We present an experimental and numerical study of the effects of geometric nonlinearities on vibrations of rotating machines support structures. The initial stiffness of a structure, computed in its unloaded state, is affected by the applied forces, the so-called geometric stiffness. Here we study these effects via experimental and numerical methods designed to evaluate previous mathematical models. Our models are a beam and a portal frame under compression supporting a DC motor. The presence of large axial compressive force will reduce the structure stiffness and natural frequencies leading to unexpected, potentially dangerous resonance states. Experimental imperfections led to observation of interesting phenomena not predicted in our theoretical and numerical studies. We also observe, as expected, occurrence of the so called Sommerfeld Effect, when underpowered excitation sources get their rotation regime stuck at resonances.