Question

Week 7 – Launch Strategy: Cost-Benefit Decision

Estimated Time: 5–6 min | PDH/CPD: 0.75 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: Two viable technical solutions exist: strengthen pier segment with 8.7 additional tonnes permanent steel (Quiz 6) or fabricate temporary 15 m launch nose. Both achieve required cantilever capacity. This classic value engineering decision weighs costs, schedule impacts, and long-term value. Launch nose is "throw-away" cost—zero value post-launch to the project.

Question: Compare launching strategies. Which option provides better overall value considering total installed costs? Given:

·        Option A: Launch Nose (Temporary)

o   Weight: 30 tonnes

o   Unit cost: $6,000/tonne (includes fabrication, transportation, installation, removal)

o   Contingency: 15%

o   Residual value: $0 (throw-away unless usable on other projects)

·        Option B: Strengthen Pier Segment (Permanent)

o   Additional steel: 8.7 tonnes (from Quiz 6)

o   Unit cost: $7,000/tonne (includes fabrication, installation as part of main girder contract)

o   Residual value: Enhanced girder strength safety margin