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Rain Load ASCE 7

Rain Load ASCE 7

Rain Load ASCE 7

(OP)
I am questioning my understanding of the applied rain load per ASCE 7...

Let's say you have a roof that was designed for 25psf snow load. You now want to know how far the main drain can be located away from your scupper (or secondary drain).

ASCE 7 says that the rain load is 5.2psf/in(h.s + h.d). Where:
d.s = depth of water on undeflected roof up to the inlet of the secondary drainage system.
d.h = additional depth of water above the secondary inlet (undeflected roof) at its design flow (hydraulic head)

It appears as though this is applied as a uniform load when viewing the examples in the commentary. However this seems odd in that the slope of the roof is likely constant and thus the load would be triangular. So then I look again at the definition of d.s and it is stated as the depth of water on the undeflected roof... So I suppose this could be interpreted to vary based on the location/distance from the secondary drain. Which do you suppose is correct?

Back to the example:

If we assume that the load is uniform, the hydraulic head is 1" and the slope is 1/4" per foot then...
h.max = (25psf - 1" * 5.2)/(5.2) = 3.8in and then L.max = 4" / 0.25 in/ft = 15.2ft

If the load is varying then that changes things more of an iterative process maybe?

Thanks!

EIT
www.HowToEngineer.com

RE: Rain Load ASCE 7

RFreund...the load is only triangular if you have a parapet at the end of the slope where water can back up. In practicality, water does not accumulate as ASCE 7 would imply. It is progressive as you proceed down the slope, as you noted. It is essentially a step function at an instantaneous point; however, it is reasonable to use the approach in ASCE 7 rather than trying to apply a more rigorous analysis that would not necessarily yield a greater accuracy in the overall design.

RE: Rain Load ASCE 7

(OP)
Ron,

So are you saying that applying a uniform load is reasonable or should some sort of triangular distribution can be applied?

Image here: LINK

EIT
www.HowToEngineer.com

RE: Rain Load ASCE 7

I have always assumed it is a triangular (or trapezoidal) load.

DaveAtkins

RE: Rain Load ASCE 7

(OP)
The more I read it, the more I think that the triangular/trapezoidal load is the intent.

EIT
www.HowToEngineer.com

RE: Rain Load ASCE 7

Keep in mind that drains get plugged by ice as well as leaves and debris.

Mike McCann, PE, SE (WA)


RE: Rain Load ASCE 7

It is only trapezoidal/triangular if there is a drainage impediment such as a parapet with scuppers through the parapet. If you have a free edge (water cascades over edge to gutter or ground), then uniform application is appropriate.

RE: Rain Load ASCE 7

Ron,

I don't agree. Even where there is a free edge, if the drain is several inches lower than the free edge, the rain load will be triangular or trapezoidal. I am assuming the drain is plugged, of course.

DaveAtkins

RE: Rain Load ASCE 7

DaveAtkins....for internal drains, I agree with you. For a monoslope roof with drainage over the side, there is no opportunity for a trapezoidal build-up of water. For internal drains, for scuppers or other concentrated slope conditions, you are correct...water is flat on the top and sloped on the bottom lol

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