Ponding analysis by manual calculation
Ponding analysis by manual calculation
(OP)
I have a project where an engineer used an in-house procedure to check some long span roof joists for ponding loading.
The roof uses primary drains only and then depends upon over-the-roof flow of water to scuppers at a far end of the building. There are about 4 to 5 valleys that have these primary drains and the water must flow over the various ridges which about 3" high or so, to get to the scuppers.
I honestly don't know what the exact procedure is but I think it is essentailly this:
1. Load up the roof joist with the initial water assuming the drain is clogged.
2. The joist deflects downward some initial amount.
3. Recalculate the new depth of water based on the now-deflected shape of the joist.
4. Re-calculate the new deflection of the joist based on the re-calculated depth of water.
5. Repeat until convergance.
I don't know the validity of this process.
I checked the joists against AISC for ponding (9th Edition and second Edition section K2 or Appendix 2 in the new 13th Edition). This check revealed that the joists were no good in terms of ponding control.
However, the engineer states that the AISC provisions only determine IF you need to further check for ponding....not whether there is a problem with ponding. He claims that his method above then satisfies the code with respect to ponding.
The actual language that AISC uses is: The roof system shall be considered stable for ponding and no further investigation is needed if both of the following two conditions are met:....
This seems to imply a "further investigation".
I've never thought of it like this before. I've always taken the AISC provisions as determining if the joists are inadequate for ponding or not.
I've never heard that they simply determine whether a further check is required.
Thoughts on this?
The roof uses primary drains only and then depends upon over-the-roof flow of water to scuppers at a far end of the building. There are about 4 to 5 valleys that have these primary drains and the water must flow over the various ridges which about 3" high or so, to get to the scuppers.
I honestly don't know what the exact procedure is but I think it is essentailly this:
1. Load up the roof joist with the initial water assuming the drain is clogged.
2. The joist deflects downward some initial amount.
3. Recalculate the new depth of water based on the now-deflected shape of the joist.
4. Re-calculate the new deflection of the joist based on the re-calculated depth of water.
5. Repeat until convergance.
I don't know the validity of this process.
I checked the joists against AISC for ponding (9th Edition and second Edition section K2 or Appendix 2 in the new 13th Edition). This check revealed that the joists were no good in terms of ponding control.
However, the engineer states that the AISC provisions only determine IF you need to further check for ponding....not whether there is a problem with ponding. He claims that his method above then satisfies the code with respect to ponding.
The actual language that AISC uses is: The roof system shall be considered stable for ponding and no further investigation is needed if both of the following two conditions are met:....
This seems to imply a "further investigation".
I've never thought of it like this before. I've always taken the AISC provisions as determining if the joists are inadequate for ponding or not.
I've never heard that they simply determine whether a further check is required.
Thoughts on this?






RE: Ponding analysis by manual calculation
Under many circumstances the differential equation governing the deflection of a beam subject to ponding is identical to the differential equation for a beam on an elastic foundation. The only difference is that the ponding case has a negative "foundation stiffness". If the resulting DE has a solution then ponding will not occur.
If you have access to any software for the BEF problem that allows you to specify negative BEF foundation stiffnesses then you might be able to use that software to solve your problem in the one pass.
Alternatively, if you have a simple beam program that allows negative nodal support springs, you could approximate the "true" BEF situation through modelling a series of closely spaced (negative) springs.
RE: Ponding analysis by manual calculation
I think the approach this engineer used is valid as long as the stress in the joist does not exceed 0.8*Fy.
DaveAtkins
RE: Ponding analysis by manual calculation
While his approach does seem reasonable, I guess I'm confused by my main question:
If the AISC ponding checks show No Good...does this imply a need for "further investigation" or does it mean the joists are No Good?
It would seem strange to have both the main body check and the AISC curves in the commentary as well - but with these ONLY serving to determine if you're close and have to check further....and then for AISC to not offer a "real" check procedure.
RE: Ponding analysis by manual calculation
I don't have ASD 9 right at this minute. By the 13th manual, I read that Section 2.1 is simplified and Section 2.2 is more exact. If Section 2.1 shows that ponding may be a problem, the engineer has the option to use the more exact method of Section 2.2 to check the roof system (this is where the curves show up in the 13th manual).
But the way I read ASCE 7 is that you are not limited to using only the AISC ponding checks (example, if SJI had a method you might choose to use it). Example, page 95 of ASCE says "it shall be investigated...." but doesn't state a required method.
So an alternate "in-house" investigation may be allowed even if both AISC methods do not work unless you are for some reason (Building Department, Goverment UFC criteria etc.) required to meet AISC. Right?
RE: Ponding analysis by manual calculation
RE: Ponding analysis by manual calculation
RE: Ponding analysis by manual calculation
RE: Ponding analysis by manual calculation
The thing that the AISC approaches tend to do is include an aspect dealing with the water flowing - sort of a dynamic effect. The engineer's approach that I listed above seems to be more static. I don't feel that water behaves itself that much....but maybe I'm wrong - I have a question posted to AISC so we'll see what they say.
haynewp - whether ASCE 7 requires the AISC procedure or not may be valid, but for a steel roof it seems to me that AISC should be the authority.
Do you all agree with the engineer, though, that the AISC checks are ONLY checks used to determine whether you need to investigate ponding?
Or are the AISC checks a check on ponding itself?
Or both?
RE: Ponding analysis by manual calculation
http://www.steeljoist.org/publications/default.asp
The AISC presents a method for checking ponding, not telling you that you have to look somewhere else if it doesn't meet AISC.
RE: Ponding analysis by manual calculation
RE: Ponding analysis by manual calculation
RE: Ponding analysis by manual calculation
RE: Ponding analysis by manual calculation
40x12/240=2" and 20x0.25=5"
RE: Ponding analysis by manual calculation
RE: Ponding analysis by manual calculation
Thus, there is a need to check for ponding with the anticipated depth of water.
RE: Ponding analysis by manual calculation
RE: Ponding analysis by manual calculation
RE: Ponding analysis by manual calculation
The maximum amount of water that will build up depends on the geographic location of the building and size of the roof as well as the size and number of scuppers at the perimeter. For a small roof you may only end up with a few inches of hydraulic head build up at the perimeter scupper where the parapet may be 1'-2' above the roof.
RE: Ponding analysis by manual calculation
The attached PDF is a good reference for dealing with hydraulic accumulation issue. It was written for HVHZ and may be a little out of date but still more than I ever wanted to know about roof drainage.
RE: Ponding analysis by manual calculation
The Scope of the AISC Specification permits alternate methods of analysis and design when such are acceptable to the authority having jurisdiction. Section 2.1 of Appendix 2 provides a simplified and conservative method for checking ponding to see if the system is adequate. This check does not necessarily mean that the system is inadequate if the conditions are not met. Section 2.2 provides one more accurate method of evaluating the ponding characteristics.
RE: Ponding analysis by manual calculation
I agree, a complete clog on drain system is highly unlikely, as well as the event occurs coincident with a major rain storm. However, if there is/are reason(s) for concern, I would rather be safe than sorry, especially when human lifes are involved. Your reasoning are sound and shall be in the first line of thoughts to determine whether there is potential for such problem, which requires further investigation.
RE: Ponding analysis by manual calculation
"The AISC ponding provisions are based on the following....Situations often occur where these conditions do not exist. For these cases, solutions can accurately be obtained by modeling the structure, adn iterating using increasing waster loads resulting from deflections until convergence..."
RE: Ponding analysis by manual calculation
RE: Ponding analysis by manual calculation
Don't know if this was mentioned, but, if the ponding is long term, the joists may end up with a permanent deflection that would be present for the next storm, deflecting further during that storm, storing even more water, causing more deflection.
This is the scenario that I worry about and why I never design flat roofs. I realize this may be an existing roof, but if there is any way to avoid ponding, do it.
Mike McCann
MMC Engineering
RE: Ponding analysis by manual calculation
I would also make sure some consideration of the hydraulic gradient was included in the analysis, unless it's too small of an area to worry about. The water has to have some positive slope to drain to the scuppers, so simply assuming 3" of water (or whatever the dimension is from ridge to valley) will not be adequate in certain areas where enough hydraulic head has to build up to slope the water to the scuppers. It's been awhile since I've looked, but from the old Q=KiA days of my hydraulic eng. class, I believe the 'i' (hydraulic gradient) is 1% or so.