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.