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Enlarged image of bridge test.
Photo Caption: UCSD Structural engineers conduct final proof tests on the treadways of a prototype mobile assault bridge made from composite materials.

Final proof testing of a revolutionary new mobile assault bridge for the U.S. Army will take place at the Powell Structural Research Laboratories on the University of California, San Diego (UCSD) campus this Wednesday, March 1 from 1 p.m. to 3:30 p.m. The prototype bridge, designed by UCSD Structural Engineering Professor John Kosmatka, boasts the highest strength-to-weight ratio ever recorded for a bridge.

The project is part of a $22 million bridge infrastructure renewal program administered through UCSD and funded by the Defense Advanced Research Projects Agency (DARPA) and the Federal Highway Administration.

The goal of the mobile assault bridge project was to design a bridge that outperforms existing metallic bridges while demonstrating new ways to cost-effectively manufacture and use composite materials. During ground invasions, mobile assault bridges are used to allow ground troops to cross man-made defensive trenches and natural ravines. The Army deploys mobile bridges carried on the backs of modified M1-A1 tanks. The lives of troops depend on how quickly bridges are set up, and how fast the deploying tank can race ahead to lay down a bridge over the next obstacle.

Following the proof test, the prototype bridge will be moved to Ft. Belvoir in Virginia where the Army will conduct fatigue tests to determine how the bridge absorbs wear and tear over time. The Army plans to have the bridge undergo 5,000 actual bridge crossings by 70-ton tanks and 100-ton transport vehicles. Pleased with the results of the project, the Army plans to encourage its contractors to begin making mobile assault bridges with composites similar to the UCSD design. In addition, the government is considering a follow-on project to design composite bridges and launch trailers that could be carried by C-130 airplanes to locations under emergency situations.

“Our project and other ongoing studies, are setting the stage for a new breed of civil and military bridges, and an emerging industry for American businesses,” said Kosmatka, who holds the Callaway Golf Chair in Structural Mechanics at the UCSD Jacobs School of Engineering.


The UCSD bridge is made from a variety of high performance composite materials and has the highest strength-to-weight ratio ever recorded for a bridge. The 50-foot-long bridge weighs less than 12,000 pounds and can carry vehicles in excess of 115 tons. In comparison, existing 50-foot metallic bridges weigh approximately 12,500 pounds but can only carry vehicles that weigh no more than 70 tons.

The mobile assault bridge is lighter, can be deployed faster, and costs less than existing assault bridges made from standard metals. The bridge (made primarily of carbon/epoxy) is so slim and light that one tank can carry two bridges. Therefore, the bridge-launching tank could go deeper into battle without restocking, or if a bridge is blown, the launcher could continue onward. In comparison, today’s launching systems can only carry one metallic bridge.

The bridge includes two five-foot wide treadways connected by a series of two-foot wide separator bars. Each treadway is composed of four critical design regions; the superstructure, the deck, the upper wear surface and two end ramps.

Various materials are used throughout the bridge to gain the optimum performance and cost-effectiveness. For example, treadway sections that have to be thin, stiff and strong are made of carbon and epoxy, but entrance and exit ramps that must withstand impact damage from rocks are made of aluminum. The bridge upper surface is coated with a thin polyurethane layer to protect from weather and vehicle wear.

The bridge prototype was fabricated by Seeman Composites of Gulfport, Mississippi using the company’s unique resin transfer molding process known as SCRIMP. Pure carbon tows, composed of 12,000 hair-thin fibers, are used to fabricate the composite portion of the bridge. These tows are woven together to form a fabric.

To construct the bridge treadways, the carbon fabric is laid into a mold. The mold is then placed in a vacuum bag to prepare for the SCRIMP process. Vacuum draws the air out of the bag and pulls in liquid epoxy resin. The epoxy is allowed to saturate the fabric and is then cured at a high temperature to form an extremely strong, light-weight component.

Seeman focused on increasing the manufacturing processing temperature because higher processing temperatures allow the use of higher performance resins, which in turn improve the performance and durability of the composite materials.

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