Minimum Snow Load

[snaggle]

In ASCE7-05 section 7.3.4 there is a formula for determining minimum flat roof designation for a hip gable roof i.e. (70/W) + 0.5

W is stated to be given in (ft), however, the end units of the equation are not stated.

It seems obvious at first glance that the result must be in degrees, since most of the numbers in the paragraph are in these units. However, with this assumption, a roof w/ a 30 ft span is not considered minimum until the pitch drops below 1.1/12. By contrast a mono-pitch roof is considered minimum at 15 deg. or about a 3.2/12 pitch.

If the the term evaluates to inches for usage in a pitch/12 designation then a roof w/ a 30 ft span is considered minimum when the pitch drops below 5.2/12. This seems more plausible.

I also think the equations in 7.3 are flip/flopped. Is there any point in applying a snow load less than 20psf? (sarcasm)

Thoughts on this would be appreciated.

Example: pg = 50psf , Importance = .8, exposure = .9, thermal = 1.2, 4/12 pitch common truss w/ 30ft span

What should the minimum flat snow load be???

 

[JAE]

Section 7.3.4: Yes, W in feet and result in degrees.
W is not the span, but the distance from eave to ridge.

[blue]Is there any point in applying a snow load less than 20psf?[/blue] Yes there is. Sometimes the a Pf less than 20 PLUS the associated drifting could control a design more than the RLL = 20 psf uniformly distributed.

 

[JAE]

Sorry, I meant the last sentence to read:

Sometimes a Pf that is less than 20 in combination with its associated drifting could control a design more than the RLL = 20 psf uniformly distributed.

 

[snaggle]

Sorry, but I do not think my question was understood. I am aware of what W represents, I made calculations above reflecting the correct W. Also, drifting has nothing to do with the min flat snow load. Drifting is a surcharge based on other factors. I am questioning the minimum snow load for a particular slope.

I have graphed the results of the two assumptions. Does anyone know what the theory behind the magic formula is?

The formula is non-linear so there must have been a certain range that the creators were trying to target??

As mentioned above the cut-off for a mono slope roof is 15 deg.