Saturday, April 11, 2009

Picking on High-Rises


I love the main author. He's done some great work, but this time he ventures into physics. The main contention is that next big Cascadia earthquake will be a high-rise breaker, mainly due to low frequencies.

I believe this author was the first to mention, what I consider a real high-rise breaker, and that's the high-velocity 'fling' pulse. These are known to crack the tall buildings. But now he is saying that a 4 minute low-frequency sinusoid can destroy high-rises.

This implies damage with a very low peak ground velocity (PGV). You'll notice in the figure that this earthquake is way the heck in the middle of nowhere town. Plus, it is a diving thrust, which can never produce any velocity. But a long rupture likes to put out the long ground motions. We can't forget that there will be one heck of a tsunami!

In order to simulate damage, he has to place an imaginary high-rise on an imaginary shake table. This allows the building to accumulate energy over 4 minutes, and with perfect resonance, the building will explode. But this is not reality, since high-rises have to be located on firm rock, or they tip over. Thus, there is radiative damping that nobody thinks about, so the building is not a perfect resonator. As long as you are on rock, there will not be any damage.

Medium-rise concrete buildings, on raft foundations, on a giant swamp are at risk. This is what happened in Mexico City.

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