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Roof Ponding 1

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civeng80

civeng80

Structural
Dec 21, 2007
745
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 ?

 
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That seems very conservative. I have not seen such requirements/consideration on portal framed buildings which are about
20-30m wide. Also why is ponding considered with Wd (I assume Wd is wind down)? It seems unlikely that wind will "push" the roof down creating a pond and more likely pull the roof up in uplift.
 
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A 2 degree pitch is not slight. I would not consider it a "near flat" roof as the slope is greater than most new roof minimums that I deal with. Drainage will easily occur and ponding is not likely unless you have an interior drainage obstruction or a back-up at a parapet.

As for deflection, stick with typical serviceability based on what is below the roof (ceiling type). L/500 is, in my opinion, much too conservative for roof deflection.
 
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What is the type of roof sheeting? What is your rain fall 100year, any peno's that could cause dumps into pans. There are a few referances that you should consider. Design of portal frame building, minimum sheeting slopes and max allowable mm/hr either stramit or lysaght.

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."
 
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The type of roof will be lysaght kliplok sheeting (a concealed deck) allowable pitch is 1 degree.
Roofing penetrations: there will be some dont know how many (try to keep to minimum)
100 year rainfall intensity (with a time of concentration of 10 minutes)for area is 85mm/hour.

Wd is dead load down
Wwl is the load of water layer in this case about 62mm not wind
Reference is Gorenc and Tinyou an old edition with deflection limits.
Couldn't find any other references.
 
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Im in Australia. Can the guidelines be downloaded ?
AITC = American Institute of Timber Construction ?
 
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Sorry I should have expanded: design of portal frame building by Woolcock et al. References an old survey done by the AISC for this situation and L/500 under dead load alone is considered ok however a "note" is also referenced, so I have attached the pdf. I would think about L/360 would be good, but would check that no slope is less than 1 deg across the beam.

60mm water layer sounds wrong and looks wrong for a 42mm high sheeting, probably tastes wrong as well.
at 85mm/hr (should be a 5min event so maybe 90mm/hr) I would expect a thinner water layer more like 5-10 mm??? and this is at the gutter end.

without doing any calcs I would be thinking a 410 (or a 360) (span/40) for a simple beam situation, what have you got worked out?.

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."
 
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Thanks alot rowing, I actually have "design of portal frame buildings" just didn't look at that referenace.
I came up with a 410UB 59.7 a total DL def of 29mm span/538 DL + LL Def of 63 span/250
I think its a reasonable deflection.
 
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If you still need the AITC guidelines, I can post 5 pages to this site from the First Edition in 1966.

It did have one statement that "When flat roofs have insufficient slope for drainage (less than 1/4"/ft), the stiffness of the supporting members shopuld be such that a 5 psf load will cause no more than a 1/2" de3flection. (See "Engineers Notebook", Civil Engineering Magazine, October, 1962.)

It al;so seemed to prefer cambering to control the ponding, apparently not what you want to do.

Mike McCann
MMC Engineering
 
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msquare I would be interested in the material. Thanks.
 
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85mm/hour for a 100-year rainfall intensity? that sounds more like 100-year rainfall depth. should be a 5 minute or less time of concentration for flat roof drainage and I would expect intensity to be more like 200 mm/hour or even more for such an intense, short duration rainfall.
 
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For a 5 min storm duration rainfall intensity for a 100 year storm is 110mm/hour.
 
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I see, you are in Australia...
I was having a difficult time finding any place in the US with less than 140 mm/hour other than Bakersfield
 
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