G3Composite special moment frames (C-SMF)

G3.4a Moment Ratio

The following relationship shall be satisfied at beam-to-column connections:

(G3-1)

where

Σ

$\sum M_{p c c}^{*}$

= sum of the projections of the plastic moments, $M_{p c c}$, of the columns

(including haunches where used) above and below the joint to the beam

centerline with a reduction for the axial force in the column. For composite columns, the plastic moment, $M_{p c c}$, shall satisfy the requirements of

Specification Chapter I, including the required axial strength, $P_{r c}$. For reinforced concrete columns, the plastic moment, $M_{p c e}$, shall be calculated based on the provisions of ACI 318, including the required axial strength,

$P_{r c}$. When the centerlines of opposing beams in the same joint do not coincide, the mid-line between centerlines shall be used.

$\Sigma M_{p b e}^{*}=$ sum of the projections of the expected flexural strengths of the beams at the plastic hinge locations to the column centerline. It is permitted to take $\Sigma M_{p b e}^{*}=\Sigma\left(1.1 M_{p b e}+M_{u v}\right)$, where $M_{p b e}$ is calculated as specified in Section G2.6d.

$\begin{aligned} M_{u v} & =\text { additional moment due to shear amplification from the location of the } \\ & \text { plastic hinge to the column centerline, kip-in. (N-mm) }\end{aligned}$

Exception: The exceptions of Section E3.4a shall apply, except that the force limit in Exception (a) shall be $P_{r c}<0.1 P_{y c}$.

G3.4b Stability Bracing of Beams

Beams shall be braced to meet the requirements for highly ductile members in Section D1.2b.

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-SMF.

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

G3.4c Stability Bracing at Beam-to-Column Connections

Composite columns with unbraced connections shall satisfy the requirements of Section E3.4c.2.

G3.5a Basic Requirements

Steel and composite members shall meet the requirements of Section D1.1 for highly ductile members.

Exception: Reinforced concrete-encased beams shall meet the requirements for Section D1.1 for moderately ductile members if the reinforced concrete cover is at least 2 in. (50 mm) and confinement is provided by transverse reinforcement in regions where plastic hinges are expected to occur under seismic deformations. Transverse reinforcement shall satisfy the requirements of ACI 318, Section 18.6.4.

Encased composite beams that are part of C-SMF shall also meet the following requirement:

(G3-2)

where

• $E \quad=$ modulus of elasticity of the steel beam
• = 29,000 ksi (200 000 MPa)
• $\begin{aligned} F_{y} & =\text { specified minimum yield stress of the steel beam, ksi (MPa) }\end{aligned}$
• $Y_{\text {con }}=$ distance from the top of the steel beam to the top of the concrete, in. (mm)
• $Y_{P N A}=$ distance from the extreme concrete compression fiber to the plastic neutral axis, in. (mm)
• $d \quad$ = overall depth of the beam, in. (mm)

G3.5b Beam Flanges

Abrupt changes in beam flange area are prohibited in plastic hinge regions. The drilling of flange holes or trimming of beam flange width is prohibited unless testing or qualification demonstrates that the resulting configuration can develop stable plastic hinges to accommodate the required story drift angle.

G3.5c Protected Zones

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

User Note: The plastic hinge zones at the ends of C-SMF 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.

User Note: All subsections of Section D2 are relevant for C-SMF.

G3.6a Demand Critical Welds

The following welds are demand critical welds and shall meet the requirements of Sections A3.4b and I2.3:

• (a) Groove welds at column splices
• (b) Welds at the column-to-base plate connections

Exception: Welds need not be considered demand critical when both of the fol- lowing conditions are satisfied.

• (1) Column hinging at, or near, the base plate is precluded by conditions of restraint.
• (2) There is no net tension under load combinations including the overstrength seismic load.
• (c) Complete-joint-penetration groove welds of beam flanges to columns, diaphragm plates that serve as a continuation of beam flanges, shear plates within the girder depth that transition from the girder to an encased steel shape, and beam webs to columns

G3.6b 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 0.04 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 0.04 rad, where $M_{p}$ is determined in accordance with Section G2.6b.

G3.6c Conformance Demonstration

Beam-to-composite column connections used in the SFRS shall meet the require- ments of Section G3.6b by one of the following:

• (a) Use of C-SMF connections designed in accordance with ANSI/AISC 358

• (b) Use of a connection prequalified for C-SMF in accordance with Section K1

• (c) The connections shall be qualified using test results obtained in accordance with Section K2. Results of at least two cyclic connection tests shall be provided, and shall be based on one of the following:

• (1) Tests reported in 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 by Section K2

• (d) When beams are uninterrupted or continuous through the composite or reinforced concrete column, beam flange welded joints are not used, and the connection is not otherwise susceptible to premature fracture, other substantiating data is permitted to demonstrate conformance.

Connections that accommodate the required story drift angle within the connection elements and provide the measured flexural resistance and required shear strength specified in Section G3.6d are permitted. In addition to satisfying the preceding requirements, the design shall demonstrate that any additional drift due to connection deformation is accommodated by the structure. The design shall include analysis for stability effects of the overall frame, including second-order effects.

G3.6d Required Shear Strength

The required shear strength of the connection, $V_{u}$, 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:

$E_{cl}=2(1.1M_{pbe})/L_{h}$ (G3-3)

where

$\begin{aligned} L_{h} & =\text { distance between beam plastic hinge locations, in. (mm) } \\ M_{p b e} & =\text { expected flexural strength of the steel, concrete-encased, or composite } \\ & \text { beams, kip-in. (N-mm). For encased or composite beams, } M_{p b e} \text { shall be } \\ & \text { calculated according to Section G2.6d. }\end{aligned}$

G3.6e Connection Diaphragm Plates

The diaphragm plates used in connections to filled composite columns shall satisfy the requirements of Section G2.6e.

G3.6f Column Splices

Composite column splices shall satisfy the requirements of Section G2.6f.