Residential Earthquake Retrofits in BC by Shear Seismic Retrofits

Proof Seismic Retrofits Work

Santa Cruz Victorian Becomes Seismic Design Laboratory

This fascinating account comes courtesy of the American Plywood Association

In 1989, at the corner of Center and Elm streets in downtown Santa Cruz, California (San Francisco area) architect Michael O’Hearn unwittingly created a laboratory for the study of seismic retrofit design. His true story follows:

On that corner, at 214 and 210 Elm Street, were two identical Victorian style homes. According to O’Hearn, the twins were built 95-100 years ago, by the same builder, with identical materials and using the same construction techniques.

When O’Hearn bought them in 1984, both houses had been through multiple remodels and O’Hearn remodeled the pair yet again. He started by retrofitting #210, installing plywood shear panels on the cripple walls and bolting the mudsill to the foundation. Unfortunately, there was no time to retrofit #214 before the 7.1 Loma Prieta earthquake hit on Oct. 17, 1989.

In a sense, 214 Elm Street was the “control element” in this amazing experiment.  “The building came apart in four sections,” O’Hearn said.

However, 210 Elm Street, with its plywood shear panels and bolted foundation, suffered only minor damage.  “The one we had retrofitted (210 Elm St.) cost us $5,000 to repair. The other one (214 Elm St.) cost us $260,000 to repair.  The whole building had to be jacked up, repaired, and slid back on a new foundation.”

O´Hearn offers this advice, “For homes more than 20 years old located in areas of seismic activity, I strongly urge owners to consider seismic retrofit.  It’s a lot cheaper to retrofit a house now than to repair it after an earthquake.”

Studies by the Association of Bay Area Governments show that a properly retrofitted home is almost one thousand percent (1000%) more resistant to earthquake damage than one that is not retrofitted.

Since earthquakes and the forces generated by them are unpredictable, seismic retrofitting cannot guarantee a house will receive no damage at all. Cracks in plaster, doors that don’t open and close smoothly, and the chimney falling down are to be expected no matter how well the house has been retrofitted.

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