Covered Bridges — Engineering Judgment and the Practical Approach

Abstract

Analytical evaluations of historic covered bridges routinely detect weaknesses of the timber structure. The trusses are found to be too weak to support desired vehicular loading – often too weak to even support just the weight of the bridge. The floor is too weak to safely support vehicles. The roof won’t safely support snow loads. And the bracing is too weak to withstand wind loading. What to do? Maybe the structure really is in trouble. Maybe there is visible evidence of distress to support such analytical predictions. Engineers are expected to protect the public from unsafe structures. Hence, the call for closure to traffic or the recommendation for reinforcement or replacement components may be prudent. At least, more investigation is in order. But what to do if there is not visible evidence of distress? Should we recommend the same solutions, when the numbers say we should? Is that “crying wolf”? Another approach is to consider the nuances of extant covered bridges within the context of complex structures that have withstood the test of time - to consider the basis for the allowable timber stresses that we use in today’s environment. There is need for more knowledge about these fascinating vestiges of our early bridge building history. Not enough is known about the statistical 95% exclusion rule for timber allowable stresses. Complex timber joinery is often not understood to the extent that it should be by those dealing with the project. While there are some interesting and hopeful developments in the use of modern materials to augment existing timber structures, there is arguably even more need for experience and sound engineering judgment by those challenged to care for these precious structures. This article delves into the complexities that confound engineers challenged to determine the safe load capacity of such structures and to determine just what ought to be addressed in rehabilitation projects. A number of popular (re)actions are considered and discussed.