| Ductile (D) | 5.0 | 1.5 | Full SFRS, seismic zones 4-5 | Pre-qualified or tested RBS/EP | | Moderately Ductile (MD) | 3.5 | 1.4 | SFRS, seismic zones 3-4 | Reduced beam section (RBS) or stiffened | | Limited Ductility (LD) | 2.0 | 1.3 | SFRS, seismic zones 2-3 | Standard moment connection | | Conventional | 1.0 | 1.0 | Non-seismic | Any moment connection |

Note: Ductile MRF (Rd = 5.0) is available per NBCC 2020 for Canadian seismic zones, but requires strict connection pre-qualification per CSA S16 Clause 27.2. Most Canadian practice uses MD (Rd = 3.5) or LD (Rd = 2.0).

Beam Design for MRF

Section Classification

All beam sections in D and MD frames must be Class 1 and laterally braced at the expected plastic hinge locations.

Beam Lateral Bracing

Per CSA S16 Clause 27.2.4.2, the distance from the beam plastic hinge to the nearest lateral brace must not exceed:

L_brace = 1.76 × ry × sqrt(E/Fy)

For W610×125 (ry = 52.1 mm, 350W): L_brace = 42.1 × 52.1 = 2194 mm ≈ 2.2 m

Bracing at the hinge location may be provided by:

• Composite slab (if capable per Clause 17)
• Beam lateral braces (angle or tee brace to adjacent beam)
• Beam bottom flange brace (if beam is composite)

Panel Zone Design

Per CSA S16 Clause 27.2.5, the panel zone (column web region at the beam-column joint) must be checked:

Panel Zone Shear Resistance

Vr_panel = phi × 0.60 × Fy × d_c × t_w × (1 + 3 × b_fc × t_fc^2 / (d_b × d_c × t_w))

Where:

• d_c = column depth (mm)
• t_w = column web thickness (mm)
• b_fc = column flange width (mm)
• t_fc = column flange thickness (mm)
• d_b = beam depth (mm)

Panel Zone Strength Demand

The shear demand in the panel zone is:

Vf_panel = M_f / d_b — V_col

Where M_f = sum of beam moments at the joint and V_col = column shear.

For MD MRF, the panel zone must be designed for:

Design force = 1.25 × Mp_beam1 + Mp_beam2

Doubler Plate Requirements

Continuity Plates

Full-depth continuity plates (stiffeners) are required per CSA S16 Clause 27.2.5.3 when:

1. The column flange thickness t_fc is insufficient for local flange bending
2. Required tp ≥ 0.4 × sqrt(b_f × Fy_beam / Fy_column)

In Canadian practice, continuity plates are provided for all D and MD MRF connections regardless of the calculated requirement, as a conservative measure.

Connection Design

Protected Zone

Per CSA S16 Clause 27.2.8, the connection protected zone extends:

• From the column face: 0.5 × d_b to 1.5 × d_b (where d_b = beam depth)
• Within this zone: No welding of attachments, no studs, no holes except for erection
• Steel surface: Slip-critical connection finish only — no paint in protected zone

Connection Types for MRF

Story Drift Limits

Per NBCC 2020 Clause 4.1.8.13:

The design drift is checked using the NBCC elastic analysis with deflection amplification factor: delta_max = delta_elastic × Rd × Ro / I_E

Worked Example — 6-Storey MD MRF

Given: 6-storey MD MRF (Rd = 3.5, Ro = 1.4). Storey height = 4.0 m. Bay width = 9.0 m. Seismic weight = 4,000 kN per floor. NBCC 2020 seismic, Vancouver (Zone 4).

Step 1 — Beam Design: Assume beam W610×125, 350W. Mp = phi × Zx × Fy = 0.90 × 3,430 × 350 / 10^6 = 1,080 kN·m Expected hinge formation at 0.5-1.5 × d_b from column face: hinge zone = 300-900 mm from face.

At the plastic hinge, the moment gradient reduces the peak moment. With RBS, the flange is reduced by 40-50% at the hinge, forcing the yield location and protecting the weld.

Step 2 — Column Design (Capacity Design): Column axial force from seismic overturning: total base shear V = (S(0.2) × M_v × I_E / (Rd × Ro)) × W

For Vancouver: S(0.2) ≈ 1.0. Approximate: V_base ≈ 4,000 × 6 × 1.0 × 1.0 / (3.5 × 1.4) = 4,898 kN For a 3-bay MRF (4 columns): axial per column ≈ 4,898 × 0.5 × (storey height) / (bay width) = 1,088 kN per storey.

Step 3 — Panel Zone Design: Column: W360×216 (tw = 12.3 mm, bf = 372 mm, tf = 25.9 mm) Beam: W610×125 (db = 612 mm) Mp_beam = 1,080 kN·m each side

Panel zone demand = 2 × 1.25 × 1,080 = 2,700 kN·m Shear = 2,700 × 10^6 / 612 = 4,412 kN (minus column shear)

Panel zone capacity: Vr_panel = 0.90 × 0.60 × 350 × 419 × 12.3 × (1 + 3 × 372 × 25.9^2 / (612 × 419 × 12.3)) / 1000 = 0.90 × 0.60 × 350 × 5154 × (1 + 3 × 372 × 671 / (612 × 419 × 12.3)) / 1000 = 0.90 × 0.60 × 350 × 5154 × (1 + 749,436 / 3,154,724) / 1000 = 0.90 × 0.60 × 350 × 5154 × 1.238 / 1000 = 1,210 kN

Panel zone shear demand (4,412 kN) >> capacity (1,210 kN). Need doubler plate.

Doubler plate: Add 16 mm each side, total effective web thickness = 12.3 + 2 × 16 = 44.3 mm Revised Vr_panel = 1,210 × 44.3/12.3 = 4,359 kN. OK.

Step 4 — Interstorey Drift Check: Elastic drift from analysis: delta_e = 30 mm (first storey) Inelastic drift: delta_max = delta_e × Rd × Ro / I_E = 30 × 3.5 × 1.4 / 1.0 = 147 mm Limit = 2.5% × 4,000 = 100 mm. 147 > 100 mm. NOT OK — increase stiffness.

Need stiffer frame. Options: (a) increase beam size, (b) add braces (hybrid frame), (c) change to LD frame (Rd = 2.0, less drift amplification).

Result: MD MRF for 6-storey building in Vancouver requires stiff columns and beams. Panel zone requires doubler plates. Drift governs — may need LD frame (Rd = 2.0) or hybrid braced-moment frame.

Frequently Asked Questions

What is the difference between D, MD, and LD moment frames in CSA S16? D (ductile, Rd = 5.0) frames use pre-qualified connections with RBS (dogbone) to force plastic hinging away from the column face. MD (moderately ductile, Rd = 3.5) frames permit RBS or stiffened connections. LD (limited ductility, Rd = 2.0) frames use standard moment connections with less ductility requirement. D frames have the most restrictive detailing: protected zones, mandatory continuity plates, strict lateral bracing requirements.

What is the panel zone in a moment frame connection? The panel zone is the region of the column web at the beam-column joint where beam moments transfer through the joint as shear. Per CSA S16 Clause 27.2.5, the panel zone is checked for shear yielding and buckling. If the column web is insufficient, doubler plates (additional web plates welded within the joint region) are added between continuity plates.

What is a reduced beam section (RBS) connection? An RBS (dogbone) connection has the beam flange width reduced by 40-50% at a location 200-300 mm from the column face. This forces the plastic hinge to form at the reduced section rather than at the column face weld. Per CSA S16, the RBS is designed so that the maximum moment at the column face does not exceed Mp of the beam, protecting the weld from fracture. RBS is the most common ductile moment connection in Canadian seismic design.

What is the protected zone in a moment frame? The protected zone is the region of the beam and column within 0.5-1.5 times the beam depth from the column face. Per CSA S16 Clause 27.2.8, no welding of attachments, shear studs, or bolt holes are permitted in this zone. The steel surface must have only a slip-critical coating — no paint or galvanising that could reduce the fracture toughness of the steel at the critical hinge location.

Related Pages

• CSA S16 Braced Frame Design
• CSA S16 Brace Connection Design
• CSA S16 End Plate Connection
• Canadian Seismic Design
• Canadian Beam Design
• Beam Capacity Calculator
• All Canadian References

This page is for educational reference. MRF design per CSA S16:24 Clauses 27.2-27.3 and NBCC 2020. Verify connection pre-qualification and panel zone detailing with project structural engineer. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent PE/SE verification.