Interpolation and Stowed Jib Deduction
Boom truck chart for outriggers fully extended, 62 ft boom: 45 ft radius 4,300 lb; 50 ft 3,700 lb; 55 ft 3,200 lb. Chart notes: deduct 350 lb for hook block and 120 lb for rigging; deduct 500 lb when jib is stowed on boom. The load weighs 3,000 lb. The operator measures 45 ft from center of rotation to the load's centerline before lifting, with the jib stowed. During the trial lift, the boom deflects and the actual radius at the load is 47.5 ft. The operator says he can interpolate between 45 and 50 ft, and because 3,000 lb is below both chart values, the lift is legal. The load must be swung 90 degrees over the side. The chart is rated for 360-degree operation, tires are clear of the ground, and outriggers are fully extended.
1. Calculate net capacity at 45 ft and 50 ft, then decide whether the lift is legal at the actual 47.5 ft radius.
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At 45 ft: 4,300 - 350 hook block - 120 rigging - 500 stowed jib = 3,330 lb net. The 3,000 lb load leaves 330 lb, about 10 percent margin. At 50 ft: 3,700 - 350 - 120 - 500 = 2,730 lb net, which is 270 lb short. The actual radius is 47.5 ft, not 45 ft. The chart has no 47.5 ft value, so unless the chart specifically permits interpolation, the operator must use the next longer tabulated radius, 50 ft. At 50 ft the net capacity is 2,730 lb, so the 3,000 lb load is not legal. Even a straight-line interpolation would give only about 3,030 lb, roughly a 30 lb margin, which is not a safe basis for a lift and is not authorized by the chart. The load must be reduced, the radius reduced by repositioning, the stowed jib removed if the chart allows, or a larger crane used.
2. What changes if the jib is removed and stored off the boom, and what must still be verified?
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Removing the stowed jib eliminates the 500 lb deduction, so at 50 ft the net capacity becomes 3,700 - 350 - 120 = 3,230 lb. That would exceed the 3,000 lb load by 230 lb, using the conservative 50 ft value for the 47.5 ft actual radius. However, the operator must confirm that the chart being used is the correct chart for the no-jib configuration, that the boom length and outrigger configuration still match, and that the swing over the side is covered by the chart. Removing the jib changes the boom configuration and may require other chart notes or reassembly checks. The radius must be re-measured after the change, and the lift should still be treated as a 50 ft chart condition unless the actual radius is reduced below a tabulated value. Do not use the 45 ft capacity merely because the pre-lift measurement was 45 ft.
3. List the conditions that could change the radius or capacity during the swing and explain why each matters.
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Boom deflection under load increases the actual radius, as this scenario shows, so a pre-lift measurement can understate the operating radius. Load center of gravity offset can increase the effective radius and put more load on one side. Swinging into a different quadrant can change capacity if the chart is not rated for full 360-degree operation, though this chart is. Outrigger settlement or tire/suspension movement can tilt the truck and change geometry or stability. Wind can increase side load and boom deflection, especially with a large air handler. A non-vertical hoist line or a load that shifts in the slings changes the radius and load distribution. Boom length and jib configuration affect the chart. The operator should monitor the load radius during the trial lift and the swing, and stop if the radius exceeds the value used for capacity.