Structural vs stability limits: meaning and why it matters

Two different reasons for the same number

Each capacity on a chart is the smaller of two things. One is how much load the crane's parts can carry without being overstressed. The other is how much load the crane can hold before it would tip, less a margin. Close in, the crane is hard to tip and the parts are the limit. Farther out, the long arm of the load makes tipping the limit well before the parts are stressed.

That is why the printed capacity is nearly flat at short radius and then drops steadily. The flat part is structural. The sloping part is stability.

Reading the difference

Many charts mark the boundary with a bold line, a shaded region or a legend note. Real charts differ, so read the chart's own legend rather than guessing from where a line falls. On our invented Practice A with an 80 ft boom, the 10 ft radius cell reads 28,000 lb, structural, and the 30 ft radius cell reads 24,000 lb, stability. The practice tool labels the basis so you can learn to see it.

OSHA 29 CFR 1926.1433 covers design, construction and testing of cranes. For what a particular cell is limited by, rely on the chart's legend and the manufacturer's instructions.

Why it matters

• Remedies differ. More outrigger extension or counterweight can help a stability-limited cell, because those add resistance to tipping. They do nothing for a structural cell, where the parts are the limit.
• Warning signs differ. A crane near its stability limit gives signs, such as the outrigger on the far side unloading. A structural overload may give no sign before a part fails.
• Margins differ. The stability limit already includes a margin below the tipping load. The structural limit has margin of its own, but exceeding it risks damaging the crane.

Common mix-ups

• Thinking the bold line is the safe side. Both sides are limits. Neither side invites you to exceed the value.
• Believing the cell above the line is "stronger". It is a different limit, not a different amount of safety.
• Assuming extra counterweight raises every capacity. It helps stability cells only.
• Skipping the legend. The legend tells you what the line means on that chart.

On the NCCCO written exam

Core tasks 3.2 and 3.3 (terminology, functions and limitations) are where you explain why capacity falls with radius, and Load Charts task 4.4 is where you read the chart. Load Charts is about 27% of Core under the 01/25 handbook. Know which kind of overload may give a warning and which may not.

Practice it

The load chart practice tool labels each cell's basis on invented charts. Read tipping axis for the stability side and gross vs net capacity for what comes off the number afterward. The crane terms glossary lists the rest.

Know the terms? Take the free 10-question check

Take the free 10-question check. It is scored by the Core outline domains and needs no signup, so you see whether site, operations, technical knowledge or load charts costs you the most points. Boom Angle's course then gives you a question bank with each answer key checked by a second solve, timed practice exams, and a study plan built from your exam date.

Sources

Show the list

• 29 CFR 1926.1433: Design, construction and testing, OSHA.
• Mobile Crane Operator Candidate Handbook, CCO, Core outline tasks and weights (01/25 revision). Task numbers on this page follow it.