Previous Quizzes
Incremental Launch Quiz Series
06 - 14 Jan 2026
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
- Option A more cost-effective by ~$145,000 but throw-away cost
- Option B more cost-effective by ~$145,000, no throw-away costs
- Option A more cost-effective by ~$145,000, provides permanent value
- Option B more cost-effective by ~$145,000 but throw-away costs
Explanation
Calculation:
- Option A: 30 × 6,000 × 1.15 = $207,000 (throw-away)
- Option B: 8.7 × 7,000 = $60,900 (permanent)
- Savings: $207,000 - $60,900 = $146,100 ≈ $145,000
Despite higher unit cost ($7k vs. $6k/tonne), Option B costs far less total because it requires only 29% of material (8.7 vs. 30 tonnes). Girder strengthening at pier (maximum negative moment location) enhances bottom flange stability. Launch nose requires 30 tonnes for 15+ m cantilever extension (structural depth for self-weight, connections, bracing).
Critical timing: Option B is only feasible if pier segment redesign is approved by owner/engineer before fabrication begins. Post-fabrication, changing plate thickness becomes impractical, forcing a launch nose solution despite $145k premium.