Seismic Engineering and Retrofitting in San Francisco
All images (c) 2003 Andrew Alden, licensed to About.com, Inc.
This new building, Moscone Center West, was engineered to a high degree—not only will it withstand a major earthquake without collapse or loss of life, but it will remain functional immediately afterward, helping the city get back on its feet. The glass panels are carefully designed and mounted with a lot of give, for instance. The steel girders holding up the three main floors inside are shaped to yield to seismic strain instead of breaking at the joints, an innovation practiced ever since the 1994 Northridge quake. But also these girders are coupled together with steel links that absorb even more strain, sacrificing themselves to preserve the girders. The links are easily replaced without needing to shut down the building.
Twenty-four seismic sensors are placed in the building, linked to recorders like the one on the left. These send data continuously to the U.S. Geological Survey offices 50 kilometers away. On the right is one of many friction joints, placed in the building's braces to absorb still more seismic energy. These engineering measures helped the building meet the "immediate occupancy" criterion at a cost of about $2 million, less than 1 percent of the building's total cost.
Just a few blocks away is the Ninth District Federal Court building, constructed just before the great 1906 earthquake and fire. This door, near the southwest entrance, shows how this corner of the building subsided in 1906, putting the doorway out of plumb. It also shows the beautiful furnishings of the building, now a historical landmark. A building like this must be retrofit rather than rebuilt. In this case a system of more than 100 seismic isolators was put in place beneath the whole structure. They are pairs of steel dishes with a specially designed bearing between them, allowing the building to move sideways as much as 35 centimeters in any direction. Here's one below. In fact, the whole building's perimeter is fitted with unobtrusive gaskets at the base of the outer walls and stairways. If you find yourself standing there during a big quake and have the presence of mind, watch the courthouse surf like a skateboarder.
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