E3Special moment frames (SMF)

E3.4a Moment Ratio

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

(E3-1)

where

$\begin{aligned} \Sigma M_{p c}^{*} & =\text { sum of the projections of the nominal flexural strengths of the columns } \\ & \text { (including haunches where used) above and below the joint to the beam } \\ & \text { centerline with a reduction for the axial force in the column, kip-in. } \\ & \text { (N-mm). It is permitted to determine } \Sigma M_{p c}^{*} \text { as follows: }\end{aligned}$

(E3-2)

When the centerlines of opposing beams in the same joint do not coincide, the mid-line between centerlines shall be used. $\Sigma M_{be}^{*}$ = sum of the projections of the expected flexural strengths of the beams at the plastic hinge locations to the column centerline, kip-in. (N-mm). It is permitted to determine $\Sigma M_{be}^{*}$ as follows: $\Sigma M_{be}^{*}=\Sigma(M_{pr}+\alpha_{s}M_{v})$ (E3-3)

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

• $F_{y c} \quad$ = specified minimum yield stress of column, ksi (MPa)

$M_{pr}$ = maximum probable moment at the location of the plastic hinge, as determined in accordance with ANSI/AISC 358, or as otherwise determined in a connection prequalification in accordance with Section K1, or in a program of qualification testing in accordance with Section K2, kip-in. (N-mm)

• $\begin{aligned} M_{y} & =\text { additional moment due to shear amplification from the location of the } \\ & \text { plastic hinge to the column centerline based on LRFD or ASD load } \\ & \text { combinations, kip-in. (N-mm) }\end{aligned}$
• $\begin{aligned} P_{r} & =\text { required axial compressive strength according to Section D1.4a, kips (N) }\end{aligned}$
• $\begin{aligned} Z_{c} & =\text { plastic section modulus of the column about the axis of bending, in. }^{3} \\ & \left(\mathrm{mm}^{3}\right)\end{aligned}$

Exception: The requirement of Equation E3-1 shall not apply if the following conditions in (a) or (b) are satisfied.

• (a) Columns with $\alpha_{s} P_{r c}<0.3 P_{y c}$ for all load combinations other than those determined using the overstrength seismic load and that satisfy either of the following:
• (1) Columns used in a one-story building or the top story of a multistory building.
• (2) Columns where (i) the sum of the available shear strengths of all exempted columns in the story is less than 20% of the sum of the available shear strengths of all moment frame columns in the story acting in the same direction, and (ii) the sum of the available shear strengths of all exempted columns on each moment frame column line within that story is less than 33% of the available shear strength of all moment frame columns on that column line. For the purpose of this exception, a column line is defined as a single line of columns or parallel lines of columns located within 10% of the plan dimension perpendicular to the line of columns.

User Note: For purposes of this exception, the available shear strengths of the columns should be calculated as the limit strengths considering the flexural strength at each end as limited by the flexural strength of the attached beams, or the flexural strength of the columns themselves, divided by $H$, where $H$ is the story height.

The available axial yield strength of column, $P_{\mathrm{yc}}$, shall be determined as follows:

$P_{y c}=F_{y c} A_{g}$ (E3-4)

and the required axial strength is $P_{r c}$ using LRFD or ASD load combinations, as applicable.

• (b) Columns in any story that has a ratio of available shear strength to required shear strength that is 50% greater than the story above.

E3.4b Stability Bracing of Beams

Beams shall be braced to satisfy 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 SMF. The placement of lateral bracing shall be consistent with that documented for a prequalified connection designated in ANSI/AISC 358, or as otherwise determined in a connection prequalification in accordance with Section K1, or in a program of qualification testing in accordance with Section K2.

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

E3.4c Stability Bracing at Beam-to-Column Connections

E3.4c.1 Braced Connections

When the webs of the beams and column are coplanar and a column is shown to remain elastic outside of the panel zone, column flanges at beam-to-column connections shall require stability bracing only at the level of the top flanges of the beams. It is permitted to assume that the column remains elastic when the ratio calculated using Equation E3-1 is greater than 2.0.

When a column cannot be shown to remain elastic outside of the panel zone, the following requirements shall apply:

• (a) The column flanges shall be laterally braced at the levels of both the top and bottom beam flanges. Stability bracing is permitted to be either direct or indirect.

User Note: Direct stability bracing of the column flange is achieved through use of member braces or other members, deck, and slab, attached to the column flange at or near the desired bracing point to resist lateral buckling. Indirect stability bracing refers to bracing that is achieved through the stiffness of members and connections that are not directly attached to the column flanges, but rather act through the column web or stiffener plates.

(b) Each column flange member brace shall be designed for a required strength that is equal to $2\%$ of the available beam flange strength, $F_yb_ft_{bf}$, divided by $\alpha _s$, where $b_f=$ width of beam flange, in. (mm) $t_{bf}=$ thickness of beam flange, in. (mm)

E3.4c.2 Unbraced Connections

Columns that do not have bracing transverse to the seismic frame at the beam-to-column connection shall conform to Specification Chapter H, except that

• (a) The required column strength shall be determined from the load combinations in the applicable building code that include the overstrength seismic load. The overstrength seismic load need not exceed 125% of the frame available strength based upon either the beam available flexural strength or panel-zone available shear strength.
• (b) The slenderness, $L / r$, for the column shall not exceed 60 , where
• $L=$ length of column, in. (mm)
• $r=$ governing radius of gyration, in. (mm)
• (c) The column required flexural strength transverse to the seismic frame shall include that moment caused by the application of the beam flange force specified in Section E3.4c.1(b), in addition to the second-order moment due to the resulting column flange lateral displacement.

E3.5a Basic Requirements

Beam and column members shall meet the requirements of Section D1.1 for highly ductile members, unless otherwise qualified by tests.

Structural steel beams in SMF are permitted to be composite with a reinforced concrete slab to resist gravity loads.

E3.5b Beam Flanges

Abrupt changes in beam flange area are prohibited in plastic hinge regions. The drilling of flange holes or trimming of the beam flange width are not permitted unless testing or qualification demonstrates that the resulting configuration can develop stable plastic hinges to accommodate the required story drift angle. The configuration shall be consistent with a prequalified connection designated in ANSI/AISC 358, or as otherwise determined in a connection prequalification in accordance with Section K1, or in a program of qualification testing in accordance with Section K2.

E3.5c 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. The extent of the protected zone shall be as designated in ANSI/AISC 358, or as otherwise determined in a connection prequalification in accordance with Section K1, or as determined in a program of qualification testing in accordance with Section K2.

User Note: The plastic hinging zones at the ends of SMF beams should be treated as protected zones. The plastic hinging zones should be established as part of a prequalification or qualification program for the connection in accordance with Section E3.6c. In general, for unreinforced connections, the protected zone will extend from the face of the column to one-half of the beam depth beyond the plastic hinge point.

E3.6a Demand Critical Welds

The following welds are demand critical welds and shall satisfy the requirements of Section 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) Complete-joint-penetration (CJP) groove welds of beam flanges and beam webs to columns, unless otherwise designated by ANSI/AISC 358, or otherwise determined in a connection prequalification in accordance with Section K1, or as determined in a program of qualification testing in accordance with Section K2.

User Note: For the designation of demand critical welds, standards such as ANSI/AISC 358 and tests addressing specific connections and joints should be used in lieu of the more general terms of these Provisions. Where these Provisions indicate that a particular weld is designated demand critical, but the more specific standard or test does not make such a designation, the more specific standard or test consistent with the requirements in Chapter K should govern. Likewise, these standards and tests may designate welds as demand critical that are not identified as such by these Provisions.

E3.6b Beam-to-Column Connections

Beam-to-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, unless equivalent performance of the moment frame system is demonstrated through substantiating analysis conforming to ASCE/SEI 7, Sections 12.2.1.1 or 12.2.1.2,

where

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

E3.6c Conformance Demonstration

Beam-to-column connections used in the SFRS shall satisfy the requirements of Section E3.6b by one of the following:

• (a) Use of SMF connections designed in accordance with ANSI/AISC 358.
• (b) Use of a connection prequalified for SMF in accordance with Section K1.
• (c) Provision of qualifying cyclic test results 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 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

E3.6d 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:

(E3-5)

where

• $L_{h}$ = distance between beam plastic hinge locations, as defined within the test report or ANSI/AISC 358, in. (mm)
• $M_{p r}=$ maximum probable moment at the location of the plastic hinge, as defined in Section E3.4a, kip-in. (N-mm)

When $E_{c l}$ as defined in Equation E3-5 is used in ASD load combinations that are additive with other transient loads and that are based on ASCE/SEI 7, the 0.75 combination factor for transient loads shall not be applied to $E_{c l}$.

Where the exceptions to Equation E3-1 in Section E3.4a apply, the shear, $E_{c l}$, is permitted to be calculated based on the beam end moments corresponding to the expected flexural strength of the column multiplied by 1.1.

E3.6e Panel Zone

E3.6e.1 Required Shear Strength

The required shear strength of the panel zone shall be determined from the sum- mation of the moments at the column faces as determined by projecting the expected moments at the plastic hinge points to the column faces. The design shear strength shall be $\phi_{v} R_{n}$ and the allowable shear strength shall be $R_{n} / \Omega_{v}$,

where

$\begin{aligned} \phi_{v} & =1.00 \text { (LRFD) } \\ \Omega_{v} & =1.50 \text { (ASD) }\end{aligned}$

and the nominal shear strength, $R_{n}$, in accordance with the limit state of shear yielding, is determined as specified in Specification Section J10.6.

Alternatively, the required thickness of the panel zone shall be determined in accordance with the method used in proportioning the panel zone of the tested or prequalified connection.

Where the exceptions to Equation E3-1 in Section E3.4a apply, the beam moments used in calculating the required shear strength of the panel zone need not exceed those corresponding to the expected flexural strength of the column multiplied by 1.1.

E3.6e.2 Panel-Zone Thickness

The individual thicknesses, $t$, of column web and doubler plates, if used, shall satisfy the following requirement:

(E3-6)

where

$\begin{aligned} d_{z} & =d-2 t_{f} \text { of the deeper beam at the connection, in. (mm) } \\ t & =\text { thickness of column web or individual doubler plate, in. (mm) } \\ w_{z} & =\text { width of panel zone between column flanges, in. (mm) }\end{aligned}$

When plug welds are used to join the doubler to the column web, it is permitted to use the total panel-zone thickness to satisfy Equation E3-6. Additionally, the individual thicknesses of the column web and doubler plate shall satisfy Equation E3-6, where $d_{z}$ and $w_{z}$ are modified to be the distance between plug welds. When plug welds are required, a minimum of four plug welds shall be provided and spaced in accordance with Equation E3-6.

E3.6e.3 Panel-Zone Doubler Plates

The thickness of doubler plates, if used, shall not be less than 1/4 in. (6 mm).

When used, doubler plates shall meet the following requirements.

Where the required strength of the panel zone exceeds the design strength, or where the panel zone does not comply with Equation E3-6, doubler plates shall be provided. Doubler plates shall be placed in contact with the web or shall be spaced away from the web. Doubler plates with a gap of up to 1/16 in. (2 mm) between the doubler plate and the column web are permitted to be designed as being in contact with the web. When doubler plates are spaced away from the web, they shall be placed symmetrically in pairs on opposite sides of the column web.

Doubler plates in contact with the web shall be welded to the column flanges either using partial-joint-penetration (PJP) groove welds in accordance with AWS D1.8/D1.8M, clause 4.3, that extend from the surface of the doubler plate to the column flange, or by using fillet welds. Spaced doubler plates shall be welded to the column flanges using CJP groove welds, PJP groove welds, or fillet welds. The required strength of PJP groove welds or fillet welds shall equal the available shear yielding strength of the doubler-plate thickness.

• (a) Doubler plates used without continuity plates

Doubler plates and the welds connecting the doubler plates to the column flanges shall extend at least 6 in. (150 mm) above and below the top and bottom of the deeper moment frame beam. For doubler plates in contact with the web, if the doubler-plate thickness alone and the column-web thickness alone both satisfy Equation E3-6, then no weld is required along the top and bottom edges of the doubler plate. If either the doubler-plate thickness alone or the column-web thickness alone does not satisfy Equation E3-6, then a minimum size fillet weld, as stipulated in Specification Table J2.4, shall be provided along the top and bottom edges of the doubler plate. These welds shall terminate 1½ in. (38 mm) from the toe of the column fillet.

• (b) Doubler plates used with continuity plates

Doubler plates are permitted to be either extended above and below the continuity plates or placed between the continuity plates.

• (1) Extended doubler plates

Extended doubler plates shall be in contact with the web. Extended doubler plates and the welds connecting the doubler plates to the column flanges shall extend at least 6 in. (150 mm) above and below the top and bottom of the deeper moment frame beam. Continuity plates shall be welded to the extended doubler plates in accordance with the requirements in Section E3.6f.2(c). No welds are required at the top and bottom edges of the doubler plate.

(2) Doubler plates placed between continuity plates

Doubler plates placed between continuity plates are permitted to be in contact with the web or away from the web. Welds between the doubler plate and the column flanges shall extend between continuity plates but are permitted to stop no more than 1 in. (25 mm) from the continuity plate. The top and bottom of the doubler plate shall be welded to the continuity plates over the full length of the continuity plates in contact with the column web. The required strength of the doubler plate-to-continuity plate weld shall equal 75% of the available shear yield strength of the full doubler plate thickness over the contact length with the continuity plate.

User Note: When a beam perpendicular to the column web connects to a doubler plate, the doubler plate should be sized based on the shear from the beam end reaction in addition to the panel-zone shear. When welding continuity plates to extended doubler plates, force transfer between the continuity plate and doubler plate must be considered. See the Commentary for further discussion.

E3.6f Continuity Plates

Continuity plates shall be provided as required by this section.

Exception: This section shall not apply in the following cases.

• (a) Where continuity plates are otherwise determined in a connection prequalification in accordance with Section K1.
• (b) Where a connection is qualified in accordance with Section K2 for conditions in which the test assembly omits continuity plates and matches the prototype beam and column sizes and beam span.

E3.6f.1 Conditions Requiring Continuity Plates

Continuity plates shall be provided in the following cases:

• (a) Where the required strength at the column face exceeds the available column strength determined using the applicable local limit states stipulated in Specification Section J10, where applicable. Where so required, continuity plates shall satisfy the requirements of Specification Section J10.8 and the requirements of Section E3.6f.2.

For connections in which the beam flange is welded to the column flange, the column shall have an available strength sufficient to resist an applied force consistent with the maximum probable moment at the face of the column, $M_{f}$.

User Note: The beam flange force, $P_{f}$, corresponding to the maximum probable moment at the column face, $M_{f}$, may be determined as follows:

For connections with beam webs with a bolted connection to the column, $P_{f}$ may be determined assuming only the beam flanges participate in transferring the moment $M_{f}$ :

$P_{f}=\frac{M_{f}}{\alpha_{s} d^{*}}$

For connections with beam webs welded to the column, $P_{f}$ may be determined assuming that the beam flanges and web both participate in transferring the moment, $M_{f}$, as follows:

$P_{f}=\frac{0.85 M_{j}}{\alpha_{s} d^{*}}$

where

• $M_{f}=$ maximum probable moment at face of column as defined in ANSI/AISC 358 for a prequalified moment connection or as determined from qualification testing, kip-in. (N-mm)
• $P_{f}=$ required strength at the column face for local limit states in the column, kip (N)
• $d^{*}=$ distance between centroids of beam flanges or beam flange connections to the face of the column, in. (mm)
• (b) Where the column flange thickness is less than the limiting thickness, $t_{\text {lim }}$, determined in accordance with this provision.
• (1) Where the beam flange is welded to the flange of a W-shape or built-up I-shaped column, the limiting column-flange thickness is as follows:

(E3-7)

where

$b_{b f}=$ width of beam flange, in. (mm)

(2) Where the beam flange is welded to the flange of the I-shape in a boxed wide-flange column, the limiting column-flange thickness is as follows: $t_{lim}=\frac{b_{bf}}{12}$ (E3-8)

User Note: These continuity plate requirements apply only to wide-flange column sections. Detailed formulas for determining continuity plate requirements for box-section columns have not been developed. It is noted that the performance of moment connections is dependent on the column flange stiffness in distributing the strain across the beam-to-column flange weld. Designers should consider the relative stiffness of the box-section column flange compared to those of tested assemblies in resisting the beam flange force to determine the need for continuity plates.

E3.6f.2 Continuity Plate Requirements

Where continuity plates are required according to Sections E2.6f or E3.6f.1, or where they are listed as an alternative in Section E1.6b(c)(4), they shall meet the requirements of this section.

• (a) Continuity plate width

The width of the continuity plate shall be determined as follows:

• (1) For W-shape columns, continuity plates shall, at a minimum, extend from the column web to a point opposite the tips of the wider beam flanges.
• (2) For boxed wide-flange columns, continuity plates shall extend the full width from column web to side plate of the column.
• (b) Continuity plate thickness

The minimum thickness of the plates shall be determined as follows:

• (1) For one-sided connections, the continuity plate thickness shall be at least 50% of the thickness of the beam flange.
• (2) For two-sided connections, the continuity plate thickness shall be at least equal to 75% of the thickness of the thicker beam flange on either side of the column.
• (3) The continuity plate width-to-thickness ratio shall be limited by

Figure description:

Mathematical Equation (E3-9:

$b/t \le 0.56 \sqrt{\frac{E}{R_yF_y}}$

where

$E=\operatorname{modulus}$ of elasticity of steel

= 29,000 ksi (200 000 MPa)

• (c) Continuity plate welds

Continuity plates shall be welded to column flanges using CJP groove welds or fillet welds on each side of the continuity plate with weld size of each fillet weld equal to at least 75% of the thickness of the continuity plate.

Continuity plates shall be welded to column webs or extended doubler plates using groove welds or fillet welds. The required strength of the welded joints of continuity plates to the column web or extended doubler plate shall be the lesser of the following:

• (1) The sum of the available tensile strengths of the contact areas of the continuity plates to the column flanges that have attached beam flanges
• (2) The available shear strength of the contact area of the plate with the column web or extended doubler plate
• (3) The available shear strength of the column web, when the continuity plate is welded to the column web, or the available shear strength of the doubler plate, when the continuity plate is welded to an extended doubler plate

E3.6g Column Splices

Column splices shall comply with the requirements of Section D2.5.

Exception: The required strength of the column splice, including appropriate stress concentration factors or fracture mechanics stress intensity factors, need not exceed that determined by a nonlinear analysis as specified in Chapter C.

E3.6g.1 Welded Column Flange Splices Using CJP Groove Welds

Where welds are used to make the flange splices, they shall be CJP groove welds, unless as otherwise permitted in Section E3.6g.2.

E3.6g.2 Welded Column Flange Splices Using PJP Groove Welds

Where the specified minimum yield stress of the column shafts does not exceed 60 ksi (415 MPa) and the thicker flange is at least 5% thicker than the thinner flange, PJP groove welds are permitted to make the flange splices and shall comply with the following requirements:

• (a) The PJP flange weld or welds shall provide a minimum total effective throat of 85% of the thickness of the thinner column flange.

• (b) A smooth transition in the thickness of the weld is provided from the outside of the thinner flange to the outside of the thicker flange. The transition shall be at a slope not greater than 1 in 2.5 and may be accomplished by sloping the weld surface, by chamfering the thicker flange to a thickness no less than 5% greater than the thickness of the thinner flange, or by a combination of these two methods.

• (c) Tapered transitions between column flanges of different width shall be provided in accordance with Section D2.5b(b)(3).

• (d) Where the flange weld is a double-bevel groove weld (i.e., on both sides of the flange),

• (1) The unfused root face shall be centered within the middle half of the thinner flange, and

• (2) Weld access holes that comply with the Specification shall be provided in the column section containing the groove weld preparation.

• (e) Where the flange thickness of the thinner flange is not greater than 2½ in. (63 mm) and the weld is a single-bevel groove weld, weld access holes shall not be required.

E3.6g.3 Welded Column Web Splices Using CJP Groove Welds

The web weld or welds shall be made in a groove or grooves in the column web that extend to the access holes. The weld end(s) may be stepped back from the ends of the bevel(s) using a block sequence for approximately one weld size.

E3.6g.4 Welded Column Web Splices Using PJP Groove Welds

When PJP groove welds in column flanges that comply with Section E3.6g.2 are used and the thicker web is at least 5% thicker than the thinner web, it is permitted to use PJP groove welds in column webs that comply with the following requirements:

• (a) The PJP groove web weld or welds provide a minimum total effective throat of 85% of the thickness of the thinner column web.
• (b) A smooth transition in the thickness of the weld is provided from the outside of the thinner web to the outside of the thicker web.
• (c) Where the weld is a single-bevel groove, the thickness of the thinner web is not greater than 2½ in. (63 mm).
• (d) Where no access hole is provided, the web weld or welds are made in a groove or grooves prepared in the column web extending the full length of the web between the k-areas. The weld end(s) are permitted to be stepped back from the ends of the bevel(s) using a block sequence for approximately one weld size.
• (e) Where an access hole is provided, the web weld or welds are made in a groove or grooves in the column web that extend to the access holes. The weld end(s) are permitted to be stepped back from the ends of the bevel(s) using a block sequence for approximately one weld size.

E3.6g.5 Bolted Column Splices

Bolted column splices shall have a required flexural strength that is at least equal to $R_{y} F_{y} Z_{x} / \alpha_{s}$ of the smaller column, where $Z_{x}$ is the plastic section modulus about the $x$-axis. The required shear strength of column web splices shall be at least equal to $\Sigma M_{p c} /\left(\alpha_{s} H_{c}\right)$, where $\Sigma M_{p c}$ is the sum of the plastic moments at the top and bottom ends of the column, and $H_{c}$ is the clear height of the column between beam connections, including a structural slab, if present.