El Centro Shake Tests Aim to Aid RC Retrofits:
Shaking a damaged existing structure will lead to reliable models for retrofitting earthquake-prone concrete structures
In an unusual experiment, a team led by University at Buffalo Assistant Professor Andreas Stavridis recently conducted shake tests on an existing, two-story, reinforced concrete (RC), infilled-frame structure in El Centro, California. Using mobile shakers from NEES@UCLA, the test severely damaged the second story of the building.
|View news coverage of the experimnents by NBC Los Angeles.|
"This is a unique test,” explained Stavridis. “We have been able to induce damage to the structure and push the building into the nonlinear regime of response. With the resulting data we want to validate the simulation models we are developing and provide guidelines for the condition assessment of infilled frames, so practicing engineers can apply them on any building of this type."
Currently, practitioners do not have reliable computer models for assessing damage in these vulnerable buildings before they attempt to retrofit them.
California is replete with such pre-Depression Era, RC infilled-frame buildings, many of which have historic value. Driving around San Francisco, Los Angeles and San Diego, one can see many of such buildings built before the 1933 Long Beach Earthquake. These buildings have sustained damage due to earthquakes; some have been retrofitted, with a variety of methods. Assessing their current condition and estimating their performance in future seismic events are major challenges for practicing engineers.
Around the world today, the RC infilled-frame is still the most common type of building constructed, even in earthquake-prone regions of the Mediterranean and Latin America. Stavridis’ NEESR project, Pre- and Post-Earthquake Damage Assessment for Infilled RC Frame Buildings, will provide techniques to assess the condition and strength of such structures before and after a major loading event. This is a necessary step towards identifying vulnerable buildings and developing effective retrofit techniques.
Located in the seismically active area of southeastern California, the El Centro building was damaged by earthquakes in 1940, 1979, 1987 and in 2010. The 2010 quake damaged the building beyond repair, and it was “red-tagged” and slated for demolition.
In a foresightful move, Stavridis and his team convinced the owners to allow the use of their building’s moribund structure for science. “It took us two and a half years of negotiations with the owners, the city, university officials, lawyers, and contractors to get to this point,” Stavridis said. After clearing many legal and safety hurdles, and with the help of the University at Buffalo in offsetting the drastically increased demolition costs, the long-awaited tests took place between November 12 and 15.
With test data gleaned from sensors and cameras, Stavridis and his team will collaborate with a Tufts University team led by Babak Moaveni, associate professor of civil and environmental engineering. Together, the researchers will develop damage-assessment models for infilled-frame buildings.
Not surprisingly, the Structural Engineering Association of California (SEAOC) is highly interested in the outcome of this project. In fact, it was during a reconnaissance trip with SEAOC members led by Fred Turner of the Seismic Safety Committee of California that Stavridis first visited this building. Depending on the outcome of the tests, the El Centro building may be used as a case study to validate analytical tools under consideration for inclusion in the next version of ASCE 41.