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'Sinking' Tower Fix: Let It Be?

Wednesday, September 21, 2016

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The Millennium Tower of San Francisco has been sparking debate all over since it was reported in August that it’s been sinking and tilting at a rate greater than ever expected. But one professor of architecture has a theory about the building that comes with a pretty simple fix.

On his blog last week, as picked up by Curbed San Francisco, Iowa State University’s Thomas Leslie (who, he admits in the piece, has not been to the site in person) offers a quick analysis of what seems to be the problem, and proposes a few fixes, including doing nothing.

The Problem

Leslie, a professor of architecture at Iowa State University, begins by noting that the tower is built on friction piles, a preferred anchoring technique that should render moot any arguments that the building needs to be anchored into bedrock. "This has always been a standard technique for building in liquid soil, and it’s why coastal construction always comes with the dulcet tones of a pile driver," he notes on his Architecturefarm blog.

Millennium Tower
By Daniel Ramirez – CC-BY 2.0, via Flickr

“As long as the settlement is slow," Leslie writes, "it may be that the building is entirely habitable for generations before things get too far out of hand.”

“This looks like it could be a classic vertical vs. rotational equilibrium problem,” Leslie writes. “While the piles under Millennium Tower are designed to resist the translational load of the tower against gravity, they’re not in the most efficient place to resist rotation. So the lean could have something to do with this.”

Leslie bases his theory in part on a case study on the building’s engineering and construction, published by the Concrete Reinforcing Steel Institute. When it was completed eight years ago, the Millennium Tower won awards for its reinforced-concrete design.

Possible Solutions

What can be done? Leslie’s preferred option, though it may not be possible, involves building another building next door, to help stabilize the building—he gives the example of Chicago’s Fisher Building, built in 1896, which saw a next-door addition built in 1907 to help keep it from leaning. The Millennium Tower, though, is in a densely built-up neighborhood, and the under-construction Transbay Center is said by some to have contributed to the problem.

There are other options, Leslie notes: Adding piles below parts of the building that need to be reinforced, or adding concrete into the soil itself, though he admits that may not be possible either.

His most novel idea, perhaps, is simply letting the building stand. (It’s not likely to go over well with residents who are already suing the building’s developers, though it’s also not completely original—28 percent of D+D readers suggested doing nothing, at least for now, in a poll on the topic last month.)

“It’s possible that doing nothing is the best option,” Leslie writes. “As long as the settlement is slow, it may be that the building is entirely habitable for generations before things get too far out of hand.”


Tagged categories: Architecture; Asia Pacific; Building design; Building Envelope; concrete; Condominiums/High-Rise Residential; EMEA (Europe, Middle East and Africa); Latin America; North America; Safety

Comment from peter gibson, (9/21/2016, 11:02 AM)

Frankly,nothing can be done. Many of the tenants are lawyers,so you can be sure they will sue. Leslie is full of ideas that don't amount to anything.

Comment from M. Halliwell, (9/22/2016, 10:57 AM)

The basic question in this matter is: does anything actually need to be done at this point? Probably not. One of my classes in university was in a steel reinforced concrete building where the floors were cantilevered out from a central core. To make a point (it was a structural class), the professor had us remain seated as the classes started to change. We could feel the deflection of the floor (as a bouncing sensation through our seats) as the masses of people worked their way from other classes toward the stairs and elevator area. We could feel it and it was a little un-nerving, but the building was structurally sound and perfectly safe. Is an additional 10" of vertical settlement or a 2" tilt un-nerving? Sure is and it might make it hair harder to hang my pictures straight. Does it mean the building is unsafe or structurally compromised? I don't have the information to say for sure, but chances are the answer is "no" at this point, otherwise the residents wouldn't be allowed to remain. Doing nothing and keeping an eye on it (monitoring it) may very well be the best option for the time being.

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