Pick-and-Carry Across a Cross-Slope with Wind
A 55-tonne rough-terrain crane is to pick and carry a 5,600 kg precast vault from a storage area to an excavation 60 m away. The crane is on 26.5R25 tires at 690 kPa. With 12.2 m of boom at an 8.5 m radius, the stationary load chart allows 8,900 kg and the pick-and-carry chart allows 6,200 kg on level, firm ground. The route crosses compacted gravel with a 4% cross-slope for 15 m and a 3 m wide soft patch where a plate load test was not done. The manufacturer's notes state: reduce pick-and-carry capacity by 10% for slopes over 2%; maximum slope 5%; do not pick and carry when wind gusts exceed 32 km/h if the load sail area exceeds 4.6 m². The vault is 2.4 m × 2.4 m (5.8 m² sail area). Gusts are 35 km/h. The crew wants to keep the load 300 mm off the ground.
1. Calculate the allowable pick-and-carry capacity for the slope and compare it with the load. Is the lift acceptable on that basis alone?
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Allowable pick-and-carry capacity = 6,200 × 0.90 = 5,580 kg. The load is 5,600 kg, so the margin is -20 kg: the lift is not acceptable on the slope derating alone. The 4% cross-slope is below the stated 5% maximum but above the 2% threshold, so the 10% reduction applies. A 20 kg shortfall is still a shortfall; there is no allowance for load swing, tire flex, or ground settlement.
2. Evaluate the wind, sail area, and soft patch. What additional checks or controls are needed?
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Wind: gust is 35 km/h, above the 32 km/h limit, and sail area is 2.4 × 2.4 = 5.76 m², above 4.6 m², so pick-and-carry must stop. Soft patch: no plate load test means unknown bearing; the crane's tire pressure and ground pressure must be checked, and mats or a geotechnical approval are needed. Also inspect the route for grade, fill, underground voids, and clearances. Keep the load as low as practical, use tag lines and a spotter, travel slowly, and avoid simultaneous travel and swing if the manufacturer prohibits it.
3. What options would you present to complete the work safely?
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Options: do the lift stationary with outriggers at a prepared pad and use a different transport method for the 60 m carry; use a larger crane with a verified pick-and-carry chart; improve the route to remove the cross-slope and soft patch; reduce sail area by removing panels or using a solid cover; wait for wind below 32 km/h. Whatever option is chosen, recheck the chart for the actual configuration and document the decision.