New Regulations for the Use of Seismic Isolation in Mexico City

Abstract

The use of seismic base-isolation in Mexican projects has increased in the last decade. Particularly, the technology has been used in several buildings with very different characteristics, located in Mexico City. Before 2023, the Mexico City Building Code (MCBC, 2019) did not include in its Complementary Technical Standard for Seismic Design (CTS-SD 2023) requirements for the analysis, design, review and construction of base-isolated structures. Because of this, international design and evaluation standards, such as ASCE 7, EN 1998 and EN 15129, were widely used. Although these international standards are widely acknowledged for their exceptional quality, they do not consider the unique characteristics of Mexico City, including its soft soil conditions and other local factors. To improve the situation, the committee responsible for updating the CTS-SD elaborated and incorporated a new base-isolation chapter into the 2023 edition of this standard. Chapter 13 explicitly establishes the basis for the use of base-isolation in Mexico City, setting a historical precedent as it is the first legal document in the country that addresses this technology. This paper discusses the particularities of the CTS-SD approach for base-isolation in a city that has soils exhibiting a wide variety of dynamic properties. Among the innovative aspects contained in Chapter 13 in relation to international standards are: 1) The definition of a design objectives matrix that relates the minimum required performance for the different seismic intensity levels relevant to the design; 2) The use, for pre-design purposes, of spectral modal dynamic analyses under the consideration of a segmented spectrum that takes into account the different damping levels associated with the fundamental mode and higher modes of vibration; 3) The requirement to carry out a performance-based evaluation to assess the design of base-isolated structures located in the Transitional and Lake Zones of Mexico City; and 4) Specific recommendations to achieve an adequate contrast of lateral stiffness between the superstructure and the isolation system.