Millennium Tower reviewed building design, but not the ground

The developers behind the sinking Millennium Tower paid for an independent review of the tower itself before it was built, but not of the site it sits on, according to new testimony at City Hall on Thursday.

The Government Audit and Oversight Committee quizzed Jack Moehle, a professor of structural engineering at UC Berkeley, whom Millennium Partners and its engineer consultants hired to conduct an independent peer review of the building design while it was being entitled.

“The interest was to do an internal review to ensure that the structural system selected was suitable,” Moehle told city lawmakers at Thursday’s hearing. “[So] that if there was a formal peer review for the city later that most questions would be dealt with already.”

Moehle says he inspected the high-rise’s design from top to bottom—but no lower than the bottom. A geotechnical review—i.e., an assessment of the condition of the soil under the building site—wasn’t part of the process, because no one ever hired a geotechnical engineer.

[Update: Note that Moehle’s testimony referred only to the lack of a geotech engineer on the voluntary review conducted by the developer ahead of time.]

“My interests went as far as the concrete mat,” says Moehle, referring to the slab that forms the base of the building.

The condition of the dirt matters because the 58-story, concrete-framed Millennium Tower’s foundation design relies on the properties of the soil to keep the building in place.

As an Iowa State University design professor explained it:

“Imagine driving a broomstick into beach sand–you can only go so far before there’s enough broomstick in contact with the sand to put up fearsome resistance. This has always been a standard technique for building in liquid soil, and it’s why coastal construction always comes with the dulcet tones of a pile driver.”

The building sits on hundreds of giant concrete “sticks” that use soil friction to hold the concrete slab in place. So the behavior of all that dirt matters.

Moehle told supervisors Thursday that all he said was that the foundation was up to code, not that it would necessarily work as intended. “I’m not qualified to do that.”

Supervisor Aaron Peskin pressed Moehle on why he didn’t insist on an analysis of the ground, saying: “[Your] letter says, I have reviewed the design criteria and find it acceptable ... But you don’t have any language in your letter: Be forewarned, you may want to get a geotechnical consultant?”

To which Moehle replied, “It wasn’t my place,” pointing out that many projects opt not to examine the soil and, since foundations must be built first, developers are usually eager to expedite its review.

Via email, a spokesperson for Millennium Partners says "It was clear from Professor Moehle’s testimony that Mission Street Development met and in fact exceeded the requirements of the city” by paying for the independent review.

• Government Oversight Committee, 2.2.17 [SF Gov TV]
• Simple Engineer’s Explanation of Tower [Curbed SF]
• Millennium Tower sinking: Everything you need to know [Curbed SF]
• SF sues Millennium Tower developer, says tilting and sinking were known since 2009 [Curbed SF]
• Jack Moehle [UC Berkeley]