Safe Installations: Code Updates and Changes for Safe Glass Guardrails

By Matt Gorny

Building codes concerning guardrails have 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. The intent of a guardrail is to guard us from a fall. Any walking surface that presents the opportunity for a fall of 30 inches or more requires a guard. There are minimum height requirements associated with this guard (42 inches or more). There are also prescribed loads that the guard must be able to support. These include 200 pounds concentrated load, 50 pound/foot linear load and wind if it’s 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 and when it breaks, there is no real warning. The point of fracture is not predictable due to the nature of how glass is made. In most structural design, we use test-based limits for well-known yield points in material. In glass design for structural purposes, we have to use probability based allowable fracture. This means that even with our best design efforts, there’s a minimal probability it could fracture before seeing the full design load. In addition, if there was a bad batch at the float glass factory, it could be like installing ticking time bombs instead of guards. This is why I believe the code needs to play catch up. With glass becoming more popular as a structural guard material, we need to apply rules that are unique to it.

A Closer Look

Let’s review the code to see how officials have worked to reduce the risk. In the 2012 International Building Code (IBC) and even prior to that:

1. Glass used as a handrail assembly or guard section had to be constructed of single fully tempered glass, laminated fully tempered glass, or laminated heat-strengthened glass. For all glazing types, the minimum nominal thickness should be ¼ inch.

2. A factor of safety of 4 had to be used in the design (24,000 psi FT / 4 = 6,000 psi).

3. Each handrail or guard had to be supported by a minimum of three balusters or otherwise supported to remain in place should one baluster panel fail.

4. Glass balusters had to be installed with a handrail or a top rail that could act as such. There is an exception—a top rail was not required where the glass baluster was laminated glass with two or more glass plies of equal thickness and the same glass type.

5. The guard or top rail mentioned above had to be designed to support a fall (200 pound concentrated load) in the event one baluster failed. This was true if the center or end lite fails.

This code excerpt above covers most every concern. However, in my opinion, there are two areas where additional information and guidance are needed:

1. The exception that a top cap is not required if laminated glass is used is open to interpretation. From an engineer’s perspective, ½-inch laminated glass is weaker than ½-inch 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. The document:

• Prescribes how to test the top cap, should one lite break;
• Prescribes that if laminated glass without a top cap is used, and one ply breaks, the remaining ply of glass is required to accommodate the load;
• Says that if you don’t include a top cap for laminated glass, you’re probably going to need to increase the thickness.

2. The second area of concern is more worrisome. If a monolithic glass rail with a top cap is installed and breaks, there is nothing stopping all this heavy glass from falling on people below the railing.

Code Evolution

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 sections must now be laminated glass composed of a fully tempered or heat-strengthened glass. For all glazing types, the minimum nominal thickness should be ¼ inch.

There is an exception. Single fully tempered glass can be used in handrails and guardrails where there is no walking surface beneath them, or if the walking surface is permanently protected from risk of falling glass.

It’s important that, as industry professionals, we understand that the code is the minimum requirement. We must ensure that we ultimately design and install a safe structure. In general, laminated glass is not as strong as the monolithic of equivalent thickness. Picture stacking two 2-by-4s on their sides, on top of one another, putting a cinder block on each end, and walking across. It’s a bit bouncy right? Well let’s scrap the 2-by-4s and grab a 4-by-4 to span this distance. It has the same overall depth, but is much less bouncy. This is essentially beam theory: the two 2-by-4s being non-composite (or not bound to each other), on one end of the spectrum, and the 4-by-4 being fully composite on the other.

But Back to Glass

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 plies together.

This is very important. In the United States, there are two accepted ways to engineer laminated glass panels: equivalent thickness and finite element analysis.

The equivalent thickness method utilizes equations to calculate an equivalent monolithic glass piece based upon the strength of the 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.

Note that the laminate interlayer strength is nonlinear, meaning its strength is time and temperature dependent. So if you’re ordering material, and you have rails inside and outside the building, don’t be sure that they can all be the same thickness.

The location of the railing also plays a major role in its strength. If your railing is outside and exposed to the elements, it can severely reduce the overall strength. As the laminate interlayer heats up, it becomes more pliable and allows more slippage between the panels. This means, the laminated glass becomes less stiff.

While architects love glass railings 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, if it’s not caught.

Matt Gorny, MAE, P.E., is the team lead, senior glazing systems engineer, for JEI Structural Engineering in Kansas City, Mo.

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