Chart Capacity vs. Actual Radius After Boom Deflection
You are operating a lattice-boom crawler crane with 110 ft of main boom and a 20 ft jib offset at 10 degrees. The lift plan calls for a 9,500 lb air-handling unit at a 70 ft radius, measured on the ground from the boom pivot to the load line. The chart at 70 ft with this configuration shows 11,000 lb. The load will be set on a roof 45 ft above grade, and the rigging (spreader bar and four-leg sling) weighs 900 lb. Wind is 12 mph gusting to 18 mph, quartering from the boom side.
1. Is the planned lift within capacity as rigged? Show the calculation.
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Gross load = 9,500 lb load + 900 lb rigging = 10,400 lb. That is under the 11,000 lb chart value at 70 ft, but the margin is only 600 lb (about 5.5%).
Now check whether 70 ft is the true radius. Radius is measured horizontally from the center of rotation to the center of gravity of the load. Two corrections matter here:
- Boom deflection: under load the lattice boom deflects, which increases the actual radius beyond the unloaded geometric radius. At 110 ft boom this can add a foot or more depending on load.
- The jib offset at 10 degrees shifts the load line outward relative to the main boom tip.
If actual radius grows to, say, 72-73 ft, the chart capacity drops. A typical chart loses several hundred to over a thousand pounds per foot of radius in this range. At 73 ft the capacity could fall below 10,400 lb, putting the lift over 100% of chart.
Conclusion: the lift is not demonstrably within capacity. Re-measure radius with the load applied (or use the manufacturer's deflection allowance), and if the corrected radius reduces capacity below gross load, reduce radius, shorten boom, or pick a different configuration.
2. How does the wind affect your judgment and what would you do?
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Wind at 12 gusting 18 mph quartering from the boom side does two things: it adds a side load to the boom (increasing bending and, on a lattice boom, side sway) and it can move the load and increase effective radius and swing. Check the manufacturer's wind limits for the boom/jib configuration and for the load's sail area. An air-handling unit is a large flat sail.
Practical actions: use tag lines on both sides, slow the swing, and be ready to lower the load if gusts exceed the limit. If the corrected radius already leaves only a few percent margin, wind-induced radius increase can push you over chart. My decision would be to defer the pick until wind is steady below the limit, or reduce radius/configuration so there is meaningful margin, and to confirm the roof set point doesn't require reaching out beyond the planned radius.