Roof Ponding 1

[civeng80]

This question has probably been asked before.
I have a roof pitch of 2 degrees.
Roof beam span about 15.6m (48 ft)
Not to keen on camber (as it is very expensive here in Australia !)
Have seen a deflection limit of L/500 to L/600 for a load of Wd + Wwl (where Wwl is load due to water layer L/250 deep)
(Gorenc and Tinyou 5th edition) This is a mighty old textbook still using working stress, but the only guide I can see.
This gives me a pretty big beam.
Anyone have comments or other recent deflection limits for near flat roofs ?

 

[WillisV]

Ponding is also covered by AISC in the steel specification - for 14th Ed. see Appendix 2 and its associated Commentary (free download here:

http://www.aisc.org/WorkArea/showcontent.aspx?id=26516)

The 1/4"/ft slope value often given as the point in which ponding can be safely ignored can be misleading and sometimes still leads to problems. If I am really worried about it I generally just do an iterative analysis on the bay using the weight of water up to the emergency scuppers plus the head as the initial load, checking the deflected shape, calculating the additional load due to the deflected shape, adding the additional load to the model and re-running the model, etc etc. until it either converges at a safe load level or fails.

 

[PSUengineer1]

I am in the states but I will reply anyway.

2 degrees is between 1/4 and 1/2 inch slope per foot (i.e. 0.25:12, 0.5:12). Take maximum slope that you coordinated with architect, in this case 1/2 per foot and use 62.4 pcf and calculate surface loading due to ponding and add it to your roof live load (uniformly or taper the load as you see fit). Don't forget, roof drains clog frequently!