Earthquake Retrofit for a Soft Story Apartment Building

The Challenge: A six-story post and beam construction apartment building in San Francisco needed an earthquake retrofit to enable it to withstand high levels of seismic forces. The street level of the building was comprised of retail spaces, parking garages and an entry lobby. In addition to the six stories above ground, the building had a basement with perimeter unreinforced masonry retaining walls left over from an older building that was on the site prior to the construction of the existing structure. Additionally, the apartment building was on sandy soil. Access to the interior of the building would be tight.

Our Solution: We assisted the engineers with the technical aspects of the design. The proposed program included encapsulating the old basement masonry retaining walls with reinforced concrete, construction of steel reinforced concrete sheer walls, and the construction a of two-story Braced frame with prefabricated diagonal braces referred to as Buckling Restrained Braces or BRBs.

Due to the sandy composition of the soil, the Geotechnical Engineer specified a soil stabilization program. We implemented this program by injecting 12’ columns of grout under the proposed grade beam. The grouting permeated the soil and solidified the ground under the 35’x9’x5’ concrete grade beam.

The massive steel frame rose from the basement floor to the second floor of the building. It took a 4-man crew of ironworkers 4 weeks to field assemble, perform welding and conduct testing of the welds.

At the end of the earthquake retrofit project, the apartment building was up to current earthquake code and much safer for its tenants.

To discuss your earthquake retrofitting project, please contact structural builders SteelCore Builders at: 415-331-0525.