Dan M. Frangopol

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.

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.

Probabilistic analysis of resistance degradation of reinforced concrete bridge beams under corrosion

Bridges subjected to environmental attack can experience changes in resistance which are time-variant. In this study, flexural strength loss in concrete bridge beams due to corrosion of steel reinforcement is considered. The sensitivity of the corrosion initiation time of steel reinforcement to changes in the main descriptors of diffusion related random variables is illustrated. Both the mean and standard deviation of the corrosion initiation time increase with an increase in the coefficient of variation of each of the diffusion related random variables for the range of values considered in this study. An example problem is given which illustrates the effect of various variables including corrosion rate and corrosion initiation time on the time-variant area of steel reinforcement and flexural strength of an existing reinforced concrete bridge beam. For the range of parameters and the damage scenarios considered, the mean value of the resistance loss function appears to increase about linearly with time. The results can be used to develop optimal lifetime reliability-based maintenance strategies for reinforced concrete bridges under environmental attack.

Reliability-based design of MEMS mechanisms by topology optimization

This paper presents a methodology for the design of micro-electro-mechanical systems (MEMS) by topology optimization accounting for stochastic loading and boundary conditions as well as material properties. This methodology combines recent advances in material-based topology optimization for compliant mechanisms undergoing large displacements and design optimization under uncertainties using first order reliability analysis methods. The performance measure approach is applied to the formulation of the optimization problem. The structural response is predicted by a co-rotational finite element formulation and the design and imperfection sensitivities are evaluated by an adjoint method. The methodology is illustrated by the topology optimization of a compliant mechanism. The results show the importance of accounting for the stochastic nature of the micro-system in the topology optimization process.

Life-cycle reliability-based optimization of civil and aerospace structures

Today, it is widely recognized that optimization methodologies should account for the stochastic nature of engineering systems and that concepts and methods of life-cycle engineering should be used to obtain a cost-effective design during a specified time horizon. The recent developments in life-cycle engineering of civil and aerospace structures based on system reliability, time-dependent reliability, life-cycle maintenance, life-cycle cost and optimization constitute an important progress. The objective of this study is to present a brief review of the life-cycle reliability-based optimization field with emphasis on civil and aerospace structures.

Resilience and Sustainability of Civil Infrastructure: Toward a Unified Approach

In recent years, the concepts of resilience and sustainability have become very topical and popular. The concept of sustainability rose to prominence in the late 1980s and became a central issue in world politics, when the construction industry began to generate the first sustainable building assessment systems with more or less equally weighted environmental, economic, and social aspects for office buildings over their life cycles. On the other hand, resilience is usually connected to the occurrence of extreme events during the life cycle of structures and infrastructures. In the last decade, it has been used to minimize specifically direct and indirect losses from hazards through enhanced resistance and robustness to extreme events, as well as more effective recovery strategies. A detailed comparison of the studies dealing with either infrastructure sustainability or resilience presented in this paper leads to the conclusion that they have a vast number of similarities and common characteristics. For instance, they both combine structural analyses with social and economic aspects; they both rely on techniques for the life-cycle analysis and decision making; they both are in an early stage, where the academic world is trying to find the best way to promote the application of the scientific results among professional engineers and the industry. Indeed, both approaches try to optimize a system, such as a civil infrastructure system, with respect to structural design, utilized material, maintenance plans, management strategies, and impacts on the society. However, for the most part, researchers and practitioners focusing on either resilience or sustainability operate without a mutual consideration of the findings, which leads to a severe inefficiency. Therefore, this paper suggests that resilience and sustainability are complementary and should be used in an integrated perspective. In particular, the proposed approach is rooted in the well-established framework of risk assessment. The impact of the infrastructure and its service states on the society in normal operational conditions (assessed by sustainability analysis) and after exceptional events (assessed by resilience analysis) should be weighted by the associated probabilities of occurrence and combined in a global impact assessment. The proposed perspective and assessment technique is applicable to various types of civil infrastructure systems, but the case of transportation networks and bridge systems is emphasized herein. A numerical application dealing with the comparative analysis of two possible bridge layouts is presented to exemplify the approach. The results show that both resilience and sustainability analyses assess a relevant amount of the impact of the bridge on the community where it is built, so neither one can be neglected.

Balancing weight, system reliability and redundancy in a multiobjective optimization framework

Abstract Redundancy is often desired in structural systems, although no redundancy definition has been widely accepted as an objective measurement. In this paper a probabilistic index is employed as a measure of structural redundancy. This index is compatible with structural system reliability analysis. The optimization problem is solved in a multiobjective framework in which the goal is to minimize and/or maximize several objectives simultaneously. Optimal structural designs are sought in the sense of least weight, highest system reliability and highest system redundancy. Numerical results show that optimal searchings balancing weight, system reliability and system redundancy produce more rational structural designs.

Reliability analysis of chloride penetration in saturated concrete

Abstract Corrosion of reinforcement in concrete is a major durability problem of reinforced concrete structures. The corrosion is initiated by chloride penetration into the concrete, which is a diffusion-controlled process involving many complex physical and chemical mechanisms. Large random variation has shown in the corrosion damage of reinforced concrete structures, and there is a pressing need to develop a reliability analysis method for chloride penetration and for the onset of steel corrosion in concrete. In order to conduct a reliability analysis, a comprehensive material model for chloride concentration is developed and described, and some of the model parameters (water–cement ratio and curing time) are selected as random variables. By including uncertainties in the selected variables, the chloride penetration front at a point in time can be represented by a time-dependent probabilistic distribution. Similar to the concepts of supply and demand used in structural reliability analysis, the chloride penetration front at a target level (depth) with respect to time can be represented by the probability distribution of crossing the target level. A detailed description of the basic concepts and numerical examples are given. To include the effect of uncertainties of the material parameters, a recently developed Monte Carlo simulation program was used in numerical examples.

System reliability and redundancy in structural design and evaluation

Abstract Several reliability-based structural design and evaluation specifications for buildings, bridges, and offshore platforms have been proposed and implemented in North America, Asia, Australia and Europe. Thus far, however, these are largely limited to individual structural component checks and do not generally include requirements of system reliability and redundanry. The main purpose of this paper is to contribute to the development of more rational system reliability-based structural design and evaluation specifications. In this context, effects of material behavior, correlations, variabilities of resistances and loads, resistance sharing, structural damage, and number of members, on both the system reliability and redundancy of a parallel system are investigated, and a framework for including system effects in structural design and evaluation specifications is proposed.

Reinforced concrete bridge deck reliability model incorporating temporal and spatial variations of probabilistic corrosion rate sensor data

The reliability of reinforced concrete (RC) bridge decks depends significantly on the rate of corrosion of the reinforcing steel. Structural health monitoring (SHM) techniques, including embedded corrosion rate sensors, can greatly improve the quantification of the steel corrosion rate, which can lead to improved estimates of structural safety and serviceability. Due to uncertainties in concrete properties, environmental conditions, and other factors, the rate of corrosion of reinforcing steel can be highly variable, both within a given structural component and over time. By placing multiple corrosion rate sensors throughout a structural component, such as a bridge deck, these spatial and temporal variabilities can be monitored and as such better predicted, for use in a reliability model. The objective of this investigation is to present a reliability model for a RC bridge deck incorporating both spatial and temporal variations of probabilistic corrosion rate sensor data. This objective is accomplished using a computational reliability model and Monte Carlo simulation. Corrosion rate sensor data is assumed for multiple critical sections throughout a RC bridge deck over time by applying empirical spatial and temporal relationships. This data is then used to improve an existing spatially invariant reliability model. The improved reliability model incorporates several sub-models to determine the changes in load effects on and resistance of a RC bridge deck slab over time, as well as spatial correlation of corrosion and a system approach to account for spatial variability. The improved reliability model incorporating both spatial and temporal variations in corrosion rate data provides a better estimate of the service life of a RC bridge deck slab.