The requirements of this section reflect the tested conditions and common allowances where appropriate. The connection is detailed to accommodate up to 0.07 rad of rotation, which, along with frame flexibility, will accommodate the expected interstory drift without affecting any connection element other than the Yield-Links. Shear plate connection welds are required to develop the strength of the shear plate, and Yield-Link material thickness may vary from nominally ½ in. (13 mm) to 1 in. (25 mm) thick and is fabricated from one of the three permitted steel grades. The previous specification of Link thickness tolerance was changed to reference ASTM A6/A6M.
The stems of the pair of Yield-Links at each connection must be fabricated from the same heat of material to ensure minimum variability in actual and for the pair of Links in a connection. This is because imbalance of the Link strengths can drive additional force into the central pivot bolt of the connection. This force is parallel to and can be cumulative with the net axial connection force in the beam, which is also resisted by the central pivot bolt. Rather than include an explicit design procedure to accommodate unbalanced Link strength, it was decided at this time to simply use material from the same heat for the stems of each pair of Links at a given connection.
In general, the topic of the potential adverse effects of unequal strength in the Links or flanges of a moment connection is not limited to just the Strong Frame connection. While the central pivot design of the Strong Frame connection in essence attempts to maintain the location of the plastic neutral axis at the centerline of the beam even if the Links are of different strengths—and thus create relatively even strain demands in each link for a given connection rotation—the same is not true for traditional built-up shapes that may have different flange strengths and form plastic hinges in the beam cross section. The neutral axis of the plastic section would shift toward the flange with higher strength, and uneven strain demands in the flanges would result. However, the effect on inelastic performance for this condition has not been studied, and currently there are no requirements to control flange strength in SMF connections using built-up sections subject to plastic hinging.
When using separate T-stub Yield-Links, beam shear is transmitted directly to the column through the single-plate shear connections and not through the Yield-Link, which affords the opportunity to use other than standard holes at the Link flange-to-column connection. When end-plate Yield-Links are used, shear is transferred from the beam to the column through the bolts that connect the end plate to the column. Because these bolts may be snug-tight, standard holes must be used in both the column and the end plate. Specific methods of fabricating bolt holes and the specification of surface roughness were removed because any approved method is acceptable, and surface roughness must already conform to the AISC Specification.
Changes to the requirements for the buckling restraint assembly reflect the addition of specific design requirements in Step 10 of the design procedure. The maximum BRP bolt diameter has also been removed because the bolts are now specifically designed in Step 10.