Design Wind Load Cases ASCE 7-10

[PT_159]

I am doing a design on an enclosed building with flat roof using the MWFRS of buildings of all heights( I have to use this method). I have all my wind loads but now Sec 27.4.6 (ASCE 7-10) states that I have to design my wind loads for load cases 1-4 on figure 27.4-8. My question is how are my wind forces applied to these 4 cases? How will I determine which case will govern and what about my Roof wind forces? Everything before this was pretty straightforward but now I have ran into a dead end. I would greatly appreciate assistance to move forward with this issue.

 

[MrHershey]

Generally there's 24 load cases that arise from the 4 depicted in Figure 27.4-8. The 12 listed below and then 12 more from taking the cases below and multiplying each by -1 (so all loads in opposite direction).

How I've understood them:
1. Case 1 in positive X direction
2. Case 1 in positive Y direction
3. Case 2 in positive X direction, positive rotation
4. Case 2 in positive X direction, negative rotation
5. Case 2 in positive Y direction, positive rotation
6. Case 2 in positive Y direction, negative rotation
7. Case 3, positive X and positive Y
8. Case 3, positive X and negative Y
9. Case 4, positive X, positive Y, positive rotation
10. Case 4, positive X, positive Y, negative rotation
11. Case 4, positive X, negative Y, positive rotation
12. Case 4, positive X, negative Y, negative rotation

 

[DCBII]

The critical case is usually Case 1, but there are times when the others are important to check. Usually you can tell which cases are pertinent for your particular structure. The torsional cases usually aren't critical, unless your lateral system is laid out in a way that is susceptible to torsional issues. The orthogonal cases only become a problem if your structure is sensitive to orthogonal effects (e.g. a column common to two perpendicular lateral systems).