Revisiting the nonlinear elastic problem of internally constrained beams in a perturbation perspective

Unshearable and inextensible planar beams, in a static regime of finite displacements, are studied in this paper. A nonlinear mixed model is derived via a direct approach, in which displacements and reactive internal forces are taken as unknowns. The elasto-static problem is then addressed, and the role of the boundary conditions is systematically discussed. The relevant solutions for selected classes of problems are pursued via a perturbation method. It is shown that each considered class calls for a specific algorithm, accounting for a proper scaling and expansion of the variables. Finally, the asymptotic solutions are compared with benchmark numerical computations, grounded on finite-element analyses. The paper is focused on the case of longitudinal force significantly smaller than the buckling load, leaving the case of large force to future developments, where a different perturbation scheme is required. • Nonlinear model of inextensible unshearable planar beam. • Mixed force displacement method. • Consistent perturbation analysis for different boundary conditions. • Discussion on the order of magnitude of the hyperstatic unknowns. • Approximate analytical response to static loads.