point supported glass

[stan45]

I am analizing a glass canopy that is point supported. All of the ASTM E1300 tables/ software call for four or two sided support for glass analysis. Any ideas on how this should be done? It is 80" x 70" (9/16" laminated glass) with (3) point supports at ea. end of the 80" length and is subject to a 65 psf winload.

 

[jedward]

Use and analysis of POINT supports for glass ,or anything, are bound to be troublesome since they give EXTREME stresses. Please define and be prepared to distribute the restraints before you can expect any reasonable answers.

 

[Ron]

I would analyze it as a plate element, supported as noted, using the properties of the laminated glass. Then compare that to a two-sided support analysis using one of the window glass analysis programs. Compare to E1300 requirements.

Problem will be the support condition. You might want to finely mesh the plate and spread the support over more than 1 element.

 

[stan45]

Unfortunately I don't have access to finite element analysis software.

 

[Ron]

Stan45...that's gonna be a problem for this analysis. You can try cutting your width into 3 sections with each point support representing the full support noted in E1300. You will analyze one section at 1/3 of the width of the supported size. This will give you a very conservative answer; however.

Check with your glass supplier. Some of them have analysis software for this purpose.

 

[stan45]

Any advise on which finite element software?

 

[Ron]

Stan45...we use RISA 3D and mTAB-Stress w/FeMAP Pre/Post Processor for general analysis. For ASTM E1300 analysis, we use Window Glass Design (WDG). There are many FEA programs that will do this analysis. You need one that does plates and is capable of differentiating surface stresses such as Von Mises top and bottom.

 

[msquared48]

I love it Ron...Just love it.

Mike McCann
MMC Engineering

 

[Ron]

Hey Mike...I'm beginning to suspect you as a co-conspirator!!(assuming you're referring to the designation change!)

 

[stan45]

I have the window glass design software using ASTM E-1300. When I ran the glass as a two sided support for the 80" span with 9/16" tempered laminated glass. The program said that it would not work. It just seems like it should for a design pressure of 65 psf.

 

[Ron]

65 psf is a relatively high load. You have a large area, almost square, with high deflection. Can you go to 5/8" glass?

 

[stan45]

I don't think its an option. It will have to be laminated glass. Are you thinking 3/8" lite laminated to a 1/4" lite?

 

[msquared48]

Think Transparent Aluminum...

Mike McCann
MMC Engineering

 

[Lion06]

If you don't have an FEA software, do you have a copy of Roark's Formulas for Stress and Strain. I'm not sure if there is point support conditions in there or not, but the other boundary conditions are exceptionally close to an FEA analysis (I ran a few examples just to check).

If you have Roark's get the stress out of there and use the max allowable stress based on your glass type. There is a provision near the back of ASTM E-1300 that gives basic allowable stresses for annealed glass, heat strengthened, and fully tempered. Use those allowables, along with the reduction factors for laminated glass or long-term loads and compare that to your max stress from Roark.

 

[Lion06]

Also, if you're using laminated glass be sure to look at the deflections using the long-term load coefficients in ASTM E-1300. You can't use the gross I for long duration loads (>3s). I would also suggest looking at DuPont's interlayer (I think it's Century Glass Plus or something, it's escaping me now, and I haven't looked at in quite a while), it has better creep characteristics than the interlayer assumed in ASTM E-1300, and can reduce long duration deflections and reduce stresses based on a higher % composite behavior (again for longer duration loads).

 

[stan45]

I do have a copy of Rourk's equations. Which tables shouls I use. Since it is poin supported I am not sure of the boundry conditions.

 

[stan45]

The wind pressure here in south florida is based on 3 sec gust. Short term loads

 

[Ron]

stan45...check your wind load. You are in a 150 mph zone, but I'm guessing you have a monoslope roof over an open structure. If that's the case, your 65 psf is high.

 

[stan45]

unfortunately the design pressure requirements are as per engineer of record.

 

[ishvaaag]

We need to forfeit the need of a proper tool for analysis, assume you have one, and then see if it is feasible within the scope of the contract or whatever. I mean, is it that there is not such thickness of laminated steel being sold that you can meet the structural demands? If there is but too expensive as to think it exceeding the scope of the contract the boss should tell the client. Or you may look at alternatives such dividing the span (I wouldn't love to part it with vertical bands of glass plate, looks to me dangerous).

And this also brings to my mind that for whatever the reason and in spite of having its logic I have never had a feeling by the most commonly seen (at least in print) point supported structural glass system (I think derived from Foster's practiceship, maybe with Arup's?) Maybe ugly is not a politically correct word for technology, but well, I always found it to be ugly.