asce 7-98 drifting

huangyhg

asce 7-98 drifting
drifting is computed using the length of the roof upwind of the drift, lu per figure 7-8, asce 7-98.
if you have a parapet perpendicular to the wind direction which blows the snow off of the upper roof, can the distance to the parapet be used for lu?
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jike- is the parapet on the high or low roof?
i don't think you can take advantage of a parapet on a higher roof to "block" the snow from continuing to migrate off the higher roof.
also, note that you need to also include windward drift (see figure 7-7)
parapet only affects leeward consideration and becomes part of hd.  essentially no help from parapet.
samdamon, the parapet is on the high roof.
jae, i am aware of the windward drift requirement.  
i would think that the parapet would block some of the drifting potential, but the code does not mention whether this is allowed.
jike-  i agree with the other posters that there is no reduction in the drift load on the lower roof if the high roof has a parapet.  it might actually hurt because having the parapet means the snow can pile higher on the lower roof. the "hc" dimension in fig 7-8 would need to be measured to the top of the parapet el. rather than just to top of high roof el.  regards.