Unless someone is looking for a workout, stairwells in high-rise buildings are one of those required spaces for egress that don’t get a lot of regular use. Once a building is considered a high-rise, according to the building code the stairwells become known as, “smokeproof enclosures.” The most common way for designers to comply with these requirements is a stairwell pressurization system. The premise of stairwell pressurization is pretty simple: push a bunch of outdoor air in with a fan to keep the smoke out. This allows for building occupants to evacuate the building safely.
From an Architectural standpoint, there are some key questions for the engineer:
Question 1: Is a shaft required?
It depends on how tall the building is. It is typical for a stairwell pressurization system to require a shaft alongside the stairwell once the building is taller than 10 stories. In buildings less than 10 stories one injection point anywhere in the stairwell should be enough.
Question 2: Does the shaft have to be rated?
No, but the stairwell does--and the duct needs to be in the stair. Theoretically, the duct could be exposed running down the stairwell. Frequently the rating of the stairwell wraps around the stairwell and an adjacent shaft, while the wall that divides the two is not rated.
Question 3: Does the shaft need to go the full length of the stairwell?
No. Typically, as the duct runs down the stairwell there is a grill every few floors. If the fan and duct started at the top of the stair, the last grill could be placed as high as the 5th floor; therefore, the shaft could end there and not go down any further. Vice-versa if the fan and duct starts down near the bottom.
Question 4: What if the fan and duct must run through the building to get to the stairwell?
Any ductwork or equipment not in the stairwell needs to be in a rated enclosure.
Question 5: How can I make that shaft smaller?
The engineer should be able to push the air faster/harder to shrink the duct somewhat. To shrink it further, it is possible to have two fans (one fan at the top of the stairwell and also near the bottom); this can cut the shaft size in half. Regardless, the minimum single dimension should be about twenty inches; any smaller than that and it will flatten out the duct too much.

If you have any questions about this - or anything MEP related please email me at tlong@engr3.com

ABOUT TREY LONG