Linear system identification of the UC San Diego Geisel Library building under low-intensity seismic excitations

This paper presents a system identification (SID) study of the UC San Diego Geisel Library using acceleration data from six low-intensity earthquakes. Five SID methods, input-output and output-only, were used to cross-validate results, enhance noise robustness, increase the reliability and number of identified modes, and assess potential soil-structure interaction (SSI) effects, considering both 3-DOF and 6-DOF rigid-base seismic excitation. The identified modal parameters (natural frequencies, damping ratios, and mode shapes) were compared with those obtained from ambient vibration data in a previous study and with predictions from an uncalibrated detailed linear FE model. Modal decomposition of the seismic response was also performed using the identified linear state-space models. A total of thirteen vibration modes were identified across the five SID methods and six earthquake events. Results indicate slight decreases in natural frequencies and increases in damping ratios with increasing response amplitude, particularly between ambient and seismic responses. No clear evidence of significant SSI effects was found. The influence of rotational seismic base excitation is noticeable but small, with no consistent trends. Potential sources of discrepancies among the various comparative results are discussed in detail. This study provides a rare opportunity to investigate linear SID of a large, complex building under low-intensity seismic excitation and establishes a high-quality experimental-numerical baseline for Bayesian FE model updating and future structural health monitoring of the Geisel Library.