Load Charts, Capacity, and Rated Load Determination
Gross, net, and the chart notes
Load-chart questions punish candidates who stop at the headline number. Most mobile-crane charts publish a rated capacity that already includes the hook block and all below-the-hook rigging; the chart notes tell you exactly what is included. Other charts are net and require deductions. Either way, the usable load at the hook equals the chart capacity minus everything hanging below the boom tip that the chart does not already count: hook block, headache ball, slings, shackles, spreader beam, and stowed jib or auxiliary head if the notes require it.
Structural versus stability
Every chart has a limiting mode. Structural limits are set by boom strength, pins, and sheaves; stability limits are set by tipping. The bold line on many charts separates the two. Capacity also changes with quadrant: over the side, over the rear, and over the front can differ. Outrigger extension, counterweight configuration, and pick-and-carry travel charts change the answer again. Boom deflection under load increases actual radius, so confirm radius with the load suspended, not just before the lift.
Worked example
A chart gives 20,000 lb at a 30 ft radius. The hook block weighs 900 lb and slings and shackles weigh 250 lb. Usable load = 20,000 − 900 − 250 = 18,850 lb. A load weighed at 19,000 lb is over capacity by 150 lb; reduce radius or use a larger crane. With two parts of line, line pull at 19,000 lb is about 9,500 lb plus friction, still within a 12,000 lb hoist.
Traps
Reading radius to the boom tip instead of the hoist line; interpolating past the chart; using an on-rubber travel chart for a set pick; ignoring the quadrant; and forgetting the block and rigging are the common mark-losers.