A considerable number of buildings in the existing building stock of the United States present a risk of poor performance in earthquakes because there was seismic design code available or required when they were constructed, because the seismic design code used was immature and had flaws, or because original construction quality or environmental deterioration has compromised the original design. The practice of improving the seismic performance of existing buildings-kwn variously as seismic rehabilitation, seismic retrofitting, or seismic strengthening-began in the U.S. in California in the 1940s following the Garrison Act in 1939. This Act required seismic evaluations for pre-1933 school buildings. Substandard buildings were required to be retrofit or abandoned by 1975. Many school buildings were improved by strengthening, particularly in the late 1960s and early 1970s as the deadline approached. Local efforts to mitigate the risks from unreinforced masonry buildings (URMs) also began in this time period. In 1984, the Federal Emergency Management Agency (FEMA) began its program to encourage the reduction of seismic hazards posed by existing older buildings throughout the country. Recognizing that building rehabilitation design is far more constrained than new building design and that special techniques are needed to insert new lateral elements, tie them to the existing structure, and generally develop complete seismic load paths, a document was published for this purpose in 1992. FEMA 172, NEHRP Handbook of Techniques for the Seismic Rehabilitation of Existing Buildings (FEMA, 1992b), was intended to identify and describe generally accepted rehabilitation techniques. The art and science of seismic rehabilitation has grown tremendously since that time with federal, state, and local government programs to upgrade public buildings, with local ordinances that mandate rehabilitation of certain building types, and with a growing concern among private owners about the seismic performance of their buildings. In addition, following the demand for better understanding of performance of older buildings and the need for more efficient and less disruptive methods to upgrade, laboratory research on the subject has exploded worldwide, particularly since the nlinear methods proposed for FEMA 273 became developed. The large volume of rehabilitation work and research w completed has resulted in considerable refinement of early techniques and development of many new techniques, some confined to the research lab and some widely used in industry. Like FEMA 172, this document describes the techniques currently judged to be most commonly used or potentially to be most useful. Furthermore, it has been formatted to take advantage of the ongoing use of typical building types in FEMA documents concerning existing buildings, and to facilitate the addition of techniques in the future. The primary purpose of this document is to provide a selected compilation of seismic rehabilitation techniques that are practical and effective. The descriptions of techniques include detailing and constructability tips that might t be otherwise available to engineering offices or individual structural engineers who have limited experience in seismic rehabilitation of existing buildings. A secondary purpose is to provide guidance on which techniques are commonly used to mitigate specific seismic deficiencies in various model building types. The goals of the document are to: Describe rehabilitation techniques commonly used for various model building types, Incorporate relevant research results, Discuss associated details and construction issues, Provide suggestions to engineers on the use of new products and techniques.
Federal Emergency Management Agency, U S Department of Homeland Security