917x

001767

Cisca Tjoa, PE

2023-12-05

AISC 341-16 Moment Frame Member Design in RFEM 6

The three types of moment frames (Ordinary, Intermediate, Special) are available in the Steel Design add-on of RFEM 6. The seismic design result according to AISC 341-16 is categorized into two sections: member requirements and connection requirements.

More in-depth details on the Seismic Configuration input are covered in a separate article, KB | AISC 341 Seismic Design in RFEM 6 .

Member Requirements

The following design checks for members that are part of the seismic force-resisting system (SFRS) are available in RFEM. The sections listed refer to Seismic Provisions AISC 341-16 [1].

Width-to-Thickness Limitations [Section D1.1]
Stability Bracing of Beams - Required Strength & Stiffness [Section D1.2a.1(b) for IMF and D1.2b for SMF]
Stability Bracing of Beams - Maximum Spacing [Section D1.2a.1(c) for IMF and D1.2b for SMF]
Stability Bracing of Beams at Hinge Locations - Required Strength [Section D1.2c.1(b)]
Column Required Strength [Section D1.4a]
Column Slenderness Ratio for Unbraced Connection [Section E3.4c.2b]

Width-to-Thickness Limitations for Ductility Requirements

Members in IMF are designated as moderately ductile members according to Section E2.5a. Members in SMF are designated as highly ductile members according to Section E3.5a.

Column Flange

The column flange of SMF must satisfy the requirements of AISC Seismic Provisions Section D1.1 [1] for highly ductile members. This design check is shown as EQ 1200 in RFEM (Image 1).

KB 001767 | AISC 341-16 Moment Frame Member Design in RFEM 6

Flange Ductility Requirement

Column Web

The limiting width-to-thickness ratio for webs of highly ductile members is determined using the governing load case for axial load, as stipulated in Section D1.4a [1]. The governing load case is based on all load combinations, including gravity only CO, CO with standard seismic load, and CO with overstrength seismic load. This check is shown in EQ 1100 in RFEM (Image 2).

Web Ductility Requirement

Similar to the columns, the width-to-thickness checks are also done for the beams.

Stability Bracing of Beams

The required strength and stiffness of the stability bracings are listed in the Stability Bracing by Member tab under “Seismic Requirements” (Image 3). These values can be compared to the calculated available strength and stiffness when designing the bracing members that frame into the beam. There are no design check details available (only references).

Stability Bracing by Member

There are two different values listed for the required strengths. The first value, P_br, is applicable for the stability bracings that are located outside the plastic hinge location. P_br is defined in Equation A-6-7 of Appendix 6 of AISC 360-16 [3]:

$P_{br} = 0.02 \cdot (\frac{M_{r} \cdot C_{d}}{h_{o}})$

P_br	Required strength of the stability beam bracing
M_r	Required flexural strength of the beam. M_r = R_yF_yZ/α_s[AISC 341 Equation D1-1]
C_d	Double curvature factor = 1.0 [AISC 341 Section D1.2a(b)]
h_o	Distance between the flange centroid h_o = d - t_f

Stability Bracing of Beams (Required Strength)

The second, larger value, P_r, is specifically for the stability bracings at the plastic hinge location. It is defined in Equation D1-4 of AISC 341-16 [1]:

$P_{r} = 0.06 \cdot R_{y} \cdot F_{y} \cdot Z / (α_{s} \cdot h_{o})$

P_r	Required strength of the stability beam bracing at the plastic hinge location
R_y	Ratio of the expected yield stress to the specified minimum yield stress
F_y	Specified minimum yield stress
Z	Effective plastic section modulus of a section (or a connection) at the plastic hinge location
α_s	LRFD-ASD force level adjustment factor = 1.0 for LRFD and 1.5 for ASD
h_o	Distance between the flange centroid

Stability Bracing of Beams (Required Strength at Plastic Hinge)

The required stiffness, β_br, is defined in Equation A-6-8 of Appendix 6:

$β_{br} = \frac{1}{Φ} \cdot (\frac{10 \cdot M_{r} \cdot C_{d}}{L_{br} \cdot h_{o}}) (LRFD)$

$β_{br} = Ω \cdot (\frac{10 \cdot M_{r} \cdot C_{d}}{L_{br} \cdot h_{o}}) (ASD)$

β_br	Required stiffness of the stability beam bracing
M_r	Required flexural strength of the beam
C_d	Double curvature factor = 1.0
L_br	Maximum spacing of the stability beam bracing
h_o	Distance between the flange centroid

Stability Bracing of Beams (Required Stiffness)

The maximum spacing of the stability bracing must satisfy the requirements of AISC 341-16 Section D1.2a.1(c) for IMF and Section D1.2b for SMF.

$L_{br} = \frac{0.19 \cdot r_{y} \cdot E}{R_{y} \cdot F_{y}} for IMF$

$L_{br} = \frac{0.095 \cdot r_{y} \cdot E}{R_{y} \cdot F_{y}} for SMF$

L_br	Maximum spacing of the stability beam bracing
r_y	Radius of gyration about the weak axis
E	Modulus of elasticity
R_y	Ratio of the expected yield stress to the specified minimum yield stress
F_y	Specified minimum yield stress

Stability Bracing of Beams (Maximum Spacing)

The design check for the maximum spacing is presented together with the other member requirements under “Design Ratios of Members”. The design check detail is shown in EQ 2100 (Image 4). The braced length, L_b, is the specified effective length for lateral-torsional buckling (LTB).

Maximum Spacing of Stability Beam

Column Required Strength

All columns that are part of the seismic force-resisting system (SFRS) are required to be designed with the overstrength loads. In many cases, the amplified axial force does not need to be combined with the concurrent bending moments. The option to neglect all bending moments, shear, and torsion in columns for overstrength limit state is activated by default. This option can be deactivated in the Seismic Configuration.

For standard load combinations without overstrength from seismic load effect, the combined loading is checked according to AISC 360-16, Chapter H.

For load combinations with overstrength seismic load, Chapters F and H are not checked when the option to neglect all bending moments, shear, and torsion in columns for overstrength limit state is activated.
In Example 4.3.2 of the seismic manual [2], the controlling case from both load combinations, standard and overstrength, needs to be considered.

Bending moments resulting from a load applied between points of lateral support can contribute to column buckling. Therefore, they are required to be considered concurrently with the axial loads by deactivating the option to neglect the moments.

Column Required Strength

Column Slenderness Ratio for Unbraced Connection

For columns in SMF with no transverse member bracing at the connection, the potential for out-of-plane buckling at the connection shall be minimized by limiting the slenderness ratio L/r to be equal to or less than 60, according to Section E3.4c.2b [1]. Unbraced connections occur in special cases, such as in a two-story frame without an intermediate floor.

In all other cases, the option to meet this requirement can be deactivated in the Seismic Configuration.

Column Slenderness Ratio

The connection requirements are covered in the article KB | AISC 341-16 Moment Frame Connection Strength in RFEM 6 .

Knowledge Base

KB 001767 | AISC 341-16 Moment Frame Member Design in RFEM 6

Author

Cisca is responsible for customer technical support and continued program development for the North American market.

Links

References

American Institute of Steel Construction. (2016). AISC 341-16, Seismic Provisions for Structural Steel Buildings. Chicago: AISC.
American Institute of Steel Construction. (2018). Seismic Design Manual, (3rd ed.). Chicago: AISC.
American Institute of Steel Construction. (2016) Specification for Structural Steel Buildings, ANSI/AISC 360-16. Chicago: AISC.

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