The paper presents a concise framework studying the coupled vibration of curved
beams, whether the curvature is built-in or is caused by loading. The governing
equations used are both geometrically exact and fully intrinsic, with a maximum
degree of nonlinearity equal to two. For beams with initial curvature, the equations of
motion are linearized about the reference state. For beams that are curved because of
the loading, the equations of motion are linearized about the equilibrium state. A
central difference spatial discretization scheme is applied, and the resulting linearized
ordinary differential equations are cast as an eigenvalue problem. Numerical examples
are presented, including: (1) validation of the analysis for both in-plane and
out-of-plane vibration by comparison with published results, and (2) presentation of
results for vibration of curved beams with free-free, clamped-clamped, and
pinned-pinned boundary conditions. For coupled vibration, the numerical results also
exhibit the low-frequency mode transition or veering phenomenon. Substantial
differences are also shown between the natural frequencies of curved beams and
straight beams, and between initially curved and bent beams with the same
geometry.