Wind Uplift Forces on Cold Storage Roof 1

[Delmarva_Struct]

For an enclosed building, the wind uplift force on a roof is caused by difference of pressures above and below the roof membrane. The motion of the wind blowing over the roof creates a area of low pressure and the air under the roof membrane is at a higher pressure and so the wind tries to move from below the roof membrane to above the roof membrane and this creates the uplift force.

Now my question is related to cold storage facilities. Considering that these facilities are constantly keep below freezing. The pressure within the building will be very low. I don't see how this pressure can be higher the pressure over the roof to cause an uplift force.

In this case, can I ignore uplift forces on roof membrane for cold storage facilities?

 

[XR250]

Why is the pressure lower because it is cold?

 

[Compositepro]

How do you think that pressure and density are related in this specific case? Please take a moment to think a little, before you dig your hole deeper.

 

[Pinwards]

So, even if that were true, what about if the wind blows during construction before it's chilled ... or if the freezer is taken offline for repairs/renovation. Would you be ok with the roof lifting off then with your stamp on the design?

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[driftLimiter]

This point of view to my knowledge isn't present in the building code.

 

[Delmarva_Struct]

I thought about it for a moment and have more question than answers.

What causes the positive or negative internal pressure i.e. the term GCPi within a building.

 

[SWComposites]

Depends on the ratio of managers to workers, the higher the ratio, the higher the temperature and pressure.

 

[Delmarva_Struct]

I guess what I was tryin to get at is that we will not have the internal pressure calculated using the positive GCPi term in case of cold storage facilities.

 

[EngDM]

I mean I see what you are getting at with the whole ideal gas PV=nRT equation, but I disagree with your conclusion. The low pressure zone creating by high velocity wind deals with bernoulli's principle which doesn't really depend on whether or not the vessel is closed and is more applicable to atmosphereic or large system applications.

To see any real measurable difference in the ideal gas equation you would need an air-tight or roughly air tight building that would not allow for any pressure equalization with the outside. If we are assuming that the pressure is lower due to temperature being lower, then it is a negative pressure room and any leaks would just equalize the pressure.

 

[Delmarva_Struct]

Thank you EngDM!

 

[Aesur]

GCPi is a concept of a pressure tank with high and low pressures with varying degrees of air leakage, but also has to do with building movement and deflection as wind hits a wall and compresses the air inside or pulls on a wall and expands the air inside. Now to get to your question, can you guarantee the building will ALWAYS be cold, even in the event of a power interruption or potential future use changes of the building. I would be very surprised if you could answer yes to that. The way I look at it is, why make complicated decisions that in the grand scheme of things probably make little to no impact on the design. Designing to code offers a degree of protection or at least justification for your designs should something happen, so why risk it.

I really hope a code writer/PHD doesn't see this and think, "oh great idea, let's add temperature differential to the wind loading", it's already complex enough and only gets worse every cycle.

 

[lexpatrie]

No, you can't ignore uplift forces because you don't feel like it.

This discussion does not need to go any deeper. This nonsense about the building not always being cold due to service interruptions, power failures, whatever isn't even valid, because the "no uplift" situation is NOT occurring in the first place. Cold building seeing lower or no uplift is not a recognized phenomenon.

Deviating from the design standard should only be done with documentation justifying it. Some kind of incorrectly thought through "air pressure" argument is not sufficient.

 

[XR250]

This got me thinking. Say you have adhered membrane roofing on top of say a concrete slab as the substructure. Wind blows over the roof and creates a pressure drop. What is the pressure differential on the membrane itself? Is there atmospheric pressure in the glue between the membrane and concrete?

 

[human909]

Technically yes there is atmospheric pressure in the glue. Just like there is atmospheric pressure on the membrane and also on a cube of steel, a rock or your body.

But unless there is a gas, or dissolved gas or another substance that is going to expand in volume significantly then nothing exciting happens. (Eg the solid expands ever so slightly.)

Remember we are talking pressure changes if around. 1% here so state changes are unlikely and the amount of free has in a glue would be minimal unless some body only glued a fraction of the area.

 

[JStephen]

"The pressure within the building will be very low."
That assumption is incorrect. When there is no wind, pressure within the building will be atmospheric pressure exactly the same as outside the building. When there is wind, differences in velocity etc. due to Bernouli's effects will cause differential pressures regardless of the temperature.

 

[Once20036]

This got me thinking. Say you have adhered membrane roofing on top of say a concrete slab as the substructure. Wind blows over the roof and creates a pressure drop. What is the pressure differential on the membrane itself? Is there atmospheric pressure in the glue between the membrane and concrete?

Me too. There was a conversation here a while ago about uplift on a topping slab on a precast double tee.
Is the uplift applied to the topping slab, such that it might need to be anchored back to the double tee? Or is the uplift applied to the topping slab/precast assembly, such that the topping slab cannot fail in uplift independently of the double tee?
I don't think I was convinced one way or another back then, and this feels like the same question.

I`m not sure if this is the exact thread that I`m remembering, but its definitely got the right vibe:

https://www.eng-tips.com/viewthread.cfm?qid=478619

 

[EngDM]

Or is the uplift applied to the topping slab/precast assembly, such that the topping slab cannot fail in uplift independently of the double tee?

Consider that the uplift is due to the velocity of wind on top being higher creating a lower pressure zone, the uplift is generated from a force on the bottom (high pressure zone) pushing up, not the low pressure zone grabbing the top and lifting it. If it was grabbing, then you'd see ballasts and other loose elements just being lifted off the roof.

It's like a balloon being filled, the air is pressing out; the outside air isn't pulling on the balloon.

For the topping to lift the high pressure zone would have to be between the membrane and the slab.