Coordinate earthquake-resistive design with the project
Professional Structural Engineer for appropriateness and
Both SMACNA and NUSIG list conditions under which seismic
bracing may be omitted. However, a design professional may
revoke these omissions on an individual project basis.
CONFORMANCE WITH SMACNA:
Requires a design professional to develop a Seismic Hazard
Level (SHL) for a specific project for use of its manual. The
SHL is derived from a combination of factors such as
applicable building codes, seismic zones, and importance
factors into a single system for determining appropriate
braces or restraints. A building with an SHL of "A" requires
the strongest restraints, whereas, a building with SHL of "C"
requires the least restraints. The design professional is
responsible for specifying the SHL of "C" requires the least
restraints. The design professional is responsible for
specifying the SHL, not the contractor.
Bracing in SHL "A" is designed to resist 48% of the weight of
the pipes. Bracing in SHL "B" is designed to resist 30%, and
in the bracing in SHL "C" is designed to resist 15% of the
weight of the ducts or pipes. In addition to the horizontal
seismic force, a vertical seismic force equal to one-third of
the horizontal force is included in the analysis.
Current building codes require most structures and their
components to be designed for a horizontal seismic force that
is a given percent of the supported weight. Each code has its
own method for determining the percent. The formulas are in
Fp - Cs x Wp
Where Wp is the weight of the ducts or pipes, Fp is the
seismic force, and Cs is a seismic coefficient which
represents a combination of factors that varies with the
building code. Use more severe of the Local Code and the
Uniform Building Code (UBC) to determine seismic force, Fp.
The percent horizontal seismic factor is:
%F = (Fp/Wp) x 100
= Cs x 100
If %F is greater than 48, then manual cannot be used, and an
independent analysis and design is required for all bracing
Use SHL "A" if %F is between 31-48, use SHL "B" if %F is
between 16-30, and use SHL "C" if %F is 15 or less.
CONFORMANCE WITH NUSIG:
NUSIG has developed guidelines as a service to the design
professionals. The guidelines are not intended as a substitute
for design. The design of seismic bracing remains the