What Seismic Elements Should I Be Concerned About?
What’s This About Earthquakes?
Buying property in the Bay Area is not only an expensive endeavor but one that inherently implicates earthquake concerns when you’re thinking about protecting your safety and investment. This is one of those situations where each property presents its own set of unique characteristics that it’s hard to generalize what’s right and what’s not so right. Because are so many issues that come along with how a sound a property is when the ground shakes, if you really want to see that a property is up to par, you’re best to consult a licensed professional structural engineer (or two) and get some professional opinions. That said, we’ve been along for so many property inspections and have seen so many examples of what can, should and must be done to buildings that we wanted to at least give a high-level view of what to look out for. Here are a few questions and points to consider.
Of Bricks and Soil…
Are You On The List?
SF has a rolling program whereby buildings are being upgraded to make it through earthquakes so that people will have enough time to evacuate a building safely — this is different than surviving an earthquake without any damage. Brick buildings (ones with unreinforced masonry foundations) had a mandatory seismic retrofit upgrade program in the early- to mid-200s. Brick foundations were either replaced with modern concrete and steel rebar foundations, encapsulated with concrete, or framed with wood with steel ties being inserted). With most of those multiple-unit buildings being complete there are still single-family homes and 2-unit buildings that didn’t get upgraded. Now the program is focused on 5-unit buildings with ‘soft-story’ foundations, i.e. 5 unit buildings with 2 floors above street-level garages. That program is now having folks add steel header beams, new steel posts and moment frames (see below);
Shaky Ground or Solid as Bedrock?
Location matters in this case more than usual. SF was developed over time east to west with lots of areas being filled in — inlets drained (Mission Bay), streams being buried or forced to go underground (Mission Dolores) or bays being filled up with trash (the Marina). Thus, the main concern you want to be worried about (at least at first glance) is if the property sits atop a liquefaction zone which means that either the soil itself will act like a liquid if the ground started shaking or that water will literally slosh around with the fill which both will compromise an otherwise rigid foundation causing a foundation to fail.
A Matter of Time…
A Newer Vintage May Be Better in This Case
This is the most important question. If your building was built since the 1990s you’re much better off than those built in the 1890s. The City took a much more aggressive stance on seismic preparedness after the 1994 Loma Prieta quake and 1995 Kobe earthquake. Also the hope is that the newer 20+ unit condo buildings (despite all their ills that NIMBYs may espouse) have the most modern engineering elements built into them — literally. Foundations sit atop a field of metal pylons that have been literally pounded into the ground so that the building’s foundation will ultimately sits on bedrock (that thunk, thunk thunk soundtrack in Mission Bay during the mid-2000s and early 2010s? That was actually that sound of those pylons being driven down to the bedrock). Sometimes developers take shortcuts (see the Millennium Tower for example) but most usually do go through this step as it’s the right thing to do.
A Matter of Time…
A Few Of These Things May Bolster Your Property’s Position (Literally)
Seismic structural elements have been evolving just as engineering knowledge and building material technology has developed with smarter polymers, steel and construction technology have come a long way since the diagonal wood studs you’ll see in buildings dating from the early 1900s. Anyway here are some things that should be on your check list or your to-do list if you’re upgrading:
- A Moment Frame (or two). These are retrofit frames of concrete and rebar that run perpendicular to the building’s main spine (the main beam which tends to be made up of old-growth wood that is denser and sturdier than younger lumber stock). The moment frame provides rigidity to support a building’s structural integrity against lateral shaking during an earthquake. Usually new steel, wood or engineered wood ceiling beams are placed at a 90-degree angle to the building’s main beam secured to that beam while spanning the width of a garage. The beam(s) are held up by new posts that are sunk into new concrete-base posts or the beam is connected and embedded into walls that connect with the perimeter’s foundation elements
- Metal Straps for Wood Support Beams and Joist Hangers. If you don’t have to replace wooden posts in garages or foundations completely your engineer may recommend that the connection to the ground (not the slab) sit in its own concrete pedestal. Contrary to looks, the concrete slab doesn’t have as much to do with soundness than the perimeter walls and moment frames.
- Mudsill-Concrete Anchor Bolts. This system works by having a contractor drill a hole through the very last horizontal wood framing element into the concrete exterior foundation wall. The hole gets filled with epoxy and a bolt and washer is drilled through the wood into the hole with epoxy with it all being screwed down with an air hammer. These bolts are added every 18 inches (thereabouts) all around the foundation. After it’s all said and done inspectors may go around and randomly try to unscrew those bolts with high-psi air hammers.
- A Finished Plywood/Sheetrock Garage/Foundation Area. After the bolting is done then exposed wood framing is covered up with plywood and/or sheetrock that adds just another layer of rigidity.
- Steel Rebar Concrete Foundations. New construction or replacement sections of old foundations should have steel rebar framework inside newer and better concrete formulations.
- Rebuilt Foundations (either partial or complete). Like filling in a decayed cavity, progressive maintenance will see parts of foundations replaced over time whereby the mudsill-connection to the foundation is raised up and new concrete poured in so that the mudsill sits above grade.
- French Drain Systems with Sump Pumps. While a home’s surrounding grade is supposed to be sloped away from its foundation if runoff water can’t flow away fast enough, some properties need a buried PVC drain/catch system that’s ultimately aided by a sump pump, which will pump water into the City’s wastewater system if not elsewhere.
- Steel header beams over garages and main beams. Over garage doors especially with reinforced posts and framing around as garages are the weakest structural parts of a foundation.
- Wood posts with their own rebar-and-concrete footings that are strapped and/or bolted to that footing. A combination of c above.
- Engineered wood. While younger wood, by itself, is not as dense and ‘solid’ as their old redwood growth counterparts. But if that wood is engineered then we have something else to consider. This happens when multiple layers of wood are heated, layered on top of each other, glued and compressed. That or when the strands (grains) are oriented in concert with each other. This type of material is denser and may be preferred to steel beams in a fire as steel can get all molten and droopy.
While that’s not a complete list of everything that’s possible it should give you a good start. Contact us if you need recommendations for structural engineers or contractors.