ASCE 7-10 Question

[spats]

I am designing a MWFRS per Chapter 28, Part 1 (envelope procedure) and using Figure 28.4-1. I have a single slope 1:12 roof. Per Figure 28.4-1 Note #8, Zone 2/2E applies for one half the width or 2.5 times the eave height of the windward wall, whichever is less. The remainder of the roof is Zone 3/3E. This provision seems obvious if the low side is the windward wall. However, if the high side is the windward wall, does Zone 2/2A still apply, or is the entire roof area Zone 3/3E because it pitches away from the wind direction. I believe 2/2A still applies, but I want to make sure.

 

[Hokie93]

I don't believe the Envelope Procedure is intended to be used with monoslope roofs. The User Note at the beginning of Chapter 28 indicates the procedure is applicable for "flat, gable, or hip roof" configurations. I recommend using Chapter 27 (Directional Procedure) for your situation. Monoslope roofs are included in Part 2 of Chapter 27 (Enclosed Simple Diaphragm Buildings with h ≤ 160 feet), assuming your building meets the "simple diaphragm building" definition.

 

[TehMightyEngineer]

I honestly don't have a good answer but my estimated guess would be either an entirely zone 3 roof for the leeward wall or perhaps treat it like a flat roof and make one half a "windward side" (zone 2) and the other half the "leeward side" (zone 3). But, like I said, I'd have to dig into the code to figure out what is the most appropriate.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[mike20793]

I second the directional procedure. It's much easier to set up a spreadsheet for this, in my opinion.

 

[spats]

Thanks for the responses. I actually misspoke. My roof slope is 1/2:12. It's hard for me to believe that such a low slope wouldn't behave the same as a flat roof. Besides, when I look at Chapter 27, Part 2, the roof wind from Table 27.6-2 is shown as "NA". Does this mean to use internal pressures only on the roof? That doesn't seem right.

 

[dukengnr79]

I think the "NA" is referring to zones 1 & 2, which show up in all the various "sloped roof" conditions (i.e. gable, etc.) The "flat roof" figure (theta < 10 deg... which would be your condition) only has regions 3, 4, and 5 and those all have non-NA values in the table 27.6-2.

 

[spats]

Thanks duke, however, it still doesn't make any sense. No zone 1 & 2 exterior pressures for a gable, hip, monoslope or mansard for transverse wind when the roof slope is less than 10 degrees??? This is just plain dumb! No wonder I've never used this section of the code.