sfmacaws
Sep 10, 2003, 9:48 PM
Post #5 of 6
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I'm not a contractor but I have lived in earthquake country most of my life. In general, masonry buildings are not where you want to be in a big one. I worked for many years in a 7 story concrete block building (SF Hall of Justice) that was built in the 1960's. It has actual rollers underneath it to allow the building to move with an earthquake. I was on the 7th floor during the Loma Prieta earthquake and let me tell you, that building does roll! Meanwhile, across the street a brick warehouse became immediate rubble killing several people on the street in front of it. I watched as it's outside wall peeled off like a wave breaking.
I spent the next week or so, driving and walking around the marina district of SF for 12 hours a day. This area was badly damaged in the quake. Most of the buildings are masonry with a single floor over a sometimes partially sunken garage. There were other reasons that this area sustained so much damage, it is built on sand dunes for one, but the type of construction also played a major role. With the back of the garage sunk into the ground and the front at grade level and a lot of nothing in between, the supports torqued during the quake. In effect, twisting the house off its support and often collapsing the living area into the garage.
We have used plywood to strengthen the corners of our house in the basement. Doing that and anchoring the house to the foundation is the first major thing you can do for any house in an earthquake zone. What you want is for the whole house to move with the quake but for the corners to be strong enough to prevent it from torquing and for the house to move with the foundation, not move off of it.
>>Is that the answer, making a foundation so strong and rigid that the ground slides underneath it? <<
I think that is what you DON'T want, if the foundation is too rigid and can't flex with the ground you will have more damage. If you think of your house as a ship on a wave, as long as the deck stays attached to the hull the whole thing can ride the wave. If it's anchored to the bottom, it will pull it apart. I don't think it is possible to build a house strong enough to resist an earthquake, the ticket is to build one that is supple enough to ride the quake.
The rollers built into the Hall of Justice were the latest technology in the 1960's. I don't know how they do it now but I know that it is the same principle still being used in the skyscrapers of downtown SF. Those suckers were whipping back and forth in the quake, I wouldn't have enjoyed the ride but they did come through it pretty well.
Jonna - Mérida, Yucatán
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