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 that our high-rises not 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, Houston-based developer Hines unveiled plans for a 1,066-foot apartment building at 50 Main St. that would be just 4 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 (984 feet) — would be the centerpiece of the redevelopment of the full downtown block that hosted PG&E’s headquarters until last year.

A scale model of the high-rise development at 50 Main St., in black, with the surrounding downtown skyline. (CPP)

A scale model of the high-rise development at 50 Main St., in black, with the surrounding downtown skyline. (CPP)

“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 6 to 8 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.)

Kevin Moore, a structural engineer at the firm Simpson Gumpertz & Heger and the president of the Structural Engineers Association of Northern California, says MKA is “very experienced with high-rise design. They’ve done probably more than anybody else in the seismic region.”

Moore is not able to comment on the specifics of 50 Main without having seen detailed plans, but he reiterated the idea of anchoring the building to bedrock should limit sinking.

“If you talk to a baker, you’ll see that the big wedding cakes actually do have internal columns,” Moore said. Those columns “bear on the top of the table,” just like the caissons of a high-rise should “bear on something that is stiffer than the surrounding soil.”

Of course, sinking is just one of the issues structural engineers need to anticipate. The 50 Main building site, set on a reclaimed portion of the Bay, is geologically complex, with layers of landfill, sand and bay mud all sitting atop bedrock.

A parking garage currently stands at 50 Main St., where a 1,000-foot building is planned. (Kevin N. Hume/The Examiner)

A parking garage currently stands at 50 Main St., where a 1,000-foot building is planned. (Kevin N. Hume/The Examiner)

To plan for the inevitable earthquake, Klemencic’s team is working with the geotechnical engineering firm ENGEO to build a computer model of the building and the soil layers beneath it. With this model, the engineers will simulate earthquakes of varying intensity from the region’s faults, looking at how both the ground and structure respond. The building will be designed to withstand the largest earthquake — the “maximum considered earthquake” as determined by the American Society of Civil Engineers — that local faults are capable of generating, or about magnitude 8.0.

Wind is also a factor for a tower of this height. At worst, high-rises can cause “vortex shedding,” or differential wind speeds on different sides of the building, which can compromise its structural integrity. Other wind issues are simply a matter of comfort: If improperly designed, supertall towers can experience imperceptible swaying that can give people motion sickness.

Noise can be a factor, too, as the public recently learned on the Golden Gate Bridge. To avoid these problems, a scale model of 50 Main as well as surrounding buildings is undergoing wind tunnel tests at a facility in Colorado.

Per San Francisco policy, 50 Main’s engineering plans will be peer reviewed by a team that includes geotechnical, structural and seismic and ground motion engineers. The Department of Building Inspection solicits outside reviewers and screens them for conflict of interest, DBI spokesperson Patrick Hannan wrote in an email to The Examiner. Reviewers are contracted by The City, but ultimately compensated by the project developer.

Before beginning construction, high-rise projects must also receive shoring and excavation permits that double-check the project’s plan to prevent damage to surrounding buildings.

Aside from being an epic engineering challenge, the development at 50 Main represents a major bet on downtown San Francisco at a time when economic conditions are highly uncertain.

“Our confidence in San Francisco is as high as it’s ever been,” said Paul Paradis, senior managing director of Hines’ San Francisco office. “Many of the fundamental, attractive aspects of San Francisco have not changed: the transit, the access to recreation, the cultural resources in The City, the great companies that are located here.”

Paradis also cites Hines’ 400-unit apartment building at 33 Tehama St. as a strong indicator of demand for downtown San Francisco living, even postpandemic. The building’s occupancy rate plummeted to the low 70s in the depths of the pandemic, but has now returned to full occupancy, Paradis says. Meanwhile, Hines is planning to break ground in the first half of this year on San Francisco’s fourth-tallest tower, an 800-foot hotel and condo complex next to the Salesforce Transit Center.

While the parcel at 50 Main is zoned for 400 feet in height, the project was able to reach its proposed height of more than 1,000 feet using the state density bonus, a law that allows for increased height in exchange for affordable housing. The program has become increasingly popular in San Francisco where the local inclusionary zoning rules — the percentage of affordable units required in each project — automatically qualifies many developments for the state density bonus.

The tower designed by Foster + Partners — the firm behind Apple’s donut-shaped headquarters in Cupertino and Oceanwide Center — is slated to include 808 apartments. Of those units, 164, or 20.5%, will be offered at below-market rates, with the majority reserved for households making less than 50% of the area median income. The project would also include the renovation of two century-old buildings facing Market Street, and a squat 1970s tower facing Mission. In the middle of the block, Hines is planning a 1.25 acre park.

The Planning Department has yet to officially weigh in on the project’s application. For his part, Paradis said Hines wants to “move ahead quickly,” beginning construction as early as 2023, and wrapping up by 2027. Regarding the proposed height, he said, “This is very much the building that we want to build. So it’s definitely not a negotiating tactic.”

As the project moves through The City’s approval process, engineering and design work is chugging along. “It’s great fun, for sure,” Klemencic said. “That said, it’s intertwined with this very serious layer of responsibility and commitment to public safety. And to make sure we get it right.”

bschneider@sfexaminer.com

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