CGI Preservation Discusses Seismic Upgrades to Cast Stone

Earthquakes are never far from the consciousness of long time California residents. This is especially so for those with a concern for the state’s historic masonry buildings, which have a special vulnerability to seismic activity. This was not fully appreciated when these structures were constructed. The built environment at that time had not been in existence long enough to demonstrate the destructive power of such events. We know better now. The result is that many historic masonry restoration projects will have a seismic retrofitting component, even if that was not the initial intent of the project.

Hollyhock HouseCast stone projects, such as the Frank Lloyd Wright-designed Hollyhock House, furnish a good example of this principle. An affordable and versatile material, it was employed widely by California builders.

Long exposure to the elements had deteriorated the cementitious “stone” units, motivating the restoration of the distinctive roof and parapet area. Dismantling revealed that these large and vulnerable masonry units were anchored together in the most casual fashion, requiring extensive engineering and retrofitting with stainless anchors to resist seismic movement.

The same fearless optimism with regard to seismic forces was demonstrated by the builders of the Wilshire Temple rose window, a massive structural element, circular, 24 feet in diameter, and constructed out of 125 separate cast stone units.

Wilshire Temple Rose WindowThe engineering of this complex structure was no simple matter and went through several iterations. The plan specified complete dismantling and coring of each unit to receive custom anchor connectors.

The takeaway? Expect any historic masonry project to require some engineering and a seismic upgrade.

By Charles Kibby, Owner of CGI Preservation