This is the Ft. Steuben Bridge, built in 1929, in Steubenville, Ohio. Credit: State of Ohio |
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
I-35W bridge—an eight-lane, steel truss arch bridge—cracked and
plummeted in 2007, one of the first thoughts was: SABOTAGE?
While sabotage was NOT the cause of the I-35 bridge collapse, sabotage CAN
be used to weaken steel plates, girders, cables, or other key
structural elements of a major bridge. And while sabotage may not be
easy to carry out, DHS takes all such threats seriously.
More
than 600,000 bridges in the U.S are 20 feet long or longer, some over a
century old, many of them 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
seminal research is discovering how materials, connection details, and
designs in aging bridges react to IEDs, other explosives, kinetic
impact, intense fires and other accidents. In addition to vulnerability
analysis, DHS 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 Departments of
Transportation and bridge authorities. Primary bridge components (tower
sections, cables, suspenders, trusses) are recovered from bridges
undergoing demolition or major renovation, including several significant
bridges, such as the Crown Point Bridge (NY State – Lake Champlain),
and the Ft. Steuben Bridge in Ohio. These are then transported to
specific facilities for testing. One such series of tests was recently
performed at test ranges in Ft. Polk, LA to determine explosive effects.
“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 Ft. Steuben Bridge—a vintage 1929 suspension
bridge in Steubenville, Ohio—was to undergo demolition in 2012.”
With
the Ft. Steuben Bridge closed to traffic, there was a unique
opportunity to conduct onsite tests. “We developed a way to perform
controlled impact testing on the bridge using a specially designed “cold
gas thruster” device,” said Coleman. “Lateral impact loads were
directed at specific locations on the main cable and forces transmitted
to the rest of the structure were monitored in real time. 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 myriad dynamic
and static forces that interact within a bridge structure. Opportunities
to conduct field tests at the Ft. Steuben have made it possible to gain
better knowledge of in situ bridge behavior. In addition to the onsite
testing, cables and steel tower sections were recovered from the Ft.
Steuben 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 (AASHTO), whose membership includes State DOTs and other
bridge owners.
The
work taking place will be shared with AASHTO 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.
Source: US Department of Homeland Security – Science and Technology