Volume 49, Issue 3 - March 2014

Coldplay

From Design to Testing to Installation, Experts Weigh in on Cold-Formed Glass
by Ellen Rogers

"What else can you do with glass?”

It’s a question that architects seem to ask continually. Requests have led to oversized, ultra-thin, super-transparent glass, among various other developments. Among the emerging technologies quietly coming on board is a technique that involves bending glass.

Bent glass itself is nothing new. Probably the most common method involves hot bending, where the glass is curved or slumped in an oven. There are other means, though, by which the glass can be formed. Warm bending consists of forcing cold plies into shape in an autoclave before laminating; having a stiff interlayer is what helps maintain the shape. There is also the cold bent method where panels are forced into shape on site, either clamped in a frame or under self-weight.

Rob Vandal, vice president, systems innovation for Auburn Hills, Mich.-based Guardian Industries Corp., says architects are exploring cold bending options, in addition to thermal bending.

“The relatively new technique can reduce the overall cost of the application, and involve [al]most any coated glass,” says Vandal, whose company produces a heat-treatable, high performance glass that can be thermally bent. “The technique was first explored with solar applications (glass under 1.5mm thick), but has now expanded into other segments.”

Cold-bending projects are rooted in Europe, where design and engineering firms have been exploring the technology for some time. Niccolo Baldassini, of the Paris-based firm RFR, presented on the concept of cold-bending at Glass Performance Days (GPD) in Tampere, Finland in 2009. He noted that the firm has been exploring the technique since the early 1990s as an alternative method for creating curved surfaces. The process, though, is not without a place in U.S. architectural projects. In fact, the San Francisco Public Utilities Commission (SFPUC), designed as a joint venture by KMD Architects and Stevens Architects, features cold-formed units installed by Benson Industries (see June 2013 USGlass, page 60).

“As you build the frame in the shop the glass is built out of square and as you install it the units are intentionally warped and anchored to the building,” says Jeff Rosenberg, SFPUC project manager at Benson, who explained this creates a curved glass detail; the out-of-square components have to be anticipated.

For many, cold-bending may be a new technology, but it’s one drawing considerable interest. It can provide both a unique aesthetic as well as cost savings compared to more traditional techniques. As architects continue to seek new design methods, contract glaziers, likewise, will also need to prepare for what may be in store.

Design Solutions
The San Francisco office of architectural firm Skidmore Owings and Merrill (SOM) is one firm that has familiarity with a couple of cold-bent glass projects. But ask Keith Boswell, FAIA, SOM’s technical director, why his company did it and you’ll learn that cold bending was not part of the initial design intent.

“We didn’t start in the design phase thinking that bending glass was the primary objective. It was a solution to achieve the form and geometry of the proposed building concept design,” says Boswell. “For us the evaluation of whether to cold- or hot-bend the glass did not enter the picture in the early design phase.”

When the firm started with the first cold-formed project, Boswell says they began with a very conservative approach.

“The framing approach consisted of two layers, a stick backbone system that defined geometry tracery lines and then a unitized frame with glass which attached to the aluminum backbone. The conservative part was utilizing structural silicone to attach the glass and also detailing a captured aluminum glazing frame on the outside. This provided redundancy during the glass mockup and tests, as well as over the life of the building if there were quality issues,” says Boswell. “The glass attachment had a primary and equivalent secondary method to hold the bent glass in place. We recognize that flat glass which is bent has a memory and is constantly trying to get back to flat.”

Speaking of the aesthetics in such projects, he adds, “The building enclosure composition and geometry design studies resulted in a 3-dimensional form—not flat. To achieve the 3-dimensional (x, y and z) enclosure required either a ‘formed’ glass panel along with a review of glass size or to triangulate the glass and add additional joinery. Triangulation was not visually satisfying from a visual design perspective.”

Maic Pannwitz, sales manager with seele sedak in Germany, has also been involved with similar projects. “One of the key advantages of the cold bending process is the elimination of visual distortions that are inherent in, and commonly associated with, traditional heat-bent glass,” he says. “With cold bending, it is possible to achieve a smooth, almost distortion-free, curved glass surface. The main limitation of the process is the allowable bending radius, which has to be reviewed … on a project-to-project basis. It freezes the physical properties of the raw glass sheet, because unlike in hot-bending, cold-bending doesn’t create a change in structure but only a change in shape. This enables structural planning certainty even when dealing with highly unusual shapes.”

For their projects, Pannwitz says his company fixes two or more lites of glass and a specialty interlayer against a framework at room temperature before laminating the layers together in an autoclave. “Once laminated in the autoclave, the composite glass/interlayer laminate permanently retains its geometric curvature,” he says.

Rigorous Requirements
No special testing is currently required for cold bent applications. Companies, however, are diligent in conducting their own test protocols.

“We conduct comprehensive quality and development tests in our laboratory and at our test site,” says Pannwitz. “In addition to theoretical verifications relating to the properties of glass when subjected to all major loads (dead weight, wind, snow, persons, etc.), we use model facades featuring original sheets to ascertain their load-bearing capacity and their wind and impact resistance, e.g. with pendulum impacts and sandbag impacts followed by tests of residual load-bearing capacity, if required and monitored by external experts.”

Pannwitz says his company’s test site features a glass facade testing stand (up to approximately 120 square meters), used first for measuring deformation during structural and dynamic tests, and secondly for establishing whether a facade is both airtight and watertight.

“The latter involves structural tests using a vacuum chamber and dynamic wind load tests involving an aircraft propeller,” he says. “A climatic chamber for material samples is used during long-term testing to simulate the impact of changing weather conditions.”

Boswell agrees that research and testing are critical.

“Before we go into actual work with an enclosure contractor we always research and, when necessary, test atypical conditions when we can’t point to other similar built examples,” he says.

As an example, he points to SOM’s first project utilizing cold-bent glass, which was designed and constructed about eight years ago.

“At that time, we could not find any historical information, built examples, etc. and so we enlisted a specialty glass engineer, Leon Jacob [of the Australian consulting and engineering firm Jacob & Associates]. We had collaborated with him before on complex glass projects to assist on computational work for glass strength and performance material to validate that a cold bending approach would make performance sense,” says Boswell. “We compiled strength values of glass from around the world and used the lower values. Then we built a test rig and we put the project glass in the test rig—without framing—and put a jack bolt on one corner of the glass to push it out of the flat plane. The glass panels were jacked to the maximum bent project condition. After testing the project bending values, the glass was bent further and further just to see when it would break. The purpose of the test was to see: would the glass break? Would the insulating edge seal tear? It was trial and error with both the biggest and smallest project glass panel size to see what would actually happen and compare this to the computational and detailing work.”

Beyond Glass
Sound complicated? Perhaps, but Pannwitz says when it comes to these application types, it’s not so much that the projects are challenging, as it is understanding the limitations.

“You can bend this cold glass just upon a several radius before it breaks,” he says, pointing out that high quality cold bent glass can reach the size of 10.8 feet x 49.2 feet with a maximum bending radius of approximately R > 30 feet (9m) biaxial R TBC.

Compared to a more traditional bend, Vandal adds there really are no immediate changes as the glass is simply forced into shape mechanically. He adds, “It must be a shape and thickness that results in stresses below design limits.”

While glass is certainly a factor, it is only part of the system. Cold bending also places added stresses on the various components, including sealants.

“Everyone talks about cold-bent glass, but you have to keep in mind that you’re bending everything else, the silicone, the glass edge seals in insulating glass, the connections, the framing, the air and weather seals, etc.,” says Boswell. “You need to look at each component in the complete assembly and review bending system wide.”

Speaking of sealants, Ken Rubis, application engineering specialist with Dow Corning explains there are a number of considerations that should be kept in mind, as cold forming can mean additional stresses upon the glass.

“Sealant suppliers for structural silicone typically require that deadloads on their sealant for both structural glazing and silicone secondary seals be kept below 1 psi,” says Rubis. “In this case, the deadload not only includes whatever is not supported by setting blocks (typically all lites are supported), but the spring force that is in the glass attempting to bring the glass back to its flat position.”

He continues, “For the polyisobutylene (PIB), the bender needs to be conscientious of the same glass spring forces that are deforming the PIB and, in some cases, potentially opening up pathways for gas loss and moisture ingress. Excessive strain on the PIB could lead to the units failing frost point or falling below acceptable gas levels sooner than units kept flat during installation.”

Jon Kimberlain, also an application engineering specialist with Dow Corning, adds that the act of bending the glass will induce loads into the silicone secondary sealant that need to be factored into the design loads of the secondary seal under wind- and dead-loading.

“Deflection of the glass plates may not happen in a uniform manner. It will cause the plates of glass to move in shear, tension and/or compression depending on the moment of bending. This allows the primary seal of PIB to potentially move as well,” says Kimberlain, explaining that “as the potential deflections increase due to the cold bending process, architects and engineers should consider the use of advanced engineering analysis related to industry test methods to ensure that the integrity of the airspace is maintained as well as the structural integrity of the silicone secondary sealant and structural silicone sealant.” He adds that recent work with finite element analysis coupled with ASTM test standards for insulating glass durability and structural performance have been used to illustrate the potential impact of cold bending.  

Curves Ahead?
Cold-formed facades may still be relatively new in North America, but as architects continue to push design capabilities the industry can expect to see increasingly complex projects. How can the industry respond? As many agree, the construction of a successful structure must involve communication and collaboration among all those involved, from the architect and engineer to the fabricator and installer.

If you’re wondering, though, if cold-forming will mean an end to thermal processes, the answer is not likely. As Vandal puts it, “Cold-bending will never replace thermal bends, as it can only achieve simple curves with a soft radius.”

Ellen Rogers is the editor of USGlass magazine. Follow her on Twitter @USGlass and like USGlass on Facebook to receive updates.

Don’t be Stuck in the Cold
When it comes to new technologies and innovations involving glazing, the bi-annual Glass Performance Days (GPD) offers a wealth of information. During the 2013 staging in Tampere, Finland, Benjamin Beer, project manager, senior façade engineer with Meinhardt Façade Technology (HK) Ltd. in the UAE, offered a look at cold-formed glass in his presentation “Complex Geometry Facades—Introducing a New Design Concept for Cold-Bent Glass.”

According to his discussion, if a project is to incorporate “a complex geometry facade by using the cold-bending technology,” it should “happen at [an] early stage of the project and in close collaboration with the facade consultant.” Here’s a synopsis of five steps he suggests should be followed for either point or line support glazing in such applications.

Step 1: Calculate the degree of cold bending (warp) for all panel dimensions as per original design intent.

Step 2: For insulating glass units (IGU) check if the degree of cold bending is within the allowable limits of the glass supplier to ensure full warranty and long-term durability of the edge seal of the IGU.

Step 3: Model the glass panels including support conditions using a finite element technique and apply the loads.

Step 4: Check if the tensile stresses in the glass due to cold bending and wind loads are within allowable limits. Note that for final stress check, both the cold bending and wind load stresses will be overlaid.

Step 5: Check if the local buckling (local warp) modes and subsequent glass reflections are acceptable and in line with the architectural design intent.

Keep In Mind … According to Keith Boswell, with the San Francisco offices of Skidmore Owings and Merrill, there are a number of design considerations when it comes to a cold-formed application. For one, he says the aspect ratio of the glass is very important.


“The bigger the glass panel, the easier it is to bend,” says Boswell. But there are more. Here’s a quick list of a few others to keep in mind:

1. How to frame or hold the glass

2. What does bending mean for glass strength characteristics “because you’re building up stress when bending?”

3. How will it perform over a long period of time?

4. How is the glass installed in the system?

5. Glass gets weaker with age; how will it perform in year 25 or 30 compared to day one?

 


USG
© Copyright 2014 Key Communications Inc. All rights reserved.
No reproduction of any type without expressed written permission.