An engineer’s short, simple explanation of Millennium Tower problem

Anyone in San Francisco who has even glanced at the news lately knows three things about the Millennium Tower: That it’s sinking and tilting, that its foundations do not extend to bedrock, and that it’s fantastically expensive.

(Everybody knew the latter point already.)

The next question is what kind of design plants a 58-story tower on sand?

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.

Understanding Construction

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.

Although we laypersons may prefer the sense of security generated by a word like "bedrock"—and, to be fair, the state of the building might be lending some credibility to our prejudices—in theory it’s a perfectly acceptable way to secure a building, and it’s worked pretty well for generations.

So why isn’t it working this time? Leslie guesses that the problem may be rotation: The building is in equilibrium with forces driving it down into the earth, but maybe not with the natural torque encouraging it to spin.

Leslie, of course, is just one academic, who cautions that he hasn’t even been to the building in question, and his educated guess could be wrong. Since his explanation was so accessible we decided to highlight it, but note that other engineers or designers may tell you differently.

And now the billion-dollar question: How do you fix it? Leslie pitches several ideas, including one that local scuttle has already deemed mostly impractical (reinforcing the soil under the building with concrete) and one that’s neat but probably impossible on this site (putting a heavier building nearby to balance the Millennium’s lean).

Both the most practical and radical idea: do nothing. If we eventually conclude that the building isn’t in any real danger (as Millennium Partners has insisted all along), it could stand at least as long as any other high-rise.

Pacifying the homeowners would be a feat perhaps beyond the means of mere engineering.

• Floating Foundations Out West [Architecture Farm]
• Leaning Tower of San Francisco [New York Times]
• Millennium Tower Case Study [CRSI]
• Comparison Of Friction Piles [Norwegian UST]
• Rotational Equilibrium [Physics Lab]