Ricardo Taborda

The Federal District Government requested the evaluation of the records obtained at the JAL building during the January 21, 2003 Tecomán earthquake. In this report the damage observed and the main results obtained in a preliminary analysis of the signals are described.

The report includes a brief description of the building and damage observed during previous earthquakes. The JAL building is a 14-story, originally concrete reinforced structure, which is founded on a concrete slab that in turn is based on 54 friction piles. JAL building suffered damage in its structural elements during the September 19, 1985 Michoacán earthquake after which it was necessary to make a first retrofit. After the occurrence of the December 10, 1994 and September 14, 1995 earthquakes, new damages were observed, leading to carry out a second retrofit of the structure. In spite of it, later events in 1999 have produced new deteriorations, which have taken the fundamental frequency of the structure to a value that coincides with the soil natural frequency of vibration (0.5 Hz).

Once the structure statues is described, damages observed in the building after the occurrence of the January 21, 2003 Tecomán earthquake are detailed. These took place essentially in its non-structural elements and included the cracking of some masonry walls. However, increments in some of the cracks that already existed in the columns of the basement and new cracks in the joints among old elements and those added in the second retrofit were detected.

Later on, the building instrumentation is described and an analysis of signals recorded during the event in study is made. It was observed that the recorded maximum accelerations were inferior to the largest ones that had been recorded before, and it is noted that interstory-drifts did not overpass the regulation limits.

A spectral analysis of the recordings allows to estimate natural frequencies of vibration and fractions of critical damping of the system, which are compared with those from previous analysis of past recordings after the second retrofit. Additionally, a time-frequency analysis by windows allows to observe that, although frequencies vary during the occurrence of the earthquake, the identified values in the first and last windows are similar. These comparisons point out the absence of significant changes in the system properties.

In its final part the report emphasize as main conclusions, the following ones:

• The building had less non-structural damages than the observed ones in 1999 earthquakes.
• Increases in the cracks dimensions of some columns, probably associated to differential settlements, were detected.

The report recommends monitoring the differential displacements at least every six months.