Modeling of coupled bending and torsional oscillations of an inclined aerial ladder

Modern aerial ladders are built as truss structures that provide limited stiffness compared to a massive boom, and therefore are susceptible to oscillations. In this paper, the linear-elastic bending of the ladder in horizontal direction and the torsion about its longitudinal axis are modeled. Oscillations in both axes are excited when the ladder rotates about its hub, and coupling is introduced by the offset between shear-center and centroid axis and by the cage at the tip of the ladder. The ladder is modeled as continuous system that consists of multiple sections, with material properties constant in each section, which yields a pair of coupled partial differential equations for each section, and additional constraints ensuring continuity of the solution. The cage at the tip is modeled as rigid body, which leads to dynamic boundary conditions at the top end. The coupling effects are demonstrated using both measurements and simulation data.