H2Composite special concentrically braced frames (C-SCBF)

AISC 341-22, Seismic Provisions for Structural Steel Buildings

Last Updated 29 Oct 2026

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H2.1 Scope

Composite special concentrically braced frames (C-SCBF) shall be designed in conformance with this section. Columns shall be encased or filled composite. Beams shall be either structural steel or composite beams. Braces shall be structural steel or filled composite members.

H2.2 Basis of Design

This section is applicable to braced frames that consist of concentrically connected members. Eccentricities less than the beam depth are permitted if the resulting member and connection forces are addressed in the design and do not change the expected source of inelastic deformation capacity.

C-SCBF designed in accordance with these provisions are expected to provide significant inelastic deformation capacity primarily through brace buckling and yielding of the brace in tension.

H2.3 Analysis

The analysis requirements for C-SCBF shall satisfy the analysis requirements of Section F2.3 modified to account for the entire composite section in determining the expected brace strengths in tension and compression.

H2.4 System Requirements

The system requirements for C-SCBF shall satisfy the system requirements of Section F2.4. Composite braces are not permitted for use in multi-tiered braced frames.

H2.5 Members

H2.5a Basic Requirements

Composite columns and steel or composite braces shall satisfy the requirements of Section D1.1 for highly ductile members. Steel or composite beams shall satisfy the requirements of Section D1.1 for moderately ductile members.

H2.5b Diagonal Braces

Structural steel and filled composite braces shall satisfy the requirements for SCBF of Section F2.5b. The radius of gyration in Section F2.5b shall be taken as that of the steel section alone.

H2.5c Protected Zones

The protected zone of C-SCBF shall satisfy Section D1.3 and include the following:

(a) For braces, the center one-quarter of the brace length and a zone adjacent to each connection equal to the brace depth in the plane of buckling
(b) Elements that connect braces to beams and columns

H2.6 Connections

Design of connections in C-SCBF shall be based on Section D2 and the provisions of this section.

H2.6a Demand Critical Welds

The following welds are demand critical welds and shall satisfy 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 met.

(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) Welds at beam-to-column connections conforming to Section H2.6b(b)

H2.6b Beam-to-Column Connections

Where a brace or gusset plate connects to both members at a beam-to-column connection, the connection shall satisfy one of the following requirements:

(a) The connection shall be a simple connection meeting the requirements of Specification Section B3.4a, where the required rotation is taken to be 0.025 rad.
(b) Beam-to-column connections shall satisfy the requirements for fully restrained (FR) moment connections as specified in Sections D2, G2.6d, and G2.6e. The required flexural strength of the connection shall be determined from analysis and shall be considered in combination with the required strength of the brace connection and beam connection, including the diaphragm collector forces determined using the overstrength seismic load.

H2.6c Brace Connections

Brace connections shall satisfy the requirement of Section F2.6c, except that the required strength shall be modified to account for the entire composite section in determining the expected brace strength in tension and compression. Applicable $R_{y}$ factors shall be used for different elements of the cross section for calculating the expected brace strength, where $R_{y}$ is the ratio of the expected yield stress to the specified minimum yield stress, $F_{y}$ . The expected brace flexural strength shall be determined as $M_{p b e}$ , where $M_{p b e}$ is calculated as specified in Section G2.6d.

H2.6d 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 complete-joint-penetration groove welds. When column splices are not made with 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 $\Sigma M_{p c c}$ is the sum of the plastic moments at the top and bottom ends of the composite column and $H$ is the height of story, in. (mm). The plastic flexural strength shall meet the requirements of Specification Chapter I, including the required axial strength, $P_{r c}$ .