The Northridge earthquake of January 17, 1994, caused widespread
building damage throughout some of the most heavily populated
communities of Southern California including the San Fernando
Valley, Santa Monica and West Los Angeles, resulting in estimated
economic losses exceeding $30 billion. Much of the damage sustained
was quite predictable, occurring in types of buildings that
engineers had previously identified as having low seismic
resistance and significant risk of damage in earthquakes. This
included older masonry and concrete buildings, but not steel framed
buildings. Surprisingly, however, a number of modern, welded,
steel, moment-frame buildings also sustained significant damage.
This damage consisted of a brittle fracturing of the steel frames
at the welded joints between the beams (horizontal framing members)
and columns (vertical framing members). A few of the most severely
damaged buildings could readily be observed to be out-of plumb
(leaning to one side). However, many of the damaged buildings
exhibited no outward signs of these fractures, making damage
detection both difficult and costly. Then, exactly one year later,
on January 17, 1995, the city of Kobe, Japan also experienced a
large earthquake, causing similar unanticipated damage to steel
moment-frame buildings. Prior to the 1994 Northridge and 1995 Kobe
earthquakes, engineers believed that steel moment-frames would
behave in a ductile manner, bending under earthquake loading, but
not breaking. As a result, this became one of the most common types
of construction used for major buildings in areas subject to severe
earthquakes. The discovery of the potential for fracturing in these
frames called to question the adequacy of the building code
provisions dealing with this type of construction and created a
crisis of confidence around the world. Engineers did not have clear
guidance on how to detect damage, repair the damage they found,
assess the safety of existing buildings, upgrade buildings found to
be deficient or design new steel moment-frame structures to perform
adequately in earthquakes. The observed damage also raised
questions as to whether buildings in cities affected by other past
earthquakes had sustained similar undetected damage and were now
weakened and potentially hazardous. In response to the many
concerns raised by these damage discoveries, the Federal Emergency
Management Agency (FEMA) sponsored a program of directed
investigation and development to identify the cause of the damage,
quantify the risk inherent in steel structures and develop
practical and effective engineering criteria for mitigation of this
risk. As the project progressed, interim guidance documents were
published to provide practicing engineers and the construction
industry with important information on the lessons learned, as well
as recommendations for investigation, repair, upgrade, and design
of steel moment frame buildings. Many of these recommendations have
already been incorporated into recent building codes. This project
culminated with the publication of four engineering practice
guideline documents. These four volumes include state-of-the-art
recommendations that should be included in future building codes,
as well as guidelines that may be applied voluntarily to assess and
reduce the earthquake risk in our communities. This policy guide
has been prepared to provide a nontechnical summary of the valuable
information contained in the FEMA/SAC publications, an
understanding of the risk associated with steel moment-frame
buildings, and the practical measures that can be taken to reduce
this risk. It is anticipated that this guide will be of interest to
building owners and tenants, members of the financial and insurance
industries, and to government planners and the building regulation
community.
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