Happenstance had me helping out on the rehab of a 19th century cape this past Friday in Contoocook Village, across and up the hill from the Train Depot and almost within sight of my old haunts on the Railroad Bridge. When we wrapped the day, I headed for the far side of Hopkinton to visit the Rowell’s. Both to compare details with those seen recently on Long Truss sister bridges, the Bement and Blair, but also to again put eyes and hands on the Childs’ living legacy.
Some photos from the visit, and observations explored in the details found.
Here is seen the west Portal, the far end has suffered some recent but minor damage. The driver apparently ignoring the impossible to ignore clearance sign, but not the screaming protests of portal trim tearing into the aluminum and fiberglass in the trucks box. Damage thankfully, limited to the Portal, and not carrying on into the Through Truss proper.
(note how the shoulders are truncated down to a common width)
Though many bridges have both systems of timber layout.
Square Rule is typically employed only on secondary systems, Rafters and Knee Bracing. Major systems such as Lateral Bracing, like the trusses, are more commonly “Scribe Ruled,” pieces literally scribed to one another in the direct transfer of information. This to compensate for changes in angles and lengths in the framing introduced by intended camber.
Unnecessary for a Long, these as the “Description” tells us, were built “flat,” with light camber being driven into them during the wedging / pre-stressing process.
The solid encased Arch comes up and kisses the Top Chord at mid-span. (See the July ’11 Archive – Children of Childs, for thoughts on the use of an Encased Arch used similarly on a Long Truss by Nichols Powers on New Yorks’ Blenheim) An Arch being harder to plan and layout than first glance might suggest.
This kiss tells us Horace was well versed in their use and that Rowell’s was likely no one-off or odd experimentation.
A unique detail I’ve seen on no other bridge is this Purlin Post / Purlin Plate system, which support the Rafters at mid-length by providing a load path to the Ties. Built of seemingly undersized timber, this is another suggestion that the Childs Brothers shared understandings learned from their cousin. This framing being kept as slight as engineering calculations allowed, to help keep the bridges dead load as light as was possible.
In some ways, this completes the circle, and in a single generation and within an extended family. The Engineer learns from the bridgewright how to achieve lightness in design and framing, the Bridgewright learns from the Engineer how to max that lightness through calculations, to keep his framing timber as slender as was possible.
Though Long is widely credited with breaking ground in engineering, in using formulas and calculations in the design of trusses, and the sizing of their webs, it is Haupt and Whipple who are widely credited, through their work and writings with disseminating this information to the wider bridge building community. Long’s influence along these lines is perhaps under realized. It clearly began with the publication of “Description” and then through the work of his agents, and their interactions with bridgewrights over widely scattered parts of a growing nation.
Preparations for, and the coming Wason Pond Covered Bridge workshop in Chester NH, are bound to distract me for a bit. We will temporarily suspend the continuing series on NH bridgewrights and patent holders. The next few posts will highlight the Chester activities and all we hope to share and learn from each other there.