Wind loading on irregular structure

[CEmonkee]

I am working on a 1 story house with a roof that is vertically offset at the center (please see the attached image). As you can see in the plan view, there are also some horizontal offsets.

For the wind loading, I would prefer to use the envelope procedure, part 2 of ASCE Chapter 28 since that is the method I am most comfortable using. However I'm thinking there are too many irregularities for this to be considered a simple diaphragm building.

Any advice would be appreciated - thanks!

 

[Simba13]

"The Analysis of Irregular Shaped Structures" by Malone and Rice is a great resource for this sort of thing.

 

[CEmonkee]

Thanks Simba13 - I bought the Kindle version of that book a few days ago, it's a great book. In section 2.3 it has the following regarding the calculation of wind loads:

2.3 Load Distribution into a Diaphragm
IBC Section 1609.1.1 allows the determination of wind loads to be in accordance with ASCE 7-05 Chap. 6 or by the “Alternate all-heights method” of IBC Section 1609.6. Any method can be used as long as the method is applicable to irregular-shaped structures and separates the wind loads onto the windward, leeward, and side walls of the building and properly accesses the internal forces in the main wind force-resisting system (MWFRS). Simplified procedures intended for rectangular structures should not be used for irregular-shaped structures.

I couldn't find any examples in the book where wind loads are calculated - the loading is just shown as xxx plf applied to the structure. Based on the paragraph above, I think I should use Chapter 28 part 1 method for determining the wind loads.

Does anyone know of some practical examples where the wind loads using this method are calculated and applied to a structure?

 

[Simba13]

Yeah, I suspect that if it went into calculating the wind loads as well that would be a whole 'nother book. I'm not expert, and others might disagree, but I feel like getting your pressures should be the same as a uniform rectangular building. You can use the envelope method as you stated, I'm a fan of the directional procedure myself (easier to make a spreadsheet for). I think the irregularities come into play more on a load path level, making sure you have an appropriately designed diaphragm, either through transfer diaphragms or just breaking up the house into sub-diaphragms (I think it's generally done this way), and then making sure you have the appropriate wind bracing (wind controls over seismic in my area, may not be the case there).

 

[CEmonkee]

Wind often controls here.... I would like to just calculate the pressures assuming a regular shaped building, but the paragraph I quoted from Malone's book made me question whether I should do that. I'll look into using the directional procedure if it's easier to setup a spreadsheet for that method. Thanks!

 

[Simba13]

I think that the simplified procedures they are referring to something like Part 2 of Chapter 28. If you look at ASCE 7-10 28.1.1 it says "Part 1 applies to all low- rise buildings ..." but then 2) is only for enclosed simple diaphragms. So I think you're good using Part 1. In the first place the house you're designing is not mega-irregular, from looking at it, it looks just slightly different from a simple rectangular diaphragm. I'd be interested to see if someone disagrees but as far as pressures go I think you'll be fine using that procedure.

 

[CEmonkee]

I think I agree - Ch 28, part 2 should not be used. Thanks for your opinion, I appreciate it. I may have further questions once I start calculating the loads (I mostly use part 2, so I need to brush up on the other method).