Rock Steady

Honeck, William; Walters, Mason; ,

Rock Steady

by William Honeck, Forell/Elsesser, San Francisco, CA,
Mason Walters, Forell/Elsesser, San Francisco, CA,

Serial Information: Civil Engineering—ASCE, 1995, Vol. 65, Issue 9, Pg. 58-61

Document Type: Feature article

Abstract:

Heavily damaged during the Loma Prieta earthquake, Oakland City Hall had to be repaired and retrofitted. Now Oakland boasts the tallest seismically isolated building in the world. The 19�story Oakland City Hall was literally head and shoulders above the rest when it was completed in 1914. At that time, this landmark structure was the tallest building in the western U.S., and the first of a genre of public buildings that attempted to combine the features of a modern high�rise office tower with those of a traditional grand rotunda building. The resulting tiered structure, which was only possible with the advent of riveted steel construction, features a broad podium base that houses the ceremonial rotunda, topped by a slender 11-story office building and a five-story clock tower. The perimeter framing is infilled with massive brick, granite, and ornamental terra cotta. The multiple setbacks in the width of this building resulted in an ungainly transfer of loading from top to bottom of the 324 ft high building. This posed a daunting challenge to the original structural engineer�each successively narrower portion of the building required massive transfer trusses and girders to spread the loads to the broader podium portion below. This challenge was matched by the one faced by Forell/Elsesser, San Francisco, as retrofit engineer, who had to devise a stiff new structural steel skeleton inside the historic building to continuously transfer lateral loads from the top of the clock tower down to the new seismic isolators atop the old concrete mat foundation. After the Loma Prieta quake, engineers determined that by seismically isolating the Oakland City Hall, the ultimate seismic acceleration could be reduced by a factor of more than 3. This significant reduction in lateral forces on the building superstructure results in a reduction of building drift (lateral movement), which translates into a major reduction in future damage to the brittle archaic infill wall materials. A further benefit of base isolation is that shear walls can be limited to the central portion of the podium and office tower, thereby preserving historic interior finishes.

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