Columns of A large roof-Effect of Snow load reducible? 5

[IJR]

Pals

For Live loads, reduction is based on tributary areas for columns and girders. Which makes good sense.

I am curious, is snow load reduction possible for columns with very large tributaries? If not,what is the rationale for not reducing it?

respects
IJR

 

[csd72]

If there is no wind, snow will fall on the whole roof evenly.

You should be asking what is the rationale for reducing it?

 

[JAE]

Agree - snow simply falls and there is no statistical basis for snow NOT happening over large areas vs. small areas.

 

[dik]

In Canada... where 0.8 of the Ground Snow Load is typically used, for larger roofs, there is no 0.8 reduction to accommodate snow falling on the larger area.

Dik

 

[connect2]

In Canada there is no tributary area reduction permitted for snow loads. The rational for this not sure, I would presume the uncertainty in prediciting the load?

 

[connect2]

Just thinking looked at ASCE 7-02 no tributary area reduction for snow either. The reason is, I believe, is that for multiple floors the probability of all being loaded to full specified load simaltaneously is low, ie the permitted tributary area reduction for live load, however you only have one roof so no reduction. I reduce the live load due to use and occupancy for a building with only one floor though. Go figure.

 

[csd72]

Next time it snows without wind go out and have a look around if there was 2" of snow there will be 2" everywhere it is flat and that there was not an obstruction.

same would apply if it was 3' and no wind.

Take a step back from the books for a reality check every now and then.

 

[connect2]

your point csd72? I think I've missed something?

 

[csd72]

My point is that the snow load is uniform when there is no drift, this is a physical fact that can be observed in the real world regardless of code books.

 

[ajh1]

The other aspect of live load on a roof is that it is there primarily to accomodate the potential of maintenance workers on the roof, at least in a post-construction timeframe. There is a good likelihood of a few workers congregated in one area, say around a roof top unit, but it is highly unlikely that you would ever have enough workers on the roof to require the small trib area level forces over a significantly large trib area.

 

[connect2]

Thought we were talking about reduction of snow load based on tributary area? In any case no reduction, based on tributary area, is permitted for structural members carrying snow load. Makes sense. Might have misunderstood the original question. Snow drifts, Ive been shovelling them since the beginning of December, I far prefer snow without wind, rarely see it though.