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Thetford and the Sayers Bridge
From Spanning Time Tour 12 - Woodstock (and north)
Thetford Town was chartered in 1761 and settled by people from both New Hampshire and Connecticut. In the 1860s, historian Isaac Hasford remarked that while the Connecticut River town was favored and above average in thrift and population, nothing had happened there to claim space in history. "We have, beside farming, a riotous mill stream, the Oompompanusuc, bisecting the town, giving life and power to three smart villages . . . . It has, on occasion, [washed] half their bridges and sometimes their mills down to the Connecticut and the town below."
Along the course of the "mill stream," were eight saw mills, four grist mills, a straw-board and paper mill, two flannel factories, a carriage shop and bedstead factory, and an edge tool and trip- hammer works. Two covered bridges survive and continue to span the Oompompanusuc. The Sayers Bridge is one of them. The Union Village Bridge is the other, but that is another story.
When Sayers Bridge is viewed from the top of the falls, the scenics are hard to equal. The truss used in the span is a unique adaptation of Herman Haupt's 1839 patent. The date-of-build is unknown
The bridge is thought by some to be a Haupt Truss span, mostly because some of the diagonals cross more than one truss panel. Covered bridge builder and restorer Jan Lewandoski holds that there is no evidence that the builders knew they were constructing a Haupt Truss. What ever truss it is, it is the only one of its kind in New England. While the names of the builders of the Sayers are lost, the Haupt Truss designer is remembered by Civil War buffs as the Colonel who built and ran the U.S. Military Railroad in the South for Union forces.
The "Haupt" design as implemented here, resembles a multiple kingpost truss. It differs in that it is assembled from planks instead of square timbers and is joined with treenails rather than with mortise and tenon. The builder integrated the whole with a segmented plank arch.
The chords are constructed with two four-by-eight-inch planks sandwiching the vertical members and the braces. The vertical members and braces are four-by-eight-inch timbers pegged together between the chord planks with their long dimensions parallel to the bridge span. The arch segments are four-by-twelve-inch timbers treenailed to both sides of the vertical members. The open spaces between the chords and between the arch segments are filled with lengths of wood, the whole structure is pegged together with treenails.
The 129-foot Sayers bridge was strengthened in 1963. The existing roadway was replaced with a nail-laminated timber deck on four steel beams supported in mid-span by a concrete pier. It is easily reached from I-91 Exit 14. Drive west two miles on Route 113, then take Tucker Hill Road south through Thetford Center. Sayers Bridge crosses the Ompompanoosuc River above a millpond. The river flows over a ruined dam and cascades down terraces of bedrock.
The Haupt Truss
The Sayers Bridge in Thetford is said to be the only example of the Haupt truss in New England. Curiously, the Sayers' truss lacks the defining lattice structure of the braces shown in Haupt's patent drawings. Instead of the lattice-like assembly of braces, it has a long and high timber arch similar in construction to Burr's arch. See below.
Note that the brace configuration in Haupt's truss implementation distributes, at least for the first several truss panels, compression forces more directly to the abutments than does the Sayers Bridge truss. This is a key feature of the Haupt patent. The Sayers' truss, rather, performs like a "multiple kingpost truss with auxiliary arch." The differences between the two truss implementations, then, are more than trivial.
Herman Haupt was born in Philadelphia, Pennsylvania in 1817. He died in 1905 in Jersey City, New Jersey. He graduated from West Point in 1835. He resigned his commission to become district superintendent and chief engineer for the Pennsylvania Railroad. In 1839, he designed and patented the Haupt bridge truss. When the Civil War began, he was drafted to serve as superintendent of military railroads. He rose to the rank of Major General. Civil engineers may remember him for his text books on bridge construction and his work on the five-mile railroad tunnel through the Hoosic mountain range in northwest Massachusetts.
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