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001768

Cisca Tjoa, PE

2023-12-05

AISC 341-16 Moment Frame Connection Strength in RFEM 6

Moment frame design according to AISC 341-16 is now possible in the Steel Design add-on of RFEM 6. The seismic design result is categorized into two sections: member requirements and connection requirements. This article covers the required strength of the connection. An example comparison of the results between RFEM and the AISC Seismic Design Manual is presented.

The member requirements are covered in a separate article, KB | AISC 341-16 Moment Frame Member Design in RFEM 6 .

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

Connection Requirements

The "Seismic Requirements" include the Required Flexural Strength and the Required Shear Strength of the beam-to-column connection. They are listed in the Moment Frame Connection by Member tab. The design check details are not available for the connection strength. However, the equations and standard references are listed. The symbols and definitions are summarized in the table below (Image 1).

KB 001768 | AISC 341-16 Moment Frame Connection Strength in RFEM 6

1 Moment Frame Connection by Member

AISC Seismic Design Manual – Example 4.3.7 SMF Bolted Flange Plate (BFP) Connection Design

For simplicity, the RFEM model consists only of a single frame instead of the entire building that is presented in the AISC example (Image 2). The gravity load on the beam = 1.15 kip/ft.

2 Example 4.3.7 of AISC Seismic Design Manual

The numbering of the steps in this example follows the step-by-step design procedure outlined in AISC 358-16 Section 7.6 [3].

Step 1. Compute probable maximum moment at the plastic hinge location, M_pr

M_{pr} = C_{pr} \cdot R_{y} \cdot F_{y} \cdot Z_{e}

M_{pr} = 1.15 \cdot 1.1 \cdot 50 ksi \cdot 200 {in}^{3}

M_{pr} = 12650 kip \cdot in

M_pr	Probable maximum moment at the plastic hinge
C_pr	Factor to account for peak connection strength (strain hardening) per AISC 358. C_pr= (F_y+F_u)/(2F_y) ≤ 1.2
R_y	Ratio of expected yield stress to the specified minimum yield stress
F_y	Specified minimum yield stress
Z_e	Effective plastic section modulus of section at the plastic hinge

Probable Maximum Moment

Steps 2 to 5 contain the bolt requirements and are outside the scope of the Steel Design add-on.

Step 6. Compute shear forces at the beam plastic hinge location, V_pr + V_g

V_{pr} + V_{g} = \frac{2 \cdot M_{pr}}{L_{h}} + \frac{w_{u} \cdot L_{h}}{2}

V_{pr} + V_{g} = \frac{2 \cdot 12650 kip \cdot in}{299.8 in} + \frac{1.15 \frac{kip}{ft} \cdot 299.8 in}{2}

V_{pr} + V_{g} = 84.4 kips + 14.4 kips

V_{pr} + V_{g} = 98.8 kips

V_pr	Shear required to produce the maximum probable moment at the plastic hinge V_pr = 2M_pr/L_h
V_g	Shear from gravity loads at the plastic hinge location V_g= w_uL_h/2
M_pr	Probable maximum moment at the plastic hinge location
L_h	Distance between plastic hinge locations L_h = L_beam - d_c- 2S_h= 360.0 in - 15.20 in - 2*22.50 in = 299.8 in L_h is equal to L_cf (clear length of beam) when the plastic hinge location is omitted
w_u	Gravity loads on the beam

Shear Forces at Plastic Hinge

Step 7. Determine moment expected at the face of the column flange, M_f

M_{f} = M_{pr} + M_{extra}

M_{f} = M_{pr} + \frac{(V_{pr} + V_{g})}{S_{h}}

M_{f} = 12650 kip \cdot in + \frac{98.8 kips}{22.50 in}

M_{f} = 14872 kip \cdot in

M_f	Moment expected at the face of the column
M_pr	Probable maximum moment at the plastic hinge location
M_extra	Extra moment from the shear force at the plastic hinge location
V_pr+ V_g	Shear forces at the plastic hinge location
S_h	Distance from the face of column to the plastic hinge location

Moment Expected at Column Face

The above equation neglects the gravity load on the small portion of the beam between the plastic hinge and the face of the column (1.15 kip/ft*1.875 ft = 2.16 kips*22.5 in = 48.6 k-in). This value is permitted to be included [3].

Step 14. Determine the required shear strength at the face of the column, V_u

The required shear strength at the face of the column is used to design the beam web-to-column (single-plate) shear connection.

V_{u} = V_{pr} + V_{g (at face of column)}

V_{u} = \frac{2 \cdot M_{pr}}{L_{h}} + \frac{w_{u} \cdot L_{cf}}{2}

V_{u} = 84.4 kips + \frac{1.15 \frac{kip}{ft} \cdot 344.8 in}{2}

V_{u} = 84.4 kips + 16.5 kips

V_{u} = 100.9 kips

V_u	Required shear strength at the face of the column
Vpr	Shear required to produce the maximum probable moment at the plastic hinge location
V_{g (at face of column)}	Shear from gravity loads at the face of the column
w_u	Gravity loads on the beam
L_cf	Clear length of the beam L_cf = L_beam - d_{c =} 360.0 in - 15.2 in = 344.8 in

Required Shear Strength at Column Face

To be more precise, the calculation above shows V_g taken at the face of the column instead of at the centerline (as shown in the AISC example [2]). The small difference can be seen in the shear diagrams (Image 3).

3 Shear Forces at Centerline and Face of Columns

The values obtained from the formulas above can be compared to the result produced by RFEM under the “Seismic Requirements” (Image 1). Small discrepancies are due to rounding. The result can also be included in the printout report (Image 4).

4 Printout Report

The detailed procedures to design bolts, flange plates, single-plate, continuity plates, and doubler plates are not part of the scope. Therefore, the steps for these checks were omitted in this article.

The moment and shear demand based on the worst-case scenario of the overstrength load combinations, Ω_oM and Ω_oV are also listed. For the design of ordinary moment frames (OMF), the potentially limiting aspects of the connection strength include the overstrength seismic load [AISC Seismic Design Manual Section 4.2(b)].

Knowledge Base

KB 001768 | AISC 341-16 Moment Frame Connection Strength 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). AISC 358-16 Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications. Chicago: AISC.

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