When Is Stairwell Pressurization Required?

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When Is Stairwell Pressurization Required?

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 a stairwell pressurization required to be provided? 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 to the ‘rules’ of the code; in fact, stairwell pressurization is itself an alternative!

Definitions Applicable to Stairwell Pressurization

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.”

So…When do I Need Stairwell Pressurization on my Projects?

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.

What is the Purpose of a Smoke Proof Enclosure?

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 though 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 in accordance with the building code. 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.

The Smoke Control Rational Analysis

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.

What is a rational analysis? Be on the lookout for our future blog: Key Components That Must be Included in a Rational Analysis.

Sample Smoke Control & Stairwell Pressurization Projects

Check out our past work with smoke control and stairwell pressurization systems here

If you want to learn more about stairwell pressurization systems and system design, please feel free to reach out to us at info@pbfpe.com, or directly to any of our fire protection engineers; the Society of Fire Protection Engineers (SFPE) offers a course just on this topic!  Check it out at this link!

Reference code sections based on the 2018 International Building Code:

IBC 403.5.4 Smokeproof Enclosures.

IBC 405.7.2 Smokeproof enclosure.

IBC 909.20 Smokeproof enclosures.