MWFRS wind loads on multiple parapets

[Wandering Wallaby]

Hi everyone, first time posting to this site but have used it for years for guidance, so thank you! Anyways, I am in the process of gathering my MWFRS diaphragm loads and am looking for advice on how to model the parapet loads when there are multiple parapets along the length of the roof. The project is a large 8500 square foot flat roof residence with 11 separate roof diaphragms with multiple vertical offsets separated by parapets. In one area there are 8 different parapets that the wind would flow over. Per ASCE 7-16 C29.5 “There should be no reduction in the positive wall pressure to the leeward parapet caused by shielding by the windward parapet because, typically, they are too far apart to experience this effect.” My question is, how far is “too far apart?” A lot of these parapets are around 16 ft apart and about 3-4 ft tall. Obviously, the load increases significantly if treated as 8 individual parapets with no shielding effect, so I’m just trying to see what other engineers have done in this situation.

Thanks in advance for any help.

 

[EngineeringEric]

I would assume that shielding doesn't happen at that aspect ratio (4ft tall to 16ft apart, 4:1). i think there is a table that discusses reducing Cp due to distance from adjacent object but I don't have my ASCE-7 book

 

[Aesur]

I typically design considering the wind on each parapet and don't even think about shielding effects. I'm not sure it's worth the time and effort to try to justify a slightly lower wind loading on parapets, especially considering the project is residential which are typically the more complex structures with very low budgets in the first place.

 

[Wandering Wallaby]

Yes, I agree; probably not worth the effort trying to reduce loads for shielding. If I get into a problem where I think the shear loads are unreasonably high then I may look into it a bit further, but for now will consider each parapet receiving full wind pressures. Thanks for all of your responses.