Recently a cyber acquaintance visited the Gilpin’s Falls, a bridge I, as part of a team of three, helped restore for many long months back in ’09. His photographer’s eye for detail saw him notice and ask after the now unusual way the floor of this bridge is assembled…
It’s a good story, and I thought I’d elaborate here, and use it as a catalyst to finally kick this blogging thing off and up a notch.
The floor as constructed, exists because knowledgeable people found historic evidence, made a case and lobbied for a construction detail to be returned to the bridge. It also helped that this evidence did not fall on deaf ears.
The floor system on the Gilpin’s had been almost wholly replaced in the 20’s, the last decade the bridge carried traffic. This in a failed attempt to increase its load carrying capacity, all the Floor Beams but one, all the Sleepers, (longitudinal joists which run from Floor Beam to Floor Beam) and all the Flooring, had been switched out from softwood to mixed species hardwood. The Flooring had also been installed on a slight bias, (though so slight as to not lend any triangulation and bracing effect) and this proved to have been done simply to avoid any need to cut the material to length, it was simply laid down in the raw rough slightly random lengths provided by the sawmill.
Occasionally things fall together as they should, and as we disassembled the floor system, it became obvious that the hardwood replacements had suffered massive infestations of Powderpost Beetles and would again require replacement, this was fortuitous in that the huge increase in weight and dead load they introduced had been directly responsible for much of the distortions to the bridges framing.
As we dismantled the Floor we found long multiple panel / bay sleepers, with longitudinal rebates and unused bolt holes. The use of these “rabbits” had been long abandoned, but we found their intended original use obvious, having read about such systems, most notably in the very descriptive hand written proposal documents of Indiana bridgewright J.J. Daniels and in the writings of Col. Long.
They are part of a simple but ingenious flooring clamping system, which obviously once saw use over a wide geographic area. Its advantages are based in a simple understanding of the materials being used, the intent being both to allow the clamp to be loosened after the flooring seasons and shrinks, rows of flooring to be tightened up, and gaps filled by adding additional pieces. This system also avoids the damage and reduction to service life in Sleepers, which comes with the repeated spiking down of flooring as wear demands its repeated replacement.
We returned these pieces to their intended use, and it is possible, if not probable, that the Gilpin’s Falls Covered Bridge is the only existing example of this system in use today.