Life-Cycle Cost Analysis and Design of Civil Infrastructure Systems

Pavements and bridges are the main components of a road infrastructure system. Both essentially perform a structural function in that they transfer the traffic loads to the foundation level or sub-grade below. In order to achieve a common rational approach for determining and funding their maintenance expenditure, it is necessary to understand the overall engineering processes involved in predicting and planning maintenance work in the two areas, their differences and similarities. Clearly, some of the differences will be fundamental, while others are nothing more than the same processes in a different guise. This paper is intended to provide an overview of the similarities and differences in the maintenance procedures used for road pavement and structures.

In a world where financial resources do not keep pace with the growing demand for maintenance of deteriorating civil infrastructures, it is imperative that those responsible for maintenance decisions make the best possible use of limited financial resources. Decision makers have to evaluate the expected maintenance cost of deteriorating civil infrastructures and to use benefit/cost techniques for finding the optimal resource allocation. This paper proposes a methodology for the evaluation of expected maintenance cost of deteriorating civil infrastructures by considering uncertainties associated with the application of subsequent maintenance actions. The methodology can be used to determine the expected number of maintenances of a deteriorating structure, or a group of deteriorating structures, during a specified time interval and the associated expected maintenance costs. The method can be easily programmed and incorporated into an existing software package for life-cycle costing of civil infrastructures.

The concepts of Transferred Force and Potential Transferred Force through various parts of the structure to its supports are explained. Methods for Conceptual (topology) design of linearly elastic structures based on the concepts are investigated. The methods do not require any formal optimization techniques, and so they are quite simple to use. Non-dimensional parameters that indicate overall performance of the structure are defined and monitored during the redesign process. The methods are applied to truss structures and 2-D plates. Improved shapes and topologies are obtained from the viewpoint of force transfer. It is concluded that the concepts of Transferred and Potential Transferred Forces for redesign of structures are viable approaches and should be developed further for practical applications. Also, redesign methods with the combined use of Transferred and Potential Transferred Forces give better solutions than the methods with the use of only Transferred Force. These methods can be used to design better civil infrastructure systems.

An importance sampling scheme to determine the time-variant reliability of structures subjected to Poisson processes is presented. The suggested approach combines the first order reliability method with standard importance sampling techniques. It becomes therewith feasible to determine extremely low probabilities of failure with a moderate number of samples. As numerical examples a fatigue crack growth problem and an oscillator subjected to exponentially distributed random noise are investigated and the computational efficiency of the proposed method is demonstrated.

Frequently NDT of bridges has been undertaken on a negotiated basis, relying upon goodwill from both the employer and the NDT test house in order to achieve value for money and quality. When competitive tendering has been introduced into this arena, problems have arisen due to lack of a quantified basis for comparison. The objective of this paper is to address the issues of drawing up competitive tenders in order to obtain the best value for money price and more importantly improved quality from competing test houses. This paper focuses on NDT techniques to identify voiding in grouted post-tensioned bridge tendon ducts; and scour measurement at bridge sites — in order to illustrate the procedures for competitive tendering of an NDT contract. The procedure for preparing tender documents has been given. In addition, guidance has been given to the client as to how to monitor the role of an independent consultant undertaking the tendering process. Each step in this process will not only yield a more competitive price, but also yield a better quality of investigation as required by the Frangopol model.

The aim of this paper is to develop an appropriate decision supporting system for planning the rational maintenance scheduling of reinforced concrete decks that are one of the structural elements composing steel highway bridges. Emphasis is placed on how to appropriately decide the repair order for some deteriorating RC decks among many existing highway bridges, taking into account the traffic characteristics of road network and the environmental condition at the construction sites. The concept of life cycle cost analysis is applied to evaluate the fitness of repair method for the damaged RC decks. Genetic Algorithm (GA) is used to search a practical and rational combinatorial optimization solution for the maintenance problem because GA has such an advantage that it can provide us with useful solutions for large and complex combinatorial scheduling problems with discontinuous or vague objective functions.

In order to establish a rational maintenance program, it is necessary to predict the future changes of structural characteristics and environments. In this paper, an attempt is made to develop a method that can provide us with several practical maintenance plans, considering the prediction uncertainties of the structural capacity and environments. Applying the genetic algorithm developed by Tamaki to the maintenance planning problem, it is possible to obtain practical solutions that are robust to the variety of structural and environmental conditions. Some numerical examples are presented to demonstrate the applicability of the method developed here.

In recent years, environmental problems have become serious issues in the whole world. Researchers and practical engineers in civil engineering have to pay enough attention to the environmental impact in addition to the function, safety, cost and aesthetics at all lifecycle stages of civil infrastructures. In this study, the estimation is made for the lifecycle CO₂ emission, and lifecycle cost of both the conventional bridges and minimum maintenance bridges. According to the results concluded from this research, the lifecycle environmental analysis and the lifecycle cost analysis may deduce various conflicting bridge design and management schemes so that the bridge engineers and government managers should consider the environmental impact as a fundamental criterion in addition to the lifecycle cost during the decision-making process for the bridge lifecycle management. In addition, the characteristics of lifecycle environmental impact and lifecycle cost of bridges using new technologies have advantages compared with those of the conventional bridges.

This paper focuses on some practical issues on the design and maintenance of urban highways with a case study conducted on a selected route of the Hanshin Expressway in Kansai, Japan. Statistical data on the structures such as their types and component percentages are provided and the trend of yearly changes in the maintenance expenditure is analyzed. The study shows that steel structures constitute the majority of the structures inventory, and that damage in these steel structures typically involve high-tension bolts, bearings, expansion joints, corrosion and fatigue cracking among others. Some repair methods and modification in design codes are also addressed. Finally, lifecycle cost analysis has been conducted to identify useful features from estimated expenditure curves generated by fitting the real data using the least-squares and logarithmic method. Correlations between the lifecycle cost and other parameters including lifecycle, economic loss and fitting method, are analyzed. The study indicates that lifecycle cost depends significantly on the lifecycle selected, and that the long lifecycle does not necessarily gives the minimum lifecycle cost.

The aim of the present study is to develop a concrete bridge rating expert system for deteriorated concrete bridges, constructed from a hierarchical neural network in order to carry out fuzzy inference and machine learning. The proposed system evaluates the performance of concrete bridges on the basis of a simple visual inspection and technical specifications. The neural network applied in this study facilitates refinement of the knowledge base by use of the Back-Propagation method, and prevents the inference mechanism of the system from becoming a black box. In this study, the training set for machine learning is obtained from inspection of actual in-service bridges and questionnaire surveys of bridge experts. Furthermore, comparisons between the diagnostic results of bridge experts and those of the proposed system are presented so as to demonstrate the validity of the system's learning capability.

An attempt is made in this paper to develop a new bridge management system (J-BMS) integrated with the Concrete Bridge Rating Expert System that can be used to evaluate the serviceability of existing concrete bridges. The proposed J-BMS not only evaluates the performance of bridges but also offers the rehabilitation strategy based on a combination of maintenance cost minimization and quality maximization. In this system, Genetic Algorithms (GAs) technique was used to search for an approximation of the optimal maintenance plan, and was constructed by using Visual Basic and C language. Furthermore, a comparison of the results of applying this system to some actual in-service bridges with the results of questionnaire surveys to experts shows that optimal maintenance planning as well as bridge rating can be predicted accurately using this system.

The prediction of the life time of steel structures is an important task for the owners. The usually used models show only small reliability. The models consists of a chain of a load model, a system transfer model and a damage model. The results of these coupled models are usually unreliable, especially the influence of the unsafe load- and damage models controls the reliability of the result. It is shown that monitoring at critical points and a time reduced parallel testing leads to very sufficient results.

Corrosion and fatigue are important considerations which affect the structural performance of bridges. This applies to structural steel, reinforced concrete, and prestressed concrete. The optimum management of bridges requires the prediction of the remaining life for partially deteriorated structures. Therefore, this paper deals with the development of reliability-based models for time-varying changes in structural performance due to corrosion and fatigue. Fatigue limit state is one of the important considerations in the design of bridges. Accumulated load cycles can cause cracking or even failure. Reliability analysis is performed for the fatigue limit state function of flexure. Prediction of the remaining fatigue life for steel and concrete beams is illustrated on examples of existing bridge girders. Corrosion causes a loss of the section and reduction of the load carrying capacity. Rate of corrosion is a random variable. Three levels of corrosion are considered. Changes in reliability are evaluated as a function of time, covering the life-time of the bridge.

This paper presents probabilistic models of costs and an approach to cost-benefit analyses to be used for planning interventions on existing structures. Costs due to inadequate performance with respect to both safety and serviceability are modelled and combined in the approach. Costs are modelled considering time dependent effects, such as deterioration, and the total cost is determined by summation over a given reference period, which is normally taken to be the expected remaining life of a structure. An approach is presented for determining the optimal intervention strategy for a structure with respect to time, place and level of intervention. The models and approach are illustrated with a simple example.

Major deterioration mechanisms for infra-structural systems such as bridges, tunnels and dams are chloride initiated corrosion of reinforced concrete and fatigue damage in steel structures. Corrosion of the reinforcement is a significant problem for a large number of reinforced concrete structures since it can result in very expensive maintenance and repair actions. Further a substantial decrease of the load-bearing capacity may occur leading to an unacceptable level of safety for the structure. One mode of corrosion initiation is that the chloride content around the reinforcement exceeds a critical threshold value. In the present paper a statistical model is described by which the chloride content in a reinforced concrete structure can be predicted. The model is used in reliability-based inspection and maintenance planning for concrete structures. For steel structures a simplified and generic approach for risk-based inspection planning of fatigue sensitive structural details is described. Fatigue sensitive details are categorized according to their Fatigue Design Factor (FDF) and SN curve. Using a fracture mechanics model calibrated on a probabilistic basis to the appropriate SN-curve, cost-optimal inspection and repair planning can be performed. The procedure is illustrated by an example considering inspection planning of welded longitudinal stiffeners in steel bridges.

The life cycle cost includes the initial construction cost, the maintenance cost, the member exchange and rehabilitation cost and the user cost. These costs are sometimes shown concretely in several cases, but the user cost has been shown only the word, not the concrete value. However, in the relation of the road network around the bridge, it is anticipated that the user cost becomes large, and in the life cycle cost calculation, it will be important to show the concrete value of user cost. For the user cost, the evaluation of the increase by the detour traffic in running time, the economic evaluation that the transport traffic takes, and the social evaluation of traffic accident are included. And here, only the time increase due to detour traffic is evaluated. This user cost can be regarded as the value index of the bridge and also the full index of the detour road network around the bridge. It can be positioned as the important index to determine the order of bridge treatment. So, in this paper, it is also proposed the BMS applying the numerical result of user cost.

Cost/benefit assessment of bridge maintenance strategies is possible if related expenditures can be correctly evaluated over an appropriate life cycle and if they produce a known effect on structural performance. The proposed method evolves from the inspection records of New York City bridges and their components. Actual and recommended annual expenditures are examined. "Full maintenance" is defined in terms of fifteen distinct maintenance tasks with known costs. The corresponding benefits are expressed in terms of bridge life extension. Alternative maintenance strategies are compared with and without discounting, pointing to a distinction between structurally and economically admissible ones. Conclusions are drawn regarding the implications of the Present Worth Method (PWM).

An international workshop on structural health monitoring was held at Stanford University in September of 1999. Results of this workshop are summarized in a subjective manner herein. We believe that future efforts should be concentrated on the analysis and interpretation of collected data. Because many professional engineers are capable of assessing the damage of existing structures, attempts have been made to build expert systems in the subject area of condition evaluation of existing structures. Moreover, fuzzy sets have been applied in civil engineering research including damage assessments. Recently, efforts are being made to develop and apply symptom-based reliability to evaluate the safety of existing structures. Potentially, symptom-based reliability can be used to monitor the structural condition of existing structures. Wong et al., also advocated a holistic approach to structural health monitoring in the form of a "value chain" achieving the value of the stakeholder. These works are summarized and discussed herein.