The shake table in Richmond
After leaving Professor Fenves's office, I drove a few miles west to the university's Richmond Field Station, home to the Pacific Earthquake Engineering Research Center, itself. The site is near the Interstate 80 freeway, which runs alongside the muddy shoreline of the San Francisco Bay. Except for the nearby traffic, the place was quiet—unlike in 1860 when the California Blasting Cap Company did business here.
On the day I visited, a group of researchers wearing hard hats had gathered around PEER's shake table—one of the largest in the United States. Stephen Mahin, professor of structural engineering, explained that the group was testing seismic isolators—a kind of structural shock absorber. The technology is expensive and is more commonly used in Japan than in the United States. Above the table, the isolators were mounted on a simple structure that resembled the scaled-down skeletal base of a skyscraper. "Here, we are looking at putting isolators at the top of the first floor columns, Mahin said. "This configuration would make them more palatable for construction in the U.S."
I asked Mahin about the relationship between this kind of physical simulation and the software counterpart. "We integrate the two," he said. "We can model some parts quite well analytically, often because we have lots of data. But in other cases, we are looking at new concepts, so that only experimental testing will do. Many of the things we test here are scale-dependent—scaled down structures don't necessarily deliver results we can believe. At the same time, even a detailed finite element analysis doesn't convince everybody that it is reflecting the real world. Physical simulation is an exercise in humility."
Sometimes, the two types of simulation are combined into a single hybrid analysis. "Say you have a big structure like a suspension bridge, and you want to analyze one of the bridge piers. So you use an actual pier, then model the rest of it in software: the tower, deck, cables, foundations, sea bed, and wave propagation." The FEA application operates as it always does, but every time data is needed on the base pier, an actual physical structure gets moved the specified distance and the actual resulting force is returned to the program. "So on this component, instead of having finite elements, we have physical elements." Mahin said that this day's physical simulation was, itself, a preliminary test for later hybrid testing. "The shake table test represents what the structure would do in an earthquake. We are then going to move the specimen to another building and hook it up to large, computer-controlled hydraulic actuator that see if we can replicate what we saw here." Inside the control room, a technician had his eyes on a computer screen and his hand, over his shoulder, poised on a dial. A flick of that dial would trigger a set of 75,000-pound hydraulic actuators, eight horizontal and four vertical, built into a pit below the table—creating an "earthquake" on demand. As if opening a safe, he pulsed the dial, and it took me a split second before I realized the obvious: that the shaking would not be confined to the table. For a few nanoseconds, my native California brain, with its first-hand knowledge of earthquakes, insisted that this was the real thing.