C-G4G4 Composite partially restrained moment frames (C-PRMF)

G4.1 Scope

Composite partially restrained moment frames (C-PRMF) consist of structural steel columns and composite steel beams connected with partially restrained (PR) composite joints (Leon and Kim, 2004; Thermou et al., 2004; Zandonini and Leon, 1992). In PR composite joints, flexural resistance is provided by a couple incorporating a conventional steel bottom flange connection (welded or bolted plates, angles, or T-stubs) and the continuous reinforcing steel in the slab at the top of the girder, as shown in Figure C-G4.1. The steel beam and the concrete slab are connected by steel anchors, such as steel headed anchor studs. Shear resistance is provided through a conventional steel frame shear connection (welded or bolted plates or angles). The use of the slab reinforcing steel results in a stronger and stiffer connection, a beneficial distribution of strength and stiffness between the positive and negative moment regions of the beams, and redistribution of loads under inelastic action. In most cases, the connections in this seismic force-resisting system at the roof level will not be designed as composite.

C-PRMF were originally proposed for areas of low to moderate seismicity in the eastern United States with seismic design categories A, B, and C; however, with appropriate detailing and analysis, C-PRMF can be used in areas of higher seismicity (Leon, 1990). Tests and analyses of these systems have demonstrated that the seismically induced loads on PR moment frames can be lower than those for fully restrained moment frames due to (a) lengthening in the natural period due to yielding in the connections and (b) stable hysteretic behavior of the connections (Nader and Astaneh-Asl, 1992; DiCorso et al., 1989). Thus, in some cases, C-PRMF can be designed for lower seismic loads than OMF.

Figure description:

Subject: Composite partially restrained connection (C-PRMF

Elevation View Components:

• Steel column: Central vertical structural element.
• Slab reinforcement: Embedded horizontal bars within the concrete slab.
• Web angles: Connectors joining the beam web to the column flange.
• Composite beam: Horizontal structural member supporting the slab.
• Seat angles: Bottom brackets supporting the beam flange at the column interface.

Plan View Components:

• Transverse slab reinforcement: Rebar running perpendicular to the beam.
• Longitudinal slab reinforcement: Rebar running parallel to the beam.
• I-beam section: Shows the central steel column profile surrounded by the reinforcement grid.

Fig. C-G4.1. Composite partially restrained connection.

G4.2 Basis of Design

Design methodologies and standardized guidelines for C-PRMF and connections have been published (Ammerman and Leon, 1990; Leon and Forcier, 1992; Leon et al., 1996; ASCE, 1998). In the design of PR composite connections, it is assumed that bending and shear forces can be considered separately.

G4.3 Analysis

For frames up to four stories, the design of C-PRMF should be made using an analysis that, as a minimum, accounts for the PR behavior of the connections by utilizing linear springs with reduced stiffness (Bjorhovde, 1984). The effective connection stiffness should be considered for determining member load distributions and deflections, calculating the building’s period of vibration, and checking frame stability. Different connection stiffnesses may be required for these checks (Leon et al., 1996). Frame stability can be addressed using conventional procedures. However, the connection flexibility should be considered in determining the rotational restraint at the ends of the beams. For structures taller than four stories, drift and stability need to be carefully checked using analysis techniques that incorporate both geometric and connection nonlinearities (Rassati et al., 2004; Ammerman and Leon, 1990; Chen and Lui, 1991). Because the moments of inertia for composite beams in the negative and positive regions are different, the use of either value alone for the beam members in the analysis can lead to inaccuracies. Therefore, the use of a weighted average, as discussed in Specification Chapter I Commentary, is recommended (Zaremba, 1988; Ammerman and Leon, 1990; Leon and Ammerman, 1990; AISC, 2022d).

G4.4 System Requirements

The system should be designed to enforce a strong column-weak beam mechanism, except for at the roof level. Leon et al. (1996) suggest using the following equation, analogous to Equation E3-1 for SMF, to achieve this behavior:

(C-G4-1)

where $M_{p c c}^{*}$ is as defined in Section G3.4a and $M_{P R e}^{*}$ is the expected flexural strength of the PR connections framing into the joint. In calculating $M_{P R e}^{*}$, appropriate factors for expected strength from Section A3.2 should be used. The value of 1.2 is intended so that a strong column-weak beam mechanism results.

G4.5 Members

G4.6 Connections

The connecting elements should be designed with a yield force that is less than that of the connected members to prevent local limit states—such as local buckling of the flange in compression, web local crippling of the beam, panel-zone yielding in the column, and bolt or weld failures—from controlling. When these limit states are avoided, large connection ductility should result in excellent frame performance under large inelastic load reversals.

Figure description:

Key Information:

• Subject: Concrete slab bearing force transfer mechanism to a structural steel column.
• Column: Centrally located wide-flange (I-beam section.
• Reinforcement:
  • Longitudinal reinforcement: Parallel horizontal bars with force arrows pointing away from the column.
  • Transverse reinforcement: Vertical bars intersecting the longitudinal bars.
• Force Transfer:
  • Diagonal arrows represent compressive forces acting towards the column flanges.
  • Uniformly distributed load arrows indicate bearing pressure applied directly to the exterior faces of the column flanges.

Fig. C-G4.2. Concrete slab bearing force transfer.

Figure description:

• Diagram Subject: Fig. C-G4.3. Solid slab to be provided around column.
• Key Entities:
  • Concrete Slab: Features a variable profile (ribbed transitioning to a solid section at the column interface.
  • Column: Vertical structural member passing through the slab.
  • Nominal slab depth: Indicated as the full thickness from the top of the slab to the bottom of the ribs.
  • Solid Slab Width Requirement: Minimum of 12 in. (300 mm of solid concrete slab extending from the face of the column before the ribbed profile begins.

Fig. C-G4.3. Solid slab to be provided around column.