Canadian Steel Bridge — CSA S6 CHBDC Design Reference

Complete reference for CSA S6:19 Canadian Highway Bridge Design Code (CHBDC) steel provisions (Section 10 — Steel Structures). Covers steel girder design for I-girders and box girders, fatigue life assessment per Clause 10.17, orthotropic steel decks, bearing stiffeners, bridge classification by importance, and a comparison of highway vs pedestrian bridge loading. Includes worked examples.

Quick access: CSA S16 Guide | Canadian Steel Beam Sizes | Canadian Steel Grades

CSA S6 Canadian Highway Bridge Design Code — Overview

CSA S6:19 (Canadian Highway Bridge Design Code, also known as CHBDC) is the governing standard for bridge design in Canada. Section 10 covers steel structures, applicable to highway bridges, pedestrian bridges, and railway bridges. Bridges in Canada are classified by importance category per CSA S6 Clause 1.2.

Bridge Importance Categories

CSA S6 Section 10 — Steel Structures Content

Steel Girder Design — I-Girders (CSA S6 Clause 10.4)

Steel I-girders are the most common superstructure type for Canadian highway bridges in the 20-60 m span range. The sections are typically built-up plate girders (not rolled W-shapes) with dimensions optimised for the specific bridge geometry.

Proportioning Limits

Web slenderness: d/w ≤ 200 (unreinforced), d/w ≤ 300 (with stiffeners)
Flange slenderness: bf / (2 × tf) ≤ 200 / √(Fy) (compact)
Overall depth: L/25 (simply supported) to L/30 (continuous) typical
Flange width: bf ≥ L/85 for lateral torsional stability

Flexural Resistance (Positive Moment)

For compact sections in positive bending:

Mr = φ × Fy × Zx

where:
  φ = 0.95 (resistance factor for flexure in CSA S6 — different from CSA S16 φ = 0.90)
  Fy = yield strength of steel (MPa)
  Zx = plastic section modulus (mm³)

The φ = 0.95 value is specific to CSA S6 and is higher than CSA S16 φ = 0.90, reflecting the stricter quality control requirements for bridge fabrication and the lower consequence of overload for the larger bridge section.

Flexural Resistance (Negative Moment)

In negative bending regions (over supports of continuous girders), the compression flange must be braced against lateral-torsional buckling:

Mr = φ × Fy × Zeff

where Zeff is the effective section modulus reduced for:
- Slender web effects (web bend buckling per Clause 10.4.3)
- Lateral-torsional buckling of the compression flange (Clause 10.4.4)
- Hybrid girder effects when flanges and web have different strengths

Shear Design (CSA S6 Clause 10.5)

I-girder web shear resistance depends on the web slenderness and stiffener layout. The CSA S6 shear provisions are closely aligned with the AASHTO LRFD Bridge Design Specifications.

Web Shear Resistance

For webs with transverse stiffeners:

Vr = φ × C × 0.60 × Fy × d × tw × (1 + 0.87 × (1 - C) / (1 + (a/d)²))

where:
  φ = 0.95 (resistance factor for shear)
  C = ratio of shear buckling strength to shear yield strength
  a = stiffener spacing (mm)
  d = web depth (mm)
  tw = web thickness (mm)

The tension field action term (0.87 × (1 - C) / (1 + (a/d)²)) accounts for the post-buckling shear resistance of the web panel. This tension field is only mobilised when the stiffeners are proportioned to anchor the diagonal tension field.

Bearing Stiffeners

Bearing stiffeners are required at supports and concentrated load points:

Width: b_stiffener ≥ bf_flange / 2
Thickness: t_stiffener ≥ b_stiffener × √(Fy / (3 × E))
Tolerance: stiffener-to-flange contact fit

Bearing check (Clause 10.5.6):
Br = φ × 0.66 × Fy × A_bearing

Fatigue Design (CSA S6 Clause 10.17)

Fatigue is a critical design consideration for steel bridges, particularly at welded details. CSA S6 Clause 10.17 follows the AASHTO fatigue categories A through E, with stress range as the governing parameter.

Fatigue Life Calculation

Fatigue life: N = (A / Δf)³ × 10⁶

where:
  N = number of cycles to failure
  A = fatigue constant (depends on detail category)
  Δf = stress range (MPa)

Detail Categories

Fatigue Load

CSA S6 uses the fatigue truck (Clause 3.8.4) for the fatigue limit state — a single design truck with 80% of the CL-625 truck weight, applied at specified load cycles. The number of cycles depends on the ADTT (average daily truck traffic):

N_cycles = ADTT × 365 × 75 years (design life) × growth factor

Predominant details in Canadian steel bridge girders — stiffener-to-web fillet welds (Category C or C'), flange-to-web fillet welds (Category B), and field splice cover plates (Category D or E) — must be checked individually.

Orthotropic Steel Decks (CSA S6 Clause 10.11)

Orthotropic steel decks consist of a steel deck plate stiffened by longitudinal ribs and supported by transverse floor beams. They are lighter than concrete decks and are used in long-span bridges where self-weight is critical.

Deck Plate

Minimum thickness: 12 mm
Design for tyre contact pressure:
t_deck ≥ a × √(3 × γ_p × P / (φ × Fy × w_eff))

where:
  a = rib spacing (mm)
  γ_p = load factor for fatigue
  P = wheel load
  w_eff = effective width of deck plate (mm)

Rib Types

Highway vs Pedestrian Bridge Loading

Bridge Steel Grades — CSA S6 Material Requirements

Grade 350W is the standard steel grade for Canadian bridge girders. Grade 350A (weathering steel) is specified for uncoated bridges in non-aggressive environments.

Charpy Requirements for Bridges

Charpy testing to CSA G40.21 is mandatory for all bridge steel per CSA S6 Clause 10.2.2. For fracture-critical members (tension components whose failure would cause collapse), Charpy testing at the design minimum temperature plus 15°C margin is required.

Worked Example — I-Girder Positive Moment Check

Problem: Check a simply-supported I-girder for a 45 m span highway bridge. The girder is built-up, Grade 350W. Section properties: d = 1,800 mm (web), tw = 14 mm, bf = 500 mm (flange), tf = 40 mm (flange). Maximum factored positive moment Mf = 28,500 kN·m. Bridge importance: Normal (IE = 1.00).

Section Properties

Web: h = 1,720 mm (clear depth = 1,800 - 2 × 40), w = 14 mm
Flanges: bf = 500 mm, tf = 40 mm

Ag = 2 × (500 × 40) + 1,720 × 14 = 40,000 + 24,080 = 64,080 mm²

Ixx = 2 × (500 × 40³/12 + 500 × 40 × 920²) + 14 × 1,720³/12
Ixx = 2 × (267,000 + 16,928,000) + 5,933,000
Ixx = 2 × 17,195,000 + 5,933,000 = 40,323,000 × 10³ mm⁴

Zx = 2 × (500 × 40 × 920) + 14 × 1,720²/4
Zx = 2 × 18,400,000 + 10,354,400 = 47,154,400 mm³ = 47.15 × 10⁶ mm³

Sx = I / (d/2) = 40,323 × 10⁶ / 900 = 44,803 × 10³ mm³

Step 1 — Section Classification (CSA S6 Clause 10.4.2)

Flange: bf / (2 × tf) = 500 / (2 × 40) = 6.25
Limit for compact: 200 / √350 = 10.69 → 6.25 < 10.69 → Compact ✓

Web: 2 × d / tw = 2 × 1,720 / 14 = 246
For plastic design (compact): 2 × d / tw ≤ 150 → 246 > 150 → Non-compact web

The web is non-compact. Effective section modulus Sxe must be calculated per Clause 10.4.3.

Step 2 — Flexural Resistance

For the non-compact web, the moment resistance uses the effective section modulus limited by web bend buckling:

R_b = 1 - 1,200 / (2 × d / tw) × (a_r / (3 + a_r))

where a_r = 2 × bf × tf / (h × tw) = 2 × 500 × 40 / (1,720 × 14) = 1.66

R_b = 1 - 1,200 / 246 × (1.66 / (3 + 1.66))
R_b = 1 - 4.88 × 0.356
R_b = 1 - 1.737 = -0.737 → use R_b = 0.95 (reduction factor limit)

Sxe = R_b × Sx = 0.95 × 44,803 × 10³ = 42,563 × 10³ mm³

Lateral-torsional buckling reduction (assume L_b = 7.5 m cross-frame spacing):

R_h = 1.0 (homogeneous girder — same grade for web and flange)

Fe = C_b × π² × E / (L_b / rt)²
rt = bf / √(12 × (1 + h × tw / (6 × bf × tf)))
rt = 500 / √(12 × (1 + 1,720 × 14 / (6 × 500 × 40)))
rt = 500 / √(12 × (1 + 0.201)) = 500 / 3.80 = 131.7 mm

Fe = 1.0 × π² × 200,000 / (7,500 / 131.7)²
Fe = 1.0 × 1,974,000 / (56.95)²
Fe = 1,974,000 / 3,243 = 609 MPa

λ_f = √(Fy / Fe) = √(350 / 609) = 0.758

LTB reduction factor R_ltb = 1.0 for λ_f < 0.8 → No LTB reduction at L_b = 7.5 m

Step 3 — Factored Moment Resistance

Mr = φ × Fy × Sxe × R_ltb = 0.95 × 350 × 42,563 × 10³ × 1.0
Mr = 14,152 × 10⁶ N·mm = 14,152 kN·m

Mf = 28,500 kN·m > Mr = 14,152 kN·m → NOT OK

The girder is significantly overstressed. Options:

• Increase flange thickness to 60 mm
• Increase flange width to 600 mm
• Add intermediate cross-frames at closer spacing (L_b = 3.5 m)
• Change to continuous girder over multiple spans
• Consider hybrid girder with higher-grade flanges (400W)

Revised Design — Try tf = 60 mm, bf = 550 mm

Zx = 2 × (550 × 60 × 930) + 10,354,400 = 61,380,000 + 10,354,400 = 71,734,400 mm³
Sx = recalculated with new Ixx: approximately 65,000 × 10³ mm³
Sxe ≈ 0.95 × 65,000 × 10³ = 61,750 × 10³ mm³

Mr = 0.95 × 350 × 61,750 × 10³ × 1.0 = 20,532 kN·m → still not adequate

The 45 m span with simply supported girders requires a deeper section. Increase girder depth to d = 2,200 mm or add an intermediate support to create a two-span continuous girder.

Bridge Deck Types — Comparison

Frequently Asked Questions

What is the difference between CSA S6 and CSA S16 for steel bridge design? CSA S6:19 (Canadian Highway Bridge Design Code) governs bridge design, while CSA S16:19 governs building design. Key differences include: CSA S6 uses φ = 0.95 for flexure (vs φ = 0.90 in CSA S16), has more stringent Charpy requirements for fracture-critical members, includes fatigue design provisions (Clause 10.17) that do not apply to building structures, and references CL-625 design trucks instead of building floor loads. Bridges also require 75-year design life vs 50-year for buildings.

What steel grades are used for Canadian steel bridges? Grade 350W (Fy = 350 MPa) is the standard bridge steel grade per CSA S6 Clause 10.2. Grade 350A (weathering steel) is used for uncoated bridges in non-aggressive environments. Grade 400W and 480W are used for high-stress applications (long-span girders, arch ribs). All bridge steel must meet CSA G40.21 Charpy requirements, with 350WT (Charpy tested at -20°C or -45°C) specified depending on the location climate zone.

How is fatigue checked for steel bridge details under CSA S6? Fatigue per CSA S6 Clause 10.17 uses the stress range method. Each detail (stiffener-to-web weld, flange splice, cover plate) is assigned a fatigue category (A through E') based on its geometry and weld detail. The stress range at the detail under the fatigue truck (80% of CL-625) must be below the constant amplitude fatigue limit for that category over the 75-year design life. The fatigue truck is applied at a number of cycles determined from the ADTT.

What is the typical span range for steel I-girder bridges in Canada? Steel I-girder bridges in Canada are most economical for spans of 20-60 m. For spans below 20 m, precast concrete girders are typically more economical. For spans above 60 m, steel box girders, trusses, or segmental concrete become competitive. Canadian provinces set standard girder depths for their typical span ranges: Ontario uses standard I-girders up to 50 m, while British Columbia uses plate girders up to 65 m for highway overpasses.

Related Pages

• Canada CSA S16 Guide — Full CSA S16:19 steel design reference
• Canadian Steel Beam Sizes — W, WWF, HSS sections per CISC
• Canadian Steel Grades — G40.21 300W to 480W
• CSA S16 Beam Design — Flexure, LTB, shear checks
• CSA S16 Connection Design — Bolted and welded connections
• Fillet Weld Size Chart — Weld capacities per CSA W59
• CSA S6 Seismic Design Reference — Seismic design per NBCC
• Load Combinations Calculator — Canadian load case calculator

This page is for educational reference. CSA S6:19 bridge design must comply with the current edition of CSA S6, CSA G40.21, CSA W59, and NBCC 2020. Steel bridge design requires specialist knowledge of fatigue, fracture control, and fabrication quality assurance. All results are PRELIMINARY — NOT FOR CONSTRUCTION without independent verification by a licensed Professional Engineer (P.Eng.).