Today’s post is going to be a short add-on to yesterday’s post about the sinking Millennium Tower in San Francisco. Today, the New York Times published the below map showing the areas of the city likely to “liquefy in an earthquake.” It goes on to note that “at least 100 buildings taller than 240 feet were built in areas that have a “very high” chance of liquefaction.”
The article might leave you with the feeling that current building codes are inadequate for the pending “Big One” in San Francisco. So I thought I would reblog this post from last fall which talks, in more detail, about how one of the best structural engineering firms in the world designed the tallest building in San Francisco.
Image: New York Times
Many of you are probably aware of the 58-storey Millennium Tower in San Francisco which is estimated to have sunk about 17 inches and to have tilted about 14 inches to the west since it was built.
Well today it was announced that they may have a fix. Here is what is apparently being proposed as a retrofit (image from SFGate):
The tower was originally built on top of a 10 foot thick raft or mat foundation, which was then supported by concrete piles that went down 60-90 feet into soft clay. Notably, the piles didn’t reach bedrock.
The proposed solution involves drilling 275-300 new micropiles into the bedrock below. But here’s where things get really interesting: The plan is to stabilize the west side of the building first and allow the east side of the building to continue sinking. In theory, this will give the building an opportunity to level out before they fully stabilize it.
According to SFGate, the entire retrofit is expected to take anywhere from 2 to 5 years, and cost somewhere in the range of $200 to $500 million. The original tower cost $350 million to build. (I’m assuming that’s just the hard cost number.)