Guardrail with baseshoe and Taper-Locs® (glass grips)
Really happy to see you again for this part 2 of the series “And so you think you’re safe?"
This time, it wasn't very difficult to find the subject because guardrails with baseshoes are very popular in residential and commercial settings. We deal with it regularly.
On the other hand, what I want to address here is mainly cases that do not pass into the hands of an engineer. We sometimes hear stories of residential guardrails made of monolithic (non-laminated) glass with thicknesses like 10 or 12mm. And of course, without handrails. I won't go into too much detail about the need for a handrail or using proper laminated glass because I discussed it in Part 1:
I will just remind you that the CAN/CGSB-12.20 standard (for the calculation of glass) is referenced in part 4 and part 9 (small buildings) of the construction code and this standard requires loads to be carried by another element in case of breakage. This is called the principle of redundancy. We will explain further how we can respect this principle.
In this article, I will mainly discuss the thickness of the glass panes and the impact of the position of the Taper-Locs® (glass grips)… which serves to secure the glass in the baseshoe.
Above, we can see what we call Taper-Locs® or glass grips... Taper-Loc® being a trademark.
Let's take an example right away, a rectangular glass 1220mm wide, 1070mm high and 12mm thick, with Taper-Locs® at 340mm center to center.
We would have 134.38 MPa of stress in the glass for this example, and the standard only allows 96.26 MPa for tempered glass of this size. Which gives 140%... or 40% too much stress in the glass. And we didn't put any wind load yet. If we look for a height of 915mm because this would be the height prescribed by the code inside a house or an appartment:
It’s better, but still 122.04 MPa out of the possibility of 96.3Mpa… 127% → 27% too much stress in the glass. Now let's look with more Taper-Locs® (glass grips). Higher up, we had 4, at ±340mm c/c.
Still for a glass 915mm high (interior of a home or house) and 12mm thick, but with 5 Taper-Locs® (255mm c/c). It's better, i.e. 110.76 MPa out of the possibility of 96.3 MPa, but still 115% → 15% too much stress in the glass.
If we add yet another Taper-Loc® in the baseshoe at ±204mm c/c, we continue to reduce the stress in the glass.
With 6 Taper-Locs® in the baseshoe, we obtain 104.63 MPa out of a possibility of 96.3 MPa → 109%. We are getting very close, but we can no longer continue to increase the number of supports, because it becomes impossible to install in the baseshoe due to the necessary tools.
And for those who have not had the chance to consult my previous article on guardrails with spigots, the fundamental problem comes from the fact that the Canadian standard (CAN/CGSB-12.20-M89) requires the carryover of the charges by another element in the event of breakage (redundancy)... therefore we cannot design or install a guardrail with a single layer of glass without a handrail. In fact, this standard, which is referenced in the building code, does not even allow a guardrail to be made without a handrail. Here's a quick reminder:
But, as I mentioned in the previous article, there is a “relatively new” standard, the CSA A500 specific to guardrails which allows the design of a guardrail without a handrail/top rail. But in no case does this standard allow a monolithic glass guardrail (a single layer of glass) without a handrail. And this also applies to houses and appartments (part 9 of the code). We must ensure that the loads can be carried out by another element in the event of breakage. Which can be done with laminated glass under certain considerations.
If we take the same case as the last simulation, we had 104.63 MPa for a 12mm tempered glass of 1220mm width and 915mm height (interior of a home) with the factored point load.
When I talk about point load, it is because the code requires to consider 2 types of loading at the level of the guardrails, namely a point load and a load distributed at the top of the guardrail (linear load). Remember that the point load (222 pounds) is the same in residential and commercial. On the other hand, the linear load is lower in residential applications considering that the occupancy rate is different.
Now, with the same dimensions, but with a 2x6mm tempered laminated glass and PVC type interlayer, we obtain 168.9 MPa out of the possibility of 96.3 MPa (175%), or 75% too much stress in the glass...
Remember also that the CSA A500 standard does not allow two tempered glass to be laminated with a PVC type interlayer. So the previous example is for illustrative purposes only. Tempered glass must be combined with a rigid interlayer such as Sentryglas (SGP) or equivalent.
And for those who are wondering if it works with 2x6mm tempered laminated glass and Sentryglas...
Unfortunately no, since we have 123.93 MPa out of a possibility of 96.3 MPa. Which makes 129%... or 29% too much stress in the glass. So 2x6mm laminated glass tempered with Sentryglas (SGP) does not work without a handrail even for a residential application in Quebec.
And that’s not all! We said above that according to the standards, it is necessary to ensure that the loads can be carried over by another element in the event of breakage (principle of redundancy). When using laminated glass without a handrail, design should take into account a broken layer with reduced loads on the intact layer.
To give ourselves a chance, let's use clamps to connect the glasses at the top like this:
By placing the clamps at the top to connect adjacent glass panes, with a broken layer, the 2x6mm laminated glass tempered with Sentryglas does not work (124%...24% too much stress).
So we can say that even for a residential application, a guardrail with a baseshoe, with a height of 915mm, a width of 1220mm, and clips between the glass panes in the upper corners, the 2x6mm laminated glazing tempered + Sentryglas (SGP) does not work... no more than a 12mm monolithic tempered glass in fact.
Important note
It should be mentioned that I have not shown any glass other than rectangular. So I should mention that glass panes in staircases which usually come in the shape of parallelograms are much more critical than rectangular glass panes. This aspect should therefore not be neglected in the design.
It should also be mentioned that the glass panes at the edges can be really problematic... If you opt for clamps between the glass panes, you will also need to attach the top of the last glass to the wall.
Where it becomes problematic is when our guardrail is near a staircase. The first glass at the top of the stairs can only be connected to one glass. We will therefore sometimes make this glass shorter so that it transfers its load more easily to the adjacent glass in the event of breakage. Unfortunately, this is the sort of thing that needs to be worked out on a case-by-case basis by a qualified engineer.
Conclusion
We saw that the position of the Taper-Locs® (glass grips) had an impact on the stress in the glass for a guardrail in a baseshoe. Closer supports will generate less strain in the glass.
We also saw that the glass height had an impact on the constraint (1070mm vs 915mm).
We mentioned the fact that a glass guardrail without a handrail must be able to take up part of the load after a layer of glass breaks. So, we cannot use monolithic glass (single layer)… we must use laminated glass and the CSA A500 standard requires a rigid interlayer (Sentryglas or equivalent) when combined with tempered glass. With laminated glass, placing clamps between the glass panes in the upper corners greatly helps transfer loads. This allows better optimization of the glass thickness.
We also talked about the glass shapes (parallelograms vs rectangles) and the fixing of the glass panes at endpoints should not be neglected.
On the other hand, for a guardrail without handrail in an interior residential application, even with 2x6mm tempered laminated glass + Sentryglas (SGP) and additional clips between the glass panes, the resistance was not sufficient (normal and post-breakage condition). The glass must therefore be thicker and be calculated by a qualified engineer according to the specific conditions of the project.
Hope this will help you with your design.
PS: For those who would be opened to compromise a bit on the visual aspect with a stainless steel profile as a top rail over the glass panes, it is then possible to use a monolithic glass (under certain considerations).
We have developed a tool that allows you to validate 1. the thickness of the guardrail glass according to the position of the Taper-Locs® and the type of building (residential or commercial) and 2. both rectangular and shaped glass panes for staircases.
Avoid hassle when it comes time for the inspection!
Contact us for more information.