Improvement of Nonlinear Static Seismic Analysis Procedures (FEMA 440)

One of the primary goals of the Department of Homeland Security's Federal Emergency Management Agency (FEMA) and the National Earthquake Hazards Reduction Program (NEHRP) is to encourage design and building practices that address the earthquake hazard and minimize the resulting damage. This document, Improvement of Nonlinear Static Seismic Analysis Procedures (FEMA 440), reaffirms FEMA's ongoing efforts to improve the seismic safety of new and existing structures in this country. Knowledgeable engineers have long recognized that the response of buildings to strong ground shaking caused by earthquakes results in inelastic behavior. Until recently, most structural analysis techniques devised for practical application relied on linear procedures to predict the seismic behavior of buildings. With the publication of the ATC-40 Report, Seismic Evaluation and Retrofit of Concrete Buildings, in 1996, the FEMA 273 Report, Guidelines for the Seismic Rehabilitation of Buildings, in 1997, and the FEMA 356 Report, Prestandard and Commentary for the Seismic Rehabilitation of Buildings (which replaced FEMA 273), in 2000, nonlinear static analysis procedures became available to engineers providing efficient and transparent tools for predicting seismic behavior of structures. Both the ATC-40 and FEMA 356 documents present similar performance-based engineering methods that rely on nonlinear static analysis procedures for prediction of structural demands. While procedures in both documents involve generation of a "pushover" curve to predict the inelastic force-deformation behavior of the structure, they differ in the technique used to calculate the inelastic displacement demand for a given ground motion. The publication of the above cited documents resulted in the widespread use of these two methods, and engineers have since reported that the two procedures often give different estimates for displacement demand for the same building. Hence the Applied Technology Council (ATC) proposed to the Federal Emergency Management Agency (FEMA) in 2000 that a study be conducted to determine the reasons for differing results and to develop guidance for practicing engineers on improved application of these two methods. FEMA agreed to fund the investigation, and in October 2000, ATC commenced a project to provide guidance for improved applications of these two widely used inelastic seismic analysis procedures (ATC-55 Project). The ATC-55 Project had two objectives: (1) the development of practical recommendations for improved prediction of inelastic structural response of buildings to earthquakes (i.e., guidance for improved application of inelastic analysis procedures) and (2) the identification of important issues for future research. Intended outcomes of the project included: 1. Improved understanding of the inherent assumptions and theoretical underpinnings of existing and proposed updated inelastic analysis procedures. 2. Recognition of the applicability, limitations, and reliability of various procedures. 3. Guidelines for practicing engineers to apply the procedures to new and existing buildings. 4. Direction for researchers on issues for future improvements of inelastic analysis procedures. This report (FEMA 440) is the final and principal product of the ATC-55 Project. The document has three specific purposes: (1) to provide guidance directly applicable to the evaluation and design of actual structures by engineering practitioners; (2) to facilitate a basic conceptual understanding of underlying principles as well as the associated capabilities and limitations of the procedures; and (3) to provide additional detailed information used in the development of the document for future reference and use by researchers and others.