Microtremor measurements for assessing the influences of non-structural components on the modal properties and vulnerability of steel structures

Abstract

Non-structural components (NSCs) in the majority of the FE models are commonly considered as a concentrated/distributed mass, and their stiffness along with damping ratio is ignored. Likewise, NSCs have significant efficacy on the accurate vulnerability assessment and select an appropriate structural system versus the seismicity of the area. The chief aim of this research is to present a simple theoretical method in conjunction with a practical empirical technique for assessing the influences of NSCs on the modal properties and vulnerability of buildings, exclusively steel structures. In this regard, the ambient vibrations were measured on four buildings with concentrically steel braced frame structural system under the construction process in three independent stages: (1) after the erection of the structural and skeletal systems; (2) after the construction of the interior and exterior partition walls; (3) after the installation of the facade and parapet elements. Utilizing two signal processing techniques (i.e., floor spectral ratio and random decrement method), the acquired microtremors were analyzed to extract the modal properties and compute the vulnerability indices of the structures. It is perceived that the values of the modal properties of the steel structures, namely, natural frequency and damping ratio, have considerably increased during the construction process with taking the influence of NSCs into account. In addition, the obtained results disclosed that the vulnerability index has significantly decreased in the second stage over the first one. However, it has not noticeably fluctuated in the third stage over the second one.