainability and life-cycle assessment in parallel with the classical structural and geotechnical issues. Also, from the economical viewpoint, the progressive transfer of operational duties from the National Road Authorities to private companies in the context of design, build and operate contracts, reinforces the need for an integrated approach.It is the purpose of the present paper: (i) to present an integrated methodology for a life-cycle and sustainability analysis (LCA and LCCA); and (ii) to apply such an integrated approach to a case study of a composite bridge. The proposed approach, besides structural considerations, contemplates environmental aspects (energy consumption, raw materials, and environmental impacts), economical aspects and a brief discussion of degradation and maintenance aspects. The case study consists of a three-span continuous bridge with spans of 27.2 m + 35.0 m + 27.2 m, thus totalling 89.40 m. Finally, some comparisons are presented between alternative solutions.     

Risk-based maintenance strategy for deteriorating bridges using a hybrid computational intelligence technique: a case study

The current bridge inspection and maintenance protocol that is used in most countries focuses primarily on the visible aspects of bridge fitness and underestimates the invisible aspects, such as resistance to scouring and earthquake hazards. To help transportation authorities to better consider both aspects, the present study developed a new computational intelligence system, the so-called risk-based evaluation model for bridge life-cycle maintenance strategy (REMBMS). This model considers the three main risk factors of component deterioration, scouring and earthquakes in order to minimise the expected life-cycle cost of bridge maintenance. Monte Carlo simulation is used to estimate the probability of bridge maintenance. The evolutionary support vector machine inference model (ESIM) was applied to estimate the risk-related maintenance cost using historical data from the Taiwan Bridge Management System (TBMS) database. The time-influenced expected costs were obtained by multiplying each maintenance probability with its associated cost. Finally, the symbiotic organisms search (SOS) algorithm is used to identify the bridge maintenance schedule that optimises the life-cycle maintenance cost. The present study provides to bridge management authorities an effective approach for determining the optimal timing and budget for maintaining transportation bridges.     

Constraint-based clustering model for determining contract packages of bridge maintenance inspection

When a number of bridges are to be maintained yearly, contract engineers need to package certain bridges based upon the specified conditions. Traditional contract packaging is performed subjectively and manually. The purpose of this paper is to develop an objective and automatic decision-making process for contract packaging. Contract Packaging Technique (CPT), which is derived from k-prototypes, can determine appropriate contract packages by considering the inherent conditions of the infrastructure to be maintained. It can simultaneously handle user-specified constraints and mixed data types such as bridge locations, bridge types, bridge construction materials and maintenance costs. Validation was performed in this study to illustrate that the CPT can effectively divide a huge bridge maintenance plan into several appropriate contract units.     

网壳结构寿命周期总费用的计算方法研究

现代结构系统在灾害中产生巨大经济损失的特点,使人们意识到在结构设计之初就对结构的寿命周期总费用进行有效评估、并反过来指导工程设计具有重要意义。对网壳结构寿命周期总费用的计算方法进行研究,分别提出初始造价、维护费用和失效损失的计算公式,并考虑寿命周期内网壳结构可能受到的荷载发生的概率,建立寿命周期总费用的计算方法,编制网壳结构寿命周期总费用的计算程序。通过一具体的网壳结构工程算例验证上述方法和程序的有效性。研究结果表明,网壳结构初始造价的追加投资会由于结构使用过程中失效损失费用的减小而得到补偿,而寿命周期总费用最小正是结构设计的最优状态,按照现行规范设计方法得到的结构配置方案不一定是寿命周期总费用最小的方案。     

Life cycle cost of flexible pavements and climate variability: case studies from Virginia

Abstract

Climate factors have not become a typical metric to consider for pavement life cycle cost analysis (LCCA). Changes in climate may affect pavement rutting, roughness, and cracking and lead to consequent changes in maintenance decision-making and life cycle costs. This study develops a methodology to incorporate the effects of climate variability into flexible pavement LCCA and to derive the additional life cycle costs incurred due to changes in climate. Case studies were performed for three road sections in Virginia (US) to demonstrate the methodology, using approximate mean climate change trends predicted for the investigated regions. It is estimated that climate change will incur additional vehicle operating costs ranging between US$2.30 and $4.40 on average per vehicle/annum if roads are used under a 2050 high greenhouse gas emission scenario and without being maintained. Assuming responsive maintenance, the budget demand for maintenance will arrive much earlier in the pavements’ life cycles (7–11?years earlier under the 2050 high-emission scenario). This is found to add up to 64% of agency costs (net present value) to repair each kilometre of the investigated roads in a 40-year design life. Agencies need to be aware of earlier or more frequent demands on their maintenance budgets.     

Bridge life-cycle performance and cost: analysis,prediction, optimisation and decision-making

The development of a generalised framework for assessing bridge life-cycle performance and cost, with emphasis on analysis, prediction, optimisation and decision-making under uncertainty, is briefly addressed. The central issue underlying the importance of the life-cycle approach to bridge engineering is the need for a rational basis for making informed decisions regarding design, construction, inspection, monitoring, maintenance, repair, rehabilitation, replacement and management of bridges under uncertainty which is carried out by using multi-objective optimisation procedures that balance conflicting criteria such as performance and cost. A number of significant developments are summarised, including time-variant reliability, risk, resilience, and sustainability of bridges, bridge transportation networks and interdependent infrastructure systems. Furthermore, the effects of climate change on the probabilistic life-cycle performance assessment of highway bridges are addressed. Moreover, integration of SHM and updating in bridge management and probabilistic life-cycle optimisation considering multi-attribute utility and risk attitudes are presented.     

Life cycle cost considerations in project appraisals of collision-free roads

In the last decade, many single carriageway roads in Sweden have been converted to collision-free roads as a cost-effective alternative to conventional motorways. Investigations have concluded that the road type has been successful in reducing the number of fatal accidents, despite increased operation and maintenance costs. In recent years, the focus has shifted to converting narrower roads which are anticipated to further increase operation and maintenance cost but also complicate traffic management during road works. There are concerns that when life cycle cost is considered in the investment assessment the socioeconomic profitability could be reduced. This article examines this issue by first assessing changes in costs for operation and maintenance using a life cycle cost analysis approach applied on a case study. The results from the analysis were thereafter integrated into a cost–benefit analysis to assess changes in costs in relation to benefits in improved traffic safety and travel time. The analysis indicated profitability even with substantial increase in operation, maintenance and road user work zone costs. Results are discussed from project implementation and road management perspectives.     

A quantitative model for efficient maintenance of plastered and painted façades

Building owners are faced with increasing maintenance costs incurred to rectify defects on different building elements and systems. With a significant proportion of maintenance costs spent on upkeep of building façades, there is a need for cost‐effective approaches for façade maintenance. A quantitative model is developed for effective evaluation, optimization and management of costs of maintenance of plastered and painted façades. Visual surveys were used to collect information about defects on the external plaster and paint finish for public housing apartment blocks in Singapore. The data were used to develop ‘defect index models’ for each type of defect to predict its onset and propagation. The output from these models was incorporated into a life cycle cost model that can be used to predict and optimise the timing and magnitude of maintenance/repair costs incurred on plastered and painted façades during their intended lifetime.     

Optimal maintenance strategies for bridge networks using the supply and demand approach

This paper presents a probability-based approach for optimising the management of bridge networks. Most of the Bridge Management Systems are focused on condition features to ensure a minimum safety level for each individual bridge. Their location on the road network, the consequences of inadequate service due to maintenance actions are therefore not taken into consideration. These multiple criteria should be considered when scheduling maintenance activities. To overcome these limitations, a probabilistic supply and demand strategy is proposed for determining the optimal maintenance planning for each interconnected bridge. The problem is solved with genetic algorithms. One objective function is first introduced, corresponding to the summation of all the maintenance, failure, and user costs. Then, two conflicting objective functions are considered, the total user costs and the maintenance and rehabilitation costs. Safety and serviceability aspects are taken into account in the methodology and the theoretical and numerical developments are applied on a part of the French national network.     

Decision analysis for elevating bridge decks with steel pedestals

Steel pedestals have been proposed to elevate bridge decks to reduce the likelihood of collisions of overheight vehicles. However, the use of steel pedestals might also increase the bridge vulnerability to seismic events. This paper develops a formulation for decision analysis for elevating bridges with steel pedestals. The formulation accounts for the expected life-cycle costs (LCCs) associated with vehicular impacts to bridge decks and seismic events. The optimum height of the steel pedestals is then obtained by minimising the total expected costs. The expected LCC associated with a bridge retrofitted with the optimum pedestal height is compared with that of the same bridge before the installation of the steel pedestals to determine whether elevating the bridge with steel pedestals is justifiable. The proposed framework is applied on a typical two-span slab-on-girder bridge at different locations in the USA.     

Multiobjective optimisation algorithm for sewer network rehabilitation

Understanding of deterioration mechanisms in sewers helps asset managers in developing prediction models for estimating whether or not sewer collapse is likely. Effective utilisation of deterioration prediction models along with the development and use of life cycle maintenance cost analysis contribute to reducing operation and maintenance costs in sewer systems. This article presents a model for life-cycle maintenance planning of deteriorating sewer network as a multi-objective optimisation problem that treats the sewer network condition and service life as well as life-cycle maintenance cost (LCMC) as separate objective functions. The developed model utilises Markov chain model for the prediction of the deterioration of the network. A multi-objective genetic algorithm is used to automatically locate an appropriate maintenance scenario that exhibits an optimised tradeoff among conflicting objectives. Monte Carlo simulation is used to account for LCMC uncertainties. The optimisation algorithm provides an improved opportunity for asset managers to actively select near-optimum maintenance scenario that balances life-cycle maintenance cost, condition and service life of deteriorating sewer network. A case study is used to demonstrate the practical features of developed methodology.     

基于寿命函数的斜拉索系统维修更换策略研究

展开寿命周期内拉索劣化规律和维修更换策略的研究,获得桥梁结构的最优成本-效益比,是缆索桥梁养护维修的重要研究内容之一。根据拉索寿命函数和拉索系统的串并联模型,研究斜拉桥拉索系统失效概率变化即结构劣化的规律,分析全寿命周期内不同维修更换措施对结构失效概率的影响。运用周期成本理论,进一步分析不同维修更换时间和更换策略的经济性。结合一斜拉桥模型,验证了模型的适用性,并且以延长使用寿命和累积维修费用最低为目标,获得该桥的最优维修策略。研究表明,寿命函数和周期成本理结合是确定斜拉桥拉索更换优化策略的有效途径,为工程拉索更换策略提供有益的参考。