Residents of Millennium Tower have one more reason to shake their fists at the sky and/or file another lawsuit as a report comes to light from a structural engineering expert that suggests that the computer models used to test the seismic safety of the design of the tower are now considered outdated and unsophisticated. As NBC Bay Area reports, structural engineer Ronald Hamburger, who was engaged by the developer to re-review the building's seismic safety, said in a January memo that based on new models, some concrete support structures called outriggers ringing the tower in two places are subject to failure in the event of an earthquake.
The outriggers are bands of steel-reinforced concrete that bind the tower's concrete core with its outside walls. Eight of the tower's twelve outriggers are not as seismically sound as they should be, Hamburger says, with the upper group of four nearest the top of the tower about half as resilient as they should be by modern standards.
According to Hamburger, who was asked to answer questions from a city-appointed panel of experts, this does not mean the building is going to collapse in the event of an earthquake, but it does have a high risk of getting red-tagged following such an event, which is worrying for obvious reasons to homeowners in the tower, who up to now have been told that the building was seismically sound despite the widely reported sinking and tilting issues.
As for how strong an earthquake it would have to be to cause the outriggers to fail, that is something that has not yet been determined.
As another structural engineer contacted by NBC Bay Area, Joe Maffei, explains it, "That’s one of the drawbacks of an outrigger design. Outrigger designs are wonderful for wind you have to be very careful with them for earthquakes."
It's been long reported that the design of Millennium Tower included a large amount of concrete, making it one of the heaviest tall buildings constructed in that part of the city to date. While other buildings including nearby Salesforce Tower have drilled 200 feet down to bedrock to support the structure, Millennium's developer believed that 80-foot pilings would be sufficient, something that will continue to be up for debate as the flood of lawsuits rolls in over the building's design and construction.
The developer, Millennium Partners, maintains that dewatering of the soil for the next-door Transbay Transit Center is to blame for their building's excessive settlement. A spokesperson for the devleoper, PJ Johnston, issued a statement about the new findings saying, "At every step of the way the building has been determined to be safe for occupancy."