Telescopic Boom Capacity at an In-Between Radius
A 60-ton telescopic-boom crane is positioned to set a 12,000 lb air-handling unit on a roof. The operator plans a 32 ft radius with 78 ft of main boom. The chart shows 17,300 lb at 30 ft and 14,700 lb at 35 ft. The hook block weighs 550 lb and the four-leg sling and shackles weigh 400 lb. The site's lift plan requires that no pick exceed 85 percent of the manufacturer's capacity. The unit is 20 ft long and the rigging is attached so the hook is over the load's center of gravity. A tag line will be used. The foreman wants to know whether the pick is legal and whether the crane can handle it without a critical lift plan. Wind is forecast at 12 mph, and the load has a 4 ft x 8 ft solid face perpendicular to the boom. No jib is installed.
1. Determine the capacity the operator must use and the total load to be compared with the chart. Show the calculation.
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Use the chart capacity for the next longer radius, not an interpolation. At a 32 ft radius, the governing capacity is the 35 ft value, 14,700 lb. Total load = 12,000 lb load + 550 lb hook block + 400 lb sling and shackles = 12,950 lb. Utilization = 12,950 ÷ 14,700 = 0.881, or 88.1%. That is under the crane's chart capacity, but it is over the site's 85% limit: 0.85 × 14,700 = 12,495 lb, and 12,950 lb exceeds that by 455 lb. So the pick is not permitted under the lift plan as proposed. It would need a critical-lift plan or approval, or a change that lowers utilization.
2. What options would bring the pick within the 85 percent limit, and what else must be checked before choosing one?
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Reducing the radius to 30 ft gives a chart capacity of 17,300 lb. Utilization becomes 12,950 ÷ 17,300 = 74.9%, within the 85% limit. That may be the simplest fix if the crane can get closer or the load can be set with less radius. Other options: use a larger crane, reduce the load, or remove unnecessary rigging weight. Before choosing, verify the 30 ft radius is measured to the load's center of gravity, that the chart is for the same boom length and outrigger configuration, and that the boom will not have to reach farther as it deflects. Also check the 4 ft × 8 ft sail area against the manufacturer's wind limits; 12 mph may be acceptable, but the chart may require a reduction for the load's sail area.
3. If the operator decides to use the 30 ft radius, what must be verified about the load chart and site before the lift?
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Verify the chart page is the correct one for 78 ft of main boom, full outriggers, the installed counterweight, and no jib or stowed attachment that requires a deduction. Confirm the crane is level, outriggers are fully extended and preloaded, and tires are clear of the ground if the chart requires it. Measure radius at the load's center of gravity, not at the boom tip, and recheck it after the load is lifted because boom deflection and swing can increase radius. Confirm the hook block and rigging weights match the estimate, and that the load will not swing or drag. If any of these change, recalculate before lifting.