A new model for resilient skyscrapers
Designers and clients are increasingly aware of the need to create buildings and infrastructure to withstand future shocks. A new tower in San Francisco — “probably the most resilient tall building on the West Coast,” according to Arup structural engineer Ibrahim Almufti — demonstrates one way that this is playing out in practice.
Business as usual?
181 Fremont, an 800ft mixed-use building currently rising in San Francisco’s SoMa neighborhood, will be subjected to strong wind and earthquakes. The typical engineering design solutions to deal with these two forces are often at loggerheads.
One way to reduce wind vibrations is to stiffen the structure by adding more steel. This is less than ideal from the perspective of seismic performance (not to mention cost and sustainability) because stiffer buildings vibrate more in earthquakes.
An alternative method involves incorporating a tuned mass damper (TMD) on or near the top floor. Essentially an 800-ton ball of steel, it counteracts forces acting on the structure by shifting in the opposite direction during a windstorm. By reducing perceptible vibrations, it keeps building occupants comfortable on blustery days.
But TMDs also have drawbacks. They’re expensive and require developers to sacrifice valuable penthouse real estate, and they don’t reliably improve seismic performance.
In most of California, seismicity is a bigger concern than wind. But while all structural designs that comply with the state’s building codes (or those of any US state, for that matter) are intended to keep occupants safe during earthquakes, most make few to no provisions for ensuring the building’s viability after a major shock. “What you get today is well below what the public expects,” Almufti said.
Today’s most common structural solution for tall buildings, something known as a reinforced-concrete core wall, relies on damage to dissipate seismic energy. When an earthquake strikes, the building’s base, walls, and structural beams absorb the energy by cracking and yielding as needed. This prevents the building from collapsing during the tremor but can leave it beyond repair after the shaking subsides.
This situation results from a lag in building codes. “The seismic codes came out, let’s say, 50 years ago,” Almufti said. “Technologies have advanced well beyond the range of where we can keep these buildings not only safe but usable. The codes just haven’t caught up, and they aren’t likely to for several more decades.”
When Arup was brought on to design the structure for 181 Fremont, Almufti and his colleagues met with developer Jay Paul Company to describe their options. “We said, ‘Just so you know, this is how a normal tall building would perform in an earthquake. Are you okay with this?’ And they said no, they weren’t. They had made a commitment to design a LEED Platinum building — they wanted to do all they could to make sure it would be usable after a disaster.”
After carefully examining design options from the perspective of
resilience and sustainability, Arup recommended controlling wind vibrations by embedding viscous dampers in the building’s structural steel exoskeleton in lieu of adding extra steel or a TMD. This solution drew on the firm’s experience retrofitting London’s Millennium Bridge (also known as the Wobbly Bridge) and incorporating dampers in the Las Vegas High Roller.
The engineers chose to place the viscous dampers in the building’s megabraces — structural beams that travel over many stories at a diagonal. Viscous dampers are relatively common; placing them within structural systems is not. In this configuration they function like a car’s shock absorbers, albeit at a massive scale (each measures approximately 14ft in length and 14in in diameter).
In the final design for 181 Fremont, four viscous dampers attach to one end of each of the megabraces, for a total of 32. A piece known as the primary brace lengthens or contracts in response to wind or seismicity, while the dampers are activated to dissipate the energy.
For added protection, the engineers also gave the megacolumns — the vertical members spanning the building’s height at the corners — the ability to lift up slightly at their base during strong earthquakes, then return to their normal position without damaging the building. This reduced demands on the structure’s foundation.
Taken together, these structural solutions address concerns related to both wind and seismicity. The resulting building should emerge from a 475-year earthquake largely unscathed.
The resilience story for 181 Fremont extends far beyond the structure. The design strategy was informed by Arup’s work on a new seismic resilience rating system known as REDi, which is based on extensive research into building responses to earthquakes around the world.
When setting out to understand what factors most influenced the amount of time a building required to return to full occupancy and functionality, the team soon realized that factors ranging from emergency food storage to post-disaster repair financing played a huge role. This list of variables became the basis for REDi’s design criteria and risk assessment methodology.
Buildings that achieve REDi Gold certification, as 181 Fremont aims to do, should be reoccupiable almost immediately after a 475-year earthquake. As soon as utilities are restored, they should function more or less normally.
To achieve this, the project team adopted measures ranging from enhanced façade testing (to prove that air and water intrusion would be avoided after a big earthquake) to helping the owner prepare contingency plans for post-disaster inspection and restarting elevators quickly.
Resilience and recovery
The long-term benefits of this design are tremendous. Aside from the obvious reduction in life-cycle carbon emissions associated with keeping a building standing rather than demolishing it, the tower’s continued operation aids community recovery on many fronts. It provides an anchor point for the neighborhood and lessens financial and societal stresses.
The design had many short-term benefits as well. A TMD would have cost the developer approximately twice as much as all the dampers used in the building combined. Placing the mechanical equipment on the roof rather than in the penthouse, as would have been required with a TMD, maximized rentable penthouse space. The design also cut the amount of steel used by 25% compared to a standard similar building.
When the building is completed in 2017, it will serve as an example throughout San Francisco and beyond of the potential to create resilient towers in seismic zones.
Published on Doggerel in February 2016.
Doggerel was an online magazine published by design firm Arup.
Article co-authored by Andrew Wade.