Assessing local-scale damage in reinforced concrete frame structures using dynamic measurements

• Experiments on a RC structure consisting of two columns and one beam.
• Development of a technique which can characterize local damage.
• Using dynamic measurements to identify the structure’s dynamic characteristics.
• Using fixity factors to assess the decrease of stiffness in the connections.

The applications of dynamic measurements on existing buildings are numerous: assessment of their seismic vulnerability, assessment of the structure’s capacities in post-earthquake situations or after changes in the vicinity, etc. At present, this type of measurement enables structural diagnosis on a global scale (the whole structure), while the identification and the assessment of local damage (each element of the structure) remains to be explored. Herein, diagnosis at the local scale was studied in the laboratory on an instrumented reinforced concrete structure consisting of two columns and one beam. It was loaded in the central part of the beam in several stages corresponding to different damage states. Displacements were measured simultaneously using displacement sensors and image correlation. After each load/unload cycle, dynamic measurements were taken using accelerometers.In the first part of this paper, the observations from the experiment were presented, with the appearance of damage and the decrease in natural frequencies that occurred simultaneously with stiffness reduction. Thereafter, the technique characterizing damage that was developed taking into account the semi-rigid connections of the frame was presented. The stiffness of the connections was identified by calibrating the dynamic responses of the structure with respect to a model. The fixity factors were used to assess the loss of stiffness in the semi-rigid connections. The validity of the identified fixity factors was evaluated using the static experimental results. This study shows that dynamic measurement coupled with finite element analysis can provide a fast and effective method to assess the quality of connections of reinforced concrete structures.