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C12.2 STRUCTURAL SYSTEM SELECTION

In a moment-resisting frame system, moment-resisting connections between the columns and beams provide lateral resistance. In Table 12.2-1, such frames are classified as ordinary, intermediate, or special. In high seismic design categories, the anticipated ground motions are expected to produce large inelastic demands, so special moment frames designed and detailed for ductile response in accordance with Chapter 14 are required. In low Seismic Design Categories, the inherent overstrength in typical structural designs is such that the anticipated inelastic demands are somewhat reduced, and less ductile systems may be used safely. Because these less ductile ordinary framing systems do not possess as much toughness, lower values of R are specified.
The values for R, Ω₀, and C_d at the composite systems in Table 12.2-1 are similar to those for comparable systems of structural steel and reinforced concrete.

C12.2.5.6 Steel Ordinary Moment Frames: Steel ordinary moment frames (OMFs) are less ductile than steel special moment frames; consequently, their use is prohibited in structures assigned to Seismic Design Categories D, E, and F (Table 12.2-1). Structures with steel OMFs, however, have exhibited acceptable behavior in past earthquakes where the structures were sufficiently limited in their structural height, number of stories, and seismic mass. The provisions in the standard reflect these observations. The exception is discussed separately below. Table C12.2-1 summarizes the provisions.

12.2.4 Combination Framing Detailing Requirements

Structural members common are intrinsic to different framing systems used to resist counteract seismic forces in any direction shall be all directions and are designed using the detailing requirements of ASCE 7-16 Chapter 12 required by the highest response modification coefficient, R, of the connected framing systems.

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