The Rigors of Firestop Tests – Part 2

Posted on 7/9/2018 1:25:28 PM By Sharron Halpert
  

When you look at a firestop detail and see the F rating, T rating, L rating and W rating. What does that all mean? Last post, located HERE, we talked about the L rating and the code implications.  This post we will talk about the T ratings a little bit, explain what they are and why they are important.  This is an interesting topic. Let’s start with definitions. 

F rating is the hourly rating of the assembly.  This is the time the assembly can prevent the passage of flames from the fire-side of the assembly to the non-fire side of a floor or wall assembly during a laboratory controlled test. 

T rating ensures that the assembly prevents the passage of heat through the assembly.  The heat is measured with thermocouplers (TC) that are placed in specific locations around the assembly and connected to a computer system.  If the temperature rise on the non fire side of the assembly exceeds 325F before the hourly duration is met, then the assembly will fail. 

The reason for the F rating is obvious. The whole point of a rated assembly is to keep fire at bay for the hourly duration required by the building code. The reason for the T rating is not so obvious, so let’s take a moment to explain this. 

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The fire triangle (above) is an easy way to explain how fires survive, thrive and grow…or die. To maintain a fire, you need all three elements of the fire triangle.  Elimination of one element will kill the fire. 

Imagine you have a piece of paper in one hand and a lit match in the other. If you were to bring the lit match closer and closer to the fuel it will not be the flame toughing the paper that will cause it to scorch and then burn, but rather it will be the heat transferring from the burning match to the paper.  As the temperature of the paper increase it will reach what is called its flashpoint, which is the temperature at which something will ignite.  

Clearly when you have all three of these together, you can actually create a fire.. That means that if there is oxygen and a fuel source in a room, all you need to start a fire is sufficient heat.  Have you heard the term “flashover”? This is when the temperature increases in a room to a point where the items inside ignite. If you are not familiar with it, this video will help you better understand it better. View an example of this in this video: https://www.youtube.com/watch?v=BtMmymOxdjc

This is why the T rating is critical in fire prevention and why it is a requirement in both ASTM E119, which is the test for rated assemblies such as walls, floors or floor ceiling assemblies.  It is also a requirement in ASTM E1966, which is the test for rated joints.  

It makes sense, that if we want the individual assemblies to have both and F rating and a T rating, that we would want the connection of these to also meet the same requirements.  Why?

Let’s look at another scenario.  Imagine you are in a hotel room.  The walls separating your room from the adjacent room will typically be designed as a one-hour wall.  Think about what different combustible items would be next to the wall? It could be curtains, chairs maybe a desk or even a headboard for your bed. Now when we put all these pieces together, you understand that if that wall separating the two rooms cannot prevent both fire and heat from passing through the wall, then it is not an effective fire barrier.  The same is true for the rated floors that are covered with various flooring materials as well as the furniture. 

The T ratings are a requirement for assemblies such as rated floors, rated walls or the joints that connect these assemblies. T ratings are not however a requirement for passing the test for through penetration firestop (ASTM E814).  The exception to this is a code related issue.  There are two sections of the building code where through penetration firestop applications require that T ratings match the F ratings and we will delve into that in our next post. 




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