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The Town Lattice Covered Bridge

Town Lattice Bridge Truss Diagram

Ithiel Town, an architect by education, obtained his first patent for a unique type of timber truss in 1820. All the other trusses principally rely on large and heavy timbers that require skilled artisans to properly craft the rather elaborate joinery between the various components. Town sought a means of constructing bridges that would rely on an easily adapted design and would require less skilled labor. His patented truss developed a configuration that could be extended to a wide range of span lengths with relatively little modification of the configuration. In the opinion of many informed bridge aficionados, his patented truss represents arguably the most important development in the history of covered bridges, and one that remains a popular and enduring style.

Town's lattice configuration relies on assembling relatively short and light planks that were available and easy to handle. He connected the overlapping intersection of members with round timber dowels or pegs, termed treenails, pronounced trunnels. The plank intersections in the web may have from one to three trunnels. Where chord members intersect with web or lattice members, the overlapping zone may contain as many as four trunnels. The dowels are often 38 to 51 millimeters (mm) (1.5 to 2 inches) in diameter. The parallel and closely spaced web members are joined to chords along both the top and bottom of the trusses. Two levels of chords commonly are used as the bottom chords. The top chords may have one or two levels of members. The lowest bottom chord provides the seat for the transverse floor beams.

Town, or lattice, trusses are most commonly comprised of thin members with pairs of chords on each side of the lattice webs. In this case, the truss is sometimes termed a plank lattice. The chord members generally are not spliced to abutting pieces at their ends, but the terminations are staggered so that any panel of chord has at least one unspliced member. A few Town lattice trusses were fabricated of heavier components using single chord members on each side of the lattice. In this case, the truss is termed a timber lattice. The chord members require splices at their ends.

The Town Lattice truss has been described as an uninterrupted series of crisscrossed diagonals forming overlapped triangles. These web members were fastened at their points of intersection so that independent action of any one triangle was impossible. This was the secret of the strength of the Town truss. Pine or spruce plank was the usual material used in the Town truss and holes were bored into the planks at every lattice intersection and at the places where the lattices were secured to the upper and lower chords. Wooden connecting pins called treenails or "trunnels" made of oak were soaked in oil and driven into these holes with a long-handled mallet called a Beetle (pronounced "biddle"). There were approximately 2,592 holes in a 100' lattice highway bridge requiring 912 trunnels to pin it together. These lattice trusses were laid out in a field adjacent to the bridge site and assembled and then the truss was jockeyed out over the river on falsework. The Town Lattice truss was very popular for both highway and railroad use. For railroad use, the lattice trusses were doubled for extra strength. This truss type was built in all the New England States, most of the Southern States and the Mid-West.

There remain about 135 bridges supported by Town lattice trusses. Town lattice trusses support varying span lengths, from relatively short (only 7.6 m (25 ft)), up to some of the longest covered bridge spans in the world. Individual Town lattice trusses span up to 49.4 m (162 ft). The oldest surviving Town lattice bridge (the Halpin Bridge in Middlebury, VT) was purportedly built about 1824. New examples of Town lattice covered bridges are still being built.