How highrises are now built stronger at core
For the people who design tall buildings — and the people who review those designs for seismic safety — there are two ways to measure the perceived safety of the towers that have transformed San Francisco’s skyline over the past decade.
One is to run thousands of lightningfast computer simulations that measure each set of design calculations. The other? Run the tests — and then go with your gut.
“There’s no test equal to an actual fullscale earthquake. I’m not going to contest that,” said Jack Moehle, a professor of structural engineering at UC Berkeley. “But barring an earthquake, the very best thing we can
do is what we’re doing now.”
Moehle helped craft the guidelines for what is known as performancebased seismic design. Thirty years after the Loma Prieta earthquake and nearly a decade after being introduced in 2010, the approach has become the norm for highrises along the West Coast, allowing designers more leeway while meeting the standards set in municipal building codes.
In simple terms — and little about the design of tall buildings is simple — performancebased design concentrates the seismic strength of a tower in its spine, the central core.
This is different than traditional highrise design, which relies on structural columns and beams along a building’s perimeter to resist the force of major earthquakes. Instead, nearly all the structural support is embodied in the thickwalled core — the columns outside it mainly hold the floor immediately above them in place.
“Performancebased design frees things up architecturally,” said John Hooper, director of earthquake engineering for Magnusson Klemencic Associates, the Seattle firm that has served as structural engineer for at least a dozen recent towers in San Francisco.
Whatever shape the exterior takes, though, whatever curves or angles are added to catch the eye, the shear core design from one new tower to the next is pretty much the same.
The boxlike walls can be as much as 4 feet thick, formed by an intricate web of steel bars and concrete that Moehle compares to “a thick soup” — it’s poured around and through the steel web so that the result is a solid mass. Inside the box are such elements as elevators and fire stairs, but the corners of the box must be solid, so as not to erode its inherent strength.
The other key factor in strengthening the core comes with horizontal beams above each large opening. They tie into the walls and are intended to absorb the force when faults come to life.
“The beams perform the majority of the work,” Hooper said. “Ultimately, what makes the core robust are the beams that hold the walls together and dissipate energy.”
Another obvious concern is not whether a tower will collapse, but what might happen to its skin. After all, you don’t need an overactive imagination to conceive a scenario where intense seismic vibrations send glass raining down on passersby.
This won’t happen, engineers say, because towers are clad in panels that form what is known as a curtain wall. Some are all glass. Others might include windows framed in masonry or metal. In either case, they’re assembled in a factory and then attached to the tower. Rubber is applied between the panels as a sealant, protecting what’s behind them from water and wind.
Before any panels are installed, fullsize mockups are tested by being attached to machines that juggle and twist them according to the seismic forces expected at a particular site. To pass muster, no element of a panel may fall off or become deformed during the test.
“Think of the panels as fish scales, hanging on while the building shakes,” Moehle said.
The tests of a tower’s seismic strength take place within a computer, in a detailed procedure that might take nine to 12 months and can easily cost $150,000.
The reason: Each case where performancebased design is used on a highrise must be vetted by a review panel that consists of outside experts and is led by a structural engineer. First they critique whether or not the design approach is technically sound. The panel and the designers continue to meet while computer simulations are run and modifications are made. The results and the panel’s comments are sent to the city’s building department, which decides whether to sign off.
While each tower design is unique — no two sites in San Francisco have the same soil condition, for starters — each must meet two basic standards.
Simulations are conducted to see how a tower would perform in a magnitude of roughly 6.5 on the San Andreas or Hayward faults, the type of earthquake that’s anticipated to strike the region every 50 or so years. The expectation is there will be cosmetic damage, nothing deeper.
And then there is the “maximum considered earthquake” — an 8.0 temblor with its epicenter near the city, which is projected to have a force at least five times times more powerful than the Loma Prieta earthquake of 1989. A quake on this scale is likely to strike the Bay Area no more often than once every 1,000 to 1,500 years.
According to the standards set by the San Francisco Department of Building Inspection, the aim of these maximum simulations is “to verify that the structure has an acceptably low probability of collapse under severe earthquake ground motions.”
Some observers and experts say this isn’t enough — that the code’s aim of preventing structural collapse or major injuries brushes aside the possibility that newer towers could require timeconsuming repairs after a 1906scale earthquake.
Along these lines, a city report on how to better prepare downtown for earthquakes went so far as to recommend that the building code be updated to set standards that, in theory, would allow new towers to be back in service within a month of the most severe temblor. Any change to the code, however, likely is several years away.
Structural engineers and architects agree that higher standards could be met if the state or city raises the bar. But taken in context — performancebased standards as well as code updates over the years with regard to sprinklers, fire stairs and other elements — they see considerable progress nonetheless.
“There is no doubt in my mind that these buildings are much safer, in so many ways, than what came before them,” said Mark Schwettmann, a design director in the San Francisco office of Skidmore, Owings & Merrill. One of his projects: a 39story residential tower under construction at Mission Street and South Van Ness Avenue.
“I’d much rather be standing next to a new tower in the next big earthquake than my 1920s apartment building in Pacific Heights.”