MWFRS Question 4

[Lion06]

This is probably going to sound silly, but I have to ask.
I was taught in school (and have seen others do it this way) that for MWFRS pressures, you neglect the internal component because they will counteract each other (the windward wall internal pressure counteracting the leeward wall internal pressure).
That assumes that the internal pressure is either causing pressure or suction on both walls at the same time. Isn't it possible for the interior pressure to be causing pressure on one and suction on the other? That seems to me to be the worst case.
Additionally, if ASCE 7 intended to have the internal pressures ignored for MWFRS then why is it included in the calc?
So the first question is this:
Do you consider what is shown as case 1 or case 2 in the attached sketch when calc'ing MWFRS pressures?

The second question is this:
The minimum pressure for MWFRS is given in 6.1.4.1 as "The wind load to be used in the design of the MWFRS for an enclosed or partially enclosed buliding or other structure shall not be less than 10 psf multiplied by the area of the building of structure projected onto a vertical plane normal to the assumed wind direction....."
I take this to mean that for an enclosed (or partially enclosed) structure that the minimum wind pressure (for both windward and leeward combined) is 10 psf, not 10 psf minimum for windward and 10 psf minimum for leeward.
An open structure is another story, but my question is for enclosed structures.

 

[COEngineeer]

EIT, you either have positive or negative pressure inside a building. Lets say there is no window on the windward wall, and there is a window on the leeward wall. This will create suction. It will create positive pressure if you flip the walls. Just like pressure in a tank, you cant have positive pressure and negative pressure in the same tank.

Never, but never question engineer's judgement

 

[Lion06]

That's what I thought, but why is it in the equation?

 

[StructLin]

My understanding is that in the real world, in one enclosed space, internal air pressure will either in outward pressure or in suction. No such thing as one wall outward, another wall suction, which is against natural law.
If there is two room, it is possible that one room in outward pressure, another room in suction, but it will be in two enclosed spaces.

For Second question, my understanding is 10psf is the total (includes both windward and leeward).

If I am wrong, please correct me.

 

[KBVT]

Internal wind pressures cancel out for wall pressures but not for roof pressures.

 

[Lion06]

I definitely agree with that, but does anyone know why it is in the equation?

 

[COEngineeer]

Probably for partially enclosed system. And it is 10 psf combined.

Never, but never question engineer's judgement

 

[miecz]

StructuralEIT-

Agree with the previous posts. For buildings shaped like the one in your sketch, the pressures will cancel. For gabled roof buildings, where one wall is taller than the other, they may not.

 

[dougantholz]

One - remember that the writers of ASCE 7 have written in such a way that the more a person hones in on a design, the lower the forces will get. If someone doesn't know enough to take it out of the equation then they end up with a conservative pressure.
Two - you can look at the MWFRS on a single wall (not the whole building) and in looking at a single wall you would have internal and external pressure acting on it.

 

[COEngineeer]

But doug, if you look at single wall, then you should probably look at components and cladding. I think EIT knows the concept. He was just wondering why we have equation for internal pressure or suction since we dont use it. I think it is for partially open building MWFRS.

Never, but never question engineer's judgement

 

[Lion06]

I am evaluating some new software to determine if we are going to buy it or not and some of the things it is doing with wind pressures was really throwing me for a loop and I had to question everything I thought I knew about wind!

Thank you gentleman, it's been a pleasure....... as always!

 

[COEngineeer]

Hmm.. I am still wondering what Miecz said though. If one wall is taller than the other, I would think the internal pressure should cancel eachother anyway. I dont know.. i feel dumber and dumber everyday.

Never, but never question engineer's judgement

 

[Lion06]

COEngineer-
I believe they will. Not necessarily from the walls, but from the vertical projection of the sloped roof. If one wall is taller than the other the roof is either sloped or stepped. Either way, there are equal vertical projections on both the windward and leeward side of the building to cancel each other out.

 

[PMR06]

I think KBVT is on to the right idea. The lateral components may cancel out, but what about the vertical component?

The vertical component of internal pressure on the roof does not cancel out with the pressure on the ground without first going thru a vertical element (walls, columns, etc). You have to include the extra tension or compression in the vertical element when analyzing the MWFRS.

 

[dougantholz]

There are instances where you are looking at a large segment of wall that MWFRS is appropriate. The rule used to be 700 sf or more was main wind, less was C&C. I don't think that's code-ified anywhere, but in 95 when I was in school that was in the code.
Remember that formulas are written to be general and conservative - so you can use it in a variety of situations. If you have a copy of the "Wind Provisions for ASCE7-xx" a book put out by ASCE Press, it shows how internal pressure effects design.

 

[Lion06]

I understand how internal pressures affect design and I understand that the roof pressures need to be considered.
The latter was not part of my question, and the former was only to be sure I was understanding the MWFRS pressures properly, not how they relate to MWFRS for a large component/cladding.

 

[KBVT]

Another possibility could be if you have a discontinuous diaphragm between the windward and leeward walls. If the diaphragm restraining the windward face does not extend to the leeward wall, it will not have the benefit of the internal pressures on the leeward wall. So if you have suction pressures on the windward wall they will have to be added to the windward external pressure.

 

[nutte]

KBVT has it. This is why they have internal pressures in the MWFRS equations. For most buildings, where the diaphragm is continuous and the windward and leeward walls are connected, the internal pressures will cancel, having no net effect on the MWFRS forces. If the windward and leeward walls are not connected, one wall will see outside pressure and internal pressure, without the benefit of the counteracting internal pressure on the opposite wall, since it is not connected to it.

 

[UcfSE]

Saying the wall mwfrs internal pressures cancel assumes the walls are parallel. If resultant forces on the walls are not parallel then the internal pressures will not just cancel out.

 

[miecz]

On second thought, the shape of the building really shouldn't matter, now. Toss my original reply (At least I said "may".)