E2Intermediate moment frames (IMF)

AISC 341-22, Seismic Provisions for Structural Steel Buildings

Last Updated 29 Oct 2026

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E2.1 Scope

Intermediate moment frames (IMF) of structural steel shall be designed in conformance with this section.

E2.2 Basis of Design

IMF designed in accordance with these provisions are expected to provide limited inelastic deformation capacity through flexural yielding of the IMF beams and columns, and shear yielding of the column panel zones. Design of connections of beams to columns, including panel zones and continuity plates, shall be based on connection tests that provide the performance required by Section E2.6b and demonstrate this conformance as required by Section E2.6c.

E2.3 Analysis

There are no analysis requirements specific to this system.

E2.4 System Requirements

E2.4a Stability Bracing of Beams

Beams shall be braced to satisfy the requirements for moderately ductile members in Section D1.2a.

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 IMF. The placement of stability 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 of lateral bracing provided adjacent to plastic hinges shall be as required by Section D1.2c.

E2.5 Members

E2.5a Basic Requirements

Beam and column members shall satisfy the requirements of Section D1 for moderately ductile members, unless otherwise qualified by tests.

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

E2.5b Beam Flanges

Changes in beam flange area in the protected zones, as defined in Section E2.5c, shall be gradual. The drilling of flange holes or trimming of beam flange width is not permitted unless testing or qualification demonstrates that the resulting configuration is able to 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.

E2.5c 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 IMF 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 E2.6c. 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.

E2.6 Connections

E2.6a Demand Critical Welds

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

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

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

(1) Column hinging at, or near, the base plate is precluded by conditions of restraint.
(2) There is no net tension under load combinations including the overstrength seismic load.
(c) Complete-joint-penetration groove welds of beam flanges 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 should govern. Likewise, these standards and tests may designate welds as demand critical that are not identified as such by these Provisions.

E2.6b Beam-to-Column Connection Requirements

Beam-to-column connections used in the SFRS shall satisfy the following requirements:

(a) The connection shall be capable of accommodating a story drift angle of at least 0.02 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 0.02 rad.

E2.6c Conformance Demonstration

Beam-to-column connections used in the SFRS shall satisfy the requirements of Section E2.6b by one of the following:

(a) Use of IMF connections designed in accordance with ANSI/AISC 358.
(b) Use of a connection prequalified for IMF 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 are permitted to 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

E2.6d 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 determined as follows:

E_{c l}=2\left(1.1 R_{y} M_{p}\right) / L_{h}

(E2-1)

where

$L_{h}=$ distance between beam plastic hinge locations, as defined within the test report or ANSI/AISC 358, in. (mm)

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

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

Exception: In lieu of Equation E2-1, the required shear strength of the connection shall be as specified 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.

E2.6e Panel Zone

There are no additional panel-zone requirements.

User Note: Panel-zone shear strength should be checked in accordance with Section J10.6 of the Specification. The required shear strength of the panel zone should be based on the beam end moments computed from the load combinations stipulated by the applicable building code, not including the overstrength seismic load.

E2.6f Continuity Plates

Continuity plates shall be provided in accordance with the provisions of Section E3.6f.

E2.6g Column Splices

Column splices shall comply with the requirements of Section E3.6g.