It took only 13 seconds for the bridge to collapse into the Mississippi River in a thunderous rain of concrete and steel. When the Minneapolis Interstate 35 bridge—an eight-lane, steel-truss arch bridge—cracked and plummeted in 2007, one of the first thoughts was sabotage.

While sabotage wasn't the cause of the bridge 's collapse, sabotage can be used to weaken steel plates, girders, cables, or other key structural elements of a major bridge. And while sabotage might not be easy to carry out, the U.S. Department of Homeland Security takes all such threats seriously.

More than 600,000 bridges in the U.S are 20 feet long or longer. Some are more than a century old, and many of them are national iconic monuments. The Department of Homeland Security's Science and Technology Directorate (S&T) has joined forces with the Federal Highway Administration and the U.S. Army Corps of Engineers Engineer Research and Development Center to conduct series of experiments that assess potential vulnerability of critical structural components of aging steel bridges.

This research is discovering how materials, connection details, and designs in aging bridges react to explosives, kinetic impact, intense fires, and other accidents. In addition to vulnerability analysis, the S&T is funding several complementary efforts that investigate advances in effective and affordable ways to strengthen bridges. Data obtained through such research will help update computational models and may be integrated into engineering software for construction of more durable bridges.

Bridge specimens for research have been obtained from state transportation departments and bridge authorities. Primary bridge components (tower sections, cables, suspenders, trusses) are recovered from bridges undergoing demolition or major renovation. These components are then transported to specific facilities for testing. One such series of tests recently was performed at test ranges in Fort Polk, La., to determine explosive effects.

These structures include several significant bridges, such as the Crown Point Bridge, at Lake Champlain, in New York state, and the Fort Steuben Bridge, in Ohio.

"Bridges slated for demolition are hard to come by," says S&T Transportation Security Laboratory's Tom Coleman. "However, last year, the research team found out that the Fort Steuben Bridge—a vintage 1929 suspension bridge in Steubenville, Ohio—was to undergo demolition in 2012."

With that bridge closed to traffic, there was an opportunity to conduct onsite tests.

"This testing, along with similar experiments performed a few years ago on the Waldo Hancock Bridge in Maine, will help us learn bridge behavior and develop mitigation measures to better prevent damage."

In a laboratory, it's nearly impossible to replicate the dynamics of a bridge structure, Coleman says. Opportunities to conduct field tests at the Ohio bridge have made it possible to gain better knowledge of bridge behavior, he says. In addition to the onsite testing, cables and steel tower sections were recovered from the bridge following its recent demolition. These specimens will be assessed to determine material characteristics and vulnerabilities during blasts—and to further develop mitigation measures.

"Our current work is quite unusual because we are testing actual vintage bridge components from long-span bridges, as opposed to newly manufactured samples," says John Fortune, S&T's bridge vulnerability project manager. "The results will help us predict susceptibility to different threats and develop effective, feasible technologies to protect the nation's bridges. We are developing innovative approaches that will protect iconic bridges from hazards, and also aid in building smarter, more secure bridges for the future."

Most U.S. bridges are owned by regional authorities, state departments of transportation, private authorities, county authorities, or local municipalities. These bridges are designed using codes and standards approved by the American Association of State Highway Transportation Officials, whose membership includes State DOTs and other bridge owners.

The work taking place will be shared with that association and specific bridge owners and operators to ensure that project results will be available to engineers responsible for building new bridges and renovating existing ones.