Previous Quizzes
Winner of the Quiz
Himanshu Sawhney
Force Structural Engineers pvt ltd
Incremental Launch Quiz Series
13 - 19 Nov 2025
Week 1 - Girder Flexural Capacity (No LTB)
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)
Engineer's Mind: Before analyzing stability or sequencing, establish baseline section strength at the pier—the elastic flexural capacity in negative moment, assuming lateral-torsional buckling (LTB) doesn't govern.
Question: Estimate the elastic moment capacity of the girder pier segment (ignore LTB and resistance factors). Use the following section for the pier segment:
• Web = 2750 mm × 16 mm
• Flanges (top and bottom) = 700 mm × 65 mm
• F_y = 350 MPa
- 42 MN·m
- 48 MN·m
- 58 MN·m
- 52 MN·m
Explanation
This theoretical elastic capacity ignores Lateral Torsional Buckling (LTB) and resistance factors. The actual factored resistance (per CSA S6-19 Clause 10.5.7 requires φ = 0.90 and consideration of stability effects, addressed in subsequent quizzes.
Calculation:
- Flange area = 0.700 × 0.065 = 0.0455 m²
- Distance to Neutral Axis (NA): d = 1.375 + 0.0325 = 1.4075 m
- Moment of interia (web), I_web = (1/12) × 0.016 × 2.75³ = 0.0277 m⁴
- Moment of intertia (flanges), I_flanges = 2 × (0.0455 × 1.4075²) = 0.1802 m⁴
- Moment of intertia (total), I_total = 0.0277 + 0.1802 = 0.2079 m⁴
- Section modulus, S = 0.2079 / 1.4075 = 0.1477 m³
- Flexural capacity (strong axis), M_y = 350,000 × 0.1477 = 51.7 MN·m ≈ 52 MN·m
Learning Resources:
- CSA S6-19 – Flexural Resistance
- CISC Handbook of Steel Construction – Section Properties