Vibration analysis of pultruded FRP frames with semi-rigid connections

Vibration frequencies and mode shapes of pultruded FRP plane and space frames with semi-rigid joints are analyzed using two-node locking-free Hermitian finite elements. A nonlinear beam displacement field, accounting for shear deformation effects due to non-uniform bending and torsion, is adopted. The equation of motion is obtained via Hamilton’s principle, and the second-order work of initial stresses is included. Flexibility of three-dimensional joints is simulated by modifying the element stiffness matrix without introducing additional degrees of freedom. The first natural frequencies of portal frames, two-bay single-storey and single-bay two-storey frames are reported as a function of in-plane rotational joint stiffness, and a mode change between flexural and flexural–torsional vibrations is sometimes observed. Finally, a pultruded space frame is analyzed, which shows the effects of semi-rigid connections, column orientation and shear deformation on the natural frequencies.