Building Code Review
The
rules and requirements for the seismic restraints
are published in the model building codes.
Historically, there have been three independent
codes used in various areas of the United States. In
the Northeast, the predominant code was developed by
the Building Officials Code Administration (BOCA),
which aimed to address concerns specific to the
region, such as high snow loads. In the Southeast,
the Southern Building Code Congress International (SBCCI)
developed the Standard Building Code (SBC), which
tended again to focus on local concerns, like wind
design. While seismic requirements were addressed by
these codes, the design criteria were not
particularly severe. West of the Mississippi and
more commonly used internationally is the Uniform
Building Code (UBC). Because of the more severe
conditions present in the western United States, the
UBC was required to address snow, wind, and seismic
loads on a more equal footing.
The first code version significantly affected by the
seismic data collected in the early 1990's was the
1997 UBC. Several new factors were introduced to the
design equations to account for soil, fault
proximity and type, specifics of the equipment
location within the building structure with respect
to grade, and typical dynamic response
characteristics for particular kinds of equipment.
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In 2000, the International Building Code or IBC
was released. This new code was developed as a
collective effort by the three independent code
bodies. The IBC was intended to, and has been
replacing the three independent codes
nationwide. The driving force behind the
development of this code has been FEMA. While
this code most closely resembles the 1997 UBC,
it is a considerably refined version using,
among other things, new and considerably more
comprehensive seismic hazard maps.
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BOCA and SBC will not be updated. Instead, these
older generation codes and the smaller seismic
design loads predicted by them are being phased
out during the next ratification cycle.
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Within the Western Hemisphere, the other code
that is most frequently used is the Canadian
National Building Code or NBC. The 1995 version
of this code, when introduced, was ahead of the
U.S. building codes in addressing the location
of equipment in buildings. While not currently
listing requirements as severe as those
identified in the 1997 UBC or IBC, the force
development equations specified by the code are
quite similar to those of the newer U.S. codes.
It is important to recognize that the newer codes
predict a significantly higher seismic design load
than past codes. This is particularly true for
equipment located in upper levels of buildings. In
some instances using the newer code criteria it will
be found that attaching heavy, unstable equipment
located on an upper floor will not be practical with
concrete anchorage. A connection directly to steel
will be required.
Depending on the geology of the installation site, a
vertical force component may also need to be
considered when evaluating seismic loads. When
incurred in the codes, this force is typically a
fixed percentage of the horizontal seismic load. |