Field NotesJuly 31st, 2020 | Category: Architects' Guide to Glass and Metal
The Less-is-More Challenge
By Chuck Knickerbocker
With the introduction of jumbo glass sizes, we’re seeing designs that push the capabilities of glazing, taking a minimalist approach to the framing. For example, we are seeing more configurations where two edges are supported by framing, but the other two edges are without a vertical or horizontal behind the glass.
Going Vertical or Horizontal?
Supporting glass on two edges isn’t a deal breaker, but it depends on whether the design eliminates the verticals (easier to do) or the horizontals (harder, see below). Eliminating verticals is common in interior applications. On exteriors, it’s a different ballgame.
Now, before you think that glazing without framing is a common, everyday practice, we need to take a step or two back from the edge of that cliff. This looks really great on paper but, in actuality, requires ways of looking at the glass that a lot of folks aren’t aware of. So, if you’re not talking to whoever might be spec’d to supply and/or fabricate the glass, you may end up with a design that is radically different at substantial completion than what was originally envisioned.
Challenges to Consider
What are the technical challenges? Mostly, they have to do with eliminating horizontals when the design calls for the glass to be supported on the vertical edges only. When you eliminate horizontal framing, you are left with the question: How is the deadload weight of the glass going to be carried? There is no horizontal there to hold the setting blocks at the eighth- or quarter-points along the bottom edge of the lite. Glass cannot be stacked on the edge of the lite(s) below.
Given these considerations, the design might come in with setting blocks right at the corner of the lites, supported by minimal shelf angles attached to the verticals. However, this puts the stress on the glass where it’s the weakest, in the corners. That’s not in keeping with current industry best practices. The glass fabricator may also have to thicken the glass plies (regardless of IGU, monolithic or laminated), heat-strengthen the glass or both.The other challenge, technically, has to do with insulating glass units (IGUs) and the durability of their edge seal over time. The frame deflection limit of L/175 or ¾ of an inch or less originally was developed to help prevent over-stressing the IGU’s edge seals. That concern is still there, and now the design eliminates the framing. Having unsupported glass edges in IGUs is not an acceptable industry practice.
An additional challenge with glass is that a product with a thickness of, let’s say ¼ inch, will deflect the same when it’s annealed, heat-strengthened or tempered. Yes, it is stronger and takes more force to break it, but it will deflect the same amount under load. So, the only recourse to limit that deflection is to either decrease its size (counter to the jumbo lite movement), or thicken the glass ply.
Given these challenges, it might be beneficial to stick with a monolithic or laminated lite in these applications. We’re all having to learn to make the tough choice and keep energy concerns a priority. So, the IGU stays in.
Navigating the Cliff
Are there glass fabricators endorsing these designs, and supplying material for them? You bet. But, whether a glass fabricator will or will not furnish the glass is not the point. If the design calls for these frameless approaches, there ought to be a technical sign-off even as early as the bidding process by a reputable glass supplier/fabricator. And, if the spec’d glass supplier gets changed out in favor of a cheaper alternative during the procurement process, the alternate needs to have been vetted out as well.
These designs are right on the edge of the cliff. Step one way and the design could be truly amazing. But, if you step another way and overlook the technical aspects required, the results are not going to be favorable. I love the line, near the end of Death of a Salesman, that goes “respect must be paid.” Never more so than with this application.
Chuck Knickerbocker is the curtainwall manager for Technical Glass Products (TGP). He can be contacted at firstname.lastname@example.org
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