Structural Health Monitoring (SHM) systems are of great importance in the aftermath of catastrophic events, such as earthquakes, as they enable rapid and remote assessment of structural integrity. Among SHM techniques, Operational Modal Analysis (OMA) is widely used due to its non-intrusive nature, as it relies on the identification of modal properties from ambient acceleration data. This study explores the complementary use of strain gauges (SGs) for modal identification, highlighting their potential to enhance the quality and reliability of vibration-based monitoring. In the first part of the work, the effect of structural damage on modal properties identified by both sensing systems is analysed through a series of progressive damage scenarios applied to a laboratory-scale steel frame. In the second part, modal data from both sensor types are jointly used to update a finite element model (FEM) through a multi-objective optimization approach. The results demonstrate the ability of the proposed multi-sensor system to consistently detect and track the evolution of structural damage, as well as the greater sensitivity of strain-based mode shapes to damage. Furthermore, the calibrated numerical model successfully reproduces the identified modal properties, confirming the benefits of combining accelerometer and SGs data. These findings pave the way for future applications of multi-sensor SHM systems in real steel structures in post-earthquake scenarios.