ASCE7-05 "projection" snow drift question

[mbullism]

If I set an AHU on a curb directly on a roof, ASCE7 requires that I carry a windward drift on any side 15 feet wide or over. If I set the same AHU on a frame, equal to or less than the base snow depth above the roof I'd use the same drift...

How far above the calculated base snow would the bottom of the frame have to be before you would assume the snow drifts under it and continues on across the roof? It seems like at an inch clear of the base snow it would fill the gap and drift anyway...but then I think the base snow is a design storm that may happen once or twice in a service "lifetime"...(never say never)

Anyone have any rules of thumb, or should I just keep it at at least base snow depth? Any thoughts would be appreciated.

MB

 

[jike]

This comes down to engineering judgement and I am sure you will get a variety of answers.

 

[JAE]

Anywhere where the wind laminar flow is disrupted is an opportunity for drifting. I would assume the drift even if it was 4 ft. clear. The columns extending up will affect it as well as the wind rushing around, under and over the unit. Since there is no direction in ASCE 7 on this I'd tend to go conservative.

Also, I'd envision this dialogue someday on your roof:

[blue]Two guys standing on the roof, scratching their heads.

Frank: "What do you suppose they put this unit up in the air like that for?"

Joe (stroking his chin): "I'm not sure....do you suppose they thought there'd be another unit go under it?"

Frank: "Maybe. But maybe they wanted to get this unit up in the air to be able to work on its underbelly."

Joe: "Ya. That must be it. I think this new unit we're puttin' in here doesn't have any maintenance access under so let's put it right on the roof on its curb".

Frank: "Sounds good. Plus this new unit is twice as heavy and twice as large so puttin' it up on those coly-ums would be hard."

Joe: "OK - on the curb then." [/blue]

 

[mbullism]

"This comes down to engineering judgement and I am sure you will get a variety of answers."

No doubt, but that's what I'm looking for to see if there might be a general conscensus...

I FULLY understand the possibility of that future dialogue, lol, but in the present tense my charge is to support this unit on an existing roof. I'm not sure I have to anticipate a future unit any more than worry about plumbers notching "my" joists in the future. As for the columns and wind flow, by the code they represent a projection less than 15'-0" wide so no drift is required... by "code".

I guess I'm looking to see if anyone has ever pondered this, and their rationale. You all know that architects want to put this stuff on the roof to hide it... raising it higher and putting it further into their sightlines is out of the question unless there is a very real upside.

 

[DaveAtkins]

I think for your case, I would raise the unit a little higher than the balanced snow depth, and then ignore any drifting. I have no Code rationale to back me up, other than my general feeling that Codes tend to be conservative.

For example, on a recent project, I had large snow drifts (69 psf) on two sides of a rooftop unit, because it was 16' long. I find it hard to believe that if my unit had been 14'-11" long, there would have been no snow drifts, but that is what the Code says.

DaveAtkins

 

[JLNJ]

I'm in agreemnet with Dave Atkins here.

If you get the frame up high enough to roof under it then I'm going to say you won't have drift. Regardless, you don't want to set the unit so close to the roof that when they re-roof it is difficult or impossible to do so properly.

 

[mbullism]

Thanks for the replies...

Seems like a billion years since Fluids class, lol. Wouldn't the wind hitting the unit, trying to go over,under and around the unit actually tend to scour out a hollow spot beneath the unit as it accelerates through the restriction?

That said, the variety of answers above not withstanding, the general concensus in our office is to carry the drift anyway. Not because of any engineering judgement, more a question of (and I hate this part)what a lawyer could do with it should it come back to bite you. Somewhere in between 1" above and 12 feet above is the right answer, I'm just not sure you could come to it even with wind tunnel testing (though I guess that would be more "defensible"...).