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The Apparent Reserve Load Carrying Capacity
of Covered Bridges

by John H. Weaver, P.E.

March 20, 2000 - I have often been intrigued by the apparent reserve load carrying capacity of covered bridges, that is, their apparent ability to carry loads beyond their analyzed limits to do so. I realize that there may be material properties which explain this phenomenon to some extent, such as moisture content, superior mechanical properties of old growth timber, etc. I participated in the evaluation review of some of these material properties when we sponsored materials studies of the Hopkins and Paper Mill bridges.
       I think, however, that secondary structural and appurtenant members of these bridges may also explain some of the reserve capacity. These members are curbs, upper chord truss bracing, the roof system, wood flooring and floor beams acting as a plate unit, outside nailers attached to the trusses, etc.
       Here is my theory: Although we at VAOT (myself included) have often (conservatively) only analyzed (for laid bearing capacity)the trusses of these bridges, we should make some estimate of the structural contributions from secondary and appurtenant components. On many covered bridges floor beams interlock with the trusses and the longitudinal flooring is attached to these members. Also upper lateral cross-bracing between trusses and roof rafters (with boarding) contribute to structural integrity by shear connection to the trusses.
       Are these reasonable assumptions? I think they are, if the covered bridge components and attached members are in good condition with no obvious signs of detachment. I think this applies to many cases. Also shear connection of secondary or appurtenant members does not require anything more than spikes or notches to develop only a small fraction of structural enhancement. However, the better the shear connections, the greater the potential enhancement.
       Were these secondary and appurtenant members originally intended to structurally enhance the covered bridges? Probably not. Their primary purpose was to keep the bridge dry or to provide bracing or to channel traffic. Nevertheless I think they do provide structural enhancement.
       To apply my theory I analyzed the Sanderson Bridge: Plank lattice type with effective span of 111 feet. Just for comparison, I assumed the existing bridge members in new condition and compared an analysis of live load capacity of the trusses alone to capacity with enhancement due to the composite contribution from a small portion of the secondary and appurtenant member sections. I limited the contribution to what could be expected from very limited shear transfer at rafter fastenings. The live load capacity of the composite (enhanced) section was 40% greater than that of the trusses alone. I consider this to be a substantial increase.

[John Weaver, of VTrans Structures (Vermont Agency of Transportation) was the design engineer on the Irasburg covered bridge project.]

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Joe Nelson, P.O Box 267, Jericho, VT 05465-0267
This file posted March 13, 2001, revised April 26, 2007