If I were to define what it is I do, the description I see as mayhaps that one most accurately describing it, with both eloquence and in as few words as is possible, is simply to suggest that “I join timber”
My work is to take one of our planets greatest blessings, Trees. And to take their only modestly converted stems, (With variation, but in essence with only four sapwood slabs removed) and then join this blessing into useful configurations. Be it, houses, barns, or bridges.
In most such efforts joining timber, I find myself working to wholly avoid the use of large metal fasteners – This is almost one of the sub-definitions of what it is to be a Timberframer. Wood to wood is what we do.
Bridgewrighting, as allied a trade as it is, is a bit different. Even those Truss types without iron in them – Longs, Paddlefords, Burrs and Towns, still are often peppered with a smattering of bolts…
Many Truss types, share iron as almost an equal partner, equal in effort if not proportion of material used. A Symbiont of sorts, necessary to allow a largely wooden truss, to do what is asked of it.
Pratt’s are one of these “types” – So, echoed in the lyrics of one of my favored songs, with The First Day in August, last years local Timber Framers Guild project – The Wason Pond Bridge marked the passing of its first year, (see July / August ’11 archives) and I recently found myself willingly engaged in a once common bridgewrighting chore – A first of several, wrench in hand scheduled visits, to tighten Truss Rods and assorted Bolts to compensate for expected and predictable shrinkage as the Timber in the Through Truss seasons.
Joints again fully seated, camber re-tuned. All went as hoped for and expected, with but one small exception. The washers on some of the smaller Bolts sunk into the now dry White Pine wood grain as an attempt was made to re-tighten. I chose to replace these. My first thought was to go to the cast Ogee’s found so commonly in this application. Their cost and limited availability saw me turn to the second most common washer type found on Wooden Bridge Bolts – Large Square Flats.
As such explorations often do, I went looking for contemporaneous Rules of Thumb in what was seen as a norm for such hardware. In AJ DuBois’ – The Strains in Framed Structures – We find these not only crunched number, but period proven suggestions in his list of specifications.
As always, I like to point out how White Pine was and is, the favored Species for the framing of Wooden Bridges – Also from the DuBois List – Section VIII
In Jacoby’s – Structural Details or Elements of Design in Timber Framing – A wealth of information is found on the seemingly mundane subject of washers, as they are related to Timber Work.
The restoration of Maryland’s Gilpins Falls, is the only time we’ve worked with “Special Countersunk Washers” and their funky headed friction dependent Bolts. This bolt & cast washer type was also that chosen and used by Nichols Powers in The Blenheim.
The Wason Pond Bridge is fitted with a number of “Malleable Iron Washers” on both the smaller section Truss Rods at Mid-Span, and at the Tie Beam Bolts.
And like many other wooden spans, it is now home to a large handful of Squares.