Case Study: San Francisco City Hall in California
Engineers consider more than the height of a building when evaluating the earthquake-resistant technologies they might include. They must also weigh the civil or cultural significance of a structure. For example, a hospital may warrant more attention than a warehouse. After the 1989 Loma Prieta earthquake, which killed 3,500 people and damaged 100,000 buildings across San Francisco, Oakland and Santa Cruz, engineers and city planners retrofit several important structures with earthquake-resistant technologies, like San Francisco City Hall. Engineers cut the two-block-long building from its foundation and floated it on 530 base isolators. If seismic waves roll its way in the future, the building will sway horizontally up to 26 inches (66 centimeters) without shaking apart.
The Future of Earthquake-resistant Construction
The goal of earthquake-resistant buildings is to preserve life. That means a building that doesn't collapse and allows its inhabitants to escape is considered a success -- even if it ends up being demolished. But what if a building could experience deformation during a quake, then return to its original shape? For some researchers, such as Greg Deierlein of Stanford University and Jerome Hajjar of Northeastern University, that's the future of seismic engineering.
Deierlein and Hajjar have teamed up to develop an innovative technology known as the rocking frame, which consists of three basic components -- steel frames, steel cables and steel fuses. Here's how it works: When an earthquake strikes, the steel frames rock up and down to their heart's content. All of the energy gets directed downward to a fitting that houses several toothlike fuses. The teeth of the fuses gnash together and may even fail, but the frame itself remains intact. Once the shaking has stopped, the steel cables in the frame pull the building back into an upright position. Workers then inspect the fuses and replace any that are damaged. The result is a building that can be reoccupied quickly after an earthquake.
Another innovation is something that's been dubbed the seismic invisibility cloak, suggesting a building could be made transparent to the surface waves produced by an earthquake. To accomplish this, engineers would bury a series of up to 100 concentric plastic rings beneath the foundation of a building. When waves encounter the rings, they enter and then become compressed as they are forced into a bottleneck. The waves basically zip by, just beneath the building's foundation, and exit the rings on the other side, where they resume their original speed and amplitude.
Interestingly, much of the future of seismic engineering involves looking back, not forward. That's because retrofitting old buildings with improved designs and materials is just as important as constructing new buildings from scratch. Engineers have found that adding base-isolation systems to structures is both feasible and economically attractive. According to the National Earthquake Hazards Reduction Program, more than 200 buildings in the United States, including many city government and fire and emergency buildings, now feature isolation systems. After the 1989 Loma Prieta quake alone, engineers retrofitted several buildings, including the city halls of San Francisco, Oakland and Los Angeles. The earthquake-resistant structures in these buildings will most certainly face a test in the form of a serious seismic event. The only question is when and to what extent.