Computations of the Load Carrying Capacity of a Deficient Bridge Structure

Abstract

The original Nahr Al Fidar bridge structure in Lebanon was built in 1967 and was replaced with a new bridge in 2007. Like all other bridges, the structure is now carrying heavier loads due to the progression in the gross weight of vehicles. This is a serious problem especially in a country like Lebanon where enforcement of law on overloads is not stringent. Consequences may range from distress in the structure and can sometimes end up in catastrophic failure. This paper develops computational methods for evaluating the load carrying capacity of the as-built new structure. This mainly consists of calculating the operating and inventory rating factors as per AASHTO (American Association of State Highway and Transportation Officials). Computations are first performed using closed form solutions from AASHTO and then verified using more sophisticated computer software analyses. The main advantage of the computer application is that the existing bridge is modeled in three-dimension which allows enhanced computation of the distribution of live loads on the structure. Comparisons between the two methods are then made in order to identify the impact of the 3-D computer modeling on reducing the forces induced in the structure. Recommendations for assessing the load carrying capacity of similar bridge structures are finally made. These range from posting the bridge for specific live loads, strengthening and up to replacement.