NBCC 2020 Seismic Hazard Framework

Seismic Hazard Values

NBCC 2020 provides seismic hazard values based on the 5% damped 5th percentile spectral response acceleration for a 2% probability of exceedance in 50 years (approximately 1-in-2,475 year return period):

Site Classification

NBCC 2020 defines six site classes (A through F) based on average shear wave velocity in the top 30 m (V_s30):

Most Canadian cities on glacial till (Toronto, Calgary, Winnipeg) have Site Class C or D. Vancouver's downtown peninsula is on glacial till with bedrock near surface (Class B/C), while the Fraser River delta (Richmond, Delta) can be Class E requiring site-specific response analysis.

Seismic Hazard by Canadian City

Design Spectrum and Base Shear

Design Response Spectrum

The NBCC 2020 design spectrum is constructed from the site-adjusted spectral accelerations:

S(T) = F_a × S_a(0.2) for T ≤ 0.2 s S(T) = F_v × S_a(0.5) for T = 0.5 s (and interpolated in between) S(T) = F_v × S_a(1.0) for T = 1.0 s S(T) = F_v × S_a(2.0) for T = 2.0 s S(T) ∝ 1/T² for T > 2.0 s

Where F_a and F_v are site coefficients from NBCC 2020 Tables 4.1.8.4 and 4.1.8.5.

Minimum Base Shear

V = S(T_a) × M_v × I_E × W / (R_d × R_o)

But not less than: V_min = S(2.0) × M_v × I_E × W / (R_d × R_o) (for T_a > 0.5 s — typically governs for steel buildings) And not more than: V_max = S(0.2) × I_E × W / (R_d × R_o) (for buildings taller than 60 m)

Where:

CSA S16 Clause 27 — Seismic Force Resisting Systems

CSA S16:24 Clause 27 classifies steel Seismic Force Resisting Systems (SFRS) into four ductility levels:

Ductility Levels

The choice of ductility level is an economic decision. Ductile (R_d = 5.0) provides the lowest design base shear but requires the most expensive detailing and member sizing. Conventional Construction (R_d = 1.5) uses simpler detailing but higher design forces. For low-seismic zones (Toronto, Calgary), Conventional Construction or Limited-Ductility is typically cost-optimal. For high-seismic zones (Vancouver, Montreal), Ductile or Moderately Ductile systems are generally required.

SFRS Types and Applicable Ductility Levels

Ductility Detailing Requirements

Ductile Moment Frames (D-MF, R_d = 5.0)

CSA S16 Clause 27.3 specifies:

• Protected zones: The plastic hinge region (typically 1.5× d from each beam-to-column joint) must have no welds, holes, or attachments other than the beam-to-column connection.
• Beam slenderness: h/w ≤ 440/sqrt(F_y) — ensures compact section for stable hinge formation.
• Column slenderness: K×L/r ≤ 60 (minimises P-Delta effects and ensures column remains elastic).
• Column-beam moment ratio: ΣM_c / ΣM_b ≥ 1.3 (strong-column/weak-beam principle).
• Lateral bracing: Beams must be braced at 2× d maximum within the protected zone.
• Beam-to-column connections: Must satisfy the AISC 341 / CSA S16 cyclic qualification protocol. Reduced Beam Section (RBS) and bolted flange plate (BFP) connections are common.
• Panel zone shear: Must be checked for the full plastic hinge moment. Panel zone yielding is permitted if it does not exceed 5% story drift.

Moderately Ductile Concentric Braced Frames (MD-CBF, R_d = 4.0)

CSA S16 Clause 27.5 specifies:

• Brace slenderness: K×L/r ≤ 200 (limits brace buckling eccentricity).
• Brace compactness: b/t and h/w limits ensure inelastic buckling capacity without local buckling.
• Balance of strengths: Brace tensile capacity must not exceed the compression capacity by more than 1.3× (ensures compression governs before tension yield).
• Gusset plate design: Must allow for out-of-plane rotation — Whitmore section check, free-edge restraint, and 2× t_p clearance from the end of the brace to the work point.
• Column design: Columns must be designed for the maximum expected tension load from the brace (1.1 × R_y × brace capacity) — capacity design principle.
• Brace connections: Designed for 1.1 × R_y × A_g × F_y (tension) or the brace compression capacity (whichever governs).

Buckling-Restrained Braced Frames (BRBF, R_d = 5.0)

CSA S16 Clause 27.6 specifies:

• Core steel: A36 or equivalent — low-yield steel (LYS) for stable hysteresis.
• Encasing: Concrete or steel tube providing buckling restraint; must maintain a gap ≤ 2 mm to accommodate Poisson expansion.
• Connection: Brace-to-gusset connections designed for 1.1 × R_y × A_core × F_y_core in both tension and compression.
• Lateral bracing of gusset: Gusset plates must have out-of-plane lateral support at 2× t_g from the beam-to-column joint.
• Maximum compression overstrength: Brace compression capacity must not exceed 1.3 × the tensile capacity at 2% story drift.

Steel Plate Shear Walls (SPSW, R_d = 5.0)

CSA S16 Clause 27.7 specifies:

• Web plate: Minimum thickness 5 mm, h/t ≤ 1,500/sqrt(F_y). Must yield before the boundary elements.
• Boundary elements: Columns (vertical boundary elements — VBEs) and beams (horizontal boundary elements — HBEs) designed per capacity design at 1.1 × R_y times the tension field yield of the plate.
• Tension field anchor: The boundary frame must resist the tension field forces from the web plate per the strip model.
• Fish plate: The infill plate is welded to a fish plate (shop-welded to the boundary frame) with a fillet weld designed for the plate yield capacity.

NBCC Seismic Design Procedure (Simplified)

Step 1: Determine Seismic Hazard

From NBCC 2020 Appendix C or the NBCC seismic tool (available from NRC/CCBFC):

• S_a(0.2), S_a(0.5), S_a(1.0), S_a(2.0) for the site coordinates
• Site class from geotechnical investigation (Class C or D if unknown)

Step 2: Select SFRS and Ductility Level

Choose the SFRS type (moment frame, braced frame, shear wall) and ductility level (D, MD, MF, BD) based on:

• Seismicity level (high: use D or MD; low: MF or BD may suffice)
• Building height (taller buildings need stiffer systems — braced frames often govern)
• Architectural constraints (moment frames allow open facades)
• Height limits per NBCC Table 4.1.8.9

Step 3: Calculate Design Base Shear

V = S(T_a) × M_v × I_E × W / (R_d × R_o)

With minimum V as applicable for T_a > 0.5 s.

Step 4: Distribute Lateral Forces

F_x = (V - F_t) × (W_x × h_x) / Σ(W_i × h_i)

Where F_t = 0 if T_a ≤ 0.7 s, else F_t = 0.07 × T_a × V ≤ 0.25 × V.

Step 5: Design and Detail Per CSA S16 Clause 27

Each SFRS must be designed and detailed per the relevant Clause 27 subsections including:

• Member capacity design (beef up columns and connections)
• Protected zones (no field welding or attachments in hinge zones)
• Brace and connection compactness limits
• Column slenderness limits
• Diaphragm force transfer requirements (Cl. 27.13)
• Inter-storey drift limits (Cl. 27.10)

Step 6: Verify Drift Limits

• Inter-storey drift ≤ 0.025 × h_s for post-disaster buildings (I_E > 1.0)
• Inter-storey drift ≤ 0.020 × h_s for other buildings
• Drift computed using R_d × R_o applied to the elastic displacement (Δ_max = Δ_elastic × R_d × R_o / I_E)

Ductility Level Comparison

Note: Base shear factor comparison assumes S(T_a) = S(0.2) = 0.94g (Vancouver), I_E = 1.0, M_v = 1.0. Actual values depend on building period and site class.

Related Pages

• Canada CSA S16 Steel Design Guide — Full CSA S16 design reference
• CSA S16 Load Combinations — NBCC ULS & SLS — Canadian load combination guide
• Canadian Wind Load — NBCC & CSA S16 Reference — Wind load calculation guide
• Canadian Snow Load — NBCC Ground & Roof Loads — Snow load calculation guide
• Canadian Steel Beam Sizes — W Shapes, HSS — Complete section tables
• CSA S16 Beam Design — Flexure, LTB & Shear — Beam design per CSA S16
• Seismic Load Calculator — Free seismic load calculator

Frequently Asked Questions

What ductility level should I use for a steel building in Vancouver?

For Vancouver (high seismicity, S_a(0.2) ≈ 0.94g), Ductile (R_d = 5.0) or Moderately Ductile (R_d = 4.0) systems are typically required. Ductile moment frames (D-MF, R_d = 5.0) provide the lowest base shear but require stringent detailing (protected zones, reduced beam sections, 0.04 rad joint rotation capacity). Buckling-restrained braced frames (BRBF, R_d = 5.0) are also common — they provide high ductility with simpler framing but add fabrication cost for the brace assembly. Moderately Ductile concentrically braced frames (MD-CBF, R_d = 4.0) are cost-effective for mid-rise buildings up to 60 m. Limited-Ductility (R_d = 3.0) systems may be used for buildings under 60 m but the higher design forces typically make them uneconomical in high-seismic zones.

What is the strong-column/weak-beam principle in CSA S16?

The strong-column/weak-beam principle (CSA S16 Clause 27.3.3.2) requires that columns in a ductile moment frame have at least 1.3 times the flexural strength of the beams framing into them at any joint: ΣM_c / ΣM_b ≥ 1.3. This ensures plastic hinges form in the beams (controlled, ductile response) rather than in the columns (could lead to a soft-storey collapse mechanism). The column moment capacity is calculated at the axial load level from the seismic combination including the gravity load. For columns with axial load exceeding 0.3 × C_r (compression resistance), the moment capacity is reduced per the beam-column interaction equation.

How are gusset plates designed for ductile braced frames?

CSA S16 Clause 27.5.4.3 requires gusset plates in MD-CBFs to accommodate 0.025 rad of out-of-plane rotation during brace buckling. This is achieved by providing a 2 × t_p clearance from the end of the brace to the nearest fold line of the gusset plate (the "2t" rule). The gusset plate must be checked for: (1) Whitmore section yielding at brace tension capacity (×1.1 × R_y); (2) Block shear at brace compression capacity; (3) Buckling from the brace compression (K×L/r limit); (4) Connection eccentricity per Cl. 20.5.2. Gusset plates are typically 10-20 mm thick for low-rise frames and 20-35 mm for high-ductility frames in high-seismic zones.

What are the drift limits for Canadian steel seismic design?

NBCC 2020 Clause 4.1.8.13 limits inter-storey drift to 2.5% of storey height (0.025 × h_s) for post-disaster buildings and 2.0% (0.020 × h_s) for all other buildings. The drift is computed as the inelastic lateral displacement, which is the elastic displacement from the design seismic forces multiplied by R_d × R_o / I_E. For a 4 m storey height: maximum drift = 0.02 × 4,000 = 80 mm. Second-order effects (P-Delta) must be considered when the inter-storey drift exceeds 0.01 × h_s per CSA S16 Cl. 27.10. P-Delta amplifies the base shear and member forces — if the P-Delta factor exceeds 1.4, the building is considered unstable and must be stiffened.

This page is for educational reference. Seismic provisions per NBCC 2020 Division B Clause 4.1.8 and CSA S16:24 Clause 27. Verify seismic hazard values against current NBCC Appendix C for the specific site coordinates. Provincial building codes may have amendments. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent P.Eng. verification.

Design Resources

Calculator tools

• Seismic Load Calculator
• Steel Brace Frame Calculator
• Steel Moment Frame Calculator
• Steel Shear Wall Calculator

Design guides

• Seismic Load Worked Example