Seismic Performance Factors for Cold-Formed Steel Special Bolted Moment Frames

The American Iron and Steel Institute (AISI) recently issued a standard on the seismic design of cold-formed steel special bolted moment frames (SBMF). It is expected that ductility is provided through bolt slippage and bearing in the moment connection region; both the beams and columns are to be designed based on the capacity design principles. This paper provides background information for the proposed seismic performance factors. Based on cyclic testing results, a value of 3.5 was proposed for the response modification coefficient, $R$ . A statistical evaluation through nonlinear time-history analysis, which considered the unique structural characteristics of this framing system, showed that the Newmark-Hall ductility reduction rule is conservative. Based on the analysis results, a revised rule was proposed, which resulted in a deflection amplification factor, $C_d$ , of $R/1.2\left(\approx 3.0\right)$ . A design procedure in the AISI seismic standard allows the designer to directly calculate the maximum seismic force in the moment connection at the design story drift level, thereby an empirical system overstrength factor, ${\Omega }_0$ , is unnecessary for the design of SBMFs.