Monthly Archives: May 2011

In a Long tradition of wooden bridge geekdom

I went off on a tangent recently, of sorts anyhow, more of a refocus.

One of my areas of interest is New Hampshire’s connection to the history of wooden bridge building, a history which is rich and long, yet still continues. This beginning with what was the longest of Timothy Palmer’s Trussed Arches, known to history as the “Great Arch” , a 244′ span over the Piscataqua, part of a 2,362 feet long and 38 feet wide bridge which allowed locals to navigate the Great Bay, an amazing first, dating to 1794.

A decade later, and in the century that followed, New Hampshire was particularly well represented in bridge building in a century of massive change . This change driven, by a rapidly expanding country, and the spirit of innovation found in new ideas and new beginnings.

New Hampshire was the birthplace of a number of Truss patent holders and bridge building innovators, some whose life’s work was done here, some who traveled far on that new spirit of American innovation.

This tangential trip, was into the life and work of New Hampshire native son Col. Stephen Harriman Long and his patent trusses.

I recently did a structural condition assessment on a Long Truss Bridge which took on a modest lean this past winter. As such hard looks often do, this prompted a little burst of renewed research on not just this bridge in particular, but on the truss type as a whole. Always a favorite of mine not just for the local connection, but as a bridgewrighting carpenter. A Long is a framers bridge, something of a timber frame carpenters dream.

Though I’ve long had a photocopy of an original from the collection at the Thayer School at Dartmouth, (Long’s Alma mater, the college, the engineering school did not yet exist – He left Dartmouth in 1809 having spent $94.56 on his education, and still $15.33 in debt upon graduation ) this research led me to discover that Long’s book on bridge building has both been recently digitized and is also unexpectedly back in print.

Stephen’s college debt was about what I paid for my new hardcopy of his long out of print thesis. In reading yesterday, the chapters titled “A Series of Directions to Bridge Builders” in section XVII – OF THE BRIDGE FLOORING – I was happy to find another contemporaneous description of the system we replicated at The Gilpin’s, which I described in my last entry.

The Gilpin's Falls Covered Bridge - Photo by James Walsh

Sometimes hard to read, all the same, I like contemporary descriptions as source material and try to defer to them in assigning names to parts and pieces, and use the terminology chosen by the builders and patent holders unless some other term had since become the term of choice amongst bridgewrights in the years which have followed.

A snippet, in the mans words –

“The floor guards consist of timbers about 3 by 12 inches, placed edgewise in contact with the posts and main braces, and on both sides of the road-way, the lower edge being coincident with the lower sides of the flooring plank, which last are prevented from slipping endwise by reason of their confinement between the guards.

The floor binders are ribands of any convenient dimensions, attached to the guards in a manner to confine the ends of the flooring plank, and keep them in their proper places.

In cases where all the parts of the flooring here considered, are applied, no nails spikes or treenails, will be required as fastenings for the floor.”

From – Description of Colonel S.H. Long’s Bridges: Together With A Series Of Directions To Bridge Builders – This was originally released in 1836 by the publishing house of J.F. Brown in Concord NH, during a brief period while the Colonel returned to and resided in his hometown of Hopkington – This revision and re-release reflects changes and addendum’s born of the 1839 patent

A link to the digital copy – http://tiny.cc/yz30s

Yes, I guess that puts me squarely in the Bridge Geek camp.


119′ Clamps

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.

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