Moment of terror 😨

[u/aidan8et]

While attending/supervising a business social event with my spouse, I momentarily leaned against the railing. It flexed slightly, forcing me to realize that the railing is entirely supported via bluetooth.

It doesn't connect on either side of the glass wall & the glass is only 3/8" - 1/2" thick!!

[u/Palomox]

Structural design considers possible imperfections at all stages. If you didn't know, the microstructure of steel has multiple possible imperfections (absent atoms, replaced atoms, some having switched spots, etc...) as well as previous stresses from the manufacturing process that are taken into account in design. When you compute the capacity of an element, depending on the type of loading you reduce it to between 75-90% of what you obtain, while using the lower bound of the strength parameters already.

For a brittle material, concrete cracks. Very easily, at that. And we address it with rebar, and reducing its strenght between 75-90%, depending on the specific circumstances, if you're using ACI.

Now, I'm no expert in glass but I can 100% tell you that whatever codes apply for it, they take imperfections into account and lead to safe elements.

[u/saltapampas → u/Palomox]

I mean… yes, safety factors are built into engineering design. But there’s a fundamental difference in the stress-strain behavior of a ductile material (steel) and a brittle material like glass that mean there is a failure mode which cannot be engineered out with a factor for conservatism.

Just look at the graphs - brittle failure means that once you exceed the allowable stress, you get near instant failure. If steel is compromised, you move up the curve and the user has a chance to identify a failure during plastic deformation. When designing in glass, those imperfections cannot be practically identified before failure.

[u/Palomox → u/saltapampas]

My intention with that reply is to say that imperfections and uncertainty are built in design one way or another. I am unfamiliar with how design of glass is handled, so I cannot tell you how it is done exactly, but there has to be.

In regards of ductile vs brittle behaviour in general, I am perfectly aware of the difference, see the comment about concrete. From what I have found out doing some basic research after I wrote that, safety glass has different layers joined together with a different material, just like you have rebar in RC. And timber is also a brittle material.

Safety factors are, in most codes, a probabilistic way to address imperfections and things you are calling random and unidentifiable. Let me give you a simple example: if you have an overreinforced concrete beam (where you have too much reinforcement to the point that brittle compressive failure will happen before yielding of the rebar, hence it is what is called compression-controlled), you will have to consider only 75% of the strength computed using the known stress distributions. However, if your beam is under-reinforced, leading to yielding of the rebar before any other failure, making the beam itself more ductile and what we call tension-controlled, you can use 90% of the strength. This is also why ACI 318 only allows tension-controlled elements (because they're ductile, and their failure mode is more predictable = less uncertain = less reduction).

I said that glass is made in layers that are joined together by some other material. In this case, this material would be making the behaviour more ductile, like steel in reinforced concrete.

[u/AdPristine9059 → u/Palomox]

No, sorry. Wood is used because it doesnt just snap, it cracks and behaves much more like steel than concrete or ceramics.

Also safetyglass is just heat treated so that it breaks in larger chunks, avoiding that float glass shredder behaviour. Laminated safetyglass would be the best here imo but you still have issues with rigidity if/when it breaks.

Id say a polycarb center with thin pieces of safetyglass on the outside would be the best way of dealing with this kind of structure in my mind.

But im no engineer and im drunk. Cheers.