slab offset effect on FEM model

[Yorgen]

Hello from abroad.
I am currently working on a concrete slab with elevation gaps and changing thickness. when using plate elements .i have noticed that changing the offset of the slab (even thou the thin slab is still fully fixed within the thick slab) yields different moment results(by 15% approx). i assume the difference in the results is due to boundary condition applied by the program (atir strap). I am having hard time understanding the difference in results since the thin slab is still fully fixed to the thicker slab.

any source of information explaining the gap in results by statics would be helpful. also any paper explaining when offset should be used.

Hopefully my english was clear enough.
Thnak you.

 

[Celt83]

Some programs will automatically apply a series if rigid links between the element centroids at the offsets.

For two-way slab structures there is incidental axial loading developed within the slab to balance the end shear in the columns generated by the bending reactions. This axial loading will generate additional bending at the rigid links similar to P*e of post-tensioning. In your second sketch the element centroids are aligned such that the internal axial loading is applied at both element centroids such that e=0.

 

[Celt83]

Also import to remember that most shell/plate elements are not true 3d bricks they are 2d plane elements with constant thickness such that the element nodes are modeled at the shell centroidal axis. So two shells require link elements between their nodes to enforce compatibility across an elevation/thickness change.

 

[Yorgen]

thanks for the detailed reply. i actually used a 2d model without columns and just now noticed that the eccentricity give rise to horizontal reactions at the supports (vertical loads applied only).

sound kind of backward

as far as i can think the boundary condition should be similar in both offsets (as long as the thin slab fully within the thick slab)

thick floor in red, offset thin floor in blue.