Trolley Out With a Long Load
You are operating a hammerhead crane with a 50 m jib. The task is to land 9 m long precast wall panels onto a slab. The panels are slung with a two-leg sling at 60 degrees included angle, hook to panel, and a spreader beam is available but the rigger says it 'takes too long'. The landing point is at 38 m radius. The panel mass is 2.8 t. The load chart at 38 m gives 4.1 t with the trolley in that position. Wind is light. The slab edge has a 1.2 m high edge protection and the panel must be lowered so its base sits 200 mm from the slab edge, between two starter bars 1.5 m apart. The dogger is standing on the slab guiding by hand.
1. Evaluate the rigging proposal and the load chart use, and state what you will require before the lift.
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The rigging proposal is not acceptable as described, and the load chart figure alone does not clear the lift.
- Sling angle. A two-leg sling at 60 degrees included angle means each leg carries more than half the load; the tension in each leg rises as the angle opens. The rigger's 'quick' option increases leg tension and applies a horizontal compression into the panel, which can crack precast units at the lift points. The spreader beam exists precisely to keep the legs vertical and remove that compression. Use it.
- Lift points. Precast panels have designated lift points and a designed lifting arrangement. Hand-sligning at an arbitrary angle is not the same as the designed lift.
- Load chart. 2.8 t against 4.1 t looks fine, but the chart value must be for the actual configuration: correct jib length, correct counterweight, correct reeving, and the radius measured to the centre of the hook with the load hanging free. At 38 m on a 50 m jib the chart is already reduced by jib deflection; adding a long panel that swings and catches wind changes the dynamic load. I want the chart confirmed for the panel's actual radius including any trolley travel during landing.
- What I require: spreader beam fitted, lift points verified, tag lines on both ends, the dogger out of the landing zone with the panel controlled by tag lines, and a agreed signal sequence for the final 200 mm placement.
Only then do I take the lift.
2. Describe how you will place the panel within 200 mm of the slab edge without contact with the starter bars or the edge protection.
Read the worked answer
Precision placement at 38 m radius is a planning problem, not a reflexes problem.
- Set up before the pick. Confirm the slew and trolley controls are smooth at low speed, and that the panel's long axis will be oriented correctly before it leaves the ground. Rotate on the ground where there is room, not over the slab.
- Approach from outside the slab. Bring the panel in from beyond the edge, base first, so the operator can see the gap to the edge protection. Never bring it in over the top of the edge protection and lower it into the gap.
- Use the tag lines and the dogger's signals. The dogger controls the final positioning by hand signals or radio, standing where they can see both the gap and the operator's line of sight. The dogger must not stand between the panel and the starter bars.
- Slow, stepped movements. Slew in small increments with the trolley stationary, then trolley in small increments with the slew stationary. Doing both at once at 38 m makes the hook path unpredictable because the panel swings out under slewing.
- Stop short and check. Land the panel with the base clear of the bars, check alignment, then lower to the bearing surface. Do not use the crane to lever or drag the panel into position.
- Keep hands off. The dogger guides with tag lines, not hands, until the panel is landed and stable.
If at any point the panel starts to swing or the gap cannot be judged, I stop, lift clear and reset rather than trying to correct mid-air.