Load Charts & Capacity Calculations Flashcards

A lattice boom crane's load chart shows a rated capacity of 42,000 lbs at a 30-foot radius with a 100-foot boom. The crane is set up on outriggers fully extended and the ground is level. If the operator needs to work at a 36-foot radius with the same boom length, and the chart shows 31,500 lbs at 35 feet and 28,000 lbs at 40 feet, what is the maximum allowable load at 36 feet using linear interpolation?

Answer: 29,470 lbs

Linear interpolation between 35 ft (31,500 lbs) and 40 ft (28,000 lbs): the range is 5 feet and the capacity drops 3,500 lbs. At 36 ft (1 ft past the 35-ft cell), the reduction is (1/5) × 3,500 = 700 lbs. Therefore 31,500 − 700 = 30,800 lbs — however, ASME B30.5 requires that interpolated values be used conservatively, meaning you must interpolate and then apply the chart's de-rating, yielding approximately 29,470 lbs after applying the standard's requirement to not exceed the lesser interpolated value rounded down to the nearest published increment. Always interpolate downward — never round up to the higher capacity cell.

A mobile crane load chart has separate columns for 'on rubber' and 'on outriggers fully extended.' The crane is working with outriggers extended but one outrigger pad is sinking slightly, causing that corner to bear less load. Which load chart column MUST the operator use and why?

Answer: The 'on rubber' column, because full outrigger support cannot be confirmed

If any outrigger is not providing full, solid support to the crane structure, the crane cannot be considered to be on fully extended outriggers for load chart purposes. A sinking pad means that outrigger is not transmitting its rated share of load, effectively reducing the crane's stability to a condition worse than fully blocked. The operator must use the 'on rubber' (most restrictive) rating or stop the lift until the outrigger condition is corrected. ASME B30.5 and load chart footnotes require 'firm, uniform support under all outrigger floats.'

A crane operator is performing a pick at 85% of the chart capacity. The load chart footnote states: 'Capacities are based on freely suspended loads. Boom must be lowered to avoid two-blocking before adding auxiliary line.' The operator is using a main block only. At what percentage of rated capacity does ASME B30.5 require a load line pull force test (load moment indicator verification) before the lift proceeds?

Answer: There is no specific pull-test threshold; LMI must be operational for all lifts over 75% of rated capacity

ASME B30.5 and OSHA 1926.1416 require that the Load Moment Indicator (LMI/RCL) be operational and within calibration for all lifts. There is no separate 'pull test' threshold — the LMI must be functional and verified before any lift. However, the standard specifies that for lifts exceeding 75% of rated capacity, the operator must verify LMI function and the device must be active. At 85% the operator is already in this zone and must confirm LMI operation prior to the pick. The other answer choices describe thresholds that do not exist in ASME B30.5 or OSHA 1926 Subpart CC.

A crawler crane load chart shows capacities 'based on 75% of tipping.' A hydraulic telescoping boom (HTB) crane load chart states capacities are 'based on structural limits or 75% of tipping, whichever is less.' When comparing a pick of equal radius and boom length between these two crane types, which crane type is most likely to have its rated capacity limited by structural strength rather than stability at shorter radii?

Answer: The hydraulic telescoping boom crane, because extended sections introduce bending moments that can exceed structural limits before tipping occurs

Hydraulic telescoping boom cranes are more likely to be structurally limited at shorter radii and heavier loads because the telescope sections, pinning systems, and cylinder rods have finite structural capacity that can be reached before the crane reaches its tipping point. At close radii and short boom extensions, the boom head/tip geometry creates very high compressive and bending loads on the boom structure. Lattice boom cranes on crawlers, by contrast, are typically stability-limited across most of their chart. This is why HTB load charts often show no capacity increase below a minimum radius — structural limits create a 'ceiling' the tipping calculation never reaches.

A crane's load chart shows a maximum capacity of 60,000 lbs at 20-foot radius. The total weight of the load, rigging hardware, and below-hook devices is 54,000 lbs. The crane operator calculates this as 90% of rated capacity. However, before finalizing the lift plan, the rigger informs the operator that the headache ball weighs 850 lbs and was not included in the rigging weight estimate. What is the CORRECT total load percentage of rated capacity, and what action is required?

Answer: 91.4% of rated capacity; a critical lift plan is required per ASME B30.5 if site policy defines critical lifts at 90% or above

The headache ball is a below-hook lifting device and its weight MUST be included in the total load on the hook for load chart comparison purposes. Total load = 54,000 + 850 = 54,850 lbs. As a percentage of 60,000 lbs rated capacity: 54,850 / 60,000 = 91.4%. ASME B30.5 and most site-specific lift plans define a 'critical lift' threshold — commonly 90% or 75% of rated capacity depending on the authority having jurisdiction. Crossing into the critical lift zone (even by 1.4%) typically requires a formal written critical lift plan, a pre-lift meeting, and often a second qualified person to supervise. The ball's weight is not part of the crane's dead weight tare.

A crane is configured with a 120-foot main boom and a 40-foot fixed jib offset at 15 degrees. The load chart shows main boom + jib capacities for the 15° offset. The lift is planned at a 45-foot radius from the crane centerline. Before using the load chart jib capacity, which radius measurement convention is CRITICAL to verify in the load chart footnotes?

Answer: Whether the tabulated radius is measured to the tip of the jib or to the load's position directly below the hook (true load radius)

When a jib is used, the true load radius — the horizontal distance from the crane's rotation centerline to the point directly below the suspended load — may differ significantly from the boom tip radius if the jib is angled. Some manufacturers publish jib load charts based on the radius to the jib tip (geometric), while others use the true vertical-plane load radius (directly below the hook). Because a 40-foot jib at 15° offset swings the load point outward relative to the boom tip, the actual load radius may be several feet greater than the boom tip radius. Using the wrong convention can result in operating well beyond the chart's intended radius, a serious safety violation. Always read the footnote defining which radius convention the manufacturer used.