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.

 

[phamENG]

@phamENG may be able to convey this concept better than I can,

You're far too kind. I'll give it a shot. Warning: I've only had one cup of coffee, it's Monday, and I haven't read through everything in detail...

I think the point is that the source of this ponding load is not only the water, but also the stiffness of the joist. An infinitely stiff joist will have no ponding. Therefore, the stiffness becomes an input to the derivation of the load, which is life safety. So should that input to a life safety parameter carry the same statistical handicap (5th percentile) as other life safety parameters (reference bending stress, for instance), or should it be treated as "normal" deflection is treated, with average stiffness?

Emin affects the Fb* and Fc*

This is true, and though I agree with ChorasDen on the direct application here, I think it could give some insight. Where member stability is a factor in determining the strength of a member, we look to Emin as our stiffness. Ponding is a question of system stability impacting capacity - maybe Emin with a repetitive member factor would be the most appropriate method?

 

[lexpatrie]

I understood what you were asking already.

Alright fine, for the curious..... I'll retract my previous "I was disappointed by it." But it's pretty abstruse, and there's even discussion in the commentary.






So that kind of follows with what Pham was going at.

This isn't the whole thing because copyright. It hasn't been touched since 1995.

https://ascelibrary.org/doi/book/10.1061/9780784400418

 

[L_Bey]

Further, the OP describes JOISTS (i.e. either wood joists, or open web steel joists), Open web steel joists are NOT under AISC and AISC doesn't apply. SJI publishes a guide for how to do ponding checks on joists. Wood joists are not under AISC either.

More to the point, though, is the OP mentions normally designing for snow loads and demonstrates a lack of familiarity with the design standard for ponding.

The concern I have with that is ponding is required for snow loads, too. So they should already be doing this for their northern state projects.

View attachment 15104
Source: ASCE 7-22

And for the compulsive, Chapter 7 is "snow loads"

Thanks for the comprehensive reply! This thread languished for a bit and then I went on vacation, but I appreciate the insights.

I'm in the North East, and for the most part we have internally drained roofs with two redundant internal drain systems separated by the code required 2" (and the secondary daylighted somewhere it's obvious that the primary is blocked), and the roof sloped by more than 1/4" per foot specified to limit the ponding susceptibility. In that condition, even with the primary drain blocked, we're only looking at maybe 3" or 4" of water and ~20 psf where the water accumulates. With snow load of 35 psf at the low end, up to 50 psf +, it just doesn't factor into the design of the roof. It's one of those things you check a few times to be sure, then specify a sufficient slope to make it not an issue and don't look at it again (besides reminding the architect that yes we need 1/4" slope minimum). Alternatively we have steep pitched roofs and it's really not an issue!

This one was particularly weird because it's a fairly large building, with minimal internal drains, a normal pitch of 1/4" per foot, and the secondary drains are scuppers located 26 ft away from the drain. So I've got 6" of slope, 2" of hydraulic head, and 2" of roof deflection to consider (and the roof drains are arbitrarily located). I don't think a 10" bathtub if the main drain is blocked is a great plan, but I'm picking this job up from another engineer and the architect hasn't been hugely responsive to feedback. I want to make sure we're providing a safe roof design and considering all the loads, but in the end I can't control what's provided for drainage. In reality that water is likely to go through the roof before it makes it to the scuppers, but that's the specified secondary drainage system so that's what I need to use for design.

 

[L_Bey]

I will agree with that, ChorasDen. If you account for the additional water due to the deflected shape of the members, then I suppose you don't have to follow AISC Appendix 2.

But it is an iterative process. The deflected shape allows a little more water to accumulate, which causes a little more deflection, which allows a little more water to accumulate, etc. That is what AISC Appendix 2 checks. It makes sure the deflection converges to basically nothing.

My theory was that if I account for 2" of water due to the deflected shape, and then end up with a maximum total deflection of less than 2" under the DL + RL combination, I will have accounted for the progressive nature of ponding, and could ignore any additional analysis. I'm trying not to dump too much time into a job that should already be done, but wanted to make sure my "short cut" wasn't missing anything - it seemed like a reasonable approach analytically, but a gut check from others is always good.

Thanks for the feedback and lively discussion!

 

[DaveAtkins]

That seems reasonable.

Also, I have found that if you limit snow load or live load deflection of a roof member to L/360, the AISC Appendix 2 check typically works - assuming you have overflow drains and/or scuppers.