HVAC Screen Wind Load

[cjw99]

I am designing the lateral bracing system for a rectangular building (approx. 200'x 160'x 40' tall). Seismic deisgn does not apply for this building. The roof has a 90'x90'x 8' tall U-shaped screen wall around the hvac units (no roof over the units). My coworkers and I are debating the proper wind load to apply to the screen walls.

Typically our engineers are applying the combined windward + leeward parapet wind pressures to each of the screen walls. My combined parapet wind pressure is 47psf (28.8psf windward + 18.2psf leeward). Under this methodology, I need to add 33.8 kips (47psf x90'x8')of wind load in the direction with 1 wall and an 67.7 kips (47psf x90'x8'x2) of wind load in the direction with 2 walls to account for the screen.

This seems very conservative to me. How are other engineers approaching this situation? If you disagree, can someone point out a code reference that would lead to using a reduced load. Thanks.

 

[structuralengr89]

I don't think you are suppose to be adding the windward and leeward winds together.

For an HVAC screen, I would get my pressure by going to ASCE7-05:
Then 6.5.15 Design Wind Loads on Other Structures

Determine force coefficient, Cf from Figure 6-21
Chimeys, Tanks, Rooftop Equipment, and Similar Structures.

Your windload seems a bit high.

 

[DaveAtkins]

If you are interested in the wind load for the design of the screen wall itself, it would be considered "rooftop equipment" as mentioned above.

If you are interested in the wind load on the screen wall which will be added to your lateral braces, it would be considered "main wind force resisting system" and will be a much lower load.

DaveAtkins

 

[jike]

I have always use "Free Standing Walls and Solid Signs" for the C&C design of parapets but the Cf is very similar in value to that for "Chimeys, Tanks, Rooftop Equipment, and Similar Structures".

Dave makes a good point to make sure you are using the lower loads of MWFRS to add forces to the bracing of the building. Under MWFRS, use GC = +1.8 for windward parapet and -1.1 for leeward parapet.

 

[cjw99]

Well I may have mispoke slightly in my original post. I'm technically not adding my windward and leeward pressures. I am subtracting them, but my leeward pressure is negative, which makes the overall result additive (28.2 psf - -18.2 psf) = 47 psf.

I could see the logic in classifying the screen as a "rooftop structure" per section 6.5.15.1 of ASCE7-05. This could reduce my overall wind load on the bracing system substantially.

Out of curiousity, what would make this wall different from a true parapet? Both are walls projecting up higher than the roof line. It seems to me that they would experience similar loads.

 

[MiketheEngineer]

If it is truly a screen - some air will permeate through??

Might ask mfg for suggestions. I agree - Loads seem high....

 

[DaveAtkins]

cjw99,

I think you are still missing our point. Use MWFRS loads for the design of your bracing system.

DaveAtkins

 

[cjw99]

Dave,

I think there's 2 different issues here. First off, I already am using MWFRS wind loads to design the bracing system. My error appears to be in the application of the MWFRS parapet wind load. After rereading section 6.5.12.2.4 of ASCE7-05, I realize that the pp is already a combined load on the parapet and therefore I don't need apply both the loads from a windward parapet and a leeward parapet.

For my building, qp=18.8psf. What I had been doing was : (Using ASCE7-05)

Total parapet design presssure = 1.5(18.8psf) - (-1.0)(18.8psf) = 47psf

After realizing that pp is already a combined net pressure, what I would do now is:

Total parapet design pressure = 1.5(18.8psf) = 28.2 psf

Anyway, regardless of all of this, the original question is whether or not anyone else would use MWFRS parapet loads on the screen walls when designing the lateral bracing system. It appears that everyone who responded would classify this as a "Rooftop Structure" and use the provisions in section 6.5.15.1.

Thanks for everyones imput.