So you think you are safe? (part 1)
Recently, I’ve challenged myself to create a new post every week to try to help people to understand better how glass works and what are the traps. To be honest, this is a huge challenge for me… with my two daughters, my wife working on 12 hours shifts as a nurse, my two businesses, the committees I’m involved in, the time to create content for a post is limited. But I really think people need it with all the conflicting information we see all around.
So, yesterday evening while I was reading, before going to bed, I had an idea for a post. Recently, I had two different conversations regarding glass guardrail with spigot. Like the one we see below.
The first conversation I had was with my associate, Christian, regarding a guardrail without any handrail and with non laminated glass and of course spigots. I was amazed to hear about that because without any calculation, we were able to say that the guardrail was not compliant with codes and standards.
I know what you’re probably thinking, yeah, but what if it’s for a residential building. Unfortunately, the part 9 of the canadian building code, which is for small residential like building, refer to the canadian standard for glass calculation CAN/CGSB-12.20-M89 or the american standard ASTM E1300… Same as for commercial buildings (refer to 22.214.171.124 and 126.96.36.199.). According to the canadian standard, we cannot even supply a glass guardrail without handrail:
This is based on the fact that glass is a fragile material with variable properties. So we need redundancy when we design a structural glass element. Redundancy is just a fancy term to say that structural glass must be designed considering an alternative path in case of failure of one or more piece of glass.
Without going too far, there is now a relatively new standard CSA A500 for guardrail, which is not yet adopted by the national building code, but still provide great guidance for glass guardrails. According to this standard, we can design a guardrail without handrail, but under certain considerations. The first one being that the glass will have to be laminated with a certain thickness to allow for redundancy. The standard also mention that we cannot use fully tempered glass combined with pvb. We can only combine fully tempered glass with a rigid interlayer like SentryGlas (SGP) or other similar products.
So first issue with the glass guardrail with spigots above: no handrail + non laminated glass → which is obviously incorrect.
Second issue is regarding the thickness, which I did not mentioned. The glass was supposed to be made of a 12mm fully tempered glass. I already new it was incorrect for the kind of fixing brackets (spigots/clamps)…
Even for a residential building, because the loads for residential buildings are not that different from commercial buildings. In fact, the point load (1kN [±225 lbf ]) is the same for residential (part 9) and commercial buildings (part 3). The linear load is different (0.5kN/m vs 0.75kN/m) because of occupancy rate of course.
So I decided to make a simulation with a FEA (finite element analysis) software, another fancy word to say that we put all the information in a software and let it work for us!
Without going into details, I’ve got 209 MPa and the Canadian standard only allow for 96 MPa for the dimension analysed. So we get 217%, more then two times the maximum allowed by the standard with point load!
So maybe they planned to add some clamps between glass… okay, fair enough!
Then I verified with the clamps. This time, the line load was more critical.
So, with the linear factored load (1.5*0.5kN/m) and a portion of the wind load as requested by the NBC, we get 118.5Mpa (123%) for a 1220mm wide piece of glass. The result could vary ±5% depending on the properties of the neoprene used between the spigot and the glass.
In case you wonder what it would be like with a laminated glass 2 x 6mm fully tempered with pvb, for the same load case (line load + wind). Instead of 123% with the 12mm fully tempered glass, we get 273% for the laminated one (263 MPa ÷ 96 MPa)… 2.7 times what the glass can handle… far from a valid solution. And anyway, the A500 standard do not allow to use fully tempered glass with pvb. Furthermore, a 2 x 6mm would not resist the post breakage requirements… even with a SentryGlas.
If the guardrail with spigots, 12mm glass and clamps is inside (without any wind loads), we get 93.9 MPa (98%), see below:
So in theory, when positioned inside, with two spigots (clamps), a 1220mm wide monolithic 12mm fully tempered glass pane and the clamps between the glass panels at the top corners, the stress would be ok… below 100%, but and this is a big but… We don’t have any alternative path in case of failure (no redundancy). Which is incorrect according to CGSB 12.20 and as mentioned this standard is referenced by the Canadian building code for both part 9 (residential) and part 3 buildings (commercial).
As we discussed previously, we cannot use a single layer of 12mm fully tempered glass because it does not respect the redundancy principle which is the basis of all structural glass calculation.
We also discussed about the fact that a 2 x 6mm fully tempered laminated with SentryGlas (SGP) would not be enough to carry the post-breakage load even with clamps at the top corner. So the glass thickness will have to be higher than 2 x 6mm… to be verified depending on the project conditions (number of spigots (clamps), the rigidity of the neoprene pad between the glass and the spigots, wind loads, glass width, etc.).
The spigot has to be verified or tested to make sure it is suitable for guardrail loads application.
Make sure also that the spigot can carry the weight of a laminated glass.
Then, if it’s the case, and if you don’t want to have any handrail or top rail, calculations will have to be made with a laminated glass with rigid interlayer (SentryGlas or equivalent).
I would also say that the glass clamps between glass panels at the top corner is a no brainer because it will lower the stress in post-breakage condition by transferring a portion of the load to the other glass panels. Which will reduce the overall thickness of the assembly.
Each project as to be verified by a qualified engineer specialized in structural glass calculation because glass is a different material requiring special considerations and knowledge. Make sure to consider it in your budget.
Hope this will help.
See you in the second part of ‘’So you think you are safe’’