Structural engineer Ron Klemencic got more than he asked for a few years back when he was called in by architect Solomon Cordwell Buenz on its One Rincon Hill condo project in San Francisco. Klemencic, co-founder of Seattle-based Magnusson Klemencic Associates, was somewhat surprised by the 61-story tower's wind problems.
“Usually the seismic concerns are so much greater than the wind issues, especially in San Francisco,” he said. The tower was to be built on top of one of the city's seven hills, adjacent to the Bay Bridge off-ramp, with no protection from winds from the south. The curved shape of the structure would create the same lift effect as with an airplane wing, resulting in eddying and swaying—technically known as vortex shedding.
Wind tunnel studies showed that the building would produce back-and-forth accelerations of 25 one-thousandths of the force of gravity. The acceptable rate for a residential structure: 15-18 thousandths of the force of gravity.
To reduce the swaying, Klemencic turned to a technology MKA had used in South Korea, the Philippines, and Hong Kong—the tuned liquid damper. “It's simply a water tank,” he says. “Just fill it to the right level and as the tower sways one way, the water sloshes the other way. It doesn't cancel the sway. It dampens it down to an acceptable level.”
Of course, it's a little more complicated than that, which is why the tanks are still being tested at the University of Western Ontario, London. As the building goes up—the shell is half-finished—MKA will install electronic sensors to measure actual vs. predicted vortex shedding. Only then will the damper's design be finalized and a determination made as to exactly how much water to put in the tank.
Path-breaking seismic solution
Rendering of One Rincon Hill, which recently topped-out at 641 feet.
Beyond the wind problem, the Building Team knew that it would have been difficult if not impossible to achieve the unique geometry of One Rincon Hill using conventional code-prescriptive seismic solutions. So Klemencic turned to a technology MKA had used in a 42-story office building in Seattle—buckling restrained braces. These “BRBs” (made by Star Seismic, Park City, Utah) will be attached to the tower's core wall at the 30th and 50th levels and linked to steel “outrigger columns,” forming a K-shaped brace.
The technology has been used in mid-rise buildings in Japan and California, but Klemencic says One Rincon Hill will be the first high-rise concrete residential structure in the U.S. to use it. The design has been validated in full-scale tests at the University of California, Berkeley, to resist seismic forces two-and-a-half times greater than code requirements.
Klemencic estimates that the liquid damper will save $2-4/sf over stiffening the building with more concrete, while the seismic bracing will cut $5/sf in costs compared to conventional code-prescribed designs.