huangyhg

floor vibrations
i am in the process of checking a composite floor per the aisc design guide #11 for floor vibrations. i have a situation where my joist spans differ on each side of my beam. here’s the question. when calculating bg(page 18), what dj value do i use if my joists differ on one side of the beam to the other? should i ratio the dj values? thanks for your help.
joemarch - one way to look at the problem is to "bracket" the solution - make calculations with different assumptions and compare the results.
for starters, you get a good break. the computed value of bg is dependent on the 4th root of dj. by the time you take the 4th root on any number, relatively small changes in the value of dj get "lost".
i suggest that you assume all the joists (on both sides) are the same (say the small joists). repeat the calcs for the other size joists (the large joists). choose the worst case; but there will probably not be much difference.
best wishes
thanks slideruleera. thats exactly what i did.
joemarch,
since floors usually give more problems when their natural periods are longer (more flexible floor system), the shorter joists might give an apparently more rigid answer. i would try using the longer joists for calculation, just so the upper bound on flexibility can be investigated also. i hope this is timely enough to be of help.
dave
david w. anderson

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