Picking the Chart Before the Pick
A 90-ton hydraulic truck crane is set on a stabilized construction entrance with outriggers fully extended on 4 ft square pads over compacted fill. The planner needs to lift a 14,000 lb chiller from a flatbed at a 34 ft radius and swing it 180 degrees to a roof curb at a 62 ft radius. Boom length is 95 ft with no jib. The site plan shows a 6 in. slope across the pad area and a buried utility vault with a 20 ft span located 8 ft behind the rear outrigger. The operator's chart for the 95 ft boom shows 16,500 lb at 35 ft radius and 9,800 lb at 65 ft radius, with a note that capacities above the bold line require outriggers fully extended and no travel.
1. What is the governing capacity for this pick, and does the lift work as planned?
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The controlling radius is the far radius, not the pickup radius. The load must be carried at 62 ft, so interpolate between 35 ft and 65 ft: at 62 ft the chart gives roughly 10,300–10,600 lb. The 14,000 lb load exceeds that, so the lift fails at the far position even though 16,500 lb at 35 ft looks comfortable. Correct action: reduce radius (relocate the curb or reposition the crane closer to the set point), shorten the boom, or split the load. Never plan on the near-radius number for a swing pick.
2. How do the slope and the buried vault affect the setup, and what would you require before the lift?
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A 6 in. slope across the pad means the outrigger floats are not on a common plane; the crane can list, and the chart assumes a level machine (typically within 1% grade). The vault 8 ft behind the rear outrigger is a bearing-capacity and collapse hazard - outrigger loads are concentrated and can crush a buried structure. Require: verify the vault's cover and load rating with the owner, use a mat sized for the outrigger reaction (not just 4 ft pads), re-level with cribbing, and get the manufacturer's or a qualified person's confirmation of ground pressure. If the vault cannot be proven adequate, relocate the crane.