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Re: Structual Design of Glass

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I guess it relates to what you consider a "structural component".
Obviously every display window carries some load when people push on it
and when wind or seismic forces are in play. I seem to recall that the
original question related to a balcony enclosure. I design quite a few
store fronts and we don't have any mullions on most of them.

Stan Scholl, P.E.
Laguna Beach, CA



On Tue, 11 Jul 2006 13:56:19 -0400 "Jordan Truesdell, PE"
<seaint1(--nospam--at)truesdellengineering.com> writes:
> I know this is late, but I really don't trust glass as a structural 
> component. The performance is too tightly tied to fracture 
> mechanics, 
> and is highly dependent on flaws in the product. This may be too 
> paranoid when looking at tempered products (most of my experience is 
> 
> with annealed), but it's a poor choice of product where a single 
> point 
> failure can cost a life.
> 
> Glass fails by propagation of cracks, often over many sub-critical 
> cycles.  The common theoretical construct is the penny shaped crack 
> 
> (flaw) which grows over time under cyclic loading until the section 
> is 
> no longer viable and a catastrophic failure occurs. Any scratch can 
> 
> initiate a fracture point, and impacts can do the same. Sometimes 
> the 
> fracture cannot be easily detected by visual methods.  
> Unfortunately, 
> architects and owners don't want to hear this. Nor do they want to 
> hear 
> that the proper use of glass should require periodic proof testing; 
> not 
> something that can be easily done. The classic scenario is that a 
> panel 
> is analyzed for the critical flaw size and associated failure 
> stress. 
> Then a test is performed which , by analysis, will induce a stress 
> high 
> enough to essentially guarantee failure, give that a flaw of the 
> critical size exists.  By estimating a loading time history for the 
> 
> installed panel, it may be determined how quickly a crack of the 
> critical size above will grow to the point where a code-level load 
> will 
> cause failure. That interval is your testing frequency.
> 
> Over time, a glass panel which proof tests fine at installation will 
> see 
> a reduced strength.  As I said above, my experience is with annealed 
> 
> glass (mostly for optics in vibration/pressure environments), so 
> this 
> may not be applicable for tempered. Just food for thought.
> 
> dan
> 
> 
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