Commonnly vulnerable SF building types.
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Liquefaction (red) and landslides (yellow)
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Editor's Note: This is one of many stories about earthquakes past and future that San Francisco is publishing over the coming weeks, all part of our October cover package, "Cracks in the Earth." To see the rest of the issue's contents, and to read stories as they become available online, click here.
Though the most immediate and common danger in a quake is flying and falling objects (remember, duck and cover!), you’d no doubt also prefer to avoid being crushed—or coming home to a flattened domicile. Take these steps to assess the soundness of your home, office, grocery store, school, or church, to find out whether it stands on sturdy ground, and to keep it from falling down.
Step One: Identify Your Building
The five most common structures most susceptible to earthquake damage in San Francisco
1. Wood-framed soft-story
Wood-framed buildings are the most common residential structures in San Francisco, and many of them come with a soft story—a first-floor garage or storefront that provides little load-bearing support—which means that they could go from four stories to three in an instant.
2. Pre-1980 concrete building
Before 1980, concrete frames were more brittle— what engineers refer to as nonductile—tending to snap rather than bend in a quake. They’re common among old apartment buildings. Finally, some good news for the non-rent-controlled tenant—your building is (probably) in the clear!
3. Concrete tilt-up
Formed by piecing together enormous precast concrete panels, these structures are like large-scale Ikea furniture—cheap and easy, but with shaky support. If one slab of the building falls during a quake, so can the rest of the structure. The technique is often used for retail stores and warehouses, like those in SoMa.
4. Pre-1994 welded steel moment-resisting frame
The Northridge Quake of ’94 in Los Angeles taught us that certain procedures used to weld together steel beams and columns are not seismically adequate. Collapse is unlikely, but a serious tremor could crack the welding—not a cheap fix for the owner.
5. Unreinforced masonry
Walls made of brick or other masonry materials can be brittle (see: Downtown Napa). The state passed retrofitting legislation in 1986, but some structures lacking the finances to retrofit, like churches, filed for exemptions. And even when masonry buildings are reinforced, a big enough shake could still peel off bits. so beware of falling bricks!
Step Two: Know the Terrain
Mapping the City's Shakiest Ground
This phenomenon occurs when loose, sandy soil, saturated with water, responds to applied stress by losing its shear strength, causing it to behave like a liquid. if your home or office sits on sandy or otherwise granular soil exposed to water—like the old marshlands under SoMa and the Mission or the Bay-lined landfill beneath the Marina—proceed to step three. (See map in slide two above.)
Anytime you build something on sloping ground, the risk of a landslide is increased. and, as we all know, San Francisco is riddled with hills. The steeper the slope, like those on Twin Peaks or Telegraph Hill, the higher the likelihood of a slide. (See map in slide two above.)
Step Three: Fix It or Move
How to Remedy Your Seismic Circumstances.
If You're a Tenant... First, ask your landlord to detail recent retrofitting work. If he dodges your questions, you have a few options. You can look up past permits for construction and retrofitting of your building on the Department of Building Inspection’s website to see if preventive measures have been taken. Or you can simply call the DBI (415-558-6088) and leave an anonymous tip to ensure an inspection. For those living or working in a soft-story complex, the new mandatory soft story retrofit ordinance mandates that owners of buildings with three-plus stories and five-plus units that were permitted before 1978 must have submitted an assessment of the building’s seismic stability, completed by a licensed architect or engineer, to the DBI last month. Wwners who failed to do so will be slapped with a notice of violation that could lead to a whole slew of fees. But there’s a catch: if your landlord does retrofit, he or she could hike up your rent to mitigate its cost—sticking you and your fellow tenants with the bill. (You can apply for financial hardship status through the San Francisco Rent Board.)
If You’re an Owner... Due to building inspections during the purchasing process, you should already have a lot of information about your home. But if you notice any red flags (like unreinforced load-bearing walls or a brick foundation), it’s time to hire a civil engineer or architect to inspect your place. Good news: There are a lot of them. Bad news: you’re going to have to do some research. For recommendations, ask contractors with whom you’ve worked in the past— they’ll likely know architects they can vouch for—or visit the Structural Engineers Association of Northern California’s website. As for budget, don’t expect to skimp. A small residence could cost around $10,000 to quake-proof, and some of the structures falling within the city’s Soft Story Ordinance Program are looking at closer to $100,000. It’s a hefty price tag, but one that you’ll appreciate when your house doesn’t turn to rubble.
Originally published in the October issue of San Francisco