An 87 Percent Lift on Ground That Is Out of Level
Company policy defines a critical lift as any lift at or above 75 percent of the crane's rated capacity. Today's load is a rooftop unit weighing 13,800 lb with 700 lb of rigging. The crane is a 90-ton AT with 115 ft of main boom at a 78 ft radius; the manufacturer's chart for that configuration on outriggers shows 16,600 lb. The crane is set on compacted gravel that a surveyor's digital level shows is 1.5 percent out of level side to side. The operating manual requires the crane to be level within 1 percent in all directions for the published chart to apply. A ground-level anemometer reads 18 mph with gusts to 24 mph. The unit has a 14 ft by 6 ft sail area and must be held steady against louver alignment at the set point. It is 06:30 and the plant wants the roof closed by noon.
1. Compute the utilization and explain what that number triggers.
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Total suspended load is 13,800 + 700 = 14,500 lb. Utilization is 14,500 / 16,600 = 87 percent.
That is above the 75 percent critical lift threshold, so the lift is a critical lift under policy. The consequences are procedural, not cosmetic: a written, engineered lift plan specific to this pick, a pre-lift meeting with everyone involved, additional supervision, defined hold points, and verification of the actual configuration against the chart - boom length, counterweight, outrigger spread, and radius measured on the ground rather than estimated.
It also means the margin for error is thin. At 87 percent, a small increase in radius, a wind-induced swing, or load on the wrong side of the chart moves the crane past its rating. Every assumption in the plan has to be true at the moment of the pick, which is why the next two findings matter more than the percentage itself.
2. Identify the conditions in the setup that independently invalidate the plan, and explain the mechanism of each.
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Two conditions each invalidate the published chart, and either one alone is enough to stop the lift.
First, level. The crane is 1.5 percent out of level against a 1 percent limit. The chart is published for a level crane. Out of level, the load swings downhill, the boom is side-loaded, and the effective radius grows on the low side - at 78 ft, a 0.5 percent excess tilt moves the load roughly 5 in. further out and adds a lateral component the boom was not designed to carry. At 87 percent there is no capacity to absorb it.
Second, wind. At 18 mph gusting to 24, measured at ground level, the actual wind at the boom tip and at the load is higher. The unit presents 84 sq ft of sail. That produces a side force on the load that translates into swing, rotation, and boom side load, and holding the unit against the louver for alignment converts the swing into a lateral pull on the boom. The manufacturer's wind limit for this boom and load configuration is unknown, which by itself means the lift is not planned.
3. What must be done before this lift is approved, and who decides?
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Level first, because it is a manufacturer's requirement and cannot be planned around. Use engineered cribbing or steel mats sized for the outrigger reactions, re-check with the digital level on both axes until within 1 percent, and re-measure the radius afterward. If the pad cannot be brought into tolerance, the load must be repositioned or the crane changed; a manufacturer's derated chart for out-of-level operation is the only other route.
For wind, get the manufacturer's maximum allowable wind speed for this boom length and load configuration and compare it to a measurement taken at boom tip height, not at ground level. Set a stop criterion in the plan and give the operator and spotter authority to halt. Reduce the sail effect by planning the rotation and the louver alignment so the unit is not held against the structure in a gust, and use tag lines from two directions.
If the company wants margin, drop below 75 percent by reducing radius or using a larger crane; that removes the critical lift classification on capacity, though the wind and level controls stay. The lift director approves the revised plan, and the decision to proceed after 06:30 belongs to the lift director, not the plant schedule.