A novel three-dimensional seismic assessment method (3D-SAM) for buildings based on ambient vibration testing

Most of current detailed seismic evaluation methods for buildings are based on numerical approaches. However, there is a need to use state-of-the-art interdisciplinary technologies and techniques to further facilitate such evaluations and improve their reliability, especially in many situations where detailed design documentation is not available. This study introduces a novel approach for seismic assessment of buildings, 3D-SAM, based on in-field ambient vibration measurements using acceleration/velocity sensors located on building platforms (floors and roofs). In the experimental phase of this project, sixteen low and mid-rise irregular buildings designated as emergency shelters in Montreal, Canada were subjected to ambient vibration tests (AVT) and their lowest natural frequencies, corresponding mode shapes and estimates of modal damping ratios are reported. The rate of success of AVT in this study to capture at least the three lowest natural frequencies/modes is unlike previous studies where difficulty of performing AVT and modal extractions in low-rise buildings were reported. Furthermore, the measured natural periods of concrete structures and braced steel frames of the database are compared with those obtained from the Canadian building code period formulas and the results show agreement in the case of braced steel frames. Due to the fact that in-situ experimental modal tests are low cost, and also owing to advances in sensing techniques and analysing procedures to derive the essential structural characteristic of buildings (operational modal analysis is well accepted in other engineering disciplines), the author developed a new three-dimensional seismic assessment method and software, called 3D-SAM in short form, that use this information to perform seismic assessment. The method incorporates torsional effects in predicting response, and therefore can deal with existing structural irregularities, an important limitation of other existing simplified methods. It does not require the creation of any artificial numerical model and can easily be integrated into existing modal identification software. Applications of 3D-SAM to four buildings, low to high-rise, located in Montreal are presented in this study to illustrate and validate the proposed method; results are compared with those obtained using detailed and updated linear dynamic analysis of finite element models of the buildings. Next, a modified 3D-SAM is introduced that incorporates modification factors for the adjustment of modal properties to further extend the application of the method to stronger ground motions that may cause nonlinear response. Finally, the method is used for deriving the dynamic amplification portion of natural torsion on all floors of 16 low to mid-rise irregular buildings located in Montreal, Canada.

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