Exterior column stiffness modeling SAFE

[EngAndy]

Hi,
Some collegues told me today that they usually apply a property modifier of 0.3 to bending stiffness of exterior columns when modeling a slab without beam in SAFE. They apply no modifiers to the slab. They told me the reason they do this is because they try to match the direct design method for the exterior columns (0.26 Mo), but I don't like that explanation. I feel this procedure is unsafe for the verification of punching shear + design of columns and it is not prescribed in any code or any textbook that I own. Since moments at exterior columns are very big if we use gross inertia of columns I was wondering if a non-linear cracked analysis of the slab could be done to have the design moments. I did not test it yet, but I feel the negative moment at the exterior support would be smaller with a cracked analysis, since the effective inertia of the slab in the negative moment region would reduce. Usually I only use that cracked analysis for deflection verification, but I was wondering if it would give good results to design for shear and bending.

 

[TheDW]

I usually envelope it...design for punching shear/slab top reinforcement without stiffness modifiers on the columns, and use 0.7 on all columns for deflection checks/slab bottom reinforcement.

Its discussed a bit in this thread...JAE has an approach to stiffness modifiers that makes sense, albeit for service checks, not strength like you're asking about:

https://www.eng-tips.com/viewthread.cfm?qid=438408

Trying to shoehorn the direct design 'approximation' method into an FEM 'exact' method doesn't feel right to me