Ponding Load

[L_Bey]

I am designing a building with minimum snow load (South Carolina) and am looking at the rain load requirements. I typically work in the north east where roof snow loads can easily be in the 50 psf+ range so rain loads typically aren't an issue.

Looking at ASCE 7-16, rain load needs to be evaluated based on 15 minute duration/100 year return period, with the static head of the difference between the low point of the roof and the rim of the secondary drainage system (assuming that the primary drains are blocked), plus the hydraulic head associated with the secondary drain system. My secondary drains are scuppers at the exterior, with each drainage area having a secondary drain. Using the tables in the commentary it's pretty straightforward to calculate the hydraulic head for the given scuppers based on the 5.2(d_s+d_h) equation. However it does then say "account for ponding instability and ponding load", which would cause additional load on the roof. This additional load would be based on the deflection of the roof under the regular rain load, say 1" of deflection adding 1" to the water depth we're designing for. Then check that load, make sure the deflection isn't greater than the 1" allowed for, etc.

ASCE 7-22 has an additional term in that equation, d_p, to account for the ponding load. Is it reasonable to set the ponding depth to say 2" maximum, calculate the required rain load based on that number, and then design the roof framing to not exceed 1" deflection each for joists and beams (2" deflection maximum)? Would that satisfy the requirement to consider ponding? Or is there another consideration that I'm missing?

I've applied my total load (static head, hydraulic head, and ponding depth) to the whole roof, which results in a total load of 55 psf (6.5 in static head due to roof slope between internal drain and scupper, 2 in hydraulic head, 2 ponding). This is a bit conservative, since the actual depth of water between the internal drain and the scupper will vary with the roof slope. The ponding load will also vary between a maximum in the center of the bay and nothing at the columns. But trying to do a variable load doesn't seem like it would be worth the engineering time, or going to special joists.

Thoughts? Anyone else have a standard practice for bar joists and ponding? I'm beginning to see why all our flat roof designs have secondary internal drains at +2-3" above the primary instead of scuppers.

 

[lexpatrie]

I think your approach deviates from the design standard approach, but 2" of rain applied across the entire roof would be a lot more that what a "normal" ponding situation would create. There are points where the structure can't move downward (i.e. walls), so it is likely conservative.

As this is technically a stability analysis, the conservative approach needs to be looked at carefully, but it can be done this way safely. You may put "extra" material in to the roof, but that's not all that harmful. We just want to avoid the situation where the roof is heavier than it would normally be AND it still has a ponding defect.

I'd caution you that while a ponding check may create a structurally safe roof, it may (if adequate slope is not provided) violate the roofing manufacturer's installation instructions, AND the code.

The more recent approach I've seen tends to be to slope the bar joists 1/4" per foot, or the steel, AISC, as you know, has a ponding check, and SJI has a different ponding check. Keep in mind that sloping multiple bays TOWARD each other is now defined as a susceptible bay (in ASCE), but perhaps not in the actual building code, I recommend using the more stringent definition of the two....

Conversationally, there are two kinds of articles on ponding, the really technical ones that get wildly abstruse (cough Molina was it? Marino, et. seq.), and the basic "water bad, slope roof!" articles by, say, Patterson and his compatriots. Lawson has a few good articles (at least two) that strike the middle ground, which help understand the more abstruse technical and design articles, but aren't enough on their own to design with.

I thought I had links to some of the ponding papers in the FAQ.

1966 - Ponding of Two-Way Roof Systems - Marino

Not exactly comprehensive, sorry.

 

[DaveAtkins]

Designing for blocked drains and designing for ponding are two different things. What you have described is designing for blocked drains, but not a typical ponding check. A ponding check verifies that the members you choose are stiff enough so they will not progressively collapse due to water accumulating on their deflected shape. I use AISC Appendix 2 to check ponding.