Read our article in USGlass Metal & Glazing November 2020 Issue.
Glass Guardrails
As structural engineers it’s important to pick our heads up from the code to understand its intent. When it comes to guardrails, the code has been updated quite a bit in the recent past. Monolithic, laminated, cap, no cap…. It’s a lot to keep up with.
Let’s talk about “intent” next. The intent of a guardrail is to guard us from a fall. Any walking surface that presents the opportunity for a fall of 30in or more, requires a guard. We have minimum height requirements associated with this guard (42in or more). There are also loads prescribed loads that the guard must be able to support (200lb concentrated load, 50lb/ft linear load and wind if its on the outside of a building). These are required of every guard, regardless of material.
However, glass is very different than most other building materials. Glass is brittle. This means when it breaks there is no real warning, it can just go. The point of fracture is also not predictable due to the nature of how glass is made. In most structural design, we use test-based limits for well know yield points in material. In design of glass for structural purposes, we have to use probability based allowable fracture. Meaning, even our best design efforts, there’s a minimal probability it could fracture before seeing the full design load. Further meaning, if there was a bad batch at the float glass factory, we could be installing ticking time bombs instead of guards. Not very settling. This is why I believe the code has needed to play catch up over recent years. As glass becomes more popular as a structural guard material, we need to apply rules that are more unique to it.
Now that we’ve discussed risk associated with glass, let’s do a code review to show what the officials have worked towards to reduce the risk.
2012 IBC or earlier:
- Glass used as a handrail assembly or guard section shall be constructed of single fully tempered glass, laminated fully tempered glass, or laminated heat strengthened glass. For all glazing types, minimum nominal thickness should be 1/4in.
- A factor of safety of 4 must be used in design (24,000psi FT / 4 = 6,000psi).
- Each handrail or guard shall be supported by a minimum of three balusters or shall be otherwise supported to remain in place should one baluster panel fail.
- Glass balusters must be installed with a handrail or top rail that can act as such
- Exception: A top rail shall not be required where the glass baluster is laminated glass with two or more glass plies of equal thickness and the same glass type
- The guard or top rail mentioned above, must be designed to support a fall (200lb concentrated load) in the event one baluster fails. This is true if the center or end lite fails.
So let’s break this down a bit by talking about these safety measures….
Heat treated glass (fully tempered and heat strengthened) is stronger and has smaller break patterns, meaning less chance of injuring someone after it breaks. Fully tempered glass fractures into thousands of little pieces when it breaks. So. we’re creating some level of safety here.
Laminated glass is not stronger, but also offers a layer of security in that if the glass breaks, it’s still glued to the interlayer and ideally would remain intact. Again, less chance of injury in the case of failure.
Finally, the top cap. If you’ll notice, the two previous safety measures still involve glass breaking. Well if the glass breaks, what’s stopping us from toppling over the edge? That’s what the main role the top cap plays. It’s something more predictable in the equation.
This code excerpt is pretty good and covers most every concern. However, in my opinion, there are two areas where additional information and guidance are needed:
- The exception previously mentions that a top cap is not required if laminated glass is used, is left open to interpretation.
I can tell you, from an engineer’s perspective that 1/2in laminated glass is weaker than 1/2in monolithic glass.
If you interpret the code directly, you could be open to weakness in your design.
ICC AC439 Acceptance Criteria for Glass Railing and Balustrade Systems sheds some light to this potential misinterpretation in the building code. A brief summary is given:
- This document prescribes how to test the top cap, should one lite break.
- It also prescribes that should you use laminated glass without a top cap, and one ply breaks, the remaining ply of glass should be able to accommodate the load.
- In short, if you ditch the top cap for laminated glass, you’re probably going to need to thicken your glass.
- The second area of concern is worse in that, if a monolithic rail with a top cap is installed, and breaks, there is nothing stopping all this heavy glass from falling on people below the railing. It’s for this reason, I believe the 2015 code updated this section as discussed below.
2015 IBC or later:
In addition to the previous requirements in the 2012 IBC, the update in 2015 added important safety measures for bystanders. Glass used in handrail, guardrail or guard section shall be laminated glass constructed of fully tempered or heat-strengthened glass. For all glazing types, minimum nominal thickness should be 1/4in.
Exception: Single fully tempered glass shall be permitted to be used in handrails and guardrails where there is no walking surface beneath them, or the walking surface is permanently protected from risk of falling glass.
Now the code has everyone covered, as best it can. It’s important as industry professionals we understand that the code is the minimum requirement. Judgement must be taken to ensure we ultimately design and install a safe structure. There are a few areas to unpack as it relates to this.
- Laminated glass is not as strong as the monolithic of equivalent thickness.
Picture stacking two 2×4 on their sides, on top of one another, and putting a cinder block on each end, and walking across. It’s a bit bouncy right? Well let’s scrap the 2×4’s and grab a 4×4 to span this distance. Same overall depth, but much less bouncy.
This is essentially beam theory. The two 2×4’s being non-composite (or not bound to each other), on one end of the spectrum and the 4×4 being fully composite on the other.
Back to Laminated Glass Engineering
Laminated glass is somewhere in between these two datum points, partially composite. Where it lands in between is entirely up to the glue laminate interlayer that is binding the plys together. This is very important! In the United States, there are two accepted methods to performing engineering on laminated glass panels: equivalent thickness and finite element analysis. Equivalent thickness method utilizes equations to calculate an equivalent monolithic glass piece based upon the strength of interlayer. This is a gross approximation at best, and does not accurately account for slippage between glass panels. This is critically important when modeling the laminated glass inside of a base shoe. Finite element analysis is the only true way to analyze these panels. European structural glass design has already moved away from equivalent thickness in favor of finite element analysis. Check with your engineering company before cutting them loose on a project, to see what method they use. You can receive wildly different results and open yourself up to liability concerns if they are behind the times.
It’s also important to note that the laminate interlayer strength is nonlinear, meaning it’s strength is time and temperature dependent. So if you’re ordering material, and you have rails inside and outside of the building, don’t be so sure that they can all be the same thickness. Location of the railing plays a major role in it’s strength, which leads into the next area to unpack.
- Location, location, location…
As mentioned above, if your railing is outside, and exposed to the sun it can severely reduce the overall strength. As the laminate interlayer heats up, it becomes more pliable and allows more slippage between the panels. The laminated glass becomes less stiff.
Equally as important, you can’t assume the same railing on the ground floor of a building can be used on the roof. While it may still be able to support the same guard loads, it could be horribly deficient in supporting the wind loads at the top of a building. Depending on the height of the building, the wind can become extremely high at a roof top.
- Base connections are very important!
An engineer can do everything right in the design of structural glass railing, but connection of the glass shoe to the structure can make your railing feel unsafe. And if a code official thinks its unsafe…. It is critically important that whatever the glass shoe connects to is sufficiently rigid to avoid even small rotations. Think of it this way. Its similar to having a 42” very stiff crowbar and pushing laterally at the top. This gives a mental picture of the magnitude of loads at the connections of the base shoe for handrail. If the structure that we are attaching into is thin metal like a shelf angle, its possible for the railing to feel very flexible at the top because of the prying action. In addition, shims should be minimized. The addition of shims may increase the feeling of flexibility at the top of the railing and cause safety issues.
To sum things up, it’s very important to consult with a structural engineer, well versed in glass railing design, as early in your project as possible. While architects love glass railing as beautiful and ornamental features, they are important to the safety of the building occupants. Missing a step could cost you a lot of money and delays if a deficient railing is installed. Or worse, should it not be caught.
