One of the approaches to categorizing covered bridges is by the type of truss supporting them. Some designs saw extensive use over a large geographical area while others are limited to a specific region. To keep it simple, this essay will contain a brief description of the design.
In the line sketches, timbers are shown by full lines, and iron or steel rods by dotted lines. There are many examples of variations, modifications, and refinements to these designs.
Kingpost – The most basic truss design, is the Kingpost which has been used since the Middle Ages. It is based on an equilateral triangle with a central post, known as the kingpost. The two diagonal timbers are braced on the ends of the lower chord and transmit loads from the center of the bridge towards the abutments. It is used for short bridges up to about 40 feet in length.
Queenpost – The second basic design is called the Queenpost. The design has been in use at least as far back as the Italian Renaissance. The Queenpost truss is an expansion of the Kingpost design adding a rectangular panel in between the two triangles. It is often reinforced by placing vertical and diagonal timbers or rods in the open center panel (shown by the dashed lines). It was typically used to span distances up to 75 feet.
Multiple Kingpost – The Multiple Kingpost truss is an expansion of the Kingpost design. The diagonal timbers carry the load from the center of the bridge outward to each successive vertical Kingpost which transfers it to the next diagonal timber. Used for spans up to about 100 feet long. Most have an even number of panels. With an odd number of panels, the center one may be open or have crossed braces as shown by the dashed lines in the lower image.
Burr Truss – Theodore Burr (1771–1822) was an inventor from Torrington, Connecticut who patented his first bridge design in 1806. On April 4, 1817 he received another patent for his arch and truss bridge design. The patent drawing shows a Multiple Kingpost truss resting on stone abutments, superimposed with an arch whose ends are seated against the abutments below the lower chords. Today, about one-quarter of the remaining historic covered bridges use this design.
Town Lattice – Ithiel Town (1784–1844) was an architect who made a significant contribution to the field of engineering when, in 1820, he patented a truss bridge consisting of two layers of overlapping planks forming a lattice fastened together with wooden pins or treenails at each intersection. Town trusses are erected with sawn planks instead of heavy hewn timbers making the timbers somewhat easier to work with. As Town explained, it was designed to be “the most simple, permanent, and economical, both in erecting and repairing.” Quebec employs a variation of the design. Some railroad bridges had double lattices to carry the heavier loads.
Long Truss – Col. Stephen H. Long (1784–1864) became an army engineer in 1814. He undertook surveys for the U.S. Army Topographical Engineers of canals and led expeditions in the West. As a consulting engineer for the Baltimore & Ohio Railroad, Long became interested in the design and construction of bridges patenting his Long truss design in 1830. A key to the design was driving wedges between the counter-braces and chords, which prestressed the structure, and added substantially to its capacity. Very few Long truss bridges remain, primarily in Maine and New Hampshire.
Howe Truss – William Howe (1803–1852) designed the Howe truss and patented it in 1840. By substituting adjustable iron rods for the wooden posts of the Long truss, Howe’s design was much stronger. He simplified the process of erecting and repairing them making the design well suited for railroads. In 1878, the American Society of Civil Engineers called it “the most perfect wooden bridge ever built.” Those in the eastern states typically have wooden diagonal braces in both directions forming an “X”. Those in western states typically only have braces angled upward towards the center of the bridge. With an odd number of panels, the center one may have braces in both directions or none at all. Adaptations of the design are found in New Brunswick. Howe trusses became popular in parts of Europe where some still stand in Austria, Germany and Switzerland.
Smith Truss – Robert W. Smith (1834–1898) was the son of an Ohio cabinet maker. On July 16, 1867, he received a patent for a design which reduced the amount of timber in the structure. Smith received a second bridge patent in 1869. He continued to modify and refine his designs without applying for additional patents. As a result, many Smith truss bridges do not exactly follow the patented designs. The variety makes categorizing them somewhat challenging. These descriptions are intended to follow the designs and drawings used by Smith when promoting his bridges.
Single web – resembles a series of inverted Vs. Three examples remain in Ohio.
Double web – the second web was an offset copy of the first giving the appearance of Xs.
Triple web – a third web duplicating the first added for additional strength in longer spans.
Paddleford Truss – A local New England builder, Peter Paddleford (1785–1859), developed the Paddleford truss. Although Paddleford never patented the design, it dominated construction throughout northern New England for over half a century. Examples of this design are found in northeastern Vermont, northern New Hampshire and northwestern Maine. Some Paddleford bridges also include arches which may have been added later for additional support.
Other Trusses – Many more designs exist beyond those described in this limited space. Hopefully, this essay has kindled your interest and prompts you to learn more about the designs and builders.
This essay is provided courtesy of the The National Society for the Preservation of Covered Bridges, whose mission is to preserve covered bridges, gather and record the history of covered bridges, collect and preserve covered bridge pictures ephemera of historical interest concerning covered bridges. This essay was drawn with their permission from “An Introductory Guide To Covered Bridges.”