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SFGate: Tall, skinny ... stable/Using novel technology, S.F. tower should resist quakes, gales

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Sunday, July 2, 2006 (SF Chronicle)
Tall, skinny ... stable/Using novel technology, S.F. tower should resist quakes, gales
Carl Nolte, Chronicle Staff Writer

   When the San Francisco building inspectors looked at what the architects
and engineers had in mind for a 55-story condominium tower on top of
Rincon Hill, they were intrigued.
   The first of two buildings was a tall, slim tower, a building developer
Michael Kriozere said was designed to change the city's skyline.
   It would be built on a narrow site right where the Bay Bridge comes into
San Francisco. Because of its location, it would be an instant landmark.
"It will have simple, strong lines. It is meant to be seen at a distance,"
Kriozere said.
   But under the skin, the building is a startling departure from anything
the city inspectors have seen before.
   "There is not another building like this in the world," said Ron
Klemencic, the structural engineer for the One Rincon project.
   From the outside, the taller building, 641 feet from the street to the
top, looks simple enough -- a tall glass tower, round on three sides. It
won't be another Transamerica Pyramid, or a black monolith like the Bank
of America building, which are not only taller, but more massive.
   What's inside and on the top of the Rincon Hill tower is what makes it
different. The engineering, Klemencic said, "is on the cutting edge."
   It is always a challenge to build a high-rise on top of a hill in
earthquake country, particularly in San Francisco, which still harbors
dark memories of the great quake and fire that destroyed the city 100
years ago. Now the city has complex building codes, and putting up a tower
on the top of a hill has special challenges -- not just earthquakes, but
strong winds that blow off the Pacific in winter, sometimes over 75 miles
an hour, hurricane force on the Beaufort Wind Scale.
   Kriozere and his associates picked Klemencic, president of the Seattle
firm of Magnusson Klemencic Associates, as the structural engineer. The
architect is John Lahey, managing partner of Solomon Cordwell Buenz, of
Chicago. Klemencic says the design was "a collaborative effort" among the
architect, the engineer and the developer.
   Klemencic is 43, a tall man who has worked on tall buildings all over the
world. He is the chairman of the Council on Tall Buildings and Urban
Habitat, an international industry group that includes among its members
some of the top authorities on the tallest buildings in the world.
   Back in the 20th century, a skyscraper would be built around a steel
frame, the way a human is built around a skeleton. But now, many tall
buildings are built around a concrete core, poured around reinforced steel
for strength.
   Underneath this is a concrete and steel foundation based on bedrock. At
One Rincon, the foundation is 12 feet thick. The bedrock on Rincon Hill is
serpentine, a metamorphic green rock sometimes called "slickrock."
   Some engineers looked on it with suspicion, but building on serpentine
bedrock is not unheard of. The bedrock under the south tower of the Golden
Gate Bridge is largely serpentine.
   At Rincon Hill, the building's core is slowly rising out of the
foundation. The core looks like the clasped fingers of a steel hand, with
concrete poured on the steel.
   Another building using the concrete core method is the Intercontinental
Hotel, going up at Fifth and Howard streets, south of Market.
   One advantage of the core construction as opposed to the steel-frame
method is that the condos in the towers would not have structural members
obscuring the windows. This means floor-to-ceiling windows and spectacular
views. The better the view, the more the developer can charge.
   Outside of the core at One Rincon will be outriggers, tall columns made of
steel-reinforced concrete. These provide extra strength. The outrigger
design is "tried and true," said Raymond Lui, a structural engineer with
the San Francisco Building Inspection Department.
   But Klemencic introduced another element that interested the building
inspectors. These were V-shaped devices called buckling restrained braces,
installed between the outriggers and the core. These act something like
the shock absorbers in automobiles to provide an extra edge in the event
of earthquake.
   One of the problems of braces is that they tend to buckle -- fold up and
lose all strength -- in the event of some serious shock, an earthquake,
for example. But the buckling restrained braces, which are steel, are
encased in a sleeve of steel and reinforced concrete designed to prevent
   "This is the first time in the United States that these have been used in
this way," Klemencic said. Lui agrees. "I don't think anyone has used the
buckling restrained braces with outriggers before," Lui said. "It is a new
structural concept," said Hanson Tom, program manager for the city's
Building Inspection Department.
   The tower has yet another unusual feature -- on the very top are two water
tanks holding about 100,000 gallons combined. Each tank will also have two
liquid damper screens to control the flow of the water. The purpose of the
tanks is to counter the sway of the building in a high wind.
   Strong winds can make even the biggest buildings move; this one can sway
15 to 16 inches, which could be upsetting to the residents.
   "You would feel the vibration if you didn't have the damper," Lui said.
But the design idea is that if the wind tends to move the building one
way, the water would provide a counterbalance for stability.
   This concept has never been used in this country before.
   No single element in the design caused a problem, but all of the
innovations -- in what Klemencic calls "a performance-based design" rather
than a prescriptive design -- meant the city wanted to look carefully at
the tower.
   It convened a peer review with three eminent engineers -- Jack Moehle, a
UC Berkeley professor whom Tom describes as "a world-renowned specialist"
in structural engineering; Ronald Hamburger of Oakland, another famous
engineer who has been president of the Structural Engineers Association;
and Lelio Mejia, an expert in seismic engineering.
   The peer group checked the calculations and the design, and ran tests at
UC Berkeley to simulate earthquakes earlier this year. "The biggest quake
we had here was 7.8 on the scale on the San Andreas Fault," Klemencic
said. "We simulated five times that. We simulated 14 different major
earthquakes," he said. "It performed fine."
   The world of top seismic engineers is small. Klemencic had studied under
Moehle at Berkeley, so his former professor once again examined his work.
"It's like defending your Ph.D. thesis over and over again," Klemencic
said, "like doing homework over. It's pretty rigorous."
   The panel signed off on the building, and the city inspectors were
satisfied; the permit to go ahead was issued after the first of the year,
and the concrete pouring began. The first tower is scheduled for
completion in 2008. After that, a second, smaller tower is planned. In
all, there will be 695 condos and 14 townhouses, nearly all of them
   Klemencic is pleased with how it is coming out. "This is one of my
favorite examples of our engineering achievements," he said. "The building
is fantastic."
   He always wanted to be an engineer, he said. "When I was a kid (in Racine,
Wis.), I used to build cities in sandboxes."
   Klemencic's career has been in very tall buildings, but he has a flaw.
"I'm deathly afraid of heights," he said. ----------------------------------------------------------------------
Copyright 2006 SF Chronicle

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