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ASCE 7-98 7.9 Sliding Snow

ASCE 7-98 7.9 Sliding Snow

ASCE 7-98 7.9 Sliding Snow

In 7.9 Sliding Snow, it is stated, "The solid lines in Fig. 7-2 shall be used to determine the total extra load available from the upper roof, regardless of the surface of the upper roof."

How is this being done?  I do not understand how Fig. 7-2 explains this.

Thank you!

RE: ASCE 7-98 7.9 Sliding Snow

ASCE 7-98, 7.9 merely states that ALL snow sliding off of an upper roof to a lower roof shall be calculated as the total snow load on the lower roof using the solid line (which is "all other roofs") of table 7.2 (not the dashed line, which is for "unobstructed slippery surfaces") under balanced loading conditions.

"regardless of the surface of the upper roof" means whether it is considered an "unobstructed slippery surface" or "all other roofs".

RE: ASCE 7-98 7.9 Sliding Snow


If the extra load caused by snow sliding off a sloped roof onto a lower roof shall be determined assuming that all the snow that accumulates on the upper roof under the balanced loading condition slides onto the lower roof, why then should I go any further to Figure 7-2?  We have our answer.  The balanced snow of the upper roof!

Why refer to the solid lines of Figure 7-2 to determine the "total extra load" when the "extra load" has previously been determined?

The solid line at Figure 7-2, for warm roofs, decreases from 7 on 12 and steeper roofs.  Would'nt a steeper roof have more total extra load available than a shallower roof?  

What am I overlooking?

RE: ASCE 7-98 7.9 Sliding Snow


Take a close look at figure 7-2, this merely states (the solid line) that you must take Cs (slope factor) at the higher curve. Cs may not be less than 0 or more than 1.0 regardless of slope. Even though 7-2a starts at 7 on 12 slope, for a 3 on 12 slope you use Cs at 1.0.

RE: ASCE 7-98 7.9 Sliding Snow


Thank you for making this more clear.

One more question.  Would you consider the UNbalanced load sliding from the upper roof?  Why or why not?

Thank you, again!

RE: ASCE 7-98 7.9 Sliding Snow


No. There is no need to consider the unbalanced load since ALL the snow load from the upper roof is assumed to slide onto the lower roof (the reason for the steeper curve).

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