Earthquake research brings down the house
Testing a building to see how strong an earthquake it can withstand is a more delicate process than it sounds.
“If we shake it down all at once, we wouldn’t get all the data we want,” said Benson Shing, a structural engineering professor at UCSD.
At the university’s Camp Elliot Field Station, a three-story reinforced concrete and brick building sits on a hydraulic shake table. Typical of 1920s construction, the building is the largest ever to be seismically tested and promises to provide insight into how these older structures can be reinforced.
More than 250 string gauges and other devices are attached to the building collecting thousands of measurements as the building is subjected to increasingly stronger ground movement.
The data helps create models to predict this type of building’s performance during an earthquake. Currently, no models exist for these older structures because they are very complicated, said Andreas Stavridis, the Ph.D. student leading the project.
“It’s hard to catch the interaction between the bricks and the concrete frame,” Stavridis said.
In conjunction with the Great Southern California Shake Out on Nov. 13, researchers tested the building at up to 50 percent above what it is designed to withstand, which is moderate damage in a magnitude 6 to 8 earthquake.
No bricks went flying, but numerous hairline cracks snaked along the ground floor brick wall and concrete frame.
“The building is performing a bit better than expected,” Shing said after the test, but explained the damage was still severe.
“Without the walls, the structure would have collapsed,” he said.
This shows the number of walls and their location can have a critical impact on how well a building holds up in an earthquake, Shing said. Buildings with fewer walls on the ground floor could be much weaker than structures with more support, he added.
Amplifying this concern was the fact that most of the damage was on the first floor.
“We’d rather spread the damage throughout the entire structure,” Shing said. “This is more dangerous.”
The engineers planned to shake the building to failure this month (a large steel support sits just an inch away on either side of the building to keep it from falling over).
After spending several months analyzing the data, they plan to construct the same building again for a second round of testing. However, this time the “inside” of the building will be reinforced with shotcrete (mortar applied via pressure hose) mixed with fibers.
The tests are part of a larger $1.24 million study funded by the National Science Foundation and were designed with input from industry professionals so they hopefully produce practical information.
“This is really important ... because we can see where exactly our building codes are adequate or not, or too conservative,” said Mike Chapin, CEO of Geocon, a geotechnical and environmental engineering firm.