Firehouse 21st Century High-Rise Training Series: Stack Effect

A fire department is dispatched to a reported structure fire in a 52-story high-rise office building. It is a weekday, 4:30 P.M. The building is fully occupied. Upon arrival, first-due units report heavy fire intermittently showing out three windows on the exposure 1 side (front/north) and...


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A fire department is dispatched to a reported structure fire in a 52-story high-rise office building. It is a weekday, 4:30 P.M. The building is fully occupied. Upon arrival, first-due units report heavy fire intermittently showing out three windows on the exposure 1 side (front/north) and immediately start pulling their equipment off the rigs to begin a difficult firefight. It is early January in this northern city and the temperature is 26 degrees, with a slight northerly breeze blowing toward the building frontage.

Crews trudge up to the front entrance and experience some difficulty in opening the swinging entrance doors. Believing that the problem lies with the breeze blowing against his back, the first-due engine captain asks two men on the second engine to prop open the swinging doors and collapse the revolving doors to expedite getting equipment and manpower into the building. This is done in very short order.

As the crews enter the building, they feel a strong wind rushing past them into the lobby, almost like being in a wind tunnel, but again, figure it is just the slight breeze gaining velocity as it squeezes through the doorways along with the firefighters. Tenants who have already reached the lobby are either milling around or are leaving the building. Crews check the fire panel for confirmation of the alarm floor as they listen to the security guards and chief engineer excitedly bark out what they have seen or received in information from tenants. They are told that an entire tenant space of about 2,000 square feet on the 38th floor is afire in the unsprinklered 1960s-era steel frame skyscraper, along with a report that several people are missing and presumed to be trapped. They are advised that the lone freight elevator is down for repairs. From their position, the crews no longer feel the wind entering the building.

After gaining access to floor plans, stair master keys and fireman service keys to the elevators, the engine, ladder and rescue units head over to the elevator banks to ascend to the staging floor, two floors below the fire floor. They step into two cabs as wind whistles around them in the shaftways. They insert their keys, put the cars into phase-two service and push the designated floor and door close buttons to begin their rise into the burning office tower. The doors on both cabs close about three-quarters of the way as the whistling intensifies, then stop and go no farther.

The crews try to push the doors the rest of the way closed, but to no avail -- they remain partially open. Frustrated, both crews exit their respective cars with their keys and attempt to commandeer cars across the elevator lobby in the same bank. They experience the same situation. Time passes and stress mounts with every second they remain in the lobby while the blaze races out of control above them with occupants trapped. The decision is made to then go for the stairs and initiate a long, grueling hike up to the fire, knowing that precious time will be wasted.

After 25 minutes of intense climbing, they reach the fire floor, sweating profusely in gear that does not let their bodies breathe. One exhausted crew begins stretching the first attack line from the standpipe on the 37th floor in the now-designated "attack stair" (the north stair, closest to the fire), while the search crew deploys out onto the fire floor from the opposite stair in a concerted effort to find the missing tenants. Other occupants continue to stream down from the upper floors, trying to stay out of the firefighters' way while still keeping their momentum going in an attempt to flee the light to moderate smoke now entering the attack stairwell as the door is propped open on the 38th floor for the deployment of a 2½-inch handline. The engine captain radios the command chief in the lobby that the north stair is becoming smoky.

The battalion chief at the fire command station realizes that the wind entering the windows blown out from the fire on the windward side of the building must be increasing the Venturi effect of the smoke working its way into the stairs. He pauses for a moment to try and decide if it might be best for his attack crew to hold off the assault on the fire until all tenants coming down from the upper floors have passed them, or try to ventilate the stair by opening the lobby stairwell door and the door leading to the roof at the top of the north stairs.

The building's pre-fire plan stair diagram has shown the north stair terminates at roof level. The chief decides to vent the stairwell and continue the fire attack. After the lobby stair door is propped open and two firefighters from the fire floor make it to the roof to open the stair door there, the rush of air up into the attack (north) stair intensifies. Meanwhile, the engine company on the fire floor is advancing the initial handline out of the attack stair on to the floor. A severe wind-tunnel effect then takes place and conditions rapidly become untenable in the attack stair above the fire floor as heavy black smoke now enters the stairwell and rises towards the tower's upper floors. The attack team is experiencing extreme difficulty making any headway against the fire roaring at them, even with the large handline flowing well over 200 gpm.

Within minutes, the dispatcher relays numerous reports from people calling on their cell phones that they are trapped in the north stair above the fire floor, cannot breathe in the super-intense smoke, and cannot exit the stairs or find a re-entry floor that is open. The chief glances down at the pre-plan and notes that all stairwell doors are mechanically locked, except for re-entry/crossover floors every five levels. Knowing that nearly all his initial resources are committed to the fire attack and search efforts on floor 38, a sense of apprehension grips the chief as he begins to ponder if he hasn't made a mistake in opening the stair doors. He wonders how he will address the mass-rescue effort on those upper floors as the second- and third-alarm crews are just now arriving at the building, passing through the opened lobby entrance doors after fighting their way through heavy rush-hour traffic and major congestion around the building site. They then have to face the daunting task of climbing 40 to 50 floors of stairs, maneuvering their way past panicked tenants to reach the trapped victims, hoping they will still be alive when they get there.

What happened at this fire to cause the problems presented to the first responders, seemingly confounding their every effort? Is it possible that the natural phenomena of "stack effect" could be involved?

What Is "Stack Effect"?

All buildings experience some degree of stack effect. Even one-story ranch homes use soffit vents to pull air into the attic during warm weather to assist in ventilating the hot air trapped in this enclosed space by way of static or mechanical venting units mounted on the pitched roof. The hot air contained within this space naturally rises and is discharged to the exterior via these roof vents. This process demands "make-up air," which is drawn into the attic through the soffit vents at the base of the roofline. In a high-rise building, the same method of air movement holds true, except the effect is more complicated and pronounced. The impact this has on firefighting operations is significant, yet poorly understood.

Consider looking at a high-rise as a "chimney", which is what it truly is for all intents and purposes. Since hot air rises and cool air descends, when a building's interior is heated or cooled by its heating, ventilating and air conditioning (HVAC) system, a natural draft takes place within the core area where stair, elevator and other various shafts are typically located. Consider these shafts to be the "flues." Especially within shafts that are not well sealed (such as elevator banks), air races in and out and moves up or down, depending on the temperature/pressure differential between inside and outside the building and the location/level within the tower. This movement of air within the building is enhanced by the temperature of the outside air, as air will either be attempting to enter or escape the building, depending on the temperature/pressure difference, through any available opening, generally lobby entrances. Consider these entrance doors at the base of the building to be the "dampers." It is important to look at why a high-rise building acts like a chimney and understand air travel within the building into and out of vertical shafts.

Everyone knows that warm air rises and cool air descends, but how do these rising and falling air currents affect the day-to-day operation of a high-rise building and, more importantly, how does it affect firefighting operations?

Summer Stack Effect

When tenants and visitors enter a tall building in warm weather through a swinging door, they feel cool air generated by the HVAC system rush past them from the interior to the outside environment. These buildings install revolving doors to contain the loss of cold and warm air during the summer and winter seasons and encourage their use by the public. Note that contrary to established fire codes, sometimes swinging doors can be found locked to demand use of revolving doors.

The author visited one fully occupied 40-story office building in the deep South midday last summer where every swinging door was locked and one of three revolving doors was also locked. I asked a guard why the exits were secured and was told it kept everyone funneled through one entrance point, making it easier to track visitors for security reasons and to conserve air conditioning. I then asked how they would manage a mass evacuation if a fire or other serious event were to occur, not even mentioning the need for fire personnel to make entry with gear and equipment. He then said he would unlock all the secured doors. I then asked him to locate and show me the key. He stated he did not have it, as it was with the security supervisor. I asked where the supervisor was and was told he was at lunch. Marvelous. I explained to him the dangers of this practice, that it probably violated local fire codes and that they had a responsibility to create a safe, efficient means of egress from the building during emergencies for the occupants. He assured me he would discuss it with his supervisor when he returned. Common practice there, I suppose.

The reason why this summertime downward draft (or "reverse stack effect") is important to be aware of during fires is that if the lobby doors are opened to expedite the movement of firefighters and equipment into the building, it must be considered that if the fire is below the "neutral pressure plane" (typically somewhere between half and two-thirds of the building height, where the pressure differential between inside and outside the building is almost even, or "neutral"), the smoke can easily be drawn down through the core of the building -- sometimes many floors below its origin. The fire department's staging floor, typically two floors below the fire, can then be expected to be lost, as smoke migrates out onto these floors, mostly through elevator seals, mail chutes and stair doors being opened, sometimes even by HVAC systems that are not being effectively managed.

Repositioning the staging floor farther down below the fire creates myriad logistic problems for personnel attempting to maintain a continuous assault on the fire, perform searches and carry out other activities. They are getting farther and farther away from where they are needed. During summer, lobby doors should be kept closed as much as possible. Remember that what you do at the bottom of the building may affect what occurs at or near the fire floor. One fire on the 12th floor in a Texas high-rise on an extremely hot and humid summer day resulted in untenable conditions in the lobby due to the "reverse stack effect" (downward airflow or downdraft), forcing the command post to be relocated in the street.

Winter Stack Effect

When tenants and visitors enter a tall building in cold weather through a swinging door, they feel cold air rush past them into the building's lobby. These doors are acting as "dampers" for the "chimney" (the building).The bottom of the chimney is temporarily opened; warm air is allowed to rise rapidly through internal shaftways (the "flues" within the "chimney") as cold air acts as replacement air, entering the building's base. The air flow rises with increased velocity, mostly due to the substantial temperature differential as well as the tower's height and internal/external pressures. If it is 26 degrees outside and 70 degrees inside, there is a 44-degree temperature differential between inside and outside air. Thus, the air movement (stack effect) is very pronounced. If lobby entrance doors are opened and the stair doors opened as well (if they indeed exit into the lobby -- some exit to the outside and even below grade), then one can expect a rush of air to enter the building and shoot up the stairwells.

Keep in mind that the strategy of venting stairs using this method may not work in many cases, as a greater volume of smoke will enter the stairs from the fire floor, due to it being drawn to these shafts by the convection currents, thus contaminating the stairs even more than just leaving the stairs sealed at lobby and roof levels and having some (but not significant) smoke migration/contamination from the fire floor occur in the stairwells. This venting action/tactic with internal staircases in an enclosed core would essentially turn the stairwell into a "smoke tower/fire tower" (a designed evacuation/escape stairwell system where there is a vestibule between the floor's tenant space and stairwell that has either an open shaft with a railing or a dedicated smaller air shaft on the wall next to the walkway to prevent smoke from following fleeing tenants into the exit stairwell). In this scenario, however, the stairwell itself would then become the smoke shaft.

It is widely agreed upon that using a smoke/fire tower as the attack stair is poor strategy, due to the chimney effect that will occur since the smoke and fire will be rapidly drawn toward that stairwell location, almost always overwhelming the attack crew (even with 2½-inch hoselines). This has occurred in several major city fires where the attack teams were completely overwhelmed due to this action, including the use of 2½-inch lines. The same thing can occur when ventilating a common, standard stair shaft -- a flue is created. This action may also pull smoke from the fire floor into elevator shafts and move it to upper floors, endangering building occupants as the fire and its byproducts are drawn towards the core area -- the same area where people are congregating and fleeing.

There may then be significant pockets of carbon monoxide (CO), a poisonous gas, now present on upper floors and points in between, increasing the risk of multiple fatalities occurring, mostly in the stair shafts or in stalled elevators on these floors where people may be trapped. Remember that CO is a lighter-than-air gas and will easily rise to floors well above the fire. A 2% concentration in air in as little as two minutes can be fatal. Rapid contamination of upper floors will occur and many tenants will be endangered. In the past 10 years alone, there have been two multiple fatality fires where the victims were found well above the fire floor in the stairwell, perishing from CO poisoning.

There may also be a stratification taking place, where in tall buildings the smoke rises up until it cools and levels off on a given upper floor or floors (similar to ground fog on a cool evening). This is more likely though to happen during the summer when the air conditioning will increase the probability of this occurring. This generally takes place in and around the "neutral pressure plane."

Case Study

Here's a case study of an actual incident, the 1993 World Trade Center bombing, involving upward draft/"winter stack effect":

It is February, with snow flurries. It is 35 degrees outside and 70 degrees inside the complex, a 35-degree temperature differential. Just 4½ minutes after the bomb detonated on the B2 level of the parking garage, there was heavy smoke on the 110th floor of nearby Tower 1 (the bomb destroyed the base of the skyscraper's elevator and stair shafts, exposing the shaftways up into the tower above). This meant that smoke traveled 112 floors and almost 1,400 feet in less than five minutes! This was a classic example of "stack effect" as it relates to smoke movement. A 110-story "chimney" sat almost directly above the bomb-laden truck and acted as just that when the attack unfolded.

The importance of this occurrence escaped nearly everybody, including firefighters in countless cities with high-rise buildings. Yet, it was such a valuable lesson to be learned (and studied) in the natural stack effect that takes place in multi-story structures -- especially skyscrapers containing thousands of people. In this case, the vertical air/smoke movement could not be controlled due to the bomb's effects, but this same type of stack effect can be greatly controlled by rigidly managing the opening and closing of both lobby and stairwell doors. At a fire in Paris three years ago in a high-rise residential building, a small open window at the top of an emergency exit stairwell created a Venturi effect, turning the stair into a virtual wind tunnel and resulted in the deaths of 17 occupants.

Can you have a "winter stack effect" in the summer and a "summer stack effect" in the winter? It is important to note that you can have a winter or summer stack effect in opposite times of the year, if there is a chilly day in June or warm day in December, since the air movement will directly relate to outside vs. inside temperatures and pressures.

Oddities

I entered a high-rise office building in Atlanta on a hot summer day and the air was rushing past me into the building (as it would in the winter) as I passed through a swinging door. It suddenly occurred to me why, as I walked into the main floor to discover that there was a large open floor of equal size below me that led out onto the lawn behind the building, some 20 feet below the grade-level main entrance. This meant that the air was being drawn down to that level by the air conditioning/reverse stack effect, accentuated by doors being opened on the main lobby level, mimicking a winter stack effect on that floor in the middle of the summer!

I noted one day years ago in the 100-story John Hancock Center in Chicago (which has three exit core stairwells) while taking pictures of U-return stairs that when I walked from the stairwell onto the 13th floor, the door did not close behind me. I turned around and noticed that the self-closing door stayed open approximately one foot due to the draft taking place at that location. It was mid-winter and what was apparently occurring was the building's natural draft (openings at the top may have been present -- even stair doors) drawing make-up air from the floor into the stair shaft as the air ascended at a rapid rate with the significant temperature differential between inside and outside the building -- the draft was actually holding the door open and not allowing it to close. Although the draft was barely perceptible, it was just enough to prevent the door's self-closing device from functioning properly.

This is important to note that in this circumstance, if tenants were to flee floors at the bottom of this building, many stairwell doors may not be closing behind the escaping occupants, thereby enhancing the stack effect. In addition to pulling the fire itself toward the core area, this action also increases smoke movement to upper floors -- especially if the fire is on a lower floor. This could easily draw enough smoke into the stairwells to move a considerable amount of smoke to upper floors within minutes, while also possibly making the stairwells untenable for evacuees.

Remember what happened at the previously noted incident at the World Trade Center in 1993. The question is, would the incident commander in the lobby even think of or be aware of this strange phenomenon occurring above his head in the tower while commanding the event? Or would he just be forced to react to the ramifications of what is happening and have to dispatch precious resources to upper floors to assist trapped occupants, when the solution may be as simple as having first-arriving search and attack crews to the fire floor ensure that the only door left open is the door to the attack stair so endangered tenants above have a path of escape if they have chosen not to stay in place? Also remember that the upper portion of this tall building is probably experiencing the reverse of this effect, meaning that the air rushing up the stair shafts will be compressing at the top of these shaftways, creating above-normal back-pressure against the exit doors, making them more difficult than normal for tenants to open when fleeing their floors during a fire. Doors leading into smoke tower vestibules would have even more pronounced drafts, but it should not affect the exit stair past the second doorway if it is kept closed -- another good reason to avoid this as the attack stair.

One other item of concern (as mentioned in the scenario at the beginning of this article) is that air rushing into a lobby of a high-rise can easily prevent elevator doors from closing at lobby level, thus precluding their use by firefighters trying to reach upper floors. This draft can be incredibly strong in taller buildings. It may even prevent elevator cars from being automatically or manually recalled to the lobby if this same action is taking place on floors above with car doors not closing. The exception to this situation would be if it is a modern high-rise possessing not just stair pressurization, but elevator shaft pressurization system capabilities as well that are activated on alarm. This may likely counteract the rush of air into these shaftways. However, this would only apply to the newest of buildings (post 1980) and even then only in cities that require it.

Summary

Maintain the integrity of the base of the "chimney" as much as possible during a high-rise fire, as what you are doing in the lobby in the way of propping open lobby doors and collapsing revolving doors, can have a tremendously adverse effect on what is happening further up the tower. Cold-weather fires will push smoke upward into the building in a very pronounced, rapid fashion. If lobby stairwell doors are propped open as well, this can turn the stairwells into virtual chimneys. The attack stair where the fire floor door is propped open for hose deployment will be considerably worse, especially if windows have popped, feeding the fire fresh (make-up) air on the fire floor -- even more pronounced if the broken windows are on the windward side of the building. This will make that stair identical to what the conditions were in the Paris fire -- the stair will be a wind tunnel. Also, firefighters on the fire floor will be quickly overpowered by the fire being drawn towards this flue effect occurring and will be forced to abandon their attack, hopefully without any members being burned. (Note: In cold-weather fires, if elevator doors are not closing due to wind racing up the shaftways, make certain all entrance doors are at least temporarily secured. This should let firefighters regain use of individual cars to access upper floors. This is exactly what occurred in the story at the beginning of this article.)

Venting stair shafts may offer beneficial results on some fires, depending on the height of the building, the conditions and the use of the stair (attack or search/rescue), but they mostly will end up serving to enhance the smoke condition within these shafts, usually making a bad situation worse by drawing the byproducts of fire towards and into the shaft(s), thereby defeating its purpose as a means of egress for building occupants. If all occupants have been accounted for above the fire floor, then it may be safe to use at least one stair for smoke evacuation if the stair terminates at roof level and conditions are right for this to be done (best in cold-weather fires).

There are exceptions to every rule and fires have occurred where this tactic proved beneficial to the cause. It is mostly agreed, though, in occupied high-rise building fires, that pressurizing the stairs is a better choice than trying to vent them via the roof, but be careful not to over-pressurize them if using fire department fans, which might inhibit people in the tower from opening exit stair doors when fleeing their floors due to excessive backpressure. Also remember that many buildings' exit stairwells do not terminate at roof level, though most buildings have at least one that does. Some buildings do not even possess a roof that is accessible, with architectural spires or sloped roofs in place. This should be noted in the building's pre-fire plan, if one exists.

Stack effect plays such a major role in these fires that it should be stressed heavily in officer training for high-rise district company and chief officers, as well as being a prominent aspect of a city's existing high-rise standard operating procedures/standard operating guidelines (SOPs/SOGs) -- especially since it is a vitally important topic not well understood in the fire service. Controlling air movement will play a positive role in the outcome of a serious incident. Survivors of a fire in these edifices will owe you their lives.

CURTIS S.D. MASSEY is president of Massey Enterprises Inc., the world's leading disaster-planning firm. Massey Disaster/Pre-Fire Plans protect the vast majority of the tallest and highest-profile buildings in North America. He also teaches an advanced course on High-Rise Fire Department Emergency Operations to major city fire departments throughout the U.S. and Canada. Massey also regularly writes articles regarding "new-age" technology that impacts firefighter safety. Very special thanks to District Chief Matt Stuckey of the Houston Fire Department (ret.) and Deputy Chief Roger Sakowich of the FDNY for assisting in the technical critique of this rather complicated article.

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