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February 8, 2021
Stairwell pressurization is intended to prevent smoke from flowing into stairwells. But when designing a stairwell, it’s important to know whether it’s necessary to implement stairwell pressurization. In this blog, we look at when stairwell pressurization is needed for a building and the requirements.
Anyone who has spent time navigating a building or fire code will quickly come to realize the importance of definitions, and that is ever clear when posing this question – when is stairwell pressurization required?
The default answer is for a high-rise building. The short answer is that it is in fact not a code requirement at all. Something about the codes that we may lose sight of is that not too much is absolute. There are many design alternatives, exceptions, and sometimes different interpretations of the “rules” of the code. In fact, stairwell pressurization is itself an alternative!
Before we determine the stairwell pressurization requirements, let’s first establish some of these key definitions that will help us to answer the question.
High-Rise Building: A building with an occupied floor located more than 75 feet above the lowest level of fire department vehicle access.
Smokeproof Enclosure: An exit stairway or ramp designed and constructed so that the movement of the products of combustion produced by a fire occurring in any part of the building into the enclosure is limited.
As alluded to above, we most commonly see stairwell pressurization systems utilized in high-rise buildings.
However, just because the building meets the definition of a high-rise does not necessarily imply that every stairwell, or any stairwell for that matter, is required to be provided with stairwell pressurization. The code simply requires that every exit stair serving floors more than 75 feet above the lowest level of fire department access shall be a “smokeproof enclosure.”
For example, stairs serving only a couple of stories within a high-rise building would not be required to satisfy the requirements for a “smokeproof enclosure.”
Similarly, for underground buildings, every stairway serving floor levels more than 30 feet below the “level of exit discharge” would also require a smokeproof enclosure, with one method being stairwell pressurization.
The objective of a smokeproof enclosure is to prevent the migration of smoke from the area surrounding the stair into the stair enclosure. A simple way this can be accomplished is through providing an outside balcony or a ventilated vestibule acting as a buffer zone between the spaces where smoke can dissipate and thus not migrate into the stair enclosure.
An outside balcony serves as a natural means to ventilate any incoming smoke, and a ventilated vestibule is provided a mechanical means of ventilation through an engineered exhaust system. Although this approach would not require a mechanical system, it does have architecturally and space implications which makes it not the most sought-after approach. Note, as seen in this approach, a smokeproof enclosure is not necessarily always a stairwell pressurization system.
Depending on the geometry of the building or architectural vision, a vestibule or exterior exit balcony might not be feasible. In this case, we would consider the stairway pressurization alternative, as allowed by the building code. However, stating that stairwell pressurization is not a code requirement may be a stretch, as it is a popular “alternative” to providing the required smokeproof enclosures.
A stairwell pressurization system is considered a smoke control system and is subject to the many requirements of Chapter 909 in the building code. Taking advantage of the stairwell pressurization alternative will require that a rational analysis be prepared by a qualified design professional. You can read more about smoke control rational analysis in our blog.
Understanding when stairwell pressurization is required for a building is important because it means you may be subject to various rules and regulations.
If you aren’t sure about when to implement it, work with a fire protection engineer. They can work with modern computer modeling and simulation software to determine if stairwell pressurization is necessary and help with the next steps.
If you want to learn more about stairwell pressurization systems and system design, please feel free to reach out to Performance Based Fire at info@pbfpe.com or check out our past work.
The Society of Fire Protection Engineers (SFPE) also offers a course on this topic!
