• Field Notes 06.03.2013

    It appears some of the “old” tried and trues for glass are starting to fall by the wayside. Just when you think the world, in certain respects, is locked down and won’t ever change; well the only constant is change, isn’t it?  It dawned on me recently when talking to Bill Lingnell (who’s forgotten more about glass than I’ll ever know) that I might be leaving some very important caveats out of my responses to architects’ questions.  Changes are occurring to what I thought were some pretty basic and widely accepted practices.

    For example, a common question architects ask is, “just how big a piece of glass can be made?”  Responses, after “it depends” include:

    The standard largest size most manufacturers will produce is 84” x 144”.  Some of them will publish larger, but you need to check with them.

    1. Some of those same manufacturers will make even larger sizes, but at a premium.  There are specialty glassmakers that will produce large lites, but that option’s not cheap.
    2. The glazier who gets to install the job has to be willing to work with the larger sizes.  Some do not; that will be reflected in their pricing.
    3. Can the budget accommodate the sizes being considered?
    5. All of the above.

    For most glass-related questions, all of those answers are SOP, right?  “Wait just a second, Kemosabe,” said the masked stranger.

    Another typical question I hear is: “Can we have two IGU lites butt up against each other without a framing member behind them?”  This, also, used to be a no-brainer, with the immediate response being, “NO!”  The “L/175 or ¾” whichever is less” limit on structural deflection of frame members came from the IGU manufacturers worried about edge seal integrity, and wanting to limit how much deflection the glass actually experienced under full load.  But, it appears some of the manufacturers are loosening up on this a bit.  So again, answer #5 above applies: check with the manufacturer.  If you use their units, you might be able to delete the horizontal.

    “If the horizontals are deleted, the dead load of the glass has to be dealt with, what are your expectations for how that can be detailed?” Without a horizontal, the normal response is there is northing that should be visible.  Ok, that works on paper, but in practical terms?  Someone recently sent me shop drawings in which the manufacturer developed a shelf screwed to the face of the mullion, and the glass was set on setting blocks completely encapsulated in the glass thickness so it was not visible.  The blocks, if not at the corners, were pretty darn close to it.  A LOT closer than the 1/8 or ¼ points of the glass width dimension commonly practiced within the industry.  If the framing manufacturer will sign off on it, and answer #5 above is confirmed, then why not?

    Bill pointed out that factors impacting permissible edge deflection include the spacer material and sealants used, as well as the process of constructing the IGUs.  Putting setting blocks at corners might depend on the quality and thickness of the glass cut and/or if the glass requires heat strengthening or tempering.

    But, here’s where the light bulb came on for me in our conversation:  Not only does the manufacturer have to sign off on it, the architect MUST (and this is really a biggie) repeat, MUST specify that no exceptions are permitted:

    It is NOT reasonable to assume that while one manufacturer will allow certain detailing, all other manufacturers will, also, if their products are used instead of the first manufacturer’s.

    IF ABC Glass allows the installation of the glass with corner edge blocks, and then XYZ gets the job because they’re cheaper, but XYZ doesn’t permit the glass to be installed on corner edge blocks, and the glass later has abnormally large amounts of breakage that are directly attributable to the corner setting blocks, what happens next?

    I’ve got a dollar that says ABC Glass won’t come to the rescue.  Will XYZ take the heat?  How about the architect or the glazing sub? I don’t know the answer to these questions, but this thought comes to mind:  “Welcome to Construction Law 101, class has just begun.”

    My response to architects’ glass questions for these and similar topics is going to be, “Check with the glass manufacturer whose product you’re considering specifying, and make sure their glass actually gets on the job, or qualify the tar out of any ”or equal” glass manufacturer / supplier statements. 

    The single biggest way of doing that is to share the glass specs with the “basis of design” glass supplier, and more importantly with any “or equal” suppliers, and make sure they understand what is required for the job.

    Most good glass specs will REQUIRE technical reviews from the glass supplier.  Don’t skip this; they’ll do it during bidding if you ask them. If the specs don’t require it, do it anyway.  Reputable manufacturers will have technical support staff for this very reason. It’s been my experience those folks are capable.  Having a letter in the file that says they signed off on the application is pretty cheap insurance.  If they don’t have a technical department, don’t buy their glass for that project.

    Special glass applications keep coming.  And, are likely to keep coming.  A case in point: A presentation two years ago at BEC caught my attention: Point-supported (spider) glazing without holes in the glass for the spiders.  The spiders were structurally adhered with a special silicone sealant to the interior face of the glass.  This came to mind a couple of days ago when a sealant manufacturer announced a product they are bringing on the market for this application.  So, if a job comes up with that on it, the glass manufacturer is going to have to sign off on it, right?  Please say yes!

    Posted by Blogger @ 10:21 am

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