UCSD Jacobs School of Engineering
University of California San Diego
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Earthquake Safety Research

Building designers have added concrete and steel to strengthen the foundations and walls of new structures built in earthquake-prone regions of California . But structural engineers at the Jacobs School say the rigid, more expensive new designs are not necessarily safer. Lessons learned from recent earthquakes in California , including the 1994 Northridge earthquake, have led UCSD faculty to design and test ductile buildings and structural components that dissipate earthquake energy rather than magnify it.

Most recently, a team led by professor José Restrepo and adjunct professor and structural engineering industry leader Robert Englekirk tested an emerging theory that concrete buildings will better survive strong earthquakes if the structures have less, but well detailed, steel reinforcement. ""Many people don't realize that excessive building strength can actually promote poor structural performance and non-structural damage during an earthquake," says Englekirk. He believes such reinforced concrete construction will become the backbone for mid-rise and highrise residential structures in densely populated and seismically active regions in Los Angeles and Southern California .

Putting this theory to the test, UCSD conducted one of the largest-ever full-scale earthquake tests on the university's new 25 ft. by 40 ft. outdoor shake table at its Englekirk Structural Engineering Center . California structural engineering firms contributed more than $1 million to construct a seven-story concrete structure built with half the amount of steel reinforcement suggested by California building codes. The experiment was designed to measure the performance of vertical reinforced concrete structural walls (shear walls) during strong earthquakes. Construction was led by Highrise Concrete Systems, Inc. of Dallas , TX , which uses cast-in-place tunnelform technology to build reinforced concrete buildings throughout the U.S.

The 7-story test building stood strong after being rocked with two major earthquake simulations. After the 'big shake' (l-r): UCSD principal investigator José Restrepo; Hanns Baumann of Baumann Engineering; grad student Marios Panagiotou; dean Frieder Seible; co-PI Robert Englekirk.

The 250-ton, 65 foot-tall structure was subjected to a series of tests simulating the ground motions recorded during the 1994 Northridge earthquake, a magnitude 6.8 quake which resulted in 60 deaths and 7,000 injuries, and more than 40,000 buildings damaged in Los Angeles, Ventura, Orange, and San Bernardino Counties.

The first large seismic test took place in November 2005. Local and national TV crews recorded the historic moments, as the test structure was rocked with precisely the ground motions that struck a seven-story Van Nuys hotel 12 years earlier at 0.3 G horizontal acceleration. Then in January 2006, the test structure was subjected to 0.82 G acceleration, duplicating the motion and forces recorded closer to the epicenter near Sylmar , CA .

The test building groaned and swayed, but it developed only minor cosmetic cracks. "If this building were at the epicenter of the quake, its residents could continue to live in it with no problem," says Restrepo.

"We have begun to break the misconceived link between strength and performance. Severing this link will promote the design flexibility required to produce safer, more economical buildings," says Englekirk, who adds that since the test, building officials in Los Angeles and San Diego have already begun to approve new construction based on the design tested at UCSD.

The seven-story structure remains standing on the UCSD shake table, where researchers plan to perform a second series of experiments to test the "T" wall, a common bracing component of residential buildings. After that, the building will be demolished to make way for future prototypes.

In addition to concrete buildings, Jacobs School researchers are working to improve the earthquake safety of steel buildings. Another major cause of damage in the Northridge earthquake was cracking in the beam-to-column welds in more than 200 steel moment-frame buildings in Los Angeles . A group led by Jacobs School structural engineering professor Chia-Ming Uang recently designed and tested several improved beam-to-column connections in full-scale tests at the Powell Structural Research Laboratories. The tests have led to the adoption of improved design guidelines for retrofit of existing moment-frames and changes to building code provisions for the design of new steel buildings.

Other earthquake research findings made at UCSD have been used to fortify California bridges. UCSD researchers helped design a jacket system (picture A) that has been applied since 1992 to retrofit California bridge columns. During the Northridge earthquake, such retrofitted columns held strong, while several bridges with vulnerable columns targeted for retrofit collapsed (pictures B, C). Since then, all of the bridges managed by the California Department of Transportation have been retrofitted. However, more than 500 locally-managed bridges have not been fixed.
 
Website: Powell Structural Research Laboratories
News Release: Engineers Announce Results of Second Northridge Shake of 7-Story Building
Video: CBS Evening News coverage of the 7-story shake test
Website: Englekirk Structural Engineering Center
In This Issue
100 years of earthquakes what we've learned
Tunable lasers
 
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