The Golden Gate Bridge

Oakland City Hall

Oakland City Hall
Structural Engineer Mason Walters and his team at Forell/Elsesser Engineers recently made sure that the Oakland City Hall built in 1914 as the first high-rise government office in the United States would remain one of the country's finest examples of Beaux Arts architecture. Their achievement was remarkable, considering that the building suffered severe damage in the 1989 Loma Prieta quake and was immediately closed to occupancy.

This beautiful building, with its three-story central rotunda, 10-story base, and highly decorative 90-foot clock tower definitely deserved a second chance. In technical terms, 20 percent of the buildings lateral strength was lost in the north-south direction and 31 percent in the east-west direction, primarily due to extensive cracking of the numerous interior hollow clay tile partitions in the office tower. This was considered a significant shift given the distance of the building from the epicenter of the quake. The entire clock tower racked and shifted at its base resulting in an offset of 1 inch.

The Structural Engineers immediately took charge to redesign and repair the structure. The goal was to restore the building to its majestic self, return the government employees as soon as possible and add earthquake mitigation measures to the structure so that future temblors would not similarly disrupt the work of the city government.

The Structural Engineers acknowledged early on that, with conventional retrofit systems, the dynamic characteristics of the clock tower would be altered significantly unless extensive and functionally intrusive strengthening could be used.

Walters and his team turned to an evaluation of seismic isolation. A base- isolated structure is supported by a series of bearing pads, which are placed between the building and the building's foundation. The base-isolation scheme not only reduced the seismic force level at the base of the building, it also altered the dynamic response and the seismic movement that the structure would sustain in a major earthquake, thus reducing the required level of strengthening.

Below city hall, a base isolated structure is supported by a series of bearing pads

This renovation also turned out to be unique in an unexpected way: it represented one of the first times that FEMA, the Federal Emergency Management Agency, allowed funding for upgrades to protect buildings against future damage.

A team of Bay Area Structural Engineers and the professional organization, SEAONC, made a strong case to FEMA that damage prevention was better - and cheaper - than repair. Thanks to their efforts, FEMA changed its attitude in terms of emergency management to consider predisaster funding as well as postdisaster funding.

The Oakland City Hall (OCH) is now the tallest base-isolated structure in the world. It has a complete steel frame. The foundation system is a reinforced concrete mat over the entire basement area. There are now 112 isolators installed in the basement of the structure. The floors and roofs are constructed of reinforced concrete slabs supported on the structural steel frame. The exterior walls of the building are composed of URM faced with granite and terra cotta ornamentation.