V-Shaped Megacolumn

At the fourth floor, loads from Sesil's cascade column lines transfer to a 45-ft-tall braced megacolumn, shaped like a "V" with a 20-ft-long flat base. The composite megacolumn, made from 10,000-psi architectural concrete and an embedded steel beam, resolves in a spread footing socketed into Manhattan schist.

The megacolumn also works with the core shear walls to resist lateral loads imposed by strong winds off the Hudson.

To meet circulation-system dimensions at the seating rake of the second- to fourth-floor auditorium and allow for the transition to rounded columns above, the V-legs taper in two directions.

A 27-in.-thick, story-tall wall below the ground floor gathers the V-leg loads, acting like a grade beam or shear wall. The wall handles unbalanced live and lateral loads, working with the core.

To accomplish the cascade's long cantilevers, steps and transitions in the spaces and provide a thin slab-edge profile while accommodating the multistory-spanning structural glass-fin facade, the cascade's post-tensioned slabs, which contain steel embeds to receive curtain-wall brackets, vary in depth from 8 in. to 24 in.

Aside from the lobby, all flat floor plates, as large as 124 ft x 46 ft, are post-tensioned to minimize depth. To minimize slab weight in non-tensioned areas, crews from Difama Concrete Inc. installed rows of pumpkin-shaped void forms to lighten the slab. Weight was reduced about 8%, according to LERA.

Thanks to the cascade's cantilevers and the tight tolerances of the glass curtain wall, Sciame had to have slab elevations surveyed five times: pre-pour, post-pour, after post-tensioning, post-shoring and five months later, in January, before the start of the curtain wall. The norm is two or three surveys per job, says DaRos.

Beyond geometry, megacolumn construction ranks, to date, as the trickiest single element of the cascade. First, crews from steel fabricator-erector KPM Construction Services assembled and welded the steel flat on the basement slab.

The crane pick for the assembly, which weighed 45 tons, followed. "We had to bring in a special crane," says DaRos.

The weekend pick in February 2014 went "pretty well," considering it was -5°F, not counting a strong wind-chill factor, he adds.