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"R" value for tension only bracing in steel frames buildings

"R" value for tension only bracing in steel frames buildings

(OP)
What "R" value (overstrength/ductility coefficient) per '97 UBC would I use for the seismic design of a steel framed building (typ 1 story metal bldg)in the opposite direction of the moment frames, where tension only bracing is used to resist lateral forces. Thanks!

RE: "R" value for tension only bracing in steel frames buildings

You would use a value of R from Table 16-N for a Building Frame System for either Ordinary braced frames or Special concentrically braced frames  (R = 5.6 or 6.4 respectively) - for that directin only.  For the moment frame direction, you would use the appropriate R for that system in that direction.

However, if you are in zones 3 or higher or you are in zone 2 with an importance factor > 1, you cannot use a tension only brace system (see 2211.4, section 9.2.c for LRFD and/or 2213.8.2.3 for ASD)  

Section 2210.2.1 outlines the fact that the steel seismic provisions in the UBC are only applicable to zone 2 (with I>1) and to zones 3 and 4.  If you are lower than this, you can use a tension only brace.

RE: "R" value for tension only bracing in steel frames buildings

UBC Section 2213.8.5 carries an exception for one and two story buildings that overrides the cited requirement of 2213.8.2.3.  The caveat is that you must multiply design forces by an omega factor (2.2 for OBF).  Tension only systems are permitted with this boost.  There are similar allowances in the AISC Seismic Provisions of Ordinary Braced Frames.

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