Understanding Glass Requirements for Guards
Should we use tempered glass for IGUs (insulated glass units) acting as guardrail? Should we temper only the inside pane? If we use fully tempered glass, can we use any thickness? Here are some of the questions we receive really often from our clients. In this post, we will discuss the different types of glass, their properties, and the factors to consider when choosing the appropriate glass for windows acting as guardrails or infill panels within guardrails.
Types of Glass
Annealed Glass: Annealed glass is standard float glass that has not undergone any additional heat treatment process. It is more prone to breakage and, when broken, tends to shatter into large, sharp shards, posing a safety risk.
Heat-Strengthened Glass: Heat-strengthened glass undergoes a heating and cooling process that makes it approximately twice as strong as annealed glass. When broken, it fractures into large pieces, similar to annealed glass but with additional strength, this type of glass cannot replace tempered glass within doors for example.
Tempered Glass: Tempered glass is heat-treated to be about four times stronger than annealed glass. It is designed to withstand higher loads and, when broken, shatters into small, relatively harmless fragments, reducing the risk of injury.
Evaluating Alternatives
Many clients approach us to explore more cost-effective options, such as annealed or heat-strengthened glass, or thinner tempered glass. Unfortunately, they often struggle to find tools that can help them evaluate these alternatives. That's where we come in, offering tools to assess and validate different types of glass for guard rail loads on any building.
Infill Panels and the National Building Code (NBC)
Most clients are unfamiliar with the concept of infill panels, which can occasionally eliminate the need for tempered glass. As per NBC's article 3.3.4.7, guardrail conditions in dwelling units of buildings assessed under part 3 can be evaluated according to section 9.8. It is crucial to emphasize that this only applies to Quebec and not Ontario, necessitating careful verification with local building codes. Furthermore, the requirements may vary for common areas and other parts of a building, such as commercial offices located on the second floor of a condo tower, for example.
Understanding the National Building Code's Guardrail Requirements
The NBC lays out specific guardrail requirements for buildings covered by Part 9 (small buildings) and dwelling units in buildings covered by Part 3. Article 9.8.8.3 refers to the height of the guardrail being 900mm for a maximum fall height of 1800mm. However, this does not apply to common areas or other portions of a building covered by Part 3, such as commercial offices on the second floor of a condo tower. For these areas, insulated glass units within windows will be considered acting as guardrail if the sill of the window is lower than 1070mm from the finished floor and where the fall height is more than 600mm (see Article 3.3.1.18).
Infill Panel vs. Guardrail Glass
When a transom is located at a guardrail height or below, the glass underneath the transom will act as an infill panel. Infill panels have lower load requirements, which may allow for the use of annealed glass. Our applications can help validate the type of glass needed in different scenarios, including infill panels and guardrails. It is important to remember that our applications do not require any engineering skills. We have developed them to be quick and easy to use for glaziers, architects, technologists and other professionals involved in the building industry.
Transom Height and Its Effects
The height of the transom is crucial in determining whether the glass above and below will act as a guardrail or an infill panel. For example:
- If the transom is located at 1070mm from the interior finished floor, the glass above will not act as a guardrail, and the glass below the transom will act as an infill panel.
- If the transom is lower, say 800mm, the glass below will also act as an infill panel, but the glass above will act as a guard.
- If the transom is at 915mm from the floor, there is no need for redundancy, so the exterior glass could be annealed if the wind load allows for it.
In these scenarios, our THGC application can be used to validate the glass above the transom, while the THPR application can be used for the glass below. In fact, for an infill panel, the loads are lower according to the building code (half of the guardrail load: 0.5kN instead of 1kN), and in some cases, annealed glass can even work for infill panels. As mentionned earlier, within dwelling units, guardrail loads can be calculated according to section 9.8. In part 9 of the building code, the point load is the same for guardrail condition (1kN) and/or infill (0.5kN). But the applicable area of the load for infill panel is bigger, resulting in lower stress. Remember that this is not valid for Ontario, where guardrail loads have to be considered regarding part 4 of the building code. To be verified depending on the province building code. It's important to note that for an infill panel, the notion of redundancy is not necessary, so only the interior glass will be studied. The exterior glass will still have to conform with the wind loads though, and this part can be analyzed and validated with our THV application.
Redundancy and Exterior Glass
The CAN/CGSB-12.20-M89 standard requires redundancy in case of breakage. What is redundancy exactly? It's an alternative pathway in case of glass breakage like the belt in your car. This principle is used for structural glass, in the aviation field and more. So, for an insulated glass unit acting as guardrail, the exterior glass should also be able to handle a portion of the guardrail load. This redundancy is ensured by checking the exterior glass so that it can take a share of the guardrail load. This way, if the inner glass has a defect or scratch and breaks at a much lower stress, safety can be provided by the outer glass. For the exterior glass of an IGU acting as guard rail, the applied load is lower. For this reason, the outer glass can generally be thinner than the inner glass and still conform to guardrail loads.
Limitations of Our Applications
While our applications can validate the conformity of glazing for wind loads and guardrail loads in normal or infill panel conditions, they are not designed to validate window frames. Extruders often have UL test reports for frames vs. guardrail loads, but these tests cannot cover glazing. As glass have variable mechanical properties depending on mass defect, scratch on the surface, notch on the edge, etc. tests are not relevant for glass. The figure below illustrate the maximum breakage stress for 740 glass pane tested with the same load and retaining condition.
Building Code and Tempered Glass
Although the building code does not explicitly require tempered glass for insulated glass units acting as guardrails, annealed glass typically cannot handle these loads. As stated in our THGC user guide, "since point loads generate relatively high localized stresses and since annealed glass is four times less resistant than tempered glass, the application does not allow for annealed glass to be considered in the presence of guardrail loads."
Practical Examples
- A 1000mm x 1000mm sealed unit composed of two 6mm annealed glass panes would result in a 260% utilization of the resistance on the inner glass pane and 195% utilization on the outer glass pane.
- For a 500mm x 1000mm sealed unit, the inner glass pane would be at 212% with the same composition (2 x 6mm annealed).
These examples show that even with 6mm annealed glass, we greatly exceed the limit of the glazing. At 212%, we are at more than twice the strength limit of annealed glass. As a consequence, no other choice than tempered glass is presented in the application THGC for insulated glass units acting as guardrails.
Using Tempered Glass
When using tempered glass, it's important to consider not only the full tempering process but also the thickness adjustment according to the window size and wind pressure, because the building code requires to combine a portion of the wind load with guardrail loads. Our THGC application allows for the validation of each glass thickness, ensuring the glass fully conforms to guardrail loads.
Conclusion
The building code does not explicitly require tempered glass for insulated glass units acting as guardrail. However, as demonstrated in this blog post, annealed glass is unable to withstand these loads, making tempered glass the preferred choice for safety reasons. When it comes to infill panels, the loads are lower, and annealed glass may sometimes be sufficient. Our applications, such as THGC, THPR, and THV, can help you assess and validate the type of glass needed for different load cases, including guardrails, infill panels and wind loads only.
It's essential to carefully consider the glass type, thickness, and thermal treatment when selecting glazing for window acting as guard rail or infill panel. Understanding the National Building Code's guardrail requirements and the differences between infill panels and guardrails can help you make informed decisions that prioritize safety while remaining cost-effective. For exemple, if you choose to position a transom at 1070mm from the floor, the glass above will only have to handle wind load, which will probably only require annealed glass. For the glass below the transom, if the window is located within a dwelling unit for a building in the province of Quebec, you might be able to find a glass composition with only annealed glass depending on the size of the glass and the wind load. Our THPR application will help you deal with that.
In summary, while tempered glass is not always necessary according to the building code, its use is often justified for guardrail applications due to its strength and safety characteristics. Our applications can assist in making the right choices when it comes to glass thickness and heat treatment, ensuring the safety and satisfaction of building code. So don't wait to give it a try, go right now look at our collection of applications. Do yourself a favor and buy the tools you've been looking for months!