Sunday, March 27, 2016

Everybody should do seismic noise tests for your house

I'm on the verge of my biggest breakthrough yet in the Mad Labs.  But first, some background.  I have said before that the soil foundation dominates seismic hazard for your house over being in California or Oklahoma.  You can be on the solid footwall of the worst earthquake California has to offer, and only be woken up by the sounds of transformers exploding down in the valley.

Similarly you can be on quicksand like High Park in a soft brick house, and have the worst risk of death in the world, even in staid old Toronto.  How do you know?  By doing a seismic noise analysis.

My new Raspberry Pi 3 will have 4 accelerometer chips chained up like a seismic array, and it will be good enough for a seismic noise test.  Basically, you run it in your house during a work day, but without people and pets stomping around.  Then you run it in your basement, or on a slab outside.  You only need an hour or so.

Then you calculate the power spectra of the noise.  A good house or concrete building will be fairly stiff at maybe 5 Hz.  So you should see a 5 Hz peak.  On the basement you should ideally see no peak, that would be very hard ground, and we are assuming the daily noise is flat.  A very bad case is that your building and ground peak are strong at 1-2 Hz.  Then you better glue down everything and get earthquake insurance.

The big mistake of earthquake engineering is that everything is sinusoidal and matching frequencies cause resonance.  Not true if you look at the physics and simulate everything with finite differences.  Then you find that matching frequencies makes one big bullwhip, with your house at snap end.  Not good.

I'll soon have it set up and try it on my house.  That's all I can do, since I live in the basement.  It will up to others to do a lot and publish the results.  The next big earthquake is the test.

ps. here's a picture with all 4 accelerometers wired up.

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