Abstract | Buckling and post-buckling behavior of beams resting on nonlinear foundation is addressed in the present study, as a decisive step towards investigating upheaval buckling of onshore buried pipelines. The adopted mechanical model is that of a beam with fixed boundary conditions supported laterally by uniformly distributed uniaxial springs that model vertical downward and upward pipeline movement in the trench. An internal hinge equipped with an elastic rotational spring in the beam middle span models the introduced flexible joint. The beam under investigation is subjected to constant compressive axial force over its length. Linear Buckling Analyses (LBAs) are initially conducted to obtain eigenmodes that are then adopted as imperfection shapes. Then, geometrically and materially nonlinear analyses with imperfections (GMNIAs), incorporating soil nonlinearity, are carried out, indicating unstable post-buckling behavior. Obtained results are of importance regarding the use of flexible joints in pipelines crossing areas prone to large ground differential movement.