Different values for max Mx / My 1

[Bill2001]

Hi friends!
I've modeled a simple supported slab of concrete 1 m x 1 m x 0.25 m in STAAD.Pro
then I applied a joint load of 100 kN at the center of the slab (in the middle of the slab).

The 1 m x 1 m slab is then
1) Meshed up 4x4 mesh (0.25 m x 0.25 m each plate element).
2) Meshed up 20x20 mesh (0.05 m x 0.05 m each plate element).

Max Mx and My (Summary) are now quite different for 4x4 mesh and 20x20 mesh !!!
Can we accept that different mesh sizes produce so different values for Mx / My?

Could you please help me out to understand the results?
Thanks a lot in advance and I do appreatiate your helps!
Bill

 

[RAYC1]

You are looking at the plate element center moment resultants table. The plate element centers are farther from the slab center and slab edge in the 4x4 model than in the 20x20 model.

If you look at the corner stresses table, then the moments are 6.30 t-m/m in the 20x20 model and 4.49 in the 4x4 model. The moments are near zero at the edges in the 20x20 model and approx. zero in the 4x4.

The center moments are different partly because of the point load at the center. A joint load, in effect, is distributed on the area that is nearest to the loaded joint. This area is much smaller in the 20x20 model than it is in the 4x4 model. So the moment at the center is more intense in the 20x20 model.