huangyhg

mwfrs question
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.
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
that's what i thought, but why is it in the equation?
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.
internal wind pressures cancel out for wall pressures but not for roof pressures.
i definitely agree with that, but does anyone know why it is in the equation?
probably for partially enclosed system. and it is 10 psf combined.
never, but never question engineer's judgement
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.
one - re
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
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!

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