The Case of the Relative Railing
Dear Glass Detective,
I am trying to determine the relative strength of half-inch tempered glass versus a composition of two quarter-inch tempered glass pieces with a .060-inch interlayer between. The application is for a glass guardrail. Can you advise me?
Thank you for making contact with the Glass Detective with your question as to the “comparative strength” of a single pane of half-inch tempered glass versus a composite glass panel made of two pieces of quarter-inch tempered glass laminated together using a .060-inch polyvinyl butyral (PVB) inner-layer.
To begin with, I am going to suggest that you take a quick look at a previous submission of ours that we prepared, comparing laminated glass to tempered glass (not multilayered). I think this document will provide you with the pros and cons of both products and is a good place to start as we deal with your specific question. Both types of glass have some very positive attributes but there are also some negatives to be considered.
On a somewhat personal note, I have long been a proponent of laminated/tempered assemblies in glass railings. While you don’t provide the size of the railing glass you are considering (you referred to it as a glass guardrail), I am going to assume that the glass sizes will be 50-inches or less in width and approximately 42-inches in height. I am also going to assume that the glass will be secured with some type of a glazing system (channel, clips or similar) that will properly and adequately hold the glass in a secure manner.
We too often put a great deal of focus on the glazing product while not putting enough thought into what is going to secure the glass in place. The glass is totally dependent on the system being used. In my experience, now that I have offered up my preference for the multilayer tempered/laminated glass approach, the price for this method is what usually discourages people from using it. I feel this is a little short-sighted because the tempered/laminated glass product is going to be much less likely to break than the laminated alone product. In a railing, it will be much safer than a tempered-only glass product. The combination of tempered/laminated provides what might be considered the best of both glass products.
You specifically asked for a “relative strength” comparison between a fully tempered piece of guard rail glass versus a tempered/laminated piece. You should also check the appropriate building code to see if the application you are planning on allows for tempered only glass. For insurance reasons, we cannot provide certain engineering calculation or design/use recommendations.
Please remember though that tempered glass vacates the opening when broken and this is a concern for me in guard rails of any kind. Thicker laminated glass will stay in the opening (at least for a while) but is more susceptible to breakage. The tempered/laminated approach, in my opinion, is the preferred methodology. While I cannot provide you with specific load/strength calculations, you will be able to get the information you seek from the glass manufacturer you get quotations and fabrication information from. I think you will be pleased with the information they will provide to you.
Additionally, I could not help but note that you referred to a specific PVB thickness (.060”) in your question on this matter. The typical consumer, unless they have had experience with PVB products in the past or have performed a little research on their own, would not ordinarily reference a PVB thickness and in particular, not a .060” PVB thickness. Typically, a .030” PVB interlayer is used for what is commonly referred to as “architectural laminated safety glass.” However, when tempered glass pieces are laminated together, manufacturers often prefer to use a thicker PVB (like .060”) because it will absorb/fill-in any minor deflections/warping or distortions that might have occurred during the glass tempering process. PVB is used in a range of thicknesses depending on the product being manufactured and its intended use.
I hope this response has been of some value to you and I again thank you for contacting the Glass.com Glass Detective. Good luck with your project.