Covered bridges are complex structures to analyze as they are constructed with timber, which is highly variable and with designs that are redundant with material properties influencing the behavior. When combined, it is easy to understand why some analyses conclude that some bridges should not even be standing. As noted in the Federal Highway Administration (FHWA) publication FHWA-HRT-04-098, Covered Bridge Manual, there are inconsistencies with the assumptions of the traditional, simple, static analysis of trusses. Generally, in most covered bridge analyses, it is assumed that the primary truss elements behave in a manner similar to steel trusses. However, the fact that timber trusses are more significant means that covered bridge members tend to behave more like frame members (i.e., have both axial and bending forces) than as truss members (i.e., axial forces only). Some of the complexities involve eccentric connections at the joints, various load paths from the stringers and floor beams to the truss, uncertainty of the connections between the trusses and the arches, and interaction between the trusses and their housing. Most engineers are relatively comfortable with completing a standard truss analysis but are unprepared to complete a sophisticated two-dimensional (2-D) or three-dimensional (3-D) frame analysis. As such, much simplified and inaccurate analyses are often performed with overly conservative safety factors applied to account for known inaccuracies. In addition, many covered bridge trusses consist of multiple kingpost trusses superimposed with an arch that has complex connectively between the various elements. An accurate analysis of such systems clearly requires sophisticated analysis techniques with which most engineers are not familiar. Because of the issues discussed above, it is clear that engineers need assistance with the analysis and modeling of covered bridges. This is especially true as they conduct repair and load rating calculations.
This study will develop guidelines for improving the analysis of covered bridges.