HAER Animation Illustrates Arlington Memorial Bridge Bascule Span In Operation

Описание: A fact unknown to most people who cross the Arlington Memorial Bridge today is that the center span was once a draw bridge, specifically a two-leaf bascule span, that opened to allow rather large vessels to reach the port in Georgetown. After the gradual decline of this traffic, the draw span was closed in 1965.

Since the entire bridge was, and is, considered to be a monument as well as a functioning bridge, the draw span engineer, Joseph Strauss, was required to design a steel structure that resembled the stone arches. To accomplish this, Strauss employed a “Chicago-type” bascule structure and mechanism, but one that has all of its structure and mechanism concealed below the deck. The entire 3,800-ton weight of each leaf and counterweight was carried by its two trunnion posts. His design compliments the rest of the bridge so well that most people fail to notice it as anything different.

This video shows the structure and mechanism of one leaf with the road and fascia surfaces removed to reveal the complex truss Strauss designed. The main components are identified, followed by an animation of the leaf opening and closing, a process that took about 90 seconds each way. Two 75-horsepower electric motors furnished the opening and closing power. Acting through reduction gears, they turned pinions that rotated a rack on each main truss.

The counterweight, with an internal truss frame and several hundred small blocks to precisely balance the span’s weight, was cast with a specially formulated heavy concrete because of the limited space available inside the abutment (not shown). The counterweight hangs from one end of each main truss and rotates to maintain a vertical alignment as the span moves.

The matching second leaf that meets this one at the bridge’s center is a mirror image. Together they comprise one of the largest and heaviest examples of a Chicago-type bascule bridge ever built and the only one that forms part of a monument. Though no longer opened, all of this structure and mechanism remains intact, yet out of sight and rarely noticed.

The Arlington Memorial Bridge Recording Project was conducted under the general direction of Richard O'Connor (Chief, Heritage Documentation Programs and Acting Chief, HAER). J. Lawrence Lee (HAER Engineer-Historian) supervised the project. The recording team consisted of Brianna Kraft, Julia Rine, and Ashley Walker (HAER Architect Interns). Dana Lockett, Paul Davidson, Jason McNatt, and Jeremy Mauro (HAER Architects) performed LIDAR scanning, and Jet Lowe and Todd Croteau (HAER Photographers) produced the large-format photographs. This project was sponsored by the George Washington Memorial Parkway, National Capital Region, National Park Service; and funded by the Federal Highway Administration.

The 3D model of the bascule span was created by Ashley Walker and Jeremy Mauro. Animation by Jeremy Mauro using 3dsMax. A complete drawing set of this recording project, as well as the history and large format photographs, will be made available online at the HABS/HAER/HALS collection at the Library of Congress http://www.loc.gov/pictures/collectio...

This video contains no spoken word.