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San Francisco's Old City Hall, built in the late 1800s, took 27 years to construct and only 28 seconds to crumble during the 1906 Earthquake. Only the dome remained intact.
Following much controversy, City Hall was finally rebuilt to the west side of Civic Center in 1915, to coincide with the Pan American Exposition that year. It was one of the finest examples of French Renaissance architecture in the country. |
History almost repeated itself in 1989, when another earthquake subjected City Hall to peak ground acceleration of up to .10 gravity forces. The signature 302-foot-tall dome twisted on its steel frame like a cap on a bottle. Cracks in the walls and concrete floor slabs occurred at all levels. While the building was never in danger of collapsing, Structural Engineers determined that its seismic resistance had to be improved or the structure would not withstand the next big quake.
After Loma Prieta, the City's power brokers learned there was damage to most of the signature buildings in the Civic Center district an area that had received National Historical Landmark status just 10 years earlier. Thus began a long-term collaboration between city government and six of the City's Structural Engineering firms to strengthen, rebuild, and do earthquake risk mitigation on more than 190 city buildings. |
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In June 1990, San Francisco voters passed a $332.4 million general obligation bond for repair and seismic retrofitting of 191 city buildings damaged in the Loma Prieta earthquake, including City Hall. In November 1995, San Francisco voters approved a $63.5 million general obligation bond issue for funding for additional improvements to City Hall.
The two-block-long building was cut from its foundation and made to "float" on 530 isolators shock absorbers designed to dissipate earthquake motion and allow the building to sway horizontally up to 26 inches without shaking apart. To install the isolators, the engineers jacked up the supporting columns of the building one at a time, cut the columns and positioned the isolators under them, and then restored the weight to the structural members. The superstructure strengthening included a new ground floor, constructed above the isolators, concrete shear walls around the light courts, steel collectors to deliver seismic forces to the new shear walls, reinforcing of the rotunda tower walls, and installation of steel braces and shotcrete walls at various levels of the dome. The dome was reinforced with 1,200 tons of steel to prevent a repeat of the four inch "corkscrew" twist it received in the 1989 quake.
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The retrofit project also included the careful removal and cleaning of some 30,000 pieces of marble from the building's walls and floors. The Structural Engineers and other professionals worked from copies of the original blueprints, created on linen by the designer Arthur Brown, Jr., and stored in UC Berkeley's Bancroft Library.
Today, this structure stands by itself as one of the finest examples of French Renaissance architecture and as one of the most seismically safe municipal buildings in the country.