21.2 Crane Charts, Suspended Weight and Capacity Deductions

The correct chart comes before arithmetic

A crane load chart states capacity for a specific crane configuration and operating condition. Determine the actual model, boom/jib arrangement, counterweight, outrigger or crawler configuration, operating area and radius. A capacity printed elsewhere in the chart does not apply merely because it is large enough.

Load radius is generally the horizontal distance from the crane's axis of rotation to the load's vertical line of action, interpreted according to the chart. Boom length is not radius. Deflection under load can change radius, so the operator must use the correct loaded geometry and chart instructions.

The rigger supplies reliable load weight, approximate CG, pick and set positions, rigging arrangement and relevant device weights. The operator verifies the crane's capacity and permissible operation. These responsibilities complement each other. A Level II rigger must understand the calculation without pretending to qualify as the operator of every crane type.

Inventory every applicable weight

The crane may support payload, slings, shackles, lifting beam, hoists, hook block or ball, load line and other items identified by the chart. Some charts express gross capacity and require deductions; others define entries differently. Read the chart's notes so each item is counted once and the same accounting basis is used in the comparison.

Do not omit a device because it has its own WLL. Do not deduct a block from net capacity and then add the same block again to demand. An arithmetic result is only meaningful when its inventory and capacity definitions match.

Gross-capacity example

Assume a supplied chart gives 24,000 lb gross capacity at the governing radius. Its instructions require deduction of a 900-lb hook block, 500-lb lifting beam, 200-lb slings/hardware and 100-lb applicable load-line allowance. These stated example values are not universal equipment weights.

$$\text{Net payload capacity}=24{,}000-(900+500+200+100)=22{,}300\text{ lb}$$

A 21,000-lb payload is below the resulting 22,300-lb payload limit. Equivalently, total demand is 22,700 lb compared with 24,000 lb gross. The gross-basis utilization is $22{,}700/24{,}000\approx94.6\%$. The comparison still does not approve weather, ground, structural attachments, operating sector or an employer's lower utilization limit.

Find the governing stage

Check the full pick-to-set route. Capacity can decrease at a greater radius or in a different operating sector. A load safely picked close to the crane may exceed the chart during placement. The limiting stage may also occur while transferring support between cranes or moving a rotating load.

Use only chart interpolation or rounding methods expressly permitted. If a radius falls between entries, do not create a higher interpolated capacity unless allowed. Follow the maker's specified next-entry or other procedure. Likewise, do not apply a fully extended-outrigger chart to a partially extended setup without its approved chart.

Changed-radius example

The same load has 22,700 lb gross demand. At the final radius the applicable supplied chart permits only 21,500 lb gross. Initial pick capacity does not authorize this destination. Change the setup, crane, load or route through an approved revised plan, then repeat checks. A stronger sling cannot increase the crane's chart capacity.

Understand what utilization does not mean

A project may define a critical lift at a particular utilization percentage. Other triggers can include personnel hoisting, multiple cranes, unusual consequences or proximity to occupied facilities. There is no single federal percentage that universally defines all critical lifts, and a critical-lift designation does not replace capacity verification.

OSHA 1926.1417 requires operation within rated capacity and reliable weight verification. Its provisions permit specific alternatives for verifying load weight; they do not mean any uncertain load can be lifted until an overload alarm sounds. The operator must follow the applicable procedure, including the conditions on using load-weighing indications.

A crane's rated-capacity limiter, anti-two-block device or load indicator provides a particular function. Such aids do not replace chart understanding, required equipment checks or the rigger's load information. An absence of alarm is not proof of safe lifting points, approved sling geometry or suitable ground.

Additional limits remain separate

Ground support and level condition affect the crane's stability. Wind can affect a large sail-area load even below a weight limit. Side pulling or dragging can introduce forces outside chart assumptions. Follow manufacturer limits and the approved plan rather than reducing a number by an arbitrary percentage to compensate.

For personnel hoisting, OSHA applies a specific 50% criterion to the total defined load, except proof testing. Do not use ordinary payload capacity as the sole personnel-hoisting calculation. Multiple-crane operations need each crane's own load inventory, reaction and changing radius; a universal tandem derating does not substitute for those checks.

Source: CCO B30.5 reference and responsibilities, OSHA crane operation, OSHA ground conditions.