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Condominium association, engineers dig deep to reverse tower tilt

The Millennium Tower Homeowners Association has outlined a plan for the building at 301 Mission Street, San Francisco, to halt settlement and reverse its tilt by installing new foundation piles tying the structure to bedrock. Mission Street Development LLC, the original developer, has agreed to perform and warrant work for the retrofit, which is anticipated to be paid for by a settlement pursuant to an ongoing, confidential mediation.

In a permit application filed with the city and county of San Francisco, the Millennium Tower Association (MTA) outlined the “Perimeter Pile Upgrade” plan to install 52 concrete piles that will transfer a portion of the building’s weight from its existing foundation to bedrock about 250 feet below. That shift will relieve stress on soils that have compressed beneath the building, causing it to settle more than anticipated and to tilt.

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San Francisco code authorities are reviewing the Perimeter Pile Upgrade engineering proposal. If approved as presented, the project is expected to take about 18 months. GRAPHIC: Millennium Tower Homeowners Association

Engineers who designed the upgrade plan said the new piles along the building’s north and west sides will limit future settlement of the tower to minimal levels and reverse the tilt. “We have high confidence in the engineers who designed this retrofit and we look forward to working with the city and county of San Francisco to get the project underway,” says MTA Center Board of Directors President Howard Dickstein. “While the city and other experts have always certified the building as safe, this solution will eliminate any lingering questions about its stability and ensure future settlement is within the normal range.”

Mission Street Development has been working with the MTA for months to reach agreement on an appropriate plan to address greater than anticipated settlement as well as tilting associated with surrounding construction activities. “We have made clear from the beginning that getting to a ‘fix’ has been Mission Street Development’s top priority,” affirms Millennium Partners Principal Philip Aarons. “This plan will relieve pressure on the soils and reverse much of the tilt. It will also increase the building’s already superior performance in the event of a major earthquake. This will be one of the safest buildings in California.”

“We firmly believe Millennium Tower was built to state-of-the-art standards and to code, appropriate for its height, weight and location,” he adds. “We also believe subsequent excavation activities impacted the soil and led to unexpected settlement. Working with the MTA to implement the retrofit plan is now the most prudent course of action.”

ENGINEERING HEFT

The Perimeter Pile plan was conceived by internationally recognized engineer Ronald Hamburger of Simpson Gumpertz & Heger, Boston. Hamburger has nearly 45 years of experience in civil and structural engineering, building design and code development. A member of the National Academy of Engineering, he is an expert in earthquake-resistant design and structural performance evaluation.

Hamburger notes that the plan halts settlement along Fremont and Mission streets, while allowing Millennium Tower to level itself over time. “It is an effective and practical approach to the settlement and tilting issues, and it preserves and enhances the building’s safety,” he adds.

“We know that settlement of the building has already slowed down significantly over the last 12 to 18 months,” notes MTA’s Dickstein. “Now it’s time to ensure any future settlement is negligible and within normal range, reverse the building’s tilt and restore its reputation.”

BEDROCK BORE

Millennium Tower sits on a 10-ft.-thick concrete mat foundation, held in place by 950 reinforced concrete piles driven up to 90 feet deep. Under the retrofit plan, the 52 new piles will extend into bedrock beneath the compressible soils currently causing the settlement and tilt, and will be structurally connected to the existing foundation by an extension of the concrete mat.

That anchoring connection will transfer a portion of the tower’s weight to bedrock, halting any further primary compression of soils beneath the building. Primary compression is a decrease in soil volume caused by a reduction of soil water content, and it occurs when the load placed on saturated clay soils exceeds what was present in the past.

Primary compression accounts for the bulk of the settlement beneath Millennium Tower. After halting primary compression on the north and west sides of the tower—the direction of the past tilt—the retrofit will allow soils on the south and east sides to undergo the normal and gradual process of secondary compression. The process is expected to slowly lead to two to three additional inches of settling on the building’s east and south sides, substantially correcting its tilt.

In 2017, an expert panel convened by the city and county of San Francisco concluded that Millennium Tower is seismically safe under the city Building Code and that settlement had not changed its “capability of resisting major earthquakes.” The Perimeter plan will enhance that capability, enabling the tower to resist earthquake motions that are even more severe than those anticipated by city code. The 52 piles will provide additional lateral capacity to resist shaking in a major quake, and improve existing piles’ performance by decreasing their loads.