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Name: Professor Robert Fleischman
Email: rfleisch@engr.arizona.edu

Earthquake Engineering Research Will Save Lives and Billions of Dollars

The 6.7 magnitude earthquake that struck the Los Angeles community of Northridge at 4:30 a.m. on Jan. 17, 1994, killed 57 people, injured more than 5,000, and caused an estimated $20 billion in damage, making it the costliest seismic disaster in U.S. history.

Structures that should have withstood the quake, such as parking garages and freeway overpasses, collapsed and set in motion a major overhaul of building codes.

“If the earthquake had happened three hours later…” The potential horror of what could have been causes Professor Robert Fleischman’s speculation to trail off.  Fleischman, who holds the Delbert R. Lewis Distinguished Professorship in Civil Engineering and Engineering Mechanics, is about to finish the first phase of a 5-year, $2.5 million earthquake engineering study that will lead to improved building codes, which will protect buildings—and their occupants—from earthquakes.

The project involved building a 500-ton concrete parking garage and then subjecting it to a magnitude 7 earthquake while measuring the stresses that almost reduced it to rubble.

A birds-eye view of one of the shake tests in the series of 15 conducted on the test structure demonstrates just how much it would move during a magnitude 7 earthquake.

The internal workings of the shake table are seen in this video, which zeroes in on specific parts of the test structure to see closely how it responds to a magnitude 7 earthquake.

The million-dollar test required the construction of a 3-story, half-scale precast concrete structure on a giant shake table at UC San Diego’s Englekirk Structural Engineering Center. About 600 instruments throughout the structure measured its responses to fifteen 20-second quakes simulated by the shake table.

Time-lapse photography shows the construction of the test parking garage at the Englekirk Structural Engineering Center. Approximately two months of construction are condensed into less than 5 minutes of video.

Many of the structures that failed in the Northridge earthquake were built using precast concrete. This is a very economical construction technique because the component slabs are made offsite. “Quality is much easier to control, and therefore high,” says Fleischman. “And the precast pieces last a lot longer than concrete that is cast in situ at a construction site.”

Precast concrete’s strength derives from prestressing. We’ve all passed construction sites and seen steel rebar sticking out of unfinished concrete structures. Prestressing involves stretching this steel in a mold, or bed, and then pouring the concrete to embed it. Once the concrete is dry, the stretched steel is cut. “It’s like a big rubber band,” says Fleischman. “The steel tries to shorten and it squeezes the concrete.”

After managing to withstand 15 major earthquakes, a portion of the top floor of the test parking garage finally collapses (two different views are shown).

These precast and prestressed slabs are transported to the construction site by truck and dropped into a concrete framework by crane. The slabs sit on ledges in the framework, and are usually welded to adjacent slabs where the steel protrudes. These joints are weak points in the structure, and can come apart like a zipper during an earthquake. “When building a precast structure in California, you have to actually pour cast-in-place concrete on top of it to hold it together,” says Fleischman. “And that kills the economic advantage.”

This close-up video of a joint between two precast concrete slabs shows how the slabs shear apart. The sensors fixed to the slabs are recording the seismic stresses caused by the shake test.

The enormous structure that Fleischman’s group almost leveled on the shake table was built based on computer models derived from hundreds of tests and simulations carried out in the first phase of the project. In addition to the spectacular shake table test, the group has also conducted many hybrid tests. “It’s called hybrid because it’s part analytical, part physical,” Fleischman says.

For instance, the group wanted to test a critical floor joint to study the behavior of some reinforcing they had been working on. The joint components were fabricated in Pennsylvania and assembled in a lab at Lehigh University. Attached to the components were actuators that could be made to shear and open and close the joint, which allowed the researchers to simulate how the joint would behave in a real earthquake. “That joint did all the things it would do if it were sitting in a building during a quake,” says Fleischman.

While Lehigh did the physical part of the test, UA did the analytical part. Although the actual structure was in a lab at Lehigh, researchers in Fleischman’s lab in Tucson controlled the actuators and monitored the stresses and forces within the critical floor joint.

Computer animation generated by the modeling software that predicts how a currently designed parking garage might behave under a California maximum considered earthquake, which is defined by the United States Geological Survey as the biggest earthquake likely in a reasonable amount of time; “reasonable” is defined by historical and geological records.

These tests have yielded “gobs and gobs of data,” says Fleischman. Like the million-dollar shake table test, the hybrid tests yield high-quality data from actual physical simulations, but at a fraction of the cost. “You build the part that’s key and the rest is virtual,” says Fleischman. “The data enables us to understand how structures behave, and we can use it to create computer models.”

The second phase of the project was given the green light, says Fleischman, “because the computer models we’ve developed are so advanced, and my students and I can do all this significant research in the next couple of years refining our results.” Fleischman says his doctoral students Ge Wan and Dichuan Zhang have done “a tremendous job at developing three-dimensional models of structures and modeling everything that breaks.”