Question

Week 10 – Alternative Counterweight: Concrete Lock Blocks

Estimated Time: 6–7 min | PDH/CPD: 1.0 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 9 showed 19 precast deck panels provide needed counterweight. However, contractor considers alternative: concrete lock blocks. Lock blocks offer operational advantages—easier handling, quicker maneuvering as launch progresses and reusable on other projects post-launch. Trade-off: many more units needed since each weigh less than deck panel. Determine equivalent lock blocks required.

Question: If contractor uses concrete lock blocks instead of 19 precast deck panels (Quiz 9), how many blocks required for equivalent counterweight capacity? Blocks placed at girder tail, over a 6m length. Given:

• Required restoring moment: 95,372 kN·m (from Quiz 9)
• Each block: 0.75 m × 0.75 m × 1.5 m, ρ_c = 2400 kg/m³
• Load factor: α_D = 0.90 (restoring)