Bending stocky S4R beam

Hello

I'm working on stocky S4R I-beam(nonlinear analysis). Span length is 1.7m and 0.5m heigth. Beam is simply spported and loaded at mid-span by concentrated force.

Unfortunately it looks like my beam is to stiff.

Deflection is smaller than I expected.

Critical load is higher than I expected.

Is this problem connected to bending and shearing stocky element?

Is it about timoshenko/bernoulli beam theory? In my case I use shell elements.

How to solve this problem? How to calibrate the model? Thank You for any advice!