E2.2 Basis of Design
Intermediate moment frames (IMF) are intended to provide limited levels of inelastic rotation capacity and are based on tested designs. Due to the lesser rotational capacity of IMF as compared to special moment frames (SMF), ASCE/SEI 7 (ASCE, 2022) requires use of a lower seismic response modification coefficient, , than that for SMF and places significant height and other limitations on its use.
While the design for SMF is intended to limit most of the inelastic deformation to the beams, the inelastic drift capability of IMF is permitted to be derived from inelastic deformations of beams, columns, and/or panel zones.
The IMF connection is based on a tested design with a qualifying story drift angle of 0.02 rad using the loading protocol specified in Section K2. It is assumed that this limited connection rotation will be achieved by use of larger frame members than would be required in an SMF, because of the lower and/or higher values used in design.
Commentary Section E3 offers additional discussion relevant to IMF.
E2.4 System Requirements
E2.4a Stability Bracing of Beams
See Commentary Section D1.2a on stability bracing of moderately ductile members and Commentary Section E3.4b for additional commentary.
E2.5 Members
E2.5a Basic Requirements
This section refers to Section D1, which provides requirements for connection of webs to flanges as for built-up members and requirements for width-to-thickness ratios for the flanges and webs of the members. Because the rotational demands on IMF beams and columns are expected to be lower than for SMF, the width-to-thickness limitations for IMF are less severe than for SMF. See Commentary Section E3.5a for further discussion.
E2.5b Beam Flanges
The intent of the requirements in this section are identical to those in Section E3.5b. See Commentary Section E3.5b for further discussion.
E2.5c Protected Zones
For commentary on protected zones, see Commentary Section D1.3.
E2.6 Connections
E2.6a Demand Critical Welds
The requirements in this section are identical to those in Section E3.6a. See Com- mentary Section E3.6a for further discussion.
E2.6b Beam-to-Column Connection Requirements
The minimum story drift angle required for qualification of IMF connections is 0.02 rad while that for SMF connections is 0.04 rad. This level of story drift angle has been established for this type of frame based on engineering judgment applied to available tests and analytical studies, primarily those included in FEMA 355C (2000e) and 355F (2000d).
ANSI/AISC 358 (AISC, 2022b) describes 11 different connections that have been prequalified for use in both IMF and SMF systems and one connection that has been prequalified for use in SMF systems only. The prequalified connections for both IMF and SMF systems include the reduced beam section, the bolted unstiffened extended end plate, the bolted stiffened extended end plate, the bolted flange plate, the welded unreinforced flange-welded web, the cast bolted bracket, the ConXtech® ConXL™, the SidePlate®, the Simpson Strong-Tie® Strong Frame®, the double-tee, and the DuraFuse Frames moment connection. The slotted web moment connection is limited to SMF systems. In a few cases, the limitations on use of the connections are less strict for IMF than for SMF, but generally, the connection design procedures are the same.
E2.6c Conformance Demonstration
The requirements for conformance demonstration for IMF connections are similar to those for SMF connections. Refer to Commentary Section E3.6c for further discussion.
E2.6d Required Shear Strength
The requirements for shear strength of the connection for IMF are similar to those for SMF. See Commentary Section E3.6d for further discussion.
E2.6e Panel Zone
The panel zone for IMF is required to be designed according to Specification Section J10.6, with no further requirements in the Provisions. As noted in Commentary Section E2.2, panel-zone yielding is permitted as part of the inelastic action contributing to the drift capacity of the IMF, and the requirements of the Specification are considered adequate to achieve the expected performance.
E2.6f Continuity Plates
The requirements in Section E3.6f are referenced in this section. See Commentary Section E3.6f for further discussion.
E2.6g Column Splices
The requirements in Section E3.6g are referenced in this section. See Commentary Section E3.6g for further discussion.