What are the effects of Barometric pressure drop on a structure?

[Ron247]

I saw in a report where damage to a house structure was partly attributed to the drop in barometric pressure due to a tornado. I know barometric pressure drop indicates the intensity of a tornado or hurricane is getting greater. The increase in intensity causes an increase in wind speed which in turn creates more pressure/suction on the surfaces.

My main question is does the barometric pressure drop directly cause structural damage of any significant amount? I think Hurricane Wilma dropped the normal 14.7 psi to 12.8 psi and that was considered an extreme drop. That is a change of about 278 psf in the barometric pressure. I am asking this based on a building not being 100% air-tight.

 

[r13]

I think the change in barometer pressure causes pressure imbalance between in-house and the open environment. But have no clue how much the drop will cause significant differential pressure on the building structure.

 

[WARose]

If we are talking a building/structure that isn't air-tight....I would think the net effect of any atmospheric pressure would be zero. Granted a sudden change (before there was time for the pressure to even out) is possible.....but you've probably got bigger problems than that when it happens.

 

[3DDave]

Part of the barometric pressure drop in tornadoes is the matching dynamic pressure increase. The inside of the building sees only the static pressure, while at least one side of the structure is seeing the total pressure - that 278 psf that is mentioned.

 

[Ron247]

I guess a better way of posing the question is:
Assuming there was only a barometric decrease in pressure and there was not any wind blowing, is it reasonably possible to sustain damage to a house? Again the house is not air-tight and the pressure decrease is not instantaneous.

A pressure differential of 278 psf between 2 faces of a typical wall or roof would tend to explode the surface.

 

[EdStainless]

And in a tornadic event this may happen in seconds.
This might loosen some things, but it really is the lateral winds that tear things apart.
I recall a case of being able to see 'fog' forming because of the sudden pressure drop.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[1503-44]

Drops of nearly 200 minibars (3 psi) have been recorded by storm chasers. Even if a house or building is not "air tight", the drop in exterior pressure takes place faster than the building can depressure, so the building explodes due to the 3psi x 144in2/ft2 = 432 psf interior surface load. Close explosive charge blasts can reach 300mb, but typically last less than 0.5 sec.

 

[human909]

Hurricanes and tornados are very different beasts, best not to confuse them.

"I think Hurricane Wilma dropped the normal 14.7 psi to 12.8 psi and that was considered an extreme drop." The pressure drop would generally be more of a concern for causing a severe storm surge which can be as bad or worse than the wind damage.

 

[WARose]

Assuming there was only a barometric decrease in pressure and there was not any wind blowing, is it reasonably possible to sustain damage to a house? Again the house is not air-tight and the pressure decrease is not instantaneous.

I'd say: no. At least, I have never seen this noted in any wind loading text I've seen (as being a issue that outweighs the normal design wind loads).

The way I've seen barometric pressure changes demonstrated (physically) is something like a sealed (as in air-tight), thin water bottle at different elevations on a mountain.

 

[phamENG]

Generally, pressure drops (even for hurricanes) are not instantaneous. So a typical building will adjust with the ambient closely enough to prevent significant damage. But a tornado coming by can, as others have stated, create a sudden drop that happens so fast that the building doesn't have a chance to reach equilibrium. So the differential pressure creates it by blowing out a window or ripping off a roof.

So to your original question, yes it seems plausible.
To your second question, no, if the drop isn't instantaneous (or nearly so, as is possible with a tornado) it shouldn't cause notable damage.

Does anyone have a copy of ICC-500 or the comparable FEMA document? Is there a comentary that may go into it? ICC-500 is supposed to be a good resource for tornado and hurricane shelters.

 

[BridgeSmith]

Seems plausible to me that the atmospheric pressure change could cause damage. Design wind load for most structures is around 40psf, about 1/10 of what could be generated by a tornado, if the house was tightly sealed and the drop was quick. So, even if it's not tightly sealed, and the drop is a little slower, it's plausible that a pressure 2 to 3 times the design wind pressure could be generated.

I also remember being told to open windows if a tornado is coming, to minimize damage from windows blowing out.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[WARose]

I also remember being told to open windows if a tornado is coming, to minimize damage from windows blowing out.

I was watching the Weather Channel once.....and they took on "twister myths". One of them was the old wise tale that if you open the windows/doors of a structure during a tornado.....that will protect it. Well they debunk that one (even though it's just a single door open).

https://www.youtube.com/watch?v=XCjxNJQmWnM

On a personal note: debunked by none other than my old fluids professor Dr. Reinhold. I don't think he's aged a day in 20+ years!

 

[3DDave]

In looking at hurricane damage one frequent initiator was breeching of the garage door which led to dynamic/stagnation pressure inside the structure blowing the roof off followed shortly by the now unsupported walls leaving. Similar effects from losing large windows from debris.

Hypothetically, nothing in the described situation will happen because the hypothetical situation won't happen; there won't be a sudden pressure drop without induced wind velocity.

 

[1503-44]

There are two effects, static pressure, independent of winds, and dynamic pressure, generated by wind. Hurricane low pressures (static pressures) are measured within the eye of a hurricane, a place where winds are calm. Static pressure changes in tornados happen quickly, hurricanes slowly. The bulk of damages in either type of storm have been caused by dynamic wind pressures.

Storm Shelter Design

Design wind speeds found in ICC 500 are based on records of measured wind speeds in hurricanes and estimated wind speeds for tornadoes based on a 10,000 year Mean Recurrence Interval (MRE – the inverse of the probability that the event will be exceeded in any one year). Contour maps are presented in ICC 500 for both tornadoes and hurricanes for the continental U. S. and its territories. For site-built storm shelters, design wind speeds may be determined using these maps. For factory-built, or relocatable storm shelter structures that may be installed in any location, the ‘worst case’ design wind speed should be used. These design wind speeds are as high as 250 miles per hour for tornado shelters and 235 miles per hour for hurricane shelters. These elevated speeds can result in wind pressures that are nearly five times as great as the wind pressures typically required for the design of buildings.
Wind-Induced Pressures

Wind pressures for storm shelters are calculated using ASCE 7-10, Minimum Design Loads for Buildings and Other Structures. Due to the unique nature of extreme wind event loadings, ICC 500 specifies the variables required to calculate the Main Wind Force Resisting System (MWFRS) and Components and Cladding (C & C) loadings. Values are specified for the directionality factor, exposure category, and topographic factor and guidance is given on selecting the proper internal pressure coefficient, GCpi.

Wind-induced internal pressures from winds acting through openings are added to external pressures. Internal pressures have a large effect on the forces acting on the shelter’s structure and its connections to the foundation. Atmospheric pressure change due to the reduced pressure in the vortex of a tornado can also result in internal negative and external positive, or ballooning, pressures. While significant in tornadoes, atmospheric pressure change is not a large factor in hurricanes where winds are more straight-line in behavior (due to the storms diameter) and somewhat cyclic in nature.

The internal pressure coefficient, GCpi, accounts for these pressure induced positive and negative load changes. The value of this coefficient varies with the ratio of openings to shelter volume or the degree of enclosure as defined by ASCE 7, i.e., whether the building is considered enclosed or partially enclosed. Small residential shelters are usually designed as enclosed spaces. Community shelters are typically defined as partially enclosed buildings due to the ICC 500 requirement for the largest operable opening on a windward side of the shelter to be considered open during peak wind speed of the storm.

https://www.structuremag.org/?p=8849

 

[phamENG]

Thanks, 1503-44

To pull a key sentence out of that:

Atmospheric pressure change due to the reduced pressure in the vortex of a tornado can also result in internal negative and external positive, or ballooning, pressures. While significant in tornadoes, atmospheric pressure change is not a large factor in hurricanes where winds are more straight-line in behavior (due to the storms diameter) and somewhat cyclic in nature.

 

[Ron247]

Thanks 1503-44,
That excerpt appears to directly address the topic. The only part of that I am curious about is

Atmospheric pressure change due to the reduced pressure in the vortex of a tornado can also result in internal negative and external positive, or ballooning, pressures.

I would think the reduced barometric would cause internal positive and external negative to be "ballooning".

 

[BridgeSmith]

Well they debunk that one (even though it's just a single door open).

I'm not sure the video really 'debunks' the 'damage due to pressure change' effect. They only mention applying wind. With an opening on the windward side, it's not surprising that it's worse; the effect that 3DDave mentioned:

In looking at hurricane damage one frequent initiator was breeching of the garage door which led to dynamic/stagnation pressure inside the structure blowing the roof off followed shortly by the now unsupported walls leaving.

Granted, as he also mentioned, you don't get the sudden pressure change without the high winds, so it's probably a moot point for most houses. It could matter for stronger and/or heavier structures, which would withstand the wind, but with windows that would not withstand the pressure.

There's also the leeward side of the building to consider, for both static pressure changes and dynamic wind-induced pressure increases. Again, I think 3DDave hit on a scenario where opening windows and doors could help, by minimizing the pressure buildup inside the structure from wind blowing in through an opening on the windward side. If there's enough open on leeward side the pressure buildup is minimized.



Rod Smith, P.E., The artist formerly known as HotRod10

 

[WARose]

Granted, as he also mentioned, you don't get the sudden pressure change without the high winds, so it's probably a moot point for most houses. It could matter for stronger and/or heavier structures, which would withstand the wind, but with windows that would not withstand the pressure.

Well the point is: any damaging atmospheric pressure change is going to be so confined to the event (i.e. the vortex of a tornado for example).....it would be hard to separate that from the rest of it.

 

[1503-44]

If there is negative differential pressure, than those forces are contributing to stress. Perhaps the issue is if those negative pressure forces are responsible for breaking the building, or not. The answer seems to depend significantly on the design of the structure itself, but in any case once the windows go, the dynamic pressure is inside, so that too creates outwardly directed forces. In this atomic bomb test video

https://m.youtube.com/watch?v=DXg2P9dx-GM

there appear to be some buildings getting knocked down, whereas others seem to explode. Either way, the end result is destruction.

Man... those poor guinea pigs in the trenches.... Gotta' feel for them.