An already applied Millennium Tower retrofit passes engineering review

The saga of San Francisco's Millennium Tower continues. An engineering design review of Ronald Hamburger's new plan to only use 18 pilings, vs the prior 52 piling design, found no reason to withhold approval of the proposed fix, seeing as the pilings are already installed.

Apparently, Ron considered going with fewer pilings, but as they were already in the ground…

This seems a bit like approving a plan to not do the rest of the work they said was necessary before, but I am easily confused. Anyways 18 is 3×6 or 6+6+6 so I am still scared.


"Subject to continued monitoring of the building settlement and tilt, through construction and following completion of the 18-pile PPU, we see no technical reasons to withhold approval of the proposed revisions to the building permit for the voluntary foundation retrofit," the review team members wrote.

The 18 piles have already been installed, as they were included in the approved 52-pile design, too, says Ronald Hamburger, a senior principal with Simpson Gumpertz & Heger and the engineer-of-record on the project. Now the contractor, Shimmick Construction, is in the process of installing excavation shoring so they can excavate 25 ft below grade on two sides of the building. Then they will tie the existing foundation to the new piles with rebar coupled onto the existing reinforced steel for a new 10-ft-thick concrete mat on those two sides that will be tied into the existing mat. Once that's done, they can jack the load onto the piles.

The revised design reduces the amount of work required on the building, the amount of shoring and the amount of excavation needed. Hamburger says they looked at installing as few as six new piles before settling on 18.