Last time we talked about the requirement of angle on non-insulated non-dampened ducts. This time we will talk about WHY it is required. For the sake of easy calculations this time we are talking about a non-insulated duct that is 24″x48″ and running through a 1 hour rated assembly. It doesn’t matter if it is a block or gypsum wall or even if it is a floor, the requirement will be basically the same. The one exception is, that a floor is firestopped from the top side, so angle is required on the top side. A wall will require both firestop AND angle on BOTH sides of the wall.
This is a big liability for your company, if the angle is not installed and today we will talk about WHY this is so important. If there is a fire, you are pretty much guaranteed that your firestop application around the duct will fail if your 24″x48″ duct does not have retaining angle properly installed. There are a few things we have to explain. First, is how assemblies are tested. Second is about buttons on shirts. Third is a painless physics lesson dealing with the Coefficient of Linear Thermal Expansion. Fourth we will tie it all together for you.
ASTM E119 allows everyone to burn something and compare it something else using very specific criterion. This way you can ensure that materials will perform as expected in a fire scenario. This is one of the standards that is used for any of this firestop stuff we are talking about,and much more. One section of the test is a “time temperature curve” that details what temperatures must be reached inside the test furnace at specific time intervals. The temperature at the 1 hour mark its required to be 1700F. Why am I telling you this? It will make sense in a moment.
Physics of Buttons:
If you are wearing a button up shirt that only has two buttons and you pull on each side of your shirt; there will be a considerable gap between the two buttons. How much of a gap will depend on how hard you pull and how far apart the buttons are. If you have more buttons on your shirt, say six buttons; then rather than have one big gap you will have a series of smaller gaps between each button. Stay with me now, on to a physics lesson and then we will relate your shirt buttons to physics and back to firestop, I promise!
Coefficient of Linear Thermal Expansion:
What happens when you heat something up? Many things respond the same way (at least prior to ignition). They expands, right? But how much something expands depends on three things, the initial temperature, the end temperature, and the coefficient of that item, in our case steel ductwork. So, if we have a room that is 70F degrees and we look only at the duct length of 48″ and we go back to the ASTM E119 time temperature curve that is used to test firestop, then at 1 hour the temperature in the furnace will be 1700F. If you ran the calculations then a 48″ duct will expand to become 48.9389″. The formula is below if you want to play with it.
Pulling it all together:
So, the same way that your two-button shirt has a big gap, the duct with no retaining angle will have a gap because the extra length, created by the expansion, will have to go somewhere. This movement will cause the duct to bow inward, because it can not bow outward because it is contained by the concrete or the metal framing of a gypsum wall assembly. The bowing that occurs will (at the apex) create a gap of almost 1”. Add that to whatever the annular space was before the increase in temperature (which I’m most cases is maximum 2″). The firestop material required on these UL listed details is intumescent, meaning that it will expand when exposed to significant temperature. However, the expansion will not be enough to fill the void of the combination of the pre-existing annular space as well as the gap created by the bowing duct. So, the angle is required to prevent this large gap from occurring. The same way the extra buttons reduced the gap on your shirt, but may vary based on the spacing-the requirements for the retaining angle will call for specific spacing and for the screws to go into the duct through the angle. This creates a series of small gaps with smaller gap apices (yes, I had to google plural for apex). This creates a scenario where the fire, smoke and toxic gas can not get to the non-fire side of the assembly prematurely.
So, at this point you should be asking about how to install (or inspect) the angle, the screws and the rest of it.
Our next blog will talk about where to find the requirement for the angle and what exactly is required and what you should be looking for so you can confirm that the installation is correct. You can find that here and the following one here. If you missed our first blog posts on this topic, please review here Part 1.
Remember the more you know the better you can be at your job! Together we all can be part of a movement- “Saving Lives for the Life of the Building.”tm If you have questions or comments please email us at email@example.com. We would love to hear from you.
Here is the formula or you can go on line for it.
dl = L0 α (t1 – t0) (1)
dl = change in object length (m, inches)
L0 = initial length of object (m, inches)
α = linear expansion coefficient (m/moC, in/inoF)
t0 = initial temperature (oC, oF)
t1 = final temperature (oC, oF)
Compliments of www.engineeringtoolbox.com
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