F3Eccentrically braced frames (EBF)

AISC 341-22, Seismic Provisions for Structural Steel Buildings

Last Updated 29 Oct 2026

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F3.1 Scope

Eccentrically braced frames (EBF) of structural steel shall be designed in conformance with this section.

F3.2 Basis of Design

This section is applicable to braced frames for which one end of each brace intersects a beam at an eccentricity from the intersection of the centerlines of the beam and an adjacent brace or column, forming a link that is subject to shear and flexure. Eccentricities less than the beam depth are permitted in the brace connection away from the link if the resulting member and connection forces are addressed in the design and do not change the expected source of inelastic deformation capacity.

EBF designed in accordance with these provisions are expected to provide significant inelastic deformation capacity primarily through shear or flexural yielding in the links.

Where links connect directly to columns, design of their connections to columns shall provide the performance required by Section F3.6e.1 and demonstrate this conformance as required by Section F3.6e.2.

F3.3 Analysis

The required strength of diagonal braces and their connections, beams outside links, and columns shall be determined using the capacity-limited seismic load effect. The capacity-limited horizontal seismic load effect, $E_{c l}$ , shall be taken as the forces developed in the member assuming the forces at the ends of the links correspond to the adjusted link shear strength. The adjusted link shear strength shall be taken as $R_{y}$ times the nominal shear strength of the link, $V_{n}$ , given in Section F3.5b.2, multiplied by 1.25 for I-shaped links and 1.4 for box links,

where

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

Exceptions:

(a) The capacity-limited horizontal seismic load effect, $E_{c l}$ , is permitted to be taken as 0.88 times the forces determined in this section for the design of the portions of beams outside links.
(b) It is permitted to neglect flexural forces resulting from seismic drift in this determination. Moment resulting from a load applied to the column between points of lateral support must be considered.
(c) The required strength of columns need not exceed the lesser of the following:
(1) The forces corresponding to the resistance of the foundation to overturning uplift
(2) Forces as determined from nonlinear analysis as defined in Section C3

Analyses shall be performed for each direction of frame loading. For systems that include columns that form part of two intersecting frames in orthogonal or multi-axial directions, the analysis shall consider the potential for link yielding in both directions simultaneously.

The inelastic link rotation angle shall be determined from the inelastic portion of the design earthquake displacement. Alternatively, the inelastic link rotation angle is permitted to be determined from nonlinear analysis as defined in Section C3.

User Note: The seismic load effect, E, used in the design of EBF members, such as the required axial strength used in the equations in Section F3.5, should be calculated from the analysis in this section.

F3.4 System Requirements

F3.4a Link Rotation Angle

The link rotation angle is the inelastic angle between the link and the beam outside of the link at the design earthquake displacement, $\delta_{D E}$ . The link rotation angle shall not exceed the following values.

(a) For links of length $1.6 M_{p} / V_{p}$ or less: 0.08 rad

(b) For links of length $2.6 M_{p} / V_{p}$ or greater: 0.02 rad

where

$M_{p}=$ plastic moment of a link, kip-in. (N-mm)

$V_{p}=$ plastic shear strength of a link, kips (N)

Linear interpolation between the link rotation angle values given in (a) and (b) shall be used for links of length between $1.6 M_{p} / V_{p}$ and $2.6 M_{p} / V_{p}$ .

F3.4b Bracing of Link

Bracing shall be provided at both the top and bottom link flanges at the ends of the link for I-shaped sections. Bracing shall have an available strength and stiffness as required by Section D1.2c for expected plastic hinge locations.

F3.5 Members

F3.5a Basic Requirements

Brace members shall satisfy width-to-thickness limitations in Section D1.1 for moderately ductile members.

Column members shall satisfy width-to-thickness limitations in Section D1.1 for highly ductile members.

Where the beam outside of the link is a different section from the link, the beam shall satisfy the width-to-thickness limitations in Section D1.1 for moderately ductile members.

User Note: The diagonal brace and beam segment outside of the link are intended to remain essentially elastic under the forces generated by the fully yielded and strain-hardened link. Both the diagonal brace and beam segment outside of the link are typically subject to a combination of large axial force and bending moment, and therefore should be treated as beam-columns in design, where the available strength is defined by Chapter H of the Specification.

Where the beam outside the link is the same member as the link, its strength may be determined using expected material properties as permitted by Section A3.2.

F3.5b Links

Links subjected to shear and flexure due to eccentricity between the intersections of brace centerlines and the beam centerline (or between the intersection of the brace and beam centerlines and the column centerline for links attached to columns) shall be provided. The link shall be considered to extend from brace connection to brace connection for center links and from brace connection to column face for link-to-column connections, except as permitted by Section F3.6e.

F3.5b.1 Limitations

Links shall be I-shaped cross sections (rolled wide-flange sections or built-up sections) or built-up box sections. Hollow structural sections (HSS) shall not be used as links.

Links shall satisfy the requirements of Section D1.1 for highly ductile members.

Exceptions: Flanges of links with I-shaped sections with $e \leq 1.6 M_{p} / V_{p}$ are permitted to satisfy the requirements for moderately ductile members, where $e$ is the length of link, defined as the clear distance between the ends of two diagonal braces or between the diagonal brace and the column face. Webs of links with box sections with link lengths, $e \leq 1.6 M_{p} / V_{p}$ , are permitted to satisfy the requirements for moderately ductile members.

The web or webs of a link shall be single thickness. Doubler-plate reinforcement and web penetrations are not permitted.

For links made of built-up cross sections, complete-joint-penetration (CJP) groove welds shall be used to connect the web (or webs) to the flanges.

Links of built-up box sections shall have a moment of inertia, $I_{y}$ , about an axis in the plane of the EBF limited to $I_{y}>0.67 I_{x}$ , where $I_{x}$ is the moment of inertia about an axis perpendicular to the plane of the EBF.

F3.5b.2 Shear Strength

The link design shear strength, $\phi_{v} V_{n}$ , and the allowable shear strength, $V_{n} / \Omega_{v}$ , shall be the lower value obtained in accordance with the limit states of shear yielding in the web and flexural yielding in the gross section. For both limit states,

$\phi_{v}=0.90$ (LRFD) $\quad \Omega_{v}=1.67$ (ASD)

(a) For shear yielding

V_{n}=V_{p}

where

\begin{aligned} V_{p} & =0.6 F_{y} A_{l w} \text { for } \alpha_{s} P_{r} / P_{y} \leq 0.15 \\ & =0.6 F_{y} A_{l w} \sqrt{1-\left(\alpha_{s} P_{r} / P_{y}\right)^{2}} \text { for } \alpha_{s} P_{r} / P_{y}>0.15 \end{aligned}

\begin{aligned} A_{l w} & =\text { web area of link (excluding flanges), in. }^{2}\left(\mathrm{~mm}^{2}\right) \\ & =\left(d-2 t_{f}\right) t_{w} \text { for I -shaped link sections } \\ & =2\left(d-2 t_{f}\right) t_{w} \text { for box link sections } \end{aligned}

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

$\begin{aligned} P_{r} & =\text { required axial strength using LRFD or ASD load combinations, } \\ & \text { kips (N) }\end{aligned}$

\begin{aligned} P_{y} & =\text { axial yield strength, kips (N) } \\ & =F_{y} A_{g} \\ d & =\text { overall depth of link, in. (mm) } \\ t_{f} & =\text { thickness of flange, in. (mm) } \\ t_{w} & =\text { thickness of web, in. (mm) } \end{aligned}

(b) For flexural yielding

V_{n}=2 M_{p} / e

(F3-7)

where

\begin{aligned} M_{p} & =F_{y} Z \text { for } \alpha_{s} P_{r} / P_{y} \leq 0.15 \\ & =F_{y} Z\left(\frac{1-\alpha_{s} P_{r} / P_{y}}{0.85}\right) \text { for } \alpha_{s} P_{r} / P_{y}>0.15 \end{aligned}

$\begin{aligned} Z & =\text { plastic section modulus about the axis of bending, in. }^{3}\left(\mathrm{~mm}^{3}\right) \\ e & =\text { length of link, defined as the clear distance between the ends } \\ & \text { of two diagonal braces or between the diagonal brace and the } \\ & \text { column face, in. (mm) }\end{aligned}$

F3.5b.3 Link Length

If $\alpha_{s} P_{r} / P_{y}>0.15$ , the length of the link shall be limited as follows:

When $\rho^{\prime} \leq 0.5$

e \leq \frac{1.6 M_{p}}{V_{p}}

(F3-10)

When $\rho^{\prime}>0.5$

e \leq \frac{1.6 M_{p}}{V_{p}}(1.15-0.3 \mathrm{p}^{\prime})

where

$\rho' = \frac{P_{r}/P_{y}}{V_{r}/V_{y}}$ (F3-12) $V_r =$ required shear strength using LRFD or ASD load combinations, kips (N) $V_{y} =$ shear yield strength, kips (N) $= 0.6F_{y}A_{lw}$ (F3-13)

User Note: For links with low axial force there is no upper limit on link length. The limitations on link rotation angle in Section F3.4a result in a practical lower limit on link length.

F3.5b.4 Link Stiffeners for I-Shaped Cross Sections

Full-depth web stiffeners shall be provided on both sides of the link web at the diagonal brace ends of the link. These stiffeners shall have a combined width not less than $\left(b_{f}-2 t_{w}\right)$ and a thickness not less than the larger of $0.75 t_{w}$ or $3 / 8$ in. (10 mm), where $b_{f}$ and $t_{w}$ are the link flange width and link web thickness, respectively.

Links shall be provided with intermediate web stiffeners as follows:

(a) Links of lengths $1.6 M_{p} / V_{p}$ or less shall be provided with intermediate web stiffeners spaced at intervals not exceeding $\left(30 t_{w}-d / 5\right)$ for a link rotation angle of 0.08 rad or $\left(52 t_{w}-d / 5\right)$ for link rotation angles of 0.02 rad or less. Linear interpolation shall be used for values between 0.08 and 0.02 rad.
(b) Links of length greater than or equal to $2.6 M_{p} / V_{p}$ and less than $5 M_{p} / V_{p}$ shall be provided with intermediate web stiffeners placed at a distance of 1.5 times $b_{f}$ from each end of the link.
(c) Links of length between $1.6 M_{p} / V_{p}$ and $2.6 M_{p} / V_{p}$ shall be provided with intermediate web stiffeners meeting the requirements of (a) and (b) in the preceding.

Intermediate web stiffeners shall not be required in links of length greater than $5 M_{p} / V_{p}$ .

Intermediate web stiffeners shall be full depth. For links that are less than 25 in. (630 mm) in depth, stiffeners shall be provided on only one side of the link web. The thickness of one-sided stiffeners shall not be less than $t_{w}$ or $3 / 8$ in. (10 mm), whichever is larger, and the width shall not be less than $\left(b_{f} / 2\right)-t_{w}$ . For links that are 25 in. (630 mm) in depth or greater, intermediate stiffeners with these dimensions shall be provided on both sides of the web.

The required strength of fillet welds connecting a link stiffener to the link web shall be $F_{y} A_{s t} / \alpha_{s}$ , where $A_{s t}$ is the horizontal cross-sectional area of the link stiffener, $F_{y}$ is the specified minimum yield stress of the stiffener, and $\alpha_{s}$ is the LRFD-ASD force level adjustment factor $=1.0$ for LRFD and 1.5 for ASD. The required strength of fillet welds connecting the stiffener to the link flanges is $F_{y} A_{s t} /\left(4 \alpha_{s}\right)$ .

F3.5b.5 Link Stiffeners for Box Sections

Full-depth web stiffeners shall be provided on one side of each link web at the diagonal brace connection. These stiffeners are permitted to be welded to the outside or inside face of the link webs. These stiffeners shall each have a width not less than $b / 2$ , where $b$ is the inside width of the box section. These stiffeners shall each have a thickness not less than the larger of $0.75 t_{w}$ or $\frac{1}{2}$ in. (13 mm).

Box links shall be provided with intermediate web stiffeners as follows:

(a) For links of length $1.6 M_{p} / V_{p}$ or less, and with web depth-to-thickness ratio, $h / t_{w}$ , greater than or equal to $0.67 \sqrt{\frac{E}{R_{y} F_{y}}}$ , full-depth web stiffeners shall be provided on one side of each link web, spaced at intervals not exceeding $20 t_{w}-\left(d-2 t_{f}\right) / 8$ .
(b) For links of length $1.6 M_{p} / V_{p}$ or less and with web depth-to-thickness ratio, $h / t_{w}$ , less than $0.67 \sqrt{\frac{E}{R_{y} F_{y}}}$ , no intermediate web stiffeners are required.
(c) For links of length greater than $1.6 M_{p} / V_{p}$ , no intermediate web stiffeners are required.

Intermediate web stiffeners shall be full depth and are permitted to be welded to the outside or inside face of the link webs.

The required strength of fillet welds connecting a link stiffener to the link web shall be $F_{y} A_{s t} / \alpha_{s}$ , where $A_{s t}$ is the horizontal cross-sectional area of the link stiffener.

User Note: Stiffeners of box links need not be welded to the link flanges.

F3.5c Protected Zones

Links in EBF are protected zones and shall meet the requirements of Section D1.3.

F3.6 Connections

F3.6a 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 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) Welds at beam-to-column connections conforming to Section F3.6b(c)
(d) Where links connect to columns, welds attaching the link flanges and the link web to the column
(e) In built-up beams, welds within the link connecting the webs to the flanges

F3.6b 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 assembly is a simple connection meeting the requirements of Specification Section B3.4a where the required rotation is taken to be 0.025 rad
(b) The connection assembly is designed to resist a moment equal to the lesser of the following:
(1) A moment corresponding to the expected beam flexural strength, $R_{y} M_{p}$ , multiplied by 1.1 and divided by $\alpha_{s}$ , where

$M_{p}=$ plastic moment, kip-in. (N-mm)

(2) A moment corresponding to the sum of the expected column flexural strengths, $\Sigma\left(R_{y} F_{y} Z\right)$ , multiplied by 1.1 and divided by $\alpha_{s}$ , where
$F_{y}=$ specified minimum yield stress, ksi (MPa)
$Z=$ plastic section modulus about the axis of bending, in. ${ }^{3}\left(\mathrm{~mm}^{3}\right)$

This moment 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.

F3.6c Brace Connections

When oversized holes are used, the required strength for the limit state of bolt slip need not exceed the seismic load effect determined using the overstrength seismic load.

Connections of braces designed to resist a portion of the link end moment shall be designed as fully restrained.

F3.6d Column Splices

Column splices shall comply with the requirements of Section D2.5. Where groove welds are used to make the splice, they shall be CJP groove welds. Column splices shall be designed to develop at least 50% of the lesser plastic moment of the connected members, $M_{p}$ , divided by $\alpha_{s}$ .

The required shear strength shall be $\Sigma M_{p} /\left(\alpha_{s} H_{c}\right)$ ,

where

$H_{c}$ = clear height of the column between beam connections, including a structural slab, if present, in. (mm)

$\Sigma M_{p}=$ sum of the plastic moments, $F_{y} Z$ , at the top and bottom ends of the column, kip-in. (N-mm)

F3.6e Link-to-Column Connections

F3.6e.1 Requirements

Link-to-column connections shall be fully restrained (FR) moment connections and shall meet the following requirements:

(a) The connection shall be capable of sustaining the link rotation angle specified in Section F3.4a.
(b) The shear resistance of the connection, measured at the required link rotation angle, shall be at least equal to the expected shear strength of the link, $R_{y} V_{n}$ , where $V_{n}$ is determined in accordance with Section F3.5b.2.
(c) The flexural resistance of the connection, measured at the required link rotation angle, shall be at least equal to the moment corresponding to the nominal shear strength of the link, $V_{n}$ , as determined in accordance with Section F3.5b.2.

F3.6e.2 Conformance Demonstration

Link-to-column connections shall meet the preceding requirements by one of the following:

(a) Use a connection prequalified for EBF in accordance with Section K1.

User Note: There are no prequalified link-to-column connections.

(b) Provide qualifying cyclic test results in accordance with Section K2. Results of at least two cyclic connection tests shall be provided and are permitted to be based on one of the following:
(1) Tests reported in research literature or documented tests performed for other projects that are representative of 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 material properties, within the limits specified in Section K2

Exception: Cyclic testing of the connection is not required if the following conditions are met.

(1) Reinforcement at the beam-to-column connection at the link end precludes yielding of the beam over the reinforced length.
(2) The available strength of the reinforced section and the connection equals or exceeds the required strength calculated based upon adjusted link shear strength as described in Section F3.3.
(3) The link length (taken as the beam segment from the end of the reinforcement to the brace connection) does not exceed $1.6 M_{p} / V_{p}$ .
(4) Full-depth stiffeners, as required in Section F3.5b.4, are placed at the link-to-reinforcement interface.

F3Eccentrically braced frames (EBF)

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.

F3.1 Scope

Eccentrically braced frames (EBF) of structural steel shall be designed in conformance with this section.

F3.2 Basis of Design

EBF designed in accordance with these provisions are expected to provide significant inelastic deformation capacity primarily through shear or flexural yielding in the links.

F3.3 Analysis

where

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

Exceptions:

(a) The capacity-limited horizontal seismic load effect, $E_{c l}$ , is permitted to be taken as 0.88 times the forces determined in this section for the design of the portions of beams outside links.
(b) It is permitted to neglect flexural forces resulting from seismic drift in this determination. Moment resulting from a load applied to the column between points of lateral support must be considered.
(c) The required strength of columns need not exceed the lesser of the following:
(1) The forces corresponding to the resistance of the foundation to overturning uplift
(2) Forces as determined from nonlinear analysis as defined in Section C3

F3.4 System Requirements

F3.4a Link Rotation Angle

(a) For links of length $1.6 M_{p} / V_{p}$ or less: 0.08 rad

(b) For links of length $2.6 M_{p} / V_{p}$ or greater: 0.02 rad

where

$M_{p}=$ plastic moment of a link, kip-in. (N-mm)

$V_{p}=$ plastic shear strength of a link, kips (N)

Linear interpolation between the link rotation angle values given in (a) and (b) shall be used for links of length between $1.6 M_{p} / V_{p}$ and $2.6 M_{p} / V_{p}$ .

F3.4b Bracing of Link

F3.5 Members

F3.5a Basic Requirements

Brace members shall satisfy width-to-thickness limitations in Section D1.1 for moderately ductile members.

Column members shall satisfy width-to-thickness limitations in Section D1.1 for highly ductile members.

Where the beam outside of the link is a different section from the link, the beam shall satisfy the width-to-thickness limitations in Section D1.1 for moderately ductile members.

Where the beam outside the link is the same member as the link, its strength may be determined using expected material properties as permitted by Section A3.2.

F3.5b Links

F3.5b.1 Limitations

Links shall be I-shaped cross sections (rolled wide-flange sections or built-up sections) or built-up box sections. Hollow structural sections (HSS) shall not be used as links.

Links shall satisfy the requirements of Section D1.1 for highly ductile members.

The web or webs of a link shall be single thickness. Doubler-plate reinforcement and web penetrations are not permitted.

For links made of built-up cross sections, complete-joint-penetration (CJP) groove welds shall be used to connect the web (or webs) to the flanges.

F3.5b.2 Shear Strength

$\phi_{v}=0.90$ (LRFD) $\quad \Omega_{v}=1.67$ (ASD)

(a) For shear yielding

V_{n}=V_{p}

where

\begin{aligned} V_{p} & =0.6 F_{y} A_{l w} \text { for } \alpha_{s} P_{r} / P_{y} \leq 0.15 \\ & =0.6 F_{y} A_{l w} \sqrt{1-\left(\alpha_{s} P_{r} / P_{y}\right)^{2}} \text { for } \alpha_{s} P_{r} / P_{y}>0.15 \end{aligned}

\begin{aligned} A_{l w} & =\text { web area of link (excluding flanges), in. }^{2}\left(\mathrm{~mm}^{2}\right) \\ & =\left(d-2 t_{f}\right) t_{w} \text { for I -shaped link sections } \\ & =2\left(d-2 t_{f}\right) t_{w} \text { for box link sections } \end{aligned}

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

$\begin{aligned} P_{r} & =\text { required axial strength using LRFD or ASD load combinations, } \\ & \text { kips (N) }\end{aligned}$

\begin{aligned} P_{y} & =\text { axial yield strength, kips (N) } \\ & =F_{y} A_{g} \\ d & =\text { overall depth of link, in. (mm) } \\ t_{f} & =\text { thickness of flange, in. (mm) } \\ t_{w} & =\text { thickness of web, in. (mm) } \end{aligned}

(b) For flexural yielding

V_{n}=2 M_{p} / e

(F3-7)

where

\begin{aligned} M_{p} & =F_{y} Z \text { for } \alpha_{s} P_{r} / P_{y} \leq 0.15 \\ & =F_{y} Z\left(\frac{1-\alpha_{s} P_{r} / P_{y}}{0.85}\right) \text { for } \alpha_{s} P_{r} / P_{y}>0.15 \end{aligned}

F3.5b.3 Link Length

If $\alpha_{s} P_{r} / P_{y}>0.15$ , the length of the link shall be limited as follows:

When $\rho^{\prime} \leq 0.5$

e \leq \frac{1.6 M_{p}}{V_{p}}

(F3-10)

When $\rho^{\prime}>0.5$

e \leq \frac{1.6 M_{p}}{V_{p}}(1.15-0.3 \mathrm{p}^{\prime})

where

$\rho' = \frac{P_{r}/P_{y}}{V_{r}/V_{y}}$ (F3-12) $V_r =$ required shear strength using LRFD or ASD load combinations, kips (N) $V_{y} =$ shear yield strength, kips (N) $= 0.6F_{y}A_{lw}$ (F3-13)

User Note: For links with low axial force there is no upper limit on link length. The limitations on link rotation angle in Section F3.4a result in a practical lower limit on link length.

F3.5b.4 Link Stiffeners for I-Shaped Cross Sections

Links shall be provided with intermediate web stiffeners as follows:

(a) Links of lengths $1.6 M_{p} / V_{p}$ or less shall be provided with intermediate web stiffeners spaced at intervals not exceeding $\left(30 t_{w}-d / 5\right)$ for a link rotation angle of 0.08 rad or $\left(52 t_{w}-d / 5\right)$ for link rotation angles of 0.02 rad or less. Linear interpolation shall be used for values between 0.08 and 0.02 rad.
(b) Links of length greater than or equal to $2.6 M_{p} / V_{p}$ and less than $5 M_{p} / V_{p}$ shall be provided with intermediate web stiffeners placed at a distance of 1.5 times $b_{f}$ from each end of the link.
(c) Links of length between $1.6 M_{p} / V_{p}$ and $2.6 M_{p} / V_{p}$ shall be provided with intermediate web stiffeners meeting the requirements of (a) and (b) in the preceding.

Intermediate web stiffeners shall not be required in links of length greater than $5 M_{p} / V_{p}$ .

F3.5b.5 Link Stiffeners for Box Sections

Box links shall be provided with intermediate web stiffeners as follows:

(a) For links of length $1.6 M_{p} / V_{p}$ or less, and with web depth-to-thickness ratio, $h / t_{w}$ , greater than or equal to $0.67 \sqrt{\frac{E}{R_{y} F_{y}}}$ , full-depth web stiffeners shall be provided on one side of each link web, spaced at intervals not exceeding $20 t_{w}-\left(d-2 t_{f}\right) / 8$ .
(b) For links of length $1.6 M_{p} / V_{p}$ or less and with web depth-to-thickness ratio, $h / t_{w}$ , less than $0.67 \sqrt{\frac{E}{R_{y} F_{y}}}$ , no intermediate web stiffeners are required.
(c) For links of length greater than $1.6 M_{p} / V_{p}$ , no intermediate web stiffeners are required.

Intermediate web stiffeners shall be full depth and are permitted to be welded to the outside or inside face of the link webs.

The required strength of fillet welds connecting a link stiffener to the link web shall be $F_{y} A_{s t} / \alpha_{s}$ , where $A_{s t}$ is the horizontal cross-sectional area of the link stiffener.

User Note: Stiffeners of box links need not be welded to the link flanges.

F3.5c Protected Zones

Links in EBF are protected zones and shall meet the requirements of Section D1.3.

F3.6 Connections

F3.6a 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 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) Welds at beam-to-column connections conforming to Section F3.6b(c)
(d) Where links connect to columns, welds attaching the link flanges and the link web to the column
(e) In built-up beams, welds within the link connecting the webs to the flanges

F3.6b 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 assembly is a simple connection meeting the requirements of Specification Section B3.4a where the required rotation is taken to be 0.025 rad
(b) The connection assembly is designed to resist a moment equal to the lesser of the following:
(1) A moment corresponding to the expected beam flexural strength, $R_{y} M_{p}$ , multiplied by 1.1 and divided by $\alpha_{s}$ , where

$M_{p}=$ plastic moment, kip-in. (N-mm)

(2) A moment corresponding to the sum of the expected column flexural strengths, $\Sigma\left(R_{y} F_{y} Z\right)$ , multiplied by 1.1 and divided by $\alpha_{s}$ , where
$F_{y}=$ specified minimum yield stress, ksi (MPa)
$Z=$ plastic section modulus about the axis of bending, in. ${ }^{3}\left(\mathrm{~mm}^{3}\right)$

F3.6c Brace Connections

When oversized holes are used, the required strength for the limit state of bolt slip need not exceed the seismic load effect determined using the overstrength seismic load.

Connections of braces designed to resist a portion of the link end moment shall be designed as fully restrained.

F3.6d Column Splices

The required shear strength shall be $\Sigma M_{p} /\left(\alpha_{s} H_{c}\right)$ ,

where

$H_{c}$ = clear height of the column between beam connections, including a structural slab, if present, in. (mm)

$\Sigma M_{p}=$ sum of the plastic moments, $F_{y} Z$ , at the top and bottom ends of the column, kip-in. (N-mm)

F3.6e Link-to-Column Connections

F3.6e.1 Requirements

Link-to-column connections shall be fully restrained (FR) moment connections and shall meet the following requirements:

(a) The connection shall be capable of sustaining the link rotation angle specified in Section F3.4a.
(b) The shear resistance of the connection, measured at the required link rotation angle, shall be at least equal to the expected shear strength of the link, $R_{y} V_{n}$ , where $V_{n}$ is determined in accordance with Section F3.5b.2.
(c) The flexural resistance of the connection, measured at the required link rotation angle, shall be at least equal to the moment corresponding to the nominal shear strength of the link, $V_{n}$ , as determined in accordance with Section F3.5b.2.

F3.6e.2 Conformance Demonstration

Link-to-column connections shall meet the preceding requirements by one of the following:

(a) Use a connection prequalified for EBF in accordance with Section K1.

User Note: There are no prequalified link-to-column connections.

(b) Provide qualifying cyclic test results in accordance with Section K2. Results of at least two cyclic connection tests shall be provided and are permitted to be based on one of the following:
(1) Tests reported in research literature or documented tests performed for other projects that are representative of project conditions, within the limits specified in Section K2

Exception: Cyclic testing of the connection is not required if the following conditions are met.

(1) Reinforcement at the beam-to-column connection at the link end precludes yielding of the beam over the reinforced length.
(2) The available strength of the reinforced section and the connection equals or exceeds the required strength calculated based upon adjusted link shear strength as described in Section F3.3.
(3) The link length (taken as the beam segment from the end of the reinforcement to the brace connection) does not exceed $1.6 M_{p} / V_{p}$ .
(4) Full-depth stiffeners, as required in Section F3.5b.4, are placed at the link-to-reinforcement interface.