The skinny spaghetti box was the culprit. At five levels, concrete outrigger shear walls with structural steel embeds, or slabs with drop beams, connect the tubes and stiffen the building. The engineer also increased the mass and reduced acceleration—a change in the velocity of a building's natural sway—by thickening to 18 in. the uppermost 10 slabs.
The original shape also presented issues regarding adverse acceleration due to winds. To reduce the acceleration, the designers altered the architecture and created double-height openings—by eliminating windows—at the tower's five two-story mechanical levels.
The openings let the wind flow through, instead of around, the tower. A central drum around the mechanical equipment further minimized the wind's vortex shedding. The design change reduced acceleration by as much as 15%, says Marcus.
Finally, a roof-level-tuned mass damper, currently under installation, will lower the acceleration to be on par with top-quality residential structures, says Marcus.
White Matters
LL was so concerned about meeting the performance specifications for the white concrete that it hired a non-bidding contractor to cast a column-spandrel mock-up. "It didn't look great, but it taught us a lot and convinced us to get column-only mock-ups from the three serious bidders," says LL's Peters.
For Roger, the white-mix design was the first priority. For this project, Roger cast 12 mock-ups—11 more than is typical for architectural concrete.
During each mock-up, the team modified the mix to improve finish and color, reduce shrinkage and cracking, and minimize voids, bug holes and honeycombing. "At first, the concrete was coming out blue because of the slag, though it faded," says Rodrigues.
For the final mock-up, crews experimented for color and finish on a permanent below-grade column, which otherwise would have been gray.
Superstructure work began in October 2012. Crews first started to form the core, which stayed two floors above the perimeter tube. "The core walls were traditional," says Rodrigues.
