The Impact of Air Movement on High-Rise Commercial Fires

BY CURTIS MASSEY

Every high-rise building breathes. Air moves in and out and up and down. It is very important fire personnel responding to events in high-rises understand where the air is going. This will give the incident commander (IC) a better idea of where the smoke and toxic gases also may be going. If you understand how a building behaves and breathes, you will have a firmer grasp on where the problem areas are going to be and where the danger zones are within the confines of the building. You can literally predict where the smoke is most likely to go and, in turn, where you should dedicate your resources to address those problem areas, sometimes even before they begin to expose themselves. You can minimize or even avoid potential casualty counts just by knowing how the building will be acting during the fire. Staying ahead of the game and not allowing yourself to have to play catch-up are vital to the successful outcome of a serious fire. Understand your theater of operations.

Air balancing is directly associated with the concept of stack effect. With stack effect, the temperature and pressure differentials between outside air and inside air dictate where the air currents will flow (horizontally and vertically) and where smoke is likely to follow. We know in cold weather that air is rushing up into the tower from the bottom of the building and out onto upper floors, whereas in warm weather the reverse is true: Air is rushing down through the tower and out of the bottom. In cold weather, fires in high-rises act completely differently than they do in warm weather. What causes this flow of air to occur within the building relative to temperature and pressure? The answer is the “air balance.” Let us more closely examine this unique phenomenon and the reasons fire departments must understand it.

AIR BALANCE AND THE NEUTRAL PRESSURE PLANE (NPP)

Air balancing is easier to explain in relation to high-rise commercial buildings that are modern and use a “sealed envelope” approach to energy conservation. Hence, I will break down that occupancy type. With newer buildings, windows do not open, and the curtain wall wraps the building up in a protective envelope, which allows for more cost-effective heating and cooling. However, the windows and wall systems are rarely tightly sealed. Because of wind pressure or thermal air differences, the air squeezes in around tiny cracks that exist on each floor’s exterior that are under slight negative pressure (the bottom half of the building in the winter/the top half in the summer). The air, then, is drawn into the core penetrations, because of a greater negative pressure, and pulled up (or down-summer) to floors not under negative pressure and then pushed back out onto those floors by the now greater positive pressure that exists in the core after the current of air has passed the NPP, thus pressurizing those upper/lower floors to a positive nature. The combined wind pressure and thermal differential may shift the NPP in the building.

In the negative-pressure range of the building, the air is “pulled” into the building; in the positive-pressure range, the air discharges or “leaks” to the outside. When significant wind pressure is involved, it superimposes on the thermal balance of the NPP and “shifts” it (depending on leakage rates).

The NPP shift also affects the building’s pressurization by mechanical HVAC (heating, ventilating and air-conditioning) systems-more outside air is brought into the building than is exhausted to the outside. Most buildings are slightly pressurized (“bias”) by the HVAC systems to reduce cold air infiltration, particularly in wintertime, which would affect the comfort conditions of occupants at the building perimeter zones. The building pressurization by HVAC systems is usually more significant than the thermal stack effect in moving the NPP. It is intentionally high to prevent the inrush of cold air into the ground-floor entrance through lobbies and into elevator shafts (“whistling” doors) and stairwells. In highly pressurized buildings, the surplus of outside air assists in pushing the thermal stack effect air back to the building’s lower levels, and thus the NPP has mechanically been shifted downward in the building.

From the aforementioned, it should be clear that the air balance and NPP in a building shift up or down from the theoretical NPP midpoint in the building when air is moving horizontally as a result of wind pressure and through leakage on nearly every floor. It also occurs vertically as a result of stack effect and mechanical air pressurization. The NPP is typically found for about one to three floors, depending on the height of the building. There is virtually little to no air exchange in and out of vertical shafts in that area; therefore, it is deemed to be neutral. It is under neither positive nor negative influences.

On floors above and below this zone, air is rushing in and out of the elevator shafts and stairwells because of temperature and pressure imbalances. The farther floors are from the NPP, the greater the differential air pressure is relative to the outside and, thus, the velocity of air movement onto and off of building floors. In cold weather (Figure 1), the air is being drawn into the building through unsealed cracks at windows and spandrel panels on each floor as well as through lobby entrances with revolving and swinging doors (and even loading docks). This is why when you enter a high-rise building through a lobby swinging door, you instantly feel a rush of air going past you into the building as the door is pulled open-this is “makeup” air as a result of differential thermal pressures entering the “chimney” (the building).

Figure 1. Cold Weather Fire
Note the intake
Note the intake “makeup” air being drawn into the core shafts, rising above the neutral pressure plane (NPP), and pushing back out onto the upper floors. Air velocity into and out of core penetrations intensifies as the distance from the NPP increases.

In cold weather, air is sucked into the core because of the negative pressure that exists on all floors below the NPP. On floors above that zone, however, it is pushed out of the core because of a positive-pressure influence. The infiltration, exfiltration, and air movement are relatively low just above and below the NPP and are much greater at the top and bottom of the tower. The air travels up above the NPP and is pushed out of upper-floor curtain wall openings and roof access doors, elevator machinery penthouses, and so on. The greater the differential pressure (outside vs. inside) caused by the three influences (thermal/stack, wind pressure, and building pressurization), the higher will be the rate of air movement, leakage, and effect on the vertical location of the NPP within a building. This also explains why super tall high-rise buildings experience such severe stack effect problems while a typical 10-story building does not: The taller the tower, the greater the air intake and discharge caused by the significant height involved and floors distant from the NPP. Positive-stack effect will be present in cold weather; negative-stack effect in warm weather. The higher the building, the more pronounced the effects.

The temperature difference between inside and outside and the height of a building play a major role-for example, if it is 34°F outside on a cold winter day and it is 74°F inside a high-rise building, there is a 40°F temperature differential. It can be expected that there will be a significant positive vertical stack effect taking place within the core penetrations with air channeling up and onto floors above the NPP while negative-pressure influences will be in the bottom half of the tower, allowing the draw of “makeup” air into the building to replace the warm air that is rising.

In super tall high-rises during cold weather, the velocity of air channeling into the core from lower floors and lobby doors can be high enough to prevent elevator doors in the lobby from opening or closing (photo 1) because of the sideward pressure on the elevator doors, tracks, or sliders as air is trying to rush into and up the elevator shafts. On lower floors, opened stairwell doors can be prevented from self-closing because of the pressure differential and air flow from the floor into the shaft (photo 2). This should be a very real concern if the fire is on a lower floor (below the NPP) in cold weather. When evacuating these floors, the doors may not self-close behind the fleeing tenants. This presents a serious problem for the IC from two perspectives:

  1. The air (and smoke) being sucked into the stairwells is, of course, going to be drawn up to the NPP (located roughly midway up the building) and discharged beyond the NPP onto occupied floors many levels above the actual fire area, thereby increasing the life threat to occupants who may be in a “defend-in-place” posture as directed by fire command.
  2. If one or both stair exit doors (in a two-stair core configuration) are not closing behind fleeing tenants (no stairwell pressurization), this ensures that the fire will also be drawn toward these openings, as the negative partial pressure of the shaftway will pull the fire and smoke toward it. Fire and smoke take the path of least resistance. Compromising one or both stairs on the fire floor where this situation is present would essentially turn both stair shafts into smoke towers and greatly endanger the people on upper floors staying put or descending the stairs. Even though the attack has to be mounted through one of the stair openings, fire personnel must verify that the other stair door is closed unless search teams are passing through it.
Elevator doors unable to close as a result of the winter stack effect-air rushing into the shaft from the lobby entrances. The doors will have to be forcefully closed by firefighters to begin Phase 2 operation. Lobby doorways must be closely controlled and supervised.
(1) Elevator doors unable to close as a result of the winter stack effect-air rushing into the shaft from the lobby entrances. The doors will have to be forcefully closed by firefighters to begin Phase 2 operation. Lobby doorways must be closely controlled and supervised. (Photos by author.)
This stair door is unable to self-close because of severe winter stack on Floor 13 of a 100-story building. These open doors well below the NPP can pull fire/smoke toward occupied exit stairs, endangering evacuees on the floors above. Crews must check for these open doors to keep the stairway secure; stair pressurization is critical.
(2) This stair door is unable to self-close because of severe winter stack on Floor 13 of a 100-story building. These open doors well below the NPP can pull fire/smoke toward occupied exit stairs, endangering evacuees on the floors above. Crews must check for these open doors to keep the stairway secure; stair pressurization is critical.

Thinking through this equation with tall buildings, stair doors on lower floors would easily be pulled into the open position and may not close without human assistance while doors at the very upper floors will be fairly hard to open for tenants choosing to leave their floors since the strong air currents caused by positive pressure will be pushing against the doors at these levels unless stairwell pressure relief dampers are present and functioning. Fortunately, stair pressurization in modern buildings will minimize this effect once it is activated by the automated alarm system (or manually at the lobby fire command center or created by fire department fans).

THERMAL STACK EFFECT

One of history’s most prominent examples of cold/winter stack effect was the 1993 bombing of the World Trade Center in New York City. It occurred in February; the outside air temperature was 37°F, and there were light snow flurries. The temperature inside the towers was approximately 75°F. Four-and-a-half minutes after detonation on the Basement 2 level in the parking garage, there was a heavy smoke condition on the 110th floor of Tower 1 because the bomb blast exposed the base of the elevator and stair shafts of the nearby tower.

How did the smoke travel so far vertically so fast, approximately 1,400 feet, in less than five minutes? Stack effect. First responders could not control it that day because of the effects of the vehicle bomb, but it can be controlled to a great degree in typical high-rise fires by minimizing the time lobby entrance doors (and external exit stair discharge doors if present) are opened. Keep the “dampers” (street-level doors) to the “flues” (core shafts) within the “chimney” (the building) closed as much as absolutely possible (Figure 2).

Figure 2. Stack Effect
The building is a chimney, the core shaftways are flues, and the lobby entrances are dampers. The dampers must be tightly controlled during high-rise fires to avoid pushing/pulling smoke throughout the tower. The taller the building, the more pronounced are the stack effect and air exchange.
The building is a chimney, the core shaftways are flues, and the lobby entrances are dampers. The dampers must be tightly controlled during high-rise fires to avoid pushing/pulling smoke throughout the tower. The taller the building, the more pronounced are the stack effect and air exchange.

At this event, misguided media advised people to break windows on upper floors for fresh air (when, in fact, all but the top-most floors were only slightly affected by smoke that truly impeded breathing). This just further exacerbated the situation, as it provided a more efficient discharge point for the stack effect on floors well above the NPP, where the outgoing pressure from core poke-throughs is greatest. It also did not help the cause that the elevators that served the lobby were being automatically recalled to lobby level and self-opening their doors, creating a more efficient intake point for the “makeup” air being drawn into these same shafts at the base of the “chimney”-110 stories tall.

How many fireground commanders would be contemplating this threat from stack effect as part of their high-rise standard operating procedures (SOP) checklist? Why is it not in every high-rise SOP that efforts need to be made to attempt to control the stack effect in these types of fires?

How does the air get into and out of the shaftways within the core of a building? The same way it squeezes through unsealed or improperly sealed details of the curtain wall (and lobby entrance doors). Air moves around stairwell doors and elevator hoistway landing doors at a higher rate than you might imagine. The better the elevator and stairwell door seals, the less potential for vertical air movement as a result of stack effect. Even poor fire stopping on core poke-throughs (cable risers, for example) can feed the vertical air flow going up or down within the core area.

The winter air drawn into the building through curtain wall leakage below the NPP into the ceiling plenum area near the curtain wall is another concern relative to stack effect in buildings in cold climates. In countless cases, this has resulted in the freezing and bursting of fire protection branch lines and piping systems supplying the heating coils of perimeter fan-powered variable air volume boxes or fan coils; the lines do not contain antifreeze additives. As noted, at the WTC complex during the 1993 bombing, numerous windows were broken during the fire. Despite sprinkler piping being insulated within 10 feet of the perimeter, the freezing/bursting of these lines was a very valid concern in this midwinter incident after the utility company shut down the steam service to perform necessary testing, which took a week. With the heat now down amid freezing temperatures and despite the loss of 300 windows, not even one sprinkler line froze. The management of the complex accomplished this by turning on all the fluorescent lights on all floors once power was restored in the evening on the day of the bombing. The lights created enough ambient heat to prevent a crisis within a crisis. Sometimes ingenuity proves its value in aces.

In warm weather, outside air is drawn into the air-conditioned building on the upper floors (caused by negative-pressure influences by the descending of dense cool air), where it is quickly cooled by the HVAC system, pulled downward, and discharged onto lower floors with the greatest “release” occurring on the lowermost floors and at the lobby level (Figure 3). When you enter a lobby through a swinging door in the summer (or a warm winter day), you can feel cool air rush past you LEAVING the building-the opposite of what happens during cold weather, when, at these entrances, the air rushes past you and ENTERS the building. The natural flow of air is important to note because it dictates where smoke will travel. Also, as previously noted, as air enters these shafts, the stair doors on the lower floors may not close behind fleeing tenants. Unbeknownst to the command post, the opposite can be true at the lobby level during warm weather fires: Lobby doors may not be closing behind people leaving the building or firefighters entering it (photo 3). Allowing this open channel for air to escape the tower will draw smoke downward and further enhance the stack effect as this opening becomes a notable “release point” for the negative draft. It is important that responders be aware of this.

Figure 3. Hot Weather Fire
Note the air being drawn into the core shafts and discharged below the NPP out onto the lower floors. Testing for carbon monoxide on the staging floor below NPP should be a standard operating procedure.
Note the air being drawn into the core shafts and discharged below the NPP out onto the lower floors. Testing for carbon monoxide on the staging floor below NPP should be a standard operating procedure.
(3) Hot weather fire. The lobby entrance door is being held open by the outflow of cool air on a warm day. Failure to control these doors can cause the loss of the staging floor below the NPP and possibly also the lobby command post.
(3) Hot weather fire. The lobby entrance door is being held open by the outflow of cool air on a warm day. Failure to control these doors can cause the loss of the staging floor below the NPP and possibly also the lobby command post.

TRACKING OF CARBON MONOXIDE (CO)

The tracking of CO must be a part of the overall scope of the incident command process at high-rise fires. Tracking CO above the fire and noting floor intervals where it may be collecting are vital to understanding where areas of great danger might exist for fleeing tenants or tenants performing a “defend-in-place” procedure dictated by the incident command post when it is deemed unnecessary for upper floors to evacuate. Firefighters and command officers tend to focus mostly on what is happening at and above the fire floor. However, we must monitor what is taking place below the fire floor as well.

FIRE FLOORS AND THE NPP

With an understanding of the air balance, NPP, and air movement above the fire floor, we should ask ourselves the question, What happens in high-rise buildings below the fire floor in hot weather? We know that in cold weather, a positive stack effect is taking place, where air is rushing in at the bottom of the tower, up past the NPP, and back out on floors above. But, what is taking place when a fire strikes in summertime? If air is rushing into the core from the upper floors, traveling down past the NPP, and pushing out the base because of negative-temperature/pressure differentials between inside/outside air, what happens in fires where the air outside is much warmer than the air inside?

Example: It is 95°F outside in July; inside, it is an average of 75°F on tenant floors. There is a 20°F temperature imbalance in play, and cooler air, being more dense, descends within the tower. Let’s say the fire is on floor 15 of a 40-story building, below the NPP. If the smoke becomes significant and enters the core area, something important will occur. Surely, some of the smoke will rise because of the temperature of the heated gases from the fire. However, the air being channeled down from the upper floors with the negative stack effect will also pull some of the smoke (and toxic gases) down BELOW the fire floor, sometimes many floors below the fire.

Why would this be an important concern? Where is the staging floor? Typically, two floors below the fire. You can see where this area can easily be compromised, especially since it is below the NPP, where air is being forced back out of internal shaftways and poke-throughs. This air pushing out onto lower floors can channel a fair amount of smoke onto the staging floor, contaminating it to the point of being untenable (especially if the HVAC system is turned off or not in “pressurization” mode for the floors above and below the fire). Fire crews would then be forced to drop down and restage several floors below, in turn moving equipment and relief crews farther from where they are needed. This includes rapid intervention teams standing by “off air.” Losing your staging floor is not a recipe for success; that is why it may be wise to also take CO readings BELOW the fire floor in warm weather fires, especially on the staging floor, to ensure high-level readings of CO do not exist where a great deal of smoke may not be present. This very real threat-is the staging floor safe to occupy?-is easily overlooked in fires during warm weather. What about other tenant floors in this area where people may be maintaining a “defend-in-place” posture because of the fire above them? This underscores the importance of tracking CO movement at various intervals above and below the fire area, in warm weather and cold weather alike. In several high-rise fires in the United States, the lobby command post was lost because of the reverse stack effect’s pulling smoke down well below the fire floor and discharging it onto the lobby level, which was likely caused by open elevator cabs standing by in Phase 1 recall mode and probably open stairwell doors that discharged into the lobby.

With a firm grasp of how air moves within a high-rise building in winter relative to temperature/pressure differentials, if you could choose where your fire would be the day you have to fight it, what would be the best option? If it is at the base of the tower, smoke will get sucked into the core shafts and channeled up and out onto upper floors, so the best place to have the fire would be on an upper floor-not just because the smoke does not have far to go as it rises but also because the air is pushing OUT of the upper-floor penetrations. This air movement will assist in keeping much of the smoke out of critical core chases since, in effect, it is naturally “pressurizing” the upper floors and assisting in the containment of smoke spread.

On a hot summer day, the descending cool interior air will be rushing out of the vertical shafts below the NPP and onto the lower-level floors. This partial positive pressurization can be a major asset to smoke travel and containment although it does provide one slight drawback: Air flowing onto the fire floor will be at a greater rate and will feed the fire to some degree once the attack stair door is propped open (the same is true on upper floors in a cold-weather fire). However, the benefits of smoke containment outweigh this factor. There may be some smoke migrating possibly because of greater positive pressure from the fire into the attack stair if no stair-pressurization capability is present and the fire is close to the core. This smoke will likely be drawn downward to floors below (and sometimes well below) the fire in some proportion in these warm-weather events.

PRESSURIZATION SYSTEMS

Newer, modern commercial office buildings typically possess stair (and sometimes elevator) pressurization systems, as well as at least floor pressurization capability during fires and possibly full smoke removal capability, too, for the fire floor. Such systems provide a new air balance and a shift in the NPP of the building, and they assist tremendously in containing vertical smoke movement in the building during a working fire.

Some building codes in milder climates require high-rise buildings to have open vented elevator shafts for smoke purging. Elevators serving the alarm floor would normally be recalled to the ground floor and remain out of operation (for civilian use) during a fire. In some high-rise buildings, (heated) elevator pressurization systems have been installed (i.e., the 52-story Manulife Centre in Toronto) to lower the NPP and reduce the significant stack effect, the air pressure on elevator doors (which can prevent doors from opening because of sideway pressure on doors and tracks), and vertical air movement from the lobby into the elevator shafts. It also curbs smoke going into the shaft and the machinery room above during a fire.

WAYS TO CONTROL STACK EFFECT

You can control stack effect by doing the following:

  • Leave the building HVAC system on (unless it is clearly proving detrimental to the cause), and allow it to maintain positive pressure to floors above and below the fire.
  • Provide supervision for the lobby entrance doors. Minimize open times, and verify throughout a warm-weather fire that they are closing after each use.
  • Keep ground-level stair doors (interior or exterior discharge) closed when not being used by tenants or fire crews.
  • Mechanically pressurize stairwells with fire department fans if a pressurization system is not present and operating (no gas-powered fans in the lobby, though, unless exhaust is vented to the exterior).
  • Send elevator cars (except for one or two cabs) to the second floor (if no atrium is present) and place in hold, monitored/controlled by a firefighter. Avoid opening lobby hoistway doors, which will draw air into these shafts and enhance the existing stack effect and smoke movement.
  • Ensure stairwell doors on the floors in the negative-pressure zone are closing behind fleeing tenants and are not oscillating in the open position.

QUICK TIPS

  • Cold weather: Expect rapid smoke spread to the upper floors from fires on the lower floors.
  • Hot weather: Expect fires on the lower floors to contaminate several floors below the fire. Expect possible loss of the staging floor and possibly even the lobby command post. Most importantly, test early and often for CO at designated floor intervals. Track it as you would your personnel.

TESTING FOR HYDROGEN CYANIDE (HCN)

One other danger commonly overlooked is the threat of hydrogen cyanide (HCN) from the burning synthetic (foam, rubber) and natural (paper, wool) products present during the combustion process. This gas is even deadlier than CO. Since many hospitals may not have the capability of testing for HCN poisoning after a fatal fire, victims may often be pronounced dead from CO exposure when it could very well have been more directly related to the toxic gas HCN. Newer gas sensors can test for and monitor the presence of HCN in addition to CO. Strongly consider having rapid ascent teams and other crews take sample readings to check for high levels of both of these deadly gases throughout the firefighting effort.

•••

It is a widely acceptable standard practice that when a fire occurs in a high-rise building that the fire department ensure the evacuation of two floors above the fire floor, the fire floor, and two floors below the fire floor. The remainder of the occupants should remain in place. This is also commonly taught in fire warden training, as dictated by fire prevention bureau guidelines. Having a complete grasp on how far and how fast smoke (and deadly CO/HCN) can travel remote from the immediate fire area because of air imbalance and stack effect, it can be argued that these parameters must be reassessed very early in a working incident.

Empowered with the knowledge of how air travels within the confines of a multistory building, you can now predict where the greatest threat from smoke spread will occur. If you can predict that, you can also predict and plan for where the greatest threat exists relative to occupant survival and where you must concentrate your rescue teams. CO monitoring, again, cannot be emphasized strongly enough. The next time you look at a 30-story building, look at it as a living/breathing 30-story chimney. Air balancing should be included in the training regimen of every department that responds to fires in high-rises.

Author’s note: Thanks to Jack Smits, managing director of engineering and technical services, Manulife Financial, Real Estate Division, and Alan Reiss, former director of the New York World Trade Center and current construction manager of the new Freedom Tower/One World Trade Center rebuilding project.

CURTIS MASSEY is a former ladder company officer and manages Massey Enterprises Inc., which preplans buildings for fire department operations in the United States and Canada. He has been a high-rise instructor and a lecturer and writer for many years. His concept of rapid ascent teams has been adopted and successfully used by departments across the country.

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