The high-rise mixed-use tower at 181 Fremont in downtown San Francisco houses Facebook’s new headquarters and residents of 55 multi-million-dollar condominiums, as well as a top floor $US42 million penthouse.
What is instantly recognisable about the tower is its encasement of large ivory-coloured beams zig zagged along its exterior before tapering on one side into a skyward spire. The aluminium exoskeleton serves as a giant shock absorber, improving the chances of the building’s survival in case of high winds and seismic events. As a result, the tower has come to be lauded as being invincible to earthquakes.
But risk and resilience engineer Ibrahim Almufti at Arup, the company that designed the tower, told Business Insider that’s not entirely true.
“Every time I read [earthquake-proof] I cringe,” Almufti said, a sentiment he said is echoed by most engineers.
Still, the high-rise might just be the tallest most resilient residential building in a seismic zone.
Here’s how it works:
The mixed-use tower is divided into two sections around the middle: above the V-shaped zone are the 55 condos, priced at or north of $US4 million each. Below is occupied by the tower’s sole commercial tenant, Facebook.
The ivory-coloured exoskeleton hugging the tower is architectural and purely for aesthetic purposes. Behind the facade is where the heavy-duty, earthquake-fighting hardware is.
Tucked away inside those zig-zagged beams are sets of three parallel steel braces that run diagonally up the building’s sides.
The braces make up the secret sauce to the building’s resilience against seismic activity. Within the outside braces of the sets of three are viscous dampers, which in this case are devices used to cushion the blow of a seismic event.
They act as giant shock absorbers to combat the impact of a rumbling earthquake and allow the tower some room to sway. They’re not unlike ones you would find on your car, except that they’re stories tall.
The braces in the V-shaped zone, where a Sky Lounge for residents is located, are also structurally equipped.
The braces here work to transfer the seismic forces from the residential levels above into the commercial levels below, since viscous dampers are only installed below the V-shaped zone.
The braces above the middle V-shaped zone that run through the resident floors are equipped with more conventional dampers.
Below the V-shaped zone, there are Teflon pads affixed to each point where the braces and a floor level intersect. These Teflon pads are tasked with preventing the braces from buckling out when stimulated by vibrations.
The braces are constantly moving and sliding against them, even outside of an earthquake. It’s not uncommon for tall structures to be in a constant state of swaying.
The engineers accounted for the amount of friction these pads can withstand with the steel braces constantly moving against them. Almufti said the risk of the pads being worn and deteriorated by that friction is low.
The building isn’t rectangular in shape.
Since the building tapers at the very top, that means every floor is different and so is each vertical angle of them, disrupting the building’s would-be rectangular shape.
Almufti said accounting for the non uniformity was a “nightmare:” They had to make sure the braces passed through each floor, while accounting for the floors’ different angles, so that the braces could run up and down the length of the building without obstruction.
Besides the exterior braces, there’s an internal core of them that you don’t see.
The building’s basement is five levels deep, with concrete pillars burrowing all the way into bedrock.
The San Andreas Fault, sketched out roughly below, runs 800 miles along the California coast, with some sections expanding 10 miles into the earth.
The San Andreas Fault Line is capable of an 8.0 earthquake, so the engineers were able to predict the impact it would have on a 800-foot tower and designed it with that in mind.
Their primary goal was to make it as easy and quick as possible for residents and occupants to move back in after an earthquake.
Considering the potential of the nearby San Andreas Fault, and the strengths of Bay Area winds, they developed a building that could be reoccupied almost immediately in the case of a “500-year earthquake,” or a quake that has a 10% chance of occurring over a 50-year lifetime.
It’s the more cataclysmic earthquakes, like ones whose magnitude is so great they would only occur every 500,000 years, that are impossible to account for.
“Designing for the worst isn’t probable,” Almufti said.