Civil Engineering Reference
In-Depth Information
NORTHRIDGE, OVIATT LIBRARY
(1994)
The Magnitude 6.4 January 17, 1994, Northridge earthquake in Southern California,
although not a major event in terms of total energy release, was relatively shallow
with its epicenter in a developed urban area. Consequently the local ground shaking
was very intense, the damage to constructed facilities was severe and the economic
loss was huge. First indications were that only minor damage had been sustained by
steel-framed structures. One such building was the four story steel braced frame
Oviatt Library building on the campus of California State University, Northridge. As
is common with many flexible structures, architectural finishes were found to have
been damaged but this was ascribed to the inherent incompatibility of the relatively
flexible steel skeleton and the stiffer cladding. The observation that substantially
greater, readily apparent, architectural damage occurred in some of the aftershocks
resulted in a more thorough inspection being undertake. This revealed fractures in the
steel column base plates and in various other welded connection in the steel
framework.
Since steel framed structures comprised the seismic resisting system of choice
for tall buildings, the established fact that this public building had not performed
satisfactorily, coupled with unconfirmed reports of several unexpected failures of
other steel frames, prompted a rigorous inspection of all the buildings falling into this
category in the zone of strong to moderate ground shaking.
It was found that in more than 100 buildings brittle fractures had occurred in
welded connections, including the moment resisting joints between beams and
columns. Detailed analyses indicated that these buildings had been designed and
constructed in conformity with normal industry standards.
The disturbing conclusion drawn from these observations was that structural
engineers could no longer have confidence in the established procedures used to
design and construct steel framed buildings in earthquake prone areas,
notwithstanding that these buildings were in compliance with the appropriate codes.
Additionally, of even greater concern in the short term, no consensus existed as to an
acceptable procedure for repair of the damaged frames.
Faced with an exceptionally challenging compound technical problem, those
concerned with the re-establishment of steel moment resistant frame buildings as a
viable alternative amongst the several structural choices available to earthquake
engineers, cooperated to devise and initiate a program with the focused objective of
leading to the development of standards for the repair, retrofit and design of steel
moment resisting frame buildings so that they will provide reliable, cost-effective
performance in future earthquakes.
The experience of the directed program of professional practice development
coupled with problem focused investigations is likely to be reviewed with great
interest by those who may wish to consider the applicability of this approach to other
engineering challenges.
