Phased evacuation was first presented as a possible egress approach in 1935 by the National Bureau of Standards, which later became the National Institute of Standards and Technology (“NIST”). The same report also gave credence to the “capacity” design method. Interestingly, this “capacity” method involved sizing the stair(s) to hold all the occupants of the building motionless within the stairwell, similar to a refuge area. The thinking behind this approach was that occupants would tire out if required to descend a significant number of steps; if one person stops on the stairs, then egress from the stair would be hindered. Therefore, promoters of this method believed stairs should be sized to hold all the occupants of the building to preemptively account for people stopping.[1] Using this method, a traditional 44 in. wide stair would allow 28 occupants on each elevated floor. While this approach seems silly today, buildings did not have as many floors or occupants, so this was a more viable option than it is in present times.

On that note, has anyone read a code requirement and wondered to themselves: “Why?” Today, we asked “Why?” concerning the International Building Code requirement for high-rise buildings to notify occupants on “at least the alarming (fire) floor, floor above and floor below.”[2] Why not require more floors? Luckily, egress modeling software allows the investigation of how varying numbers of evacuating floors affect egress duration.

We ran four egress models based on a hypothetical 10-story building with 100 occupants on each floor. The first model only evacuates the fire floor; the second model evacuates the fire floor, floor above, and floor below; the third model evacuates the fire floor, two floors above, and two floors below; and the fourth model evacuates all 10 floors. The results are shown in the table below:

Scenario

Egress Time from Fire Floor (mm:ss)
Fire floor 2:12
Fire floor, floor above, and floor below 4:11
Fire floor, two floors above, and two floors below 5:33
Full building 6:28

As you can see, even evacuating the floor above and below the fire floor almost doubles the egress time. The code recognizes that evacuating multiple floors slows egress, and that is why it only requires alarming the fire floor, floor above, and floor below.

The video below shows a comparison of only the fire floor evacuating (left) and the full building evacuation (right). The fire floor is shown with occupants wearing white, and if you watch the colors mingle within the stair, the effect of merging (too many) levels becomes apparent.

The reasoning behind alarming the floor above and floor below is that these floors can also be directly impacted by the effects of a fire either through smoke/flame spread or structural collapse. NFPA 101 provides the following guidance:[3]

In high-rise buildings, the exit stairs, at any level, are designed to accommodate the egress flow of only a very small portion of the occupants — from only one or a few stories, and within a relatively short time period — on the order of a few minutes. In case of a fire, only the immediately affected floor(s) should be given priority use of the means of egress serving that floor(s). Other floors should then be given priority use of the means of egress, depending on the anticipated spread of the fire and its combustion products and for the purpose of clearing certain floors to facilitate eventual fire service operations. Typically, this means that the one or two floors above and below a fire floor will have secondary priority immediately after the fire floor.

The section goes on to state that, in most cases, persons entering a stairwell on lower floors should defer to people already descending the stairs to clear the upper levels more quickly. This is recommended to avoid the exact condition that is shown in the full building evacuation model.

 

[1] Bukowski, R.W., Tubbs, J.S., Chapter 56: Egress Concepts and Design Approaches, SFPE Handbook of Fire Protection Engineering, 5th Edition, Morgan J. Hurley ed., NFPA 2016.

[2] International Building Code Section 907.5.2.2, 2021 edition.

[3] NFPA 101, Life Safety Code, Section A.4.8.2.1(3), 2018 edition.