5.3 Worked Load-Chart Problems
Key Takeaways
- Solve every chart problem with the same sequence: confirm the chart and configuration, find gross capacity at the radius and boom length, subtract all deductions, total the actual load, then compare
- Percent of capacity = (total load ÷ net capacity) × 100; the safest answer is usually the configuration with the lowest percent of capacity
- A structural-strength limit is set by the crane's components and does not change with outrigger spread, while a stability limit can change with counterweight, outriggers, and quadrant
- When a load passes through more than one work area or quadrant, rate the lift for the least favorable (lowest-capacity) condition it will encounter
- The most common chart-reading errors are using boom angle instead of radius, ignoring footnotes, comparing bare load to gross capacity, and forgetting that radius grows as the load is picked and the boom deflects
A Repeatable Method for Every Chart Problem
Every NCCCO load-chart question, no matter how it is worded, can be solved with the same six steps. Memorize the sequence and apply it identically every time:
- Confirm the chart and configuration. Right crane, counterweight, outrigger state, on-outriggers vs on-rubber, quadrant.
- Determine the operating radius and boom length for the planned lift.
- Read gross rated capacity at that radius and boom length from the correct chart.
- Subtract all applicable deductions (hook block, ball, jib, aux head, excess wire rope) to get net capacity.
- Total the actual load: object + slings + shackles + spreader/beam + any below-hook devices.
- Compare: if total load ≤ net capacity and all footnotes are satisfied, the lift is within capacity. Otherwise it is not.
Percent of Capacity
Many questions ask for percent of capacity, which is how heavily the crane is loaded relative to what is allowed:
Percent of capacity = (Total load ÷ Net capacity) × 100
Lower is safer. When a question gives several setups and asks which is best, compute percent of capacity for each and choose the lowest percentage. A value over 100% means an overload.
Worked Problem 1: Is This Lift Safe?
(Illustrative values only.)
| Given | Value |
|---|---|
| Object weight | 17,400 lb |
| Slings + shackles | 600 lb |
| Hook block | 500 lb |
| Gross capacity at planned radius/boom | 20,000 lb |
| Footnotes | All conditions met |
Step 3: Gross capacity = 20,000 lb.
Step 4: Net capacity = 20,000 − 500 (hook block) = 19,500 lb.
Step 5: Total load = 17,400 + 600 = 18,000 lb.
Step 6: 18,000 lb ≤ 19,500 lb, and footnotes are satisfied → the lift is within capacity.
Percent of capacity: (18,000 ÷ 19,500) × 100 = 92.3%. This is within limits but heavily loaded; small radius increases or load swing could push it over, so it warrants extra caution and a test lift.
Worked Problem 2: Radius Grows During the Pick
A frequent trap: the planned radius is read on paper, but in reality the radius increases when the load is picked because the boom deflects and the crane settles slightly. Always use the largest radius the load will actually reach, not the smallest.
| Given | Value |
|---|---|
| Object + rigging | 12,000 lb |
| Gross capacity at 30 ft radius | 13,000 lb |
| Gross capacity at 35 ft radius | 11,200 lb |
| Hook block | 400 lb |
If the load is set down at 35 ft but lifted at 30 ft, you must rate the lift at the larger radius the load passes through (35 ft):
- Net capacity at 35 ft = 11,200 − 400 = 10,800 lb
- Total load = 12,000 lb
- 12,000 lb > 10,800 lb → overload at 35 ft. The lift is not safe as planned, even though it would have passed at 30 ft.
Structural vs Stability Limits
Load chart ratings are governed by one of two limits, and the exam expects you to tell them apart:
| Limit type | Set by | Changes with |
|---|---|---|
| Structural strength | Strength of boom, hoist, and crane components | Does not change with outrigger spread or counterweight; fixed by the machine |
| Stability (tipping) | The crane's resistance to tipping | Changes with counterweight, outrigger extension, quadrant, and ground support |
Charts often draw a bold line, asterisk, or shaded area separating the two. Short-radius, high-capacity cells are usually structurally limited (the steel is the limit). Long-radius cells are usually stability limited (tipping is the limit). A note such as "capacities above the line are limited by structural strength" tells you that adding counterweight or widening outriggers will not raise that number, because the component, not tipping, is the constraint.
Worked Problem 3: Which Setup Is Best?
A 21,000 lb total load can be lifted three ways. Which has the lowest percent of capacity?
| Setup | Gross cap. | Deductions | Net cap. | Percent of capacity |
|---|---|---|---|---|
| A: short boom, 18 ft radius | 30,000 | 800 | 29,200 | 71.9% |
| B: mid boom, 25 ft radius | 26,000 | 800 | 25,200 | 83.3% |
| C: long boom, 25 ft radius | 24,000 | 1,900 | 22,100 | 95.0% |
Setup A loads the crane to only 71.9% of net capacity, the lowest of the three, so A is the safest configuration. Setup C is technically within capacity but at 95% it leaves almost no margin for load swing, deflection, or radius growth.
Common Chart-Reading Errors
These are the mistakes the exam is built to catch:
- Using boom angle instead of operating radius to enter the capacity grid.
- Comparing the bare object weight to gross capacity and ignoring deductions and rigging.
- Skipping footnotes (minimum boom angle, parts of line, counterweight, quadrant).
- Reading the on-outrigger chart when the crane is actually on rubber.
- Ignoring radius growth from boom deflection and ground settlement during the pick.
- Rating the strongest quadrant when the load swings through a weaker work area.
- Forgetting the load chart, not the operator, is the legal authority — "it felt fine" is never the standard.
If you internalize the six-step method and always rate the least favorable condition, almost every Load Charts question becomes a disciplined arithmetic check rather than a guess.
Object + rigging = 18,000 lb, hook block = 500 lb, gross capacity at the planned radius = 20,000 lb. What is the percent of net capacity for this lift?
A load is picked at a 30 ft radius but must be set down at a 35 ft radius. Gross capacity is 13,000 lb at 30 ft and 11,200 lb at 35 ft, hook block is 400 lb, and total load is 12,000 lb. How should the lift be rated?
A load chart note states that capacities above a bold line are limited by structural strength. What does this tell the operator?
Three setups can lift a 21,000 lb total load: A has 29,200 lb net capacity, B has 25,200 lb, and C has 22,100 lb. Which setup is safest by percent of capacity?