One of the primary objectives of the model building codes is to minimize the impact of fire, including reducing to the greatest extent possible the spread of fire and smoke floor-to-floor. This objective is achieved using a combination of building fire safety features, including passive fire/smoke separations and pressurization smoke control systems. Pressurization systems may include stair pressurization, elevator pressurization, or zoned smoke control.
The design requirements for pressurization smoke control systems included in the International Building Code (IBC) and NFPA 92, Standard for Smoke Control Systems, specify a minimum and maximum pressure differential across smoke boundaries. The minimum pressure differential ranges are from 0.05 in H2O (sprinklered building) to 0.1 in H2O (non-sprinklered building or IBC stair pressurization), and are meant to counteract the anticipated buoyancy force resulting from a compartment fire adjacent to the stair, incorporating appropriate safety factors. The maximum pressure differential specified is 0.35 in H2O, and is derived from the maximum allowable opening force for doors entering the stairs, which is typically specified to be 30 lbf; however, it can be less in some jurisdictions depending upon the interpretation of ADA door opening force requirements during operation. The door-opening force must be low enough to allow the majority of building occupants to be capable of opening the door to the stairwell in an emergency event. Excessive force against the door could prevent occupants from entering the stairwell, which could be a dangerous condition even in the event of a small fire in a high-rise building.