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Field Notes

August 10th, 2020 | Category: Architects' Guide to Glass and Metal

The Pirate’s Life for Me: Considerations for Specifying Timber Curtainwall

By Chuck Knickerbocker

“Wood is universally beautiful to man,” Frank Lloyd Wright once said. Timber, since it’s a renewable resource (and therefore eligible for LEED credits), is now cropping up as a primary framing material on some curtainwall and window projects. We are starting to see an uptick in inquiries about wood framed construction, and I thought I’d see if there was more of a consensus on how to approach these jobs.

Understanding the Material

Most metal/frame suppliers are good at engineering and fabricating their particular systems. Some suppliers’ forte is steel, others aluminum systems. And still, other materials, such as fiber reinforced plastics, “pultrusions” and more, are working their way into the marketplace. Given enough time and exposure, one might be able to cross over into these other materials, as well.

But first, there’s the issue of understanding the material. We once had a customer ask us if they could do the fab on our material, as they had an opening in their production schedule. They faced challenges they weren’t prepared for. You can saw aluminum, and you can saw steel, but in many ways, they’re way different. The saws on steel run at a much slower speed than aluminum saws. The equipment is, therefore, different, too. Many of the other fabrication methods are material-dependent, and if they’re not accounted for in the fabrication planning, well, you’re in for a shock. Finishing is also different.

Such is the case for wood. Some suppliers have glazing adaptors that can be applied easily to the front of a wood horizontal or vertical. Screwing it on is pretty straightforward. Making sure there are enough fasteners spaced appropriately and concentrated at glass setting block locations is fairly simple engineering, and is dependent on the species of the wood used in the framing. No issues there, right?

Wood Engineering

The engineering and fabrication—and I’m going to guess installation—of these timber frames is another matter. With steel windows and curtain-walls, the material holds to the same performance (or better) than aluminum systems adhere to for deflection limits, for air- and water-resistance, etc. Unless there’s something about wood-framed curtainwalls I’m not familiar with, I’d venture a guess that all the performance criteria we see in any glazing system spec would have to be met, regardless of the framing material and/or the project location. Criteria such as windload and gravity deflection, thermal expansion/contraction, U-value, condensation, seismic and so forth, would still hold, even with wood, to ensure the system(s) deliver the same level of performance as their metal systems counterparts now do.

Once the requirements are understood, one would have to find a professional engineer (PE) who is familiar with the material and willing to prepare the calculations. Present engineering resources may or may not be willing to take that on. If not, they’d have to find someone who is. Don’t let a PE-stamped set of calculations slip through the cracks when considering how to do one of these projects.

Fabrication Considerations

What is also different is having the expertise in-house to do shop drawings and fab docs for the wood members. Then, there’s the fab itself. Many metal (steel) fab shops and aluminum shops couldn’t process wood even if they wanted to. And I got a dollar that says the anodizing, PVDF (liquid or powder coat) finishes that we all know and love can’t be applied to wood. So now, everyone has a whole different set of finishing types (paint, stain, varnish, etc.) to learn about.

There might be a simple approach to this: team up with a vendor who can engineer, prepare shops, fab docs and fabricate the wood. Then, combine your expertise in glazing with theirs in wood.

Getting that process started will require a degree of communication different than what most of us have had to deal with. And tolerances for fabricating and installing the wood have to be understood and resolved in order to cut down on the lead time of ordering not only the wood framing, but to also coordinate procurement and delivery of any metal and glass components. My guess is that carpenters who would normally set these frames don’t work in as tight a tolerance as what the metal framing side of the profession sees.

Where and when the glazing system is applied to the timber frame has to be addressed. Will the timber frame have to be set so that “verify in field” dimensions can be obtained, before the sizes for those other materials (specifically glass and metal) will be released for fabrication?

We personally haven’t closed the sale on one of these systems yet, but when that does happen, our perspective will surely be altered by the experience (definition: experience is what you get just after you needed it). And everyone will be the better for it. For now, the idea of working with wood frame timber or glued laminated timber (glulam) members is intriguing. I’m looking forward to it.

And we’ll find out if “Argh, argh, argh, shiver me timbers” can be said with a degree of confidence the next time through.

Thanks for reading.

This column was adapted from Chuck Knickerbocker’s June 20, 2019, USGNN™ “Field Notes” blog post.

Chuck Knickerbocker is the curtainwall manager for Technical Glass Products (TGP), a supplier of fire-rated glass and framing systems, along with specialty architectural glazing products. With more than 35 years of curtainwall experience, he has worked successfully with numerous architects, building owners and subcontractors from development of schematic design through installation. He can be contacted via email at chuckk@tgpamerica.com.

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