An independent Engineering Design Review is backing a proposed seismic upgrade and foundation stabilization for the 301 Mission Street Tower in San Francisco.

The four engineers also cleared an associated shoring and excavation design and indicator pile program for the residential high rise, which also is known as Millennium Tower.

The decade-old building has been sinking and leaning more than had been expected, prompting calls for foundation repairs and upgrades.A $100 million plan to fix the leaning building was submitted to city officials for approval in December. Plans call for engineers to install 52 concrete piles to transfer a portion of the building's weight from its existing foundation to bedrock 250 ft below.

The 58-story skyscraper sits on a 10 ft thick concrete mat foundation held in place by 950 reinforced concrete piles, which do not reach bedrock.

Since its opening in 2009, the building has sunk 18 in, and leans 14 in to the west and 6 in to the north.

The Engineering Design Review Team consists of Shahriar Vahdani, Craig Shields, Marko Schotanus, and Gregory Deierlein, chair.

A 2017 report prepared by the four said that the settlements "have not compromised the building’s ability to resist strong earthquakes" and have not had a "significant effect on the building’s safety."

The Millennium Tower complex consists of two structures, identified as the Tower and the Mid-rise on structural drawings. The drawings describe the Tower as a 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 in 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.

Crane footing, concrete core and construction equipment in August 2006.The two structures use different foundation systems. The Tower foundation consists of a 10 ft thick pile cap supported by about 950 precast concrete piles, measuring approximately 80 ft in length. The Mid-rise structure rests on a mat foundation that varies between 6 to 8 ft in thickness.

Tie-downs resist hydrostatic uplift pressures under the three-story portion of the Mid-rise building. The original design anticipated 1 in 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 in. 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 raised concerns regarding the structural integrity of the building and its safety against earthquakes.

Structural upgrade

The main features of the structural upgrade are the addition of new piles, extending down to rock on the north and west sides of the building, which will be attached to the building through an extension to the existing mat foundation.

Associated with the foundation retrofit is installation and testing of an indicator pile and installation of temporary shoring to retain the sides of the excavation required to construct the foundation upgrade. The structural upgrade is designed to meet the requirements of the San Francisco Existing Building Code (SFEBC), with the intent to reduce future building settlement and improve the seismic performance of the foundation.

The number and size of piles to be added is limited mostly by site constraints and the ability of the existing construction to transfer loads to the new piles.

The engineering design review team focused its work on ensuring that the altered structure would be no less conforming to provisions of the San Francisco Building Code with respect to earthquake design than it was before the upgrade. The team also worked to ensure that the alterations would not create structural irregularities.

The review team said that, to date, all of its comments on the geotechnical and structural design have been adequately addressed by a separate design team, and no outstanding or unresolved issues exist. The team said that once the foundation retrofit is constructed, the building is expected to have performance "consistent with the stated design objectives" and pertinent sections of the city's building code.

"On the basis of our review we see no reason to withhold approval of the building permit for the structural upgrade of the foundation and the associated permits for shoring and excavation and the indicator pile program," the team said.

However, given the "inherent uncertainties" in the foundation settlement and response, the engineering review team recommends that building performance be monitored during and upon completion of the proposed construction.

It said that due to the characteristics of the Old Bay clay, which underlies the building foundation, the maximum stress developed within the existing mat and its extension due to uplift forces imposed by the new piles could occur over months, if not years, after jacking the new piles has been completed. The engineer of record has proposed a system of monitoring the mat settlement, pile forces and building movement during jacking of the new piles and continuing for 10 years after completion of construction. Monitoring is to be performed by a Geotechnical Engineer and reported to the engineer of record and the San Francisco Department of Building Inspection.

The Engineer of Record (EOR) for the project is Ronald Hamburger, S.E., of Simpson, Gumpertz & Heger Inc., assisted with supporting geotechnical engineering by John A. Egan, GE and Slate Geotechnical Consultants.