Earthquakes, sinking and wind: Here’s what it takes to build a skyscraper in S.F.
1,000-foot ‘supertall’ tower proposed for downtown
Here in earthquake country, we prefer our high-rises not to sink or tilt. So, the more Millennium Tower settled into soft bay landfill, the more unsettled San Franciscans became.
Yet even as Millennium undergoes an expensive, complex foundation fix, a developer has proposed the tallest tower of the post-sinkage era — and it’s eager to share the engineering techniques it will employ to avoid a similar fate.
In December, the Houston-based developer Hines unveiled plans for a 1,066 foot apartment building at 50 Main Street that would be just four feet shorter than nearby Salesforce Tower, The City’s tallest structure. The Foster + Partners-designed supertall tower — a technical designation for buildings higher than 300 meters — would be the centerpiece of the redevelopment of the full downtown block that hosted PG&E’s headquarters until last year.
“We’re still in the formative stages of design,” says Ron Klemencic, whose firm, Magnusson Klemencic Associates (MKA), is the lead structural engineer on the project. “But there’s no question about what the foundation will be. We’re going to be on large diameter drilled shafts that extend roughly about 250 feet down to bedrock.” That technique, Klemencic says, should essentially eliminate the potential for sinking.
The roughly 40 drilled shafts, also called caissons, that will support the tower will be between six and eight feet in diameter and filled with reinforced concrete. Rather than standing on bedrock, the caissons will “socket” into it at a depth between 10 and 30 feet. The same technique was used in other recent downtown high-rises, including Salesforce Tower, Park Tower, and the stalled Oceanwide Center project. Klemencic and his firm worked on all of those buildings and more — “with one notable exception,” he adds. “We had nothing to do with Millennium Tower,” whose foundation does not reach to bedrock. (Though it has sunk and tilted a few inches, Millennium Tower is considered safe by structural engineers and city officials.)
