Monday, August 17, 2015

Earthquake Stupidity #4 -- Shake Tables

As an early warning system is to a seismologist, so a shake table is to an engineer.  In fact, there is holy war on shake tables, getting bigger and bigger ones.  It's not enough that they can get an entire house on one, we look forward to entire towns!

So, why are we all into shake tables?  A long time ago there was an earthquake in Venezuela that affected some really bad buildings on soft soil, but as usual, the PGA wasn't that high.  Ergo, the fault was not in the bad soil,  but rather that the buildings had resonated like a porch swing.  Engineers went nuts with this resonance thing, introducing modal analysis and response spectra.  They could fling model buildings on shake tables and make them do the shimmy-shake.  It was obvious that resonance was the worst thing ever.

Never mind that PGV on soft basins is amplified a 100 times, they applied response spectra to every building.  I remember those days, seismic analysis used up most of the computing power of the corporation.  It was magnificent.

And it was all make-believe.  People studied hundreds of earthquakes with structures on average soil, and there was never any sign of resonance.  It never existed.   But when you bolted a springy structural model onto a solid steel slab, you could vary the frequencies on your shake table and get lots of resonance.

If you do a real analysis of a reasonable building on a reasonable foundation, you find the seismic energy goes in and then goes out again.  I used finite-difference computer models with realistic time histories and it was always the same.  There is no resonance.  There's a factor of two for the top, but that's the same for the free surface, and the building is on a foundation that has a lower initial PGV.

To get on my top stupidity list, you have to kill people in the next earthquake.  This is linked with PGA because they scale the shake tables based on PGA.  Buildings do not pancake any more, they tip over, thanks to shake table analysis.  All such buildings should have a sign:  "Designed on a shake table and built proudly on soft soil.".  Then all your furniture should be made of foam.  No books, and not too much booze in glass bottles.  Appliances should all be built-in, then you are good to go!  You can survive a tip-over.