Free Steel Moment Frame Calculator — SMF/OMF Design
Design steel moment frames for seismic and wind loading — special moment frames (SMF), ordinary moment frames (OMF), and intermediate moment frames (IMF). The calculator checks beam-column flexural and axial interaction, panel zone shear, story drift, strong-column weak-beam ratio, and welded unreinforced flange (WUF) connection provisions per AISC 341-22, AS 4100 Section 8, EN 1993-1-1, and CSA S16 Section 27.
How to Use
- Select frame type: SMF (R=8), IMF (R=6), OMF (R=3.5), or wind-only frame.
- Enter story geometry: bay width, story height, column and beam sections.
- Define loads: gravity, seismic base shear (from ELF or modal analysis), wind loads.
- Review member checks: beam-column interaction per AISC H1, panel zone per J10.6.
- Check drift: elastic story drift limits per AISC 341 Table C1.5.2a.
- Verify strong-column weak-beam ratio per AISC 341 Section E3.4a.
Design Code Requirements
| Check | AISC 341-22 | AS 4100 | EN 1993-1-1 | CSA S16 |
|---|---|---|---|---|
| Strong-column/weak-beam | E3.4a (sum Mc ≥ sum Mg) | Cl 8.5.3 | Cl 6.3.2 | Cl 27.8 |
| Panel zone shear | E3.6e (J10.6) | Cl 8.6.2 | Cl 6.2.6.1 | Cl 27.9 |
| Story drift limit | C1.5.2a (0.020*hsx) | Cl 8.6.4 | Cl 6.3 | Cl 27.10 |
| Beam-column interaction | AISC 360 H1 | Cl 8.4 | Cl 6.3.3 | Cl 13.8 |
| Connection ductility | E3.6 (pre-qualified) | Cl 8.7 | Cl 6.2 | Cl 27.11 |
Strong-Column Weak-Beam Ratio
Per AISC 341-22 Section E3.4a, the sum of the expected flexural strengths of columns at a joint must exceed the sum of the expected flexural strengths of beams by at least 20%:
Sum(Mpc*)/Sum(Mpb*) ≥ 1.2 (for SMF) or 1.0 (for IMF/OMF)
This ensures a stable plastic mechanism forms in the beams rather than a soft story in the columns. When violated, the beam hinge locations shift to the column — a catastrophic failure mode that must be avoided.
Frequently Asked Questions
What is panel zone shear and why is it critical in moment frames? Panel zone shear is the shear force in the column web within the beam-column joint region. Under seismic loading, the panel zone must either remain elastic (for OMF) or be allowed to yield in a controlled manner (for SMF). AISC 341-22 Section E3.6e requires panel zone thickness to be adequate to resist the shear from the expected moment strength of the beams. Thin panel zones can lead to excessive joint deformation and loss of frame stiffness.
What is the strong-column weak-beam requirement? The strong-column weak-beam (SCWB) requirement ensures that plastic hinges form in beams rather than columns during a seismic event. Per AISC 341-22 E3.4a, the ratio of column-to-beam flexural strengths must be at least 1.2 for SMF. A column-sway mechanism (soft story) concentrates drift at a single level, while a beam-sway mechanism distributes energy dissipation across the frame height.
What is the difference between R factors for SMF, IMF, and OMF? AISC 341-22 Table C1.5.2a lists R = 8 for SMF (highest ductility, lowest design force), R = 6 for IMF (intermediate), and R = 3.5 for OMF (lowest ductility, highest design force). Higher R factors mean lower seismic design forces but require stricter detailing. SMF requires pre-qualified connections per AISC 358, while OMF permits simpler connections.
Is this moment frame calculator free? Yes, completely free with unlimited calculations.
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Disclaimer (educational use only)
This page is provided for general technical information and educational use only. It does not constitute professional engineering advice. All structural designs must be verified by a licensed Professional Engineer (PE) or Structural Engineer (SE). The site operator disclaims liability for any loss or damage arising from the use of this page.