San Francisco Has Its Very Own Sinking, Leaning Tower


San Francisco's Leaning Skyscraper Raises Alarm AP News
San Francisco's Leaning Skyscraper Raises Alarm AP News

It can be tough when you're buying a multimillion-dollar luxury condo in the heart of a thriving city and something goes wrong. Like maybe the stainless steel appliances aren't smudge-proof. Or perhaps the floor-to-ceiling windows only get western exposure. Or the entire 58-story building is sinking not-so-slowly into the cushiony, soft soil of the San Francisco Bay.

Many of us may not know the ins and outs of luxury home buying, but most of us can at least relate to not wanting our house to fall into a sinking abyss. And naturally, in a city like San Francisco where earthquakes are not just a possibility but an extreme susceptibility, people are really quite wary of wobbly buildings.

While it's going to take a trip to court to decide if the occupants of the Millennium Tower deserve recompense for the lowered housing values that come with a building that sunk, we're here to talk about the nitty-gritty: How the heck can a building sink 16 inches (41 centimeters) since 2009? AP News reports that it's leaning 2 inches (5 centimeters) at the base, and even more at the top.

Peter Mackenzie-Helnwein is a research associate professor of structural engineering and mechanics at the University of Washington. Mackenzie-Helnwein says any structure will add load to the soils it's built on, thus causing settlement (the preferred term over "oh no, oh no, oh no, we're sinking"). While an evenly dispersed settlement is not a huge issue, "if settlement varies greatly over an area, that would cause stress on the structure," Mackenzie-Helnwein says in an email. He points out that a differential settlement might be "large enough to tilt the building and thus destabilize it." Think the Leaning Tower of Pisa.

And the truth is that quite a few things can be done to mitigate building on "soft" soils. There are many soil improvement strategies, says Mackenzie-Helnwein, including "pre-compaction, adding gravel columns, adding steel or concrete piles to reach deeper, stronger soils, etc. In addition, we choose different types of foundation. Last but not least, slabs allow the building to even out uneven settlement."

Who exactly should be raising the red flag and telling everybody that larger-than-expected settlement might be an issue? Well, it's not easy to pinpoint one group who's responsible for an unstable building, and Mackenzie-Helnwein stresses he doesn't have intimate knowledge of the Millennium Tower project or the roles played. But generally speaking, "most likely it's the geotechnical engineer who will raise the first flag, if there are any. That flag goes to the structural design team who may or may not be able to mitigate any issue within the financial constraints of the project," Mackenzie-Helnwein says. From there, it's the building's owner's responsibility to decide how much money to — ahem — sink into the problem.

It's not that settlement is always terrible; adjusting a few utility connections or entry doors isn't a huge deal. But a big building with a lot of space to settle differently or create tilt can be a big problem. While Mackenzie-Helnwein can't speak to the future of the Millennium Tower building without a full settlement history, he does say that the settlement and lean of the building is something to closely monitor.

So when you're looking to buy your multimillion-dollar high-rise condo, inspect the paint swatches carefully, squint at the view to make sure you've got the one you want and perhaps check in with the engineers, just for good measure.