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Re: Point supported glass design

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You are right; as far as I know the best way to design glass is to perform a flaw growth analysis (there was/is a NASA developed program called FLAGRO for this), where a "penny shaped" crack is subjected to analysis via fracture mechanics and the allowable tension stress on a surface under cyclic loading is backed out of the model by finding how many cycles it takes for rupture to occur. The number of cycles and maximum stress are, naturally coupled.  The technical way to determine if glass will survive under service via proof loading is to determine what crack size, x, will allow your service life to be met under your anticipated loading. Then rerun the glass analysis to determine what pressure will cause a crack of size x to propagate to failure instantly (in an engineering sense).  Proof the glass to that pressure and if it doesn't fail, you don't have any cracks of a critical size.

Now, that's not really practical for architectural work. For handrails, I've made a FE model of the glass with the holes and then applied my forces and restraints as in real life. There are allowables out there fore different glasses and manufacturing methods, and I generally take the bottom of the range and keep my peak von Mises stresses below that level. I can't say that it's a perfect method, but short of a real fracture analysis (which I was only tangentially involved in when I learned about it) it's all I've got.
Jordan


Garner, Robert wrote:

I had reason to study this once a long time ago.  What I found, and it may be out of date, is that glass is "designed" on the basis of statistical failure due to imperfections.  That is, glass fails by a flaw in the glass and not from "overstressing" the glass.  Glass manufacturers know they need a certain quality level to produce say, a 4'x8' plate glass pane that will survive 80 mph winds.  You can reduce that to an equivalent stress level to extrapolate your design but you aren't really designing to this stress as much as you are designing to a standard that has proven reliable in the past.

 

So that's what I learned long ago.  I'm interested in hearing the latest. 

 

 

Bob Garner, S.E.

 


From: Jim Wilson [mailto:wilsonengineers(--nospam--at)yahoo.com]
Sent: Wednesday, March 11, 2009 2:29 PM
To: seaint(--nospam--at)seaint.org
Subject: Point supported glass design

 

Are there any simplified methods for designing point-supported glass?  I'm looking for applications such as glass handrails, stair treads, etc.  I've searched and found some generic glass articles online, but nothing technical.  Is this the kind of thing that requires a detailed analysis by an industry insider?

 

It seems like there could be some standardized analyses available for a single 1/2" or 3/4" pin through a piece of 3/4" glass.

 

Thanks,

Jim Wilson

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