Modelling soft impacts for vibrational excitation of civil engineering structures

Experimental modal analyses of civil engineering structures are performed for various reasons. One is to assess the condition of the structure. Another is to verify the behavior of the structure under dynamic loads. An accurate method to excitate the structure is to use vibrators and random noise. For large structures in field conditions this may be quite expensive, and requires heavy equipment. Excitation by impact is often an effective alternative. In the report an impact equipment consisting of a striker and a mobile 'lift and drop' arrangement is used and some problems with impact excitation are discussed. The striker consists of a falling weight, a damper and a load cell. A soft interface between the load cell and the surface of the structure is used to prolong the contact time and filter out high frequency components. The contact force history is measured for 'soft' impacts on a stiff floor, for various choices of the mass and initial velocity of the striker. The floor is considered to be at rest during the impact event. Under these assumptions an analytical model of the contact force history is fitted to the measurements and used to solve a differential equation governing the striker system. From the solution the nonlinear stiffness of the striker system is estimated.