Team Building 1,397-ft 432 Park Avenue Tower Went to Extremes to Make It White
Few know more about deceptive appearances than the team producing the world's first high-strength, white-concrete, exposed-perimeter structure for the tallest residential tower in the Western Hemisphere. On the face of it, Manhattan's 1,397-ft 432 Park Avenue—the first supertower designed by Rafael Viñoly Architects PC—looks straightforward. The upended spaghetti box, enclosed and 74% complete, has no bedeviling setbacks, twists or swoops. Columns and spandrel beams form a consistent checkerboard pattern. The white color never changes. Every window is the same size.
But it is the very regular, repetitive—and white—perimeter columns and spandrel beams that had the contractors, who already were stressed by the challenges inherent in building a supertower, climbing the walls. "It looks very simple," says Justin Peters, project executive for the construction manager-at-risk, Lend Lease (LL). "It isn't."
The pale complexion of the unclad perimeter columns and beams—achieved using white, instead of gray, cement—was the tower's single most mind-boggling feature. "The exterior was the make-or-break part of the job," says Peter Rodrigues, executive project manager for superstructure concrete contractor Roger & Sons Concrete Inc.
The white, high-strength concrete stacks up as the greatest challenge ever requested by a ready-mix producer, adds veteran mix developer Andreas Tselebidis, director of sustainable concrete technology and solutions for mix designer-chemical supplier BASF Corp.
White cement, which is finer than gray, changes the mix chemistry but not in a good way (see story, p. 25). Yet the self-consolidating concrete, pumped to 1,397 ft, had to pass muster in terms of strength, modulus of elasticity, fluidity, pumpability, color and appearance.
The tube's specified strength, as high as 14,000 psi, didn't help the flow. Because of the height, there were pumping challenges related to weight and time, even for the interior's gray concrete, says Kenneth Krautheim, technical director of Ferrara Bros. Building Materials Corp., the ready-mix concrete producer. The exterior was worse, thanks to the temperamental high-strength white concrete, which becomes tacky and thus difficult to mix, deliver, pump and place.
"Initially, every time we did a mock-up, the pump clogged," says Peters. "But we didn't place the first floor columns—most important because they are the most visible—until we got it right," he adds.
It took eight months to bid the concrete work and six months to get the white mix down—two months longer than expected. "It set us back, though it was before we had guaranteed a schedule," says LL's Peters.
Overall, LL says it achieved its October 2014 topping-out milestone with the help of two-shift, six-day work weeks. Initial occupancy is set for August; substantial completion is set for early next year.
Developers CIM Group and Macklowe Properties Inc., which declined requests for interviews, do not allow the building team to provide cost data and do not release sales data. Condos start at $7 million, and the penthouse sold for $95 million, a spokesman confirms.
The condominium tower contains 104 units, beginning at level 18. Up to 18, floors are devoted to retail use, residence amenities and mechanical space.
The design grew from the inside out. "We started with the core and the floor plan, rather than the height and shape," says Jim Herr, partner in charge for Viñoly.
The tower's footprint is a 93.5-ft square, an efficient size for residential were it not for the height. Because of the height, structure and services eat up a great deal of livable space, says Gloria Glas, a partner of executive architect SLCE Architects LLP.
That reality pressured the designers to maximize saleable floor space. For example, the architect introduced a double-switchback core stairway because it takes up less room than a single switchback. The move freed up 200 sq ft—enough for another bathroom.
The switch also increased floor-to-floor heights to 15.5 ft from 12 ft. "It's a luxury condominium, so the developer can sell the extra floor space to compensate for the cost of the extra height," says Herr, adding that a bonus of the double switchback is 12.5-ft ceilings.
For building services, the strategy was to break the tower into five stacked, 12-story zones, each served by a two-story mechanical level. "We pull in fresh air at the mechanical levels, instead of at the street," says Gary Pomerantz, executive vice president for WSP USA Building Systems. "That reduced riser size."
WSP also devised a system to reduce the size of the electrical conduits. A basement transformer boosts to 5,000 volts the utility's incoming 460-volt service. Then, at the mechanical floors, the electricity is reduced to 208 volts. "This reduced a 100-square-foot riser to 10 square feet and cut the amount of copper wire by 90%," says Pomerantz.
The structural engineer also minimized member sizes by using up to 14,000-psi concrete and up to 97-ksi, large-dia rebar. The building has 2,915 tons of 97-ksi vertical rebar, which is the most extensive use in a high-rise anywhere, says Silvian Marcus, director of WSP USA's Building Structures unit.
The 2.5-in.-dia, 97-ksi rebar brought its own challenges. For example, it required a template for accuracy in positioning. And at large diameters, the code does not permit splicing, only mechanical connections; the threaded 97-ksi bars are connected by sleeves.
In terms of space savers, Marcus says the exposed structure and consequent absence of cladding added 8 or 9 in. to the usable floor plate and eliminated a cost. But to achieve the specified 44-in.-wide columns and 44-in.-deep spandrel beams, WSP had to increase depths. At lower floors, columns are as deep as 6.5 ft.
The primary concrete structure is a square tube—the central core—within the expressed perimeter tube, which has seven columns on a side, 16 ft on center. Flat-plate floor slabs are typically 10 in. thick.
The foundations consist of spread footings, with tie-down rods into rock to resist overturning. Only the core sits on a mat.
The skinny tower's ratio of width to height, called the slenderness or aspect ratio, is nearly 1:15. "The New York City code considers anything in excess of 1:7 [as being] slender," says Marcus.
Wind-tunnel tests at RWDI revealed unacceptable dynamic motions and acceleration when the engineer followed the building code's standard stress requirements (see p. 26). "Initially, it was quite a lively building," says Derek Kelly, RWDI's project manager.
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.
For the exposed perimeter tube, crews eventually achieved a four-day typical-floor cycle until floor 45; above that, a floor took three days. Outrigger floors took five to eight days, as anticipated.
To keep pace on the perimeter tube, rebar maker Retech Systems LLC prefabricated the 97-ksi rebar, supplied by SAS Stressteel Inc., into 15.5-ft-tall column modules, following a system patented by consulting engineer Felix E. Ferrer. A template holds the bars in position. Roger fabricated spandrel rebar into cages. For the first 20 floors, crews site-assembled a 93.5-ft-long spandrel cage—a full building side— leaving out stirrups at column locations.
The first cage "took 15 minutes from the time it was picked to the time we disconnected the crane hook," says Rodrigues.
At level 20, the crane picked, per side, two spandrel cages, which were prefabricated at Roger's yard in Brooklyn because there was no longer room available on-site. "If we had to pick the longer cage at the top of the house, it would have been a disaster because of the high winds," says Rodrigues.
To reduce congestion that could lead to honeycombing and voids, spandrel rebar was 80 ksi, instead of a lower strength. Overlapping corner cages were the most complicated to install, says Rodrigues.
Roger crews first set the column rebar cages. Workers then installed self-climbing column-and-beam custom steel formwork with stainless-steel face sheets and built-in chamfers, reveals and drip notches. "We fabricated the forms to the exact shape of the columns and spandrel beams," says Michael Schermerhorn, senior account manager for Doka USA Ltd.
Next, crews set the slab forms and closed column forms; slab rebar and spandrel rebar cages followed. When the spandrel rebar cages were set into the form, long bars for the columns stuck up through the spandrel cage. Crews then placed concrete in the columns. The next day, they placed slab concrete and spandrel concrete. A day later, crews would set the column cages on the floor above and open and unfold the column and spandrel forms. Then, workers rolled the forms outbound, 2.5 ft onto the top level of a four-tiered work platform. Once preliminary work was completed, the form climbed to the next level.
Pumps were on the street, riser pipes ran up the core wall, and the placement boom was set on a gantry on the core. The boom had to reach to the perimeter, more than 30 ft below.
Though the concrete was self-consolidating, "we vibrated the heck out of it to make sure there were no voids," says Roger's Rodrigues.
Pumping white concrete uphill was a big challenge. Crane buckets were at the ready in case of pump clogs, especially for spandrels, which were cast a complete level at a time to avoid cold joints.
The team leaders closely monitored the concrete and adjusted the mix when necessary, especially for the 10,000-psi concrete in the building's upper reaches.
Casting started in the winter. "With every pour, we wanted to change to gray concrete, which enjoys cooler weather," says Chuck LeRoux, LL's senior project manager. "When it got warmer, the operation got better," he adds. Still, the "white mix is less forgiving than gray" and was always "looking for a hug," he says.
Microclimates
Above 800 or 900 ft, the weather was often different. One day, there were 40 mph winds at the top but not at the base. Another, it snowed only on high. Once, the top was in a rain cloud. "We had to guarantee the schedule with no idea of what was going to happen with the weather," says LL's Peters.
At high altitudes, it's not just the weather that can get turbulent. With so much pressure and so many parties involved for more than three years, there were many battles, says Peters.
"We were dealing with some rough individuals," especially for the superstructure, Peters adds. "Every day ended with a screaming match."
Peters says his biggest lesson learned on the job is to pay closer attention to group and individual dynamics. "We had to learn to deal with the same people in an intense atmosphere for a longer period than on most projects," he says.
LL may get a chance to apply that lesson and then some. The construction manager is gearing up for four more Manhattan supertowers. At 1,775 ft, the tallest would dwarf 432 Park Avenue.
