Slab loads to beams

[stroxy]

I have a finite element model consisting of 2 way spanning slabs supported by beams.  After viewing the panel cuts for the self weight shear force diagram, I notice that cut A1 and A3 are not behaving as I would expect, which is that the shear force diagram would change signs over beams 2549 and 2553. As a result, the beams are very lightly loaded. Is there a logical explanantion or am I missing something.

[Artur.Kosakowski · Accepted solution]

The beams being not significantly stiffer than the slab? Try to offset them below the slab or increase its moment of inertia as described:

http://forums.autodesk.com/t5/Autodesk-Robot-Structural/T-slab/m-p/3185418

http://forums.autodesk.com/t5/Autodesk-Robot-Structural/Beam-offsets/m-p/3332921

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[stroxy]

I kind of suspected that.  OK Thanks!!

[stroxy]

One issue comes to mind.  In seismic analysis we are supposed to actually reduce the moments of inertia of frame members to increase the building period.  Actually my beam elements have reduced moments of inertia in the analysis module and that may also be further contributing to the problem.

Am i correct to reduce for vertical elements and maintain that of beams.  

[Artur.Kosakowski]

Then I think this approach should be applied to both beams and slabs (as well as columns).

[stroxy]

Sounds like a plan!! Thanks

[stroxy]

in reference to your post, I have a uni-directional box floor with stiffnes matrices as attached and I want to reduce the stiffness matrix by 50%.  do I reduce all clouded values in the attachment by 50% to be code compliant?

[Artur.Kosakowski · Accepted solution]

Correct.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.