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[SEAOC] Re: [SEAOC] Steel frame design

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Re: industrial structures with knee brace to mid height of column.

I try to avoid this configuration, but sometimes must provide a design.  An
Rw of 4 is the general default for non-building structures.  With an Rw of 4
and a non-building structure, many prescriptive requirements, such as
strong-col weak-beam, tension-compresion bracing, are not required.  With an
Rw = 4, only the connection requirements of concentric braced frames apply.
 Note that when the connection requirement is "strength" based (i.e. develop
3*Rw/8 times the seismic force in the brace) the allowable stress may be
increased by a 1.7 factor as per UBC 2211.4.2.  Thus, with an Rw=4, the
connection requirement is not much higher than if the brace force were from
wind loading. 

For an added measure of reliability, An Rw=3 may be used.  An Rw = 3 should
theoretically keep your forces at about or below yield (considering safety
factors and overstrength provided).  While the prescriptive requirements of
higher Rw buildings are not required for this catagory of structure, they
should be at the very least qualitatively considered during design as
desireable qualities.  Adding redundant locations of the bracing is also

I presented a paper titled "Use of Building Code Seismic Provisions for
Non-Building and Industrial Structures" at the Dec 1994 EERI regional seninar
in St. Louis MO.  It is in the proceedings, or I could forward you a copy on

Thomas F. Heausler, S. E.
Kansas City, MO
(816) 822-1180