Basic Truss Designs for Covered Bridges

Basic Truss Designs for Covered Bridges. . , King Post , The king post was the first and simplest truss structure used in America. Its basic design consisted of a center post, the king post, with compression beams slanting downward and outward toward each shore. The original king post design was somewhat limited in size due to the length of the compression beams. , Queen Post , The queen post was an adaptation for building longer bridges than the king post design permitted. 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” brace rather than a “V” in the middle of the queen post truss resulting in a much stronger design. , All three, king post, queen post, and Warren truss, were commonly used in building barns; thus they made an easy transition to covered bridges. , Truss Design Variations , Burr Arch , In 1817, Theodore Burr patented the Burr arch truss. In later years, Burr became known as the “Father of American bridge building” due to the large number of bridges that can be traced to Burr and his designs. Burr's design used a series of king posts to make the bridge longer and added an arch to support the truss. , Joseph Elliot, in building the Union Covered Bridge, put in two arches instead of one. This design is commonly referred to as a double Burr arch. , Town Lattice , In 1820, Ithiel Town obtained a patent for his lattice truss. 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. , 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 vertical braces, while the timber continued to serve as diagonal braces. This design was very popular after 1850 and was strong enough to be adapted for railroad use. Three of the four covered bridges in Missouri employ the Howe truss design., 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 on the ground and fastened loosely together with trunnels 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 chord or cross beams and then working beam by beam to complete the truss structure.