Mode shapes are strange

[Robert-32]

I Etabs model of multistory building, I get mode shapes showing large deformation of the slab, each mode shows different part of the slab moving up and down in no realistic way. Does anybody have a solution for this? The slab was modeled as ribbed slab using the deck section.

 

[structSU10]

What you describe may be correct - note that a mode shape as shown in a model has no direct physical meaning. The mode shapes are normalized based on the way they must be calculated - so a modal deflection of 3.2 is only with respect to a control point set to 1 or whatever the program assumes. They are more for understanding problem areas, and they are used with a forcing function to determine the structural response.

 

[MrHershey]

Do you have the slab modeled as a membrane or with very small stiffness modifiers? If so, that could be the source. I've had this issue when I would adjust wall stiffness modifiers to force all load into in-plane bending and all of a sudden my major mode shapes are all out-of-plane bending of the walls.

 

[Robert-32]

my slab modeled as deck to represent one way ribbed slab.

 

[Robert-32]

Solved.
It was loose connections of some beams not slab, that drag the slab with them.