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.)
Take a look at this photo from Central in Hong Kong and note the MEP (mechanical, electrical, and plumbing) systems running up the cutout in the middle of the tower. Also note the bamboo scaffolding next door and the epic terrace on top of the ground floor bank. The building on the opposite side put parking on top of its podium.
I was told that systems are commonly run on the outside of buildings here to maximize interior square footage. Again, space is a precious commodity in this city. But it also speaks to not having to worry about freeze-thaw cycles. Winter in Hong Kong has so far equaled me walking around in a t-shirt.
These exposed systems look ugly as all hell, but I suppose they also mean not having to break open drywall when you have a problem.
I am fascinated by the relationship that buildings have between interior and exterior space. In cold cities like Toronto we are forced to hermetically seal off our buildings from the elements. We have to worry about thermal bridging and about heat tracing cold spots.
But in a city like Hong Kong I would imagine that the greater concern is stifling heat and humidity. All of this comes through in the built form.
Alexis C. Madrigal recently published a piece about the Salesforce Tower in San Francisco called: The Tower at the Heart of the Tech Boom. At 61 floors and 1,070 feet, it is now the tallest building in San Francisco and the second tallest building west of the Mississippi River after the Wilshire Grand Center in Los Angeles.
Hines and Boston Properties are the developers of the building. Pelli Clarke Pelli is the architect. And Salesforce is the anchor tenant. In April 2014, it was announced that they had leased 714,000 sf on floors 1, 3-30, and 61. (Get that top floor.) So almost half of the building.
Perhaps not surprisingly, Madrigal calls the Salesforce Tower the “the most visible monument to the industry [tech] in the region and the country.” It is a demonstration of the power and reach of Silicon Valley. San Francisco has a new symbol. The TransAmerica Pyramid now feels inadequate.
Though interesting, this is actually not what I want to talk about today. I’d like to talk about what it took to build such a tall building in a seismically active city like San Francisco. Unfortunately, this feels timely given that the sinking Millennium Tower is getting so much attention right now.
The structural engineer for the Salesforce tower is Seattle-based Magnusson Klemencic Associates (MKA). They are a world-renowned structural and civil engineering firm that have been around since the 1920s. Other projects they are currently working on include the third tallest building in Chicago.
The tower’s seismic force-resisting system is made up of reinforced concrete shear walls that surround the central elevator and exit stair core. These walls are 24 to 48 inches thick. Here is a plan taken from a STRUCTURE Magazine post written by Ron Klemencic of MKA:
The tower’s foundations have been well documented, or at least frequently mentioned, because of how deep they had to go down. The site has poor soil conditions (fill, sand, San Francisco “old bay clay”, and weak bedrock), and so given the weight of the tower the only option was to go down to bedrock – approximately 250 feet below grade.
The foundation system they ended up going with uses something called Load-Bearing Elements (LBEs). The typical LBE measures 5′ x 10.5′. The entire foundation system uses 42 LBEs and a mat foundation that varies in thickness from 14′ around the core to 5′ around the perimeter. (See image below.) The LBEs were brought down to rock. And in some cases, they went down more than 310 feet below grade.
As a condition of buying the site, the Transbay Joint Powers Authority required proof that any future tall building would not negatively impact the surrounding structures – including the adjacent Transbay Transit Center – and that it would perform under a Maximum Considered Earthquake (MCE) event.
So while the tower itself may be a symbol for the new world, its structural system also achieves many firsts in terms of how to build a supertall in a seismically active region.
Please keep in mind that I am not a structural engineer. I just pretend to be an architect sometimes. If you’re interested in more of the details, check out the post by Ron Klemencic. All of the above information was taken from there.
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