Free Steel Moment Frame Calculator -- SMF/OMF Design

Design steel moment frames for seismic and wind lateral load resistance -- Special Moment Frames (SMF), Intermediate Moment Frames (IMF), Ordinary Moment Frames (OMF), and non-seismic wind moment frames. The calculator checks beam-column flexural and axial interaction per AISC 360 Chapter H, panel zone shear per AISC 360 J10.6, strong-column weak-beam ratio per AISC 341-22 Section E3.4a, story drift against ASCE 7-22 limits, continuity plate requirements, and welded unreinforced flange (WUF-W) connection capacity per AISC 358. The analysis covers AISC 341-22, AISC 360-22, AISC 358-22, AS 4100 Section 8, EN 1993-1-1 Section 6.3, EN 1998-1, and CSA S16 Section 27.

Moment-resisting frames resist lateral loads through flexural action in beams and columns, creating a rigid frame with full or partially restrained moment connections. Unlike braced frames that create a vertical truss, moment frames provide unobstructed interior space and are preferred for architectural layouts requiring open bays, atria, or flexible floor plans. The tradeoff is that moment frames are less stiff per pound of steel than braced frames, resulting in larger member sizes and higher drift.

Frame types supported:

What this calculator does not cover: connection fracture mechanics (Charpy V-notch requirements per AISC 341), gusset-plate moment frame connections (proprietary systems), and deep-column sections requiring doubler plates thicker than the column web.

How to Use This Calculator

Step 1 -- Select frame type. Choose SMF, IMF, OMF, or wind-only frame. The calculator automatically applies the correct R factor, drift limits, and detailing requirements. For SDC D-F, SMF is permitted without height limit per ASCE 7-22 Table 12.2-1. OMF is limited to 35 ft height in SDC D and not permitted in SDC E or F.

Step 2 -- Enter frame geometry. Define the number of bays and stories, bay widths, story heights, and column splice locations. The column splice is typically located 4 ft above the floor (above the anticipated plastic hinge zone). Enter the number of moment-resisting bays in each direction -- not all bays need to be moment-resisting; gravity-only bays have simple shear connections.

Step 3 -- Assign member sections. Enter column and beam sections for each story and bay. For preliminary sizing, beam depth ≈ L/24 to L/30 for SMF (slightly deeper than gravity beams to control drift). Column sections typically increase in size from roof to base to resist accumulated overturning moment. The calculator checks whether the sections satisfy strong-column weak-beam ratio at each joint.

Step 4 -- Define loads. Enter gravity loads (dead, live, roof live/snow) and lateral loads (wind per ASCE 7-22 Chapter 27, seismic equivalent lateral forces per Chapter 12). The calculator generates load combinations per ASCE 7-22 Section 2.3.1 (LRFD) or 2.4.1 (ASD). For seismic, include the redundancy factor rho (1.0 or 1.3) and the overstrength factor Omega_0 (2.5 for SMF, 3.0 for IMF and OMF) for collector and column splice design.

Step 5 -- Review beam-column interaction. For each beam and column, the combined axial-flexural interaction is checked per AISC 360 Section H1.1. For Pr/Pc ≥ 0.2: Pr/Pc + 8/9 x (Mrx/Mcx + Mry/Mcy) ≤ 1.0. For Pr/Pc < 0.2: Pr/(2*Pc) + Mrx/Mcx + Mry/Mcy ≤ 1.0. Beams in SMF must satisfy the span-to-depth ratio (clear span/d ≥ 7 per AISC 341 D1.2) and lateral bracing requirements (Lb ≤ 0.086 ry E / (Ry Fy) near plastic hinge zones).

Step 6 -- Verify panel zone shear and strong-column weak-beam. The panel zone shear check per AISC 360 J10.6 compares the shear demand from beam moments to the panel zone capacity. For SMF, limited panel zone yielding is permitted (phi = 1.0 for 0.75Pc > Pu; otherwise phi = 0.90). The strong-column weak-beam check sums expected plastic moments of columns and beams framing into each joint. For SMF: Sum(Mpc) / Sum(Mpb*) ≥ 1.2.

Engineering Theory -- Moment Frame Design

Strong-Column Weak-Beam (SCWB) Rationale

The strong-column weak-beam requirement of AISC 341-22 Section E3.4a is the single most important design principle for seismic moment frames. The objective is to force plastic hinges to form in the beams rather than in the columns. A column-sway mechanism (soft story) concentrates all inelastic drift in one story, imposing plastic rotation demands that exceed the capacity of column sections, which have inherently lower rotational ductility than beams due to higher axial load. A beam-sway mechanism distributes plastic rotations across many beams at multiple levels, keeping individual rotation demands within acceptable limits.

The SCWB check sums the expected flexural strengths of all beams and columns framing into a joint:

Sum(Mpc*) / Sum(Mpb*) ≥ 1.0  (for OMF, IMF)
Sum(Mpc*) / Sum(Mpb*) ≥ 1.2  (for SMF)

where Mpc* = Zc x (Fy - Puc/Ag) is the column plastic moment reduced for axial load, and Mpb* = 1.1 x Ry x Fy x Zb + Muv is the beam expected plastic moment including the additional moment from shear amplification at the plastic hinge location. The 1.2 factor for SMF accounts for material overstrength and strain hardening that can increase beam moment above the expected value.

When SCWB fails at a joint, several corrective strategies are available: increase column size, reduce beam size (check drift impact), use higher-strength column steel (Gr 65 or Gr 70), or add a second column line to share overturning demand.

Panel Zone Shear

The panel zone is the column web region within the beam-column joint. Under lateral load, the beam moments at the column face transfer shear into the panel zone. The nominal panel zone shear strength per AISC 360 J10.6 is:

Rn = 0.60 x Fy x dc x tw  (when no axial load effect, Pr ≤ 0.4*Pc)
Rn = 0.60 x Fy x dc x tw x (1.4 - Pr/Pc)  (when Pr > 0.4*Pc)

For SMF where panel zone yielding is permitted, the available shear strength is phiRn with phi = 1.0 when the column axial demand is low (Pr ≤ 0.75Pc), reflecting the excellent ductility of panel zone shear yielding. For OMF where panel zones must remain elastic, phi = 0.90.

The panel zone shear demand is Vu = Sum(Mb/(db - tf)) - Vc where the first term represents the shear from beam moments and Vc is the column shear above the joint. In SMF, the beam moments are the expected plastic moments (1.1 x Ry x Mp), not the factored design moments, to ensure the panel zone can resist forces from the fully yielded beams.

If the panel zone is inadequate, options include: provide doubler plates (welded to the column web to increase effective tw), increase column depth (increases dc, though this also increases beam moment), or use a heavier column section with a thicker web.

Beam Lateral Bracing in SMF

AISC 341-22 Section D1.2b requires that beams in SMF be laterally braced at a spacing not exceeding 0.086 x ry x E / (Ry x Fy) near the plastic hinge location (within a distance d from the column face). For a W24x76 beam (ry = 1.92 in, E = 29,000 ksi, Ry = 1.1, Fy = 50 ksi): Lb_max = 0.086 x 1.92 x 29,000 / (1.1 x 50) = 86.7 in = 7.2 ft. This tight bracing requirement ensures the plastic hinge can develop and sustain its full plastic rotation capacity without lateral-torsional buckling. Beyond the hinge zone, standard AISC 360 Chapter F bracing limits apply.

Worked Example -- 4-Story SMF

Problem: Design a 4-story SMF for an office building in SDC D. Bay width = 30 ft, story height = 13 ft (1st), 12.5 ft (typical). Seismic base shear V_base = 320 kips (Cs = 0.10, W = 3,200 kips). Story forces per ASCE 7-22 Eq 12.8-12: F4 = 128 kips, F3 = 96 kips, F2 = 64 kips, F1 = 32 kips. Overturning moment at base = 128x50 + 96x37.5 + 64x25 + 32x12.5 = 6,400 + 3,600 + 1,600 + 400 = 12,000 kip-ft. Design per AISC 341 and 360, LRFD.

Step 1 -- Preliminary beam sizing. Try W24x76 (A992, Fy=50 ksi, Zx = 200 in^3). Span/depth = 360/23.9 = 15.1 > 7 (SMF minimum). OK. Expected plastic moment: Mpb* = 1.1 x Ry x Fy x Zx = 1.1 x 1.1 x 50 x 200 / 12 = 1,008 kip-ft. Design moment from seismic plus gravity: Mu = M_gravity + M_seismic. Gravity moment ≈ wL^2/8 = 1.2 x 1.0 klf x 30^2/8 = 135 kip-ft. Seismic moment at each end from lateral analysis ≈ 320 kip-ft (from frame analysis, approximate). Mu_total ≈ 455 kip-ft. phi_Mn = 0.90 x 50 x 200/12 = 750 kip-ft. DCR = 455/750 = 0.61. Passes for strength. Connection design: WUF-W connection per AISC 358 must develop the expected plastic moment of 1,008 kip-ft.

Step 2 -- Preliminary column sizing at 1st story. Try W14x311 (A992, Ag = 91.4 in^2, Zx = 604 in^3, ry = 4.13 in). Axial load from gravity: 4 floors x (1.2 x 80 psf DL + 0.5 x 50 psf LL) x 30 ft x 15 ft trib per column = 4 x 121 psf x 450 sf = 218 kips per column line. For two moment frame columns sharing overturning: P_ot = M_ot / (lever arm). With 30 ft bay: P_ot = 12,000 / 30 = 400 kips per column (one up, one down). Total compression column: Pu = 218 + 400 = 618 kips.

Column plastic moment reduced for axial: Pr/Pc = 618 / (0.90 x 50 x 91.4) = 618/4,113 = 0.15. Mpc* = Zc x (Fy - Puc/Ag) = 604 x (50 - 618/91.4) / 12 = 604 x 43.2 / 12 = 2,175 kip-ft.

Step 3 -- SCWB check at 1st story joint. Sum Mpc* = 2 x 2,175 = 4,350 kip-ft (columns above and below joint). Sum Mpb* = 2 x 1,008 = 2,016 kip-ft (beams each side of joint, plus beam on opposite side). Ratio = 4,350 / 2,016 = 2.16 > 1.2. Passes easily.

Step 4 -- Panel zone check at 1st story. Vu = Sum(2 x Mpb* / (db - tf)) - Vc = 2 x 1,008 x 12 / (23.9 - 0.83) - column shear above. Column shear ≈ 320 kips base shear / 2 columns / 4 stories roughly = 40 kips at that story (approx). Vu = 2 x 1,008 x 12 / 23.1 - 40 = 1,047 - 40 = 1,007 kips.

For W14x311: dc = 17.1 in, tw = 1.41 in. Pr/Pc = 618/(0.9 x 50 x 91.4) = 0.15. Panel zone capacity: Rn = 0.60 x 50 x 17.1 x 1.41 = 723 kips. Since Pr ≤ 0.4*Pc, no axial reduction factor. phi = 1.0 (SMF, low axial load). phi_Rn = 1.0 x 723 = 723 kips. DCR = 1,007 / 723 = 1.39 -- FAILS panel zone.

Step 5 -- Add doubler plate. Required additional shear capacity = 1,007 - 723 = 284 kips. Try 3/4-inch doubler plate (Fy = 50 ksi): additional tw = 0.75 in. Effective panel zone thickness with doubler = 1.41 + 0.75 = 2.16 in. Rn_doubled = 0.60 x 50 x 17.1 x 2.16 = 1,108 kips. phi_Rn = 1,108 kips. DCR = 1,007/1,108 = 0.91. Passes with doubler.

Step 6 -- Story drift check. From frame analysis (first-order), roof drift ≈ 4.5 in under seismic forces. Cd = 5.5 for SMF per ASCE 7 Table 12.2-1. Deflection amplification factor is applied to first-order drift: delta_xe amplified = Cd x delta_xe / I = 5.5 x 4.5 / 1.0 = 24.75 in (elastic displacement). This is used for P-Delta stability checks.

Inelastic story drift (design level): delta = Cd x delta_xe / I = 5.5 x 4.5 / 1.0 = 24.75 in. Drift limit = 0.020 x hsx per ASCE 7 Table 12.12-1 for SDC D-F (Risk Cat II) = 0.020 x 50 x 12 = 12.0 in for the building height (total), or per-story limit 0.020 x 12.5 x 12 = 3.0 in per story.

From the first-order analysis, worst-story drift ≈ 1.2 in. Cd-amplified = 5.5 x 1.2 = 6.6 in > 3.0 in limit. Drift FAILS. The frame must be stiffened.

Step 7 -- Increase beam and column sizes to control drift. Increase beams to W27x94 (Zx = 278 in^3, Ix = 3,270 in^4 vs W24x76 Ix = 2,100 in^4, 56% stiffer). Increase columns at lower two stories to W14x398 (Ix = 3,210 in^4 vs W14x311 Ix = 2,260 in^4, 42% stiffer).

After restiffening, reanalyze: worst-story drift reduces to approximately 0.85 in (inelastic). Cd-amplified = 5.5 x 0.85 = 4.68 in > 3.0 in. Still fails. Check: The SMF design is drift-controlled. Additional strategies: increase column sections further, reduce beam span by adding a column line, or add a supplemental damping system.

Alternative: Instead of increasing sections, change the frame type to dual system (moment frame + braced frame) at the central bays. The braced frame takes 75% of the lateral load, reducing moment frame drift to 0.88 in (inelastic). Cd-amplified = 5.5 x 0.88 = 4.84 in -- still marginal but can be mitigated with larger columns. In a dual system with SCBF (R=6), the base shear is slightly higher (Cs based on R=6 vs 8), but the drift is significantly reduced.

Result: The SMF design for this 4-story building is drift-controlled rather than strength-controlled. The final design uses W27x94 beams, W14x398 columns at the lower two stories, W14x311 at the upper two stories, with 3/4-inch doubler plates at the 1st and 2nd story panel zones. Column splices at 4 ft above each floor are designed for 1.1 Ry Fy Ag per AISC 341 D2.5.

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 capacity to resist shear from the expected moment strengths of the beams. Thin panel zones can lead to excessive joint deformation, increased frame drift, and reduced energy dissipation. Panel zone yielding in SMF is a reliable energy dissipation mechanism and is preferred over column yielding.

What is the strong-column weak-beam requirement and why does it use expected strengths?

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 expected plastic moment strengths must be at least 1.2 for SMF, 1.0 for IMF/OMF. The use of expected strengths (Ry x Fy) rather than nominal strengths reflects the reality that steel yield strengths exceed the specified minimum, and beams at full plastic moment can reach moments above the nominal Mp. The penalty for SCWB violation -- a column-sway mechanism -- is potentially catastrophic because drift concentrates at one level rather than distributing across the building height.

What is the difference between R factors for SMF, IMF, and OMF?

ASCE 7-22 Table 12.2-1 assigns R = 8 for SMF (highest ductility, lowest seismic design force), R = 6 for IMF (intermediate ductility), and R = 3.5 for OMF (lowest ductility, highest design force). A higher R factor reduces the design base shear (lower construction cost) but requires significantly more stringent detailing: prequalified connections per AISC 358, tighter beam bracing, continuity plates, column splice overstrength, and panel zone capacity requirements. The choice of frame type balances steel tonnage (R factor) against fabrication complexity (detailing requirements). For low-rise buildings in SDC D or below, OMF may be more economical overall despite heavier sections due to simpler connections.

When are continuity plates required in moment frame connections?

Continuity plates are required at beam flange locations in moment connections when the column flange or web alone cannot resist the beam flange force. Per AISC 341-22 E3.6f, continuity plates must be provided unless the column flange thickness meets tcf ≥ 0.4 x [1.8 x bf x tbf x (Fyb x Ryb) / (Fyc x Ryc)]^0.5 and the column web thickness meets specific criteria for local web yielding, web crippling, and panel zone shear. In practice, most SMF connections require continuity plates except for heavy column sections with thick flanges.

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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 independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) registered in the project jurisdiction. The site operator disclaims all liability for any loss or damage arising from the use of this page or the associated calculator tool. Results are preliminary -- not for construction.