Is it in danger of collapsing? In a word: NO.
Based 5% on inside information and 95% on the laws of physics, San Francisco’s 58-story Millennium Tower is not in danger of tipping. In this age of overstimulated media, the rabid coverage of this issue has cast doubt in the minds of ordinary citizens about the competence of those of us who develop, design and build great things.
What really went wrong?
The building collapsed and is, in fact, tipping over. Most reports indicate that the building has settled about 17 inches and is tilting 14 inches to the west and 6 inches to the north at the top. The settlement is normal (more on that later), but what about the tilt? Let’s do some math: at the top, the horizontal displacement is 15.2 inches (hypotenuse of a 14:6 triangle), and the building is 645 feet tall, so the Millennium Tower tilts 0.11 degrees to the west-northwest.
What does this matter? At its most precarious, the famous Leaning Tower of Pisa was tilting about 5.5 degrees, but it has recently been stabilized to tilt “only” 4.0 degrees. Currently, its top is displaced by about 13 feet. The Tower of Pisa is only 183 feet tall; if the Millennium Tower tilted 4.0 degrees, its top would be displaced 45 feet! London’s Big Ben, which is 315 feet tall, is also tilted, but by only 0.26 degrees, or a little more than twice the tilt of the Millennium Tower.
It’s pretty mathematical, but it shows that the San Francisco tower is almost not leaning at all, especially compared to the Tower of Pisa (and you probably didn’t even know about Big Ben). Yet 60 Minutes showed some San Franciscans arguing that they can clearly see the tower leaning, almost ready to topple over, and advising others to stay away.
Unfortunately, they have fallen prey to a suggestion based on the inability to distinguish between an a priori truth and the opinion of any person who blurts out a tasty sound.
The story of the tower’s collapse was revealed by a suburban geologist hired by the homeowners’ association, a licensed practitioner, but one with no obvious expertise in the design of underground structures for tall buildings. He pointed out that the foundation piles do not go all the way down to bedrock, but instead rest on the sands and clay of the upper stratum. This may sound scary, especially when repeated by lawyers, but friction piles driven into strata above bedrock have been used successfully to support massive buildings in San Francisco for years.
Bedrock, at least in San Francisco, is overrated. The word itself suggests strong, reliable, and dependable attributes, and in many large cities bedrock must be destroyed with dynamite. But the Franciscan complex in the Bay Area can be removed with a pickaxe. The problem here is geological complexity; our bedrock is technically called a mélange by geologists, because it is a tangle of metamorphic rocks. There are all sorts of layers of different densities below the surface; the strata capable of supporting the weight of a building are sometimes close to the surface and sometimes very deep.
This geological complexity is the source of our amazing topography, and it requires sophisticated engineering. Bedrock is stronger than sand or clay, and for a tall building in general, fewer piles are needed when founded in bedrock rather than in the upper strata. The choice between fewer long piles and more short piles is usually dictated by economic considerations, but both systems have proven reliable.
Within days of the Millennium Tower deal being announced, marketers of nearby buildings under construction announced that their foundations rested solidly on bedrock. This is exaggeration: a huge structure can be just as stable with sand or clay foundations.
There is no question, however, that the Millennium Tower has sunk further than expected. Most foundations settle during construction as the site is loaded with the dead weight of the structure, and once occupied, a building can continue to move slightly – usually no one notices. In this case, something unusual has occurred, causing a larger than expected shift after the building is completed. The most likely explanation is a change in the water table.
Imagine that what lies beneath the surface in San Francisco looks like a piece of Swiss cheese, and the voids are filled with all sorts of things, including water. In fact, the cheese itself is more of a mixture of hard and soft materials, and there are many cracks through which water can migrate underground. Over the past decade, a construction boom has boosted San Francisco’s economy, and many large-scale projects have been built near the Millennium Tower.
Excavations for these projects may have caused a shift in the water table and overloading of the clay layers beneath the tower’s piles, resulting in greater than expected settlement. Most of the underground work near the Millennium Tower is now complete, and credible experts have concluded that the building is safe. Given the relatively small amount of displacement, the tower will not collapse.
Not even in an earthquake.
In fact, most Bay Area residents weren’t too concerned about THE BIG ONE until the New York Times published an article in April commemorating the 112th anniversary of our famous 1906 earthquake and fire. Following a pattern perfected by Fox News, the authors cited various sources to question the wisdom of allowing San Francisco to build towers, much less one like the Millennium Tower. In their view, San Francisco has taken a great seismic risk. The most disturbing quote is from a Caltech professor who said of our buildings, “It’s kind of like getting on a new airplane that was only designed on paper but no one has ever flown in it.” What could that possibly mean? Since the Gothic era, buildings have been “designed on paper” rather than through trial and error, and some of the most remarkable large-scale human creations owe their existence to mathematical abstractions that have resulted in a constructed reality. Designs are largely tested using computer models, which, by the way, is also how aircraft are designed today.
Since the days of the Barbary Coast, those of us who live on fault lines have developed coping mechanisms to deal with the possibility of a major earthquake in our lifetime. Some have left – most famously Enrico Caruso, who was here for the 1906 event and vowed never to return (and never did). Right now, oddsmakers estimate that a major earthquake of magnitude 6.7 or greater on one of our three active seismic faults has a 72% chance of occurring in the next 30 years. In the meantime, most of us will continue to enjoy our extraordinary urban scenery, which would not even exist without ground motions whose behavior is impossible to control.
See the article in French
