FLSD High-Rise Fire Behavior and Smoke Control — Questions and Answers
Question 1: The 'stack effect' in a high-rise building refers to:
- The stacking of fire suppression equipment in mechanical floors
- The natural upward movement of warm air and smoke through vertical shafts caused by temperature and pressure differences (Correct answer)
- The accumulation of heat on upper floors when the HVAC system is shut down
- The layering of firefighting personnel on multiple floors during a simultaneous multi-floor response
Correct 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.
Question 2: During a high-rise fire, an HVAC system that is NOT placed in smoke control mode can:
- Automatically suppress the fire through its air filtration system
- Distribute smoke from the fire floor to other floors through the ductwork (Correct answer)
- Reduce the fire's oxygen supply by recirculating stale air
- Provide a clean air supply to the fire floor, aiding firefighter visibility
Correct 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.
Question 3: In a high-rise building fire, why is it generally unsafe to evacuate ALL occupants to the street level simultaneously?
- Simultaneous full evacuation is always the preferred strategy in high-rise fires
- Stairwells can become dangerously congested, slowing firefighter access and creating panic-related injuries (Correct answer)
- Occupants on upper floors should shelter-in-place permanently during any fire event
- Street-level evacuation is prohibited by FDNY unless the entire building is fully involved
Correct 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.
Question 4: Smoke entering a pressurized stairwell from the fire floor is MOST likely caused by:
- The stairwell pressurization system operating correctly at design specification
- A stairwell door being held open, reducing pressure differential and allowing smoke infiltration (Correct answer)
- Firefighters opening hose valves on the standpipe, reducing air pressure
- The building's generator activating and reversing airflow through the stairwell shaft
Correct 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.
Question 5: The 'defend in place' strategy for occupants in fire-resistive high-rise buildings is MOST appropriate when:
- The fire is confirmed to be on the same floor as the occupants being considered
- Occupants are on floors remote from the fire and evacuation would increase their exposure to smoke in stairwells (Correct answer)
- Occupants have been unable to locate their floor warden
- The FDNY has not yet arrived on scene and the fire's location is unknown
Correct 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.
Question 6: 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:
- Thermal expansion of the door frame due to heat from the fire below
- Reverse stack effect creating negative pressure in upper-floor stairwells that pulls doors shut (Correct answer)
- The building's automatic locking system engaging during the alarm
- Debris accumulation in the door tracks from years of deferred maintenance
Correct 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.
FLSD High-Rise Fire Behavior and Smoke Control
The 'stack effect' in a high-rise building refers to: