How to Read a Mobile Crane Load Chart: A Step-by-Step Guide for Riggers and Lift Planners

The Most Dangerous Piece of Paper on a Lift Site

The mobile crane load chart — sometimes called the crane rating chart or capacity chart — defines the maximum rated load the crane can handle at every combination of radius, boom configuration, and operating sector.

Misreading a load chart does not just result in a failed lift. It results in crane tip-overs, dropped loads, and fatalities. The OSHA data on mobile crane accidents consistently identifies incorrect load chart application as a primary contributing factor in tip-over events.

This guide provides a systematic method for reading load charts correctly, understanding the critical variables, and applying the appropriate derating factors.

What a Load Chart Actually Tells You

A load chart does NOT give you the total weight you can pick up. It gives you the maximum rated capacity at the hook at a specified radius, in a specified configuration, under ideal conditions.

Everything that reduces from ideal conditions reduces the number you can use from the chart. The typical variables:

The Anatomy of a Load Chart

Most mobile crane load charts (following SAE J765 for telescoping cranes; ASME B30.5 governs US operations) contain these sections:

1. Configuration Header

At the top of each table or chart page: the exact crane configuration the chart applies to. Always check:

• Boom type: Main boom only; main boom + jib; lattice boom; hydraulic telescoping
• Boom length: Length of main boom in metres or feet
• Counterweight: Total counterweight fitted (e.g., 12,000 kg; 24,000 kg). Wrong counterweight = wrong chart.
• Outrigger position: Full extension (typically stated in feet/metres span); partial extension; rubber (no outriggers)
• Operating sector: 360° / 0°–360° or restricted front/rear sectors (some cranes have higher rear capacity)

Never assume the chart you are reading applies to your configuration. Flip to the correct section first.

2. The Capacity Table

The main body of the chart is a matrix:

• Rows: Working radius (typically in 1m or 2ft increments from minimum to maximum)
• Columns: Boom length (various configurations)
• Cell value: Maximum rated load at that radius / boom length combination, in tonnes or US tons

(Illustrative values — always use the crane's actual rated load chart)

Two critical points about reading this table:

3. The Load Line and Deductions

Hook block weight: Every crane has a rated hook block weight. This is always deducted from chart capacity. Example: A crane with 25t chart capacity at a given radius, using a 5-sheave hook block weighing 800 kg, gives a net load capacity of 24.2 tonnes.

Below-the-hook equipment deduction: Spreader beams, lifting frames, magnets, and all other below-the-hook devices reduce net capacity. A 500 kg spreader beam further reduces the above example to 23.7 tonnes.

Formula:

Net Load Capacity = Chart Capacity – Hook Block Weight – Rigging Weight – Below-Hook Device Weight

4. The Jib / Fly-Jib Section

When a fixed or luffing jib is installed, the crane operates from an entirely separate load chart section. Jib charts:

• Have significantly lower capacities than main boom charts
• Are sensitive to jib offset angle (typically 0°, 15°, 30°)
• Have minimum and maximum operating radii different from the main boom

A common error: using the main boom chart when the jib is fitted. The jib is structurally separate from the main boom; applying main boom chart values risks structural overloading of the jib.

5. The Structural / Tipping Boundary Line

Load charts contain two limits, and the lesser always governs:

• Structural limit: Based on boom, jib, and structural member stress limits. Usually governs at shorter radii.
• Stability (tipping) limit: Based on crane tipping about the fulcrum point (outrigger or crawler track edge). Usually governs at longer radii.

The changeover point between structural and stability limits is visible on some charts as a line or shade change. Chart values are already the minimum of the two at each point.

ASME B30.5 applies a derating factor to published SAE rated loads: Chart values for crawler cranes are published at 75% of tipping load. For wheel-mounted cranes on outriggers: 85% of tipping load. These safety margins are built into the published numbers — do not add a further general safety factor on top of chart values. The safety factor is already applied.

Working Out the Radius: The Step People Get Wrong

The working radius is measured from the crane's centre of rotation (slewing centre pin, not the outrigger or the hook position on the ground) to the point directly below the hook.

Common errors:

• Measuring from the side of the crane body to the load (underestimates radius; over-estimates capacity)
• Measuring to the load's centre of gravity rather than the sling attachment point (usually negligible but check for wide loads)
• Forgetting that boom deflection under load increases the effective radius (especially important for long boom configurations; manufacturers provide boom deflection tables)

For critical lifts, calculate the maximum radius the hook can be at for the load you are lifting, then add 500mm as a safety buffer before reading the chart. This accounts for minor position variations and boom deflection.

Outrigger Configuration and the Capacity Table You Must Use

This is where the most dangerous load chart errors occur. Operating with partially extended or unextended outriggers uses a completely different — and significantly lower — capacity chart.

Ground bearing pressure is a separate issue from outrigger extension. Even with full outrigger extension, if the ground conditions cannot support the outrigger pad load, the outrigger will sink, increasing radius and reducing stability. Always calculate outrigger pad pressure using the crane's maximum outrigger load chart and verify against geotechnical data for the site.

Wind Load Derating

Most load charts are published for wind speeds at or below 9.8 m/s (approximately 35 km/h or Beaufort 5). Above this threshold:

• The wind imposes additional overturning moment on the crane
• ASME B30.5 requires the lift to be suspended or the crane to be de-rigged above 40 km/h for most configurations
• At wind speeds between 25–40 km/h, evaluate whether the lift should proceed using the Appointed Person / lift supervisor's judgement

Large, flat, or sail-area loads (panels, pre-cast elements, tanks) are significantly more wind-sensitive than compact, dense loads of the same weight.

A Practical Step-by-Step Checklist for Load Chart Use

Key Takeaways