Hyo-Nam Cho

A risk assessment methodology for incorporating uncertainties using fuzzy concepts

Abstract This paper proposes a new methodology for incorporating uncertainties using fuzzy concepts into conventional risk assessment frameworks. This paper also introduces new forms of fuzzy membership curves, designed to consider the uncertainty range that represents the degree of uncertainties involved in both probabilistic parameter estimates and subjective judgments, since it is often difficult or even impossible to precisely estimate the occurrence rate of an event in terms of one single crisp probability. It is to be noted that simple linguistic variables such as ‘High/Low’ and ‘Good/Bad’ have the limitations in quantifying the various risks inherent in construction projects, but only represent subjective mental cognition adequately. Therefore, in this paper, the statements that include some quantification with giving specific value or scale, such as ‘Close to any value’ or ‘Higher/Lower than analyzed value’, are used in order to get over the limitations. It may be stated that the proposed methodology will be very useful for the systematic and rational risk assessment of construction projects.

Reliability-based fatigue failure analysis for causes assessment of a collapsed steel truss bridge

Abstract This paper is intended to demonstrate conventional and reliability-based approaches to the collapse cause assessment in order to identify the effects of mis-installed bracket and H-beam members on the collapse of a steel truss bridge over the Han river in Korea only 15 years after opening to traffic. Based on extensive numerical investigations with parametric studies on various possible failure causes in terms of failure probability and expected fatigue life, it has been found that the mis-installation of bracket and H-beam members accelerated the fatigue failure of the vertical pin-connected hanger. Moreover, it may be observed that both the conventional and reliability-based S-N and linear elastic fracture mechanisms (LEFM) approaches in terms of the expected fatigue life and the fatigue failure probability provide about similar and compatible results. This indicates that any of the reliability approaches could be used as effective and rational techniques for the quantitative investigation of the complex collapse causes, together with the aids of conventional S-N/LEFM fatigue analysis.

An Experience of Practical Reliability-Based Safety Assessment and Capacity Rating

Nowadays, reliability-based methods have been widely used for the safety and capacity rating of deteriorated and/or damaged bridges. This paper is intended to suggest practical reliability-based assessment models and methods for the safety assessment and capacity rating of various kinds of actual existing bridges. And the application of recently developed a new approach, called equivalent system-strength, for reliability-based capacity rating is illustrated using the data obtained from actual bridges. This paper also summarizes various approaches to reliability-based safety assessment and capacity rating, and investigates their application to various existing aged bridges. It will be systematically shown that the proposed bridge reliability models and methods could be effectively used in practice for the safety assessment and capacity rating of existing old bridges in conjunction with static and dynamic field load tests, nondestructive tests, and visual inspections.

Field load testing and reliability-based integrity assessment of segmental PC box girder bridges before opening to traffic

The main objective of this study is to suggest practical methods for field load testing and a reliability-based approach for the assessment of safety, durability, and load carrying capacity of newly constructed PC box girder bridges before opening to traffic. The proposed field proof-load and behavior tests such as static and dynamic proof-load tests, crack behavior test, and the test for the measurement of external post-tension forces are proved to be very practical and effective for the integrity assessment of PC box girder bridges. The response ratios acquired from proof-load tests are incorporated into the limit state model for the assessment of realistic safety and load carrying capacity of the type of bridges. This paper also suggests practical strength and crack durability limit state equations for the reliability-based integrity assessment of segmental PC box girder bridges. The AFOSM and MCS (Monte-Carlo Simulation) are used as reliability methods for the reliability analysis using the proposed models. The proposed reliability-based approach with the results of extensive field static and dynamic tests are applied for the integrity assessment of a new segmental PC box girder bridge, namely, New Haengju Grand Bridge, which have been reconstructed after the collapse during construction 4 years ago.

Risk assessments of long-span bridges considering life-cycle cost concept and near-fault ground motion effect

The influence of the near-fault ground motion on the response of long-span bridges must be considered as a critical factor for seismic design because the response indicates different aspects from existing earthquake characteristics. Also, it is important to note that the safety index for the risk assessment of long-span bridges is determined based on the minimum expected life-cycle cost E(LCC). In this study, earthquake characteristics are analyzed by creating elastic and inelastic response spectrums with actual measurement records (Chi-Chi earthquake records) and then the numerical analysis of the long-span bridge in Namhae, Korea is performed according to the increase and reduction of the member stiffness based on the standard design., the reliability evaluation of the long-span bridge considering aleatory uncertainties is performed on the basis of the combined results of static analysis and seismic response analysis. Also, the minimum LCC is estimated based on failure probabilities by the different alternative design. Because of epistemic uncertainties, the results of reliability evaluation and the LCC of optimal design are selected as random variables; the safety index, failure probability and expected minimum LCC are re-evaluated with regard to critical percentage values for a risk-averse design of the long-span bridge, and are presented graphically using cumulative percentages. It is, therefore, expected that this study will provide the basic information for the risk assessment and optimal design method in performing seismic design of the long-span bridge considering earthquake characteristics.

Reliability-based failure cause assessment of collapsed bridge during construction

Abstract Until now, in many forensic reports, the failure cause assessments are usually carried out by a deterministic approach so far. However, it may be possible for the forensic investigation to lead to unreasonable results far from the real collapse scenario, because the deterministic approach does not systematically take into account any information on the uncertainties involved in the failures of structures. Reliability-based failure cause assessment (reliability-based forensic engineering) methodology is developed which can incorporate the uncertainties involved in structural failures and structures, and to apply them to the collapsed bridge in order to identify the most critical failure scenario and find the cause that triggered the bridge collapse. Moreover, to save the time and cost of evaluation, an algorithm of automated event tree analysis (ETA) is proposed and possible to automatically calculate the failure probabilities of the failure events and the occurrence probabilities of failure scenarios. Also, for reliability analysis, uncertainties are estimated more reasonably by using the Bayesian approach based on the experimental laboratory testing data in the forensic report. For the applicability, the proposed approach is applied to the Hang-ju Grand Bridge, which collapsed during construction, and compared with deterministic approach.

Seismic Risk Analysis of Frames with Uncertain Support and PR Connection Conditions

A hybrid reliability evaluation procedure is proposed to estimate the risk of steel frames considering the rigidity of connections and supports as accurately as possible. Nonlinearities due to geometry, material, partially restrained (PR) connections, and flexible supports are considered in the proposed algorithm. The four-parameter Richard model is used to represent the flexibility of connections and supports. All major sources of uncertainty in the loading and resistance-related parameters and the parameters in the Richard model are incorporated in the algorithm. The unique feature of the algorithm is that the earth-quake loading can be applied in the time domain, providing an alternative to the random vibration approach. The proposed algorithm intelligently integrates the concepts of the finite element method, the response surface method, the first order reliability method, and the iterative linear interpolation scheme. The algorithm is verified using Monte Carlo simulation. With the help of an example, it is shown that the proposed algorithm can be used to estimate risk for both the serviceability and strength limit states. The presence of PR connections and/or flexible supports alters the dynamic properties (stiffness and damping) of the structure and adds a new and major source of energy dissipation. The serviceability limit state may become the controlling limit state, particularly for seismic loading. Thus, the common practice of considering all connections and supports to be rigid and designing the frame for the strength limit state may not be appropriate. The flexibility of connections and supports and the uncertainty in modeling them have a considerable influence on the overall behavior of frames, particularly under seismic loading. An efficient, robust, and accurate method is proposed to evaluate the reliability of such frames.

PRACTICAL APPLICATION OF LIFE-CYCLE COST EFFECTIVE DESIGN AND REHABILITATION OF CIVIL INFRASTRUCTURES

The demand on the practical application of life-cycle cost effectiveness for design and rehabilitation of civil infrastructure is rapidly growing in civil engineering practice. In the 21st century it is almost obvious that life-cycle costs together with value engineering will become a new paradigm for all engineering decision problems in practice. However, in spite of impressive progress in the research on LLC, most of the research has focused on the theoretical point and has not fully incorporated the critical issues for the practical implementation. This paper intended to suggest a systematic integrated approach to the practical application of various LLC methodologies for the design and rehabilitation of civil infrastructure.

LIFE-CYCLE COST EFFECTIVE OPTIMAL SEISMIC DESIGN FOR CONTINUOUS PSC BRIDGES

A systematic procedure is developed for the reliability-based seismic safety level for bridge life-cycle cost design criteria of continuous PC bridges. The paper proposes a set of cost function models for life-cycle cost analysis of bridges. The total life cycle cost function consist of initial coast and direct/indirect damage costs considering repair/replacement costs, human losses, property damage costs, road user costs, and indirect regional economic losses. The damage costs are successfully expressed in terms of Park-Ang median global damage indices and damage probabilities. To computer Park-Ang median global damage indices and damage probabilities, SMART-IDARC-BRIDGE, a modified version of the well-known IDARC-BRIDGE, incorporating simplified Monte Carlo simulations and system reliability for multi-unit PC bridges is used. Earthquake ground motions used to simulate the realistic loading condition are obtained from non-stationary filtered Gaussian processes with both frequency and amplitude modulations based on the Yen-Wen stochastic ground input motion model. The proposed approach is applied to model bridges of moth moderate seismicity regions like Korea and high seismicity regions like Japan and various sensitivity analyses are performed to identify governing parameters. From the results of the numerical investigation, the proposed approach can be effectively utilized for optimal seismic safety levels for bridge life-cycle costs design criteria.

PRACTICAL LIFE-CYCLE COST EFFECTIVE OPTIMUM DESIGN OF STEEL BRIDGES

A practical Life-Cycle Cost (LCC) formulation and procedure for the LCC-effective optimum of steel bridges is presented in this paper. A general LCC model for optimum design of steel bridges consists of initial cost and direct/indirect rehabilitation costs of steel bridges including repair/replacement costs, loss of contents or fatality and injury losses, road user costs, and indirect socio-economic losses. This paper focuses on the presentation of a general formulation of LCC models and LCC effective design system models for steel bridges suitable for practical implementation.