D1Member requirements

AISC 341-22, Seismic Provisions for Structural Steel Buildings

Last Updated 29 Oct 2026

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Members of moment frames, braced frames, and shear walls in the seismic force-resisting system (SFRS) shall comply with the Specification and this section.

D1.1 Classification of Sections for Ductility

When required for the systems defined in Chapters E, F, G, H, and Section D4, members designated as moderately ductile members or highly ductile members shall comply with this section.

D1.1a Section Requirements for Ductile Members

Structural steel sections for both moderately ductile members and highly ductile members shall have flanges continuously connected to the web or webs.

Encased composite columns shall comply with the requirements of Section D1.4b.1 for moderately ductile members and Section D1.4b.2 for highly ductile members.

Filled composite columns shall comply with the requirements of Section D1.4c for both moderately and highly ductile members.

Concrete sections shall comply with the requirements of ACI 318, Section 18.4, for moderately ductile members and ACI 318, Sections 18.6, 18.7, and 18.8, for highly ductile members.

D1.1b Width-to-Thickness Limitations of Steel and Composite Sections

For members designated as moderately ductile, the width-to-thickness ratios of compression elements shall not exceed the limiting width-to-thickness ratios, $\lambda_{m d}$ , from Tables D1.1a and D1.1b.

TABLE D1.1aWidth-to-Thickness Ratios: Compression Elements—Diagonal Braces

	Case	Description of Element	Width-to- Thickness Ratio	Limiting Width-to- Thickness Ratio		Example
	Case	Description of Element	Width-to- Thickness Ratio	λ_hd Highly Ductile Members	λ_md Moderately Ductile Members	Example
Unstiffened Elements	1	(1) Flanges of rolled or built-up I-shaped sections	b/t d/t	0.30 √ E / (R_yF_y)	0.38 √ E / (R_yF_y)
		(2) Flange and stem of rolled or built-up tees
		(3) Flanges of rolled or built-up channels
		(4) Legs of single angles or double-angle members with separators
		(5) Outstanding legs of pairs of angles in continuous contact
Stiffened Elements	2	(1) Walls of rectangu-lar HSS^[a]	b/t h/t	0.65 √ E / (R_yF_y)	0.76 √ E / (R_yF_y)
		(2) Flanges and side plates of boxed I-shaped sections
		(3) Walls of box sections

For members designated as highly ductile, the width-to-thickness ratios of compression elements shall not exceed the limiting width-to-thickness ratios, $\lambda_{h d}$ , from Tables D1.1a and D1.1b.

D1.2 Stability Bracing of Beams

When required in Chapters E, F, G, and H, stability bracing shall be provided as required in this section to restrain lateral-torsional buckling of structural steel or concrete-encased beams subjected to flexure and designated as moderately ductile members or highly ductile members.

TABLE D1.1a (continued)Width-to-Thickness Ratios: Compression Elements—Diagonal Braces

	Case	Description of Element	Width-to- Thickness Ratio	Limiting Width-to- Thickness Ratio		Example
	Case	Description of Element	Width-to- Thickness Ratio	λ_hd Highly Ductile Members	λ_md Moderately Ductile Members	Example
Stiffened Elements	3	Walls of round HSS [a]	D/t	0.053 E / (R_yF_y)	0.062 E / (R_yF_y)
	4	Webs of rolled or built-up I-shaped sections and channels	h/t_w	1.49 √(E / (R_yF_y))	1.49 √(E / (R_yF_y))
	5	Walls of filled rectan-gular HSS and box sections [a]	b/t h/t	1.4 √(E / (R_yF_y))	2.26 √(E / (R_yF_y))
	6	Walls of filled round HSS sections [a]	D/t	0.076 E / (R_yF_y)	0.15 E / (R_yF_y)

[a] The design wall thickness shall be used in the calculations involving the wall thickness of hollow structural sections (HSS), as defined in Specification Section B4.2.

User Note: In addition to the requirements in Chapters E, F, G, and H to provide stability bracing for various beam members such as intermediate and special moment frame beams, stability bracing is also required for columns in the special cantilever column system (SCCS) in Section E6.

D1.2a Moderately Ductile Members

D1.2a.1 Steel Beams

The bracing of moderately ductile steel beams shall satisfy the following requirements:

TABLE D1.1bWidth-to-Thickness Ratios: Compression Elements—All Members Except Diagonal Braces

	Case	Description of Element	Width-to- Thickness Ratio	Limiting Width-to-Thickness Ratio		Example
	Case	Description of Element	Width-to- Thickness Ratio	λ_hd Highly Ductile Members	λ_md Moderately Ductile Members	Example
Unstiffened Elements	7	(1) Flanges of rolled or built-up I-shaped sections (2) Flange and stem of rolled or built-up tees (3) Flanges of rolled or built-up channels (4) Legs of sin-gle angles or double-angle members with separators (5) Outstanding legs of pairs of angles in continuous contact	b/t	0.30 √E/(R_yF_y)	0.38 √E/(R_yF_y)	b b t t b b t t HPL ttitt t d t d b b t t b t b PA t I► b t
	7		d/t
	8	Horizontal legs of double-angle members with separators or in continuous contact	b/t	0.47 √E/(R_yF_y)	0.54 √E/(R_yF_y)	b +++
	9	Flanges of H-pile sections per Section D4	b/t	not applicable	0.45 √E/(R_yF_y)	I

TABLE D1.1b (continued)Width-to-Thickness Ratios: Compression Elements—All Members Except Diagonal Braces

	Case	Description of Element	Width-to- Thickness Ratio	Limiting Width-to-Thickness Ratio		Example
	Case	Description of Element	Width-to- Thickness Ratio	λ_hd Highly Ductile Members	λ_md Moderately Ductile Members	Example
Stiffened Elements	10	Webs of H-pile sections	h/t_w	not applicable	1.50 √E / R_yF_y
	11	For moment frames, where used in beams or columns, as webs in flexure, or combined axial and flexure Webs of rolled or built-up I-shaped sections and channels	h/t_w	2.5 (1 − C_a)^2.3 √E / R_yF_y^[b]	5.4 (1 − C_a)^2.3 √E / R_yF_y^[b]
	12	Where used in beams or col- umns as flanges in uniform compression due to flexure or combined axial and flexure (1) Flanges of rectangular HSS^[a] (2) Flanges of boxed I-shaped sections (3) Flanges of box sections	b/t	0.55 √E / R_yF_y	1.00 √E / R_yF_y

TABLE D1.1b (continued)Width-to-Thickness Ratios: Compression Elements—All Members Except Diagonal Braces

	Case	Description of Element	Width-to- Thickness Ratio	Limiting Width-to-Thickness Ratio		Example
	Case	Description of Element	Width-to- Thickness Ratio	λ_hd Highly Ductile Members	λ_md Moderately Ductile Members	Example
Stiffened Elements	13	Where used in beams, columns, or links, as webs in flexure, or combined axial and flexure (1) Side plates of boxed I-shaped sections (2) Webs of rectangular HSS^[a] (3) Webs of box sections (4) Except for moment frames, webs of rolled or built-up I-shaped sections and channels	h/t	For C_a ≤ 0.113^[b] 2.45(1 − 1.04C_a) √E/(R_yF_y) For C_a > 0.113 2.26(1 − 0.38C_a) √E/(R_yF_y) ≥ 1.56 √E/(R_yF_y)	For C_a ≤ 0.113^[b] 3.76(1 − 3.05C_a) √E/(R_yF_y) For C_a > 0.113 2.61(1 − 0.49C_a) √E/(R_yF_y) ≥ 1.56 √E/(R_yF_y)	Multiple diagrams: Boxed I-sections, Rectangular HSS, Box sections, I-sections, and Channels
	14	Flanges of box sections used as link beams	b/t	0.55 √E/(R_yF_y)	0.64 √E/(R_yF_y)	Hollow square section with width 'b' and thickness 't'
	15	Webs of box sections used as EBF links	h/t	0.64 √E/(R_yF_y)	1.67 √E/(R_yF_y)	Hollow square section with height 'h' and thickness 't'
	16	Walls of round HSS^[a]	D/t	0.038 √E/(R_yF_y)	0.07 √E/(R_yF_y)	Hollow round section with diameter 'D' and thickness 't'

TABLE D1.1b (continued)Width-to-Thickness Ratios: Compression Elements—All Members Except Diagonal Braces

	Case	Description of Element	Width-to- Thickness Ratio	Limiting Width-to-Thickness Ratio		Example
	Case	Description of Element	Width-to- Thickness Ratio	λ_hd Highly Ductile Members	λ_md Moderately Ductile Members	Example
Composite Elements	17	Flanges and webs of filled rectangular HSS and box sections^[a]	b/t h/t	1.4 √(E⁄R_yF_y)	2.26 √(E⁄R_yF_y)
Composite Elements	18	Walls of filled round HSS sections^[a]	D/t	0.076 E⁄R_yF_y	0.15 E⁄R_yF_y

[a] The design wall thickness shall be used in the calculations involving the wall thickness of hollow structural sections (HSS), as defined in Specification Section B4.2.

$\alpha_{3}$

$P_{r}$

(b) $C_{a}=$

RyFyAg

where

$A_{g}=\text { gross area, in. }^{2}\left(\mathrm{~mm}^{2}\right)$

$E=\operatorname{modulus}$ of elasticity of steel $=29,000 \mathrm{ksi}(200000 \mathrm{MPa})$

$F_{y}=$ specified minimum yield stress, ksi (MPa)

$P_{r}=$ required axial strength using LRFD or ASD load combinations, kips (N)

$R_{y}=$ ratio of the expected yield stress to the specified minimum yield stress

$\alpha_{\mathrm{s}}=\mathrm{LRFD}-\mathrm{ASD}$ force level adjustment factor

= 1.0 for LRFD and 1.5 for ASD

(a) Both flanges of beams shall be laterally braced or the beam cross section shall be braced with point torsional bracing.
(b) Beam bracing shall meet the requirements of Appendix 6 of the Specification for lateral or torsional bracing of beams, where $C_{d}$ is 1.0 and the required flexural strength of the member shall be

$M_{r}=R_{y}F_{y}Z/\alpha _{s}$ (D1-1)

where

$R_y$ = ratio of the expected yield stress to the specified minimum yield stress $Z$ = plastic section modulus about the axis of bending, $\text{in.}^3$ ( $\text{mm}^3$ ) $\alpha_s$ = LRFD-ASD force level adjustment factor = 1.0 for LRFD and 1.5 for ASD

$r_{y}=$ radius of gyration about y-axis, in. (mm)

D1.2a.2 Encased Composite Beams

The bracing of moderately ductile encased composite beams shall satisfy the following requirements:

(a) Both flanges of members shall be laterally braced or the beam cross section shall be braced with point torsional bracing.
(b) Lateral bracing shall meet the requirements of Appendix 6 of the Specification for lateral or torsional bracing of beams, where $M_{r}=M_{p b e}$ of the beam, where $M_{p b e}$ is defined in Section G2.6d, and $C_{d}=1.0$ .
(c) Member bracing shall have a maximum spacing of

L_{b}=0.17 r_{y} E /\left(R_{y} F_{y}\right)

(D1-3)

using the material properties of the steel section and $r_{y}$ in the plane of buckling calculated based on the elastic transformed section.

D1.2b Highly Ductile Members

In addition to the requirements of Sections D1.2a.1(a) and (b) and D1.2a.2(a) and (b), the bracing of highly ductile beam members shall have a maximum spacing of $L_{b}=$ $0.086 r_{y} E /\left(R_{y} F_{y}\right)$ . For encased composite beams, the material properties of the steel section shall be used and the calculation for $r_{y}$ in the plane of buckling shall be based on the elastic transformed section.

D1.2c Special Bracing at Plastic Hinge Locations

Special bracing shall be located adjacent to expected plastic hinge locations where required by Chapters E, F, G, or H.

D1.2c.1 Steel Beams

For structural steel beams, such bracing shall satisfy the following requirements:

(a) Both flanges of beams shall be laterally braced, or the member cross section shall be braced with point torsional bracing.
(b) The required strength of lateral bracing of each flange provided adjacent to plastic hinges shall be

P_{r}=0.06 R_{y} F_{y} Z /\left(\alpha_{s} h_{o}\right)

(D1-4)

where

$h_{o}=$ distance between flange centroids, in. (mm)

The required strength of torsional bracing provided adjacent to plastic hinges shall be

M_{r}=0.06 R_{y} F_{y} Z / \alpha_{s}

(D1-5)

(c) The required bracing stiffness shall satisfy the requirements of Appendix 6 of the Specification for lateral or torsional bracing of beams with $C_{d}=1.0$ and where the required flexural strength of the beam shall be taken as $M_{r}=R_{y}F_{y}Z/\alpha_{s}$ (D1-6)

D1.2c.2 Encased Composite Beams

For encased composite beams, such bracing shall satisfy the following requirements:

(a) Both flanges of beams shall be laterally braced, or the beam cross section shall be braced with point torsional bracing.
(b) The required strength of lateral bracing provided adjacent to plastic hinges shall be

P_{u}=0.06 M_{p b e} / h_{o}

(D1-7)

of the beam, where

$M_{p b e}=$ expected flexural strength of the steel, encased, or composite beam, kip-in. (N-mm), determined in accordance with Section G2.6d.

The required strength for torsional bracing provided adjacent to plastic hinges shall be $M_{u}=0.06 M_{p b e}$ of the beam.

(c) The required bracing stiffness shall satisfy the requirements of Appendix 6 of the Specification for lateral or torsional bracing of beams, where $M_{r}=$ $M_{u}=M_{p b e}$ of the beam is determined in accordance with Section G2.6d, and $C_{d}=1.0$ .

D1.3 Protected Zones

Discontinuities specified in Section I2.1 resulting from fabrication and erection procedures and from other attachments are prohibited in the region of a member or a connection element designated as a protected zone by these Provisions or ANSI/AISC 358.

Exception: Welded steel headed stud anchors and other connections are permitted in protected zones when designated in ANSI/AISC 358, or as otherwise determined with a connection prequalification in accordance with Section K1, or as determined in a program of qualification testing in accordance with Sections K2 and K3.

D1.4 Columns

Columns in moment frames, braced frames, and shear walls shall satisfy the require- ments of this section.

D1.4a Required Strength

The required strength of columns in the SFRS shall be determined from the greater effect of the following:

(a) The load effect resulting from the analysis requirements for the applicable system per Chapters E, F, G, and H.
(b) The compressive axial strength and tensile strength as determined using the overstrength seismic load. It is permitted to neglect applied moments in this determination unless the moment results from a load applied to the column between points of lateral support.

For columns that are common to intersecting frames, determination of the required axial strength, including the overstrength seismic load or the capacity-limited seismic load, as applicable, shall consider the potential for simultaneous inelasticity from all such frames. The direction of application of the load in each such frame shall be selected to produce the most severe load effect on the column.

Exceptions:

(a) It is permitted to limit the required axial strength for such columns based on a three-dimensional nonlinear analysis in which ground motion is simultaneously applied in two orthogonal directions, in accordance with Section C3.
(b) Columns common to intersecting frames that are part of Sections E1, F1, G1, H1, H4, or combinations thereof, need not be designed for these loads.

D1.4b Encased Composite Columns

Encased composite columns shall satisfy the requirements of Specification Chapter I, in addition to the requirements of this section. Additional requirements, as specified for moderately ductile members and highly ductile members in Sections D1.4b.1 and 2, shall apply as required by Chapters G and H.

D1.4b.1 Moderately Ductile Members

Encased composite columns used as moderately ductile members shall satisfy the following requirements.

(a) The maximum spacing of transverse reinforcement at the top and bottom shall be the least of the following:
(1) One-half the least dimension of the section
(2) 8 longitudinal-bar diameters
(3) 24 transverse-bar diameters
(4) 12 in. (300 mm)
(b) The maximum spacing of transverse reinforcement at the top and bottom shall be maintained over a vertical distance equal to the greatest of the following lengths, measured from each joint face and on both sides of any section where flexural yielding is expected to occur:
(1) One-sixth the vertical clear height of the column
(2) Maximum cross-sectional dimension
(3) 18 in. (450 mm)
(c) Spacing of transverse reinforcement over the remaining column length shall not exceed twice the spacing defined in Section D1.4b.1(a).
(d) Splices and end bearing details for encased composite columns in composite ordinary SFRS of Sections G1, H1, and H4 shall satisfy the requirements of the Specification and ACI 318, Section 10.7.5.3. The design shall comply with ACI 318, Sections 18.2.7 and 18.2.8. The design shall consider any adverse behavioral effects due to abrupt changes in either the member stiffness or the nominal tensile strength. Transitions to reinforced concrete sections without embedded structural steel members, transitions to bare structural steel sections, and column bases shall be considered abrupt changes.
(e) Welded wire fabric shall be prohibited as transverse reinforcement.

D1.4b.2 Highly Ductile Members

Encased composite columns used as highly ductile members shall satisfy Section D1.4b.1 in addition to the following requirements:

(a) Longitudinal load-carrying reinforcement shall satisfy the requirements of ACI 318, Section 18.7.4.
(b) Transverse reinforcement shall be hoop reinforcement as defined in ACI 318, Chapter 18, and shall satisfy the following requirements:
(1) The minimum area of hoop reinforcement, $A_{s h}$ , shall be

A_{s h}=0.09 h_{c c s}\left(1-\frac{F_{y} A_{s}}{P_{n}}\right)\left(\frac{f_{c}^{\prime}}{F_{y s r}}\right)

(D1-8)

where

$A_{s}=$ cross-sectional area of the structural steel core, in. ${ }^{2}\left(\mathrm{~mm}^{2}\right)$
$\begin{aligned} F_{y} & =\text { specified minimum yield stress of the structural steel core, } \\ & \text { ksi (MPa) }\end{aligned}$
$F_{y s r}=$ specified minimum yield stress of the transverse reinforcement, ksi (MPa)
$\begin{aligned} P_{n} & =\text { nominal axial compressive strength of the composite column } \\ & \text { calculated in accordance with the Specification, kips (N) }\end{aligned}$
$h_{c c}=$ cross-sectional dimension of the confined core measured center-to-center of the transverse reinforcement, in. (mm)
$f_{c}^{\prime} \quad=$ specified compressive strength of concrete, ksi (MPa)
$\begin{aligned} s & =\text { spacing of transverse reinforcement measured along the } \\ & \text { longitudinal axis of the structural member, in. (mm) }\end{aligned}$

Equation D1-8 need not be satisfied if the nominal strength of the encased structural steel section alone is greater than the load effect from a load combination of $1.0 D+0.5 L$ ,

where

$D=$ dead load due to the weight of the structural elements and permanent features on the building, kips (N)
$L=$ live load due to occupancy and moveable equipment, kips (N)
(2) The maximum spacing of transverse reinforcement along the length of the column shall be the lesser of six longitudinal load-carrying bar diameters or 6 in. (150 mm).
(3) Where transverse reinforcement is specified in Sections D1.4b.1(c), D1.4b.1(d), or D1.4b.1(e), the maximum spacing of transverse reinforcement along the member length shall be the lesser of one-fourth the least member dimension or 4 in. (100 mm). Confining reinforcement shall be spaced not more than 14 in. (350 mm) on center in the transverse direction.
(c) Encased composite columns in braced frames with required compressive strengths greater than $0.2 P_{n}$ , not including the overstrength seismic load, shall have transverse reinforcement as specified in Section D1.4b.2(b)(3) over the total element length. This requirement need not be satisfied if the nominal strength of the encased structural steel section alone is greater than the load effect from a load combination of $1.0 D+0.5 L$ .
(d) Composite columns supporting reactions from discontinued stiff members, such as walls or braced frames, shall have transverse reinforcement as specified in Section D1.4b.2(b)(3) over the full length beneath the level at which the discontinuity occurs if the required compressive strength exceeds 0.1Pn, not including the overstrength seismic load. Transverse reinforcement shall extend into the discontinued member for a minimum length required to fully develop the encased structural steel section and longitudinal reinforcement. This requirement need not be satisfied if the nominal strength of the encased steel section alone is greater than the load effect from a load combination of 1.0D + 0.5L.
(e) Encased composite columns used in a composite special moment frame (C-SMF) shall satisfy the following requirements:
(1) Transverse reinforcement shall satisfy the requirements in Section D1.4b.2(b)(2) at the top and bottom of the column over the region specified in Section D1.4b.1(b).
(2) The strong-column/weak-beam design requirements in Section G3.4a shall be satisfied. Column bases shall be detailed to sustain inelastic flexural hinging.

(3) The required shear strength of the column shall satisfy the requirements of ACI 318, Section 18.7.6.1.1.

(f) When the column terminates on a footing or mat foundation, the transverse reinforcement as specified in this section shall extend into the footing or mat at least 12 in. (300 mm). When the column terminates on a wall, the transverse reinforcement shall extend into the wall for at least the length required to develop full yielding in the encased shape and longitudinal reinforcement.

D1.4c Filled Composite Columns

This section applies to columns that meet the limitations of Specification Section I2.2. Filled composite columns shall be designed to satisfy the requirements of Specification Chapter I.

D1.5 Composite Slab Diaphragms

The design of composite floor and roof slab diaphragms for seismic effects shall meet the following requirements.

D1.5a Load Transfer

Details shall be provided to transfer loads between the diaphragm and boundary members, collector elements, and elements of the horizontal framing system.

D1.5b Nominal Shear Strength

The nominal in-plane shear strength of composite slab diaphragms shall be taken as the nominal shear strength of the reinforced concrete above the top of the steel deck ribs in accordance with ACI 318, excluding Chapter 14. Alternatively, the composite diaphragm nominal shear strength is permitted to be calculated according to AISI S310 or determined by in-plane shear tests of concrete-filled diaphragms.

D1.6 Built-Up Structural Steel Members

This section addresses connections between components of built-up members where specific requirements are not provided in the system chapters of these Provisions or in ANSI/AISC 358.

Connections between components of built-up members subject to inelastic behavior shall be designed for the expected forces arising from that inelastic behavior.

Connections between components of built-up members where inelastic behavior is not expected shall be designed for the load effect including the overstrength seismic load.

Where connections between elements of a built-up member are required in a protected zone, the connections shall have an available tensile strength equal to $R_{y} F_{y} t_{p} / \alpha_{s}$ of the weaker element for the length of the protected zone where $t_{p}$ is the thickness of the element being considered.

Built-up members may be used in connections requiring testing in accordance with the Provisions provided they are accepted by ANSI/AISC 358 for use in a prequalified joint or have been verified in a qualification test.

D1Member requirements

AISC 341-22, Seismic Provisions for Structural Steel Buildings

Last Updated 29 Oct 2026

Hover any section to copy its link or text, ask Copilot about it, or bookmark it.

Members of moment frames, braced frames, and shear walls in the seismic force-resisting system (SFRS) shall comply with the Specification and this section.

D1.1 Classification of Sections for Ductility

When required for the systems defined in Chapters E, F, G, H, and Section D4, members designated as moderately ductile members or highly ductile members shall comply with this section.

D1.1a Section Requirements for Ductile Members

Structural steel sections for both moderately ductile members and highly ductile members shall have flanges continuously connected to the web or webs.

Encased composite columns shall comply with the requirements of Section D1.4b.1 for moderately ductile members and Section D1.4b.2 for highly ductile members.

Filled composite columns shall comply with the requirements of Section D1.4c for both moderately and highly ductile members.

Concrete sections shall comply with the requirements of ACI 318, Section 18.4, for moderately ductile members and ACI 318, Sections 18.6, 18.7, and 18.8, for highly ductile members.

D1.1b Width-to-Thickness Limitations of Steel and Composite Sections

TABLE D1.1aWidth-to-Thickness Ratios: Compression Elements—Diagonal Braces

	Case	Description of Element	Width-to- Thickness Ratio	Limiting Width-to- Thickness Ratio		Example
	Case	Description of Element	Width-to- Thickness Ratio	λ_hd Highly Ductile Members	λ_md Moderately Ductile Members	Example
Unstiffened Elements	1	(1) Flanges of rolled or built-up I-shaped sections	b/t d/t	0.30 √ E / (R_yF_y)	0.38 √ E / (R_yF_y)
		(2) Flange and stem of rolled or built-up tees
		(3) Flanges of rolled or built-up channels
		(4) Legs of single angles or double-angle members with separators
		(5) Outstanding legs of pairs of angles in continuous contact
Stiffened Elements	2	(1) Walls of rectangu-lar HSS^[a]	b/t h/t	0.65 √ E / (R_yF_y)	0.76 √ E / (R_yF_y)
		(2) Flanges and side plates of boxed I-shaped sections
		(3) Walls of box sections

For members designated as highly ductile, the width-to-thickness ratios of compression elements shall not exceed the limiting width-to-thickness ratios, $\lambda_{h d}$ , from Tables D1.1a and D1.1b.

D1.2 Stability Bracing of Beams

TABLE D1.1a (continued)Width-to-Thickness Ratios: Compression Elements—Diagonal Braces

	Case	Description of Element	Width-to- Thickness Ratio	Limiting Width-to- Thickness Ratio		Example
	Case	Description of Element	Width-to- Thickness Ratio	λ_hd Highly Ductile Members	λ_md Moderately Ductile Members	Example
Stiffened Elements	3	Walls of round HSS [a]	D/t	0.053 E / (R_yF_y)	0.062 E / (R_yF_y)
	4	Webs of rolled or built-up I-shaped sections and channels	h/t_w	1.49 √(E / (R_yF_y))	1.49 √(E / (R_yF_y))
	5	Walls of filled rectan-gular HSS and box sections [a]	b/t h/t	1.4 √(E / (R_yF_y))	2.26 √(E / (R_yF_y))
	6	Walls of filled round HSS sections [a]	D/t	0.076 E / (R_yF_y)	0.15 E / (R_yF_y)

[a] The design wall thickness shall be used in the calculations involving the wall thickness of hollow structural sections (HSS), as defined in Specification Section B4.2.

D1.2a Moderately Ductile Members

D1.2a.1 Steel Beams

The bracing of moderately ductile steel beams shall satisfy the following requirements:

TABLE D1.1bWidth-to-Thickness Ratios: Compression Elements—All Members Except Diagonal Braces

	Case	Description of Element	Width-to- Thickness Ratio	Limiting Width-to-Thickness Ratio		Example
	Case	Description of Element	Width-to- Thickness Ratio	λ_hd Highly Ductile Members	λ_md Moderately Ductile Members	Example
Unstiffened Elements	7	(1) Flanges of rolled or built-up I-shaped sections (2) Flange and stem of rolled or built-up tees (3) Flanges of rolled or built-up channels (4) Legs of sin-gle angles or double-angle members with separators (5) Outstanding legs of pairs of angles in continuous contact	b/t	0.30 √E/(R_yF_y)	0.38 √E/(R_yF_y)	b b t t b b t t HPL ttitt t d t d b b t t b t b PA t I► b t
	7		d/t
	8	Horizontal legs of double-angle members with separators or in continuous contact	b/t	0.47 √E/(R_yF_y)	0.54 √E/(R_yF_y)	b +++
	9	Flanges of H-pile sections per Section D4	b/t	not applicable	0.45 √E/(R_yF_y)	I

TABLE D1.1b (continued)Width-to-Thickness Ratios: Compression Elements—All Members Except Diagonal Braces

	Case	Description of Element	Width-to- Thickness Ratio	Limiting Width-to-Thickness Ratio		Example
	Case	Description of Element	Width-to- Thickness Ratio	λ_hd Highly Ductile Members	λ_md Moderately Ductile Members	Example
Stiffened Elements	10	Webs of H-pile sections	h/t_w	not applicable	1.50 √E / R_yF_y
	11	For moment frames, where used in beams or columns, as webs in flexure, or combined axial and flexure Webs of rolled or built-up I-shaped sections and channels	h/t_w	2.5 (1 − C_a)^2.3 √E / R_yF_y^[b]	5.4 (1 − C_a)^2.3 √E / R_yF_y^[b]
	12	Where used in beams or col- umns as flanges in uniform compression due to flexure or combined axial and flexure (1) Flanges of rectangular HSS^[a] (2) Flanges of boxed I-shaped sections (3) Flanges of box sections	b/t	0.55 √E / R_yF_y	1.00 √E / R_yF_y

TABLE D1.1b (continued)Width-to-Thickness Ratios: Compression Elements—All Members Except Diagonal Braces

	Case	Description of Element	Width-to- Thickness Ratio	Limiting Width-to-Thickness Ratio		Example
	Case	Description of Element	Width-to- Thickness Ratio	λ_hd Highly Ductile Members	λ_md Moderately Ductile Members	Example
Stiffened Elements	13	Where used in beams, columns, or links, as webs in flexure, or combined axial and flexure (1) Side plates of boxed I-shaped sections (2) Webs of rectangular HSS^[a] (3) Webs of box sections (4) Except for moment frames, webs of rolled or built-up I-shaped sections and channels	h/t	For C_a ≤ 0.113^[b] 2.45(1 − 1.04C_a) √E/(R_yF_y) For C_a > 0.113 2.26(1 − 0.38C_a) √E/(R_yF_y) ≥ 1.56 √E/(R_yF_y)	For C_a ≤ 0.113^[b] 3.76(1 − 3.05C_a) √E/(R_yF_y) For C_a > 0.113 2.61(1 − 0.49C_a) √E/(R_yF_y) ≥ 1.56 √E/(R_yF_y)	Multiple diagrams: Boxed I-sections, Rectangular HSS, Box sections, I-sections, and Channels
	14	Flanges of box sections used as link beams	b/t	0.55 √E/(R_yF_y)	0.64 √E/(R_yF_y)	Hollow square section with width 'b' and thickness 't'
	15	Webs of box sections used as EBF links	h/t	0.64 √E/(R_yF_y)	1.67 √E/(R_yF_y)	Hollow square section with height 'h' and thickness 't'
	16	Walls of round HSS^[a]	D/t	0.038 √E/(R_yF_y)	0.07 √E/(R_yF_y)	Hollow round section with diameter 'D' and thickness 't'

TABLE D1.1b (continued)Width-to-Thickness Ratios: Compression Elements—All Members Except Diagonal Braces

	Case	Description of Element	Width-to- Thickness Ratio	Limiting Width-to-Thickness Ratio		Example
	Case	Description of Element	Width-to- Thickness Ratio	λ_hd Highly Ductile Members	λ_md Moderately Ductile Members	Example
Composite Elements	17	Flanges and webs of filled rectangular HSS and box sections^[a]	b/t h/t	1.4 √(E⁄R_yF_y)	2.26 √(E⁄R_yF_y)
Composite Elements	18	Walls of filled round HSS sections^[a]	D/t	0.076 E⁄R_yF_y	0.15 E⁄R_yF_y

[a] The design wall thickness shall be used in the calculations involving the wall thickness of hollow structural sections (HSS), as defined in Specification Section B4.2.

$\alpha_{3}$

$P_{r}$

(b) $C_{a}=$

RyFyAg

where

$A_{g}=\text { gross area, in. }^{2}\left(\mathrm{~mm}^{2}\right)$

$E=\operatorname{modulus}$ of elasticity of steel $=29,000 \mathrm{ksi}(200000 \mathrm{MPa})$

$F_{y}=$ specified minimum yield stress, ksi (MPa)

$P_{r}=$ required axial strength using LRFD or ASD load combinations, kips (N)

$R_{y}=$ ratio of the expected yield stress to the specified minimum yield stress

$\alpha_{\mathrm{s}}=\mathrm{LRFD}-\mathrm{ASD}$ force level adjustment factor

= 1.0 for LRFD and 1.5 for ASD

(a) Both flanges of beams shall be laterally braced or the beam cross section shall be braced with point torsional bracing.
(b) Beam bracing shall meet the requirements of Appendix 6 of the Specification for lateral or torsional bracing of beams, where $C_{d}$ is 1.0 and the required flexural strength of the member shall be

$M_{r}=R_{y}F_{y}Z/\alpha _{s}$ (D1-1)

where

$r_{y}=$ radius of gyration about y-axis, in. (mm)

D1.2a.2 Encased Composite Beams

The bracing of moderately ductile encased composite beams shall satisfy the following requirements:

(a) Both flanges of members shall be laterally braced or the beam cross section shall be braced with point torsional bracing.
(b) Lateral bracing shall meet the requirements of Appendix 6 of the Specification for lateral or torsional bracing of beams, where $M_{r}=M_{p b e}$ of the beam, where $M_{p b e}$ is defined in Section G2.6d, and $C_{d}=1.0$ .
(c) Member bracing shall have a maximum spacing of

L_{b}=0.17 r_{y} E /\left(R_{y} F_{y}\right)

(D1-3)

using the material properties of the steel section and $r_{y}$ in the plane of buckling calculated based on the elastic transformed section.

D1.2b Highly Ductile Members

D1.2c Special Bracing at Plastic Hinge Locations

Special bracing shall be located adjacent to expected plastic hinge locations where required by Chapters E, F, G, or H.

D1.2c.1 Steel Beams

For structural steel beams, such bracing shall satisfy the following requirements:

(a) Both flanges of beams shall be laterally braced, or the member cross section shall be braced with point torsional bracing.
(b) The required strength of lateral bracing of each flange provided adjacent to plastic hinges shall be

P_{r}=0.06 R_{y} F_{y} Z /\left(\alpha_{s} h_{o}\right)

(D1-4)

where

$h_{o}=$ distance between flange centroids, in. (mm)

The required strength of torsional bracing provided adjacent to plastic hinges shall be

M_{r}=0.06 R_{y} F_{y} Z / \alpha_{s}

(D1-5)

D1.2c.2 Encased Composite Beams

For encased composite beams, such bracing shall satisfy the following requirements:

(a) Both flanges of beams shall be laterally braced, or the beam cross section shall be braced with point torsional bracing.
(b) The required strength of lateral bracing provided adjacent to plastic hinges shall be

P_{u}=0.06 M_{p b e} / h_{o}

(D1-7)

of the beam, where

$M_{p b e}=$ expected flexural strength of the steel, encased, or composite beam, kip-in. (N-mm), determined in accordance with Section G2.6d.

The required strength for torsional bracing provided adjacent to plastic hinges shall be $M_{u}=0.06 M_{p b e}$ of the beam.

(c) The required bracing stiffness shall satisfy the requirements of Appendix 6 of the Specification for lateral or torsional bracing of beams, where $M_{r}=$ $M_{u}=M_{p b e}$ of the beam is determined in accordance with Section G2.6d, and $C_{d}=1.0$ .

D1.3 Protected Zones

D1.4 Columns

Columns in moment frames, braced frames, and shear walls shall satisfy the require- ments of this section.

D1.4a Required Strength

The required strength of columns in the SFRS shall be determined from the greater effect of the following:

(a) The load effect resulting from the analysis requirements for the applicable system per Chapters E, F, G, and H.
(b) The compressive axial strength and tensile strength as determined using the overstrength seismic load. It is permitted to neglect applied moments in this determination unless the moment results from a load applied to the column between points of lateral support.

Exceptions:

(a) It is permitted to limit the required axial strength for such columns based on a three-dimensional nonlinear analysis in which ground motion is simultaneously applied in two orthogonal directions, in accordance with Section C3.
(b) Columns common to intersecting frames that are part of Sections E1, F1, G1, H1, H4, or combinations thereof, need not be designed for these loads.

D1.4b Encased Composite Columns

D1.4b.1 Moderately Ductile Members

Encased composite columns used as moderately ductile members shall satisfy the following requirements.

(a) The maximum spacing of transverse reinforcement at the top and bottom shall be the least of the following:
(1) One-half the least dimension of the section
(2) 8 longitudinal-bar diameters
(3) 24 transverse-bar diameters
(4) 12 in. (300 mm)
(b) The maximum spacing of transverse reinforcement at the top and bottom shall be maintained over a vertical distance equal to the greatest of the following lengths, measured from each joint face and on both sides of any section where flexural yielding is expected to occur:
(1) One-sixth the vertical clear height of the column
(2) Maximum cross-sectional dimension
(3) 18 in. (450 mm)
(c) Spacing of transverse reinforcement over the remaining column length shall not exceed twice the spacing defined in Section D1.4b.1(a).
(d) Splices and end bearing details for encased composite columns in composite ordinary SFRS of Sections G1, H1, and H4 shall satisfy the requirements of the Specification and ACI 318, Section 10.7.5.3. The design shall comply with ACI 318, Sections 18.2.7 and 18.2.8. The design shall consider any adverse behavioral effects due to abrupt changes in either the member stiffness or the nominal tensile strength. Transitions to reinforced concrete sections without embedded structural steel members, transitions to bare structural steel sections, and column bases shall be considered abrupt changes.
(e) Welded wire fabric shall be prohibited as transverse reinforcement.

D1.4b.2 Highly Ductile Members

Encased composite columns used as highly ductile members shall satisfy Section D1.4b.1 in addition to the following requirements:

(a) Longitudinal load-carrying reinforcement shall satisfy the requirements of ACI 318, Section 18.7.4.
(b) Transverse reinforcement shall be hoop reinforcement as defined in ACI 318, Chapter 18, and shall satisfy the following requirements:
(1) The minimum area of hoop reinforcement, $A_{s h}$ , shall be

A_{s h}=0.09 h_{c c s}\left(1-\frac{F_{y} A_{s}}{P_{n}}\right)\left(\frac{f_{c}^{\prime}}{F_{y s r}}\right)

(D1-8)

where

$A_{s}=$ cross-sectional area of the structural steel core, in. ${ }^{2}\left(\mathrm{~mm}^{2}\right)$
$\begin{aligned} F_{y} & =\text { specified minimum yield stress of the structural steel core, } \\ & \text { ksi (MPa) }\end{aligned}$
$F_{y s r}=$ specified minimum yield stress of the transverse reinforcement, ksi (MPa)
$\begin{aligned} P_{n} & =\text { nominal axial compressive strength of the composite column } \\ & \text { calculated in accordance with the Specification, kips (N) }\end{aligned}$
$h_{c c}=$ cross-sectional dimension of the confined core measured center-to-center of the transverse reinforcement, in. (mm)
$f_{c}^{\prime} \quad=$ specified compressive strength of concrete, ksi (MPa)
$\begin{aligned} s & =\text { spacing of transverse reinforcement measured along the } \\ & \text { longitudinal axis of the structural member, in. (mm) }\end{aligned}$

Equation D1-8 need not be satisfied if the nominal strength of the encased structural steel section alone is greater than the load effect from a load combination of $1.0 D+0.5 L$ ,

where

$D=$ dead load due to the weight of the structural elements and permanent features on the building, kips (N)
$L=$ live load due to occupancy and moveable equipment, kips (N)
(2) The maximum spacing of transverse reinforcement along the length of the column shall be the lesser of six longitudinal load-carrying bar diameters or 6 in. (150 mm).
(3) Where transverse reinforcement is specified in Sections D1.4b.1(c), D1.4b.1(d), or D1.4b.1(e), the maximum spacing of transverse reinforcement along the member length shall be the lesser of one-fourth the least member dimension or 4 in. (100 mm). Confining reinforcement shall be spaced not more than 14 in. (350 mm) on center in the transverse direction.
(c) Encased composite columns in braced frames with required compressive strengths greater than $0.2 P_{n}$ , not including the overstrength seismic load, shall have transverse reinforcement as specified in Section D1.4b.2(b)(3) over the total element length. This requirement need not be satisfied if the nominal strength of the encased structural steel section alone is greater than the load effect from a load combination of $1.0 D+0.5 L$ .
(d) Composite columns supporting reactions from discontinued stiff members, such as walls or braced frames, shall have transverse reinforcement as specified in Section D1.4b.2(b)(3) over the full length beneath the level at which the discontinuity occurs if the required compressive strength exceeds 0.1Pn, not including the overstrength seismic load. Transverse reinforcement shall extend into the discontinued member for a minimum length required to fully develop the encased structural steel section and longitudinal reinforcement. This requirement need not be satisfied if the nominal strength of the encased steel section alone is greater than the load effect from a load combination of 1.0D + 0.5L.
(e) Encased composite columns used in a composite special moment frame (C-SMF) shall satisfy the following requirements:
(1) Transverse reinforcement shall satisfy the requirements in Section D1.4b.2(b)(2) at the top and bottom of the column over the region specified in Section D1.4b.1(b).
(2) The strong-column/weak-beam design requirements in Section G3.4a shall be satisfied. Column bases shall be detailed to sustain inelastic flexural hinging.

(3) The required shear strength of the column shall satisfy the requirements of ACI 318, Section 18.7.6.1.1.

(f) When the column terminates on a footing or mat foundation, the transverse reinforcement as specified in this section shall extend into the footing or mat at least 12 in. (300 mm). When the column terminates on a wall, the transverse reinforcement shall extend into the wall for at least the length required to develop full yielding in the encased shape and longitudinal reinforcement.

D1.4c Filled Composite Columns

This section applies to columns that meet the limitations of Specification Section I2.2. Filled composite columns shall be designed to satisfy the requirements of Specification Chapter I.

D1.5 Composite Slab Diaphragms

The design of composite floor and roof slab diaphragms for seismic effects shall meet the following requirements.

D1.5a Load Transfer

Details shall be provided to transfer loads between the diaphragm and boundary members, collector elements, and elements of the horizontal framing system.

D1.5b Nominal Shear Strength

D1.6 Built-Up Structural Steel Members

This section addresses connections between components of built-up members where specific requirements are not provided in the system chapters of these Provisions or in ANSI/AISC 358.

Connections between components of built-up members subject to inelastic behavior shall be designed for the expected forces arising from that inelastic behavior.

Connections between components of built-up members where inelastic behavior is not expected shall be designed for the load effect including the overstrength seismic load.