Page:America's Highways 1776–1976.djvu/425

 movable spans were used. These were most likely single or double leaf bascules (hinged sections) operated manually. Floating bridges were provided with sections that could be hinged or pulled out of place to permit the passage of waterway traffic.

Some short span stone arches were built, mainly in the colonies north of the Potomac River.

Reproduction of historic timber beam and pile bent bridge at the original site in Concord, Mass.

King-post or queen-post trusses (actually simple braced beams) were in early use for short spans. However, the very common pier washouts experienced by multispan beam bridges, plus the increasing knowledge of the society in general, led to the development of timber trussed arches that could span wide rivers. A New England millwright named Timothy Palmer built a series of patented trussed arches very like one of those illustrated in Palladio’s Treatise on Architecture (1570). The best known was the 244-foot span across the Piscataqua River near Portsmouth, New Hampshire, built in 1794. The roadways were supported on the lower chords of the structure, resulting in steep grades ascending to the center from each end of the spans. These bridges being true arches, provision was made for their horizontal thrust to be transferred to the substructure.

The Upper Ferry Bridge over the Schuylkill River, Philadelphia, Pa., a covered timber trussed arch created by Lewis Wernwag in 1805.

Besides Timothy Palmer, two other men, Louis Wernwag and Theodore Burr, stand out as the first professional bridge builders in the United States. These men shared several things in common—they were, of course, contemporaries, their structures were all highly indeterminate combinations of trusses and arches, and they most likely did not have the 419