Jib Capacity and Deductions at Long Radius
A 250-ton crawler is configured with 150 ft of main boom and a 30 ft jib at 20 degree offset, setting steel on the far side of a foundation. The main boom chart gives 46,000 lb at 45 ft radius. The jib chart gives 11,000 lb at 60 ft jib radius and 9,800 lb at 65 ft. The notes state: jib capacities include the weight of the jib and its pendants; deduct the jib hook block and all rigging; deduct the jib weight from main boom capacity at the boom point radius. The jib assembly weighs 3,400 lb, the jib hook block 500 lb, the pick 9,000 lb, slings 300 lb. The operator plans to set the main boom at a 45 ft radius, which puts the pick point at about 62 ft jib radius.
1. Walk through both chart checks and decide whether the pick is legal.
Read the worked answer
Check 1 - main boom structural (the jib hangs on it):
- Main boom at 45 ft radius = 46,000 lb.
- Deduct jib assembly 3,400 lb -> ~42,600 lb available. The jib's load reaction is nowhere near that. Main boom passes.
Check 2 - jib (this one governs):
- Gross load on the jib = 9,000 + 300 + 500 = 9,800 lb.
- The pick point is at 62 ft. The chart gives 11,000 at 60 ft and 9,800 at 65 ft. Unless the notes expressly permit interpolation, you use the next longer radius: 9,800 lb at 65 ft.
- 9,800 lb of load against 9,800 lb of capacity = 100% of chart.
Verdict: not legal. A chart value is a maximum, not a target. At 100% there is no allowance for wind, for swing and stopping the load, for the fact that radius is measured on the deflected boom, or for the jib flexing under load - which will push the working radius past 65 ft, where the chart may not even exist.
Fixes: reposition so the jib radius is 55-58 ft; reduce the jib offset angle; pick in two pieces; or bring in a larger boom/jib combination and build a fresh chart. Never extrapolate beyond the last chart line, and never interpolate unless the notes say you may.
2. The operator says the radius was measured with a tape on the ground before the boom was loaded, and that the 62 ft figure already has a foot of pad in it. How do you respond?
Read the worked answer
Ground-tape radii taken on an unloaded crane are not operating radii.
- Load chart radius is horizontal from the crane's rotation center to the center of the hook, with the load on and the boom deflected. That is how the chart was developed.
- On 150 ft of main boom plus a 20 degree offset jib, deflection under load at that radius can easily move the hook a couple of feet outward. So the true radius is bigger than 62 ft, not smaller.
- The jib also flexes and the offset angle effectively changes under load.
What to do instead:
- Re-measure the loaded radius - use the boom-angle/boom-length geometry or the LMI radius reading, verified against an independent angle check, with the actual load hanging.
- Set up so the working radius lands in the middle of a chart step, not at a break point.
- Verify the LMI is calibrated and reading the correct boom/jib configuration (a wrong configuration entry gives a confident, wrong number).
- Keep the load low and slow through the swing; every acceleration adds radius and load.