G2.2 Basis of Design
ASCE/SEI 7 limits the use of composite intermediate moment frames (C-IMF) to seismic design categories A through C. The provisions for C-IMF, as well as the associated and values in ASCE/SEI 7, are comparable to those required for reinforced concrete intermediate moment frames and between those for steel intermediate moment frames (IMF) and OMF.
The inelastic drift capability of C-IMF is permitted to be derived from inelastic deformations of beams, columns, and panel zones. This is more permissive than the design requirements for composite special moment frames (C-SMF) as defined in Section G3, which are intended to limit the majority of the inelastic deformation to the beams.
The C-IMF connection is based on a tested design with a qualifying story drift angle of 0.02 rad.
G2.4 System Requirements
G2.4a Stability Bracing of Beams
The requirement for spacing of lateral bracing in this section is less severe than that for C-SMF in Section G3.4b because of the lower qualifying story drift angle for C-IMF as compared to C-SMF. In this case, the required spacing of bracing is approximately double that of the C-SMF system.
G2.5 Members
G2.5a Basic Requirements
This section refers to Section D1.1, which provides requirements for moderately ductile members. Because the rotational demands on C-IMF beams and columns are expected to be lower than C-SMF, the requirements and limitations for C-IMF members are less severe than for C-SMF.
G2.5b Beam Flanges
For relevant commentary on changes in cross section of beam flanges, see Commentary Section E3.5b.
G2.5c Protected Zones
For commentary on protected zones, see Commentary Section D1.3.
G2.6 Connections
G2.6a Demand Critical Welds
There are no demand critical welds in C-IMF members because the story drift angle is 0.02 rad, which is half the value for C-SMF members, and ASCE/SEI 7 limits the use of C-IMF to seismic design categories A through C.
G2.6b Beam-to-Column Connections
The minimum story drift angle required for qualification of C-IMF connections is 0.02 rad, which is half the value for C-SMF connections, reflecting the lower level of inelastic response that is anticipated in the system.
G2.6c Conformance Demonstration
Requirements (a), (b), and (c) for conformance demonstration for C-IMF connections are the same as for C-SMF connections, except that the qualifying story drift angle is smaller. Refer to Commentary Section G3.6c.
G2.6d Required Shear Strength
The requirements for shear strength of the connection for C-IMF are comparable to those of SMF, with the exception that the calculation of the expected flexural strength must account for the different constituent materials. Refer to Commentary Section E3.6d.
G2.6e Connection Diaphragm Plates
Connection diaphragm plates distribute the beam flange forces to the hollow structural section (HSS) or box-section column webs, or sidewalls, within the column panel zone. Whether they are internal or external to the column, they should be proportioned to transfer the beam flange force that could be imposed on the connection considering the flow of forces into the plate and providing enough room for the flange force to flow to the sides of the HSS or box-section column webs.
When an internal diaphragm plate is provided, a hole in the diaphragm plate is required for the placement of concrete. These Provisions do not provide specific criteria related to the size of the hole, so some engineering judgment must be exercised. From a practical standpoint, the size of the hole will be dictated by the column geometry, the concrete mix fluidity, and the concrete placement operations. This needs to be balanced with providing enough plate material at the widest hole section to transfer the beam flange forces through the joint to the sidewalls or the far side of the column as required by the specific design.
G2.6f Column Splices
The requirements for column splices for C-IMF are comparable to those of SMF, with the exception that the calculation of the expected flexural strength must account for the different constituent materials.