MWFRS Question
MWFRS Question
(OP)
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.
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.






RE: MWFRS Question
Never, but never question engineer's judgement
RE: MWFRS Question
RE: MWFRS Question
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.
RE: MWFRS Question
RE: MWFRS Question
RE: MWFRS Question
Never, but never question engineer's judgement
RE: MWFRS Question
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.
RE: MWFRS Question
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.
RE: MWFRS Question
Never, but never question engineer's judgement
RE: MWFRS Question
Thank you gentleman, it's been a pleasure....... as always!
RE: MWFRS Question
Never, but never question engineer's judgement
RE: MWFRS Question
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.
RE: MWFRS Question
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.
RE: MWFRS Question
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.
RE: MWFRS Question
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.
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It seems like there are a lot of iterations that can be had with this.
UcfSE-
I believe that if the diaphragm is connected to all walls that the orientation of the walls does not matter. The projection of the walls in each direction will be the same.
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When designing for base shear I usually add the internal pressure to the windward wall, nothing to the leeward wall.
RE: MWFRS Question
If you have any shape container and pressurize the inside, and have the base on a frictionless surface it will not move. It doesn't matter the shape.
RE: MWFRS Question
Never, but never question engineer's judgement
RE: MWFRS Question
Okay, so the forces cancel laterally. But as KBVT mentioned way back, the vertical forces don't cancel out. That has to be why the pressure forces are written into ASCE7's formulas. So if you have any shape container and pressurize the inside, and have the base on a frictionless surface it will move. Up. (Just ask Dorothy.)
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Agreed, but my question was really geared toward the walls. This thread has meandered well beyond its intended path.
RE: MWFRS Question
Perhaps you should email the code writers instead of continuing posting here. After nearly 30 replies you are no closer to what you want. Let us know what they say.
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RE: MWFRS Question
Keeping the aforementioned in mind, I agree with the previous poster in that the internal pressures should always cancel out for net MWFRS lateral pressure, but should be considered for uplift. I always take the roof pressure plus the positive internal pressure when looking at MWFRS uplift, which I usually only reserve for foundation elements. Is there a reason why I should do otherwise?
RE: MWFRS Question