Page:America's Highways 1776–1976.djvu/424



History teaches that all sciences and arts, including bridge design and construction, benefit from the innovations, successes and failures of the past. This presupposes a tradition of learning not always available to the American colonists. The first bridges in this country were often built by untrained persons inexperienced with the difficulties and problems of constructing a bridge of adequate strength and reasonable durability. Instead, they substituted determination, native ingenuity, and more determination.

Economics and material availability generally limited the colonial bridges to timber or stone construction with timber being predominant. This was largely due to the greater time and labor required to quarry and transport stone. From this lowly beginning, the complex design, construction and material sciences have developed.

The existing bridges in the American colonies in 1776 were few and minor in size. At that time, all major cities and most towns and villages were located on navigable waterways, since the waterways offered the most practical transportation between populated areas. Barges, boats, canoes and fords were mainly used where roads crossed the waterways. As the population centers developed, some timber bridges were built over adjacent narrow waterways by local authorities to facilitate access by travelers and commercial goods.

Generally, the bridges had log beam spans and were limited to the length of timber available from local trees. Abutments at the stream banks were timber mud sills, wooden cribs, or dry stone masonry where stone was available. Where more than one span was required, timber pile bents, wooden cribs or wooden mud sills were used for piers in the stream bed. The cribs were usually braced and filled with rock or compacted earth. The life of these bridges was usually short due to the rapid deterioration of the timber and the washing out of the foundation structures. Some floating bridges were constructed, usually of large logs fastened together.

It is interesting to note that the Concord Bridge, at which the homespun New England “Horatius” fired “the shot heard round the world,” was a timber beam and pile bent structure very similar to the Sublician Bridge defended by his Roman prototype nearly 2,000 years before.

Since the waterways were the main and pre-existing arteries of travel and commerce, any bridges built over navigable waterways required openings and clearances adequate for the passage of the waterway traffic, whether sailing ships, barges, canoes or log rafts. Where the horizontal and vertical clearances of fixed spans were not adequate for such passage, 418