If you haven’t read our IBC Stair Width Calculation article, I recommend reading that first so that you can see the IBC codes we reference. If you have already read it, below is a scenario in which we calculate the required width of multiple stairways that serve multiple floors.
Scenario: We need exit stairs for a building that has 3 floors that have varying occupant loads. There is a sprinkler system, but the system was not installed per code requirements. The occupant loads are as follows:
Floor 1: 40 occupants
Floor 2: 480 occupants
Floor 3: 110 occupants
The required total stairway width per floor is as follows:
Floor 1: 40 occupants x 0.3” = 12” (BUT the minimum stairway width for a floor serving less than 50 occupants is 36”. Can you imagine trying to walk down a 12” wide stairway?)
Floor 2: 480 occupants x 0.3” = 144”
Floor 3: 110 occupants x 0.3” = 33” (BUT the minimum stairway width for a floor serving more than 50 occupants is 44”.)
What do we do when each floor requires a different width? We use the largest minimum width because we don’t want to create a bottleneck effect where a wide stairway leads into a narrower stairway.
Therefore, we want to use the 144” measurement for the minimum combined stair width.
We will need two stairways because we have an occupant load between 1 and 500 per story. The 144″ width can be divided between the two stairways. BUT in case a stairway is destroyed or inaccessible during an evacuation, the remaining stairway needs to be at least half of the required minimum (144″). We cannot have a stairway that is 60” wide and the other one is 84”. Why? If we lose the 84” stairway to a fire, we only have the 60” stairway which is less than half the required width.
Summary: We calculate the minimum required combined width by multiplying the occupant load of each floor by 0.3″ and using the largest number, which is 144″. We need two stairways, so we can divide the 144″ between the two stairs. However, since we only have two stairways, each stairway must be at least half of the 144″.