SYSTEM RELIABILITY ASSESSMENT OF STEEL GIRDER BRIDGES

Artur A. Czarnecki and Andrzej S. Nowak

University of Nebraska

W181 Nebraska Hall, Lincoln, NE 68588-0531, USA
E-mail: arturcz@umich. edu, anowak1@unl. edu

Abstract

Reliability can be considered as a rational evaluation criterion in assessment of bridge structures. The traditional element-based approach to bridge design and evaluation does not allow for consideration of interaction between the components that form a structural system and, therefore, it can be conservative. Safety of the structural system also depends on the degree of redundancy (load sharing) and ductility. As a result, it has been observed that the load carrying capacity of the whole structure can be much larger than what is determined by the design of individual components. Therefore, this paper is focused on the system behavior. The objective is to formulate a limit state function for the whole bridge, identify the load and resistance parameters, and develop an analysis procedure to assess the reliability of the bridge as a structural system. The major steps of the procedure include selection of representative structures, formulation of limit state functions, development of load and resistance models, development of the reliability analysis method, reliability analysis of selected bridges, and formulation of recommendations for practical bridge assessment. The live load is considered in form of a design truck. The analysis is performed for different values of span length, truck position (transverse and longitudinal), number of vehicles on the bridge (multiple presence), girder spacing, and stiffness of structural members (slab and girders). For each combination of these parameters, the bridge resistance is determined in terms of the weight of a truck (or trucks) causing an unacceptable deflection or instability of the considered bridge. The reliabilities are also calculated for individual components (girders) and compared to system reliabilities of the bridge. The resulting system reliability can serve as a tool in the development of a rational bridge design and evaluation procedure.

Keywords: Reliability index, System reliability, Steel girder bridges, Loads, Resistance

Introduction

There is a growing interest in the development and improvement of efficient procedures for the design, evaluation, repair and rehabilitation of bridge structures. Structural reliability can be considered as a rational measure of performance, as it is a function of uncertainties associated with loads and load carrying capacity (resistance). A traditional deterministic approach is based on the analysis of individual components. The capacity of the structure is reached when any of the components reaches its ultimate capacity. However, in practice, due to ductility and redundancy, there can be redistribution of load resulting in an increased load carrying capacity. The margin safety can be assessed more accurately using the system reliability approach.

Therefore, this paper deals with calculation of the reliability for the whole bridge, rather than individual components (girders). The main objective is to compare the reliability of components (girders) and system for steel girder bridges. The limit state function is formulated for the whole bridge, load and resistance parameters are identified, and the reliability analysis procedure is developed. The developed methodology is demonstrated on representative steel girder bridges. The recommendations are formulated for a practical bridge assessment.

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M. Pandey et al. (eds), Advances in Engineering Structures, Mechanics & Construction, 699-710.

© 2006 Springer. Printed in the Netherlands.

The ultimate limit states (moment and shear carrying capacities) are considered. The structural performance is determined as a function of load components, strength of material, and dimensions. The reliability analysis is performed for different values of span length, truck position (transverse and longitudinal), number of vehicles on the bridge (multiple presence), girder spacing, and stiffness of structural members (slab and girders). For each combination of these parameters, the bridge resistance (system resistance) is determined in terms of the gross vehicle weight, GVW, of two side-by-side trucks, with axle configuration of the design truck in AASHTO (2004). It is assumed that the ultimate capacity (maximum GVW) is reached when the resulting deflection exceeds the critical limit. The analysis is performed separately for each transverse position of two trucks.