Snow Drift: Spacing RTUs to Avoid Drift

[MJB315]

Hey All,

I have a question regarding drifting snow and rooftop mechanical units. For discussion purposes, say I have three, 13' long x 5' wide RTUs that are going to be placed along the same column line on a flat roof. Since each unit is shorter than 15' feet, ASCE7-05 says that drift loads do not need to be considered.

However, there must be a unit-spacing where these three RTUs begin to act like one large projection (and attracting loads in the process). Does anyone know what that spacing distance is, or might be?

Is it 13'? Is it 5'?

I am sure this topic has been discussed in the past (I searched the closed threads), but I didn't come across a consensus or final answer.

Any guidance or discussion on the matter would be appreciated.

-MJB

"We shape our buildings, thereafter they shape us." -WSC

 

[DST148]

There is a book : Snow Loads - A Guide to the use and understanding of snow load provisions of ASCE 7 - 02 By Dr. Michael O' Rourke and Peter Wrenn. Dr. Rourke is Chair of the Task Committee on Snow and Rain Loads. There is an example of drift around RTU and also a question about installing a new RTU adjacent to existing unit. See if it helps....

 

[JAE]

I thought that if your unit is less than 15 feet wide you don't have to deal with drifting (see ASCE 7-05, section 7.8)

 

[MJB315]

I took a look through Professor O'Rourke's guide (at least what I was able to skim through via Google) and I didn't see an example where a new RTU unit was being added next to an existing one. I'll look a little further.

JAE: It's true that if the unit is less than 15' wide, ASCE7 takes drifting off the table. But I started to ask myself what the difference is between: a 42 foot wide projection and; three, 13 foot projections with a 12" gap between them.

That gap distance must make all the difference, but I don't have a feel for how wide it needs to be to prevent drifts (5 feet, 15 feet, etc.).


"We shape our buildings, thereafter they shape us." -WSC

 

[DST148]

@MJB315: I had Googled and found it in Chapter 12, FAQ, Q #2 - Installing a new RTU adjacent to an existing unit.