Twenty-Two Years of Soil–Structure Interaction Monitoring in an Instrumented Building Founded on Soft Soil

Soil–structure interaction (SSI) can affect the apparent dynamic properties of instrumented buildings, and its long-term variability may complicate structural health monitoring when SSI effects are not explicitly tracked. This study presents a long-term monitoring analysis of SSI effects for a 17-story reinforced concrete building founded on soft soil in Acapulco, Mexico, using earthquake records collected between 2001 and 2022. The simplified Luco decomposition model is combined with a recursive subspace identification method to independently estimate the frequencies associated with the soil–structure system, the fixed-base structure, foundation rocking, foundation translation, and torsional response. The estimated frequencies are assessed through comparisons with classical spectral analysis and subsequently used to evaluate the long-term evolution of the equivalent SSI stiffness against analytical and numerical models. The results reveal a gradual reduction in both the system and fixed-base structural frequencies throughout the monitoring period, accompanied by a corresponding decrease in the equivalent stiffness. The proposed methodology provides consistent component-by-component SSI estimates from seismic recordings and supports the long-term identification and interpretation of SSI-related dynamic properties in instrumented buildings founded on soft soils.