July 24, 2014

Mockups for range - for glass?

Lucky me! I finally caught strain patterns in the wild!
We all expect natural materials to vary in color, pattern, and texture. Concrete, too, and other things that are, let's face it, made of mud. We wouldn't dream of building a major project without demanding mockups for the marble panels or the "Brazilian cherry" wood floors. But we need mockups for glass, too! Who knew?

But glass is manufactured, you say? It's a precise material made and fabricated by experts in a highly controlled environment, right? Well, this is, perhaps, precisely the reason we must specify a mockup. Because glass does vary, and we expect it not to.

I'm helping a client manage some owner expectations, that is to say, disappointment, with insulating glass units (IGUs) on a new building. Little rainbow striation patterns appear in the fa├žades in certain light and at certain angles. Some seem vertical, others horizontal, and some lites have little or no pattern. 

These patterns are well-understood in the glass industry, but not well-controlled. They are called strain patterns or quench patterns, and they can be more pronounced in heat-treated glass, tempered glass, and IGUs. And GANA and the ASTM committees on glass insist that these patterns, and variations in them, are not defects and not cause for rejecting the work. Unfortunately, that doesn't make the owner feel better.

(The photo above does not depict this client's building. It does, however, depict both common strain patterns, the grid type and the edge type.)

So now GANA has put glass in the same category, from a design standpoint, as maple and limestone: things that vary, even though sometimes we wish that they wouldn't. And that means mockups, my friends. 

With wood and stone, we might settle for sets of three or five samples representing a range of possible visual effects. But quench patterns in glass depend on light and view angles and orientation - we could have twelve samples of the same IGU makeup and never see a single rainbow, if we didn't bother to take them outside. So GANA suggests a mockup, on site, oriented the same direction as the major facade, and available for comparison throughout construction. And, reluctantly, I am forced to agree. For if GANA and ASTM C1048 insist that quench patterns are not defects, then it's up to the design team to narrow the limits of acceptable variation. 

Thers's an extra added bonus, of course, to a mockup. It shows the client what's reasonable to expect the project to look like. So if the mockup is covered in little wavy rainbows and the client objects, it's not the whole facade he objects to, just the mockup. There's still time to make a change, or, if not, the client has time to adjust to the idea. 

So here are the pieces you need, if you want to defend against excessive variation in the appearance of glass. 

First, you must specify a fenestration mockup on site, oriented the same as the major facade, and, ideally, shaded on the interior side. This might be in addition to the performance mockup - we are talking about exerting tight control, after all, not building a shed in the backyard. And be sure that the mockup specified is for visual effect and for the expected range of variation in the glass. Make sure it is clear that the owner and client reserve the right to reject glass outside the ranges represented on the mockup.

Next, trace the mockup requirements back into Division 01, to be sure the mockup is required to remain available for review during construction of the assembly it represents. 

And finally, check the construction contract to understand who has the right to reject work. The Architect has that right in AIA contracts, but your mileage may vary with other contract forms.

Then make sure the owner attends the mockup review, and that it's viewed more than once, at different angles and times of day, if possible.

For this client, we may be chalking the variations up to lessons learned, or we may be able to make a few strategic corrections. But we have certainly learned.

P.S.: Even if the owner does not want to pay for a mockup, offering one opens the opportunity to educate about what to expect. Thanks, Liz O'Sullivan, for the twitter conversation!  

Photo Credit: Vivian Volz, copyright 2014


  1. Hi Vivian, I represent a glass fabricator and find your blog post interesting. I agree that owners should have mock ups for glass, especially given the typical scope size for exterior glass. Worth noting (I hope) is that float glass and high performance glass manufacturers are not always the fabricator. That said, there is a benefit to the specifier in understaNing and familiarizing themselves with regional IGU and heavy glass fabricators. Another suggestion is to require a fabricator's tempering certificate from GANA to ensure they are qualified to provide tempered glass that meets ASAP C1048. Also, a plant tour for specifiers is highly recommended. Thanks for bringing up such an important topic. -Lauren Anderson, Conners Sales Group, CSI, CDT

    1. Hi, Lauren, thanks for commenting, and also for tweeting!

      Yes, the fact that the fabricator is not always the manufacturer is a really big, messy issue - a lot of architects don't understand that. I see a lot of them call out the low-e coating as if it's the only identifying characteristic of the glass. I suspect that's a whole separate blog post - maybe we should talk about it and co-author something?

    2. First, so sorry for all the typos! That will teach me to post from my phone and let autocorrect take over. I do think there is a benefit to architects and specifiers alike understanding "the supply chain process" of manufacturing raw glass and coatings to the ultimate product. It's very murky to someone who isn't involved on a daily basis in the glass industry. I'd love to partner on co-authoring a post. Just the other night, I attended the Building Enclosure Council meeting in DC, and the panelists included a building envelope QC specialist from Clark Construction. With regards to mock-ups, Clark will now require a building envelope mock-up on every project and require it to be visibile throughout the course of the project so that all interfaces and issues such as glass distortion are addressed prior to a project start. I think they've made a move that we may see going forward from many major general contractors.

  2. Of course, we could use smaller panes of glass that have less strain. Or deliberately set each panel as a slightly different angle so variations from panel to panel are part of the design. Or...

    1. Hi, Michael, thank you for commenting!

      Yes, there are lots of ways the designer can obscure the variations in glass. There's a nifty building going up in San Francisco with the panels undulating inward and outward in ribbons up the building. I daresay there was a mockup for that effect, too. But for mere mortals, designing to be less demanding of glass consistency is certainly an alternative to tightening our requirements.

  3. You can't have too many mockups. It will be harder to explain to the owner later why you didn't use them, than to explain ahead of time why the owner should pay for them.

  4. Hi, Sheldon, thanks for commenting!

    I've been trying to use a mockup report from preliminary spec sets as a tool for discussing which mockups to require. When I'm working through the architect, the owner does not always see this report, but I do encourage sharing. I'd rather have that conversation early than late, just as you say.

  5. I apologize if this posts twice or three times now.... Great post... and I agree with a lot of what Lauren noted above. No question also there's nothing worse than standing with an owner and seeing this. Been there... not fun. Also a big key here is the phenomenon is not consistent. It can show at different times of year and day. So that has to play in to the thought process as well. All in all though your direction is dead on and needed. And I think when you do part 2 and address the who phenomenon of why/when/where this is happening, that will help make people understand more as well.

    1. Hey, you made it! Thank you for your comments and for your link from your blog!
      The why will be the hardest part. I think I've got to dig up some optics textbooks...


You may comment with an online profile, your own URL, just your name, or anonymously. All comments are moderated.