High-Rise Fire Behavior and Smoke Control Flashcards

The 'stack effect' in a high-rise building refers to:

Answer: The natural upward movement of warm air and smoke through vertical shafts caused by temperature and pressure differences

Stack effect is the movement of air (and smoke) upward through vertical shafts (elevator shafts, stairwells, utility chases) driven by the temperature difference between building interior and exterior. In winter, strong upward stack effect can rapidly carry smoke from lower floors to upper floors.

During a high-rise fire, an HVAC system that is NOT placed in smoke control mode can:

Answer: Distribute smoke from the fire floor to other floors through the ductwork

Standard HVAC systems recirculate and distribute air across multiple floors. Without being placed in smoke control mode (which typically seals dampers and shifts to exhaust), the system can draw smoke from the fire floor into return ducts and redistribute it to unaffected floors throughout the building.

In a high-rise building fire, why is it generally unsafe to evacuate ALL occupants to the street level simultaneously?

Answer: Stairwells can become dangerously congested, slowing firefighter access and creating panic-related injuries

High-rise fire strategy typically involves selective (phased) evacuation — the fire floor, the floor above, and often the floor below — rather than total building evacuation. Simultaneously evacuating a 40-story building creates dangerous stairwell congestion, impedes firefighter ascent, and can cause injuries.

Smoke entering a pressurized stairwell from the fire floor is MOST likely caused by:

Answer: A stairwell door being held open, reducing pressure differential and allowing smoke infiltration

Pressurized stairwells maintain positive air pressure to prevent smoke entry — but only when doors are closed. A propped-open or wedged stairwell door destroys the pressure differential, allowing smoke to enter freely. FLSDs must enforce door discipline during evacuations.

The 'defend in place' strategy for occupants in fire-resistive high-rise buildings is MOST appropriate when:

Answer: Occupants are on floors remote from the fire and evacuation would increase their exposure to smoke in stairwells

In fire-resistive high-rise construction, the building's compartmentalization often makes it safer for occupants on remote floors to stay in place rather than enter potentially smoke-contaminated stairwells. FDNY directs when and where to evacuate; occupants should not self-initiate descent unless instructed.

During a high-rise fire, the FLSD notices that stairwell doors on the upper floors are difficult to open against their frame. This is MOST likely caused by:

Answer: Reverse stack effect creating negative pressure in upper-floor stairwells that pulls doors shut

Reverse stack effect (which can occur in summer or in mechanically pressurized buildings) creates negative pressure at upper floors relative to the building interior, making stairwell doors difficult to pull open. Understanding this phenomenon helps the FLSD anticipate evacuation challenges and communicate them to FDNY.