Issues on the Seismic Retrofit of a Building near Resonant Response and Structural Pounding

Paper presents an analytical study on the possible application of commonly used retrofit schemes for an existing nine-story apartment building in Mexico City. The building, located in Mexico City's lake-bed region, was originally conceived as a RC waffle-flat slab structural system and was built in the late sixties. The building suffered important structural damage during the March 14, 1979 Petatlán Earthquake, primarily caused by structural pounding with neighboring structures due to its flexibility and resonant response with the ground. The building was retrofitted shortly after the Petatlán Earthquake with steel bracing in the direction of pounding. The retrofitted building survived the September 19, 1985 Michoacán Earthquake with light structural damage. Some evidence of pounding with one neighboring structure was again observed.

Because of the chronic pounding problems with adjacent structures, which are separated eight to fifteen cm from the apartment building, it is felt that an additional upgrade of the structure may be needed for the safety of the building if a strong earthquake similar to the 1985 Michoacán Earthquake may strike Mexico City soon again. Different retrofit schemes were studied, which can be grouped as follows: a) weight reduction, b) column and waffle-slab jacketing, c) addition of energy dissipation devices, d) removal of top floors, e) replacement of diagonal bracing with newer bracing and, f) a combination of previous solutions. The effectiveness of the studied retrofit schemes is discussed by comparing their dynamic structural response with respect to the response of the actual retrofitted structure using nonlinear dynamic analyses for a postulated M_s=8.1 earthquake, as well as construction process, property value, and building use considerations. It is concluded that many aspects need to be evaluated in a seismic retrofit plan for a given building structure and often, the best alternative from the structural viewpoint may not be the best alternative overall.