An acceleration and strain response based method to evaluate the effectiveness of seismic retrofitting steel braces under small to moderate earthquakes

Seismic retrofitting is vital for aging structures, with steel bracing serving as a high-performance solution. However, the behaviour of these braces under low-intensity seismic events remains under-researched. This study presents a method to assess the in-situ performance of steel braces in a nine-story reinforced concrete building using cost-effective, asynchronous wireless units equipped with MEMS accelerometers and semiconductor strain gauges. Using the “effective local stiffness ratio” as a primary metric, the study analyzes structural responses from 34 earthquakes. Results indicate that the vibration mode of the retrofitted frame involved significant rigid-body rotation, accounting for 63% of the total inter-story drift. This transition from shear-dominant to bending-dominant behaviour resulted in an effective local stiffness ratio of approximately 42%, significantly lower than the 100% efficiency typically assumed in seismic design. The findings demonstrate that such discrepancies, often overlooked in standard seismic evaluation methods, can lead to premature structural failure or inadequate torsional stiffness. The proposed methodology provides a robust framework for detecting these performance degradations in strength-based retrofits. Furthermore, the ability to isolate the actual contribution of inter-story drift to story shear deformation suggests high potential for assessing the in-situ effectiveness of damping-based seismic retrofits.