OMA-Based Evaluation of Seismic Damper Retrofitting Effects on the Dynamic Characteristics of a Scaled Steel Frame Structure

Seismic retrofitting is widely used to enhance the dynamic performance of steel structures. This study experimentally investigates the effect of seismic damper retrofitting on a scaled steel frame using ambient vibration testing. Both as-built and retrofitted configurations were evaluated under low-amplitude excitations generated on a shake table. Modal parameters, including natural frequencies, mode shapes, and damping ratios, were identified using output-only Operational Modal Analysis (OMA) based on the Enhanced Frequency Domain Decomposition (EFDD) method. The results show a significant increase in damping ratios, with more than a 23-fold improvement in the first mode, along with mode-dependent frequency variations ranging from 17.63% to 142.74%. Mode shape comparison confirms strong consistency between configurations. The findings indicate that the integration of the damping devices enhanced the overall energy dissipation capacity of the scaled steel frame by increasing its global damping ratios and modifying the modal responses. The results suggest that output-only OMA can provide valuable insights into damper effectiveness, offering a practical alternative when controlled input excitation is not available.