Load Chart and Outrigger Setup at a Trench Edge
A 60-ton truck crane is set up beside a new building. The load chart for the current configuration - 62 ft boom, outriggers fully extended, 360-degree operation - shows 12,400 lb capacity at a 34 ft radius. The lift is a 9,800 lb mechanical skid; rigging and shackles add 250 lb, and the 600 lb headache ball is not included in the chart capacity. The operator plans to set the rear outrigger float on a 24 in × 24 in timber mat over compacted fill 12 ft from an open utility trench. The ground is level, but the fill has not been density-tested. The site superintendent wants the skid landed 6 ft farther out than planned, which would increase the radius to 40 ft; the chart capacity at 40 ft is 9,900 lb. Wind is 8 mph, and the load has a 30 ft² sail area. The crew has a tag line and a signal person.
1. Calculate the net capacity at 34 ft and the percent utilization of the planned lift. Does the lift comply with the load chart?
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Net capacity at 34 ft = 12,400 − 600 − 250 = 11,550 lb. The 250 lb rigging is already deducted from chart capacity, so do not add it again to the 9,800 lb payload. Utilization = 9,800 ÷ 11,550 = 0.848, or about 85%. The planned lift is within the chart. At 40 ft, net capacity = 9,900 − 850 = 9,050 lb; 9,800 ÷ 9,050 = 1.08, or 108%, so the farther landing is not permitted. The correct decision is to relocate the crane or re-rig, not to rely on the 34 ft calculation.
2. If the superintendent insists on the 40 ft radius, what must be done before any lift is made?
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The 40 ft radius is an over-capacity lift even before dynamic effects. The operator must not boom out and try to feel the load. Options are to move the crane closer, shorten the boom, reduce the load, or use a larger crane. Recalculate using the actual loaded radius; radius increases as the boom deflects and the load swings. The 8 mph wind and 30 ft² sail area add load-control difficulty, so check the manufacturer's wind limit and use tag lines.
3. What ground and trench conditions must be resolved for the rear outrigger before setup is accepted?
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Resolve the ground before setup. A 24 in × 24 in mat is only 4 ft²; a 30,000 lb outrigger reaction would impose 7,500 psf, which many compacted fills cannot take. Obtain the outrigger reaction from the load chart or manufacturer, have the fill evaluated, use a properly sized crane mat or engineered pad, and keep the outrigger outside the trench influence zone. If the trench depth is unknown or the fill is loose, set the crane farther back or use a different setup. Barricade the trench and do not allow the crew to stand between the load and the trench.