## 1 Introduction  
The steel-concrete composite beam has achieved wide acceptance because it guarantees economic benefits, strength and stiffness improvement, and construction convenience and has been extensively used in modern high-rise buildings, stadiums, and bridges [1, 2]. In steel-concrete composite beams, the connection between the steel beam and concrete slab is commonly provided in forms of welded shear studs, bolts, or perforated plates (Figure 1), among which the welded shear studs have become the most popularly used due to the availability of detailed research and standard design methodology. The shear connectors between the concrete slab and steel girder are often subjected to serious stress, overloading, corrosion, and fatigue. All these anomalies involve a deterioration or break of shear connectors, causing a decrease in the rigidity, a reduction in resistance, and catastrophic structural failures with huge losses. Condition assessment for the local shear connectors in SCCBs is of great significance to ensure structural reliability and durability. However, the inaccessibility of the shear connectors makes direct visual inspection unusable. On the other hand, a huge number of shear connectors prevents any local Non-Destructive Testing (NDT) methods from being able to inspect the connectors one by one. From a great variety of NDT methods developed to date, the vibration-based method is recognized as one of the least expensive and simultaneously effective, with appropriate processing of measurement data. The premise of the vibration-based method is that the damage or deterioration of the structures would primarily affect the stiffness and thus the modal characteristics of structures, e.g., the natural vibration frequency [3], mode shapes [4; 5], frequency response function, etc. [6]. Thus, damage can be inferred from the changes in identified dynamic properties. Recently, methods based on changes in derived modal parameters such as modal strain energy, mode shape curvature, and flexibility have been considered more sensitive to damage than vibration frequency. [PERSON] et. al. [7] investigated the influence of shear stud damages on the dynamic characteristics of SCCBs and concluded that the bending modal is more sensitive to the damage of shear studs. [PERSON] et al. [8] conducted the finite element model updating based on vibration measurements to assess and quantify the structural damage of a high ductile steel-concrete composite frame. [PERSON] et. al. [9] proved the effectiveness of modal assurance criterion (MAC) in damage evaluation for concrete structures, and [PERSON] et al. [10] combined the modal damping ratio and coordinate modal assurance criteria (COMAC) to locate and evaluate the internal deterioration of reinforced concrete (RC) slabs at different loading stress through vibration-based tests. [PERSON] et. al. [11] proposed a modified jerk energy methodology to localize the reduction in the stiffness of a reinforced concrete shear frame. [PERSON] et. al. [12] proposed a vibration-based damage detection method for bolted connections using incomplete modal data by a Bayesian approach and model reduction technique. To the author's best knowledge, only a few studies concentrated on the detection of damage to shear connectors in SCCBs. [PERSON] et al. [3] applied the diagnostic methodology based on natural frequency and modal shapes to evaluate the damage to shear connectors of SCCBs. [PERSON] et al. [4] proposed locating the damaged shear stud by comparing the changes in curvature modes before and after the damage occurred, but no quantitative evaluation was conducted. [PERSON] and [PERSON] et al. [13; 14] reported a damage assessment for a steel-concrete beam based on vibration data in the frequency domain. Damage indicators such as the correlation of FRFs (COFRF), the relative difference of FRFs (RDFRF), and transmissibility were proposed and validated. [PERSON] et al.