The city is eyeing mandatory seismic strengthening for soft story buildings. Whether it's worth it for you depends on your building and its value.

Roughly 300,000 San Franciscans live in multifamily buildings and of those more than half live in soft story buildings-that is, wood-frame buildings constructed before 1973 with at least half of the ground floor designated open space. The Department of Building Inspection expects one in five city buildings will be uninhabitable or destroyed after a magnitude 7.2 earthquake with property damage in the $30 billion range. As many as 27,000 buildings could be condemned after such a quake, and the city could lose 85,000 housing units. If you own a soft story building yours could be one of them.

As the San Francisco Board of Supervisors considers adopting a nine-year 51 million program to make soft story retrofits mandatory: it's a good time for property owners to weight their options and learn more about this important issue.

Soft Story 101

Soft story buildings are peppered throughout San Francisco. To qualify as a soft story building your property must be wood framed and meet both of the following criteria: it was constructed before May 21, 1973 and at least 50% of the ground floor area is used for assembly business, mercantile storage, or open or enclosed parking garages. In San Francisco this most often means buildings with garages on the bottom floor. To qualify for the city's proposed Community Action Plan for Seismic Safety (CAPSS) buildings must reach these criteria and also be at least three stories and have at least five units.

A soft story building will remain a soft story building until the openings in the ground floor are strengthened. From a land-use perspective, such buildings are essential; from an earthquake-preparedness perspective, this can be your building's Achilles' heel, making it expensive to repair and vulnerable to severe damage in a quake.

A Weighty Issue

Before I explain why soft story buildings are so vulnerable in an earthquake, understand a little of how the ground floor of a multistory building helps to support the entire structure. A typical San Francisco three-story building has 3 inch by 6 inch wood studs around the perimeter of the ground floor. Each additional floor is supported by smaller wood studs, usually 2 inch by 4 inch.

While these structural members are able to support a static load-that is, the downward pressure of gravity on a building-they can't support the kind of sideward pressure that is so common in an earthquake. When sideward pressure-called lateral force-stresses a soft story building connections and joints can fail, causing the building to "pancake" or flatten. The damage can be exponentially worse when the building is built on a liquefaction area, such as the Marina, or if the building is located on a corner, where abutting buildings aren't available to provide additional bracing.

Partial and Full Retrofit

Seismic strengthening which is what the Board of Supervisors is considering requiring of soft story building owners, typically involves shoring up these connections with steel moment-resisting frames. Such frames allow buildings to transfer lateral weight more easily in the event of an earthquake, thus withstanding it better. But that's not what many owners have done to deal with the problem. Instead, they have addressed the problem with widely available hardware and shear walls.

The kink in this plan, of course, is that such fixes don't address a soft story building's weakest link: the large openings on the ground floor. To really solve this problem, you have to install steel moment-resisting frames on the ground level, usually connected to the building's existing framing. The moment-resisting frames are rigid steel frames that have been engineered to withstand a certain amount of lateral force and reinforce your building's existing structure.  Because many moment frames are prefabricated, installation can be done quick, with minimal inconvenience for the building residents, depending on what the ground floor is used for and the location of service and utilities to the building.

The Cost Conundrum

Of course, the reason many owners go with Band-Aid fixes is that they are less expensive, even if they aren't as effective and won't answer the city's strengthening requirements. And have no doubt: seismic strengthening can be costly. A 2010 economic study released by the city reported that the average cost of soft story strengthening is about $21,600 per unit for a five-unit building. That's more than $100,000 in total. As one of his last acts in office, former Mayor Gavin Newsom established the Earthquake Safety Implementation Committee, which has as part of its charge to find low-cost loans and other financial assistance for property owners targeted by CAPSS.

Bracing for the Big One

The good news is that the benefit to your building and insurance against future catastrophes far outweighs the cost of seismic retrofits. After all, your building is an investment and needs to be maintained in order to produce financial returns for a lifetime. When you bought it you probably never imagined that a repair in the tens of thousands would be part of that maintenance. But it's important to remember that the higher your property's value, the more essential the retrofits. Another way to look at it is that the risk of property value loss in an earthquake far outstrips the immediate perceived savings of not doing the work. One can purchase earthquake insurance to reduce loss exposure, of course, but that's not an appropriate substitute to seismically retrofitting a building and should be treated as a supplemental safeguard.

Of course, you won't see the benefit of seismic strengthening until an earthquake happens. And when it does happen, you're likely to be one of the lucky ones whose property value remains as strong as your buildings support.

Peter Iskander is a founder of Marina Seismic, a structural engineering and master builder construction company. Contact him on the web at www.MarinaSeismic.com