Ricardo Taborda

In its first section the paper presents reflections about the importance of building seismic instrumentation in regions of high seismic risk, the state of instrumentation of structures in Mexico and the need to increase the number of instrumented buildings with the purpose of better understanding phenomena like stiffness degradation of structures and soil-structure interaction. The goals pursued with the instrumentation of the building under study are described and its importance is highlighted, being a recently built structure, of typical characteristics in structures of its gender and in one of the highest seismic risk regions in Mexico. It is noted that the building is founded on sandy soft soil for which non-linear behavior is expected for low excitations. The soil fundamental frequency is slightly below that of the building. A brief description of criteria utilized in the building selection is also included.

The study case is a building 17-story high plus a basement level used as parking; its structure consists on reinforced concrete frames and walls which rest on 30 cast-in-place piles of 12.5 m depth. The building instrumentation is composed by 24 uniaxial servo-accelerometers of high resolution which are distributed in a free field station and in the basement, the roof and three intermediate levels. Their distribution is such that it allows to visualize the three-dimensional building response and the soil-structure interaction (SSI) effects appropriately.

Later on, the dynamic properties identified from an ambient vibration test and two low intensity earthquakes records are compared. The analysis of the response spectra of free field records reveals that the fundamental frequency of the site is around 0.75 Hz and comparison of these recordings with those corresponding to the basement shows the importance of SSI effects. Analysis in the frequency domain also points out these characteristics and it allows detecting fundamental frequencies of the structure during the seismic events and in the ambient vibration test, which are found to vary from 0.79 to 0.88 Hz in the transverse direction and from 1.00 to 1.17 Hz in the longitudinal direction. Modal shapes of vibration are also presented for the first three frequencies for each one of the translational directions as well as for the torsional component.

As final comments, the article emphasizes the changes detected in the fundamental frequencies of the structure, as well as variations and differences observed in both transfer functions and response spectra, reveal non-linearities attributable to the soil-structure system.