Scientists say there is a 63% probability of a damaging earthquake in the Bay Area in the next 30 years. Officials in quake-prone California recently took a step toward making the best of the Big One, when it does hit, by installing the densest array of accelerometers in any U.S. skyscraper to date. The 72 sensors, activated recently in the tallest tower in the U.S. built using performance-based seismic design, are expected to yield valuable data about the behavior of PBSD skyscrapers under quake loads.

"The key thing is the calibration and verification of performance-based design for high-rises," says Anthony Shakal, manager of the California Strong Motion Instrumentation Program of the California Geological Survey (CGS).

The 641-ft-tall subject of the $300,000 monitoring program is the four-year-old One Rincon Hill-South Tower (ORH) in San Francisco. The project is the first collaboration of this type between CGS and the U.S. Geological Survey.

Tall-building PBSD is a way to meet the intent of the code's prescriptive provisions for buildings 240 ft or taller by resisting lateral loads using a single framing system instead of a costlier dual system. But PBSD has yet to be tested in a major quake.

Under this project, ORH is instrumented to record floor motion, forces, frequencies and periods. If a temblor hits, the system automatically turns itself on and, "like E.T.," phones home to the Center for Engineering Strong Motion Data, where the data is recorded, says Shakal.

Even before a large quake occurs, the sensors will yield valuable data about structural behavior under wind loads. Except for the system's start-up and maintenance, captured wind data will be recorded for posterity only when weather forecasters predict high winds.

The data will provide a "good understanding of real dynamic performance under wind loads to better calibrate our computer simulations and modeling assumptions," says Ron Klemencic, president of ORH's structural engineer, Magnusson Klemencic Associates, Seattle.

ORH was a good candidate for this project because of its height, location and lateral system, says Shakal. The tower has a ductile-concrete core with supplemental outriggers in the form of buckling restrained braces. It has concrete perimeter columns and post-tensioned flat slabs.

The center records data on many structures. All of the information is available for free online.

Shakal says data collected during 1994's magnitude-6.7 Northridge quake, in Los Angeles, showed that forces farther from the fault were lower than had been expected. Consequently, there are ongoing discussions to reduce the building code requirements for structures 40 kilometers or farther from the fault, he says.

CGS is working toward monitoring more tall buildings in San Francisco, Los Angeles, San Bernardino and San Diego. "Near-fault motions—forces from big earthquakes—are unknown," says Shakal. "Potentially, the forces are high, but we have little data, internationally." Adds Shakal, "It's so hard to have [instrumented] stations in the right place."