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Painful Precision Allows Frank Gehry to Twist the Glass Envelope
Crews site-bend glass with gloved hands to curve a curtain wall that evokes waving sails
By Nadine M. Post
Nadine M. post/ENR
Three years ago, standing on a cladding factory floor watching robots lift a 10 x 5-ft window pane, Frank Gehry was surprised to see the glass sag under its own weight. It was a pivotal moment, for it led to a suggestion that freed the famed architect to sculpt his first all-glass curtain wall, full of his signature sweeps but lacking budget creep. The novel twist on the 10-story headquarters for Interactive Corp., near the Hudson River in Manhattan, is that crews purposely cold-bent each of the 1,437 flat-glass cladding panels with their "bare" hands as they installed them. It had never been done before.

The 195,000-sq-ft building's wavy form evokes ship sails. The cold-bending of the glass, instead of prohibitively pricey factory heat-bending, "was really the trick to give us the shape," says Craig Webb, Gehry's design partner. "The breakthrough was to actually think of doing it on purpose," says Webb. Warped panels are normally a "no-no" in the curtain wall sector, he adds.


Interactive Corp. Building

"Not anymore," says Mike Budd, president and CEO of cladding supplier Permasteelisa Central-South, Miami, and the person Webb credits with suggesting the cold-bending approach that day when Gehry and his team realized glass is a flexible material. Budd says the cold-warping idea came from his work on Gehry's Walt Disney Concert Hall, which has cold-warped stainless-steel panels, and from observing glass handled in the shop and in curtain-wall mockups.

Desimone Consulting Engineers
Desimone Consulting Engineers

Like most Gehry adventures, this voyage required something never attempted in the U.S. and never done to the same degree anywhere. Success relied on an outside-in design process that, in turn, depended on precision engineering and construction. Permasteelisa had to figure out just how much stress each aluminum-framed double-glass unit could take before it broke. And because the building's concrete frame served as the skin's template, the concrete contractor had to set the curtain-wall bracket embeds in the slab edges perfectly.

"The curtain wall drove construction," says William Kell, project manager for Sorbara Construction Co., Lynbrook, N.Y. Sorbara, new to Gehry's envelope stretches, built the concrete frame under an $11-million contract. "There was zero tolerance for error," adds Kell.

No Learning Curves

The $95-million building is the Los Angeles-based architect's first in New York City. True to Gehry, the only curves missing are learning curves. Cladding units, though assembled flat at the plant, are still trapezoidal, triangular or rectangular. All but 88 are different. Exterior walls slope up to 15° from the vertical. No two floor plates are the same. They are rotated slightly one above the other, and they decrease gradually from 22,000 sq ft to 15,000 sq ft, creating another twist. The top four stories are set back 15 ft on three sides. Most perimeter columns slope at different angles up to 20°, within a floor and floor to floor. Only three column lines are continuous.

Nadine M. Post/Enr
The building’s wavy and sloped concrete frame served as the template for the cold-bending of the glass curtain-wall panels.

Thanks to the undulating form, components as mundane as window-washing equipment and window shades became big headaches, says Terry L. Carbaugh, project executive for the New York City office of Turner Construction Co., the construction manager for Interactive and its partner, the local Georgetown Co.

For example, Turner tried, but failed, to get approval for a window-washing system used in Asia and Europe but never in the U.S. that would move side to side, not just up, down, in, out. "After a year of thinking, we went to a conventional system," says Carbaugh. "Some areas are impossible to reach." In addition, shade guides could not be fitted for the oddest shapes, so some windows are bare.

Eric Levin/IAC
Slab edge had to be perfect so that embeds and brackets could be perfect, allowing the warp to be perfect.

The geometry made work "particularly difficult" even for Permasteelisa, a 20-year veteran of Gehry sweeps, says Alberto De Gobbi, president of Permasteelisa Cladding Technologies LP, Windsor, Conn. The job was a collaboration between Budd and De Gobbi's groups.

The first step—even before Gehry could set the building's final shape—was to set bending limits and constructibility parameters for the units. Permasteelisa built mock-ups and ran calculations, including a finite-element analysis to determine the amount of long-term stress in the structural silicone affixing the glass to its subframe.

Eric Levin/IAC

Some panels are warped in excess of 4 in. out of plane over their height. Allowable twist was a function of panel size and geometric configuration. The typical panel is 8 in. thick. Height varies from 14.5 ft to 20 ft and width varies from 3 ft to 12 ft.

Aluminum frame extrusions are designed to allow the horizontals to handle 5° of rotation relative to each other. Vertical extrusion sets have multiple combinations that take up plus or minus 3° of rotation for each set.

After setting rules, the design team imposed, like wallpaper, the resulting skin on Gehry's 3D computer model and established the panel layout and configuration.

Desimone Consulting Engineers

The structural template followed. The engineer chose a flat-plate concrete system because of its simplicity, the highly articulated slab edges and the high steel prices of the time, says Stephen V. DeSimone, president of local DeSimone Consulting Engineers.

DeSimone used Gehry's 3D model to conceptualize the structure but not document the project. "The local contracting community would not buy into the idea of using a 3D solid model for dimensional control," he says.

Permasteelisa Cladding Technologies

Column layout was tricky. Perimeter columns wanted to be in the same location relative to each floor, but floors were different relative to one another. Connecting the dots resulted in sloping columns, which were a more pure solution than transferring out columns at every floor; but the rub is that all the columns slope in a counterclockwise direction. That introduced a horizontal twisting force that exceeded the wind and seismic forces, explains the engineer. DeSimone used an off-center shear wall core to resist the twist. Maximum twist from gravity loads and long-term effects was 5/16 in. at the northwest corner, as predicted.

To achieve 38-ft spans in flat-plate concrete, the engineer used a 1-ft slab—nearly twice the usual thickness. At the sixth floor, a 2-ft-thick slab transfers loads from setback columns to base columns.

Permasteelisa Cladding Technologies

Sorbara started the superstructure in October 2005. Work called for exact placement of 1,600 steel embeds for the glass panel brackets. If the brackets were not set to 1/100 in., site-bending would go awry. "Bracket layout was the most time-consuming activity," says De Gobbi.

To minimize error, Turner departed from custom and established direct communication between Sorbara and Permasteelisa, which set the embed locations. In addition, Turner required Sorbara to have a licensed surveyor checking work before concrete was cast, instead of after. Turner also had its own surveyor double-checking Sorbara's.

Desimone Consulting Engineers
Even the footprint is odd.

Because of the building's form, the slab edge was a segmented radius, with a 10-lb embed every 4 ft. Using a global positioning system-based 3D transit, surveyors first located every insert in theoretical space. "Then we connected the dots" to create the slab edge, says Kell.

Sorbara originally projected a concrete schedule of two weeks per floor. That included a full day for the surveyor, with no interference from other trades. But the surveyor only needed a half day. Crews soon reached a four-day cycle. Sorbara finished the superstructure Feb. 1, three weeks ahead of schedule.

Permasteelisa could only take partial advantage of the early finish. The unshored frame, which stayed shored until it was topped out, needed time to twist and shorten under its own weight before panels went up, says DeSimone.

Desimone Consulting Engineers
Eric Levin/IAC
Columns tilt this way and that, floor plates differ and the building has many faces.

Still, curtain wall installation started on Feb. 22 earlier than planned, and finished Aug. 17. Moving up instead of across the building, as is customary, crews of four or five affixed each panel to the one below. After engaging three corners to the brackets, they bent the remaining corner inward or outward to create the twist.

The installation was a success, says Turner. Though 60 units had to be replaced because of improper fritting or broken glass during handling or shipping, not a single unit cracked during site-bending.

The job reached substantial completion in November, seven months late and $5 million over the original budget. But the delay and extra cost were not caused by the novel curtain wall. In mid-2004, during excavation, crews from Urban Foundations/Engineering LAC, East Elmhurst, N.Y., hit 2-ft-sq and 30-ft-long timber walls built in cribs loaded with boulders—leftovers from a pier built before the shoreline was extended. The obstructions, missed by about 20 test borings, covered 30% of the site. The snag pushed the cost of foundations, which had to be predrilled through obstructions, from $16 million to $21 million, says Turner's Carbaugh.

Unlike the foundations, the 95,000-sq-ft skin was on time and budget. But the skin wasn't cheap. The original unit cost was $185 per sq ft instead of $115, says Carbaugh. Changes then brought the envelope's total cost to $20 million. Chalk it up to the cost of innovation.


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