Open and free standing structure

[engr567]

I am designing a free standing one story commerical shade (flat roof and no walls): 30 PSF snow load, 20 PSF dead love and 90 MPH (exposure B) wind. Since there are no side walls (it is all open on sides), I assume there is no lateral load from wind, except there will be an uplift on roof. I know there will be a lateral force acting on the structure due to dead and live load. Does IBC specify anything in this situation? How much lateral force do I consider in design, the county is looking for a calculations for it.

 

[msquared48]

I would use 10 psf minimum on the projected area for wind - there has to be some structural depth here, plus throw in the seismic forces you calculate for the structure.

You need to show something on paper, even if it is next to nothing.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[engr567]

Mike,
Thanks for the quick response. Do you know if there is a section in IBC that talks about this?

 

[Ron]

Agree with Mike. Often the roof will have a fascia that is 6 to 8 inches high as well as the exposed column faces. All of this accounts for something. I work in a higher wind area and the lateral load from small projections can often make a difference in the column size and in the footing design, particularly for OT moment.

 

[woodman88]

With a 30 psf snow load I would design the wind with a 30% of the uplift load as a lateral component. If it changes the design I would reduce it to 20% or 15%.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[AELLC]

If there is 30 psf now load you get wind lateral pressure acting on the height of snow. I normally wouldn't be so nit-picking except for the Exp B and only 6"-8" of fascia.

Aerodynamically, you get some wind drag even though there is no appreciable profile. I don't have a copy of ASCE but this was
discussed before.

You probably won't see this mentioned in IBC because so much has been referenced to ASCE.

 

[BAretired]

If the underside of roof is framed with beams, joists, trusses or other sharp-edged sections, you should consider the respective areas modified by a shielding factor. The NBC (Canada) provides shielding factors. I don't know about the IBC.

If top and bottom surfaces are dead flat it may be prudent to consider a wind drag force of, say 5 psf of plan area.

BA

 

[DST148]

@engr567: ASCE-7 gives 4 wind load cases for MWFRS. Case 2 is 75% of the wind load acting simultaneously in two orthogonal directions. This is really simplification of the wind hitting the structure at an angle. The same logic could be extended to the wind loads on the open roof structures. You can assume wind hitting roof at an angle, the vertical component would then be uplift on the roof and the other component the lateral load.
Long time ago, for an industrial structure open at both ends, my supervisor asked me to design horizontal wind bracing in plan at the roof level for 5 psf on the plan area of the roof. Of course, this load was in addition to the wind acting on the exposed roof truss members. I never questioned him the basis for his assumption but now I think it could be to account for the effect of the wind drag on the roof.

 

[spats]

You also need to consider p-delta for stability under gravity loads, and add that to whatever the wind/seismic effects are.

 

[spats]

Oops! You did kind of say that... sort of.

 

[DST148]

@BAretired: I apologize I missed your second paragraph and did not realize that you had already mentioned about the wind drag on the roof.

 

[BAretired]

No problem, DST.

BA

 

[wannabeSE]

ASCE 7-05 has provisions for wind loads on open structures in section 6.5.13. The MWFRS pressure is normal to the roof slope. Unless the roof is completely flat, there will be a lateral component with this wind load. Also, if the roof slope is less than 5°, the code states that a parpapet wind pressure should be applied to the facia.

Next time I work on an open structure, I will keep in mind BA retired's comments on wind drag.

 

[engr567]

thanks all for the input