“It’s quite astonishing to see a structure so pure,” says Hezi Mena, senior vice president of building structures at WSP in New York. “We tried to simplify the structure as much as possible so as not to conflict with the architect’s vision.”
The structural solution developed by the WSP team does have a pleasing simplicity, though this belies the extensive testing and refinement that went into developing it. 432 Park Avenue is made of two high-strength concrete tubes. The interior tube forms the building’s core, measuring 9m x 9m, containing the lifts and egress stairs. This is the backbone of the building. Its reinforced-concrete walls impart considerable stiffness, their thickness varying from 760mm at the base to 300mm at the top.
The outer tube is the tower’s perimeter beam-and- column frame, which is also incredibly stiff, even with the windows punched through it. At every 12th floor these tubes are joined by stiffening beams, accommodated in the double-height plant rooms to avoid encroaching on the apartments themselves. In fact, WSP has managed to remove all interior columns, giving the occupants complete flexibility in how the living spaces are configured.
Innovative Structural Solutions
The mechanical or ‘drum’ floors also play a crucial role in making the building more thermodynamic. Early wind-tunnel testing showed that the structure was suffering from significant ‘vortex shedding’ even under relatively low winds. This effect occurs as wind moves past a building, creating a fluctuating low-pressure area behind it and causing it to vibrate. The engineers’ solution was to leave the glazing out of the mechanical floors to allow air to pass through the building at regular intervals, effectively making it more aerodynamic. “It’s similar to making holes in the sail of a boat,” explains Silvian Marcus, director of building structures at WSP, the principal in charge of the project. “Instead of making the building even stiffer to overcome the movement, we flow with the wind.” There are also two tuned mass dampers on the 84th floor, weighing 1,300 tonnes. Supported on cables and hydraulic cylinders, these concrete weights counter the building’s movement and slow its acceleration to acceptable limits.
Floor-to- ceiling heights in the apartments are a very generous 3.8m, 1m higher than is conventional in New York. This not only adds to the prestige of the apartments, it also maximizes lateral living space. The extra height allowed the engineers to add a second return to the access stairs in the building’s core, helping to keep it compact, and resulting in 250ft2 of additional living space per floor.
