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View Full Version : How Many Foundation Bolts for Earthquake protection?



Brian Kent
07-08-2011, 10:55 AM
My relatives live in Riverside, CA - earthquake zone, USA. Their wood-frame house is bolted to the foundation. But I have never gotten a good answer to the question, "How many bolts (or earthquake straps) are enough?" I want to know in case I should recommend additional bolts or straps.

For those outside of earthquake zones, you can have huge amounts of destruction even if your home is bolted to the foundation, but being shaken off the foundation is often the difference between big repairs (10's of thousands of dollars) and total destruction (100's of thousands of dollars). Earthquake insurance can cost over $1,000 per year, with a $60,000 deductible.

I would love to find recommendations for best practices. The house passed inspection when purchased last year, but what is best?

Thanks, all.

Brian

David Helm
07-08-2011, 12:28 PM
Contact the AHJ where they live. The earthquake codes will be at that location. In my experience, though, also living in a major earthquake zone, the older houses that aren't as rigid as the newer ones handle earthquakes better. They actually flex and don't break.

Brian Kent
07-08-2011, 12:51 PM
I would love to do that, David. What's an AHJ?

Dan Hintz
07-08-2011, 1:07 PM
Authority Having Jurisdiction...

Brian Kent
07-08-2011, 3:02 PM
Thanks, Dan & David. I found the phone number to call.

Shawn Pixley
07-10-2011, 10:41 AM
Brian,

You should check but if i recall, the anchor bolts are required are 5/8" x 10" hook ends at 6'-0" O.C. There is a requirement on the washer plate, sill thickness, and edge clearance as well.

At some point the ductility of the bolt will be less limiting than the breakout of the concrete.

Joe Pelonio
07-10-2011, 4:54 PM
Don't forget about corner bracing.

When we lived near the Hayward fault I checked and found the foundation bolts to meet code, however there was no solid bracing in the corners. With a basement I was able to go ahead and install it using 1/2" plywood.

Greg Cuetara
07-14-2011, 9:57 AM
Brian,

As someone who designs anchor rods for a living I will tell you that there are prescriptive codes, those that the BCO, Building Code Official, will know about and will be able to give you the 'best practice' for your area. The other way to design is for the actual size of the structure, layout, weight etc. The latter is what I get into because I am looking at very large industrial equipment. I hope you were able to get ahold of the BCO or the AHJ because that is your best bet for residential construction. If you are really concerned you should hire an engineer to review the local requirements along with your specific house construction etc.

Greg

Joel Goodman
07-14-2011, 11:28 AM
+1 on the engineer. From what I've read earthquake evaluation and remediation (if that's the right word) for buildingsis not that simple -- even for the foundation bolting. I would have an engineer look at shear wall issues etc as well as make specific recommendations about the foundation bolting.