This is the same model as the one in the previous post except for the length. The end begins to twist when the compressive load gets very close to 2*G*A where A is the area of the cross-section and G is the effective shear modulus. The effective shear modulus is defined to be (T*L)/(J*q) where T is the torsion which causes a small angle of twist q, L is the length, and J is the polar second moment of the cross-section.