One Project, but Many Seismic Solutions

The complex and contextual de Young Museum, set in San Francisco's lush Golden Gate Park, incorporates an impressive diversity of earthquake-resisting strategies
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From Architectural Record
Joann Gonchar, AIA

Its stepping and leaning geometry makes the tower more vulnerable to ratcheting than a building with a more regular shape. The worry is that during a quake, the end walls could move two steps in the direction of tilt but move only one step back, says Lizundia. With the next jolt, it might move two more steps in the same direction and again move only one step back. "There could be more cumulative movement in the direction of lean, resulting in permanent displacement," he warns.

To counteract this potential "bunny hop" effect, the end walls are vertically posttensioned. Ducts embedded in these walls, each containing 12 unbonded tendons under 1,620 kips of force total, "make the tower ‘think' it is balanced," says Lizundia. "The gravity overturning moment is balanced by the internal posttensioning-induced moment," he says.

Special skin

The education tower and the base-isolated lower structure are clad with more than 7,000 perforated and dimpled copper panels, each unique. The size and density of the pattern is varied to help limit the amount of natural light allowed into galleries and to camouflage mechanical equipment. To determine the pattern's placement on each panel, the design-build supplier of the skin, Zahner Architectural Metal, developed software that would take into account such factors as where the 1.5-millimeter-thick material could be bent in relation to the voids and puckerings, according to Bill Zahner, C.E.O.

The panels, each about 30 inches tall and 12 feet long, fold and interlock. They are attached to the exterior stud walls of the base-isolated lower structure with continuous stainless-steel clips, but can float in one direction.

The owner's desire for a museum with as many large, column-free spaces as possible resulted in long spans, one more than 90 feet, and trusses as deep as 20 feet.

The cladding details for the tower are devised to permit even more movement. Because it is not isolated from the ground, the tower required a special solution that would prevent transfer of stress to its sheathing when the structure moves in an earthquake. The tower is "a whole different beast," says Zahner.

The tower's panels can slip relative to one another horizontally and are attached to 3-inch-diameter vertical steel pipes set about 5 feet apart. The pipe sections connect between each floor with a sleevelike joint that accommodates interstory drift, or lateral deflection due to seismic or wind loads. The joint allows the pipes to elongate or shorten as the building moves. The pipes, in turn, are secured to the edges of the tower floor slabs and the lower concrete exterior walls with anchors that permit rotation.

The objective of the engineering behind the tower's skin system is the same as that of the lower museum building's base-isolation system, explains Steve Huey, principal of Wallace Engineering, the sheathing's Kansas City−based structural engineer. Just as the superstructure of the lower structure is isolated from the its foundation, "the tower's skin is isolated from the movement of its frame."

 

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Originally published in Architectural Record.
Originally published in May 2006

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