Remember that leaning tower of San Francisco? They've adopted a plan to put the building right.

The Millennium Tower's homeowners association on Tuesday announced that a panel of independent experts, hired by the City and County of San Francisco, has given the green light to a plan to upgrade the building at 301 Mission Street, known as the Millennium Tower.

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In a permit application submitted in December, the Millennium Tower Association proposed a plan called the "Perimeter Pile Upgrade." The upgrade calls for the installation of 52 concrete piles that will transfer a portion of the building's weight from its existing foundation system to bedrock about 250 feet below ground.

Although the building's settlement and tilt essentially stopped more than a year ago, the retrofit plan will provide long-term assurance against irregular future settlement and potentially reverse the Tower's tilt. Because no foundation improvements are currently required, the expert panel noted, the upgrade is considered "a voluntary seismic retrofit."

As part of the City's approval process, the Engineering Design Review Team, chaired by Stanford University engineering Professor Gregory Deierlein, evaluated the proposed upgrade over the last seven months. The team officially reported its findings to the Department of Building Inspection on Tuesday – a critical step that moves the upgrade forward.

"To date, all our comments on the geotechnical and structural design have been adequately addressed by the Design Team, and there are no outstanding or unresolved issues. In our professional opinion, once the foundation retrofit is constructed, the building is expected to have performance consistent with the stated design objectives," the panel's report said.

The Perimeter Pile Upgrade was designed by Ronald Hamburger of Simpson Gumpertz & Heger. Mr. Hamburger has more than 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 internationally recognized expert in earthquake-resistant design and structural performance evaluation.

Before construction can begin, the final phase of environmental review under the California Environmental Quality Act (CEQA) must be completed, a process that could conclude by the end of the year.

Mission Street Development, LLC (MSD), Millennium Tower's developer, agreed to coordinate the work for the upgrade, which is expected to be paid for through a settlement of pending lawsuits, pursuant to ongoing mediations.

"After working collaboratively with the City, the Deierlein panel, MSD, and Ron Hamburger to propose an effective, long-term solution, we are pleased to reach this important milestone, which moves us closer to a retrofit of Millennium Tower," said Howard Dickstein, president of the Millennium Tower Association. "We have high confidence in the design of this upgrade and we look forward to working with the City to complete the approval process and get the project under way."

"We have made clear from the beginning that Mission Street Development's top priority has been to address and resolve the conditions that caused the building's problems, no matter what the cause," said Philip Aarons, a representative of MSD. "This plan addresses the impacts of nearby construction activities and soil conditions on the building, and is designed to protect it from similar impacts in the future."

The Millennium Tower sits on a 10-foot-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 soils 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.

In 2017, an expert panel convened by the City and County of San Francisco concluded that Millennium Tower is seismically safe and that settlement had not changed its "capability of resisting major earthquakes."

The upgrade will enhance that capability, enabling the tower to resist earthquake motions that are even more severe than those anticipated by city code. In addition, monitoring of the building will continue during the upgrade work and for at least ten years following its completion, a step proposed by Hamburger's design team and endorsed by the expert panel.