Determination of axial load and support stiffness of continuous beams by vibration analysis

Abstract

Three models are presented which predict frequencies and mode shapes of transverse vibration for a continuous prismatic Bernoulli-Euler beam on elastic supports, subjected to a compressive axial load. The first model, which approximates support stiffnesses by an equivalent elastic foundation, is found to be inaccurate for wave lengths close to the support spacing. A discrete mass model is formulated which accounts for axial load by stability functions which modify the element stiffness matrices. A continuous model is formulated which yields an exact solution for Bernoulli-Euler beam theory. The frequencies predicted by the discrete mass model and continuous model are in excellent agreement. A method of predicting axial compressive load and support stiffness based on measured frequency and phase data is presented which can be used for either the discrete mass model or the continuous model. A frequency reduction factor is derived which accounts for the effects of shear deformation and rotatory inertia. Tests are performed on an eight span beam with compressive axial load. Test results show that the models accurately predict frequencies and mode shapes of vibration. Results indicate that the method formulated can be used to determine compressive axial load and support stiffness.