ASCE 7-10 Fig. 28.4-1 Sidewalls and Flat Roofs 1

[Steel5]

I have a 2 part question about ASCE 7-10 Fig. 28.4-1. I've mostly used Ch. 27 for MWFRS wind, so I'm less familiar with Ch. 28.

First, why does load case 'A' not have side wall pressures?

Second, when you have a flat roof (zero degrees), you no longer have a definitive ridge, so which load case applies in which direction? Could you consider one load case for both directions?

If you could use one load case for flat roofs I would think 'A' is more applicable. The negative wind pressure on the roof is higher toward the windward edge and lower toward the leeward edge, which is very similar to how Ch. 27 would have you do it. But then it still makes me wonder why load case 'A' doesn't have sidewall pressures; if I were using Ch. 27 it would give me sidewall pressures.

Side note: I'm aware of footnote 7 regarding flat roofs, but that still doesn't seem to address my question.

Thanks for any help.

Edit 3/26/17 12:04pm: fixed one of the chapter references.

 

[ajh1]

We work with this chart all the time, or at least the computer programs we coded to match the chart do. Our take on your first question is that the two sidewall suctions are equal and opposite so don't have any particular impact in regard to a MWFRS solution. The magnitudes of Load Case B are in the same range and will accommodate the net forces in that direction. C&C values will be higher and control all of the individual wall components for all walls.
We don't tend to do purely flat roofs but do single slopes a lot. Notes 7 and 8 tend to cover that condition. Load Case A was originally created to deal with the primary framing orientation, i.e., frames spanning from eave to eave with a ridge in the "center". Load Case B is used in the direction where bracing as opposed to primary framing is used. At least that is our take on the process.
IF you think of wind at the corner of the building as the illustrations show, you will have a zone on the roof of higher pressure regardless of whether you are using A or B. Also Zone 2 pressures are higher than Zone 3 pressures, although not to the same extent that 2 and 2E or 3 and 3E differ.
Obviously if you have a small enough building with a flat roof and joists only spanning the full length being walls, i.e., no internal primary framing menbers, you get to pick and choose which direction is which although I would probably still run both load cases (one way or the other) just to be sure of coverage.

 

[Steel5]

Thank you for the detailed response, that makes sense.