Page:America's Highways 1776–1976.djvu/426

 knowledge to analyze such structures. They were virtually carpenters building bridges in accordance with their own experience or that related by others, and they were among the group that produced the distinctly American bridge characteristic, the covered bridge. Their picturesque practice was, of course, a sound and practical measure that extended the life of the wooden structure immeasurably.

Camp Nelson covered arch-truss over the Kentucky River during razing in 1933. The three trussed 240-foot span bridge was built by Wernwag in 1838 and closed to traffic in 1926.

The first of many covered bridges in America was built in 1800 by Palmer at Middle Ferry, Philadelphia, Pennsylvania. The covering protected the bridge members from decay to the extent that a properly maintained bridge would give many years of service. The Waterford Bridge over the Hudson River, a covered wood truss bridge built by Theodore Burr in 1803–4, had a service life of 105 years (it was destroyed by fire in 1909).

The steep roadway grades of the trussed arches were objectionable, and by the beginning of the 19th century, a combination truss and arch was developed with the roadway supported by the lower chords and having only a slight longitudinal camber. The height or rise of the timber arch was about equal to the depth of the truss, with the arch fastened to the truss web members at all intersections. The Susquehanna River bridge at McCall’s Ferry, Pennsylvania, built in 1814—15 by Theodore Burr, contained a truss-arch with a 367-foot span said to be the longest then built in America. Burr’s bridge represented a small, but significant, change—the arch was added to the truss.

Inside view of typical Indiana covered bridge, built in 1900, showing arch-truss details.

Covered timber truss bridge over the Connecticut River between Cornish, N.H., and Windsor, Vt. Built in 1866, this 460-foot long, two-span Town truss structure is said to be the longest timber bridge remaining in this country.

In 1820, Ithiel Town patented a timber lattice truss, although lattice trusses had been built in Vermont as early as 1813, and in 1829 Colonel Stephen H. Long developed a panel truss with double diagonals, similar to a Howe truss. These trusses could stand by themselves, and arches were not necessary. Thus, the truss bridge appeared in a recognizable modern form.

The web members, diagonals and verticals of the earlier trussed arches and truss-arches were made of timber. Some of the later web systems used iron rods for tension verticals, diagonals and counters in the web system. There were many other variations of the truss besides those mentioned. Due to intensive promotion, the various types were identified by the name of the developer. When iron and steel trusses were developed, the timber truss designations were given to the corresponding geometric metal trusses, i.e., Howe, Pratt, Fink, etc. Connections of members were made by bearings of timber, forgings, iron bolts and spikes, mortises and tenons, and hardwood dowels.

The layout and relative position of the trusses and arches, the proportioning of the members, the methods of support of the floor system, and type and magnitude of the connections were determined by the experience and preference of the builder. There was no published method of stress analysis available to the builders for proportioning members and connections until Squire Whipple’s publication in 1847, An Essay on Bridge Building, and Herman Haupt’s General Theory of Bridge Construction in 1851, although it appears that Colonel Long may have used mathematical theory in the design of his bridges.

While the timber bridge had evolved into an almost determinate, fully utilitarian structure, it was not free of problems. Covering the truss with a roof and siding had retarded deterioration, but there were still many failures of covered timber trusses due to lack of maintenance, fires, floods, overloads, and inadequate design. Nonetheless, the covered bridge era might have lasted until the coming of the automobile were it not for a combination of events that led bridge designing into the modern era. The newly developed truss and the fledgling analytic methods found themselves a new client, the railroad. Trains being unable to ford even small streams or tolerate the sinuous alinement and steep grades used by pedestrians and wagon traffic, a great many bridges were needed, and not only more bridges, but stronger and more durable ones. This accelerated the growth of design technology. 420