You have heard of the leaning Tower of Pisa, right? The structure is so famous that it is the ‘poster child’ for building construction failures. Everyone from physicists to geologists and structural engineers have studied the tower and its problems. They still haven’t straightened it, but they have managed to prevent any further leaning.
Unfortunately, Pisa’s tower isn’t the only tower in the world suffering from what Hackaday contributor and structural engineer Alex Weinberg refers to as ‘differential settlement’, a scenario in which a building settles unevenly. Because the settling is uneven, the building starts to lean. Like the Tower of Pisa, San Francisco’s Millennium Tower has fallen victim to differential settlement.
The powers that be in San Francisco are currently debating what should be done about the Millennium Tower. One suggestion is to employ a process known as slab jacking. However, slab jacking expert and Salt Lake City contractor The Concrete Raising Company says fixing the tower this way would be cost-prohibitive. Other options include building some sort of counterweight on the other side or doing nothing at all.
What Causes Differential Settling
Both Weinberg and the engineers at The Concrete Raising Company concur that settling is generally due to unstable soil beneath the structure in question. In the case of the Millennium Tower, it is built on reclaimed land that was once part of San Francisco Bay.
Architects chose to create the foundation via concrete friction piles. They decided against driving piles down to bedrock because, in that particular area of real estate, the piles would have been more than 200 feet deep. Friction piles seemed a more reasonable, cost-effective solution.
Unfortunately, the friction piles are driven into soil that is mainly clay. Anyone who knows anything about clay understands that it doesn’t deal with water well. Combine water content with a massively heavy structure like the Millennium Tower and you are going to get some settling. The tower’s main problem is that settling is uneven.
Settling Concrete Slabs
What is happening with the Millennium Tower is extreme but expected. Settling is a normal result of building construction. Even residential homes and concrete slabs settle over time. In fact, most never stop settling. It is just that the changes are so minor that they often go unnoticed by property owners.
According to the Concrete Raising Company, a concrete slab or a set of concrete steps settles for the same reason. Soil underneath gives way due to moisture content, poor compaction, or some other underlying issue. Uneven settling results in tilted structures and stress fractures.
Stress is perhaps the biggest concern for the Millennium Tower. It is not such a big deal for concrete slabs and steps. But again, you can repair a slab or a set of steps with slab jacking. It is fast, it is cost-effective, and it is fairly easy to do for experienced contractors.
Wait for Now
Weinberg says slab jacking is not cost effective for the Millennial Tower – at least for now. As for building a counterweight system, he says the only way to accomplish that is to build a comparable tower on the high side of the Millennium Tower. That is not feasible right now either.
What is his suggestion? To do nothing for now. The building doesn’t seem to be in serious trouble. Any observed stress damage is minor. Waiting offers the possibility that the tower will stop settling on its own or begin to settle on the other side. If and when the settling becomes a more serious issue, they can talk about slab jacking or some other repair.