Paolo Borlenghi

The influence of debonding in concrete-filled steel tube (CFST) columns on the dynamic characteristics of super high-rise buildings is a common concern that remains insufficiently understood. The abnormal vibration incident of the SEG Plaza on 18 May 2021, also known as the 5·18 incident, serves as a typical case highlighting this issue. After two decades of service, the first-order bending frequency of the building decreased by approximately 6.1%, and extensive CFST column debonding was observed, with the maximum debonding rate reaching up to 97% on certain middle floors. To investigate the influence of CFST column debonding on structural dynamic characteristics, this study first derives a theoretical relationship between debonding parameters, namely angle and distance, and the equivalent bending stiffness of CFST columns. This analytical formulation is then implemented and validated through finite element simulations at multiple scales, including planar frame analysis in ABAQUS, a thin-interlayer simulation method in ANSYS, and full-building modeling in ETABS. Results show that for a planar frame, when a CFST column debonds at 270°, the structural natural frequency decreases by 0.984%; when the debonding angle is 180° with a 2 mm gap, the first-order frequency decreases by 0.141%. Numerical simulation of the SEG Plaza structural model predicts a reduction in the first-order frequency of 0.987% under the observed debonding conditions, confirming that debonding impairs force transmission, reduces structural stiffness, and alters natural frequencies. This study provides a mechanistic basis for evaluating stiffness degradation in long-service super high-rise buildings.

The bridge crossing the Gesso River is a multi-span masonry arch bridge built in the 19th century in Cuneo, Piedmont, Italy. Due to extended local degradation and damage, the bridge recently underwent a significant strengthening intervention. Ambient vibration tests (AVTs) were performed both before and after the strengthening to assess the effectiveness of the repairs. The paper presents the results of the dynamic investigations, identifying the modal characteristics of the masonry bridge through different techniques. The pre-intervention analysis revealed clear anomalies, including a sort of "frequency splitting" phenomenon and irregularities in the mode shapes, that were localized in the regions of maximum masonry decay. After the strengthening works, the identified modal parameters showed an increase of natural frequencies, along with the resolution of previously identified mode shape irregularities, indicating a clear improvement of the bridge structural condition. As a final remark, the presented results highlight the value of operational modal analysis (OMA) as a non-destructive tool for validating the effectiveness of rehabilitation measures.