Ponding
Ponding
(OP)
I'm looking at the Ponding design requirements in AISC (9th Edition, Section K2). I don't understand where this would be a problem. It states that ponding must be checked "... unless the roof surface is provided with sufficient slope ... to prevent the accummulation of rainwater." Who wouldn't do this? All architects I know of are aware that it rains and slope roofs, using tapered insulation or just sloping the roof members. I would dare say they would be negligent if they didn't. I know there have been roof failures during heavy rainfalls, but I don't think that not providing a roof slope is the reason.
So when would this be an issue? Is there some slope that isn't sufficient?
The reason I'm asking is that I'm doing a review and formula K2-1 is about .4, but the roof has a 3/8 inch per foot slope.
So when would this be an issue? Is there some slope that isn't sufficient?
The reason I'm asking is that I'm doing a review and formula K2-1 is about .4, but the roof has a 3/8 inch per foot slope.






RE: Ponding
One case that you need to watch is beam camber. It might not cause problems in steel design, but with glulams it can be a problem. Also sometimes on jobs there are areas of roof transitions that get over looked or errors in bearing elevations.
If you have 3/8" per foot slope you should be Ok.
RE: Ponding
RE: Ponding
I recommend that you have a heart to heart talk with the plumbing designer to make sure that he has an non- adjustable drain specified and a proper overflow or scupper at the right height.
RE: Ponding
Don Phillips
http://worthingtonengineering.com
RE: Ponding
If AISC wants us to check ponding, they should give a minimum slope where this might be an issue instead of using terms like "sufficient slope." Maybe the new AISC does this.
RE: Ponding
RE: Ponding
That being said, let's get our terminology straight. Ponding has nothing to do with blocked drains or a lack of scuppers. If a roof has large bays and is sloped 1/8" per foot, the joists may deflect so much that they POND water, deflect, POND more water, deflect more, and so on, until failure.
If you have no scuppers or secondary drains, the IBC requires you to check for BLOCKED drains. This is much worse than ponding, since you must assume the roof fills up with water until the water spills over the top of the parapet. A ponding check will NOT prevent failure if you have no scuppers or secondary drains.
DaveAtkins
RE: Ponding
1. Simplified design for ponding
2. Improved design for ponding
These are contained in Appendix 2 “Design for Ponding” in AISC specifications for steel buildings dated March 9, 2005. This version of specifications supersedes all previous editions.
AISC members can download it from the web site in PDF format.
Regards,
Lutfi
RE: Ponding
Most "flat" roofs in the US are set at 1/4" per foot. I'm just making sure you didn't mean "equal to or more than" 1/4" per foot.
With most "flat" roofs at 1/4" per foot, the question then is whether or not the ponding check in AISC has any significant meaning for those roofs. I think that DaveAtkins point that you are really checking the predisposition of the roof to be susceptible to ponding and that is really the whole point.
I have to admit that JedClampett's question has been in my mind in the past.
RE: Ponding
"Ponding: Retention of water due solely to the deflection of flat roof framing"
"The roof structure system shall be investigated through structural analysis to assure adequate strength and stability under ponding conditions, unless the roof surface is provided with a slope of 1/4 in per ft or greater toward points of free drainage or an adequate system of drainage is provided to prevent the accumulation of water."
We make a living by what we get, we make a life by what we give.
Sir Winston Churchill
RE: Ponding
A 3/8" per foot slope is great for drainage, and if the drainage edge is free (gutter or direct overflow) then the structural impact is minimal. If there are internal drains or scuppers, I would check it.
RE: Ponding
I would blame that problem on the scupper design, not the lack of a ponding check. Again, what you are describing is more akin to a "blocked drain" than a ponding problem.
DaveAtkins
RE: Ponding
I think the point Ron and I are trying to make is that water can pile up on a roof even with a 1/4-inch per foot slope. Where water can pile up, I don't see how you can avoid checking for ponding stability, in addition to other things. If a blocked drain causes the water to pile up and you didn't check for stability, what will you tell your lawyer?
RE: Ponding
As far as my review project that started this thread, I looked at the architectural drainage plan and didn't like what I saw. Small scuppers, parapet roofs, one level of roof draining onto another, etc. I noted that I'm concerned about ponding issues and bounced it back to the design engineer. Luckily this is in southern California, so we don't have Florida or Texas type rains. But every once in a while....
RE: Ponding
I agree it's a scupper design problem, but since most building design is not well integrated, we often see that disconnect.....the structural engineer doesn't design the roof drainage, but sometimes has to consider that whomever did, might not have done such a great job.
RE: Ponding
If a roof is sloped at 1/4" per foot AND has proper drainage, then ponding CANNOT occur. The deflection of the joist at this roof slope is not enough to allow water to build up.
If a roof does NOT have proper drainage, the issue is not ponding. Rather, the live load must be increased (sometimes tremendously) to make the roof safe. The average roof live load in my part of the country is about 30 psf (for snow). If you don't have proper scuppers or secondary drains, and the roof is designed for this live load, then it is only good for about 6" of water. Parapets can be alot taller than 6" sometimes--for example, a 2' high parapet with no scuppers or secondary drains will result in 125 psf live load before the water spills over the top of the parapet.
My point? Unless you want to seriously overkill the roof member sizes, make sure there are adequate scuppers or secondary drains.
DaveAtkins
RE: Ponding
Properly designed drains don't account for the improper placement of dead animals, sticks or leaves. I can put whatever notes on the drawings I want and that dead raccoon won't move out of the way. You're still back to primary drains blocked. Someone else sizes the drains where I'm from, so how can I ensure proper drains? We're not even in that equation.
RE: Ponding
DaveAtkins
RE: Ponding
RE: Ponding
I think we should keep in mind time and probability when referring to all the water than can collect on the roof. It would be highly unlikely that any storm dump more than about a foot of water or so, and especially not 24 inches. I don't have actual data to back that up, but I've never heard of having that much at once. It would take several storms with no leaking, slow drainage or evaporation in between to actually fill a roof with anything larger thana small parapet. I think we should turn to the 50-year or 100-year storm if we want to determine how much water can be dumped on the roof at any given time rather than treat the roof like a pool that has a constant supply of water to fill it.
RE: Ponding
There are 2 things to consider when designing a roof for rain loads.
First is your roof pitch less than 1/4" per foot? If so, then you need to consider stability against ponding. If your pitch is 1/4" per foot or greater, then I see nothing in ASCE 7-98 or IBC 2000 that requires a ponding check. Concerns regarding blocked drains are addressed in my next paragraph.
Second, you have to design the roof for the depth of rainwater that can accumulate if the primary system is blocked, including the hydraulic head. This has to be done wheather or not ponding is a factor.
The retention of water due to the deflection of relatively flat roofs and the accumulation of water due to blocked drains are 2 separate issues.
RE: Ponding