Rear Pick, Side Set, and Chart Deductions
A 100-ton all-terrain crane is set up with 120 ft of main boom, full outriggers, and 26,000 lb of counterweight. It must pick a 22,500 lb air-handling unit from a truck at a 45 ft radius over the rear and swing it 90 degrees to set it on a roof. The rear chart at 45 ft shows 27,000 lb gross. To clear a parapet, the operator must boom out to 48 ft before swinging; the rear chart at 48 ft shows 24,800 lb, and the side chart at 48 ft shows 19,600 lb. The hook block weighs 900 lb and the sling and shackles weigh 300 lb. The operator plans to use the 45 ft rear value because the load will be at 48 ft only briefly and will be over the side for 'just a few seconds.' The LMI is functioning and shows 86 percent during the pick. The ground is firm and the crane is level. The load has a wind sail area of 90 sq ft; gusts are 16 mph, below the manufacturer's 20 mph limit.
1. Calculate net capacity at 45 ft rear, 48 ft rear, and 48 ft side. Is the planned lift acceptable?
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Deduct the hook block and rigging from each gross chart value: 900 + 300 = 1,200 lb.
- 45 ft rear: 27,000 - 1,200 = 25,800 lb net. The 22,500 lb load is acceptable with about 3,300 lb margin.
- 48 ft rear: 24,800 - 1,200 = 23,600 lb net. The load is acceptable only if the load remains over the rear and the radius truly does not exceed 48 ft; margin is about 1,100 lb.
- 48 ft side: 19,600 - 1,200 = 18,400 lb net. The 22,500 lb load exceeds net capacity by about 4,100 lb, so it is not acceptable.
The planned set requires the load to be over the side at 48 ft. The brief duration does not matter; the chart capacity applies at every point in the lift. The lift as planned is not acceptable.
2. Why can't the operator use the 45 ft rear chart value or the crane's 100-ton nominal rating?
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Mobile crane capacity is not a single number. It depends on boom length, radius, quadrant, outrigger spread, counterweight, boom configuration, and the deductions for load-handling devices. The 45 ft rear value is valid only at 45 ft over the rear. Once the load is boomed out to 48 ft, the 48 ft value applies; once it swings over the side, the side chart applies. A brief excursion still loads the boom, structure, and stability at that condition. The 100-ton nominal rating is a crane class rating, typically achievable only at a short radius with a specific configuration; it is not the capacity at 48 ft over the side. The LMI is an aid and warning device, not a substitute for reading the chart and confirming the actual radius, quadrant, and deductions. Boom deflection and load movement can also make the actual radius larger than the planned radius.
3. What are acceptable ways to proceed if the side set at 48 ft is required?
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The contractor must change the lift plan so the crane is not operating outside the side chart at 48 ft. Options include repositioning the crane to reduce the side radius, reconfiguring the lift so the set remains over the rear, using a larger crane or a different boom configuration, or breaking the load into smaller pieces. If a shorter side radius is proposed, the operator must read the side chart at that exact radius and deduct the hook block and all rigging; do not extrapolate or assume the 45 ft rear value applies. The new plan must also check ground bearing, outrigger spread, swing clearance, wind, and the actual radius with the load suspended. The operator should not proceed merely because the LMI showed 86 percent during the rear pick; that reading does not cover the side condition.