capacity of a gypsum roof

huangyhg

capacity of a gypsum roof
i am a structural engineer in the mid-west and have a project that would put snow drift load on an existing structure.  the existing roof construction is bar joists @ 4'-0" o.c. +/-, bulb-tee purlins @ 32 5/8" o.c., 1/2" gypsum board, 2" poured gypsum, 2" insulation & bur.  i'm looking for resources that would let me determine capacity of this system for additional snow loading.
gypsum strength declines with the ater to dry gypsum ratio.
wc (water/dry gypsum)    limit tensile strength
0.45                     22.7 kgf/cm2
0.5                      17.7 kgf/cm2
0.6                      14.7 kgf/cm2
0.7                      10.6 kgf/cm2
0.8                       9.2 kgf/cm2
1                         8   kgf/cm2
compressibe strengths are far bigger, maybe around 8 times the tensile.
the main thing here is to ensure a perfect impermeabilization of the roof to be kept closely watched, because water will distroy the gypsum plates, and even condensation studies need be made to avoid condensation produce the same effect. water plus gypsum plus steel also will give severe problems the mass of gypsum here being so big.
the attached papers of the gypsum board somewhat will hamper the effective contact and i wouldn't rely on composite action.
lutfi,
i know of at least one contractor in illinois that still does this type of work, so if you need a contractor, i can give you his info.
thanks in advance.
the system you described was very popular in the late 50's -60's.  unfortunately, if any water got into the system, you might as well replace the gypsum deck because it is usually worthless after it gets wet.  the other popular roof deck material was "tectum".
here are a couple of url's with information on gypsum roof deck systems from a quick google search just to get you started.  i did not download the .pdf file myself so, i really don't know how on target that one is.
thank you everyone for responding, but the info doesn't really address my question.  i need some resource for determining the capacity of this roof system for additional snow loading due to drift.
i'm aware of what can happen if water gets into the system & that is another concern.  i'm not sure yet i want to add load to this roof that will later melt and possibly get into the deck.  i know this roof is not "tectum" so that's not a concern.
jeidt2543 maybe your contractor contact can help.  if you could provide his info, that would be my next step.  my e-mail address is:
vobs,
i certainly appreciate your concern.  my thoughts are that, if you haven't already done so, you first see where you are considering the capacity of the joists.  odds are they aren't going to be adequate for additional load.  (one fix i've seen used  is to add joists in alternate (every other) deck spans to theoretically reduce the tributary width to each joist.)  if the cost of the fix for the joists can't be justified, you're done; otherwise you'll still need to figure out what it takes to reinforce the gypsum construction for the added load and, unfortunately, i can't be helpful on that aspect without doing a good bit of digging myself.
sto04078
i've considered that and will check the joists as well.  i just want to have all bases covered before i make a recommendation.
well, maybe i was not clear enough. i simply wouldn't use gypsum as a structural material. but if someone wants to use gypsum to his risk, i would recommend the limit strengths above, using only the new thickness, maybe a safety factor of 3, (that is, the values aboved divided by 3 would be the allowable stresses) and determining the stresses elastically as plates or beams.
it's not my choice to use this material, but it is part of the existing building that would get additional load due to snow drift due to a proposed addition.
now if i understand you correctly, at best i could get 22.7 kgf/cm2 tensile strength which equals 323 psi approximately. with a factor of 3, that gives me 107.667 psi for an allowable tensile stress.  let's just say 105 psi. now assuming the 1/2" gyp board plank is acting alone (non-composite), the most it can carry on a 32.625" span would be 4.75 psf.  self weight of the decking system itself is 28-35 psf depending on how much balast is present.  something else is happening here.  while your info may be technically correct, it's not getting me any closer to a solution.
as i stated before,
i have span and load tables for gypsum roof decks with bulb tees. i do not know what happened to my earlier response. i am offering this again. please contact me at