ASCE - Ponding Instability

[CURVEB]

Hi all - wondering if you would consider the framing in the attached sketch a "susceptible bay" in accordance with ASCE7-10 8.4. The issue being that while the slope is 1/4" per ft, the overflow scuppers are by their nature controlling drainage of the rainwater in the case of the primary drains being blocked. So section 8.5 applies, which sends you back to section 8.4 and requires you to investigate the framing for instabilities.

Or would you consider the overflow scuppers "free drainage" so that instability does not exist (since the roof slopes?

I appreciate the input.

 

[racookpe1978]

When it rains 4 inches per hour, how much water will drain?

 

[DaveAtkins]

Remember--ponding is not the same thing as designing for blocked drains. You must always design for blocked primary drains, but you only need to design for ponding when the slope of the roof is less than 1/4" per foot.

In your case, you must design for blocked primary drains, but not ponding.

DaveAtkins

 

[CURVEB]

Thanks DA - that's the direction I was leaning.

 

[Hokie93]

I don't believe the revised wording in Section 8.4 of ASCE 7-10 (revised compared to ASCE 7-05) supports that view. The pertinent portion states: "Bays with a roof slope less than 1/4" per foot OR on which water is impounded upon them (in whole or in part) when the primary drain system is blocked, but the secondary drainage system is functional, shall be designated as susceptible bays. Roof surfaces with a slope of at least 1/4" per foot toward points of free drainage need not be considered a susceptible bay".

In my opinion, an overflow scupper is not a "point of free drainage" unless it is very, very large. I believe "a point of free drainage" is intended to be a free edge of the roof.

I would say that ASCE 7-10 requires you to consider ponding instability for the roof system shown in your sketch.

 

[wannabeSE]

If you are not certain, check ponding instability. With experience, you will know if it could be an issue that needs to be checked, or OK by inspection.

 

[DaveAtkins]

I have never thought about it the way Hokie93 thinks about it. I have always interpreted "free drainage" to mean unblocked drains, scuppers, etc.

I did not realize ASCE 7-10 is a little more definitive.

DaveAtkins

 

[CURVEB]

In this case the overflow scupper will be sized to prevent buildup of more than 4" of water on the roof at the design rainfall rate, but I would agree: this doesn't seem to meet the requirements of ASCE7-10 for "free drainage". I think the joist needs to be checked for instability, but at this slope I don't believe it deflects enough to allow more than 4" of build-up - therefore an iterative analysis would stop at the first iteration, if you conservatively assume that the 4" of rainwater is uniformly distributed over the joist.

 

[RFreund]

I'm asking out of curiosity -
Why would a scupper not be free drainage? What makes it "not-free"? To me there is nothing blocking the drainage (i.e. a grate or some other means to screen debris). Although I suppose if you were to make the scupper 1" wide, then it doesn't seem so free.

EIT

http://www.HowToEngineer.com

 

[CURVEB]

I would say it's not free drainage because the water still develops a hydraulic head. In other words, at the design rainfall rate a scupper of any given size (say 30" wide x 6" deep) will only allow some water to pass through. The depth of water flowing through the scupper is the head (which might be something like 2" for example). Free drainage (to me) would be if you didn't have a parapet wall with scuppers at the end of the slope and instead just had a gutter. Water can freely flow off the roof and does not build up at all.

 

[msquared48]

The only situation I can think of is icing up of the scupper due to a drifting of snow and melting of snow at the parapet. Depends on whether or not this is in a cold region where that could occur. That the OP will have to address...

But with the current weather conditions in upper New York State...



Mike McCann, PE, SE (WA)

 

[wannabeSE]

In ASCE 7-10 commentary, Table C-8.1 lists the hydraulic head and flow rate for various scuppers.

 

[FootNMouth]

In an SJU publication titled "STRUCTURAL DESIGN OF STEEL JOIST ROOFS TO RESIST PONDING LOADS" the author states that

"The authors of this digest concur with the requirements of points 1 through 5.
Calculations presented in this digest indicate that ponding instability can occur
with roof slopes of 1/4 in. per ft. and greater. Thus, it is recommended in the
digest that ponding instability calculations be made even for roof slopes greater
than 1/4 in. per ft., unless the roof slope is greater than 1/4 in. per ft. and is free
draining (free to drain over the edge of the roof)"