85% Reduction in Unbalanced Snow Load

[Cambridge99]

We are doing a structural analysis of an existing fabric covered steel building (curved roof) and the original design calculations show a 85% reduction in the unbalanced snow load. We were wondering where in ASCE 7-05 this reduction is referenced.

 

[MiketheEngineer]

Not sure exactly what ASCE says but there are substantial deductions allowed for 24/7 heated roofs with backups and for highly sloped roofs. That may help. 85% seems a bit high but I didn't do the calcs.

 

[msquared48]

I guess I would have to ask what wind speed and exposure was used in combination with the snow load, as well as the assumed angle of attack. Seems very high to me too, but...

Mike McCann
MMC Engineering

 

[MiketheEngineer]

I think the absolute minimum is like 12 psf. You can easily find this in ASCE or IBC

 

[pilesmakesmiles]

BS6399-3 doesn't mention a reduction for asymmetric loading due to snow, however it does allow a reduction in the factor of safety applied to the load (use 1.05 instead of 1.6) when checking member strengths for snow that has been redistributed i.e. blown around on the roof.

The justification is that the figures given in the code are an absolute worst case scenario and unlikely to happen very often e.g. you have to have strong winds AND a full snow load on the rest of the roof for it to happen AND all the snow has to move.