ATC-62 Project

ATC-62 Project: Advanced Seismic Analysis Methods - Resolution of Issues

In October of 2004, the Applied Technology Council (ATC) was awarded a contract by the Federal Emergency Management Agency (FEMA) entitled “Advanced Seismic Analysis Methods – Resolution of Issues” (ATC-62 Project). The objective of the ATC-62 Project is to resolve a series of difficult technical issues that were identified during the preparation of the FEMA 440 report, Improvement of Nonlinear Static Seismic Analysis Procedures (ATC, 2005). The FEMA 440 report outlines improvements to inelastic seismic analysis and modeling procedures used in existing standards of practice for the seismic rehabilitation of buildings, which include the Coefficient Method in the FEMA 356 Prestandard and Commentary for the Seismic Rehabilitation of Buildings, and the Capacity Spectrum Method, as documented in the ATC-40 Report, Seismic Evaluation and Retrofit of Concrete Buildings. Unresolved technical issues include:

Component and global modeling to include nonlinear degrading response, including cyclic and in-cycle degradation of strength and stiffness. Most current design modeling procedures only recognize cyclic degradation, or cannot distinguish between these two types of structural degradation.

Soil and foundation structure interaction. While some advances were made during development of the FEMA 440 report, it is still not possible to fully understand how a building’s foundation responds to seismic ground motions. Some soil and structure characteristics remain that can not be currently accurately modeled, and there are some situations, especially for short-period or large footprint structures, where current models significantly over-predict the input ground motion, resulting in over-design.

Improvement of simplified modeling of nonlinear multi-degree-of-freedom effects. Current design procedures are based on single-degree-of-freedom models, which while simple to understand, are very limited in their ability to address complex structures as well as multiple-mode effects from input ground motions.

Currently, the ATC-62 Project is focusing on the issue of nonlinear degrading response. Project activities have included an extensive literature review and interview effort to identify and document existing research on degrading response of individual components and structural systems. Project focused studies are now building on the results of available research to quantify the significance of cyclic versus in-cycle degradation on the overall stability of a structure. The eventual goal is to develop improved recommendations for including the effects of degrading response in nonlinear analyses.

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