Life-cycle performance,management, and optimisation of structural systems under uncertainty: accomplishments and challenges 1

Our knowledge to model, analyse, design, maintain, monitor, manage, predict and optimise the life-cycle performance of structures and infrastructures under uncertainty is continually growing. However, in many countries, including the United States, the civil infrastructure is no longer within desired levels of performance and safety. Decisions regarding civil infrastructure systems should be supported by an integrated reliability-based life-cycle multi-objective optimisation framework by considering, among other factors, the likelihood of successful performance and the total expected cost accrued over the entire life-cycle. The primary objective of this paper is to highlight recent accomplishments in the life-cycle performance assessment, maintenance, monitoring, management and optimisation of structural systems under uncertainty. Challenges are also identified.     

Life-cycle reliability assessment of reinforced concrete bridges under multiple hazards

This paper proposes a novel probabilistic methodology for estimating the life-cycle reliability of existing reinforced concrete (RC) bridges under multiple hazards. The life-cycle reliability of an RC bridge pier under seismic and airborne chloride hazards is compared to that of a bridge girder under traffic and airborne chloride hazards. When conducting a life-cycle reliability assessment of existing RC bridges, observational data from inspections can provide the corrosion level in reinforcement steel. Random variables related with the prediction of time-variant steel weight loss can be updated based on the inspection results using Sequential Monte Carlo Simulation (SMCS). This paper presents a novel procedure for identifying the hazards that most threaten the structural safety of existing RC bridges, as well as the structural components with the lowest reliability when these bridges are exposed to multiple hazards. The proposed approach, using inspection results associated with steel weight loss, provides a rational reliability assessment framework that allows comparison between the life-cycle reliabilities of bridge components under multiple hazards, helping the prioritisation of maintenance actions. The effect of the number of inspection locations on the updated reliability is considered by incorporating the spatial steel corrosion distribution. An illustrative example is provided of applying the proposed life-cyle reliability assessment to a hypothetical RC bridge under multiple hazards.     

A random field based technique for the efficiency enhancement of bridge network life-cycle analysis under uncertainty

Studies associated with distributed civil infrastructure systems are usually very demanding from a computational point of view, especially when they involve life-cycle analysis, uncertainty, and optimization. For this reason, computational tools that enhance the efficiency of the analysis and make it feasible for complex practical applications are of utmost importance. In this paper, a computational technique for the efficiency enhancement of bridge network life-cycle analysis under uncertainty is presented and its impact in terms of CPU time reduction is investigated.The proposed technique consists in the joint use of random field theory and probabilistic reliability models for the simulation of the individual bridge service states over the life-cycle of the infrastructure. This random field based approach is extremely efficient and takes simultaneously into account the deterioration in time of the bridge reliability and the correlation in space of the service states of bridges belonging to the same transportation network. Compared to other techniques previously used to perform the same task, the proposed methodology is theoretically more solid and improves the computational efficiency by more than two orders of magnitude.A numerical example is provided to validate the proposed technique. Moreover, a second example involving the life-cycle performance analysis of a complex bridge network in Santa Barbara, CA, is presented.     

Maintenance and management of civil infrastructure based on condition,safety, optimization,and life-cycle cost∗

Cost-competent maintenance and management of civil infrastructure requires balanced consideration of both the structure performance and the total cost accrued over the entire life-cycle. Most existing maintenance and management systems are developed on the basis of life-cycle cost minimization only. The single maintenance and management solution thus obtained, however, does not necessarily result in satisfactory long-term structure performance. Another concern is that the structure performance is usually described by the visual inspection-based structure condition states. The actual structure safety level, however, has not been explicitly or adequately considered in determining maintenance management decisions. This paper reviews the recent development of life-cycle maintenance and management planning for deteriorating civil infrastructure with emphasis on bridges using optimization techniques and considering simultaneously multiple and often competing criteria in terms of condition, safety and life-cycle cost. This multiple-objective approach leads to a large pool of alternative maintenance and management solutions that helps active decision-making by choosing a compromise solution of preferably balancing structure performance and life-cycle cost.     

Network level bridges maintenance planning using Multi-Attribute Utility Theory

Bridge infrastructure managers are facing multiple challenges to improve the availability and serviceability of ageing infrastructure, while the maintenance planning is constrained by budget restrictions. Many research efforts are ongoing, for the last few decades, ranging from development of bridge management system, decision support tools, optimisation models, life cycle cost analysis, etc. Since transport infrastructures are deeply embedded in society, they are not only subject to technical requirements, but are required to meet the requirements of societal and economic developments. Therefore, bridge maintenance planning should accommodate multiple performance goals which need to be quantified by various performance indicators. In this paper, an application of Multi-Attribute Utility Theory (MAUT) for bridge maintenance planning is illustrated with a case study of bridges from the Netherlands road network. MAUT seeks to optimise multiple objectives by suggesting a trade-off among them and finally assigns a ranking to the considered bridges. Moreover, utility functions of MAUT appropriately account for the involved uncertainty and risk attitude of infrastructure managers. The main contribution of this study is in presenting a proof-of-concept on how MAUT provides a systematic approach to improve the decision-making of maintenance planning by making use of available data, accommodating multiple performance goals, their uncertainty, and preferences of infrastructure managers.     

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.     

Life-cycle of structural systems: recent achievements and future directions

Structural systems are under deterioration due to ageing, mechanical stressors, and harsh environment, among other threats. Corrosion and fatigue can cause gradual structural deterioration. Moreover, natural and man-made hazards may lead to a sudden drop in the structural performance. Inspection and maintenance actions are performed to monitor the structural safety and maintain the performance over certain thresholds. However, these actions must be effectively planned throughout the life-cycle of a system to ensure the optimum budget allocation and maximum possible service life without adverse effects on the structural system safety. Life-cycle engineering provides rational means to optimise life-cycle aspects, starting from the initial design and construction to dismantling and replacing the system at the end of its service life. This paper presents a brief overview of the recent research achievements in the field of life-cycle engineering for civil and marine structural systems and indicates future directions in this research field. Several aspects of life-cycle engineering are presented, including the performance prediction under uncertainty and optimisation of life-cycle cost and intervention activities, as well as the role of structural health monitoring and non-destructive testing techniques in supporting the life-cycle management decisions. Risk, resilience, sustainability, and their integration into the life-cycle management are also discussed.     

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.     

Development of the Swedish bridge management system by upgrading and expanding the use of LCC

Although many bridge management systems (BMSs) contain some forms of life-cycle costing (LCC), the use of LCC in bridge engineering is limited. Life-cycle costing in many BMSs has mainly been applied within the bridge operation phase to support decisions related to existing bridges. Life-cycle costing has several useful applications within the bridge entire life, from cradle to grave. This article introduces the Swedish Bridge and Tunnel Management System (BaTMan). A comprehensive integrated LCC implementation scheme will be illustrated, taking into account the bridge investment and management process in Sweden. The basic LCC analysis tools as well as other helpful techniques are addressed. A real case study is presented to demonstrate the recent improvement of BaTMan practically in the function of whether to repair or to replace a bridge. Cost records for 2508 bridges are used as input data in the presented case study. Considering the same records, the average real and anticipated initial costs of different bridge types in Sweden will schematically be presented.     

Life-cycle evaluation of deteriorated structural performance of neutralised reinforced concrete bridges

For existing reinforced concrete (RC) bridges, the structural performance is highly dependent on the changing properties of concrete and reinforcing steel due to neutralisation-induced corrosion. As neutralisation progresses, the corrosion could become serious enough to deteriorate not only the serviceability, but also the maintainability, of the structural performance. To study the influence of neutralisation on the existing RC bridges, the inspected data and test results collected from 21 bridges in Taiwan were examined to obtain the essential parameters through regression analyses. The regressive parameters related to service time can be employed in evaluating the variation of material and sectional properties in both reinforcements and concrete, and, accordingly, the change of structural performance from time to time could be obtained quantitatively via structural analysis. As a consequence, the performance degradation curve of an existing RC bridge can be predicted and, if necessary, the appropriate timing for repair or retrofit could be suggested. The results obtained could facilitate the minimisation of life-cycle cost for the neutralised RC bridges and enhance the functionality of a bridge management system (BMS).     

Bridge owner's benefits from probabilistic approaches

The growth of national economies in the latter half of the twentieth century has resulted in steadily increasing traffic volumes on trading routes and in an increase in the demands placed on an ageing bridge stock. Ironically, economic growth has not resulted in an increase in the budgets available to bridge owners for maintenance of their ageing resource. The approach adopted by the Danish Road Directorate in addressing this challenge has been to exploit advances in scientific methods in the management of its bridges. The use of probabilistic approaches within the areas of capacity assessment of bridges, bridge management and ship impact assessments have resulted in substantial cost savings, and future maintenance and repair needs have been minimized. Examples of the various areas where the probabilistic approaches have been applied are given, and both the technical approach and the administrative and financial benefits are presented.     

Multihazard resilience of highway bridges and bridge networks: a review

Abstract

Assessment of resilience for engineered systems has drawn ample attention from the engineering community in recent years. It has resulted in a significantly large body of literature focusing on pertinent areas of resilience. This article provides a systematic and comprehensive review of the literature addressing resilience assessment of bridges and bridge networks under single hazard and multihazard conditions. Though not much work been performed yet on multihazard resilience of bridges, relevant aspects including combinations of multiple hazards for bridge performance evaluation, methods for loss assessment and approaches taken for post-event recovery are discussed. Furthermore, maintenance is a key component if resilience is assessed in a life-cycle framework. Hence, available maintenance plans and strategies and their probable applications for bridges and bridge networks are discussed. The article concludes with a discussion on the need for further research in the focus area and challenges involved with the same.     

Life cycle utility-informed maintenance planning based on lifetime functions: optimum balancing of cost,failure consequences and performance benefit

Decision-making regarding the optimum maintenance of civil infrastructure systems under uncertainty is a topic of paramount importance. This topic is experiencing growing interest within the field of life cycle structural engineering. Embedded within the decision-making process and optimum management of engineering systems is the structural performance evaluation, which is facilitated through a comprehensive life cycle risk assessment. Lifetime functions including survivor, availability, and hazard at component and system levels are utilised herein to model, using closed-form analytical expressions, the time-variant effect of intervention actions on the performance of civil infrastructure systems. The presented decision support framework based on lifetime functions has the ability to quantify maintenance cost, failure consequences and performance benefit in terms of utility by considering correlation effects. This framework effectively employs tri-objective optimisation procedures in order to determine optimum maintenance strategies under uncertainty. It provides optimum lifetime intervention plans allowing for utility-informed decision-making regarding maintenance of civil infrastructure systems. The effects of the risk attitude, correlation among components and the number of maintenance interventions on the optimum maintenance strategies are investigated. The capabilities of the proposed decision support framework are illustrated on five configurations of a four-component system and an existing highway bridge.     

Maintenance plan optimization system for existing concrete bridge groups

In the present study a comprehensive decision support system is developed for the formation of maintenance strategies with/without annual budget limitations. This system is based on life-cycle cost analysis of an entire bridge inventory, which comprises part of a highway network. The formulation of an optimum bridge maintenance program for an entire stock of bridge structures has been impeded historically by insufficient information concerning the existing structural conditions. The capability of this optimization system was enhanced by employing Genetic Algorithms in the formation of semi-optimal solutions. This system was tested by employing to formulate a set of semi-optimal bridge management programs for a set of selected existing bridges representing a typical al bridge inventory.     

Seismic risk assessment of reinforced concrete bridges in flood-prone regions

This article presents a conditional seismic risk evaluation framework of bridges located in seismically active flood-prone regions. Flood-induced bridge scour causes loss of lateral support at bridge foundations and thus the effect of seismic hazard on bridge performance gets amplified. Two example reinforced concrete bridges located in Sacramento County in California are considered. The regional multihazard scenario is characterised by combining scour resulted from regional flood events of different intensities with a suite of earthquake ground motions that represents regional seismicity. Uncertainties in the hazard models are discussed and their influences on bridge performance are investigated. A separate set of analysis is performed to evaluate the bridge performance only under earthquake ground motions. Seismic fragility curves and risk curves for the example bridges are generated. Result shows higher seismic risk of bridges when the impact of regional flood hazard on bridges is considered in the analysis framework. This suggests the use of a combined seismic and flood hazard model for reliable seismic risk evaluation of bridges located in flood-prone regions.     

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.     

Bridge Annual Maintenance Prioritization under Uncertainty by Multiobjective Combinatorial Optimization

Abstract:   Bridge managers are facing ever-increasing tasks of prioritizing limited budgets to cost-effectively maintain normal functionality of a huge inventory of deteriorating civil infrastructures such as highway bridges over the life cycle. A satisfactory maintenance planning scenario should meet managers' specified requirements for the optimum balance between whole-life costing and structural performance. This article presents a general computational procedure to prioritize on an annual basis maintenance efforts for deteriorating reinforced concrete bridge crossheads over a designated time horizon. Within each year, none or one of the available maintenance types with different performance improvement capabilities could be applied and the time of application for any maintenance intervention is considered to be uniformly distributed within a 1-year time interval. Effects of uncertainties associated with bridge crosshead deterioration processes with and without maintenance interventions are considered by means of Monte Carlo simulation to predict probabilistically structural performance and life-cycle maintenance cost. The resulting combinatorial optimization problem is automated by a multiobjective genetic algorithm. It produces a group of different sequences of annualized maintenance interventions that lead to optimized tradeoff among condition, safety, and life-cycle cost objectives. This enables bridge managers to determine a preferred annual maintenance prioritization solution by comparing different alternatives.     

Influential fluid–structure interaction modelling parameters on the response of bridges vulnerable to coastal storms

Analysis of coastal bridges under hurricane-induced wave and surge loads is essential for safety and performance assessment of water crossing bridge inventories. A reliable numerical model that can be employed to study the behaviour of bridges in hurricane events is beneficial because it reduces the cost and effort required for experimental models. Furthermore, it is important to identify modelling parameters that have a significant effect on the simulated response in order to guide uncertainty treatment for future bridge reliability studies. To address these needs, a high fidelity numerical model for simulation of coastal bridges is utilised that takes into account the fluid–structure interaction and includes contact surfaces that permit deck shifting and unseating during surge and wave passage. After validation of the model with experimental test data, it is implemented to examine the response of a typical water crossing bridge in the Houston area, revealing the values of wave and surge loads and also the potential of deck unseating under extreme wave and surge conditions. A sensitivity study is conducted to determine the uncertain structural modelling parameters that significantly affect the bridge response when subjected to surge and wave. Concrete strength and density, coefficient of friction between super- and substructure and soil shear strength are found to influence the bridge response and should be considered in probabilistic analyses and reliability assessments of coastal bridges.     

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.     

Seismic fragility increment functions for deteriorating reinforced concrete bridges

Fragility increment functions are developed to estimate the seismic fragility of reinforced concrete (RC) bridges subject to deterioration due to the onset and progression of corrosion of the reinforcement. For each mode of failure considered, the fragility at time t of a deteriorating bridge is obtained by multiplying the initial fragility of the undeteriorated bridge by a corresponding increment function expressed in terms of the environmental conditions, the original material properties, time, a measure of the seismic demand, and a set of unknown model parameters. The developed increment functions account for the effects on the fragility estimates of the loss of the reinforcement and of the increasing uncertainty over time. As an application, the developed increment functions are used to estimate the seismic fragility of an example RC bridge. The proposed fragility increment functions are useful to estimate the fragility of deteriorating bridges without any extra reliability analysis once the fragility of the undeteriorated bridge is known. In particular, the proposed fragility increment functions can be used to assess the time-variant fragility of bridges for applications such as reliability-based design, life-cycle cost analysis, and risk analysis.