Shaft Enclosures

If a 4-story building with first floor as an assembly space with the above 3 floors as R-2, and the horizontal floor assembly rating between first and second floor is 3 hours, what is the min fire rating for an exit stair for all floors?

Per IBC, section 713 Shaft Enclosures, 713.4 Fire-resistance rating, Shaft enclosures shall have a fire-resistance rating of not less than 2 hours where connecting four stories or more… ,but need not exceed 2 hours.

With the above code verbiage, does that mean we can use 2 hours enclosure thru out the building, even though one of the horizontal floor assembly is more than 2 hours?

Hi @ellakui - each assembly is separately governed by different parts of the IBC. It’s typical that, for example, a demising wall between apartments would have one required rating, walls between apartments and corridors another, and walls surrounding the staircases another still. You don’t need to apply the most strict rating required to everything else in the project.
The logic in my above example is that the ratings get higher as an occupant moves through the building in a fire - They have, say, one hour to get out of their apartment, 1.5 hours to get from their apartment to the stairs, and 2 hours to get from the stairs out of the building.
Hope that was helpful and good luck on your next exam!