While it may seem that there’s an “anything goes” attitude toward new construction in China by foreign architects, especially in the wake of the radical structures erected for and around the 2008 Olympic Games in Beijing [record, July 2008, Special Issue], there is a real chance that a project is subjected to an intensive redesign. “We were hoping to avoid CCTV’s fate,” Nordenson says, referring to the looping headquarters tower for China Central Television designed by Office for Metropolitan Architecture with Arup, whose initial design was rejected. Nordenson adds, “Though the government pushes for radical buildings, there is an old guard of Communist-era engineers that maintains certain values of frugality and conservation of resources.”
To that end, Holl and his team looked for ways to make Linked Hybrid’s overall design as straightforward and repetitive as possible. Each building features a structural core, concrete slabs, concrete cross walls, and a perimeter concrete moment frame — all of which give it the necessary stiffness to contend with movement at the level of the skybridges.
The concrete structural components respond to the architects’ and engineers’ desire for simplicity and frugality, but the design gets daring wherever steel is introduced. The goal of the bridges’ structural design was to maximize transparency and create floating hallways of light, traversing between 65 and 197 feet between the heavy concrete towers. (Some of the one- to two-story-high bridges slope to connect towers at different levels.) Each skybridge is composed of a steel truss whose rigidly connected vertical and horizontal members form a Vierendeel frame. Though the truss is strong enough to support the skybridges, diagonal cables were added.
The bridges, which were assembled on the ground and hoisted into place, rest on friction pendulum isolators. Nordenson proposed locating the isolators on only one side of each bridge, but Xiao Congzhen of CABR upped the ante, suggesting that the isolators be located on both sides. (As constructed, an isolator sits at each corner of a bridge, for a total of four per bridge.) This essentially means that the bridges are floating between towers. “You’re counting on the fact that movement of the buildings away from the bridges is never going to be larger than the extent of the supports,” Nordenson says. “That is the case, but it’s still a fairly adventurous thing to do.”
The isolators — provided by California-based Earthquake Protection Systems — are shaped with a radius to achieve a described period of vibration that will minimize the shear transfer by reducing the resonance. In an earthquake, the bridges will move up to 15 inches relative to the buildings, sparing them, and the buildings, from the effects of lateral forces. By virtue of the curved bearing surface, the structure is lifted as it slides sideways.
It is the 33-foot-long, multistory cantilevers at the top of five of the towers, however, that determined the seismic design. The structure of the cantilevers is also steel and completely independent of the skybridges, which in some cases sneak beneath a cantilever. In those instances, a gap between the structures keeps them separate, assuring that neither is supporting the other.
The massive cantilevers exert forces that are resolved through the introduction of discrete diagonals in the towers concrete exoskeletons. Rather than creating larger concrete forms, steel members are inserted in those diagonals. Additional diagonals are located where there is increased demand on the concrete structure as a result of the skybridges, or to reinforce holes in the grid. The overall seismic design was analyzed by performing a shake-table test, a common practice in China in which structural scale models of the building are subjected to a variety of simulated ground motions.
http://continuingeducation.construction.com/article.php?L=5&C=630&P=3
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