An engineer’s short, simple explanation of Millennium Tower problem
http://sf.curbed.com/2016/9/16/12945600/why-millennium-tower-sinkingSEP 16, 2016
...Well, intrepid Iowa State University architectural design professor Tom Leslie (who noticed the building’s woes after reading the recent New York Times story) has now explained it via the simplest analogy possible, that of a mere stick.
The Concrete Steel Reinforcement Institute’s case study of the building (the Millennium is a concrete-framed building, rather than the more popular steel designs used in most San Francisco buildings) says that the Millennium’s foundations rest on 950 friction piles.
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A friction pile is simply a long, cylinder-shaped construction shoved into the ground, with the weight of the building on top of it. The Millennium Tower piles are roughly fourteen inches to a side—which may not sound very big relative to the frame, but remember that there are almost 1,000 of them.
This Norwegian University of Science and Technology paper explains that if the piles aren’t long enough to touch bedrock, they rely on the friction of the surrounding soil for support (hence the name). What does that mean? Leslie’s stick explains it all:
"Imagine driving a broomstick into sand. You can only go so far before there’s enough broomstick in contact with the sand to put up fearsome resistance." Eventually, even all of the pressure you can muster won’t push the stick in any further.
And that’s the principle that’s supposed to keep the building in place: Its own weight (14,000 pounds per square foot, according to CSRI), distributed over the 950 piles, contested by the resistance of the soil against those piles, all sitting in a big, giant, concrete-framed equilibrium....
