Vibration data processing to assess the rigidity of diaphragms in existing buildings

Ambient vibration tests are important tools for the calibration of structural models targeted at damage detection, health monitoring, seismic retrofitting, vibration control. The conceptual process of synthesizing mechanical models unavoidably involves a priori simplifications. Their a posteriori validation is a mandatory step to check the actual reliability of theoretical results. The in-plane rigidity of diaphragms is a common assumption in the seismic assessment of existing buildings, allowing simplified and cost-saving structural analyses. In this regard, the paper proposes a vibration-based procedure to assess the validity of the rigid diaphragm hypothesis. The procedure, requiring at least two bi-axial sensors, exploits a perturbative approach to distinguish and separately estimate the time histories of in-plane rotations and shear deformations of each diaphragm. A frequency domain representation allows the assessment of these variables for the identifiable modes. The effects of measurement noise, errors in sensor position and signal desynchronization are discussed through numerical simulations. The procedure is experimentally validated on a laboratory frame and applied to full-scale vibration measurements of a building. The outcomes highlight how higher modes tend to violate the rigid-body assumption.