Outrigger Setup Beside a Backfilled Trench
An operator is setting up a 28-ton boom truck to lift a 3,200 lb transformer from a flatbed to a concrete pad. The chosen radius is 18 ft, where the chart shows 9,400 lb gross capacity. The rear right outrigger float, 20 in by 20 in, lands 4 ft from the edge of a 6-ft-deep utility trench that was backfilled three weeks ago with sandy silt. The ground slopes about 3.5% down toward the trench, and there are no compaction records. The LMI estimates 14,500 lb on that outrigger. The operator proposes a single 2 in by 12 in plank under the float and says the outrigger will firm up the ground. The site is tight, and the only alternate position is 15 ft farther from the truck, which would increase radius to 24 ft. Chart capacity at 24 ft is 6,800 lb.
1. Calculate the approximate ground pressure under the float and evaluate whether the 2x12 plank is adequate. Show your units.
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- Float area: 20 in × 20 in = 400 in². Bare float pressure = 14,500 lb ÷ 400 in² = 36.25 psi (5,220 psf).
- A single 2×12 plank has actual dimensions about 1.5 in × 11.25 in. If it spans 24 in under the float and the float bears on the 11.25-in width, contact area ≈ 270 in², so pressure ≈ 53.7 psi. Even if full 20 in × 24 in contact is assumed (480 in²), pressure is ≈ 30.2 psi.
- Disturbed sandy silt fill with no compaction records should not be assumed to carry 30–54 psi (4,300–7,700 psf). Typical allowable bearing for such fill may be only 1,500–2,000 psf (about 10–14 psi). The plank is not adequate; use an engineered mat or cribbing designed for the actual outrigger load, or relocate.
2. What trench-edge, slope, and leveling checks must be completed before this lift, and who should approve the setup?
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- Trench edge: 4 ft from a 6-ft-deep trench is inside the zone of influence of the excavation. Do not accept without a competent person and geotechnical evaluation. A common rule is to keep outriggers back at least the trench depth, or greater if the soils engineer requires it; the slope also adds load toward the trench.
- Slope: about 3.5% (roughly 2°) exceeds the typical manufacturer limit of 1% or less for crane levelness. Level the crane with cribbing or mats under the outriggers, not by adjusting the outrigger beams, and verify with the machine level indicator.
- Verify all outriggers are fully extended, floats are centered on mats with no voids, no underground utilities are present, and the LMI radius is correct. The lift plan should be approved by the site competent person and, for the trench, a qualified geotechnical person.
3. If the alternate position is used, does the lift remain within capacity after deducting 420 lb of hook block and rigging? Explain.
Read the worked answer
Gross required = 3,200 lb load + 420 lb hook block and rigging = 3,620 lb. At 24 ft the chart gives 6,800 lb, so 3,620 lb is less than 6,800 lb; capacity is adequate with about 3,180 lb margin. But re-verify the actual radius at the load center, the quadrant, outrigger extension, and ground conditions at the new position. The alternate position is not automatically acceptable only because the chart number is higher.