Tandem Lift of a Precast Beam With Unequal Load Share
Two mobile cranes are to lift an 18 t precast beam that is 20 m long. Crane A is at 9 m radius and has a rated capacity of 12.5 t there. Crane B is at 15 m radius and has a rated capacity of 8.0 t there. The beam's centre of gravity is 8 m from the A end and 12 m from the B end. The slings are vertical and the rigging is symmetrical at each end. There is no tandem lift plan; the site engineer says the load shares are 'about half each' and the banksman proposes to lift slowly and watch the load. Wind is 8 m/s with a light gust, and the beam has a significant side area. During a trial lift, Crane B's load indicator reads 8.5 t while Crane A reads 10.2 t. The beam is still only 300 mm above the ground.
1. Calculate the static load share and compare it with the crane capacities. Does the calculation make the lift acceptable?
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The static share is found from the centre of gravity. Crane A is 8 m from the centre of gravity and 12 m from the B end; Crane B is 12 m from the centre of gravity. Crane A takes 18 × 12/20 = 10.8 t, and Crane B takes 18 × 8/20 = 7.2 t. Against capacity, Crane A is at 10.8/12.5 = 86%, and Crane B is at 7.2/8.0 = 90%. On paper both are below capacity, but the margins are small. The calculation does not make the lift acceptable because it is static and assumes rigid, level support and equal rigging geometry. In reality the cranes have different boom lengths and stiffness, so the load share changes as one boom deflects more. Wind and the beam's side area add dynamic load. A tandem lift needs an engineered plan, matched hoist speeds, load monitoring and a trial lift, not a verbal 'about half each'.
2. What should be done when Crane B reads 8.5 t during the trial lift?
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The lift should be stopped immediately and the beam lowered to the ground under control. Crane B is already above its 8.0 t rated capacity, and the share has shifted from the static estimate. The operator should not boom down or adjust one crane independently while the other crane is still taking load, because that can transfer more load suddenly. Both operators should lower together on the banksman's instruction until the beam is fully supported. The rigging, beam centre of gravity and crane radii should then be rechecked. Options include repositioning Crane B closer to reduce radius, using a larger crane, or changing the lifting points only with the engineer's approval. The lift should not resume until a tandem lift plan exists with calculated shares, a derated capacity, agreed communication and a new trial lift.
3. What must a tandem lift plan contain for this beam?
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The plan should identify the lifting engineer, the beam weight and centre of gravity, the rigging geometry, the initial and final radii, and the calculated load share for each crane. It should state the derated allowable load for each crane, including dynamic and wind effects, and confirm that neither crane exceeds its capacity at any stage. It should define matched hoist speeds, the communication method, the banksman's position, the exclusion zone and the emergency lowering procedure. Load cells or load indicators should be monitored during a trial lift, and the lift should stop if either crane exceeds its agreed share. The plan should also cover wind limits, ground bearing under each outrigger, and the sequence for setting the beam down. Because the beam is long and has side area, the plan should limit wind speed and use taglines to control rotation.