Week 6 – Additional Steel Tonnage

Estimated Time: 4–5 min | PDH/CPD: 0.5 hr | Difficulty: Intermediate

Background: The Sombrio bridge is a two-span composite steel plate girder bridge (40 m + 82 m) launched across a deep ravine in Port Renfrew, BC. Four plate girders were incrementally launched without a temporary pier or launch nose, using precast deck panels as tail counterweight, thickened pier-segment bottom flanges, and crane-assisted final lift. (Parameters simplified for conceptual learning. Allow +25% for cross-frames, connection & splice plates. Girders launched without formwork; ignore wind.)

Engineer's Mind: Quiz 5 determined bottom flange must increase from 65 mm to 76 mm for 81 m cantilever capacity. Quantify material impact for steel erector: estimate additional tonnage for procurement, fabrication planning, and cost. Add 20% contingency for conceptual-phase uncertainties (pier segment length, welding details, material wastage).

Question: Calculate total additional steel required (tonnes) for all four girders to strengthen pier segment by increasing bottom-flange thickness from 65 mm to 76 mm. Pier segment is 30 m long per girder. Include 20% contingency. Given:

• Bottom flange width = 700 mm
• Thickness increase: 65 mm → 76 mm (Δt = 11 mm)
• Pier segment length per girder = 30 m
• Number of girders = 4
• Steel density ρ_s = 7850 kg/m³
•   Contingency factor = 1.20