ASCE 7-10 Table 29.4-1 Wind Load Case B

[rlmhawk]

Wind loading on walls/signs in Case A of Table 29.4-1 represents a uniform wind load pattern placed on the structure with its resultant acting at the centerline of the fence/wall. Wind loading in Case C represents an oblique loading scenario applied as a stepped load pattern along the wall/sign length based upon length/height ratios. Both of these cases are pretty well defined as far as their application to these structures is concerned. However, load Case B is somewhere in between the two, representing an oblique loading scenario with its resultant acting at 20% off the centerline of the wall/sign. To me, it is not clearly defined as to how you are to apply the load along the full length of the structure as the others are and thus is rather subjective. Such load application is similar to applying a triangular load (in plan) to the wall/sign, where the resultant would be located at 1/3 of the length of wall from the windward end (approximately 7/10 of the wall). However that doesn't seem realistic for a wall wind load scenario given one end gets no load and the other a very high psf load. It seems there could be a wide range of load distribution interpretations that might place the overall resultant at this off center location. To me, a distribution that applies a minimum load to all the wall seems more appropriate than something resembling a triangular approach, most likely one that also uses an alternate stepped approach for example. How have you interpreted the application of wind in load Case B?

 

[phamENG]

Apply each and design for the worst case. I've found Case B usually applies to pylon signs and it either induces a torsion in the pole or an imbalance in a pair of support poles.

If your sign is a square, then Case C will equal Case A and you'll need Case B's "arbitrary" 0.2B to give you the eccentric load application that Case C does more accurately for longer walls/signs.

 

[rlmhawk]

I usually am looking at tall screen walls for this type of loading, and thus I guess my question applies more towards that type of application. i always apply my own interpretation of Case B loading, but I am curious how others approach it. You may well be right regarding torsion on a pylon sign.

 

[dauwerda]

Case B is to account for torsion/unbalanced load, similar to what we do with rigid diaphragms and accidental torsion (it is still a normal load, it is not accounting for oblique loading like case C is)
Keep in mind, the code does not have you calculate the load in psf to be applied over an area. It includes the area in the force equation (29.4-1) to calculate a resultant point load. The intent is not to have it act as a distributed load to design the wall/sign with, the intent is to check the effects of torsion on a global level - what forces do the sign/wall deliver to the underlying support structure and foundation. As phamENG mentioned this will mainly be a controlling issue in situations he noted.