The moment ratio between the columns and beams in Section E3.4a of AISC 341-10 is one of the requirements that distinguishes a steel SMF from an IMF or OMF. For SMF, plastic hinges are expected to form in the beams (Figure 1b). If plastic hinges occur in the columns (meaning the beams are stronger than the columns), there is a potential for the formation of a weak-story mechanism (Figure 1a).
Figure 1 - Weak-Story Mechanism
The Strong Frame special moment frame is unlike the typical SMF, which has either a reinforced connection (e.g., bolted flange plate connections) or weakened beam connection (e.g., RBS connections) where the plastic hinges are formed by the buckling of the beam flange and web (Figure 2). In the Strong Frame SMF, the stretching and shortening of the links at the top and bottom of the Strong Frame beams are the yielding mechanisms (Figure 3). So instead of a strong column — weak beam check, the Strong Frame design procedure checks for a strong column — weak link condition where the ratio of the column moments to the moment created by the Yield-Link® couple is required to be greater than or equal to 1.0.
Figure 2 - Plastic Hinge in Beam Element for Typical SMF Connection
(Ref: NIST GCR 09-917-3, NEHRP Seismic Design Technical Brief No.2)
Figure 3 — Yielding in Strong Frame Links