Unexpected movement while slewing a suspended load
You are lifting a 900 kg pallet of bricks from a truck tray to a second-storey scaffold platform using a vehicle loading crane with outriggers extended on a grass verge beside a sealed driveway. The chart allows 1 500 kg at the 6 m radius you are working at, so you have margin. The wind is gusty, around 25 km/h with stronger gusts, and the pallet has a shrink-wrapped load with a partly torn cover. As you slew toward the scaffold, the truck rocks noticeably on its suspension and the load begins to swing and rotate. The dogger signals you to keep coming. You notice one outrigger pad has sunk about 40 mm into the verge and the truck is now sitting slightly off level. The scaffold platform handrail is 1.1 m high and the pallet must pass over it.
1. What is happening, and what is the correct immediate action?
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The symptoms point to a set-up failure, not a load problem. The outrigger has penetrated the grass verge by about 40 mm, so the truck is no longer level. Once the chassis tilts, the crane's geometry changes: radius increases as the boom leans, the load moment rises, and the swing of a suspended load adds dynamic forces the chart does not cover. The rocking on the suspension suggests load is transferring between outriggers as the ground yields. The gusty wind acting on a large, partly unwrapped pallet adds a side load and explains the rotation.
The immediate action is to stop the slew and hold position, or lower the load to the ground if that can be done without further movement. I would not continue to the scaffold. I would not lower the boom or retract anything abruptly, because changing geometry under a swinging load can worsen the situation. I would bring the load down steadily at the nearest safe landing point, then shut down and reassess.
The dogger's signal to keep coming reflects the pressure to finish, not an engineering judgement. The 900 kg against 1 500 kg margin is irrelevant once the base has moved - the chart assumes a level, stable machine.
2. After landing the load, how would you re-establish a safe set-up, and what would you change about the lift?
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First, secure the crane: park the load, relieve the outriggers, and inspect the pad and ground. A 40 mm penetration on a grass verge indicates the bearing capacity is inadequate for the outrigger load, and the same spot will fail again. I would not simply re-pack the hole and carry on.
Re-set-up: relocate so all outriggers bear on the sealed driveway or on properly sized pads or mats spread over competent ground, with full extension and the machine confirmed level on a spirit level. I would check each pad for penetration after pre-loading the outriggers and before lifting. I would re-read the chart for the new radius, because moving the truck changes the geometry.
Lift changes: reduce the sail area by re-wrapping or removing the torn cover, or break the pallet down; postpone if gusts remain high, since a suspended load with a large face is vulnerable to wind; use tag lines to control rotation; and confirm the load can clear the 1.1 m handrail with adequate height without booming out beyond the checked radius. I would brief the dogger on the stop signal and make clear that a stop is not negotiable. The event and the corrective actions would be recorded and reported.