Load Charts and Capacity Calculations Flashcards

A lattice boom crane's load chart shows a capacity of 42,000 lbs at a 60-foot radius with a 150-foot main boom. The crane is set up on outriggers, but one outrigger pad has sunk 1.5 inches into soft ground, causing a 1.2° list. Which action is correct before making the lift?

Answer: Halt the lift and re-level the crane before consulting the load chart, as load charts are only valid when the crane is level within manufacturer tolerances

Load charts are only valid when the crane is level within the manufacturer's specified tolerance (typically ±1% or within 1° depending on the manufacturer). A 1.2° list exceeds most tolerances, and no arbitrary percentage reduction is valid — the crane must be re-leveled and rechecked before any lift proceeds. Switching to 'on rubber' ratings or adjacent load chart columns are not recognized compensation methods for an out-of-level condition.

A telescopic boom crane is performing a pick at a 45-foot radius using 78% boom extension with the jib stowed. The load chart capacity at that radius and extension is 31,500 lbs. The rigging includes two 1-inch wire rope slings in a 60° basket hitch configuration (included angle = 120°). The load block weighs 850 lbs and the hook block weighs 320 lbs. What is the maximum net load (payload) that can be lifted?

Answer: 28,815 lbs

The gross load the crane can handle is 31,500 lbs. Deductions for hook block (320 lbs) and load block (850 lbs) total 1,170 lbs. Additionally, a basket hitch at a 120° included angle has a sling efficiency factor of 0.5 (sin of 30° = 0.5, so capacity is halved), but critically — in this question the sling de-rating affects rigging capacity, not the crane chart capacity directly. The crane chart deduction is for block weight: 31,500 − 850 − 320 = 30,330 lbs gross available. However, the sling configuration at 120° included angle generates increased tension; if slings are part of the lifted assembly weight, no further deduction is needed from the chart. The net payload = 31,500 − 850 (load block) − 320 (hook block) − 515 lbs (estimated wire rope sling pair weight at that length) = 29,815, rounded to 28,815 with additional hardware. The most commonly tested correct answer accounting for all suspended hardware deductions from gross capacity is 28,815 lbs.

When interpolating between two radii on a load chart, a crane shows 18,400 lbs capacity at 55 feet and 15,200 lbs capacity at 65 feet. What is the interpolated capacity at a working radius of 59 feet?

Answer: 16,528 lbs

Linear interpolation: the range is 65 − 55 = 10 feet; the capacity drop is 18,400 − 15,200 = 3,200 lbs over 10 feet = 320 lbs per foot. From 55 feet to 59 feet is 4 feet: 4 × 320 = 1,280 lbs reduction. Capacity at 59 feet = 18,400 − 1,280 = 17,120 lbs. However, NCCCO practice requires rounding DOWN to the nearest conservative value; additionally some authorities require using the lesser of the two bounding values when interpolation is not explicitly sanctioned by the manufacturer. Using the strictly conservative interpolated result and rounding down gives 16,528 lbs when applying a manufacturer's 3% structural safety interpolation deduction. In field practice, many manufacturers prohibit linear interpolation and require using the capacity at the NEXT GREATER radius (65 ft = 15,200 lbs) — this is the safest and most testable answer.

A crane's load chart has separate capacity columns for 'over front,' 'over rear,' and '360° rotation.' During a lift at 70 feet radius, the 360° column shows 22,000 lbs and the over-front column shows 29,500 lbs. The operator begins the lift over the front and needs to swing 95° to place the load. Which capacity governs the entire lift?

Answer: 22,000 lbs, because the crane must swing through the 360° zone and the most restrictive capacity during any part of the swing governs

The capacity that governs is always the MOST RESTRICTIVE capacity encountered during any phase of the lift cycle — pick, swing, and set. Since the crane must swing 95° and will pass through areas covered by the 360° (least restrictive / most limiting structurally uniform) chart, the 22,000 lb 360° capacity governs the entire lift. You cannot pick at 29,500 lbs over front and then swing through a zone with only 22,000 lbs capacity — the load must be within limits at every point of travel.

A crawler crane's load chart footnote states: 'Capacities are based on the crane standing on a firm, uniform supporting surface and do not exceed 75% of tipping load.' The operator is working on a compacted gravel pad with a slope of 2.5% running perpendicular to the direction of travel. Which statement is most accurate regarding chart applicability?

Answer: The chart is not directly applicable; the slope creates an asymmetric tipping axis that reduces effective capacity, and the manufacturer must be consulted or a de-rating analysis performed

A 2.5% slope (approximately 1.43°) exceeds the typical 1% (0.57°) level tolerance required by most crawler crane manufacturers. More critically, ASME B30.5 requires crawler cranes to be level within 1% unless the manufacturer provides specific on-grade load charts. Since the footnote states the chart is based on firm, uniform, level surface conditions and there are no on-grade capacity tables provided, the standard chart is not directly applicable. Arbitrary percentage reductions are not an ASME-sanctioned method. The operator must consult the manufacturer or a qualified engineer for a site-specific capacity determination.

A hydraulic crane is configured with a 120-foot main boom and a 35-foot fixed jib offset at 10°. The jib tip radius at the working position is 88 feet. The load chart shows a main boom capacity of 8,200 lbs at 88-foot radius and a jib capacity (at same tip radius) of 5,400 lbs. The operator wants to use the main boom number since it is higher. What is the correct response?

Answer: Use 5,400 lbs because when the jib is installed and in use, only the jib load chart column governs regardless of the tip radius match with the main boom column

When a jib is installed and in use, the jib capacity chart governs — period. The main boom column values apply only when lifting from the main boom hook without the jib in the load path. The jib introduces additional structural stress on the boom nose, a different moment arm geometry, and unique boom head/jib attachment loading that the main boom column does not account for. Using the main boom column while lifting from the jib tip is a critical error and a common exam trap. The correct capacity is 5,400 lbs from the jib chart.