Snow Drift Loading ASCE 7

[ChrisKn]

SHould be any easy question, and i believe i know the answer, but would like some confirmation.

Calculation snow drift roof loading at building adjacent with different height.:

1.Calcualte hd for leeward & windward drifts per section 7.7.

2. Calculate w (width of drift) using the hd from step 1. Shall not exceed 8*hc.

3. Calcualte pd (maximum intensity of drift). **The hd from step one for this calculation can never exceed hc??? Basically the snow drift height can never exceed the height of the wall?? This is the section where the confusion lies for me as it seems it isnt clearly explained but logic leads me to believe that this is true.

Based on these equations the these situations will occur as the legth of the windward/leeward roofs increase.

The max snow drift load (pd) and width of drift (w) will increase proportionately until the drift reaches the top of the wall at the higher adjacent structure. Then the pd will remain constant (because the height of the drift cannot exceed the top of the wall at the higher adjacent structure) and the drift width (w)will increase until it reaches 8hc. At this point both the pd and w will reamain constant?

Are my thoughts on this correct??

Thank you,
Chris

 

[prsconsultant]

Use the link below for a spread sheet and verify your results:

http://www.seaofsc.org/Alex's Corner.htm

 

[csd72]

According to figure 7-8 hd is the maximum of a triangular load. As there is not flat portion mentioned in this figure then I would assume that the drift is still triangular.

Reading the commentary of ASCE7-05 they have widened the drift length so that the volume of snow is the same as if it was an unrestricted height of hd.

They also talk about actual drifts having a slope of 1 in 6.5 which also infers a triangular load.

I hope I have helped and not confused you

csd

 

[ChrisKn]

Thank you for your help. PRS i tried your online spread sheet and the spreadsheet I created comes up with the same answers.

Graphically it makes sence to me, as for the comment about ASCE 7-05, I do not have the newer code (I only have ASCE7-02). So CSD72 are you saying that the drift length has widened beyong 8hd? or have i misunderstood?

My original explination was meant to to imply that the drift load ALWAYS remains triangular and that the value of pd and w would rise proportionally until the drift reached the top of the wall. Then at this point the pd would remain constant (equivalent to max loading when the drift reachs the top of wall) and the width would increase making for a triangular load with a greater width (basically the triangle would flatten out to a degree until it reached the maximum w=8*hd). Does this make sense?


Chris

 

[csd72]

ChrisKn,

Yes in ASCE7-05 the width if hd>hc has widened out so the volume would remain the same even though hd is reduced.

Yes all snow drifts are taken as triangular according to the code.

You will probably find that your buiding code only references ASCE7-02 anyhow.

Regards
csd