Load Charts and Capacity Calculations Flashcards

A crane's load chart shows 18,500 lbs capacity at a 30 ft radius and 15,200 lbs at a 35 ft radius. The calculated load radius is 32.5 ft. Using linear interpolation, what is the maximum net capacity for this lift?

Answer: 16,850 lbs

Linear interpolation: the capacity difference between 30 ft and 35 ft is 18,500 – 15,200 = 3,300 lbs over a 5 ft span. At 32.5 ft the operator is 2.5 ft (50%) into that span, so the deduction is 3,300 × 0.50 = 1,650 lbs. Maximum net capacity = 18,500 – 1,650 = 16,850 lbs. Rounding up to the 30 ft column (18,500 lbs) is never permitted; the operator must always interpolate downward to the actual radius.

A crane's load chart at a given radius shows a structural capacity of 22,000 lbs and a stability (tipping) capacity of 28,000 lbs. A rigger argues the crane can safely handle a 25,000 lb load because it won't tip. Which statement is correct?

Answer: The rated load must not exceed the lesser of the structural or stability-based capacity, making 22,000 lbs the limit

Per ASME B30.5, a crane's rated load must not exceed the lesser of the structural capacity or the stability-based (tipping) capacity at any given configuration. Here the structural limit of 22,000 lbs governs, regardless of the higher tipping capacity. Lifting 25,000 lbs would risk structural failure of crane components even though the machine would not overturn.

On many manufacturer load charts, certain capacity values are printed in bold type or marked with an asterisk (*), while others appear in standard type. What does this distinction typically indicate?

Answer: Bold or asterisked values are stability (tipping) limited, while unmarked values are structurally limited

ASME B30.5 and most OEM load charts use a visual distinction — bold type, shading, or asterisks — to separate stability-limited capacities (where tipping governs) from structurally-limited capacities (where component strength governs). Understanding which limit governs at each configuration is critical: as radius increases, capacities typically transition from structurally limited to stability limited, meaning the crane's tipping behavior, not its steel, becomes the binding constraint.

A crane has a chart-rated gross capacity of 42,000 lbs at the planned radius. The upper block weighs 850 lbs, the lower hook block weighs 1,200 lbs, the wire-rope slings weigh 380 lbs, and the spreader bar weighs 760 lbs. What is the maximum net load that may be hoisted?

Answer: 38,810 lbs

All rigging hardware and lifting accessories are part of the suspended load and must be deducted from gross rated capacity to determine net available load. Total deductions: 850 + 1,200 + 380 + 760 = 3,190 lbs. Net capacity = 42,000 – 3,190 = 38,810 lbs. Omitting even one component (a common error with spreader bars or slings) overstates the usable capacity and can result in an overload condition.

Per ASME B30.5, load radius is precisely defined as the horizontal distance measured from:

Answer: The axis of rotation to the center of the vertical hoist line with the rated load applied

ASME B30.5 defines load radius as the horizontal distance from the crane's axis of rotation (center pin) to the center of the vertical hoist line — measured with the rated load applied. The critical phrase is 'with load applied': boom deflection and structural flex under load move the boom tip outward, increasing the working radius beyond what a no-load or geometric calculation would predict. Operators must account for this deflection when verifying chart compliance.

A crane manufacturer publishes separate load chart pages for (1) outriggers fully extended, (2) outriggers extended to 50%, and (3) on rubber. An operator needs to work with outriggers extended to 75% of full extension. Which approach is correct?

Answer: Use the chart page for the next more conservative published configuration — the 50% extension page

Load charts are rated for specific, discrete outrigger configurations only. Operating at an intermediate outrigger extension that has no dedicated chart page requires using the next more conservative (lower-capacity) published configuration — in this case, the 50% extension page. Interpolating between outrigger positions, or applying a percentage of the full-extension chart, is not permitted unless the manufacturer explicitly provides a methodology for doing so in writing. Using a more conservative chart directly protects against uncharted stability behavior.