A 58-story residential tower in San Francisco that is tilting sinking at the rate of 1 inch a year remains safe, according to a report by Gregory Deierlein, Ph.D., Marko Schotanus, SE, Ph.D., and Craig Shields, PE, GE.
The Structural Safety Review of the Millennium Tower at 301 Mission Street was ordered by the City’s 301 Mission Street Seismic Safety Study Committee to determine the building’s ability to meet structural and seismic safety standards of the San Francisco Building Code.
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 effect on the building’s safety," the report says.
As the building continues to settle at a fairly consistent rate of about 1 inch per year, the report recommends continued monitoring of the Millennium Tower and a reevaluation of the structure when the amount of expected long-term settlement has been confirmed.
The Millennium Tower complex consists of two structures, identified as the “Tower” and the “Midrise” on structural drawings. The drawings describe the Tower as a 58-story, 605-ft tall structure over a one story basement, and the Mid-rise as a 12-story, 128-ft tall structure over five below-grade levels.
The Mid-rise structure includes the three-story-tall portion between the 12- and 58-story tall towers. The Tower and Mid-rise are structurally separated by a seismic joint.
Both structures are of cast-in-place concrete construction, using post-tensioned slabs for the floors above ground level.
The seismic force-resisting system of the tower consists of a “dual system,” which is comprised of a 36-inch-thick special reinforced concrete shear wall core with outriggers and concrete special moment-resisting frames. The mid-rise relies on a special reinforced concrete shear wall system that includes the perimeter basement walls.
The two structures use different foundation systems. The Tower foundation consists of a 10-foot-thick pile cap supported by about 950 precast concrete piles, measuring approximately 80 feet in length. The Mid-rise structure rests on a mat foundation that varies between 6 and 8 feet in thickness.
Tie-downs resist hydrostatic uplift pressures under the three story portion of the Mid-rise building. The original design anticipated 1 inch of settlement under the Tower by the time of construction completion, and additional long-term settlement due to compression of the underlying clay layers of 5 inches. Settlements were expected to occur uniformly over the Tower foundation area.
Reports of the large vertical settlement, differential settlement and tilting of the Tower structure have raised concerns regarding the structural integrity of the building and its safety against earthquakes.
The report says that that the building had settled about 6 inches by the time the Tower structure was completed in February 2008. Total settlement rose to about 9 inches by the project’s completion in August 2009. It subsequently increased to about 16 inches by June 2016 and almost 17 inches by July 2017.
In addition to vertical settlement, the building mat foundation has experienced some distortion (dishing) and tilting. The maximum difference in elevation across the tower mat foundation is about 6 inches at present.
Comparative measurements of mat elevations from April 2009 to July 2017 indicate differential settlements of about 1 inch from the south to north end of the mat and about 2 inches from east to west across the mat during this time. In contrast to the total vertical settlement and rigid body tilt that continue to increase, the surveys indicate that most of the mat distortion occurred by 2009.
The mat tilting is accompanied by a building lean of about 0.18% of the building height, with total horizontal roof displacements of 14.0 inches to the west and 6.3 inches to the north. The study says that the maximum construction tolerance for out-of-plumbness (ACI 117-10) is 1/600 times the building height (about 0.17%) for buildings taller than 100 ft and is limited to 6 inches total.
"This is twice what would be considered an acceptable construction tolerance for out-of-plumb, the report says. "The continuing differential mat settlement and building tilt are obviously related."
The trio of structural consultants conclude that the foundation settlement experienced by the Tower "has not appreciatively affected the safety of the building at this time." They say, however, that because the structure is still settling, "continued monitoring and further study of the cause of the settlements is recommended to allow a better understanding of maximum future settlements."