Crane Pick at 60 Ft Radius on Loose Fill
A 90-ton truck crane is planned to set twelve 34,000 lb precast wall panels at a 60 ft radius. The load chart for the planned 110 ft boom shows 32,000 lb at 60 ft. The outrigger floats are 2 ft × 2 ft and the maximum outrigger load at that radius is 190 kips. The site is 6 ft of loose fill with an allowable bearing of 1,500 psf over stiff clay, and the crane will sit on that fill with no engineered pad. Panels are 8 ft tall and 24 ft long; lifting inserts are at the third points, and the plan is a two-leg bridle at 45° to the horizontal with a vertical drop to each insert. The panels will be set against an existing masonry wall, and the crew will guide them by hand from the far side.
1. Is the planned pick acceptable with respect to crane capacity? What must change?
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No. Required load 34,000 lb against a chart value of 32,000 lb is 106% of capacity - over the chart before anything else is counted.
Then deduct the rigging. Chart capacity must cover the total suspended load: panel plus spreader beam, slings, shackles, and hook block. That assembly is realistically 1,500–2,000 lb, so the net usable capacity at 60 ft is about 30,000 lb - the panel alone is 113% of it.
What must change (in order of preference):
- Reduce the radius by repositioning the crane so the chart value comfortably exceeds panel plus rigging. Read the chart at the new radius; capacity rises steeply as radius falls.
- Use a larger crane with a charted capacity at 60 ft that leaves a margin.
- Confirm the boom length and configuration - longer boom means lower capacity at the same radius; the 110 ft boom may be unnecessary.
A pick at more than 100% of chart is a stop-work item, not a judgment call. Do not "try one and see."
2. Evaluate the outrigger ground bearing. What mat or pad is required?
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Current condition: each float is 2 × 2 = 4 sf. At 190 kips:
190,000 lb ÷ 4 sf = 47,500 psf, against an allowable of 1,500 psf - an overstress of more than 30 times.
Required bearing area per float: 190,000 ÷ 1,500 = 127 sf
So each float needs at least 127 sf of effective mat, for example a 6 ft × 21 ft timber mat assembly, or three layers of 4 × 12 crane matting cross-laid (144 sf).
Judgment points beyond the arithmetic:
- Mats spread load at roughly 45°, so with 6 ft of fill the pressure at the fill/clay interface is far lower - but do not credit that spread without a geotechnical opinion, because the loose fill itself will punch and shear.
- The fill must be proof-rolled and level; differential settlement on one float tilts the crane and changes the radius.
- Check the travel and pick path, not just the pick position: twelve picks at varying radii need an engineered crane pad, not four mats under one setup.
- Consider removing or improving the loose fill instead of bridging over it.
3. Evaluate the rigging geometry and the crew's work position.
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Sling tension: the two inserts at the third points carry equal vertical load of 34,000 ÷ 2 = 17,000 lb each.
At 45° from horizontal: T = 17,000 / sin 45° = 24,042 lb ≈ 12.0 tons per sling.
- Shackles and slings must be rated at least 12 tons, so specify 13.5- or 15-ton shackles and slings with a 12-ton WLL. A nominal "10-ton" shackle fails.
- Horizontal component = 17,000 lb pushes each insert inward. Verify the inserts and panel reinforcement were designed for it; this is a common cause of insert pull-out.
- The load line must be vertical over the hook; a bridle angle that spreads beyond the hook's plumb line induces a side pull the crane was not charted for.
Crew position: guiding a panel by hand from the far side, between the panel and the masonry wall, puts workers in the fall zone and in a pinch zone. That is prohibited. Require tag lines, a marked exclusion zone, and no worker under a suspended load or between the load and a fixed structure.
Wind: 8 × 24 = 192 sf of sail area. Establish and enforce a stop-work wind speed from the manufacturer's and crane's limits, and a lift plan that reflects a critical lift.