Glass and steel unite
Approaching the base of the new 50-story UBS Tower in Chicago, the glass wall of the lobby comes into view. It is so transparent that you're not sure it's really there. Seen first in Europe, this dramatic example of the dematerialization of the membrane that separates interior and exterior space is the result of combining extremely clear glass with a cable net system used for the first time in the U.S.
When Chicago developer The John Buck Co. commissioned local architect Lohan Associates (now Lohan Caprile Goettsch Architects) to design the office tower in 1998, it expressed a desire for a traditional stone exterior wall, according to Lohan project manager Steve Nilles. Buck was asked if an alternate exterior material would be considered. The answer was affirmative — as long as the view into the lobby could be made so transparent that the building's core wall would be highly visible.
To accentuate the 40-ft.-tall entrance lobby, the architects specified a full height glass curtain wall. Working with the local office of Thornton-Tomasetti Group of New York City, the building's structural engineer, two proposed designs were developed. One incorporated cantilevered glass fins with point fixings that would support the glass panels. The scheme chosen, however, did not require any drilling of glass. It is a flat, prestressed cable-net system with nodes that support 5 ft.-by-5 ft. glass panels.
Each node restrains corners of four lites of glass. Cruciform metal shelf pieces extending from the nodes support two glass lites, with silicone gaskets separating the glass and metal. Restraining blocks between the face plate and back plate of the nodes clamp them to the cables, transferring the weight of the wall to the vertical cable.
The wall was developed under a design/ build contract awarded to ASI Advanced Structures Inc. of Marina Del Rey, Calif., and Trainor Glass of Alsip, Ill.
Natural light floods the 40-ft.-high lobby of the UBS Tower, even on cloudy days.
Three elevations divide the lobby: two end bays with entrance portals and seven regular bays. All bays are enclosed with monolithic glass panels supported at their corners by custom nodes. The nodes are attached to intersecting points of vertical and horizontal cable. Cables and nodes, both of stainless steel, are the only structural members that interrupt views through the glass.
Vertical cables span between floor beams, and horizontal cables between columns. The cables are attached to steel connection boxes. All cables were prestressed, and the adjoining steel columns were encased in concrete to provide necessary stiffness to accommodate the loads imposed by the cable net.
The first use of a cable net-curtain wall system a decade ago was in the lobby of the Kempinski Hotel in Munich, Germany. Chicago-based Murphy/Jahn and the German firm of Schlaich Bergermann and Partners designed the hotel. Because Nilles had worked on the hotel project while employed by Murphy/Jahn, he was familiar with the cable-net concept.
The UBS Tower lobby glass has a non-reflective coating that the hotel lacks. Nilles says this substantially improves its nonreflective character — lowering it to less than 1 percent, compared with a 7 percent reflectivity for normal float glass. People standing outside the lobby will not see their reflected image, he adds. The wall's 9,500 square feet of tempered, low-iron glass was manufactured by Schott Glas, Mainz, Germany.
Out-of-plane forces in the wall are resolved by the tension in the net, says Will Shepphird, project engineer with ASI. He likens the cable net system to a tennis racket, because its rigid outer structure rings tension members within. The weight of the glass, the cables and the nodes is taken by a floor beam on the fourth floor.
Stainless steel nodes are located at points where the vertical and horizontal cables intersect.
All glass structures — and particularly cable-net walls — require tight fabrication and installation tolerances, Shepphird notes. Because the cable intersections determined the node locations and subsequently the glass support points, precise installation of the cable connection boxes was crucial to the project's success.
The cost of a cable-net glazing system is comparable to that of a point-fixed system, Shepphird adds.
A mock-up test of a wall section was conducted by Construction Research Laboratory in Miami both to allay safety concerns of the City of Chicago and to address performance warranties for the owner.
Test rig simulates stiffness
A full-size test rig was fitted to the testing chamber. Steel truss members and wide-flange beams simulated the stiffness of boundary members. A 25-ft.-wide and 37-ft.-tall aperture with four vertical cables, seven horizontal cables and 28 nodes represented the average job site condition. The mock-up rig passed both air and water infiltration tests.
The owner and architect established a maximum deflection threshold of 7 inches. In the end, the cable net deflected less than the pre-established limit. A further test to destruction was attempted. But suction pressures exceeding 150 psf (equivalent to a wind speed of about 250 mph) failed to damage the system.