Design of long span modular bridges for traffic detours

Abstract

The oncoming large amount of bridge replacements in the next 10 to 20 years called for a detailed examination of available replacement schemes which can have variable impact on user costs. Detouring traffic with a modular bridge proved to be the most desirable scheme in terms of user costs such as traffic delays, detour distances, ultimate highway geometrics, construction crew safety, and safety of drivers. Criteria that encompassed modular bridge design were defined and two companies in North America - Acrow and Mabey - were found to provide bridges within those parameters. A brief analysis of Acrow bridges showed that maximum span lengths range in the order of 100m; this is fairly short compared to spans of many bridges that will be have to soon be replaced. The current bridge system with which modularity is achieved is a set of truss panels which are supported by abutments or piers. In order to span crossing over 100m, piers would have to be placed in the channel or on the head-slopes which is a costly and undesirable construction process. Therefore, a modular bridge which could achieve longer spans was proposed for a 2 lane and a 3 lane wide bridges using as many existing Acrow components as possible. The scheme encompasses a harp cable-stay bridge with cables spaced and sized such that they are fully interchangeable between the various bridge widths and can be built up to any span. These cables are the only additional component as the towers, the girders, and the decks are all made out of existing Acrow components.

(cont.) The pylon is balanced with an anchoring cable and ideas for modular foundations for the anchor are presented. A span of approximately 183m is possible for a 3 lane bridge limited by the maximum axial tower capacity and 250m for the 2 lane bridge based on lateral vibrations. The design fully reflects modularity and should promote the use of modular bridges for longer span crossings.