RC floor vibration analysis

[ramihabchi]

hi.is the stifness of slab used to in vibration analysis is usualy the initial stiffness of the slab or the stifness after creep and shrinkage occurs?(long term stiffness)

 

[WARose]

I generally use the effective moment of inertia (about 0.35I_g). But that is just as a median. Since the loads & f'_c of concrete are not 100% predictable, I look to see if there is a possible stiffness of the slab that will create issues. I've seen people vary the stiffness by as much as +/- 100%.