Load Charts and Capacity Calculations
Load Charts and Capacity Calculations
This domain tests your ability to read the crane's load chart for the actual configuration and calculate the net capacity available for the load. The exam focuses on radius, boom length, boom angle, outrigger state, counterweight, deductions, and structural versus stability limits.
Core rules
- Identify the chart for the exact configuration: boom length, jib stowed or erected, outriggers fully extended or not, counterweight, and track or tire condition.
- Radius is measured from the crane center of rotation to the center of the load, allowing for boom deflection. It is not the distance from the boom pivot.
- Deduct the weight of the hook block, slings, shackles, below-the-hook device, and any stowed attachment from the chart capacity. The remainder is the net load capacity.
- Use the chart values for the actual radius. If the chart does not permit interpolation, use the next lower capacity.
- A stability-limited chart and a structural-limited chart can govern different parts of the range; never exceed either.
Worked example
A chart gives 24,000 lb gross capacity at a 30 ft radius for the current boom and outrigger configuration. The hook block, slings, and shackles weigh 1,200 lb, so net capacity is 22,800 lb. The load weighs 21,500 lb. Utilization is 21,500 divided by 22,800, about 94 percent. The lift is within capacity but with little margin. If the radius increases to 35 ft and the chart falls to 19,000 lb gross, net capacity becomes 17,800 lb, so the same load is not permitted. The reasoning: compare the load to net capacity at the actual radius, not to the bare chart number.
Traps
Using gross chart capacity without deductions; reading radius from the wrong point; ignoring boom deflection; and selecting a chart for outriggers fully extended when the crane is on tires or partial outriggers.