FIREFIGHTING AND THE “HIGH-PRESSURE BACKDRAFT”

BY BRIAN M. WHITE

Over the years, firefighters have always had difficulty fighting fires. As buildings be-came larger and more complex, increasing in area, height, and energy efficiency, the inherent difficulty increased dramatically. Recently, it seems that there has been a dramatic increase in the number of catastrophic fires in low- and high-rise fire resistive buildings, especially apartment buildings. The fires have been catastrophic not only in intensity and damage but also in the increase in the number of firefighter deaths.

If only a few of these types of fires had occurred over a long period of time, it might be surmised that they were the result of bad timing-being in the wrong place at the wrong time. But the frequency and intensity of these fires indicate a more serious and insidious cause. The following hypothesis may explain the situation.

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The volume and intensity of fires in large buildings during high-wind conditions seem larger than might be expected if the wind were only fanning the flames. The unknown factor is what I call the high-pressure backdraft. Although the initiation of the event does not follow the traditional backdraft scenario, it is the result that is the same-explosive fire growth and spread. This effect causes the building’s internal pressure to become abnormally elevated, which contributes to the fire’s intensity when this pressure is released through an open window or door, or a combination of the two. This affects the internal pressure, and the wind causes erratic flow inside and outside the building, attributable to internal and external pressure differentials, the opening through which the air passes, and the building’s design.

MY THEORY

Air pressure within a building increases in proportion to the force and duration of the wind pushing against the building. This exterior pressure pushes into the building through infiltration spaces (open windows, bathroom vents, and poor insulation, among other things) until the interior pressure equals the pressure exerted by the wind. In addition to the pressure, tremendous amounts of air will flow into the building on the windward side.

“The pattern of air flow through any part of the structure depends on both the pressure difference and the area of the openings. When the pressure difference is the result of wind pressure, air will enter the building through openings in the windward walls and leave through openings in the leeward walls or through ventilating ducts in the roof.” ¹

Air pressure on the opposite side of the building where the wind does not impact the building will have the lowest pressure potential because low pressure will develop on the leeward side of the building because of the flow of air over and around the building.

“Air flow due to wind around and over a building creates regions in which the static pressure is either above or below the static pressure in the undisturbed air stream. In general, pressures are positive on the windward side, resulting in inflow of air, and negative on the leeward side, resulting in outflow of air.”²

If an opening is created on this side of the building, then the natural “wind tunnel” will be created, exhausting the internal pressure from the high-pressure side of the building to the low-pressure side through this opening.

In addition to the pressures mentioned above, there will be an added pressure buildup from the fire itself. As noted in the formula PV = RT, where P is the pressure inside the closed room, V is the volume of the room, R is the gas constant, and T is the temperature, when T increases, P will increase proportionately because V and R remain the same.

In my theory, the effect of the pressure can be compared with the effects of water on a land mass during a hurricane’s storm surge. As the wind pushes the water up against the shore, the pressure and volume of water increase. If a storm surge were pushing against a retaining wall and the retaining wall were to collapse, the resulting release of energy would be catastrophic. This is what occurs when a window on the windward side of the building is opened.

The effect of the wind on the internal pressure of the building can also be compared to a hypodermic syringe. Cover the end of the syringe with your finger and press on the plunger. The air has nowhere to go and results in an increase in the internal pressure compared with the outside. Now remove your finger while still pressing on the plunger and see how the air escapes under high pressure. Because of the restricted space, the velocity of the air that passes through the space is dramatically increased.

1. When the wind is forced against the wall of a building for an extended period of time, this “forced” air infiltrates the skin of the building and increases the internal pressure of that building. Depending on the tightness of the building and the openings in the wall, the pressure will increase proportionately to the increased speed of the wind.

2. As the duration and speed of the wind increase on the outside of the building, the more pressure is transferred to the inside of the building. Because the air is restricted, first by the window opening and then by the interior doors, the pressure slowly builds from the windward side of the building toward the leeward side.

3. Doors and partitions will allow pressure to build slowly through the building. Because areas of infiltration are relatively small, the pressure will build slowly, because of the resistance of these partitions (doors), until the pressure against the building caused by the wind transfers that pressure internally through these openings. As the pressure builds in the apartments on the windward side of the building, it will pressurize the hallways and stairways, depending on the tightness of the stairway doors on each floor, lobby, and roof.

“Pressures inside buildings due to wind action alone will depend on the resistance of cracks and openings and their locations with respect to the wind direction. In large buildings, the tightness of internal space separations may also be a factor.” ³

4. Because of the size of the infiltration points, the building pressure [static pressure (SP)] will increase slowly relative to the size of the openings. Therefore, when the internal pressure of the building reaches its maximum, then the pressure will be released through “like” openings on the opposite (leeward) side. As long as the wind remains constant, the internal pressure will remain high relative to the outside of the building on the leeward side. Because the size of the exit openings on the leeward side of the building are the same as those on the windward side, the pressure can only be released as fast as it can enter and thereby act as a type of pressure relief valve after the entire building has become fully pressurized. At some point, the pressure inside the building will reach its maximum. This maximum pressure will exist as long as the wind condition exists and the tightness of the building remains intact.

“The rate of air flow into and out of the building due to either infiltration, exfiltration, or natural ventilation, depends on the magnitude of the pressure difference between the inside and outside of the structure and on the resistance to flow of air offered by openings and interstices in the building. The pressure difference exerted on the building enclosure by the air may be caused by either the wind or by the difference in density of the air inside and outside.” 4

5. If a window is opened on the opposite side of the wind (leeward), then a release of pressure will occur out that window until the pressure inside the building equals the pressure outside the building.

6. If a window is opened on the windward side of the building, in addition to the window on the leeward side, the actual force of the wind acting on the already pressurized internal component of the building will dramatically increase the force in which the pressure is expelled on the leeward side. Even if all windows remain closed on the windward side of the building, the cumulative air infiltration of the windows, air-conditioning units, vents, and so forth will allow a tremendous amount of air flow into the building.

7. If a fire exists in the apartment on the windward side and a situation similar to (6) above occurs, then an extremely dangerous condition will occur. It will develop rapidly and without warning.

“While the internal pressures are generally less in magnitude than the external ones, their influence on the overall load on the envelope is far from insignificant. Even though the magnitudes of the internal pressures are usually less than the external ones, they are almost fully correlated over all the well-connected regions of the interior and thus their integrated effect, particularly at higher frequencies, may exceed that of the external pressures. This increased correlation is associated with the transmission speed; the internal pressures are transmitted at the speed of sound, whereas external fluctuations are convected at speeds of the order of the mean speed.”5

8. If a window in the fire apartment fails, the increase of air flow will be more than five times the normal air infiltration for the entire floor. You can expect a staggering 8,000 cubic feet of air per minute (cfm) flowing through a two- by four-foot opening with 20-mph wind. If the window is a large type, as in the case of the December 1998 Lincoln Plaza fire in Manhattan, you can expect as much as 20,000 cfm to force its way into the building. Once the door to the fire apartment is opened, this air, with the products of combustion, will flow freely down the hallway through the paths of least resistance. We have to limit the spread of fire down the hallway.

SCENE SURVEY: ITEMS TO CHECK

Presence of exterior wall penetrations. Bathroom vents, through-the-wall air conditioners, windows, and so on.

Location of openable plane penetrations. Doors-lobby, bulkhead, maintenance, and so on.

Odd-shaped structures. Normally, we would not consider the shapes of buildings or integral parts of buildings as dangers to firefighters. But, when we add the factor of high wind into the formula, we have a different situation. We are now dealing with the physics of wind, and the result can be phenomena such as the Venturi effect, high- and low-pressure zones, eddies, downdrafts, and more. More research is required to find out how the wind affects the fire but also how the wind affects the internal dynamics of the building wind flow. Testing is now being conducted; the results will be made available in the future.

Buildings with more penetration points (weak integrity) have the greatest pressurization potential. This will increase the possibility of rapid intensification in the case of developed fires. Even if the integrity of the plane is “good,” firefighters should use caution because the effect can occur if open windows and doors on opposite sides of the building are in proximity to each other.

STATIC PRESSURE (SP) FORMULA

The Static Pressure formula shows the potential danger when the building has reached the maximum static pressure (MSP). The larger the factor (A1/A2), the greater the danger. See Example 2 in which only A1 is increased.

When the bulkhead door is open prior to the arrival of firefighters, it will increase SP if oriented to the wind. It will conversely limit or reduce the SP within the building if facing away from the wind. Facing away from the wind flow may also cause a negative pressure to develop in the stairway because of the Venturi effect. If this occurs, this negative pressure could draw the fire toward the stairway. If the stairway is used as an attack stair, this would result in a very dangerous situation indeed.

Units can determine static pressure during outside activities. The numerical figure obtained can be used to compare buildings with each other to determine which buildings pose the greatest danger to members during high-wind conditions (see Figure 1). A standard wind velocity (V1) of 20 mph can be used to determine the potential static pressure for every building for comparison purposes (see Examples 1 and 2).

SP = V1 2 (A1/A2)

SP =static pressure

V1= sustained wind velocity in miles per hour

A1 = area of wind infiltration of wall in square feet (windows, a/c units, and so on.)

A2 = total area of wall in square feet

Example 1

For a six-story building with a wall 100 feet wide by 60 feet high (A2) that has 120 eight-square-foot windows (A1), SP is calculated as follows.

SP
= 20 2 (960/6000)
= 20 2 .16
=3.2

Example 2

Double the area of infiltration (A1 4 1,920 square feet)

SP
= 20 2 (1,920/6000)
= 20 2 .32
= 6.4

The orientation of the building to the prevailing wind should be noted. Buildings with the long side facing north to northwest pose the greatest potential danger.

“In almost all localities, the summer wind velocities are lower than winter wind velocity. In about two-thirds of the localities, the prevailing direction is different in the summer and winter.”6

Scenario 1

The fire apartment is on the windward side of the building. The wind is steady with a sustained velocity of more than 20 mph and has been from the same direction for an extended period of time. A window near the location of the fire apartment is opened on the leeward side of the building. There is an unobstructed path from the fire apartment to the leeward window, and the fire apartment window on the windward side fails or is open.

Given the above circumstances, anticipate that a rapid decompression of the pressurized fire floor will occur through the areas of lowest pressure, which will be intensified by the actual wind and cause severe and almost instantaneous untenable conditions. This is the synergistic effect of the forces working together, in which the effect of the combination of the forces is greater than that of the forces individually.

Scenario 2

This is the opposite situation from the above scenario. The fire apartment is on the leeward side of the building, and heat and smoke would be vented directly out of the fire apartment window. Because this situation results in less punishment for the members encountering it, this aspect of static-pressure or high-pressure backdraft may have been encountered many times before. The deleterious effects of the fire were not encountered because the fire easily vented out the fire apartment window. Because of the positive result, little attention would be paid to the cause of the relatively easy extinguishment of the fire. The situation in Scenario 1 exists, except that the fire was on the “good” side of the building and allowed the products of combustion to be vented from the building easily.

Scenario 3

This is the most dangerous of the three scenarios. The pressure inside the windward apartments has maximized, and it is before that pressure is transferred into the hallway and the apartments on the leeward side. This fire’s pressure, if the fire is on the windward side, will add to the already increased pressure in the apartment, causing an even larger pressure difference between the fire apartment and the hallway. Because of the dynamics of the structures protruding from the building, low-pressure zones can be created inside the building. This dynamic can draw the fire out of the fire apartment toward the low-pressure zone. If the low pressure developed on the leeward side is combined with the low pressure from odd-shaped building structures, the increased pressure caused by the fire, and the wind effect, the result will a potential disaster. The resulting flow from the fire apartment will carry superheated smoke and gases into the hallway at tremendous speed (close to the speed of sound). This will cause almost instantaneous zero visibility and disorientation of the members operating on the fire floor. It is analogous to the uncorking of a champagne bottle.

“The actual pressure differences that occur across the walls of the building are less than the theoretical pressure differences for most buildings and depend on the tightness of the floor separation relative to that of the exterior wall. If the resistance between the floors is low relative to that through the exterior wall, most of the pressure difference due to chimney action is taken across exterior walls. Increasing the resistance between floors reduces the pressure difference across the exterior walls but results in a corresponding increase in the pressure difference between floors and across walls of the vertical shafts. The ratio of actual to theoretical pressure difference depends on the ratio of air flow resistance across the exterior wall to the resistance to vertical flow inside the building.”7

CONCLUSIONS AND SOLUTIONS

Pressure must remain constant in the building. All doors and windows must be closed, and the need for two 21/4-inch handlines must be anticipated. The interior pressure within the building must be equalized. To equalize the pressure, doors to the hall stairway, the bulkhead doors at roof level and ground level, and the door and windows to apartments remote from the fire apartment and on the opposite side of the building must remain closed.

For a fire in a high-rise multiple dwelling, the door from the stairwell where the line is to advance must obviously be open for the line to advance. The pressure can only be equalized in that stairway if all the other doors to that stairway remain closed. The only doors that serve that stairway that can be opened are the door on the fire floor and the door on the floor below. If the stair door on the floor below the fire is opened in addition to a window on the windward side of the building on the floor below, the resulting inward pressure from the wind may neutralize the inward pressure on the fire floor. All other activity and operations requiring the use of stairs must be done using the others stairs.

The first truck must stretch a search rope from the attack stairway to the fire apartment door. The rope must be secured at both ends to ensure a reasonable means of egress. This will ensure proper egress direction if visability is reduced or disorientation occurs.

As noted in some recent fires fatal to firefighters, disorientation and loss of visibility seem to be major contributors, but not the only ones, to the fatalities. It is possible that if a clear means of egress were available at the time, then some of these deaths could have been avoided.

Stretching a compactly stored removal rope from the stairway to the fire apartment doorknob may accomplish two important tasks:

1. Rope provides members a guaranteed direction of egress (visual or by feel).

2. Fastening the self-closing loop around the fire apartment doorknob gives the forcible entry team control of the apartment door.

You can install mini lights on the rope and have a locating sound device at the fire apartment door to allow you to know if you are getting closer or farther away from the fire apartment. Also, it will help members trying to get to the fire apartment to assist in operations (nozzle team, relief companies, and so on.) and members trying to exit from the fire area.

Example. In a fire on the 10th floor, stairway A is the attack stair. In stairway A, only the doors on the ninth and 10th floors can be opened. In Stairway B, all doors except those on the ninth and 10th floors can be opened.

Because of air infiltration, a very strong flow will result from air leaking into the building. A wind of 25 mph striking the building at a right angle will allow the infiltration of approximately 8,000 cubic feet of air per minute into the building on that one floor alone if an approximately eight-square-foot window is removed. Also, it is in comparison with the natural infiltration through walls and windows of approximately 1,500 cubic feet per minute per floor (assuming a building length of 100 feet).

Measured leakage values of a nine-storybuilding8 with masonry walls with operable aluminum sash windows were approximately 115 cubic feet per hour per square foot of outside wall area. Based on an assumed leakage characteristic of 90 cubic feet per hour per foot of sash crack at .3 inches of water pressure difference, it was estimated that windows and doors contributed approximately 25 percent of the total leakage with the remainder through the masonry wall construction.”9

While the standard belief is that the “stack effect” will influence fire characteristics in taller buildings, the effect of static pressure will affect fire behavior to a much greater extent, proportionate to the speed and sustained duration of the wind. It should be noted that the stack effect will also add to the severity of fire conditions encountered.

“With outdoor air temperature lower than indoor air temperature, the neutral zone level is lowered with a corresponding increase in pressure difference across the walls of the lower floors [e.g., at a recent fire in Brooklyn on Vandallia Street, wind rushed into the upper floors of the building]. Conversely, with the outdoor air temperature higher than the indoor air temperature, the neutral zone level is raised. The change in the pressure difference due to pressurization is equal to the pressure difference required to dissipate the excess air supply through all openings in the enclosure [e.g., at another Brooklyn fire in Coney Island, wind rushed into the lower levels of the building].

If excess air supply is introduced uniformly on each floor level, the change in the exterior wall pressure difference pattern resulting from temperature difference forces is uniform. If, however, the excess air supply is not introduced uniformly [for example, if air were introduced to one floor only (as happens when a window in the fire apartment fails )], the extent of pressurization will vary from floor to floor and will depend on the resistance of the exterior restrictions. Pressurizing all levels uniformly has little effect on the pressure differences between floors and across walls of the vertical shafts, but pressurizing individual floors increases the pressure drop across these internal separations.”10

Each building presents a major problem, but the manner in which the pressure forces act can be considered qualitatively.

These fires have taken a tremendous toll on our members in recent years. It is up to us to recognize the danger these types of fires pose. Further research and testing must be done to enable us to detect the warning signs of these fires and forewarn us of the associated danger. Once we identify the potential danger, we can then be properly prepared to defend ourselves.

LESSONS LEARNED

1. Control the door. The apartment door is the No. 1 defense we have between the fire and ourselves. The ability to control this barrier has become almost as important as stretching the handline or making the primary search. To advance the line and make the search, we need to protect ourselves. We can’t help those people at the fire if we don’t get there! We must control the door to allow the engine company to set up for the attack and the truck company to prepare for the initial search. As with all things we do routinely, we may do it hundreds of times before we have to do it in a serious situation. If we do it for everything, we’ll be ready when it counts.

2. Provide a way out. If we can develop a small compact exit rope, we can always carry it as a routine-not a bulky search rope, but a thinner and stronger compact rope in a carrying case that is always available for deployment. The primary end must be secured to the attack stair door on the fire floor. This line will be played out down the hall to the fire apartment. At the fire apartment, the other end of the rope will be secured to the doorknob of the fire apartment. This will ensure control of the fire apartment door and provide a means of egress to a place of safety.

3. Check the wind effect on buildings before a fire. Visit buildings in your administrative area during high-wind situations and see what effect the wind has on your high-rise buildings. See which buildings have the long side exposed to the prevailing wind. In New York City, winter is the high-wind season, and the prevailing direction then is northwest to north. Go to the upper floor and see which way the air flows when you open the stairway, bulkhead, or compactor chute doors. When you open the compactor chute door on a windy day, the effect may be surprising. Do the same thing on the lower floor and see if there is any difference.

4. Be aware of high wind activity. Once sustained wind velocity reaches 15 mph, alert field units to sustained high-wind conditions. Use the following scale to gauge the severity of wind effect:

Level 1-Sustained winds between 10 and 20 mph with no appreciable gusts expected.

Level 2-Sustained wind between 20 and 30 mph with occasional gusts expected.

Level 3-Sustained wind above 30 mph with regular gusting expected.

To alert members, the dispatcher should use the voice alarm and the teleprinter systems. All units should be advised of the high-wind condition in the initial notification and through updates. Updates can be transmitted hourly through department radio and the voice alarm system. Such information should be part of the chief and company officer’s size-up.

5. Be prepared to have a second line ready. With the reduced staffing, emphasizing the use of two companies to stretch the first line has been in the forefront of discussion. Now, add problems associated with high wind to the mix, and we need four engine companies to stretch two handlines in the same time frame. With six or more companies on the fire floor (four engine, one ladder, and maybe a rescue or squad unit-more than 30 people), continuity and teamwork are more important than ever. With so many people in such a small area, confusion can happen any time. When the potential exists for the fire to explode out of the fire apartment and down the hall-and this is coupled with the slightest sensation of confusion-a disaster could be the result. Constant size-up and minute-to-minute reevaluation are key to our safety.

I have conducted a significant amount of research on this topic. If your department has any experiences or information related to this topic, please contacted me c/o Fire Engineering.

References

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1. ASHRAE Handbook of Fundamentals. 1972. American Society of Heating, Refrigeration, and Air-Conditioning Engineers, Inc., 333.

2. Ibid.

3. Ibid., 334.

4. Ibid., 333.

5. Vickery, B. J. “Internal Pressures and Interactions with the Building Envelope.” 1994. Journal of Wind Engineering and Industrial Aerodynamics.

6. Ibid., 344.

7. Ibid., 335.

8. G.T. Tamura and A.G. Wilson. 1966-67. “Pressure differences for a nine-story building as a result of chimney effect and ventilation system operation.” Parts 1 and 2. ASHRAE Transactions 72:180; 73:II.1.1, II.2.1.

9. ASHRAE Handbook, 339.

10. Ibid., 336.

Author

BRIAN M. WHITEis an 18-year veteran of the fire service and a captain with the Fire Department of New York City, assigned to Engine 265. He is pursuing a bachelor’s degree in earth and space science at SUNY-Stony Brook and wind research at Hunter College.

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