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.

 

[Lion06]

So, I guess the next question is do all of you take this into account when doing your wind design? do you assume wind in +X with sucktion for internal, then wind in +X with pressure for internal?

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.

 

[civilperson]

Looks like a duplicate post.

 

[Lion06]

duplicate post?

 

[UcfSE]

The wind loads act normal to the surface, not normal to the projected area, so there should be a difference.

 

[dougantholz]

UcfSE - if the wind isn't blowing, and the HVAC is pumping the most air out of the building it can, the building will not have a net shear. Doesn't matter what the configuration of the lateral force resisting system or outside walls are. You might have a very small force due to non-parallel systems if you looked at an individual component - but your sum of forces in the X dir should equal zero.
When designing for base shear I usually add the internal pressure to the windward wall, nothing to the leeward wall.

 

[Lion06]

Agreed, but if you take the components in any direction they will equal out.
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.

 

[COEngineeer]

Someone need to simplify wind calculation again. It is getting out of control!

Never, but never question engineer's judgement

 

[miecz]

Structural EIT

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.)

 

[Lion06]

miecz-
Agreed, but my question was really geared toward the walls. This thread has meandered well beyond its intended path.

 

[UcfSE]

Agreed, I was thinking of something else.

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.

 

[Galambos]

Have you considered the anchorages of a metal building that uses a girt system? I think everyone is thinking of walls that distribute directly to the diaphragm and not the columns.

 

[abusementpark]

The case where the walls are not parallel is a unique one because I do not believe the wind code actually presents a method for calculating the MWFRS for such a case, so we must bear in mind that the pressures given by code are technically not valid for such a case. Unless the magnitude of the project merits wind tunnel tests, we have to use engineering judgment.

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?

 

[jechols]

Just as Galambos touched on, the local bending of the metal building frame in a horizontal wall grit system would be affected by the internal pressure. The grits would be designed for C&C but the frame would only be designed for MWFRS pressure.