Snow Loading - Mechanical Equipment

[dik]

I have an existing roof that is exposed to wind and the client wants to install a new piece of mech equipment (relatively small, but sufficient to cause snow accumulation). If the unit is mounted above the roof to provide a few feet of clearance is it possible to consider that there will not be a snow accumulation.

Is anyone aware of literature for this type of consideration?

Dik

 

[spats]

I don't know what code(s) your working with, but Chapter 7 of ASCE 7 covers how to do drifts, including around roof projections. Regarding clearance to avoid the drift, I've always considered that if the bottom of the unit/unit support is higher than the flat roof snow load depth, it won't drift.

 

[dik]

Spats:
Thanks, for the info. That was the rational that I was considering; have you encountered any literature that indicates this type of approach?

I've not paid attention to mechanical units, but I've seen beams on roofs that are clear of snow beneath, and I've attributed this to increased wind velocity flowing beneath the beam, because of the obstruction.

Dik

 

[msquared48]

Or the snow was falling vertically with no wind. Does happen sometimes.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[dik]

Without the wind, you won't get a snow accumulation.

 

[dik]

Maybe a little hasty in the reply, if you were thinking a a 'shadow' underneath the beam. The effect is for a couple of feet on either side of the beam. I've also seen this with piping (>1'dia) on a roof.

 

[msquared48]

Yes, a shadow.

And I believe you meant you won't get drifting without wind, not accumulation. Different animals here.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[dik]

Thanks... I've noticed a 'scouring' that occurs under beams and pipes that are within a few feet of a roof surface and have attributed it to the increase in velocity caused by the slight obstruction and the air flow around it.

My problem is that the existing roof joists don't have sufficient capacity for the added load (barely accommodate building code loading at the time of construction), let alone any snow accumulation. I'm looking at options in lieu of reinforcing them (not easy because of existing electrical and mechanical stuff in the way).

I attended a discussion a couple of weeks ago about modelling snow accumulation using a building model, water flume, and sand as well as computational fluid dynamics (I've used walnut shells to model beach erosion in past which is similar; CFD was not common then). Don't have the time or budget, in this case.

Dik

 

[steellion]

Dik, I agree with your approach and I have similarly raised the bottom of screen walls and mechanical units to avoid the snow drift penalty. You will still have to design the roof underneath the unit for flat-roof snow load, because the wind can blow it underneath the unit.

 

[BAretired]

The wind flows faster under a slightly elevated obstruction and tends to scour the snow out from under the obstruction. We have had more snow than usual this year, but the area under evergreen trees seems to be almost free of snow. This may be partially because of shading by the tree but mostly because of wind action.

BA

 

[dik]

Thanks, BA... I've been trying to dig up information on this and have picked up a couple of papers... nothing specific to my issue.

Dik