Alfredo H.-S. Ang

Determination of optimal target reliabilities for design and upgrading of structures

A systematic approach is proposed for formulating risk-based cost-effective criteria for the design and upgrading of structures with special reference to earthquake protection. Target reliabilities (or acceptable risks) for damage control and life safety are determined on the basis of minimum expected life-cycle cost and from which risk-consistent criteria for design or upgrading are developed. The approach is illustrated for a specific class of reinforced concrete buildings in Mexico City.

Cost optimal design of R/C buildings

Abstract A systematic approach is proposed for evaluating the cost-effectiveness of existing or proposed design criteria from the standpoint of life-cycle cost consideration. A series of alternative designs of a model structure representing a class of R/C buildings would be developed following an existing code procedure, except that the code requirements or parameters will be varied for the alternative designs so that a suite of different structures will be obtained each with a different level of safety or reliability. For each of the designed structures, the probability of exceeding the various damage levels under a given earthquake intensity may be calculated. Aggregating and integrating all the cost components with the damage probability density functions for each of the designed structures, as well as with the probabilities of all possible earthquake intensities over a given life will yield the expected life-cycle costs for the respective structures as a function of structural reliability. From these results, the design with the minimum expected life-cycle cost may then be identified; its underlying safety or reliability can also be determined. The approach is illustrated for a class of reinforced concrete buildings under earthquake loading.

A model for the seismic reliability assessment of electric power transmission systems

A comprehensive model to evaluate the seismic reliability of electric power transmission systems is presented. The model provides information on the probability of structural failure of critical equipment at the major substations, from which the corresponding probabilities of power disruption to a given service area are determined. With the proposed methodology earthquake ground motions are defined as stochastic processes, and seismic capacities of electrical equipment are determined on the basis of available test data and simple modeling, from which fragility functions of critical equipment and specific substations are developed. Probabilities of power disruption resulting from network disconnectivity and abnormal power flow are assessed through Monte Carlo simulation. As a case study, the proposed model is applied to the electric power network in San Francisco and vicinity under the 1989 Loma Prieta earthquake, and the probabilities of power interruption are contrasted with the actual power failures observed during that earthquake.

Life-cycle Considerations in Risk-informed Decisions for Design of Civil Infrastructures

Civil structures and infrastructures, such as buildings, bridges, and other facilities, are constructed and built for long service lives; many for over 50 or 100 years. In this light, the performance of an engineered system over its useful life span is naturally of major concern. The concern must include the assurance of a minimum level of reliability of performance, which will necessarily require inspections, repair, and maybe even retrofitting or replacement. The whole-life cost associated with this life-cycle performance, therefore, is the pertinent cost that ought to be the basis for determining the cost during the design stage (in terms of present value); this must include the maintenance and potential damage costs, besides the initial cost, over the life of a system. In this regard, significant technical and economic uncertainties can be expected and are unavoidable; therefore, decisions required in the design of such systems must consider risk associated with the probability of non-performance and serious damage or failure, as well as of the financial risk. The importance of these factors in the decision process at the design stage is emphasised and a practical approach for life-cycle consideration in formulating risk-informed decisions in the planning and design of infrastructure systems is described. The approach is illustrated with specific applications in the optimal design of civil infrastructure systems.

Reliability evaluation of idealized tunnel systems

Abstract In order to discuss the overall safety of a tunnel project, it is necessary to consider the reliability of a given length of the tunnel system rather than that at a cross-section. Since the failure pattern along a tunnel axis consists of failed and unfailed sections with varying lengths, a two-state availability model is introduced. Results show that the reliability of a tunnel system within one type of rock would depend upon the correlation characteristics of the ground along the tunnel axis and the reliability of a cross-section. The effect of alternating rock types along the tunnel axis upon the reliability of the tunnel system is also investigated.

Seismic reliability of electrical power transmission systems

Abstract The reliability of electric power transmission systems is important for the probabilistic safety assessment of nuclear power plants under a given earthquake loading as it relates to the loss of off site power to the nuclear power plants. Here, a comprehensive model to evaluate the seismic reliability of electric power transmission systems is presented. The model provides probabilistic assessments of structural damage and abnormal power flow that can lead to power interruption in a transmission system under a given earthquake. With the proposed methodology seismic capacities of electrical. equipment are determined on the basis of available test data and simple modeling from which fragility functions of specific substations are developed. Earthquake ground motions are defined as stochastic processes. Probabilities of network disconnectivity and abnormal power flow are assessed through Monte Carlo simulations. The proposed model is applied to the electric power network in San Francisco and vicinity under the 1989 Loma Prieta earthquake, and the probabilities of power interruption are contrasted with the actual power failures observed during that earthquake.

On the first-order third-moment reliability method

In this article, the reliability evaluation with inclusion of random variable with unknown probability distribution will be discussed, in which the only information of the first three moments of the random variable will be used instead of probability distribution. The second-order polynomial normal transformation technique using the first three central moments is investigated and a pseudo standard normal space is defined based on the investigation. The first-order third-moment reliability analysis in the reduced space and that in the pseudo standard normal space are compared. Through the numerical examples presented, the proposed methods are found to be sufficiently accurate to include the random variables with unknown cumulative distribution functions in the first-order reliability analysis with little extra computational effort.

Estimating statistical moments of random systems based on appropriate reference variables

Purpose The purpose of this paper is to find an accurate, efficient and easy-to-implement point estimate method (PEM) for the statistical moments of random systems. Design/methodology/approach First, by the theoretical and numerical analysis, the approximate reference variables for the frequently used nine types of random variables are obtained; then by combining with the dimension-reduction method (DRM), a new method which consists of four sub-methods is proposed; and finally, several examples are investigated to verify the characteristics of the proposed method. Findings Two types of reference variables for the frequently used nine types of variables are proposed, and four sub-methods for estimating the moments of responses are presented by combining with the univariate and bivariate DRM. Research limitations/implications In this paper, the number of nodes of one-dimensional integrals is determined subjectively and empirically; therefore, determining the number of nodes rationally is still a challenge. Originality/value Through the linear transformation, the optimal reference variables of random variables are presented, and a PEM based on the linear transformation is proposed which is efficient and easy to implement. By the numerical method, the quasi-optimal reference variables are given, which is the basis of the proposed PEM based on the quasi-optimal reference variables, together with high efficiency and ease of implementation.

Cost-Benefit Model for Risk-Based Design of a Typical Marine Platform in Mexico

A reliability-based cost-benefit model for the risk management of oil platforms in the formulation of optimal decisions based on life-cycle consideration is proposed. The model is based on structural risk assessments and the integration of social issues and economics into the management decision process. Structural risks result from the platform’s exposure to the random environmental loading associated with the offshore site where it is located. Several alternative designs of a typical platform are proposed and assessed from the cost-effectiveness viewpoint. This assessment is performed through the generation of cost/benefit relationships that are used, later on, to select the optimal design.Copyright

A new point estimation method for statistical moments based on dimension-reduction method and direct numerical integration

Abstract Estimation of statistical moments of structural response is one of the main topics for analysis of random systems. The balance between accuracy and efficiency remains a challenge. After investigating of the existing point estimation method (PEM), a new point estimate method based on the dimension-reduction method (DRM) is presented. By introducing transformations, a system with general variables is transformed into the one with independent variables. Then, the existing PEMs based on the DRMs are investigated. Based on the qualitative analysis of difference in the approximations for response function and moment function, a new PEM is proposed, in which the response function is decomposed directly and the moments are calculated by high dimensional integral directly. Compared with the existing PEM based on univariate DRM, the proposed method is more friendly and easier to implement without loss of accuracy and efficiency; as compared with the PEM based on the generalized DRM, the proposed method is of better precision at the cost of nearly the same efficiency and computational complexity, further, it does hold that the even-order moments are nonnegative. Finally, several examples are investigated to verify the performance of the new method.