ASCE 7-10 Snow Load Drift Height

[mtuhusky]

When calculating the drift height and width per Section 7.7.1 I'm confused by this statement:
"If this height exceeds h_c, the drift width, w, shall equal 4h_d²/h_c and the drift height shall equal h_c."

So, what value of h_d is used in the equation for w? The value calculated in Figure 7.9 OR the value of h_c, making w=4h_c²/h_c.

 

[RobertHale]

I use the calculated value of hd in the equation. This increases the width of the drift, but if my calculations are correct it preserves the total volume of the drift. If it were otherwise I have to believe they would have stated that w = 4*hc.

 

[structSU10]

it is the hd calculated previously. Its basically trying to get the volume of drifted snow to remain the same. the volume of snow in the normal case (hc>hd) give you 4*hd^2/2. when the snow drift is truncated you get hc*(4*hd^2/hc)/2 which reduces to the same 4*hd^2/2.

 

[BSVBD]

Here in Wisconsin, i still use ASCE 7-05.

Are you all saying that the "h_d" in 7-10 is calculated by formula rather than derived from a Figure 7-9 as in 7-05?

I DO need to purchase ASCE 7-10 since Wisconsin is destined to adopt it.

 

[Hokie93]

I agree with the responses by RobertHale and structSU10; use the calculated value of h_d.

BSVBD; no, both ASCE 7-05 and ASCE 7-10 use Figure 7-9 to determine h_d. There is an equation for h_d included within Figure 7-9 but that is true for both ASCE 7-05 and ASCE 7-10 and the equation is identical in both standards.

 

[mtuhusky]

Thanks everybody. It makes sense to use the previously calculated hd value to preserve the total snow drift volume. It's worded very awkwardly to me.

 

[mtuhusky]

I've got another question to tag onto this thread. In section 7.8 it states that for parapets lu is the length of the roof upwind of the wall so obviously hd is calculated assuming a windward drift. However for projections it specifies that lu be taken as the larger of the roof lengths on either side. For a roof projection I'm not sure if hd is calculated using the windward or leeward equations. Does anybody know the answer to this?

 

[BSVBD]

To provide an absolute answer, one would need to define a "projection".

In 7.8, ASCE inadvertently defines an applicable projection as something greater than 15' wide.

But, how TALL is the projection? And is the projection a parapet wall?

If either the projection or parapet is too tall, we would then not consider the leeward drift.

I would say, in most cases, a projection would cause a windward drift.

Within project specific reason and judgment, unless one "greatly" outweighs the other, why not take the conservatively larger of the two?