ponding

huangyhg

ponding
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.
i would never assume that the architect is designing the roof for proper drainage.  they should be but sometimes they don't.
one case that you need to watch is beam camber.  it might not cause problems in steel design, but with glulams it can be a problem.  also sometimes on jobs there are areas of roof transitions that get over looked or errors in bearing elevations.
if you have 3/8" per foot slope you should be ok.  
not all rain water can freely run off of the roof.  most of the time i see it has to go through scruppers or internal drains.  even unclogged the water will develop some head as it builds up before it can get out.  if the scuppers are clogged then you'll have both static head and hydraulic head as the water builds up to get out of the overflow scuppers.  that's plenty reason to design for ponding.  many times when i'm looking at steel joists and girders ponding stability per aisc k2 will control the design.
years ago, there used to be a philosophy to use flat roofs and "controlled flow" roof drains which would restrict the flow of water to the sewer so that a larger sewer pipe did not need to be provided. the idea was to pond water on the roofs until the sewer had time enough carry the water away. unfortunately, there are still those adjustable flow drains which are sometimes specified by plumbing designer that may not know the structural implications.
i recommend that you have a heart to heart talk with the plumbing designer to make sure that he has an non- adjustable drain specified and a proper overflow or scupper at the right height.
you must also consider maintenance - or lack thereof - of the roof drains to avoid a failure, or assume some ice damming. and i have seem the plumbing and architectrual drawings not being coordinated and the roof drains are in the wrong location relative to the slopes of the flat roof. all are potential sources of evenutal failure.
don phillips
my problem is i'm reviewing a project for other engineers who are pretty sharp.  the aisc doesn't say to assume lack of maintenance or clogged drains and the scuppers are not flow restricted.  they're going to trump me with the roof slope and say it's sufficient.
if aisc wants us to check ponding, they should give a minimum slope where this might be an issue instead of using terms like "sufficient slope."  maybe the new aisc does this.
if the roof in constructed such that there is no way for water to pond on the roof, then you're fine.  think of a typical plain house with a 4:12 sloped roof.  the water just falls off the sides with no where to collect on the roof.  no ponding design needed.  if you have parapets or anything that can hold water if your drains become blocked, then you need to check ponding stability.  it doesn't matter how much slope you have if the water cannot get off the roof as fast as it can get on the roof.  the aisc should not have to tell us to assume the drains are clogged.  we should know to do that.  at some point to have to use judgment.  they can't do it all for us.  check also chapter 16 of your building code.  the fbc, based on the ibc, has am updated section for rain loads.  you may have this also.  
i read somewhere (the ibc?) that a roof need not be checked for ponding if it slopes more than 1/4" per foot.  that is the criterion i use.
that being said, let's get our terminology straight.  ponding has nothing to do with blocked drains or a lack of scuppers.  if a roof has large bays and is sloped 1/8" per foot, the joists may deflect so much that they pond water, deflect, pond more water, deflect more, and so on, until failure.
if you have no scuppers or secondary drains, the ibc requires you to check for blocked drains.  this is much worse than ponding, since you must assume the roof fills up with water until the water spills over the top of the parapet.  a ponding check will not prevent failure if you have no scuppers or secondary drains.
daveatkins
you may want to perform rigorous analysis on the primary and secondary members utilizing the equations for:
1. simplified design for ponding
2. improved design for ponding
these are contained in appendix 2 鈥淒esign for ponding鈥?in aisc specifications for steel buildings dated march 9, 2005. this version of specifications supersedes all previous editions.
daveatkins - could you please clarify your criteria for me....you said "if it slopes more than 1/4" per foot" you don't check ponding.
most "flat" roofs in the us are set at 1/4" per foot.  i'm just making sure you didn't mean "equal to or more than" 1/4" per foot.
with most "flat" roofs at 1/4" per foot, the question then is whether or not the ponding check in aisc has any significant meaning for those roofs.  i think that daveatkins point that you are really checking the predisposition of the roof to be susceptible to ponding and that is really the whole point.
i have to admit that jedclampett's question has been in my mind in the past.
aisc 2005:
"ponding: retention of water due solely to the deflection of flat roof framing"
"the roof structure system shall be investigated through structural analysis to assure adequate strength and stability under ponding conditions, unless the roof surface is provided with a slope of 1/4 in per ft or greater toward points of free drainage or an adequate system of drainage is provided to prevent the accumulation of water."
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