Comparison of Actual Truck Traffic with Different Live Load Models Used for Design of Bridges in Pakistan

Abstract

Design of bridges is primarily governed by the live load models representing the truck traffic. In Pakistan, bridges are designed as per Pakistan Code of Practice for Highway Bridges 1967 (called herein as “CPHB”) and AASHTO LRFD Bridge Design Specifications (called herein as “AASHTO Specifications”). Further, National Highway Authority (NHA) has specified legal limits on the live loads to prevent overstressing of bridges.In Pakistan, over the years, service-level truck traffic 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 by various industries. Thus, CPHB Code, NHA Legal Loads and AASHTO live load model may not be a true representation of service-level truck traffic of Pakistan. Therefore, following CPHB Code and AASHTO Specifications for designing of bridges in Pakistan needs detailed evaluation to ensure that these are safe and economical. In this study, characteristics of live loads (axle weights, axle configuration and Gross Vehicle Weight (GVW)) and live load effects (shear and moments) of actual truck traffic are compared with live load models of CPHB and AASHTO Specifications. Two samples truck traffic data recorded at Ayub Bridge Peshawar (411 trucks) and Sanjani Weigh-in-Motion Station (35034 trucks) have been used for Analysis. Live load effects due to moving loads have been determined using Influence Lines for representative bridge spans ranging from 20 to 50 meter. Since, live load effects depend on axle weights, axle spacing and resultant of axle weights, therefore, hand calculations for over 35000 trucks, having numerous axle and weight configurations, is very tedious and cumbersome, hence, a Matlab Code has been developed for all kinds of analysis. In addition, this study includes the results of violation of NHA Legal Loads limitations of Axle weights and GVW. From analysis, it is found that GVW varies from 0.84 ton to 109.26 tons with average of 41.47 ton. 6 percent of actual trucks have GVW higher than GVW of live load model of PBH Code and 71.5 percent of actual trucks have GVW higher than GVW of live load model of AASHTO. Maximum value of GVW is 97.5 and 150 percent higher than GVW of live load models ofPBH Code and AASHTO, respectively. From total