[Covered Bridge] Truss Structures and Truss Variations

[Covered Bridge] Truss Structures and Truss Variations. . , Truss Structures, Kingpost , The kingpost was the first and simplest truss structure used in America. Its basic idea consists of a center post, the kingpost, with compression beams slanting downward and outward toward each shore. The original kingpost design was somewhat limited in size due to the length of the compression beams., Queenpost , The queenpost was adapted for building longer bridges. This truss design consists of an inverted "V" between two center posts and then compressed with beams slanting downward and outward toward each shore., Warren Truss , The Warren truss was the first major variation in bridge design. This truss structure consisted of an "X" instead of a "V" in the middle of the queenpost truss resulting in a much stronger design., All three, kingpost, queenpost, and Warren truss, were commonly used in building barns so they made an easy transition to covered bridges., Truss Variations, Town Lattice Truss , In 1820, Ithiel Town obtained a patent for the lattice truss system. Besides adding strength, this system had the advantage of requiring shorter timbers. The design employed a large number of parallel diagonal timbers running in opposite directions. The resulting network supported covered bridges up to 220 feet in length., Burr-Arch Truss , In 1817, Theodore Burr, later known as the "Father of the American Covered Bridge," patented the Burr-arch truss. This design increased the length and strength of the covered bridge. Burr simply added an arch to the vertical supports - a system once compared to "...a man who wears a belt but adds suspenders for additional support.", Howe Truss , In 1840, William Howe (uncle of Elias Howe who invented the sewing machine) patented the design that bears his name. The Howe truss was the first system to routinely employ both wood and wrought iron. The wrought iron rods were used as verticals while timbers continued to serve as diagonal braces. This design was very popular after 1850 and was strong enough to be adapted for railroad use., The Building of a Bridge , There were two popular ways of constructing a covered bridge in the 1800s. With one method, the complete skeleton would be laid out upon the ground and fastened together loosely with wooden nails before raising the sections up and fastening them together. The other method, known as hoisting, involved placing each beam separately across the river starting with the bottom cord or cross beams and then working beam by beam to complete the truss structure.