Mind-Bender In Upper Manhattan
Find the load paths. That could have been structural engineer Daniel A. Sesil's mantra while seeking support for the daring southern portion of Columbia University Medical Center's 15-story Medical and Graduate Education Building in Upper Manhattan.
Though it encloses only about 100,000 sq ft, the $77-million base building, designed by Diller Scofidio + Renfro, has a split personality. The northern section is conventional. In dramatic contrast, the southern side is brazenly asymmetrical. DS+R describes the mind-bending form as a glass-enclosed "cascade" of interconnected study and social spaces that afford spectacular views of Manhattan's skyline and the nearby Hudson River.
All floors cantilever, half of them slope, and much of the concrete is exposed. "It's incredibly animated, with two- and three-story spaces," says Anthony Saby, DS+R's design leader.
For the slanted and canted cascade—which has no perimeter columns but shares a core with the north side—the hunt for supports started on high. "I drew from the top down, looking for load paths all the way," says Sesil, a partner at Leslie E. Robertson Associates (LERA).
He soon found two main load paths. The cascade's floor slabs, which cantilever 15 to 25 ft, stand up thanks to two interior composite concrete-and-steel column lines and the core. Sesil uses walls, stair ramps and the auditoriums' sloped slabs to connect and stiffen the floor plates.
A two-column-line system with folded plates sounds routine to build, if the column lines went straight to the foundation. They don't. Sesil had to zig and zag each one around various program spaces.
In addition, the column lines—offset from each other by 8 to 10 ft—are not identical. Shapes, slopes and sizes vary.
Their load paths also vary. From the top down, each line is vertical for several floors and then slopes. The east column slopes below the eighth floor; the west slopes below the seventh. About the only things in common are some embedded steel beams, 97-ksi reinforcing steel and 8,000-psi self-consolidating architectural concrete—all of which complicated construction.
The cascade's geometry and dearth of typical connections are testing the building team's mettle. "How to put it together is the fun and the challenging part," says Maddy Burke-Vigeland, a principal with the building's executive architect, Gensler.
For the contractors, the challenges have overpowered the fun. "There was a huge learning curve," says Robert E. DaRos Jr., assistant vice president for the construction manager-at-risk, Sciame Construction LLC. DaRos figures the superstructure took 50% longer to build than a regular, repetitive job.
Still, the 70%-complete building is on budget and on schedule for completion next spring, says the Columbia University Medical Center, which is funding the job with more than $70 million in gifts.
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.
Workers braced the assembly temporarily to keep it in place as the rebar and formwork went in. The removal of the braces had to coincide with the casting of the encasement and floor diaphragms.
The next "coordination nightmare" was the installation of the 97-ksi rebar in a way that wouldn't adversely impact the architectural concrete, says DaRos.
Above the basement, the basic construction sequence was two floors of steel over the weekends, followed by superstructure concrete during the week.
Sciame had to work out jurisdictional issues for the use of the tower crane. "We had different agreements for the steel and concrete contractors," says DaRos.
The slabs were another operation that required extra attention. DaRos is not a fan of post-tensioning. "It hinders co- ordination with the mechanical trades" because coring is restricted, he says.
Building information modeling helped with the mechanical coordination. Tendons could be shifted 6 in. to the left or right, under the structural engineer's guidance. Also, some of the void formers could be eliminated.
Post-tensioning also required coordination of the conduit installers and rebar crews. And it delayed form stripping until after tensioning. "It took another month per floor to do this phase," says DaRos.
Still to come is the installation of the cascade's exotic glass curtain wall. "It's probably the most complex in the city" because of its long vertical glass spans and tight tolerances, says DaRos. He expects crews to finish the work in October.
DS+R's Saby wishes the glass system, made more difficult by slab deflections, had been designed with more fit-up forgiveness. Otherwise, he is satisfied—even inspired—by the work to date: "It's probably going to push us to do more aggressive things in the future."
