BUFFALO, N.Y. -- 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
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
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 research is funded by the U.S. Federal Highway
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
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
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.
"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
"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.
MCEER, headquartered at the University at Buffalo, was founded
in 1986 as a national center of excellence in advanced technology
applications dedicated to reducing losses from earthquake and other
hazards nationwide. MCEER has been funded principally over the past
19 years with $68 million from NSF; $36 million from the State of
New York and $26 million from the Federal Highway Administration.
Additional support comes from the Federal Emergency Management
Agency, other state governments, academic institutions, foreign
governments and private industry.
The University at Buffalo is a premier research-intensive
public university, the largest and most comprehensive campus in the
State University of New York.