Stationary Chart, Boom-Out, and Suspended Travel
A crawler crane is configured with a 70-ft boom and a pile-driving lead system. The manufacturer's load chart for this configuration, with the hook block and stowed jib included, shows 24,000 lb at a 30-ft radius, 20,500 lb at 35 ft, and 17,800 lb at 40 ft. You plan to pick a 50-ft, 16-in square prestressed concrete pile that weighs 13,350 lb. The lifting bridle, shackles, and slings weigh 850 lb. The planned initial pick is at a 33-ft radius. To clear a stockpile, the operator must boom out to a 39-ft radius before swinging. The ground under the crawler tracks is firm compacted gravel, but the operator plans to travel about 20 ft with the pile suspended at a 6-ft height because the stockpile blocks the direct swing path. The site supervisor wants the pile pitched before the morning concrete delivery.
1. Calculate gross load and evaluate the stationary picks at 33 ft and 39 ft.
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Gross load is 13,350 + 850 = 14,200 lb. At 33 ft, interpolate between 24,000 lb at 30 ft and 20,500 lb at 35 ft: 24,000 - (3/5)(3,500) = 21,900 lb. Utilization is 14,200/21,900 = 65%. At 39 ft, interpolate between 20,500 and 17,800: 20,500 - (4/5)(2,700) = 18,340 lb; utilization is 14,200/18,340 = 77%. Both stationary picks are under the chart capacity if the radius is measured correctly, the crane is level, and the chart includes the lead system and hook block. The margin does not authorize travel or side loading, and the operator should treat the 39-ft pick as the controlling condition.
2. Can the operator travel 20 ft with the pile suspended at 6 ft? Explain.
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No, not on the supplied chart. That chart is a stationary capacity chart; traveling with a suspended pile is a different operation and usually has a lower rating or is prohibited for this configuration. The lead system, suspended pile, boom deflection, and travel shock can add load and side force not represented in the stationary values. The operator may travel only if the manufacturer's travel chart for the exact boom, lead, and counterweight configuration permits it, the ground is level and verified, the load is kept as low as possible, tag lines and a spotter are used, and no one is under the load. Otherwise, reposition the crane or move the stockpile.
3. What factors could invalidate the chart capacity?
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Chart capacity can be invalidated by an incorrect radius, out-of-level crane, soft or settling ground, wind, side loads from tag lines or leads, boom deflection, dynamic starts and stops, or a chart that does not include all attachments. Verify whether the lead system and hammer weight are already included; if not, deduct them. Confirm the hook block and stowed jib are the ones listed. Check ground bearing under the tracks, and use the actual radius at the load, not the boom angle alone. If any condition is outside the chart notes, stop and get the manufacturer or engineer's direction.