The variations of identified wave velocities of vertically propagating waves through the structure are investigated for a 54-story steel-frame building in downtown Los Angeles, California, over a period of 19 years since construction (1992–2010), using records of six earthquakes. The set includes all significant earthquakes that shook this building, which produced maximum transient drift ∼0.3% and caused no reported damage. Wave velocity profiles β( z) are identified for the NS, EW, and torsional responses by fitting layered shear beam/torsional shaft models in the recorded responses, by waveform inversion of pulses in impulse response functions. The results suggest variations larger than the estimation error, with a coefficient of variation about 2–4.4%. About 10% permanent reduction of the building stiffness is detected, caused mainly by the Landers and Big Bear earthquake sequence of 28 June 1992, and the Northridge earthquake of 17 January 1994. Permanent changes of comparable magnitude were identified also in the first two apparent modal frequencies, f1; app, and f2; app, which were identified from the peaks of the transfer-function amplitudes.
SUMMARYNonparametric techniques for estimation of wave dispersion in buildings by seismic interferometry are applied to a simple model of a soil-structure interaction (SSI) system with coupled horizontal and rocking response. The system consists of a viscously damped shear beam, representing a building, on a rigid foundation embedded in a half-space. The analysis shows that (i) wave propagation through the system is dispersive. The dispersion is characterized by lower phase velocity (softening) in the band containing the fundamental system mode of vibration, and little change in the higher frequency bands, relative to the building shear wave velocity. This mirrors its well-known effect on the frequencies of vibration, i.e. reduction for the fundamental mode and no significant change for the higher modes of vibration, in agreement with the duality of the wave and vibrational nature of structural response. Nevertheless, the phase velocity identified from broader band impulse response functions is very close to the superstructure shear wave velocity, as found by an earlier study of the same model. The analysis reveals that (ii) the reason for this apparent paradox is that the latter estimates are biased towards the higher values, representative of the higher frequencies in the band, where the response is less affected by SSI. It is also discussed that (iii) bending flexibility and soil flexibility produce similar effects on the phase velocities and frequencies of vibration of a building.
The case study, Los Angeles 32‐storey residential building, has an exceptionally long seismic observation history, spanning 50 years (1971‐2020), during which it has experienced many earthquakes, some causing extensive damage in the metropolitan area. Records of 13 earthquakes are analyzed, including San Fernando of 1971, to find out if permanent loss of stiffness occurred in the structure since 1971 and to describe statistically its nonlinear elastic behavior, not related to damage. It is a rare example of an instrumented pre‐San Fernando earthquake steel‐frame building, constructed with some on‐site welded column‐beam connections, type of construction that exhibited weaknesses after the Northridge, 1994 earthquake. The identification method consists of fitting equivalent uniform and four‐layer Timoshenko beams models by waveform inversion of bandpass filtered impulse responses, which results in piecewise constant profiles of shear wave velocity (the damage sensitive parameter) and the elastic moduli ratio, reflecting the distribution of structural stiffness along the height. The nonlinear elastic behavior derived from the smaller amplitude data constitutes strain‐dependent baseline for future structural health monitoring of this building. This behavior reveals more prominent strain dependency in the longitudinal direction and stronger variation with strain toward the base of the structure.
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