This paper focuses on system identification (SID) of the UC San Diego Geisel Library building using ambient vibration (AV) data, assuming that the building’s behavior can be fully described by linear models in terms of material, geometry, damping, etc. Three state-space-based, output-only time domain SID methods are applied and fully automated to identify the library’s modal properties using AV data from both a 15-day and a 486-day monitoring period. The modes identified from the AV data are higher-order coupled torsional-flexural modes. The identified modal properties are influenced by the atmospheric conditions, and the amplitude of the building’s ambient vibration. The time-varying identified modal properties show a cyclical 1-day pattern due to human activity, earth tremors, and short-term changes in atmospheric conditions such as wind speed and temperature. One of the output-only SID methods was used to estimate modal properties from ambient vibration data recorded continuously over a 486-day period, including three low-intensity earthquakes. No permanent changes in the identified modal properties were observed due to the three low-intensity earthquakes that occurred during that period. Renovations of the Geisel Library involving only non-structural components (e.g., non-load-bearing partition walls, changes in space allocations, and inertial/live loads) caused some discontinuities in the identification of the modes of interest in this study. The influence of the data window length on system identification results, in terms of identification success rate and estimation uncertainty, was investigated. The identified state-space models are also used to assess the relative contribution of the ambient base excitation to the building’s total ambient vibrational response. This research offered a unique opportunity to study linear SID of a large and complex real-world structure under ambient excitations, and the effects of changing environmental conditions on the identified modal properties. It provided insight into some of the causes of the observed temporal variation in the identified modal properties. The SID results presented in this study also provide a baseline for future structural health monitoring studies of the Geisel Library building.
