ETABS 2013 automesh affecting outrigger columns

[Pharaohnee]

Good morning,

I'm having an interesting issue with ETABS 2013

I have a concrete high rise structure with an interior core wall and perimeter mega-columns. These mega-columns are being engaged in the lateral system by a typical outrigger system (3 outriggers equally spaced up the tower).

For the first run, I assigned "rigid diaphragm only" for my slab. For my second run, I wanted to get some gravity loads, so I used a default auto-mesh. With my second run, I noticed that my outrigger columns were seeing approximately 35% more load (wind only) when using default auto-meshing for the slab.

Has anyone ever experienced this?

Thanks!

 

[Pharaohnee]

Also in addition, my total base shear from wind loads is increasing by about 26% with using the default meshing - something does not seem right

 

[asixth]

Yeah, the increased base shear isn't right.

For the automeshing. If you have haven't automeshed the slab then it will just be the wall transferring the load to the megacolumns. With automeshing the slab with engage with the wall to make a stiffer flanged section capable of transferring more of the overturning load.

 

[Pharaohnee]

Right now I have my slab bending stiffness down to 0.25. Maybe I should reduce it a bit more?