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 no 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-known 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 nonlinear methods proposed for FEMA 273
became developed. The large volume of rehabilitation work and
research now 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 not 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.
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