A consultant hired by the developer of the sinking Millennium Tower issued a new report Tuesday that presents a much more optimistic picture of the sinking building than his draft analysis just two years ago.
In his latest report, the consultant, structural engineer Ronald Hamburger, discards many of the significant findings contained in his October 2014 draft report without explanation. In both reports, however, Hamburger declares the sunken building is safe and not at risk of collapse in a quake.
Hamburger’s earlier 2014 draft report is not completely favorable, however. For example, Hamburger had warned that the settlement would trigger foundation damage that would otherwise be avoided in just a moderate quake.
He had said the settlement “significantly” increased stress on the slab, “at some locations in the foundation mat, by as much as 50 percent.”
As a result of the settlement stress, Hamburger warned back in 2014, the foundation and other parts of the building “will experience damage in somewhat lower intensity earthquakes than would be anticipated if no settlement had occurred.”
Hamburger also expressed a concern about the foundation’s intended ability to twist and hinge and act as “a type of fuse to shield the rest of the structure from damage.”
While that function was “never quantified during the original design,” Hamburger noted, the foundation’s settlement means that the foundation itself is under “more than would generally be considered desirable” stress for a new building, although “we do not believe that this represents a collapse hazard.”
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He also concluded: “We do not believe the safety of building occupants is significantly affected.”
He floated an idea that would help the building avoid the added quake damage. That approach involved reinforcing the 10 foot thick foundation slab, or mat, by running reinforcement walls out from the core of the building in the two basement levels above it, creating a triple decker foundation.
“Such reinforcement could potentially be accomplished by introducing new walls” that would extend about 20 feet outward from the core in the basement, Hamburger suggested.
Hamburger’s new report no longer mentions any walls needed to bolster the foundation in a quake. He no longer suggests that the sinking puts the foundation under greater stress or that the foundation would suffer more than expected damage because it would not be able to hinge as designed.
Hamburger then concludes: “Our analyses suggest that the amount of such hinging is acceptable.”
“The effect of settlement on most building elements is negligible,” Hamburger now concludes. In a worst-case earthquake, “most building elements continue to meet criteria commonly adopted for design of similar new buildings in the City of San Francisco today.”
“We conclude,” Hamburger’s new report opines, “that the settlements experienced by the 301 Mission tower have not compromised the building’s ability to resist strong earthquakes and have not had a significant impact on the building’s safety.”
