Use of frequency change for damage identification in reinforced concrete beams

The detection of damage will become one of the most frequent applications of identification techniques. In this paper we deal with a particular use of these techniques to identify damage in reinforced concrete beams due to cracking, based on frequencies only as measured quantities. Damage is obtained by comparing the identified model in the undamaged and damaged conditions. A simple interpretative model is adopted: the damaged zone is represented by a beam element with a lower value of bending stiffness. Three parameters are necessary to define damage: the position and the extension of the damaged element and the reduction of its flexural stiffness. The optimal values of damage parameters are determined by means of a minimization procedure based on an output error equation. The characteristics of the inverse problem are first investigated using pseudo-experimental data. Static and dynamic tests are performed on reinforced concrete beams; the static load is increased in different steps up to a value close to failure. Frequencies are measured at each load step. These experimental data and others taken from the literature are used to examine the efficiency of the illustrated procedure to identify the damage pattern.