Hi Wenlong. Thank you for you assistance.

I've made some test on much simpler model to verify how does symmetry work in this case. I extraced just middle frame and check it as a simple supported beam with symmetry and without. Conclusion is that there is no artificial stiffness.

Error isn't in the symmetry BC in my cargo hold model but in my expectations about behavior of the model. I supposed that middle frame will be in bending. After displaying displacement x5 i realized that because of linear periodic BC and constant hydrostatic pressure in the bottom middle frame just moved upward without any bending.

Thank you one more time.