What is a Fire Rating?

MeyerFire University | LS151.02

By Steven Barrett, PE

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LS151 SERIES

1. ​Purpose of Fire-Resistance-Rated Construction?
2. What is a Fire Rating?
3. Fire Partition vs. Fire Barrier vs. Fire Wall?
4. What is a Fire Barrier?
5. What is a Fire Wall?
6. What is a Fire Partition?
7. Purpose of Smoke-Resistant Construction?
8. What is a Smoke Partition?
9. What is a Smoke Barrier?
10. Crossword: Fire & Smoke Resistive Construction
11. What are fire/smoke requirements for floors?
12. Difference between floor, floor/ceiling, roof/ceiling, and roof assembly?

TRANSCRIPT

What is a Fire Rating?

INTRODUCTION

In our last segment we talked about what the purpose of having fire-resistance-rated construction.  

Fire-resistance-rated construction is a way to describe the robustness of building elements to reflect how well they resist fire and/or smoke and its used to make building safer by limiting fire spread, thereby giving occupants more time to get out, protect-in-place, and protect the structure so that firefighters the ability to manually fight a fire. 

In this segment, we’re going to talk about what a fire rating is. Let’s dive in. 

WHAT IS A FIRE RATING?

A fire rating is a duration of time, usually expressed in hours, that represents the length of time that an element will continue to perform its function when exposed to fire.  

Fire ratings can be applied to elements, like walls and floors. They can also be applied to openings in those walls and floors, such as for doors, glazing, and penetrations.

Fire ratings can range from 0.5 hours (or half an hour) to 4 hours. Common ones include ½-hour, 1-hour, 2-hour, 3-hour and 4-hour.  

Now we apply fire ratings to an “assembly”, which is a combination of elements that has been tested according to a standard. 

A “STANDARD” FIRE TEST

Fire ratings are typically determined by testing the assembly in a test furnace – exposing it to real fire, temperatures of up to 2000 degrees Fahrenheit. The most common type of test is ASTM E119. This test is considered to be a “standard fire”, but let’s not make any pretenses. There is no such thing as a “standard fire”, because the heat release rate and temperatures that a compartment experiences varies widely. That said, in order to test our fire-rated assemblies, we need a standard protocol to test towards, and so we’ve established this “standard fire” as a way to consistently test assemblies.  

In order for an assembly to pass an ASTM E119 test, the assembly must be able to prevent flaming and temperatures on the unexposed side of the assembly to not exceed 325 degrees or 250 degrees above ambient temperature at the start of the test. 

DOES A 4-HOUR FIRE WALL MEAN IT’LL LAST 4-HOURS? 

Does having a 4-hour Fire Wall mean the wall will last four hours? 

Perhaps surprisingly, no. A 4-hour Fire Wall does not mean that the wall will survive a fire for four hours.

This somewhat defies common sense – but let’s explain that. A 4-hour fire-rating for an assembly means that it passed a 4-hour test, like ASTM E119, but that’s with a “standard fire” condition. We’ve just said that a standard fire doesn’t exist in the real world.  

If there’s a fire involving liquid fuel, or battery storage, or expanded plastics, or another high hazard situation fire the heat release rate could be substantial. Also, what if our compartment is sprinkler protected, or not? What if the fire is suppressed, or not? 

There are many variables that impact the exact temperature a wall or a floor will experience in a fire, so it’s a slight misnomer to interpret testing of a standard fire to mean that an assembly will last for exactly that same amount of time with a different hazard in a different compartment. 

SIMPLIFIED FIRE-RATING SYSTEM

That being said, we need standards in order to develop testing criteria, and we need quantitative benchmarks for our codes & standards to have real meaning. So, we’ve simplified the way that we talk about fire-resistance-ratings by giving them the time duration which they’ve lasted in our “standard fire” test.  

The takeaway here is that the fire-ratings are relative. Has a 1-hour fire rating tested better than a ½-hour rating? Yes, it has.  

Will a 1-hour fire rated assembly perform better than a ½-hour fire rated assembly? Yes, it should. 

Will a 1-hour fire barrier last for an hour with a fire in a warehouse? Maybe, maybe not. That would make for a pretty advanced analysis. 

DESIGNATING FIRE RATINGS FOR SPECIFIC WALLS 

Now let’s say I build a wall that matches a tested assembly. By code, it is constructed as a fire barrier to protect a hazard within the room. Elsewhere in the building, I am using the same type of wall construction, for corridors and other rooms that are not required to have any fire resistance rating.  

Does this mean, that all of my walls are fire rated?  

Not necessarily.  

If the walls are built to match a tested assembly, they are capable of providing a fire resistance rating but that doesn’t mean they necessarily have a fire rating.  

This happens more often than you might think. 

DESIGNATION FOR FIRE-RATED ELEMENTS

Typical wall assemblies in commercial construction are built in a manner that could achieve a 1-hour fire resistance rating. If they are built in a manner that could meet a 1-hour rating, then they are 1-hour rated, right?  

No, the architectural design and life safety plans determine where the fire ratings will be provided.

Generally, fire ratings are only provided where they are required by code or in some cases, intentionally provided as good design practice, to specifically protect an area.  

Why the distinction?  

Just make it a fire barrier?  

Even though the wall assembly may be similar or even identical, calling it a fire barrier bring a lot of requirements with it, as we’ll see: it means providing a fire rated door with a closer, it means sealing penetrations and joints, it might mean providing a fire damper in ductwork and maintaining these for the life of the building.  

If the wall, however, is not required to be and thus not designed as a fire barrier, then none of these are required. This is how even if two walls are constructed the exact same, one might have a fire rating and the other does not. 

WHEN DO WE USE FIRE-RATINGS?

So then, we’ve talked about what a fire rating is. We’ve introduced testing for them, and designating them. But when do we use fire ratings?  

There are many situations when fire resistance construction is specifically required by code.  

Like in the previous example, some occupancies, like residential, require fire resistance rated corridors. Some means of egress require fire rated construction, like interior exit stairs which require fire rated construction. Often, penetrations through floors, like HVAC shafts require fire rated construction. Rooms that are more hazardous than adjacent areas often require fire rated construction. Fire rated construction is used to separate occupancies, fire areas, and to separate buildings.  

There are many more but, in each situation, the fire rated construction is required to prevent fire spread from one area to another area to ensure occupant safety.  

We’ll talk more about where code triggers fire-rating requirements later in this series. 

SUMMARY

So, what is a fire-resistance-rating? 

A fire rating is a duration of time, usually expressed in hours, that represents the length of time that an element will continue to perform its function when exposed to fire.  

Fire ratings can be applied to elements, like walls and floors, or can also be applied to openings in those walls and floors, such as for doors, glazing, and penetrations. 

In our next video, we’re going to talk about the difference between a fire wall, fire barrier, and fire partition. 

I’m Steven Barrett, this is MeyerFire University.