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Local architect Lohan Caprile Goettsch turned a lemon into lemonade not only by filling the space between the lobby and the office floors with an income-earner–a 489-vehicle garage–but by expressing the garage ramp as a spiral around the core. Then, LCG wrapped three sides of the lobby with a 335-ft-long, 45-ft-tall cable-supported glass wall, pragmatically marrying architecture and structure.

Lobby. Stepped ceiling of glass-enclosed lobby in Chicago is underside of garage ramp. Fourth in U.S. Vertical-cable glass wall joins transparency trend.
Photo left courtesy of LCG Architects/Mark Ballogg, Steinkamp/Ballogg Photography. right by Tudor Hampton for ENR

Even the garage framing, a nine-story steel megatruss, is part camouflage and part architecture: Its chevron web members are expressed.

The tower opens May 15. "As a passerby, you don’t perceive that there is a parking structure within the building," says Steven M. Nilles, principal for Lohan Caprile Goettsch.

The 15,000-sq-ft glass wall cost about $3.5 million, according to Tom Niepokoj, vice president of sales for the Bolingbrook, Ill. office of Awalls Inc., the installer, and a unit of Harmon Inc. since December.

The partial ellipse in plan hangs from the megatruss via 67 post-tensioned, stainless-steel, wire-rope cables. Designers eliminated horizontal elements, which would have reduced the wall’s transparency, except for a thin bead of silicone caulk sandwiched between the 5-ft-sq, nonreflective panels.

In addition, because the lobby wall of the 1-million-sq-ft building is curved, horizontal strands would have tightened against the glass panels and reduced their stability, says Ron Klemencic, president of the job’s structural engineer, Magnusson Klemencic Associates, Seattle.

Expressed. Chevrons of garage megatruss, columns (above, left) and garage ramp (above, right) are pragmatic design elements. (Diagram source: Magnusson Klemencic Associates; photo right courtesy of Michael Goodman for ENR)

Awalls and its design-build partner MERO Structures post-tensioned the 29-millimeter-thick cables at an average 40 kips and secured the ends with a steel saddle-and-disk assembly. Workers tensioned the cables individually at the bottom. "It was very much like tuning a piano," Klemencic says.

The effect "put a lot of pressure on the floor framing," says James F. Dushek, senior vice president at the Chicago office of Bovis Lend Lease, construction manager for the tower. The process compressed the lobby and third floors, requiring heavier beam connections in the 60-ft-sq concrete core than on other floors.

(Diagram source: Lohan Caprile Goettsh Architects)

The design team was so focused on the dynamic effects of post-tensioning on the building that it overlooked deflection when sizing the glass panels. "We had to reorder some glass," says Nilles.

Under its rated wind load of 25 psf, the wall deflects 6.5 in. In the fall of 2003, Awalls-MERO tested a full-height model, 5 ft wide, under dynamic load. During an overload test, wind pressure of 55 psf was applied without glass breaking–but the caulk burst, Niepokoj notes. Workers finished the "real" wall last May, after nearly five months of construction.

The maximum 90-ft-deep megatruss, with its chevron web members and horizontal floor trusses at top and bottom, is on three sides of the 682-ft-tall building. The megatruss, 182 ft x 160 ft in plan, cantilevers over the smaller-in-plan lobby and bears on 50 to 60-ft-tall perimeter pipe columns–two on a side–filled with 12,000-psi concrete. The stainless-steel columns, just outside the glass wall, also are expressed as architecture. Underneath the core, a 100-ft-sq, 10-ft-deep structural concrete mat ties together new and existing caissons.

Starting in August 2003, workers from Cordeck Sales Inc., Chicago, built the columns and megatruss on 12 temporary braces, each rated at 1,200 kips. Falsework came out in January 2004, reminding Klemencic of "the old table-cloth trick," he says. At most, the structure deflected 3Ú8 in.

The fourth side of the perimeter structural steel tube is set against the view-blocker. It has standard framing.

Connections were tough to detail due to complexity of the load pattern, says Andrew Vogl, project manager for steel fabricator Merrill Iron & Steel, Schofield, Wis. The megatruss diagonals tip away from the core and building corners cantilever over the lobby.

Glass cable walls are fashionable in Chicago and across the U.S. Though 111 South Wacker’s vertical-only cable wall may be the fourth of its kind, it wraps an office tower lobby of a completely different color.

he inspiration for the unorthodox stacking plan of a 52-story office tower in Chicago, which has a two-story-tall lobby topped by a nine-level parking structure topped by 38 office floors–is a 15-story "view-blocker" to the east. The building, which cuts off views of Lake Michigan, would have put a major damper on lower-level office rents of the $275-million tower, according to the local developer, The John Buck Co.