Ponding analysis by manual calculation

[JAE]

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: [blue]The roof system shall be considered stable for ponding and no further investigation is needed if both of the following two conditions are met:....[/blue]

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?

 

[JAE]

I did get a reply from AISC:

[blue]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.[/blue]

 

[kslee1000]

SteelPE:

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.

 

[WillisV]

As an FYI the previously mentioned SJI Technical Digest 2 is based primarily on the AISC provisions with jost specific guidance; however, it does contain the following statement:
"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..."

 

[WillisV]

Er...technical digest 3.

 

[msquared48]

JAE:

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

 

[JKStruct]

With regard to your orginial question, I interpret the AISC provisions as a conservative analysis for roof systems to avoid a more detailed or comprehensive ponding investigation. If you do not meet the provisions in K2, you still might be adequate for ponding stability, but you have to complete further analysis.

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.