How Will Tall Buildings React In The Big One?
San Francisco’s Tall Buildings Safety Strategy puts forth 16 key recommendations that could improve the city’s readiness for a major earthquake. A recent talk at the San Francisco nonprofit SPUR explored what those suggestions are and how to make them happen.
In San Francisco’s Financial District tall buildings tower over you like a super-sized Jenga set. Will they be safe during an earthquake? The short answer: yes. But there’s a problem. How usable are they going to be afterward? Apparently, not very.
“The first thing to know is, by design, per the building code, buildings are intended to be broken and damaged,” says David Mar, a structural engineer who focuses on seismic and sustainable design. “And under a major earthquake, they're intended to be within an inch of their life and barely avoid collapsing.”
That’s how the current building code sets it up. Instead of a building collapsing onto you during an earthquake, it basically collapses onto itself in strategic places.
“It's analogous to a car crash,” Mar adds. “When you have a car crash, an automotive engineer does all these things to keep you alive, like your steering wheel collapses, and the engine drops, and you're in a safety cage, but your car is getting totaled. So your car is sacrificing itself for your life.”
Same thing with a wrecked building. It’s broken and gets deformed and bent out of shape.
“At the end of a maximum considered earthquake, you're going to have a building that's a teardown,” says Mar. “It's optimized for safety, not for continued use or anything like that.”
For the city, that’s a problem.
“We need people back in their homes within weeks, otherwise they're going to leave,” says Ayse Hortacsu, a structural engineer who was part of the team that wrote up the Tall Buildings Safety Strategy. “Same with a major employer that might be the occupant of a 40-story steel building in downtown. We need them to be able to reopen within weeks.”
San Francisco already has 156 buildings classified as tall — that’s about the height of a full-grown sequoia tree, according to the study. If the Big One strikes tomorrow, a lot of these buildings will become unusable. The city doesn’t require them to be structurally sustainable. They just have to meet the safety code.
“Technologically, it is possible to design them better,” says Hortacsu.
When it comes to designing buildings that won’t collapse, it’s all about redistributing the forces that travel through them during an earthquake. A common practice is to use shear walls and cross braces. Imagine two walls connected by a pair of diagonal beams that create an X. That X reinforces the strength of the two walls by sharing the load of an earthquake. It’s great for safety — but not so great for building sustainability.
“This is where the building first gets damaged, and it’s the hardest part to repair,” says Mar. “And when it gets damaged and the building deforms then you have a hard time, again, realigning elevators and things like that.”
Mar wants to design buildings better. He would add what’s called post-tension cables. Basically, they’re like giant rubber bands built into the cross braces. They’d put a lot less stress on the joints between the braces and walls.
“The whole idea is that during an earthquake, the walls will flex, stretch the rubber bands, the rubber bands will realign the building,” says Mar. “That’s one of the initial big keys that you have to get the building realigned.”
What about buildings that already exist? Well, you have to retrofit them, and that’s a hard sell for building owners because it isn’t cheap. But Mar thinks it’s just about sharing the right information.
“I think where we, as a design community, have really fallen short is that we're not telling anybody that their buildings, some of them are just crap,” says Mar. They’ll keep you safe in most earthquakes, in other words, but then they’re trashed. “We don't tell our clients that they're going to get beat up, and we don't tell them what's going to happen. As long as there's no earthquake, everything's cool, and then afterwards we're going to have an earthquake and everyone's going to be so pissed off at us.”
This all comes at a time when San Francisco’s skyline continues to be redrawn. At the moment, there are 14 tall buildings recently completed or still under construction. San Francisco Mayor London Breed wants the next batch of tall buildings to be better. She’s begun putting some of these new sustainability designs into the building code. And she wants to create a disaster recovery task force to help the city get back to normal as quickly as possible. But don’t worry — when there is an earthquake, it seems pretty clear that these towering Jenga sets won’t come tumbling down.