Effective Stiffness Identification for Structural Health Monitoring of Reinforced Concrete Building using Hysteresis Loop Analysis

Structural Behavior of reinforced concrete (RC) buildings under earthquake excitation are highly nonlinear, particular for cracked RC structures with stiffness changing across elastic, pinched and hybrid regimes. This study investigates a structural health monitoring (SHM) method for an experimental scaled 12-storey RC frame building with cracks visually observed in the beam-column joints prior to a sequence of 4 shaking table tests. Hysteresis loops for every two floors are reconstructed using corrected acceleration and displacement measurements based on a multi-rate Kalman filter. The changes of nonlinear stiffness over time are calculated for divided half cycles using a hysteresis loop analysis (HLA). The effective elastic pinching stiffness are then extracted from the identified nonlinear stiffness values to examine the evolution of structural degradation during these events. A critical validation analysis is whether the final identified stiffness values match the initial values in a subsequent event, to ensure the method is robust in finding an accurate and repeatable value. In addition, fundamental frequencies, often used for SHM, are assessed using the identified effective stiffness and compared to those from modal analysis before-after each event. Results show the effective pinching stiffnesses for each floor are identified consistently between larger and small events. Final stiffness values for one event are within 9.7% of initial values for the next. The fundamental frequency results compare well further validating the method, but also show little sensitivity to large changes in effective stiffness. The overall results indicate the capability of the HLA to accurately identify damage that may not be found by vibration-based methods for highly nonlinear RC structures.