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Source:

University at Buffalo

Date:

May 10,
2007

More on:

Construction, Civil Engineering, Earthquakes, Natural Disasters, Aviation, Tsunamis

Bridges Will Rock -- Safely -- With New Earthquake
Resistant Design

Science Daily — Bridges that
"dance" during earthquakes could be the safest and least expensive to
build, retrofit and repair, according to earthquake engineers at the
University at Buffalo and MCEER.

A "rocking bridge" design methodology was successfully
tested on this model truss tower on a six-degrees-of-freedom shake table
in the university's Structural Engineering and Earthquake Simulation
Laboratory. (Credit: Image courtesy of University at Buffalo)

The researchers recently developed and successfully tested the first
seismic design methodology for bridge towers that respond to ground
motions by literally jumping a few inches off the ground.

The new methodology allows steel truss towers that support bridge decks
to be built or retrofitted at far less expense than conventional
approaches, where each leg of a bridge tower is strongly anchored to its
footing.

The design recently underwent successful testing on a model truss tower
that is 20 feet high and weighs nine tons.

Testing was conducted on a six-degrees-of-freedom shake table in UB's
Structural Engineering and Earthquake Simulation Laboratory (SEESL). One
of the world's most versatile earthquake engineering laboratories, it is a
facility within the UB School of Engineering and Applied Sciences.

"Our approach is unconventional, counterintuitive," admits Michel
Bruneau, Ph.D., director of MCEER and UB professor of civil, structural
and environmental engineering, who developed the new approach with Michael
Pollino, a doctoral candidate in the UB Department of Civil, Structural
and Environmental Engineering.

"With an earthquake, conventional wisdom dictates that the most
important thing is to anchor the bridge tower," explained Bruneau. "The
mass wants to overturn, so you have to tie it down."

To do that, he explained, the tower must be anchored with a very
expensive foundation system, which in turn, subjects it to the full force
of the earthquake. "In this scenario, something usually has to yield," he
says. "Here, we're standing that concept on its head. By letting the tower
rock, we're significantly reducing the overturning force."

The UB engineers developed a design procedure in which the legs of the
truss tower are disconnected from their base and briefly uplifted by a
small amount if significant ground motions occur.

One of the options they evaluated includes using specialized devices to
control the structure's uplift. The devices, called hysteretic or viscous
dampers, some of which were provided by Taylor Devices, Inc., were
inserted at the base of the towers to allow the tower to rock while
absorbing part of the earthquake's energy and helping to control the
amount of uplift to the structure.

During the series of tests at UB on SEESL's state-of-the-art shake
table, the experimental truss tower fitted with these devices was
subjected to ground motions simulating the 1994 Northridge, California
earthquake; testing also was conducted without any devices attached, as
the design procedure was developed to generally address performance both
with and without dampers.

Typically, during testing, the tower's legs uplifted nearly two inches
in the air for less than a second. For some of the free-rocking cases, the
tower legs lifted nearly four inches.  See the video of the shaking
bridge at http://www.buffalo.edu/news/videos/bridge_shake_close.mpg

"All of the tests were successful," said Bruneau. "The damper systems
typically reduced the magnitude of uplift and the velocity upon impact,
which may be important, in some conditions."

The methodology will not allow uplifts to exceed limits considered safe
by the design procedure and dictated by the tower design, local conditions
and the need for the tower to return safely to its original position,
according to Bruneau. The UB methodology is the first to be established
for this application, but Bruneau notes that engineers previously have
employed the concept, such as in the approach spans of the Lions Gate
Bridge in Vancouver, British Columbia.

"Professional engineers are starting to recognize that it is economical
to allow this type of rocking in their designs, as long as the structure
remains stable and the speed with which the legs come down is carefully
controlled to minimize the forces that develop during the rocking," said
Bruneau.

In addition to the cost savings in construction, this design also saves
money if seismic retrofit needs to be done, he added. "It's much easier to
fix a tower to enhance its seismic resistance if the crew only has to work
at the base, instead of having to climb 60, 80 or 120 feet to strengthen
individual members along the height of those towers," he said.

The research is funded by the U.S. Federal Highway Administration.

Note: This story has been adapted from a news release issued by
University at Buffalo.

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