Week 9 – Counterweight Requirement

Estimated Time: 7–8 min | PDH/CPD: 1.25 hr | Difficulty: Advanced

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 8 determined girders cantilever only 56 m without counterweight—25 m short of 81 m target. Solution: add counterweight at tail. The possibility of using the precast concrete deck panels serves dual purposes: provide ballast during launching and become permanent bridge components. Panels shared across all four girders (placed on top). Calculate the number of panels needed for 81 m cantilever.

Question: Determine number of precast deck panels (shared across all 4 girders) required to achieve 81 m cantilever. Panels stacked at tail end.

Given:

·        Target cantilever: x = 81 m

·        Total girder length: 122 m → Tail length: 41 m

·        Girder weight: w = 7.13 kN/m per girder (Quiz 8)

·        Each panel: 10 m × 3 m × 0.200 m, ρ_c = 2400 kg/m³

·        Panel CG: 1.5 m inward from the girder tail end

·        Load factors: α_D = 1.25 (driving), α_D = 0.90 (restoring)

·        Equilibrium: Restoring Moment (M_R) = Driving Moment (M_D)