Page:America's Highways 1776–1976.djvu/433

 Lake Washington Pontoon Bridge, Seattle, Wash., is a concrete pontoon bridge completed in 1940. The floating portion consists of 25 units 350 feet long, together with retracting sections to provide a 200-foot opening for large vessels. Passage for small boats is provided near each shore under truss spans. The pontoons are fixed laterally by long steel cables fastened to submerged concrete anchors.

The George P. Coleman Memorial Bridge over the York River near Yorktown, Va. The double 500-foot swing spans are the largest tandem swing spans in the world.

Cabin John Bridge over Cabin John Creek in Montgomery County, Md. The aqueduct for the Washington, D.C., water supply is in the spandrel fill under the roadway.

The great size of today’s structures, which evolved from the short span crossings of the Charles River and Merrimack River, is due to the growth in technical knowledge, experience, better steels, steam and electric power, and a near perfect use of the old-fashioned counterweight.

Stone arches have not been a major element in highway bridge construction in this country. However, numerous short-span stone arches have been built in areas where stone and skilled masons were readily available. These bridges are frequently seen alongside present highways where they have been left for their historic and scenic value when highway alinement, width, or grade was improved. Long- and short-span stone bridges have been built in parks and large cities to be compatible with their surroundings.

One of the most notable stone arches was built during the Civil War as a combination aqueduct and road. The 220-foot filled spandrel Meigs Arch, which carries Washington, D.C.’s water supply and MacArthur Boulevard over Cabin r Creek in Cabin John, Maryland, was the longest span of its type when built. It is still in service despite its narrow roadway.

Where stone was readily available, stone arches of nominal spans and stone arch culverts were frequently used in railroad construction before metal superstructures were developed and accepted. Some of the noteworthy stone railroad bridges still in existence are: 427
 * Carrollton Viaduct over Gynn’s Falls on the Baltimore and Ohio Railroad, near Baltimore, Maryland, was built in 1829 of granite ashlar masonry. The center span is 80 feet and the overall length is 297 feet.
 * Thomas Viaduct over Patapsco Creek on the Baltimore and Ohio Railroad, near Relay Station, Maryland, was built in 1835 of granite ashlar masonry and is 612 feet long and has eight full-centered arch spans.
 * Starrucca Viaduct over Starrucca Creek (a tributary of the Susquehanna River) on the New York and Erie Railroad, was built in 1847–48 with stone masonry made of stone quarried locally. The viaduct is 1,040 feet long with a maximum height of 110 feet above the creek.
 * The Morgan Bulkeley Bridge was constructed across the Connecticut River between Hartford and East Hartford with funds raised by subscription from all the towns around the area in 1905. This beautiful structure of pink granite has 11 elliptic arches, the largest being 119 feet long. In 1962, it was widened to carry Interstate 84.