Previous Quizzes
Incremental Launch Quiz Series
20 - 26 Nov 2025
Week 2 – Cantilever Moment
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: With baseline section strength established (Quiz 1), determine the flexural demand if girders cantilever nearly the full long span.
Question: Estimate the girder cantilever moment at the pier roller support at an 80 m long cantilever. Calculate weight based on the typical section:
• Web = 2750 mm × 16 mm
• Flanges (top and bottom) = 600 mm × 38 mm
• Add 25% for cross-frames, diaphragms, connection & splice plates
• Steel density = 7850 kg/m³
- 17.5 MN·m
- 27.6 MN·m
- 24.5 MN·m
- 21.6 MN·m
Explanation
1. Area (web), A_web = 2.75 × 0.016 = 0.0440 m²
2. Area (flanges), A_flanges = 2 × (0.600 × 0.038) = 0.0456 m²
3. Area (total), A_total = 0.0896 m²
4. Mass (m) = 0.0896 × 7850 = 703.4 kg/m
5. Weight (steel), w_steel = 703.4 × 9.81 = 6.90 kN/m
6. Weight (total), w_total = 6.90 × 1.25 = 8.62 kN/m
7. Moment M = 8.62 × 80² / 2 = 27.6 MN·m
This unfactored dead load moment (27.6 MN·m) is less than the pier section capacity (52 MN·m from Quiz 1), indicating that there is sufficient capacity. However, but girder stability effects including Lateral Torsional Buckling must be checked (Quiz 3), and load and resistance factors applied for design verification.