As far as durability was concerned, early attempts at increasing the life of timber in contact with earth and water achieved little success. Methods used included dipping, soaking or brushing the timber with creosote or salts, such as zinc chloride or mercuric chloride. These methods were ineffective due to the lack of penetration and leaching out of water soluble preservatives.
While the railroads were especially desirous of finding a satisfactory method of prolonging the life of the great number of ties and timber structures on their systems, it was not until 1865 that the first pressure creosote treatment plant was constructed at Somerset, Massachusetts, primarily to treat timber track ties.[1] Other plants were constructed soon thereafter for treatment of ties, piles and timber.
With the advent of timber treatment plants to supply the railroads, treated timber piles and lumber became available for use on highway structures, but by then the use of metal in trusses was becoming more common.
The general use of timber trusses for highway bridges continued to the 1880’s, but gradually decreased until by 1916, there were practically none. However, such construction has continued in isolated areas where timber was readily available and in scenic areas as tourist attractions.
Cast-Iron, Wrought-Iron and Composite Wood Bridges and Metal Bridges
The use of iron for incidental connecting parts was introduced early in the construction of timber bridges since it facilitated construction, improved the rigidity of the structure and reduced maintenance problems. Wernwag used iron rods for the light web diagonal of his Upper Ferry Bridge, popularly named the “Colossus,” in 1806.[2]
Soon after the railroad era started in 1827, bridge engineers realized that the timber truss-arch bridges were not serving satisfactorily under the speed and weight of railroad traffic and sought various methods to introduce iron members into the truss arch. Such structures were called combination bridges.
One of the early combination bridges was a through timber truss-iron arch bridge, a 133-foot span, carrying the Pennsylvania Central Railroad over a canal. The bridge was so constructed that the timber truss would receive the load from the ties, transfer it to the arch, and provide lateral support for the arch ribs which were the main load carrying members. However, should the arch fail, the truss was adequate for the full load. The engineer, possibly at the owner’s request, tested the completed structure by passing a 23-ton locomotive over it several times.[3]
In 1840, William Howe patented the “Howe” truss. This was a true truss with timber chords and compression diagonals but with iron bars for the vertical hangers. Iron members had been used before, but they had always played a minor role.
His patent also permitted prefabrication of parts, permitting manufacture away from the site. The ability to tighten up the hanger, via an adjustable nut should it become loose, was an added benefit. The parallel chord and X diagonal pattern soon became a familiar part of the countryside since the Howe truss was the most common of all timber trusses. However, this truss represented high tide for the timber truss and for the carpenter bridge builders, of whom Howe was the last well-known one. What had already happened in Europe was happening in America. Stress and strength were about to join stability as the criteria in bridge building.
The stone arch had been the main bridge type for nearly 2,000 years. A masonry arch, if stable enough to stand, was adequate for any conceivable load. For this reason, stress analysis had been unnecessary. With the advent of timber (and metal) beams and trusses, it could no longer be assumed that the bridge’s existence was proof of its strength. The possibility of overload and failure due to overload now existed, especially with the heavier loads of the locomotive.
The timber trusses and combination timber-metal trusses still suffered from the difficulty in joining timber—the tensile strength of the joint was always less than the strength of the timber member. This made these trusses especially susceptible to falling apart at the joints. On March 4, 1840, a Town lattice truss over Catskill Creek, New York, came apart dropping a train of boxcars into the water, resulting in the Nation’s first railroad bridge fatality.[4] The need for a metal bridge had arrived.
Other methods were used in replacing wood with iron in the arches of truss-arch railway bridges. While limited construction of short-span combination trusses with timber compression and wrought iron or steel tension members continued until the end of the 19th century, it steadily decreased.
The engineer’s first choice for a bridge metal was cast iron, and 1836 saw the first cast-iron bridge in this country, an 80-foot arch span, built over Dunlap Creek in Brownsville, Pennsylvania, by the U.S. Corps of Engineers.[5] Shortly afterward, in 1840, the first iron trusses came into existence when two highway bridges were built over the Erie Canal by Earl Trumbull and Squire Whipple. Trumbull’s truss spanned 80 feet and had a wooden floor system. It featured a parabolic bottom chord of wrought iron bars. Squire Whipple’s 72-foot bridge was the first example of the famous Whipple bowstring truss, so called because of its curved upper and horizontal bottom chord. The tension members were wrought iron and the compression members were cast iron.[6] While most of Whipple’s bowstring trusses have vanished, there are still some examples of his other design, the trapezoidal truss, in service.
In 1844, 4 years after the first two iron bridges, Thomas and Caleb Pratt patented the Pratt truss. This parallel chord truss with tension diagonals and compression verticals in the web system was well suited for metal trusses and, together with its many modifications, became the most popular type of truss for short and intermediate span trusses to the present day. Among its better known variations are the Baltimore, Parker, Pegram, Pennsylvania and Petit trusses.[7]
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- ↑ G. Hunt, A Century of Engineering in Wood Preservation, Proceedings of Wood Symposium, September 3–13, 1952 (Timber Engineering Co., Washington, D.C., 1952) p. 66.
- ↑ J. Gies, Bridges and Men (Doubleday, Garden City, N.Y., 1963) p. 107.
- ↑ H. Haupt, General Theory of Bridge Construction (D. Appleton and Co., New York, 1853) pp. 243–247.
- ↑ J. Gies, supra, note 11, p. 112.
- ↑ L. Edwards, supra, note 1, p. 75.
- ↑ Id., p. 71.
- ↑ Id., p. 74.
