Variability of the fixed-base and soil-structure system frequencies of a building-The case of Borik-2 building

Borik‐2 is an IMS (Institute for Testing of Materials, Belgrade, Serbia)‐type prefabricated 14‐story reinforced‐concrete building located in Banja Luka, Republic of Srpska (Bosnia, former Yugoslavia), and is a rare example of an instrumented building in Europe shaken by a significant number of earthquakes. This paper presents an analysis of its response to 20 earthquakes recorded in this building, and a comparison with results from previous full‐scale tests and analyses. Only one of the 20 earthquakes (8/13/1981, M=5.4) could possibly have caused damage, but no structural damage was reported. For each of these earthquakes, the building fundamental fixed‐base frequency f1 was computed from wave propagation travel times estimated by impulse response functions, and the soil–structure system frequency fsys was estimated from the peaks of the Fourier spectra of the response. The analysis suggests consistency of the estimates of fsys from the earthquake response data, from the forced vibration tests before the earthquakes, and ambient vibration tests conducted near the end of the earthquake sequence. The results suggest nonlinear but essentially ‘elastic’ behavior of the building for the amplitudes of motion covered by the data, and essentially linear soil–structure interaction. During the largest event, f1 and fsys decreased, respectively, by about 16 and 22% for EW motions, and by about 18 and 31% for NS motions, compared with the values before the earthquake from the small amplitude response. Comparison of f1 and fsys during the smaller events before and after EQ 11 event shows that f1 did not change, but fsys reduced permanently, by about 10% for EW and 15% for NS. Copyright © 2008 John Wiley & Sons, Ltd.