Reply to post: Re: Not the developer's fault.

San Francisco's sinking luxury Millennium Tower: Tilt spotted FROM SPACE

MNGrrrl

Re: Not the developer's fault.

> There seems to be a lot of ignorance here about the effects of different terrains on earthquake damage. The fact is that a thick layer of soil (or landfill) between bedrock and the buildings does not "cushion" the waves, it amplifies them.

This is why I generally shake my head and mutter whenever someone from Southern California tries to sound edumucated. The other 47 states on the continent keep praying you guys fall into the ocean so as to rid us of your stupidity, but the geologists keep telling us it's going to take too long. Sigh. So here we go again, from the top:

*** There is no bedrock in the San Francisco bay area. ***

************** 404: GEOLOGY NOT FOUND. **************

Bedrock sounds like a really nice, reassuring word, and of course everyone wants to know why every building doesn't simply drill down to bedrock. Well, there's a simple engineering answer: It's not necessary. Foundations in a building serve only one purpose: Load spreading. And the method used for constructing the tower is a proven one: Friction piles. To understand how they work, take the wooden handle of a broom and push it into the ground. Hammer it in, if you want. You're not going to get very far down, and that's just dealing with the topsoil and clay in your backyard. Friction piles operate on the same principle: They're just a lot bigger. They will easily support the weight of many, many tons without moving, because the static friction along the sides keeps it from sinking. That's how the Millenium Tower was built, and it's solid engineering practice. And there is zero risk of liquifaction here: The foundation is irrelevant, the geology of the site isn't suseptible to it unless you have an earthquake of a magnitude that would wipe out pretty much everything except a military bunker.

I have said it before, and I will say it again: The design is solid, and the site survey was done properly and to best practices. What was *NOT* done correctly was the dewatering of the whole area, which radically changed the friction and compressibility of the soil under the tower, and that's what has caused it to sink and tilt. There is no disagreement about this amongst engineers. You can't expect the engineers to have anticipated that, after construction, the next door neighbors would fuck up the geology under the building like this: Their dewatering has dropped the water table by over 20 feet. The piles that previously were in the water table are now sitting above it. Water tables do not simply drop by 20 feet in a coastal area in the span of a few months. It's not a natural process.

So please, stop acting like you know something about engineering: You don't. And as far as the waves being amplified: Learn the difference between primary and secondary waves, because one induces compressive load and the other shear load. If you build down to bedrock, your piles need to be able to withstand both as first-order effects. If you don't, the piles only need to worry about compressive load because there won't really be any sheering... the building will move with the earth underneath it (which it wouldn't, if it were nailed to the bedrock). Though in practice, all of this simply means tweaking your concrete mixture... the design will be the same.

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