One Project, but Many Seismic Solutions
Wind is also a worry
Protruding from the museum's west facade, and sheltering a café below, is a 62-foot-long and 7-foot-deep cantilever, composed of three copper-sheathed "wings" tied together at the outboard edge. Typically, gravity is the dominant design consideration for a cantilever. But at the de Young, seismic and wind forces are more critical.
The element was analyzed and designed to avoid the phenomenon of resonance, or the matching of its fundamental period with the dominant periods of vibration from seismic or wind forces. Matching the periods could impose "excessive forces and movements," says Lizundia.
To keep the cantilever's fundamental horizontal and vertical periods at about 0.5 seconds, a comfortable margin below the isolated structure's period of 3 seconds and high-energy wind vibrations with periods over 1 second, the element is composed of 10 primary trusses, spaced 24 feet on center. Spanning between these components are Vierendeel cross trusses, about 8 feet 9 inches apart. A hat truss at the cantilever's edge ties the whole together and helps stiffen the structure. In addition, a diagonally placed primary truss defines the northern edge of the center wing and acts like a "super brace," says Lizundia, further stiffening the entire assembly. The resulting cantilever meets the demands imposed by gravity, as well as seismic and wind forces, and satisfies the architects' desire for structure that would be visible through perforations in the copper skin only as an elegant shadow.
Fixed, but far from simple
Because the de Young's tower houses educational programs rather than exhibitions, the nine-story structure did not require base isolation. Unlike the lower gallery building, the reinforced-concrete tower has a fixed based. However, it is anything but straightforward.
The tower has a rectangular footprint, roughly 38 by 90 feet, at the lower three floors. The upper floor slabs then transform to parallelograms that gradually become more exaggerated as the building rises. At the same time, each of the upper six floor slabs rotates relative to the slab below to align with the city's grid beyond, so that the ninth floor's northeast and southwest corners project almost 26 feet from the lower floor corners and seem to hover over the surrounding trees.
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The tower has a "coupled shear wall system" composed of two elevator cores linked by 18-inch-wide and 5-foot-deep beams. The components absorb energy from longitudinal seismic or wind forces. End walls, 20 inches thick, function as both bearing and shear walls, taking forces in the transverse direction. Posttensioned girders support the floor slabs and span between end walls from the fifth floor up. A torsion box, made up of walls at lower floors on four sides, works in both directions.