Question

Week 12 – Crane Capacity for Final Lift

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: With ~1.0 m tip deflection (Quiz 11), crane lifts tip at 81 m position and holds while girders are pushed final 1 m to 82 m span. System becomes two-span continuous beam: 40 m (launch abutment to pier) + 82 m (pier to crane at far abutment). Using simple tributary method, crane supports approximately half the far span. Calculate end reaction to determine required crane capacity. Simplified rating: (Load × Radius) / 2.5.

Question: Estimate required crane size for final lift where the system transforms to two-span continuous beam: 40 m (launch abutment to pier) + 82 m (pier to far abutment with crane support). Use simple tributary method to determine girder tip load: crane supports half of 82 m span. Given:

• System weight: w = 28.52 kN/m
• Girder tip load: R = w × (L₂/2)
• Rigging weight (hook, slings, spreaders): 5 tonnes
• Crane rating formula: (Load × Radius) / 2.5
• Lift radius: r = 6 m