The 32-Foot Pick That Grew Six Feet
Your 50-ton rough-terrain crane is set on fully extended outriggers in a level, paved yard. Main boom is 85 ft, no jib, and the boom is at a radius of 32 ft. The load chart for 85 ft of boom, 360-degree operation, reads: 30 ft radius = 17,200 lb; 35 ft radius = 14,100 lb; 40 ft radius = 11,700 lb. On-board deductions are 780 lb for the hook block and 260 lb for slings and shackles. The item to be lifted is a skid-mounted pump whose shipping paperwork lists 11,400 lb; nobody has weighed it. Wind is 18 mph gusting to 24, and the pump is 6 ft tall by 5 ft wide. Your superintendent wants the pick in 20 minutes and asks whether you can also pick it 6 ft further out to reach the pad.
1. Compute the net allowable capacity at 32 ft and at 38 ft, compare to the gross load, and state what you will and will not do.
Read the worked answer
Interpolate the chart (linear between listed radii).
- At 32 ft: step from 30 ft to 35 ft is 5 ft and 17,200 − 14,100 = 3,100 lb, so per foot = 620 lb. Capacity = 17,200 − (2 × 620) = 15,960 lb.
- At 38 ft: step from 35 ft to 40 ft is 5 ft and 14,100 − 11,700 = 2,400 lb, so per foot = 480 lb. Capacity = 14,100 − (3 × 480) = 12,660 lb.
Gross load for both cases: 11,400 + 780 + 260 = 12,440 lb.
Utilization:
- At 32 ft: 12,440 ÷ 15,960 = 78% of chart capacity.
- At 38 ft: 12,440 ÷ 12,660 = 98% of chart capacity.
Decision. The 32 ft pick is inside the chart but not comfortable, because 78% already exceeds the common 75% threshold at which a written lift plan and a second level of review are expected, and the weight is unverified. The 38 ft pick is not acceptable: 98% leaves no room for a paperwork weight that is low by even 3%, for load sway, for boom deflection, or for a gust. I would make the 32 ft pick only after the load is verified, and I would refuse the 38 ft pick unless the crane is repositioned to bring the radius back to the original figure or a lift plan is produced with a verified weight and lower utilization.
2. How does the unverified weight and the 18-24 mph wind change your analysis?
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Unverified weight. Shipping papers list net weight and routinely omit crating, pallets, skids, oil or water left in a unit, and accessory boxes strapped on. A 10% error here is 1,140 lb, which at 38 ft radius pushes the lift from 98% to about 107% of chart - over capacity. The fix is cheap: weigh it on a platform scale, use a load cell or dynamometer in the headache ball line on a test pick inches off the ground, or have the shipper certify the weight on a scale ticket. Until then I treat 11,400 lb as an estimate, not a fact.
Wind. The pump is 6 ft × 5 ft, so it presents roughly 30 sq ft of sail. At 24 mph that is meaningful side load and, more important, it makes the load oscillate and makes the swing harder to control at 38 ft radius where the margin is already 2%. Manufacturer operating manuals typically cap normal crane operation at around 20 mph sustained, and many charts state that capacities are for no wind. Above that, the load's sail area and the crane's own boom area govern, and the pick falls outside the chart's assumptions.
Combined effect. The two uncertainties do not average out; they stack. My position is that I will not make the longer-radius pick in these conditions regardless of the paperwork, and the shorter pick waits for a verified weight or drops to a radius where utilization is under about 75%.
3. The superintendent says the pick is 'basically the same' and that a spotter will watch. What is your response and what do you document?
Read the worked answer
A spotter does not add capacity. The chart is the limit, and at 98% the margin exists only on paper - a spotter cannot arrest a boom deflection or a tipping failure. The superintendent's statement also treats radius as an opinion, when it is a measurement: radius is taken from the center of rotation to the center of gravity of the load, and the actual radius after the boom is loaded is usually slightly greater than the unloaded reading.
What I do:
- Hold the lift and restate the numbers in writing (capacity 12,660 lb vs. gross 12,440 lb at 38 ft, wind 18 gusting 24).
- Offer the practical alternative: reposition the crane so the pad is reached at 30-32 ft radius, or use a larger crane, or wait for the wind to drop.
- Ask for the weight to be verified before any pick.
- Note the refusal, the time, the conditions, and to whom it was reported in the daily log, and notify my supervisor in writing.
Refusing a lift is a licensed operator's authority and duty; signing on to a lift you believe is over capacity is not a management decision you can be directed into.