Marvelous mesh

Cable net application grows in intelligent glazing
By Terry C. Peterson
December 1, 2007
COMMERCIAL, FABRICATION : PROJECTS

 

Her face looks on gracefully from a statue in the square, soon to be an icon of a renovation. Chosen for her calm, romantic qualities, her profile will be magnified on the prominent façade of the Fifth Third Bank building at Fountain Square in downtown Cincinnati. A glass cable net will give the building a distinctive look while meshing easily with the rest of the square structures.

As industry professionals continue to push the limits of what is possible, cable nets continue to gain flexibility in form. The popularity of the application is growing as word spreads about this intelligent glazed structure solution.

A work of art
With the renovation of Fountain Square, officials at Fifth Third Bank came to FRCH Design Worldwide, Cincinnati, knowing that the building would soon be the prominent backdrop for the square. The project called for original design. The team leaders at FRCH, including Roger Stein, project manager/vice president; Paul Lechleiter, principal; Jim Stapleton, vice president; and senior professionals Eric Arter and Russell Miller set out to design and implement.

In February 2006, officials at TriVersity Group, Cincinnati, approached Novum Structures, Menomonee Falls. Wis., a specialty contractor for high-technology spatial architectural structures and enclosures. As a part of the rejuvenation of Fountain Square, TriVersity officials asked Novum to create the celestial face of a statue from within the square and superimpose it on the south side of the Fifth Third Bank building. While the overall re-facing of the building involved about 30,000 square feet of double wall glazing, the face would occupy a 21 feet high and 21 feet wide section.

“We had to keep a visual connection between the employees behind the existing façade and Fountain Square,” Stein says. “We wanted to maximize glass for that purpose and also to give it enough character that the façade was upgraded without taking too much focus from the square itself. Glass gave it a nice character and solved the problem cleanly and elegantly.”

In creating the canvas, the team decided on 25 glass panels, 4 feet 27⁄8 inches square. The challenge then became how to minimize the seams within this grid work.

Since glass, an inherently transparent material, was the canvas, the concern was about visibility and possible shadows that any structural support system might create. Novum engineered the Novum TC-System, an ultra light, stainless steel two-way cable net system with diameter of  7⁄8 inches that matched up with the joints of the glass panels. The thin support system was completely concealed behind each vertical and horizontal glass joint.

Attaching the glass to the cable net using a conventional aluminum framing concept with bulky perimeter mullions was not an option. To eliminate the aluminum edge framing, Novum engineers suggested that the glass be used structurally and offered CCG-System, a mullionless corner clamping, ECG-System or edge clamping, and PSG-System or point fixed options. FRCH officials selected the PSG-System but with the additional caveat that the circular heads of the rotules, the PSG fittings, would have to be buried within the glass thickness.
Mold for glass feature at Fifth Third Bank building, Fountain Square, Cincinnati, Ohio
A laminated layup consisted of two glass sheets. The rotules were encased within the glass sandwich and were attached to a heat strengthened inner sheet only. The outer sheet of glass, containing the art work, would cover the rotule heads. The complete sandwich was adhered by a cast-in-place interlayer.

“The biggest advantage of the cable net system is clearly the ability to minimize any perceived support or any perceived structure behind what was ultimately the feature part of the design, the cast glass element,” Stein says.

Having technically solved all structural, transparency and attachment issues, the next step was the not so minimal task of creating the contoured art work of the face.

Architectural Glass Art, Louisville, Ky., a firm well experienced in cast glass technique, matched  the exact contours of the face, making a full three-dimensional scan of the actual statue then compressing it to the appropriate glass sheet thickness, about 3 inches total for the outer glass sheet. Architectural Glass workers created 19 positive molds by a proprietary process, and aligned them for architectural viewing and final artistic adjustments. They manufactured the negative castings to create the final facial contours, added low iron cast glass or “water” white glass and produced the final sheets. Finally, the pieces were sent to the laminator to be attached to the low iron, heat strengthened, structural inner sheet of glass that contained the pre-attached rotules. 

“The installation of the cast glass panels provides some unique challenges based on tight site logistics and ability to rig glass for installation,” says Ron Haas, project manager at Pioneer Systems Cladding & Glazing in Loveland, Ohio. “The uneven face surface does not allow for conventional equipment usage. A combined effort between the manufacturer and ourselves was needed to assure a quality installation and minimize risk of damage to the finished product.”

Says Stein: “The architect needs to put aside some of his preconceptions and be willing to put faith in the folks that know these systems best. And the owner needs to be willing to let the architect and the contractor or the manufacturer work together. Working as that kind of a team—that three-legged team—makes not only the process work more smoothly but also allows the solution to benefit from each individual’s expertise. Being able to work in that fashion was positively fresh and very rewarding.”

Cable nets gain popularity
Throughout the United States, cable nets are emerging as popular artistic expression to solving modern architectural challenges. Part of their initial popularity results from the nearly seamless, highly transparent enclosures they create.

“Typically, structural wall members get strength through the depth of the member,” says Brian Vande Zande, preconstruction engineer at Novum Structures. “Conversely, cable nets get strength through pre-tensioning which results in such minimal depth that they create virtually no obstruction.”

In addition, rapid advancements in technology and sourcing during the past few years has resulted in lower costs, more varied architectural forms, alternative cable arrangements, larger glass panels and a variety of glass attachment techniques.

When being introduced to the United States marketplace about 10 years ago, cable nets carried a higher cost as compared to more conventional solutions. This limited their use to the highest-end architectural applications. More recently, unit prices, which began near $400 per square foot, have been nearly cut in half on some projects. Price varies depending on architectural form, cable arrangements, number of penetrations and specifications.

“The introduction of systemic components, more automated engineering principles, global sourcing and a better trained field force has created a significant reduction in what the end user pays,” says Ian Collins, president of Novum Structures. “In addition, by providing engineering, supply and installation as a single source, Novum is able to guarantee pricing at the project’s earliest stages. By being able to predict the eventual cost at the project’s most conceptual stages, architects have become more confident in the technology increasing usage dramatically.”

Although cable nets might cost more than alternative solutions, the notability is profoundly beneficial for appropriate projects, Stein says.

“The uniqueness of [a cable net] more than pays for itself if it’s in an application that demands something unusual, something out of the ordinary,” Stein says.

“[Cable nets] require the kind of owner or developer who can sufficiently demonstrate that his project pro forma will allow for it,” Stein says. “If you’re looking for maximum profitability on a project, it’s not going to be the answer to your needs. But if you’re looking for a clean, forward-thinking architectural solution, it is certainly something that should be at the very forefront of your considerations.”

The success of recent projects in the United States aids the recognition of the obvious benefits of cable nets, says Scott Seyer, senior associate at Goettsch Partners, Chicago. As more contractors become familiar with the technology and engineering involved with this type of application, there’ll be more of these walls in the United States, he says. In an effort to advance the technology of the approach, in an upcoming project, Goettsch Partners will explore larger glass pieces to further eliminate components of the wall and increase transparency.

“We’re always striving to use these walls because we feel it opens up the lobby and engages the pedestrian,” Seyer says. “We’re constantly striving for a more transparent and inviting lobby wall.”

Says Steve Nilles, partner at Goettsch Partners: “It goes back to [Ludwig] Mies van der Rohe: ‘less is more.’ Now you use the new technology to create something that’s even more transparent than you could create five, 10 or 15 years ago.”

Uses in intelligent glazing applications
While the Fifth Third building project is arguably the most elaborate, it is not the first cable net art that Novum has created. Several years ago, Novum officials worked with the artists at James Carpenter Design of New York to create a 35-foot wide by 60-foot tall lobby wall that serves as a huge glass prism for the IDX Tower in downtown Seattle.
The biggest advantage of the cable net system is the ability to minimize any perceived support or structure behind the cast glass element
A two-way, thin, cable net supports a vertical glass wall that seals up the opening. Novum engineers added vertical, non structural, acid etched fins to the exterior and horizontal, decorative dichroic fins on the interior. All fins line up with glass joints. Natural light shining through the fins paints the building lobby with brilliant colors that vary depending on the hour of day and season of the year.

The lobby wall also exploits flexibility of form with eye catching results. Once thought to be an application for walls of flat plane, advancing technologies have made way for other more playful solutions. One of the first projects to step out of plane was designed by Goettsch Partners at 111 South Wacker Drive, Chicago: an elliptical shaped structure spanning a horizontal distance of 340 feet long and 44 feet high. Its beauty is further punctuated by low iron glass panels coated with an “anti mirrored” material that nearly eliminates reflection.

“In terms of creating value with a glass cylinder without any structure behind it—45 feet high—you can’t really compare it to anything else,” Nilles says of 111 South Wacker Drive. “In this case, mullions or any other type of system really would be an obstacle in terms of its visual transparency. For this project it was a good value.”

In the past, cables were arranged both vertically and horizontally to create a two-way geometry. While double curved forms require such two way cabling to create the geometry, using cables in a single direction like strings stretched between the frames of a harp, such as in 111 South Wacker Drive has become much more common place. The geometry was very complex and posed a great challenge because of the curved cylinder shape of the design, Nilles says.

“You have to be very careful of what you call your boundary conditions, and be very cognisant that a cable net will impose significant structural loads. You can misuse cable net technology,” Nilles says.

Fentress Bradburn Architects took form to full fruition with the huge, double curved cable net at Seattle-Tacoma International Airport. Five highly transparent  55-foot-by-70-foot “windows” created by the saddle shaped cable nets allow unhindered viewing of the runway for waiting passengers. Insulating glass panels are point supported to the cable net.

To minimize attachment hardware for the glazing, architects at Berkebile Nelson Immenschuh McDowell Architects, Kansas City, Mo., used two closely spaced vertical cables on either side of the glass joint on the Institute of Molecular Medicine, University of Texas building in Houston. Glass connectors virtually disappear as rotules attach directly to the double cable and eliminate the need for typical spider arms. In addition, an Indianapolis architectural firm has begun to think sideways, creating the first weight-supported horizontal cable net system built in the United States at the Indianapolis-Marion County Public Library. The structure is pre-tensioned using weighted medallions suspended off lever arms hanging within a truss located at one side of the structure.

“Hanging glass on horizontal cables reminds me of mom draping sheets on a clothes line,” says Robin Halbesleben, project manager at Novum, “only glass weighs 400 times more, and that would cause the cables to really sag.”

To compensate for the glass weight and prevent excessive horizontal deflections, each glass panel has to be designed to support the one above it.

“Basically, you have to design for a complete column of glass, in this case, consisting of eight stacked panels,” says Soeren Stephan, director of engineering at Novum. “The cables only take the wind load.”

Glass fixings also have found new flexibilities as corner clamped, edge clamped and point supported systems provide attractive options in the architect’s tool box. Sometimes aesthetics is the main objective in hardware selection. In other cases, glass size and structural behavior play a role. For the initial cable nets, all glass panels were corner clamped and the nets were flexible with three times more deflection than a regular curtain wall, resulting in the use of smaller square panels in the 5-foot range. However, stress reducing, rotating head rotules are now available with the PSG-System, allowing glass to grow.

The engineering of cable nets requires a thorough understanding of the behavior of tension-only components, the sensitivity of boundary conditions and the structural characteristics of glass.

“It’s a little bit like tuning a harp,” Stephan says. “The more tension you put on the strings the less deflection you get. However, there are limits because too much tension creates huge reaction forces at the perimeter structure.”

To resist those reaction forces at the boundary, larger more expensive perimeter members are required to reduce overall economy. However, reducing the tension forces causing the cables to deflect more, can create a negative condition leading to warpage of the glass panels.

“The glass panels are particularly sensitive around a door opening or at the corners of a net wall,” Stephan says. “At these locations, two edges of the glass panels are held relatively rigid in a glazing channel, while one corner of the panel moves much more with the flexible cables.” This results in a warping action. Through investment in considerable research and development as well as extensive testing, Novum engineers have created comprehensive computer models that accurately predict cable net behavior. These developments provide further economies as individual testing is not required per job and engineering has become automated leading to greatly enhanced technical productivity.

Flexibility of cable nets has come to mean much more than higher deflections. With the introduction of art glass, exciting architectural forms, new cable geometries and glass attachment methods yielding larger glass panels, cable nets have become an architectural vocabulary. Improved and highly predictable economics have also added to its popularity. As seen in the Fifth Third Bank building, cable net wall construction has become a more intelligent glazed structure solution.

 

Sources
Roger Stein
FRCH Design Worldwide,
311 Elm St., Suite 600
Cincinnati, OH 45202
513/241-3000
rstein@frch.com

Ron Haas
Pioneer Glazing
107 Northeast Drive
Loveland, OH 45140
513/583-5925
RHaas@pioneerglazing.com
 
Scott Seyer
and Steve Nilles
Goettsch Partners
224 S Michigan Ave.
Chicago, IL 60604
312/356-0600
sseyer@gpchicago.com
snilles@gpchicago.com

Brian Vande Zande, Ian Collins, Robin Halbesleben and Soeren Stephan
Novum Structures LLC
W126 N8585 Westbrook Crossing
Menomonee Falls, WI 53051
262/255-5561
Brian.vandezande@novumstructures.com
Ian.Collins@novumstructures.com
Robin.Halbesleben@novumstructures.com
Soeren.Stephan@novumstructures.com.

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The author is vice president of sales for Novum Structures of Menomonee Falls, Wis., 262/255-5561, terry.peterson@novumstructures.com.