Engineering a Glass Floor Panel 3

[Deener]

Hey guys,
I'm designing and building my back deck. I've got an opportunity to use a 5' x 6' x 5/8" thick piece of glass that my friend salvaged from a Victoria Secret runway (high five). I have zero experience with the mechanical properties of glass. For those of you with experience, I would really appreciate some input on my questions below.
1. Are there any kind of propriety markings etched into the structural glass to indicate the supplier or methods of fabrication (i.e. annealed, tempered, laminated etc)? Could get any information about the structural glass panels with a visual inspection?
2. Since I suspect the answer to question 1 is no, is there a conservative tensile strength that I can for the design? I think this article may be a good source -->

https://www.structuremag.org/wp-content/uploads/2014/08/C-BB-Kufferman-June081.pdf

Seems to suggest 2.8 ksi allowable stress.
3. Are there more stringent requirements for deflection in the elements which support the glass? The article above mentions L/175 under live load.
4. If the glass is to be flush with surrounding wood decking, are there generic fasteners I can use? From a quick search, it looks like some prefer to use silicone caulking ( Dow Corning’s 995 structural silicone)
5. I suspect I should leave some gaps around the glass or use some kind of expansion joint to account for the thermal strains in the surrounding wood structure. Any recommendations here? I'm wondering if mixing glass with wood is just a horrible idea since I anticipate the wood shrinking as it dries.

Lots of factors to consider here. Any input is appreciated.

 

[jjl317]

I can answer a few of your questions, though to be frank, I don't necessarily think it is something I would consider -

1) In the US, tempered glass is supposed to be etched in a corner, identifying it "Tempered" or "Temp". Laminated glass should be evident visually.
2) The article you mentioned does include design values, though the values listed would not be conservative for an application supporting live loads (the values listed are based on the most typical uses of glass, vertically, with horizontal wind loads applied). For walking surfaces, much higher design factors would be required. And to clarify, glass doesn't have a yield point, it has a modulus of rupture, so glass is typically analyzed based on a design factor with an allowable number of breaks per 1000.
3) Typically glass design is based on an allowable L/175 deflection limit on the supporting members.
4) Structural silicone is one potential option, though I would be surprised if data was available for attaching to lumber (especially PT).
5) Wood shrinkage would be a concern.

My biggest concern is safety. Typically, glass walking surfaces are built-up layers of laminate, including a sacrificial outer layer designed to break without impacting the overall capacity. Tempered glass breaks into smaller square ("safety glass"), but annealed or heat strengthened will break into sharp pieces, and I would be concerned about the glass breaking into sharp pieces, while someone steps on it. Plus, you might have slip issues when wet.

Design of glass walking surfaces is very specialized, and the so I would be extremely hesitant to try something like this.

 

[IRstuff]

Seems to me that if you had two panes of this stuff you might have something; otherwise, you've just a nice piece of glass:

https://diometonline.co.uk/walk-glass-flooring

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[StrucPEng]

Normally we have always designed glass walking surfaces with a minimum of 3 glass lites laminated with a stiff inter layer material (SGP). It gets quite complicated when your glass is made to support walking live loads vs. just snow, or self-weight. You would need to review post breakage scenarios where one of the glass layers fails and you have a reduced capacity while still being able to hold a load and prevent further breaking.

As jjl317 noted glass is designed based on probability of breakage so there are stress values that should be used to make sure you are not overloading the glass but even below those levels there is a probability of breakage so redundancy is key.

Tempered glass is always the way to go in an overhead scenario and in general is a better structurally. The pre-compression from the tempering process allows it to withstand higher stresses on top of the safety aspect jjl noted.

Definitely would not use a single piece glass of unconfirmed quality in an application like that.

 

[Phil1934]

Consider it once held one 100# person, not two 250# people.

 

[3DDave]

The biggest concern I can think of is the low chance for the short original service of anyone dropping a hammer or other heavy item on it. particularly the edges. Glass can be very strong, but it has a significant ability to develop stress concentrations from any initial fracture. I would at the least put an aluminum channel with silicone caulking around the perimeter to absorb sharp impacting items from hitting the edge. Then the problem that this will cause is it will become a shallow pond whenever there is rain or snow making a seriously slippery surface until it is cleaned.

If it is tempered it will have a marking on the face somewhere indicating this. The marking is to tell glass cutters to not cut this glass as it will rupture. Additionally the edges of the glass may not be crisp with the razor sharp corners as the tempering process tends to round them over, though they may have ground the edges after cutting them to be safe to handle.

I would consider looking for some other use for the panel.

 

[klaus.]

If it is a single glass pane ...not laminated..then don't use it to walk on or as over head glass

 

[LittleInch]

Try and find out what was under the glass to support it on the "runway". Then replicate. Not sure if everyone here picked up the victoria secret runway. A picture would help!

Support just around the edges sounds like a recipe for disaster.

The models do seem to wear a lot of high heels though so maybe it will be good for point loads, but a bunch of blokes drinking beer?? - "I'm designing and building my back deck."

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[r13]

Most thin model weights less than 100 lbs

 

[Deener]

Thanks for the helpful comments everyone. I've added a picture below to better explain the occupancy loading I'm hoping for on my deck.
Lots of valid concerns here.My thoughts below. If I can't get more information on the glass piece, I think I'll scrap this idea.
1. Do not use the glass unless it is tempered (etched markings to confirm) and laminated (visual inspection).
2. Glass will be slippery. I've seen glass walkways. How is this typically avoided? Looks like there's some special non slip coatings out there.
3. Glass would be supported with joists on 24" centers. A 85 psf load (further factored) would result in stresses around under 1 ksi.
4. Check the edges for cracks. Consider protecting the edges.
5. Allow for shrinkage of the wood structure.

 

[LittleInch]

Avoid point loads / dirt / gravel on the joists.

You could lay something like neoprene on all the rafters it sits on?

Many glass walkways are inside - that's how they don't get wet.

I nearly missed the bit about the future wife!

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[STrctPono]

I have never designed with structural glass before but a few thoughts anyway...

This would no doubt be the focal point of your deck and as such would require a fair amount of attention to aesthetics and detail. Not sure what you are thinking for wood but my thoughts are that a Douglas Fir, especially if it is incised and treated for outdoor use, would just be an eye-sore and undermine everything that you are trying to accomplish. You may in fact repel women instead of attracting them! I feel at the minimum you would need to use a clear redwood or at least plane and sand the redwood joists yourself. I would think the silicone would be a better detail as this will provide a slip plane from the glass to the joist. Another thought would be to use aluminum joists.

https://www.metalsdepot.com/aluminum-products/aluminum-beams

 

[glass99]

the replacement value of that little piece of glass is like a couple hundred bucks (even if laminated), but saying that it was used by Victoria's secret is priceless. Which is the first lesson for all structural glass projects. Its got to be cool, otherwise use concrete.

 

[EdStainless]

I worked on a project once with glass roof panels that were designed for loading.
They came to us siliconed into metal channels with small gaps in the corners for drainage.
They weren't exactly channels there were more of an "F" shape. We were told that the extended leg was to get the correct stiffness.

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[r13]

F shape support, that's a great idea.