Page:America's Highways 1776–1976.djvu/432

 The rising tempo of bridge building may be noted from the fact that the Williamsburg Bridge, although 5 feet longer than the Brooklyn Bridge, was just a long bridge. The long-span title had gone across the seas to the Firth of Forth’s 1,700-foot cantilever span in 1890.

Bridges with movable spans to accommodate water traffic also date back to colonial times. The early timber bridges were opened and closed by the only available power—manpower. As a rule, they were either bascule (draw) spans or swing spans which rotated to open the channel to marine traffic. This, of course, meant that only short span openings could be used. Fortunately, the pace of life, as well as of river traffic, was sufficiently slow so that lengthy bridge opening times could be tolerated.

Some early spans were a wooden drawbridge between Boston and Charlestown over the Charles River, built in 1785–86, and the Haverhill Bridge over the Merrimack River at Haverhill, Massachusetts, built in 1794 with a wooden bascule drawspan. The 30-foot drawspan was raised by means of levers elevated on a post on each side of the draw. The Tiverton Bridge over a tidal inlet near Howland Ferry, between Portsmouth and Tiverton, Rhode Island, built in 1795, contained a sliding drawspan.

The advent of the railroads and steam-powered boats made the span weights heavier and the opening time of the movable spans more critical. Fortunately, as wrought iron, and then steel, became the material for the bridge members, steam became the motive power for opening railroad and main highway movable bridges over busy waterways. For minor waterway crossings and during power failures, movable spans were still opened manually.

As the larger rivers and tidal estuaries became more frequently bridged and river craft became wider and longer, wider channel openings were required, necessitating longer spans for movable bridges. Of the movable bridges constructed during the last half of the 19th century, two were notable. The first was a railroad bridge located between Rock Island, Illinois, and Davenport, Iowa, providing a swing span with two 120-foot channel openings. Built in 1854–56, it was the second bridge to cross the Mississippi River. The other was the Utica Lift Drawbridge, a 60-foot x 18-foot movable deck, suspended by rods from the lower chord panel points of an overhead fixed truss. The vertical lift was 11.5 feet. Opening time, using preset weights, took 10 seconds. This was one of several unusual vertical lift bridges which Squire Whipple designed and built over the Erie Canal in New York in the 1870’s.

This sketch of a manpower-operated bascule on the Pongo River Bridge in Virginia probably exaggerates the difficulties of such operations. From the Harpers New Monthly May 1858.

The development of modern movable bridge spans started in Chicago when the channels of the Chicago River and related waterways were improved at the turn of the century. Many of the existing movable bridges were manually powered swing spans, with center piers obstructing channels. These were replaced with bascule and vertical lift spans. This era was begun by two famous bridges. The Halsted Street Lift Bridge over the Chicago River in Chicago, Illinois, 1894, lifted a 130-foot truss span with a 34-foot roadway and two 7-foot sidewalks using steampower. Maximum vertical clearance was 155 feet above low water. Two light longitudinal, laterally braced, trusses connected the tops of the towers. The design of this bridge introduced another famous name in bridge engineering, J. A. L. Waddell. The other bridge, the Van Buren Street Bridge also over the Chicago River, was opened in 1895. This 115-foot span, double leaf of the Scherzer type, was the first rolling lift bridge.

From then on there was a series of new movable bridge types: the simple trunnion, or Chicago, and Strauss bascules, the Rail rolling lift, and various vertical lifts. The swing span, despite its economy and minimal power requirements, fell into relative disuse since it required a pier in the middle of the stream and blocked the channel more during opening and closing operations.

Vertical lift bridge over the Cape Fear River at Wilmington, N.C. The 400-foot lift span has a vertical clearance above mean high water of 65 feet in the closed position and 125 feet in the open position. 426