Structural behavior of the Bollman truss bridge at Savage, Maryland

摘要

The Bollman truss bridge at Savage, Maryland, is the last surviving structure of its type in the United States. While much has been written about the career of its designer, Wendel Bollman, and the basic behavior and design concept of the truss system, there remain some outstanding questions regarding the truss design. The purpose of this paper is to address some of these questions by the use of indeterminate structural analysis. It is found that the system of Bollman members performs the primary load carrying function, while the diagonal bracing system serves to distribute non-uniform live loads among the Bollman members, and provide a degree of structural redundancy to the truss. Field inspections indicate that the lower chord members are highly unusual in that they are capable of transmitting only compressive loads through their connections. In a strange type of behavior for a simply supported truss, these lower chord members are found to act in compression under a wide range of loading conditions, being most important to the performance of the bridge when non-uniform loads act. It is lastly proposed that this compression behavior of the lower chord members may provide an explanation for the observed retrofit of the lower chord members in only the end panels of the truss.