Structural engineers at UC San Diego were waiting to see how a tall building, made from recycled steel, would react in a 6.9 magnitude earthquake.
The earthquake shake table was ready to go as the test building towered above it. In about half an hour, the table would start moving, and Ben Schafer was ready to see how the 10-story tower would take it.
"Externally we’ll be looking at exactly how the building deforms. But internally we have a huge number of sensors so we can look at every aspect of the building to see if it is performing the way we want. And understand exactly how all the forces are moving inside the building,” said Schafer, a systems engineering professor at Johns Hopkins University.
The shake table started to shift and pitch. It moved up and down. The building shook and, well, it didn’t collapse. Schafer, the co-lead investigator for the shake test, said Monday's test result wasn’t quite what they expected.
“Little bit of a surprise for us in terms of the fact that the building was twisting, as well as shaking side to side. Kind of no surprise here. We saw more of that twisting in addition to shaking side to side,” he said.
The earthquake shake table reproduced the effects of the Loma Prieta Earthquake that hit the San Francisco Bay Area in 1989. And it was also testing a recycled steel building material called cold-formed steel.
The shake table has been in operation since 2004, and it’s one of very few in the world that can test multistory buildings outdoors. UCSD structural engineering professor Joel Conte has worked with the device since its inception. He says it’s gone from being only able to shaft from east to west to doing a lot more.
“So the table can now move north-south," Conte said, demonstrating motions with his smart phone. “It can go up and down. It can pitch. It can roll. So we have the full range. So like your feet would feel on the ground during an earthquake.”
The building tested was constructed from recycled steel that’s called cold-formed steel. It gets its name from the fact that its final pressing and shaping is done at room temperature.
“Cold-formed steel is a thin sheet steel. And it’s very light,” Schafer said.
Today, buildings made from the material can only be six stories high in an earthquake zone. But a successful test on the shake table may allow taller constructions.
Reducing the mass of a building is a great advantage in an earthquake. The lightness also allows a building to be built in modules, which can be lifted by a crane and put in place, rather than having to be built on site.
That saves time and money for builders and hopefully reduces the cost of housing for everyone. UCSD urban planning professor Feiyang Sun says modular construction means contractors can build more quickly, and that saves them lots of money on construction loans.
“Reducing the construction timeline by using modular, pre-made units can cut the schedule in half — and reduce loan interest payments by half,” Sun said.
There is more shake testing to come for this 10-story building. Researchers plan to turn up the force of the Loma Prieta earthquake to simulate what they call a maximum credible earthquake event that’s about 50% higher than the actual earthquake.
“That would be a very rare earthquake, but we want to generate the data to understand what would be the behavior of the building under that very rare event,” said Tara Hutchinson, a structural engineer at UCSD and lead professor on the project.
That rare earthquake is one that would only happen every 2,500 years.
