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Millennium Tower



Millennium Tower stands 645 feet and is the tallest reinforced concrete structure situated in a seismic zone 4 region, as well as the 4th tallest structure in the city of San Francisco. The $350 million project is comprised of a 59-story tower and a 12-story building connected by a 3 level podium structure, The above ground structure contains approximately 900,000 sf with an additional 250,000 sf of parking and support space located below-grade. This project required a 75-ft deep excavation, which is one of the deepest in San Francisco. The tower's immense height posed many challenges and required the creative use of technologies and cutting edge innovation. Concrete, while a unique choice for this high-rise tower, is utilized most effectively to make this project a financial success for the developer. Millennium Tower demonstrates that concrete is a safe and secure building material, even in seismically challenging situations.

STRUCTURAL FRAMING SYSTEM

To resist the large seismic forces placed on a building of this size, a dual lateral system is employed, which is comprised of a 36-inch-thick concrete shearwall core with outriggers and a partial perimeter special moment resisting frame (SMRF). Four-story tall outrigger walls connect the core walls to large outrigger columns at three elevations to control the building's lateral deflections. One-story deep, diagonally reinforced link beams connect each outrigger to the supporting column and are designed as the fuses intended to yield in the event of a major seismic event. A capacity design approach is used for the design of the outrigger columns to insure that they remain elastic even if all six connecting link beams yield at the same time.

In order to meet the owner's floor height requirements, wide-flange steel link beams are used as the shearwall core coupling beams. Conventional diagonally reinforced coupling beams require significantly more depth. The embedded region of the beam extends 4 ft into the wall and transfers the beam moment by bearing on the concrete above and below the embedded beam.

In addition to significant innovations above grade, the tower's below grade solution is also challenging. A tower of this height creates a higher bearing pressure below the core (14,000 psf). To provide support for such a high concentration of force, 950 14-inch square precast piles were used. While overturning due to earthquake is a significant factor, the number of piles was governed by gravity design. In order to provide a high-confidence against a possible foundation shear failure, vertical shear reinforcing is added in the 10 ft thick pile cap.

The project's five-story basement extends 75 feet below grade, nearly 45 feet below the site's water table. This open cut required special waterproofing. A unique shot-crete mixture containing Caltite is employed at perimeter walls, eliminating the need for a conventional adhered waterproofing membrane.

The foundation supports two very different types of structures. Overtime, the building is expected to settle as much as 5". The two towers are separated by a horizontal joint, and hinge slabs connect the two structures allowing for additional settlement to occur. The area between the towers, supports only three levels above grade and requires tie-downs to resist the hydrostatic uplift at the bottom of this deep excavation.

UNIQUE STRUCTURAL AND/OR ARCHITECTURAL DESIGN FEATURES

High-rise buildings usually exhibit significant higher-mode effects. This higher-mode effect significantly changes the load distribution applied to the building. A historical inverse triangular load distribution push-over analysis is the wrong approach for high-rise analysis. Our team explored other options such as multi-mode pushover analysis as described by FEMA 440 and non-linear time history analysis.

For the heavily reinforced walls, beams and columns, grade 75 ductile reinforcement is used to minimize the amount of rebar required. This solution reduced rebar tonnage by 25%, significantly reducing congestion and facilitating concrete consolidation. A robust testing procedure is implemented to gain confidence in the rebar and the mechanical devices used with the rebar, and demonstrates to the city of San Francisco Department of Building Inspection the safety and efficacy of this solution.

To further simplify placement of concrete during construction, a Baugrid system of welded grid reinforcement is also employed. Welded wire grid reinforcing confines shear wall boundaries and provides cross-ties at the beams and columns. This tie system eliminates all hooks and significantly reduces the volume of rebar within the shearwall boundary elements, SMRF columns, and SMRF beams. The flanges of the shearwall core are reinforced by continuous Baugrids providing resistance for confinement and shear.

CHOOSING REINFORCED CONCRETE

When the project was initially conceived in the late 1990's, it was assumed that structural steel would be selected for the project. Steel had long been the material of choice for residential and office buildings in California, as the reduced overall structural weight leads to reduced seismic design forces. By the time the project gained traction in 2003, two things had changed in the local industry.

The first change involved, the availability of high strength concrete in the local market. Prior to 2000, 7,000 psi was the highest specified strength for any building in San Francisco. However, higher strengths were required for the new Bay Bridge construction. That project eventually utilized material with strengths as high as 12,000 psi. Utilizing advice from the local construction industry, Millennium Tower's solution ultimately specified 10,000 psi elements.

The second change within the industry was the development of new slip-form technology that allowed a concrete shear wall core to be constructed rapidly and in advance of the floor plates. This innovation changed the economics on the construction side, facilitating faster concrete construction schedules.

These developments in tandem with residential construction's preference for concrete enabled the Millennium Tower concrete solution. The use of flat plate construction reduces overall floor to floor heights, thus allowing more floors to be constructed within the given building height, which is limited by zoning regulations. All of these items, combined with the clients' preference for selling a concrete structure, led to the selection of a concrete system.

Project Details

Location:

San Francisco, CA

Owner:

Millennium Partners, San Francisco, CA

Architect:

Handel Architects LLP, San Francisco, CA

Engineer:

DeSimone Consulting Engineers,
San Francisco, CA

General Contractor:

Webcor Builders, San Francisco, CA

Reinforcing Bar Fabricator:

Regional Steel Corporation, Tracy, CA

Total Project Cost:

$350 million

Total Project Size:

1,100,000 sq ft

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