Pedestrian Bridge Stringer Replacement in Vancouver, BC

Case Study 2: Pedestrian Bridge Stringer Replacement in Vancouver, BC

Scenario A 45-year-old timber-decked pedestrian bridge over a creek in Stanley Park, Vancouver, requires replacement of its primary steel stringers due to corrosion-induced section loss. Environmental constraints: high humidity, marine aerosol exposure, and freeze-thaw cycling. Design must comply with CSA S16-19 and CAN/CSA-S6-14 (bridge loading), with strict durability requirements — galvanized or weathering steel mandatory. Space is constrained: existing abutments limit depth to ≤400 mm; aesthetic integration with heritage timber deck is required.

Given Data

• Span Length: 8.2 m (measured clear span between existing concrete abutments)
• Uniform Load: 12.8 kN/m (CSA S6-14 pedestrian live load 5 kPa × 1.2 m deck width + self-weight estimate 7.8 kN/m)
• Material Yield Strength: 345 MPa (ASTM A588 Grade C weathering steel, selected for corrosion resistance)
• Modulus of Elasticity: 200 GPa (adjusted for weathering steel per ASTM standards)
• Allowable Deflection: 15 mm (L/550 per CSA S6-14 for pedestrian bridges to prevent discomfort and railing misalignment)

Calculation Inputting values into the Structural Beam Calculator:

• Maximum Bending Moment = wL²/8 = (12.8 kN/m × 8.2² m²) / 8 = 135.4 kNm
• Required Section Modulus = M / σ_y = (135.4 × 10⁶ N·mm) / 345 N/mm² = 392,500 mm³ = 392.50 cm³
• Maximum Shear Force = wL/2 = (12.8 × 8.2) / 2 = 52.5 kN
• Calculated Deflection (for candidate W310×60, S = 547 cm³, I = 8480 cm⁴): δ = (5 × 12.8 × 8200⁴) / (384 × 200,000 × 8480 × 10⁴) ≈ 13.7 mm < 15 mm ✅

Result and Decision W310×60 (305 mm depth, 165 mm flange) met all criteria: S = 547 cm³ > 392.5 cm³, VRd = 210 kN > 52.5 kN, δ = 13.7 mm < 15 mm. Its compact depth preserved clearance under the historic timber handrail. Fabricated from ASTM A588 Gr.C, hot-dip galvanized after fabrication for redundant protection. Installed with elastomeric bearings to accommodate thermal movement and isolate vibrations.

Lesson Material-specific properties — especially reduced modulus of elasticity in weathering steels and stricter deflection limits for dynamic pedestrian loads — must be explicitly entered. Using default values (e.g., E = 210 GPa) for non-standard materials introduces non-conservative error; always validate input parameters against material specifications and regional codes.