Concrete Products

JAN 2019

Concrete Products covers the issues that attract producers of ready mixed and manufactured concrete focusing on equipment and material technology, market development and management topics.

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www.concreteproducts.com January 2019 • 43 surrounding construction activities. "We have made clear from the beginning that getting to a 'fix' has been Mission Street Development's top priority," affirms Mil- lennium 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 supe- rior 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 build- ing 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 con- crete mat. That anchoring connection will trans- fer a portion of the tower's weight to bedrock, halting any further primary compression of soils beneath the build - ing. 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 direc- tion of the past tilt—the retrofit will allow soils on the south and east sides to undergo the normal and gradual process of second- ary 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 set- tlement 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 addi- tional lateral capacity to resist shaking in a major quake, and improve existing piles' per- formance by decreasing their loads. NEWS SCOPE PRACTICE

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