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Home design in earthquake country

by Steve McKee on May 30, 2004

It’s sort of a rite of passage as a Californian to experience your first earthquake. My twelve year old has one under her belt. It was the Napa quake of a few years ago and we both felt it because we were sitting still in the front room together. The house swayed a couple of times and I of course began to excitedly jabber on about what had just happened.

Though often just a brief and fun diversion, these quakes have the potential to get really big and become a life shattering event. Just ask the good people living in those big blocky stucco apartment buildings in Northridge and the San Fernando Valley ten years ago when a sizable earthquake hit.

A friend of my dad’s was in Anchorage in 1964 when hell broke loose. It was horrible, he says. The thing about the Alaska quake was that it was not only big (roughly twice as strong as the Bay Area quake of 1989) but it was long, terribly long, four minutes long. The houses were jerked back and forth. . . . . and it just kept going on and on. Most earthquakes are well under a minute.

In Benicia we’re close enough to major earthquake faults such as the San Andreas and the Hayward fault to be at risk for damage when “the big one” hits. We have our own more local faults such as the Concord – Green Valley fault, but they’re considered “minor faults” without as much risk for major movement.

When the Loma Prieta earthquake (“the pretty big one”) occurred in October of ’89, Benicia shook enough that our downtown fire department building got condemned, even though the earthquake was centered in a fairly distant piece of the San Andreas fault south of San Jose.

The good news about all of this earthquake stuff is that structures are now being built with some really good engineering in them. Engineers swarm over the damaged areas after a quake has hit to see what failed and why. One of the lessons learned from the Northridge quake of ‘94 was the importance of having really strong washers at the foundation bolts, among other things. That’s right, something as simple as making the washers on the bolts bigger and stronger would help many structures survive a quake like that one (which had an unusually destructive up and down motion compared to most earthquakes. This was due to the fact that the Northridge fault was a horizontal one, so that the pieces of earth sliding by each other created a sort of jack hammer motion.)

Over the years California designers and builders have gotten pretty good at dealing with seismic engineering. Framers have come to know earthquake hardware and techniques like builders elsewhere know hammers and nails. Words like “Shear nailing” and “stab bolts” and “HD5A’s” are the language of the California job site.

How houses resist earthquakes:

When a house is being framed and before it gets siding, it will get largely covered in plywood that is nailed on very tightly along the edges. This plywood turns a framed wall into a “shear wall” and it’s the heart and soul of earthquake design. The plywood prevents the wall from bending or racking in the direction parallel to the length of the wall. These plywood shear walls need to be helped by steel connectors designed to connect the ends of the shear walls to the foundations. When strategically placed in various locations around the house, these shear walls will take the “whiplash” energy that occurs in an earthquake and transfer it through these rigid wall connections into the ground.

What’s nifty about these shear walls is that after the house is finished, you don’t even know they’re there. They’re invisibly distributed throughout your house, disguised as regular walls.

I had someone tell me once he thought houses should sway to counteract the earthquake forces. Wrong! Sheetrock joints don’t do well in swaying houses. Windows don’t like the swaying motion either. When it comes to earthquakes, we want our houses to be rigid unwavering wooden rocks.

Local house mover Phil Joy told me about a builder friend of his who was at a job site when the Loma Prieta quake hit. The house was about half finished and had recently had its framing sheared with plywood, so the house did just fine. The builder happened to touch the edge of a shear wall and all the nails were very warm to the touch. The nails had absorbed the energy of the earthquake’s motion and transformed it into heat! Here was tactile proof you could feel as a warm glow at your fingertips of the energy transfer that I usually just encounter inside some long math problem. I love that story.

To engineer a house for earthquakes (or “lateral loads”) is to complete several pages of technical calculations based on a variety of factors, all mandated by building codes. Proximity to earthquake faults is factored in, as well as soil type, and many other fairly arcane elements. Because lateral loads include wind forces, a house is also examined for wind loads. The loads created by “worst case” wind storms are compared to the lateral loads created by earthquakes to see which one is stronger. Sometimes the wind force is deemed stronger, even in earthquake country. It has to do with how “thick” the house is, to put it very simply. (Extreme example to make the point: a billboard is more susceptible to wind force than earthquake force.)

With all this considered, the resulting force numbers are applied (on paper) sideways to the house and distributed into the various shear walls that are located throughout the house. If your architect had good foresight, there will be nice pieces of walls located at strategic places ready to become shear walls with the mere addition of a plywood skin and some earthquake bolts. If your architect or designer wasn’t so hot, there will be hair pulling and angst late in the design phase and maybe some extreme engineering to try and make the house comply with modern earthquake codes. I heard about one remodel where the homeowner learned after their drawings were completed that their vintage kitchen was going to need to be gutted to make way for shear plywood.

As a homeowner you shouldn’t have to learn about all this earthquake stuff in detail. You should at least know why you can’t have only windows along a face of the house. Having said that, I feel the need to tell you that there actually is a way to have all windows, but that it involves big bucks and a heavy duty steel framework that usually requires a crane to lift it into place. But that’s another story, for another day.


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