Key Takeway: Stair pressurization is a method for providing a smokeproof enclosure. When used, it is considered a smoke control system and must be accompanied by a rational analysis.
If you have ever worked on the design of a high-rise building, you almost certainly have come across a stairway pressurization system. Fans located on the roof, a shaft next to the stair, extra ductwork – all are common in high-rise design. But even though nearly all high-rise buildings have these systems, there are many wide-spread misconceptions and misunderstanding about stair pressurization.
For example, did you know that stairway pressurization is not actually required by the IBC?
Or did you know that there is a detailed analysis that must be conducted as part of the stair pressurization design process?
In this article, we will dispel some of these common misconceptions and misunderstandings about stair pressurization systems.
What is Stair Pressurization
Stair pressurization is the concept of pressurizing an enclosed stairway to have a higher pressure relative to the rest of the building, reducing the chance that smoke and other fire products can enter the stairway.
When is Stair Pressurization Required
As mentioned previously, stair pressurization is not required by the IBC. Rather, the IBC requires smokeproof enclosures in the following situations:
Specifically, smokeproof enclosures are for interior exit stairways serving floors more than 75 feet above the lowest level of fire department vehicle access in high-rise buildings and for interior exit stairways serving floors more than 30 feet below the level of exit discharge in underground buildings.
So what is a smokeproof enclosure?
It is an exit stairway or ramp designed to limit the movement of smoke or fire products into the enclosure.
This sounds very similar to the definition of stair pressurization, but here is the difference: the IBC offers several options to design a smokeproof enclosure. The base option is actually a ventilated vestibule at the stair openings or locating the stair next to an open exterior balcony. Pressurizing the stairway is considered an alternate approach for achieving a smokeproof enclosure as described in IBC 909.20.5.
Why is Stair Pressurization Provided if it is not Required
Simply stated – building area is valuable. Particularly in high-rise buildings, owners want to maximize the value of their floor space. Providing stair pressurization allows the building designers to eliminate the vestibule at the stairway entrances, thus saving valuable floor space on every floor.
Stair Pressurization Design Requirements
The basic design criteria for a stairway pressurization system is to provide a minimum of 0.10 inches of water pressure differential and a maximum of 0.35 inches of water pressure differential in the stairway, relative to the rest of the building.
The minimum pressure differential is the code-required threshold to limit smoke movement into the stair enclosure. The maximum pressure differential is the threshold to still allow the stairway door to open.
The IBC has a variety of other requirements for stair pressurization which can be found in Section 909, but we will focus on two here: the rational analysis report and special inspections.
What is a Rational Analysis
The IBC requires a rational analysis to support the selection, design, construction and operation of any smoke control systems. Starting with the 2015 edition of the IBC, stair pressurization systems are specifically considered “smoke control systems,” which means this analysis is required for all stair pressurizations systems (IBC 909.6.3).
The rational analysis is required to consider the following items, at a minimum (IBC 909.4):
The rational analysis is required to be submitted with the construction documents as part of the building permit submission.
Who Prepares the Rational Analysis
In my experience, the rational analysis is conducted by either a fire protection engineer or a mechanical engineer. In the past, it has been common for mechanical engineers to utilize basic rules of thumb for stair pressurization systems. Examples include providing 300-500 CFM of fan capacity per stair door and one duct inlet every 4-5 floors. Some mechanical engineers will also utilize hand calculations to determine the required fan capacities.
Unfortunately, these rules of thumb and basic calculations generally do not satisfy the requirements of the rational analysis report. For example, applying a set volumetric flow rate per stairway door does not account for stack effect or wind effect, both of which are clearly required to be considered by IBC 909.4. Similarly, I am not aware of any hand calculations for stair pressurization that can accurately account for wind effects.
To accurately account for all required considerations, some type of modelling analysis is necessary. Most commonly, CONTAM, a free program developed by the National Institute of Standards and Technology (NIST) is used. A Computational Fluid Dynamics (CFD) model could also be an option.
These models are typically prepared by fire protection engineers, though a mechanical engineer familiar with their use could also conduct the analysis.
If you are looking for a rough estimate of the required fan capacity for your stair pressurization system, check out our estimator. Note that this is for initial estimating purposes only...this estimator does not account for all factors required in the rational analysis report!
Stair pressurization systems require special inspections per IBC 909.3 and 1704. The purpose of the special inspection is to verify the proper commissioning of the stair pressurization system in its final installed condition. Frequently, the engineer who prepared the rational analysis report is also involved in the special inspections. Many jurisdictions have specific requirements for special inspections and will often send a designated inspector to conduct or witness the testing.
Stair pressurization systems are a common choice for providing the required smokeproof enclosures in high-rise and underground buildings. Stair pressurization is considered a smoke control system and must comply with the requirements of IBC 909, including the provision of a rational analysis and special inspections.
If you are working on a project utilizing stair pressurization and need assistance, including with rational analysis, our friends at Campbell Code Consulting can help. They have extensive experience with stair pressurization systems, including conducting CONTAM modelling to optimize the design of the systems. Reach out at the links below!