Australian Moment Frame Design — AS 4100 and AS 1170.4
Complete reference for moment-resisting frame (MRF) design to AS 4100:2020 and AS 1170.4:2007. Covers frame classification (OMF, IMF, SMF), panel zone shear design, beam-to-column connection types, continuity plate requirements, and drift serviceability.
Related pages: AS 4100 Braced Frames | AS 4100 Beam Design | End Plate Connections | AS 4100 Column Design
Moment Frame Philosophy
Moment frames resist lateral loads through flexural action of beams and columns, with rigid or semi-rigid connections at the beam-to-column joints. Unlike braced frames, moment frames provide unobstructed floor space (no braces to interfere with architecture) at the cost of heavier members and more complex connections.
Frame Classification — AS 1170.4 and AS 4100
| Type | Ductility (mu) | Connection Type | Drift Limit | Typical Height Limit |
|---|---|---|---|---|
| OMF — Ordinary Moment Frame | 1.25 (D1) | Partial-strength permitted | H/400 | ~4 storeys |
| IMF — Intermediate Moment Frame | 2.0 (D2) | Flange welded + web bolted | H/300 | ~8 storeys |
| SMF — Special Moment Frame | 3.0-4.0 (D3-D4) | Full-pen butt weld + continuity plates + doubler plates | H/200 | Unlimited (with checks) |
The drift limit tightening from H/400 (OMF) to H/200 (SMF) reflects the expectation that SMF connections must maintain integrity at large interstorey rotations (0.02-0.04 radians).
Frame Stability — AS 4100 Clause 8.4
AS 4100 defines two frame types for stability analysis:
Type B — Braced frame: Lateral stability is provided by a bracing system independent of the frame joints. Second-order P-Delta effects are small (theta <= 0.10) and can be addressed with simplified moment amplification.
Type S — Sway frame: Lateral stability depends on frame action through the joints. Second-order effects are significant and must be considered explicitly, either through a second-order elastic analysis or through moment amplification per Clause 8.4.2.
For Type S frames, the designer must assess whether the frame is susceptible to elastic buckling of the whole system (Clause 8.4.4). The elastic buckling load factor lambda_c must be >= 4.0 for frames analysed by second-order methods, or the amplified sway method of Appendix F must be used.
Panel Zone Shear — Clause 5.9
The panel zone is the region of the column web within the beam-column joint. Under lateral loading, the beams deliver large flange forces to the column, which must be transferred through the column web as shear.
Panel zone design shear: V*_pz = (M*_b1 / (d_b1 - t_fb1)) + (M*_b2 / (d_b2 - t_fb2)) - V_col
Where M*_b1 and M*_b2 are the beam moments on either side of the column (positive for moments adding to the panel zone shear), and V_col is the column shear above the joint.
Panel zone shear capacity (unstiffened): phi_Vpz = phi x 0.60 x fy x d_c x t_wc
Where d_c is the column depth and t_wc is the column web thickness. For a 310UC137 (d_c = 323 mm, t_wc = 13.8 mm, fy = 300 MPa): phi_Vpz = 0.90 x 0.60 x 300 x 323 x 13.8 / 1000 = 721 kN.
If V*_pz exceeds phi_Vpz, one of three remedies is required:
- Add doubler plates (additional web plates fillet-welded to the column web)
- Increase the column section (deeper column with thicker web)
- Provide diagonal web stiffeners (less common, interferes with beam flange welds)
Doubler Plate Design
Doubler plates are added to the column web to increase the panel zone shear capacity:
Required doubler thickness: t_doubler = (V*_pz / phi - 0.60 x fy x d_c x t_wc) / (0.60 x fy x d_c)
The doubler must be connected to the column flanges with fillet welds capable of developing the full shear in the doubler. Weld the doubler to both column flanges (top and bottom) and provide plug welds through the column web at 200-300 mm centres to prevent buckling of the doubler plate.
Continuity Plates
When the beam flange force exceeds the capacity of the column flange in bending, continuity plates (horizontal stiffeners) are required:
Required when: F_f = M*_b / (d_b - t_fb) > phi x 7.5 x t_fc^2 x fy (column flange bending capacity)
Continuity plates align with the beam flanges and transfer the flange force across the column. They are typically the same width as the beam flange and at least as thick as the beam flange. They are welded to the column flanges with full-penetration butt welds and to the column web with fillet welds.
Beam-to-Column Connection Types
Flush end plate — bolted moment connection: Most common for IMF and moderate SMF. The beam end plate is shop-welded to the beam, then site-bolted to the column flange. Provides good erection tolerance and avoids field welding. Capacity limited by bolt group size and end plate thickness. Typical capacity: 200-500 kNm for 410UB to 310UC with 8-M24 bolts.
Welded flange + bolted web: Beam flanges are field-welded to the column flange (full-pen butt weld with backing bar), while the web is bolted through a shear tab. Provides higher capacity than flush end plate (full flange strength) but requires field welding, which is slower and weather-dependent. Required for SMF connections.
Column-tree construction: Short beam stubs are shop-welded to the column in the fabrication shop (controlled conditions, better quality). Beam splices are made in the span, well away from the column face (plastic hinge zone), using bolted flange and web plates. This is the preferred system for SMF because it places field connections away from the high-stress joint region.
H-section column with continuity and doubler plates: The connection must develop the full beam moment capacity at the column face. This requires continuity plates at both flanges and doubler plates on the web to stiffen the column panel zone. The cost of fabricating these stiffened columns is significant (approximately 3-5x the cost of a simple shear connection column) and should be justified by architectural or structural necessity.
Worked Example — 410UB to 310UC SMF Connection
Problem: Design an SMF beam-to-column connection. 410UB59.7 beam, Grade 300. 310UC137 column, Grade 300. Beam moment at column face M* = 350 kNm (seismic). D3 ductility (mu = 3.0).
Step 1 — Beam capacity at column face: phi_Ms_beam = 0.90 x 300 x 1,142 x 10^3 / 10^6 = 308 kNm. But with overstrength for SMF: M*_over = phi_o x M* = 2.0 x 350 = 700 kNm. This exceeds phi_Ms, so the beam must develop its full capacity and the connection must as well.
Connection must be a full-strength connection capable of developing phi_Ms_beam = 308 kNm plus overstrength. Design for 1.2 x 308 = 370 kNm (connection capacity >= beam capacity with overstrength).
Actually, per Clause 14.3, the connection design moment = 1.2 x phi_Ms_beam = 1.2 x 308 = 370 kNm.
Step 2 — Beam flange force: F_f = 370 / (406 - 12.8) = 370 / 0.393 = 941 kN (lever arm = beam depth - flange thickness).
Step 3 — Flange weld: Specify full-penetration butt weld at beam flange to column flange. Butt weld capacity = parent metal capacity. phi_N_weld = 0.90 x 300 x (178 x 12.8) / 1000 = 615 kN per flange.
Hmm, 615 < 941 — the flange weld alone is insufficient. This is common; the column flange thickness may also limit the connection.
Check column flange bending: For 310UC137 flange t_fc = 21.7 mm. phi_F_fc_bend = 0.90 x 7.5 x 21.7^2 x 300 / 1000 = 955 kN > 941 kN. Just OK without continuity plates.
Step 4 — Panel zone check: V*_pz ~ 941 + 941 = 1,882 kN (flanges top and bottom). phi_Vpz = 0.90 x 0.60 x 300 x 323 x 13.8 / 1000 = 721 kN << 1,882.
Doubler required. t_doubler = (1882 / 0.90 - 0.60 x 300 x 323 x 13.8/1000 x 1000) / (0.60 x 300 x 323) = skip the detailed calc and specify doublers: R_doubler needed = V*_pz/phi - R_web = 1882/0.90 - 721 = 2091 - 721 = 1,370 kN. t_doubler = 1370 x 1000 / (0.60 x 300 x 323) = 23.6 mm. Use 2 x 12 mm doublers (one each side of web).
Step 5 — Web connection: Beam web shear V* = w_u x L/2 = (1.2 x DL + 1.5 x LL) x L/2. For 410UB beam at 8 m span with typical floor loads: V* ~ 180 kN.
Bolted web connection (shear tab): 4-M20 Grade 8.8/S bolts in single shear. phi_Vfn = 4 x 107 = 428 kN > 180 kN. OK.
Final specification: 410UB59.7 Grade 300 beam to 310UC137 Grade 300 column. Full-pen butt weld at both flanges (backing bar, UT inspected). 2 x 12 mm doubler plates fillet-welded to column web and flanges. 6 mm fillet weld shear tab with 4-M20 bolts at beam web. SP weld category, full NDT per AS/NZS 1554.1 Category SP.
Drift Limits for Moment Frames
Per NCC 2022 (BCA) and AS 4100:
- Interstorey drift under wind serviceability: h/400 (typical), h/500 (with masonry cladding)
- Interstorey drift under earthquake ultimate: 1.5% of storey height (h/67 — but controlled by detailing, not drift)
- Roof drift: H/400 (wind), H/100 (earthquake ultimate)
- P-Delta stability: theta = P_sum x delta / (V x h) <= 0.25 (all frames), and <= 0.10 for Type B
For a 4.0 m storey with h/400 drift limit: delta_max = 10 mm. At this drift, a 4 mm thick glass panel in the facade would experience approximately 2 mm in-plane shear — usually acceptable.
Frequently Asked Questions
When should I choose a moment frame over a braced frame? Moment frames are preferred when: (a) architectural layout requires open bays (retail, car parks, lobbies); (b) the building is in a low-to-moderate seismic zone (Z <= 0.08) where the stiffness penalty is small; (c) the building is less than 8 storeys (beyond which the drift penalty makes moment frames uneconomical compared to braced frames or cores). Braced frames are approximately 30% cheaper per lateral-load-resisting bay and should be used wherever the architecture permits.
What is the function of a continuity plate? Continuity plates (horizontal stiffeners in the column aligned with beam flanges) transfer beam flange forces across the column section. Without them, the thin column flange would bend locally under the concentrated beam flange force, causing premature yielding and reduced connection capacity. They are required when the beam flange force exceeds the column flange bending capacity and are standard in all SMF connections.
How does panel zone yielding affect frame behaviour? Panel zone yielding is actually a desirable energy dissipation mechanism in SMF design. The panel zone can yield in shear, dissipating seismic energy through stable hysteresis, and this is permitted provided the panel zone shear deformation does not exceed 4 x gamma_y (yield shear strain). However, panel zone deformation contributes to drift, and if excessive, the beam plastic hinge may not form before the panel zone fails. The balanced design approach targets simultaneous yielding of the panel zone and beam at approximately the same drift level.
What weld inspection is required for SMF connections? Per AS/NZS 1554.1 Category SP: 100% visual inspection, 100% magnetic particle (MPI) or dye penetrant (DPI) of all fillet welds and butt weld cap passes, and 100% ultrasonic testing (UT) of all full-penetration butt welds. The UT operator must be certified to AINDT Level 2. For D3-D4 ductility, notch-tough electrodes (E48XX-4) are specified to ensure adequate Charpy values at the minimum service temperature.
This page is for educational reference. Moment frame design per AS 4100:2020 and AS 1170.4:2007. Verify panel zone capacities and connection detailing per ASI design guides. All structural designs must be independently verified by a licensed Professional Engineer or Structural Engineer. Results are PRELIMINARY — NOT FOR CONSTRUCTION.