Building designers, first responders, and occupants must understand how smoke moves and spreads during a building fire to make informed, life-saving decisions.

According to ASHRAE Handbook of Smoke Control Engineering, six key factors drive smoke movement in buildings:

1. Expansion of Smoke: Smoke spreads rapidly as it heats and expands.
2. Buoyancy: Hot smoke rises while cool smoke stagnates, both have their own dangers.
3. HVAC Systems: If mismanaged, HVAC systems can spread smoke throughout the building.
4. Elevator Piston Effect: Elevators can push smoke through shafts, increasing risks.
5. Stack Effect: In tall buildings, temperature differences can pull smoke vertically.
6. Wind Forces: Exterior wind can drive smoke into different areas of a building.

These factors are crucial for designing smoke control systems and safe evacuation routes. Stay informed and prepared by understanding how smoke behaves in various environments.

How Smoke Expands During a Fire

When we observe large smoke clouds, they are primarily composed of air rather than combustion products. The actual volume of combustion gases released by a fire is relatively small compared to the amount of air drawn into the smoke cloud.

As the fire burns, hot combustion gases rise quickly in a plume, creating a vacuum that pulls in surrounding air. This process, known as entrainment, causes the smoke cloud to expand as more air is drawn in.

Furthermore, the combustion gases themselves expand due to the high temperatures of the fire. For instance, at a fire gas temperature of 2,200°F (1,260°C), the gases can expand to about 500% of their original volume. This expansion can force hot smoke out of the burning room while simultaneously drawing cooler air into the room.

How Buoyancy Impacts Smoke Movement During a Fire

Smoke produced by a fire is hotter and less dense than the surrounding air inside a building, causing it to rise with significant force. The strength of this upward buoyancy force varies depending on the situation. In one study, the force reached up to 0.064 inches of water gauge (16 Pa).

Hot Smoke vs Cool Smoke

As the smoke cloud travels, it mixes with and is cooled by the surrounding air. In some cases, the smoke can eventually cool to room temperature. Once the smoke cools, its movement becomes less predictable, as it no longer rises naturally.

Cool smoke behaves more like regular indoor air, which can pose challenges during evacuations. It may stagnate near the floor, greatly reducing visibility for occupants trying to escape.

How HVAC Systems Impacts Smoke Movement During a Fire

During a fire, HVAC systems can either slow down or accelerate smoke movement throughout a building via ductwork, air handling units, and return air plenums. One of the most dangerous forms of unwanted smoke transfer occurs when hot smoke enters vertical duct shafts, allowing it to travel significant distances to upper levels of the building with minimal cooling, putting occupants far from the fire at risk.

To prevent the spread of smoke through HVAC systems, building codes and sound engineering practices are designed to mitigate these risks. Fire and smoke dampers, smoke detection system shut-offs, and zoned smoke control systems are commonly implemented to help contain smoke movement.

How Elevator Piston Effect Impacts Smoke Movement During a Fire

Elevator hoistways can serve as a pathway for smoke movement, particularly due to the elevator piston, or “plunger,” effect. This occurs when a moving elevator creates pressure differences in the hoistway, displacing air and forcing smoke either upward or downward, potentially spreading it far from the fire.

For example, as the elevator ascends, it generates negative pressure below and positive pressure above, drawing smoke into the hoistway from lower levels and distributing it to other floors. The reverse happens when the elevator descends.

According to the ASHRAE Handbook of Smoke Control Engineering, one study found that the elevator piston effect created up to a 0.12 in.wg. (30 Pa) pressure difference in the hoistway.

The magnitude of the elevator piston effect is influenced by:

• Number of elevator cabs (single-cab hoistways are more affected than multi-cab hoistways)
• Speed of the elevator cabs (faster velocities create a stronger piston effect)
• Leakage area between the hoistway and the building
• Leakage area between the building and the outdoors

How Stack Effect Impacts Smoke Movement During a Fire

Stack effect and reverse stack effect occurs when air inside a building rises or falls through vertical shafts, such as ducts, elevator hoistways, and stairwells, due to temperature differences between the indoor and outdoor environments. This effect is most pronounced in high-rise buildings.

Stack effect occurs during colder weather, when the outside air is cooler than the inside air. This temperature difference leads to a difference in air density. The warmer, less dense indoor air rises through vertical openings and escapes through upper-level exterior wall openings, while cooler outdoor air enters through lower-level openings to balance the pressure.

Reverse stack effect, on the other hand, happens in warmer weather, when the outside air is warmer than the inside air. The temperature difference again causes a variation in air density. In this case, the cooler, denser indoor air falls through vertical openings and exits through lower-level exterior wall openings, while warmer outdoor air enters from the upper levels, maintaining pressure balance.

The pressure difference and airflow created between the upper and lower levels can influence smoke movement during a fire, depending on where the smoke is located within the building.

How Wind Impacts Smoke Movement During a Fire

Wind blowing on a building can significantly impact the movement of smoke during a fire. As wind presses against one side of the building, it creates pressure differentials on the other sides and roof, which can alter how smoke travels within the structure.

Because wind is unpredictable and constantly changes in direction and speed, engineers use average values when assessing wind effects. The impact of wind on smoke movement is unique to each building, influenced by several factors:

• Wind velocity: Higher wind speeds exert greater forces on the building, affecting smoke movement.
• Ground effect: Wind speeds are slower at ground level due to friction with the ground, while upper levels of a building experience stronger winds.
• Surrounding structures: Nearby buildings and trees can either increase or reduce local wind speeds around the fire, depending on their arrangement.

These factors must be considered when evaluating how wind affects smoke spread in a fire scenario.

Citations

1. National Fire Protection Association. (2008). NFPA Fire Protection Handbook (20th Edition).
2. Klote, J. H., Milke, J. A., Turnbull, P. G., Kashef, A., & Ferreira, M. J. (2012). Handbook of Smoke Control Engineering. ASHRAE.