Envelope #11: Rule of thumb for concrete thickness (over steel deck) to meet fire rating requirement
Andy Lin
June 02, 2022
Good morning! This is Back of the Envelope – the place to learn structural engineering in tiny bites 🍪.
In today’s email, I will talk about how to do a quick-n-dirty preliminary check to make sure your composite steel deck meets the fire rating requirement.
This is something that confuses me almost all the time… but there are some rule of thumb that could be applied for back-of-the-envelope checks.
Let’s dive in.
(Estimated read time = 1 minute and 30 seconds -- I told you, tiny bites)
What is the fire rating requirement?
The first step is to obtain the structure’s required fire rating from the client/architect. This is based on the type of building and other goodies per IBC Table 601.
Spray-applied fire resistive material underside of deck?
Once you know the required rating, you should then find out if “spray-applied fire resistive materials” (SFRM) will be applied to the underside of the deck.
(Some people refer to the SFRM using the product name Monokote).
Rule of thumb
Now here comes the rule of thumb:
If SFRM will be applied to the underside of the deck, then the concrete thickness above the deck would generally be 2-1/2” thick.
And it can be either normal weight or lightweight.
You’ll achieve a 1 to 4-hour rating with most UL assemblies (the architect and/or fire protection engineer need to detail that).
On the other hand, if SFRM will NOT be applied to the underside of the deck, then the thickness of the concrete varies depending on the required rating.
The legacy Verco catalog had this handy table below (it’s a rough generalization of all the UL assemblies):
For example, the thinnest configuration to get a 1-hour rating would be using 2-1/2” lightweight concrete over 1-1/2” deck (watch out for unshored span though – topic for another email).
A very common 2-hour rating configuration that I have seen is 3-1/4” lightweight concrete over 3” deck.
And there you have it. Hope that all makes sense.
Now you should be able to come up with a preliminary concrete thickness based on the required rating.
You could then move on to figure out the other design requirements (e.g., unshored span, vertical capacity, diaphragm capacity…etc.). We'll save those for another email.
Thanks for reading and enjoy the rest of your week!