University of California, San Diego researchers used an outdoor earthquake simulator to test the structural integrity of a key part of a seven story tall building. The 275 ton concrete and steel structure is the largest ever put through an earthquake simulation. Engineers want to know if a new building technique will help revolutionize the construction of mid-rise residential buildings in earthquake-prone Southern California. KPBS Reporter Erik Anderson has details.
The 65 foot tall structure towers over a nearby metal shed. It is NOT a complete seven story building. In fact, the concrete and steel edifice is really just a corner that looks like its been sliced out of a building. All but one outer wall has been stripped away. Structural Engineer Bob Englekirk says that allows researchers to assess the impact of the force applied by the shake table.
Englekirk: We're trying to make sure that the building can flex and move without causing damage."
Englekirk says it the largest structure of its kind to undergo a realistic earthquake test. The largest earthquake shake table in the United States delivers the same force felt in Van Nuys during 19-94 Northridge earthquake. But the hydraulically powered shake table has to gear up. Englekirk looks on as the table gently slides the structure back and forth.
Englekirk: It causes the top of the building to move about two inches back and forth.
This, of course, is a slow motion test. Englekirk says this particular structure features different building techniques than a typical earthquake safe building. The structure's concrete walls contain less reinforcing steel. That allows the building to move during a quake, without suffering major cracking or structural failure.
Englekirk: People have told me that they have been at the top of seven story buildings that I have designed, for example at Loma Prieta, and couldn't get up off the floor. The shaking was so bad. But the damage to the building was nothing.
After some initial testing, engineers are ready to start the simulation. Once underway, the shake table will mimic the exact movement of the Northridge earthquake that was recorded at a Van Nuys Holiday Inn in 19-94. That seven story building suffered severe damage that was expensive to repair.
The test begins with a series of violent lurches. The structure sways back and forth. Small chips of concrete flake off and fall to the floor. Its more than a minute before the structure completely stops shaking. U-C San Diego Structural Engineer Jose Restrepo says the concrete model survived the test without much visible damage, just the a few concrete chips falling off. Restrepo: One chip here or there is not going to kill anybody. I saw the chips, they are about half an inch square so, that's perfectly normal. You get a lot of dust, a lot of things coming off in an earthquake. That's fine. "
Restrepo says there were some minor cracks on an interior wall, but they were so small they couldn't be seen from more than a few feet away. He says the Van Nuys hotel fared far worse. Restrepo: The building suffered extensive damage, structural damage. It had severe failures in many, many columns.
Restrepo says THAT hotel wasn't flexible enough to endure the tembler that rocked the Los Angeles basin.
Restrepo: Buildings in the past have been made apparently overly strong, overly strong in that they take more load to bend, and what they end up doing is that they break and sheer. They just snap.
Civil engineer Bob Englekirk - who designed the Getty Center in Los Angeles - says the new approach to building medium-rise, earthquake resistant buildings is cheaper. He says using less steel rebar to reinforce concrete walls can cut costs by 40 percent. Englekirk says findings from this and future tests will influence the design of a 38 story high rise condominium project near Petco Park in downtown San Diego. He says the knowledge gathered here could also help rewrite California's buildings codes, allowing developers to build safer and cheaper buildings. Erik Anderson KPBS News.
