G2Composite intermediate moment frames (C-IMF)

G2.4a Stability Bracing of Beams

Beams shall be braced to satisfy the requirements for moderately ductile members in Section D1.2a.

In addition, unless otherwise indicated by testing, beam braces shall be placed near concentrated forces, changes in cross section, and other locations where analysis indicates that a plastic hinge will form during inelastic deformations of the C-IMF.

The required strength and stiffness of stability bracing provided adjacent to plastic hinges shall be in accordance with Section D1.2c.

G2.5a Basic Requirements

Steel and composite members shall satisfy the requirements of Section D1.1 for moderately ductile members.

G2.5b Beam Flanges

Abrupt changes in the beam flange area are prohibited in plastic hinge regions. The drilling of flange holes or trimming of beam flange width is not permitted unless testing or qualification demonstrates that the resulting configuration is able to develop stable plastic hinges to accommodate the required story drift angle.

G2.5c Protected Zones

The region at each end of the beam subject to inelastic straining shall be designated as a protected zone and shall satisfy the requirements of Section D1.3.

User Note: The plastic hinge zones at the ends of C-IMF beams should be treated as protected zones. In general, the protected zone will extend from the face of the composite column to one-half of the beam depth beyond the plastic hinge point.

G2.6a Demand Critical Welds

There are no requirements specific to this system.

G2.6b Beam-to-Column Connections

Beam-to-composite column connections used in the seismic force-resisting system (SFRS) shall satisfy the following requirements:

• (a) The connection shall be capable of accommodating a story drift angle of at least 0.02 rad.
• (b) The measured flexural resistance of the connection determined at the column face shall equal at least 0.80$M_{p}$ of the connected beam at a story drift angle of 0.02 rad, where $M_{p}$ is defined as the plastic moment of the steel, encased, or composite beams and shall meet the requirements of Specification Chapter I.

G2.6c Conformance Demonstration

Beam-to-column connections used in the SFRS shall satisfy the requirements of Section G2.6b by one of the following.

• (a) Use of C-IMF connections designed in accordance with ANSI/AISC 358.
• (b) Use of a connection prequalified for C-IMF in accordance with Section K1.
• (c) Results of at least two qualifying cyclic test results conducted in accordance with Section K2. The tests are permitted to be based on one of the following:
• (1) Tests reported in the research literature or documented tests performed for other projects that represent the project conditions, within the limits specified in Section K2
• (2) Tests that are conducted specifically for the project and are representative of project member sizes, material strengths, connection configurations, and matching connection processes, within the limits specified in Section K2
• (d) Calculations that are substantiated by mechanistic models and component limit state design criteria consistent with these provisions.

G2.6d Required Shear Strength

The required shear strength of the connection shall be determined using the capacity-limited seismic load effect. The capacity-limited horizontal seismic load effect, $E_{c l}$, shall be taken as follows:

(G2-1)

where

$\begin{aligned} L_{h} & =\text { distance between beam plastic hinge locations, in. (mm) } \\ M_{p b e} & =\text { expected flexural strength of the steel, encased, or composite beam, } \\ & \text { kip-in. (N-mm) }\end{aligned}$

For an encased or composite beam, $M_{p b e}$ shall be calculated using the plastic stress distribution or the strain compatibility method as described in Specification Section I1.2a or I1.2b, respectively. Applicable $R_{y}$ and $R_{c}$ factors shall be used for different elements of the cross section while establishing section force equilibrium and calculating the flexural strength.

User Note: For steel beams, $M_{p b e}$ in Equation G2-1 may be taken as $R_{y} M_{p}$ of the beam.

G2.6e Connection Diaphragm Plates

Connection diaphragm plates are permitted for filled composite columns both external to the column and internal to the column.

Where diaphragm plates are used, the thickness of the plates shall be at least the thickness of the beam flange.

The diaphragm plates shall be welded around the full perimeter of the column using either complete-joint-penetration (CJP) groove welds or two-sided fillet welds. The required strength of these joints shall not be less than the available strength of the contact area of the plate with the column sides.

Internal diaphragms shall have circular openings sufficient for placing the concrete.

G2.6f Column Splices

In addition to the requirements of Section D2.5, column splices shall comply with the requirements of this section. Where welds are used to make the splice, they shall be CJP groove welds. When column splices are not made with CJP groove welds, they shall have a required flexural strength that is at least equal to the plastic moment, $M_{p c c}$, of the smaller composite column. The required shear strength of column web splices shall be at least equal to $\Sigma M_{p c c} / H$,

where

$\begin{aligned} H & =\text { height of story, in. (mm) } \\ \Sigma M_{p c c} & =\text { sum of the plastic moments at the top and bottom ends of the composite } \\ & \text { column, kip-in. (N-mm) }\end{aligned}$

For composite columns, the plastic flexural strength shall satisfy the requirements of Specification Chapter I, including the required axial strength, $P_{r c}$.