The ideal setup is to use 1/4 symmetry. Use two planes through the centerline of the geometry to cut the sample in half one way and in half perpendicular to the first plane. The Symmetry Boundary Condition is X=0 on one face and Z=0 on the other face. Now the cross-sectional area is only 1/4 of what it was, but the beautiful thing is, you can use a Y = 0 on the bottom instead of the Fixed Support, because the symmetry BC will prevent motion away from the centerline. Put the pressure on the top and let the simulation run. You will get a perfectly uniform deformation profile and no pivot warnings.