Ricardo Taborda

The paper begins describing of the building under study, its location, and its structural and foundation systems. The main reasons for selecting this building are its location in one of the areas of higher seismic risk in Mexico and the fact that its foundation rests on sandy soft soil. The accelerographic network installed in the building and the location of the instruments are briefly described. Besides, as an introduction to the theme, the importance of the instrumentation of structures is noted, highlighting the most interesting topics in studies such as this.

Afterwards, a description of the selected seismic events is made, including specifics characteristics like their epicentral distance to the building, their intensities, magnitude and maximum accelerations recorded in the free-field station. Apart from the earthquake records, two ambient vibrations tests carried out with the same accelerographic network were included in the analysis.

Later on, dynamic characteristics identified by means of a frequency-domain analysis for the six selected events and the two ambient vibrations tests are detailed. For all the earthquake records up to three modal frequencies of vibration were obtained for the two translational directions (T and L) and the torsional component (R). The system presented reductions of 10, 14 and 12 % in its fundamental frequencies during the occurrence of the most intense earthquake (T, L and R, respectively). Comparison among the two ambient vibrations tests shows only a reduction in translational directions of 5 and 8 % (T and L).

In the main body of the paper, the soil-structure interaction effects in the building are studied. Stiffnesses and frequencies of the translational and rocking components of the foundation motion are estimated, starting from the experimental records, using a simplified method. Stiffnesses associated to these motions are also obtained with the proposal for the new Complementary Technical Standards of Earthquake Design of the Mexico City Construction Code (CTS) and by means of the computer program Dyna5. For this last one, different models are evaluated considering and not considering pile-soil-pile and soil-pile-cap group effects. In both analytical procedures the system is evaluated under static and dynamic conditions.

The stiffness values obtained starting from the experimental data and those of both normative and analytic procedures are compared. This assessment allows to highlight the following observations:

• For the site dominant frequency little differences between estimations of the stiffnesses under static and dynamic conditions are observed.
• Results by CTS procedures, and in less amount those obtained with the program Dyna5 not considering group effects, fit well to the experimentally obtained values.
• It was observed that considering group effects, especially pile-soil-piles interaction, lead to differences up to 50 % compared with those stiffnesses identified by the simplified method starting from the experimental recordings.