Investigation of Adequacy of Sample Bridges Under Service Level Truck Traffic of Pakistan

Abstract

Design and lifelong structural performance of bridges is primarily governed by the live load models representing truck traffic. In Pakistan, bridges are designed as per live load model of Pakistan Code of Practice for Highway Bridges 1967 (called herein as �PHB Code�). 1967 PHB Code is based on 1961 American Association of State Highways and Transport Officials (AASHTO) Bridge Design Specifications. Over the years, service-level truck traffic of Pakistan has changed significantly in axle weights, axle configuration, gross vehicle weights and traffic volume due to developments in truck industry to meet the heavier loads carrying demands of industries. Thus, live load models specified in 1967 PHB Code may not be a true representation of today�s service - level truck traffic of Pakistan. This difference in live load level and pattern may lead to rapid deterioration of bridges or in reducing their design life. Therefore, use of 1967 PHB Code for designing bridges in 2014 needs detailed evaluation to ensure that these bridges have adequate safety level. In this research, safety level of three simply supported samples prestressed I-beam concrete bridges (Muhammad Wala, Mansoor and Bagh-e-Naran) has been investigated in terms of structural reliability index which is an indirect measure of structural safety. The target reliability index of 3.5 used in the current AASHTO Bridge Design Specifications is used as the benchmark for evaluating the safety level. Truck data acquired from three weigh-in-motion sites was used for reliability analysis. For live load effects i.e., moment and shear in girders, two strengths were used in the reliability analysis: 1) strength required by 1967 PHB Code as the sum of factored design dead load and live load termed herein as PHB design-case; and 2) strength required by the 2010 AASHTO LRFD Bridge Design Specifications as the sum of factored design dead load and live load termed herein as AASHTO design-case. Actual truck data was applied on sample bridges to determine moment and shear in girders. These live load effects were then extrapolated to 75 years using nonparametric fit, and statistical parameters i.e., mean maximum values and co-efficient of variations were determined. Using these statistical parameters, values of structural reliability index were determined through First Order Reliability Methods for all the bridges. In PHB design-case, values of structural reliability indices for moment and shear analysis were found to be 2.43 and 2.32 for Muhammad Wala Bridge (Sangjani), respectively, 2.5 and 2.24 for Bagh-e-Naran Bridge, respectively and 2.24 and 2.20 for Mansoor Bridge, respectively. In AASHTO design-case, values of structural reliability indices for moment and shear analysis were found to be 3.04 and 2.75 for Sangjani Bridge, respectively, 2.74 and 2.51 for Bagh-e-Naran Bridge, respectively, and 2.82 and 2.59 for Mansoor Bridge, respectively. Values of structural reliability indices in all cases are significantly less than the target reliability index value of 3.5 used in the design codes as benchmark for evaluating the desired safety level. It can be concluded that live load model of 1967 PHB code is not the true representation of existing truck traffic on roads. Bridges being designed using 1967 PHB Code may not have desired safety level which may lead to rapid deterioration of bridges or in reducing their design life. Thus, there is a need to develop a new live load model for design of bridges in Pakistan or to enforce legal load limits.