huangyhg

ibc 2003 serviceability for steel buildings
we have a great debate waging in our office and i thought i would throw it out there to you guys to see what kind of opinions i get.
the whole issue began with the question as to whether a steel rafter supporting a drop ceiiling needs to meet a deflection of l/180 or l/240. of course the drop ceiling would qulaify as a non-plaster ceiling putting it in the l/240 category. however, the argument has been made that the drop ceiling is a flexible system and could certainly bounce back from a deflection of l/180.
this of course led to other debates concerning the amount of snow load that needs to be considered for these deflection calculations. is it dl + 1/2 50 year snow or dl + full 50 year snow. there seems to be some disagreement here between aisc, asce and mbma.
i'll leave it at that for now and see if you guys have an interest in discussing this topic.
~waytsh

i would think a non-plaster ceiling such as you describe could handle l/180 total load deflection, and l/240 live load deflection. but of course, it's not that simple.
if the roof framing is concrete, creep must be considered. if the roof framing is steel, and is a low slope roof, ponding must be considered. i size steel joists for roofs for l/360 live load deflection, since i have found this tends to control ponding.
oh, and i always use full snow load for checking deflection.
daveatkins
i follow aisc design guide 3 (second edition)recommendations. in the second edition, they talk about although acoustical pnl ceilings have a high tolerance for distortion, this isn't the case at the perimeter where the metal angle gets attached to the wall. to keep the ceiling from hanging up on the wall would have to isolate it with additional hangers. i have seen a lot of problems in pre-engineered metal buildings in the winter with ceilings hanging up on partition or perimeter walls.
i design for the recommendations in the second edition of the design guide, which have recommended deflection ratios and maximum recommended deflections. they also say to use a 50 year snow. if the ceiling does hang up on a partition and there is some distress, you could get an irate call from the owner.
thank you both for your input. it seems that there is very little chance of having the drop ceiling recieve damage although the end customer certainly may not like the look of the deflected ceiling. on the other hand there also does not seem to be much written in the codes defending the position that a deflection as high as l/180 can be used. having been undecided on this issue i think i am starting to lean towards playing it safe and using a deflection limit of l/240.
i would still be interested in hearing more opions both for or against as well as any relavent code mentions.
~waytsh
my group always designs to a deflection of l/240 for a drop ceiling under the uniform roof live or snow load case, as well as any pattern/unbalanced snow cases. we do not, however, check deflection under any pattern live load cases as these are construction live loads.
the codes just don't leave any out for anything but the l/240.

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