To Lee or Not To Lee

By: David Rickert

Many variables need to be taken into account when sizing up structures in regard to topside ventilation. One such variable that can be difficult to gauge – and may fluctuate from inconsequential to the single most important factor when determining hole location is: WIND.

Wind effects on buildings have been studied for decades, but predominantly as it relates to wind-loading of structures for engineering purposes; not for the effect it may have on structural fires and tactical fireground decisions. Only recently have strides been made involving research into wind-driven fires, in particular, wind-driven fires in hi-rise buildings. Wind-driven fire’s research was primarily based in large part to the realization that it was a significant contributing factor in several critical incidents resulting in serious injuries for firefighters; and in some cases line of duty deaths (LODD). While wind generally has a much more pronounced effect on hi-rise buildings, it should not be discounted on low-rise buildings and single family dwellings, as the effects and results may be no less dramatic given the right wind conditions.

Strong winds can cause fire and the products of combustion to be driven back into the building via any external openings. This may cause sudden and drastic changes to fire conditions within the building with little or no warning, imperiling crews or victims on the interior of the building.

The focus of this article will be on the effect wind may have on potential vent openings in pitched roofs; some of the implications it may have on tactical decisions; and ways to quickly incorporate this into your initial size-up.

GETTING THE ANSWERS

Utilizing complicated formulas and equations is necessary in the laboratory, but in the “street assessment,” requires that we are able to quickly apply attained knowledge. To that end, every attempt will be made to illustrate points simply, clearly and graphically, at the same time relating why, when, where and how we need to do these things.

Wind’s effect on roof surfaces is largely dependent on:

  1. The pitch of the roof – The steeper the pitch, the greater impact any given wind will have on an opening made in that roof.
  2. The velocity of the wind-The greater the velocity, the greater the risk of creating a wind-driven fire.
  3. The exposure the roof -The amount of roof surface exposed to the wind; the quality of that exposure; is it fully exposed? Or, is the wind flow significantly disturbed, blocked or broken up by other objects in the upwind path?

To evaluate these three key factors, it will help to have a basic understanding of some of the properties of air, wind and wind flow.

Wind is air in motion, caused by flow from an area of high pressure to an area of low pressure; the bigger the pressure differential, the higher the wind speed.

Wind generally flows horizontally across the ground unless it is obstructed or disturbed by an object (trees, buildings, hills, etc.).

Air has mass―which means when it is in motion, it has kinetic energy and inertia.

Air has viscosity―which allows it to adhere to a surface; in our case, it will be the roof surface, this is also why an airfoil/wing is able to create lift or can suddenly stall out.

Boundary Layer―a layer of slow moving air that forms close to the surface of an object exposed to a wind stream. This layer forms due to the viscosity of the air and directly influences and distorts the free stream air flow.

ROOF PITCH

It is pretty easy to understand why roof pitch plays a crucial role in how wind can effect a roof opening. Everyone has at one time or another stuck their hand out of a moving car window: Place it flat and it cuts smoothly through the wind; Lift your hand palm up, and your hand is driven up and backwards; Palm down and it is driven down and back. These physics principles are easily relatable to the way different roof pitches react to wind.

When wind is deflected by an object such as a roof, it imparts a force against that structural object. The more it is deflected, the more force it imparts. Maximum force is generated by wind striking a vertical surface perpendicular to the wind. With this in mind, it’s not too hard to conclude that the steeper the pitch of the roof the more force that is directed against it. This is highlighted in fig.1 which shows the incident wind angle of a 4⁄12 pitch roof and that of a 12⁄12 pitch roof.

The deflection tells just part of the story: once the hole is open, that part of the roof will no longer be deflecting the wind, but will potentially be allowing the wind to flow into the hole. The roof will now behave as a wind catcher – rather than a wind deflector. Since wind generally interacts with the roof horizontally, the most important factor will be the amount of vertical face (cross-section) exposed by the hole.

Fig1 shows vertical cross-sections of equally sized holes in various roof pitches. The range includes a flat roof (0%) which has no vertical exposure to the wind up to a wall (100%) – which has full exposure to the wind. This exposed cross-section is one of the primary factors in determining a roof’s true (potential) exposure to the wind.

WIND VELOCITY

Wind velocity is a pretty straightforward relationship. Higher wind speeds = greater potential force. Wind force grows at an exponential rate (doubling wind velocity will quadruple the force). The table below shows this relationship. Pressures are listed in pounds per square feet and pascals. Pascals (Pa) is the internationally recognized unit for pressure utilized in scientific literature. A Pascal is a very tiny unit of measure (1 Pa =.02 psf), which is roughly equivalent to the pressure exerted by a sheet of writing paper laying on a flat surface. This highlights the fact that very small pressure changes can significantly influence the flow of smoke, heat and fire gases inside a structure.

The pressures generated by the wind are important because it gives us an idea of when air infiltration into the vent opening may become a problem – or the point at which conditions may be present to create a wind-driven fire. Determining precise pressure variations across the hole and within the attic/crawl space/fire room is impossible for any given fire, due to the almost infinite variability of conditions present (flow dynamics of the fire building, turbulence, wind dir. and velocity, heat release rate [HRR] of the fire, internal pressures, etc); however, general pressure conditions that occur during fires can and have been determined in laboratory experiments:

NISTIR 7213-Effect of Positive Pressure Ventilation on a Room Fire-Stephen Kerber/ William D. Walton “After ignition, the fire created pressures of 34 Pa (0.005 PSI) at the top probe, 14 Pa (0.002 PSI) at the middle probe.” pg.111

NIST Technical Note 1498 Evaluating Positive Pressure Ventilation in Large Structures: School Pressure and Fire Experiments– Stephen Kerber /Daniel Madrzykowski “Table 29 shows the peak pressure in the fire room, before natural or positive pressure ventilation was applied, ranged from 12 Pa to 18 Pa.” pg.23

“The pressure peaks inside the gymnasium also increased with fuel load as would be expected. Experiment GF7’s pressure increased to 11 Pa, and GF8’s gymnasium pressure was 19 Pa. GF9 through GF11 had similar fuel loads and similar pressures of approximately 30 Pa. The last experiment had pressures of 75 Pa prior to a ventilation limited condition that had temperatures high enough to damage all of the transducers, so the pressures reported for GF12 are assumed to be underestimations, because they were already compromised by the time the fire reached its peak output. The peak pressures corresponded to the filling of the gymnasium with combustion products, with the higher pressures at the upper level transducers.” pg.1132

NFPA 92A Standard for Smoke-Control Systems Utilizing Barriers and Pressure Differences (NFPA, 2006) specifies pressure differences in non-sprinklered buildings of between 12.5 Pascal (Pa) and 44.8 Pa to overcome the pressure resulting from hot gases at a temperature of 927o C (1700o F) next to the smoke barrier. This includes a built in safety factor of ~7 Pa so the actual pressures assumed are lower ~5-37 Pa.3

The peak pressures sighted by these two studies and the NFPA standard vary in range from 12-75 Pa. with all but one of the fires generating pressures under 40 Pa. Keep in mind that these are the peak pressures achieved at ceiling level, most pressures during the tests were lower. It is safe to assume that most attic/cockloft/crawl space fires fall within the top of this range (> 75Pa) due to the limited amount of venting normally available in these areas, remembering again that these pressures will occur towards the very top of the compartment. Using 12 -75 Pa as the pressure range for room fires, we see that at the low end 12 Pa is matched by a 10 mile per hour (MPH) wind and a 20 MPH wind (48 Pa) will generate enough pressure to overcome all the other pressures generated by room fires, save one ,which was set-up specifically to create a ventilation-controlled fire. This pressure, 75Pa, can still be overcome by a 25 -30 MPH wind.

ROOF EXPOSURE

The most important part of roof size-up as it relates to the wind is determining, with reasonable certainty, how the wind will impact the roof surface; both at the time of opening, and a short but indeterminate time there after. Wind, even laminar (smooth) flow, is highly variable and prone to fluctuations both in velocity and direction. When wind flow is turbulent, the variations become even more pronounced and unpredictable.

In urban areas, and even most rural areas, the airflow that is acting on a structure will be turbulent. Airflow becomes turbulent when it has to move around or over an obstruction. Obstructions can either be man-made (buildings, fences, walls) or part of the terrain such as a trees or hills. Once the wind is clear of the object, it remains turbulent downwind of the object for quite awhile (sometimes up to 50 times the height of the object) until it regains its laminar flow characteristics once again or is acted on by another obstruction. Basically, this means that all airflow you encounter in the city will be turbulent.

Assessing the conditions that exist on the roof requires close attention paid to any signs present at the fire building (trees, flagpoles, chimneys). The best single indicator is observing smoke from the fire, especially how it is behaving once it reaches roof height, on up to the ridge-line. I am immediately suspicious when one side of the house and roof is shrouded in smoke and the other is completely clear. Some may see this as an opportunity to work “out of the dirt,” but it should be seen as a warning sign that wind velocity and pressure may be high enough to cause problems once the roof is opened.

Remembering to continuously gauge the wind – especially when you are on the roof – is paramount: being able to actually “feel” the wind and its effects at the point of any potential opening will either reinforce the visual clues you used – and the tactical decisions based on them – or will undermine these same visual clues and allow you to readjust your tactics accordingly before any real damage is done.

The graphic above illustrates basic wind interaction with a steeply pitched residential home. Generally, wind builds up on the windward side creating positive pressures and is pushed along the boundary layer over the ridge, where the boundary layer separates from the surface. This creates a negative pressure (suction) on the leeward side; so not only does the leeward side offer protection from the wind – and any possibility that wind will be driven into the vent hole – but the negative pressure (suction) on the leeward side creates an ideal condition for venting super-heated atmospheres from within the structure.

AT THE VENT HOLE

Pressure flows from areas of high pressure into areas of low pressure until a system reaches equilibrium. In a closed system (such as two containers at different pressures connected by a valve), this is a relatively straightforward matter: open the valve and the system will quickly equalize. In an open system – such as the case of a building on fire – the balance will not be achieved so quickly, predictably or even at all. The pressure within the fire building is widely variable due to the heat of the fire which will constantly be adding energy (heat and pressure) to the system; venting of windows – creating new openings – or opening of doors, may randomly and unpredictably change air flow and pressure throughout the building. The area we are most concerned with is the changing pressures (pressure gradient) across the vent opening. The highest pressures will be found toward the top of the (hole) and lower pressures toward the bottom. The bigger vertical profile a hole has, the greater the chance that wind will overcome the pressure and infiltrate into the opening. The reason for this is detailed in the following graphic:

This graphic highlights how roof pitch plays a significant role in determining possible air infiltration into the vent hole. The 4⁄12 pitch offers a very narrow vertical profile that will be much less prone to air infiltration than the steeper 12⁄12 pitch – which allows for a much wider range of pressure variations across the hole – due to its wider vertical profile. For example, say on a roof of 12⁄12pitch, the pressure generated from the fire ranges from 25Pa at the bottom of the hole to 75Pa at the top. Whereas for a 4⁄12 roof – and it’s narrower vertical profile – the range may be something more on the order of 50Pa at the bottom to 75Pa at the peak. So the low range (25Pa) of the steeply pitched roof (presents) a much greater opportunity for wind to infiltrate into the vent opening than that of the gently pitched roof which spans a much narrower range beginning at 50 Pa.

I would not be too concerned with some air infiltration or exchange at the vent opening. This occurs on a regular basis with no serious consequences; however, we do need to be concerned when some air infiltration becomes significant air infiltration, or when a sustained, penetrating wind is driven deep into the compartment we are attempting to ventilate. Think about it this way: would you make a hole in the roof and then stick a PPV fan into the opening? Of course not; regardless, making a windward opening with significant winds impacting the roof creates this exact scenario.

IN THE STREET

When you are assigned to a truck company, assessing wind conditions is an ongoing process that starts with the beginning of your shift and ends when you make a hole. Whether you are the officer, senior truckman or probie, being able to recognize visual cues and estimating wind speed is an important part of the size-up process.

This chart is based on the Beaufort Wind Force Scale. Utilizing the information on this chart while responding – and once you arrive – should give you a good idea of wind velocity; and if it will play a role in your tactical decisions concerning roof operations.

TACTICAL DECISIONS

When do we need to factor wind into our roof evaluation process? In making tactical recommendations based on the chart below it is assumed that:

  1. These are sustained winds with possibly higher gusts.
  2. The wind is striking the roof surface from a mostly perpendicular direction.
  3. The windward roof surfaces are fully exposed to these wind speeds.

SUMMATION

Many variables at a fire are beyond our control. This truism makes the variables we do have control over all the more important. This is never truer than when we arrive at a structure fire and begin “opening up.” Opening up haphazardly without regard to why, when, where and how we are doing something can have serious, even catastrophic consequences. Creating significant openings requires significant diligence: Factoring in wind and understanding what effect it may have on the vent opening is an important part of this “due diligence.” Remember, it is not always enough knowing when to do something – but in the case of determining a leeward or windward opening – when NOT to do something.

1 NISTIR 7213-Effect of Positive Pressure Ventilation on a Room Fire-Stephen Kerber/ William D. Walton
2 NIST Technical Note 1498
Evaluating Positive Pressure Ventilation In Large Structures: School Pressure and Fire Experiments– Stephen Kerber /Daniel Madrzykowski
3 NFPA 92A Standard for Smoke-Control Systems Utilizing Barriers and Pressure Differences (NFPA, 2006)

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