Revisions to Code Provisions for Site Effects and Soil-Structure Interaction in Mexico

The seismic hazard in Mexico has been re-evaluated recently and now we can estimate the maximum acceleration on rock for any given site in the country. This is the starting point for constructing site-specific earthquake design spectra that explicitly include the effects of soil dynamic amplification (site effects). The effects of soil-structure interaction (SSI) can be accounted for in two stages: first in the elastic design spectrum, considering the enlargement of the period and the increase in damping; and then in the strength reduction factor, taking into consideration the global ductility reduction. Site effects refer to the dynamic amplification of ground motion due to the local geotechnical characteristics of the subsoil. These effects are reflected in the seismic design coefficients specified by building codes in terms of siteand structure-response amplification factors. The SSI effects, on the other hand, refer to the modification of the foundation motion with respect to the free-field ground motion due to the flexibility of the supporting soil. In building codes, however, these effects are generally accounted for modifying the dynamic properties (natural period and damping ratio) of the original structure and evaluating the response of the modified structure to the prescribed freefield motion specified by a design spectrum. A new approach to determine earthquake design spectra including site and SSI effects has been incorporated in the CFE Seismic Design Code (MDOC), a model design code in Mexico (Tena-Colunga et al., 2009). The previous version of the MDOC was published by the Federal Electricity Commission in 1993, so an in-depth review was mandatory in order to update the code for 2008. In the absence of state seismic codes, the MDOC is legally used in the entire nation for earthquake design of different structure types. The MDOC provides expressions to determine earthquake design spectra at any location in the country, which makes unnecessary the use of conventional zoning maps. These spectra, which have realistic shape and size for elastic response, are then reduced by two separate factors to account for the nonlinear structural behavior and overstrength.