Ponding 2

[JedClampett]

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.

 

[UcfSE]

Dave, I agree. I see the ponding check as a stabitlity check. We still have to check for strength.

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.

 

[bjb]

I agree with Dave Atkins

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.

 

[civilperson]

The angles of rotation, (slope), on the lower end of a deflected bean or joist must be less than than the slope of the roof to prevent ponding in the well drained case.