The 82% Pick on Fill Beside an Open Trench
A 60-ton rough-terrain crane is set on fully extended outriggers with 4 ft x 4 ft composite mats, on compacted gravel fill. The pick is a precast concrete vault weighing 12,400 lb; slings, shackles, and a spreader beam add 340 lb. The required radius is 32 ft, where the manufacturer's chart supplied with the crane (76 ft main boom, outriggers fully extended, stowed jib, counterweight as configured) shows 15,600 lb. Company lift-plan policy caps planned utilization at 75 percent of chart capacity. Wind is 18 mph gusting to 26 mph from the side; the vault presents about 32 sq ft of sail area. One outrigger float sits 8 ft from the edge of an unshored 6-ft-deep utility trench cut in the same fill. The chart shows a maximum outrigger reaction of 62,000 lb for this configuration.
1. Compute the gross load and the utilization. May this pick proceed as planned under company policy?
Read the worked answer
Gross load = 12,400 lb (vault) + 340 lb (rigging) = 12,740 lb
Utilization = 12,740 / 15,600 = 81.7%
- The 75% cap is a planning limit, not the chart limit. 81.7% exceeds it, so the lift cannot proceed as planned even though it is comfortably under the charted 15,600 lb.
- Before anyone argues about the cap, confirm the chart actually matches the machine as configured: boom length, outrigger spread (fully extended vs. partial), counterweight, and whether the chart notes require deducting the weight of the stowed jib, hook block, or other attachments. If the notes require those deductions and they were not taken, the real net capacity is lower than 15,600 lb and utilization is higher still.
- Acceptable fixes: reduce the radius by re-siting the crane or repositioning the pick, remove weight from the load (set the vault in two pieces, strip rigging), or bring in a larger crane. Recompute against corrected net capacity after any change.
2. Using the 62,000 lb maximum outrigger reaction, evaluate the ground and the proximity of the trench. What must happen before the crane is operated?
Read the worked answer
Bearing pressure check: 62,000 lb over a 4 ft x 4 ft mat (16 sq ft) = 3,875 psf at the most heavily loaded corner. That is a serious load, but the pad itself is not the problem - the trench is.
Why the trench controls:
- The excavation is 6 ft deep, unshored, and cut in the same fill the outrigger is bearing on. A common rule of thumb is that surcharge must be kept back at least 1.5 times the excavation depth unless a competent person or engineer approves otherwise - here that is roughly 9 ft, and the pad edge is at 8 ft. It is inside the zone of influence, not outside it.
- Fill adjacent to a trench is often looser and may have been disturbed; vibration from the crane and from the pick can cause it to consolidate or slough, and a float that drops a few inches changes both level and radius without warning.
Before operation: relocate so no float is within the influence zone of the trench (or have the trench shored/sloped and the setup reviewed by a competent person or engineer); use a properly sized crane mat rather than scrap plywood; never place a float on trench spoil; re-walk and re-check the setup after rain or after any excavation activity nearby.
3. The wind is 18 mph gusting to 26 mph. What is the go/no-go decision and the reasoning behind it?
Read the worked answer
Decision: hold the lift.
- Most manufacturers of this class of crane publish a maximum in-service wind speed in the 20 mph range in the operator's manual, and the manual limit - not the chart - governs. A 26 mph gust exceeds it.
- The static wind force on 32 sq ft of sail at 26 mph is only on the order of 80-100 lb, which is small against a 15,600 lb chart. The hazard is not the force, it is control: a flat-sided vault weathervanes, the load rotates, the taglines cannot restrain it, and the load can swing out and increase the effective radius or side-load the boom.
- Gusts must be measured at boom-tip height with an anemometer, not estimated at ground level; wind at 60 ft is routinely higher than at the cab.
- When the lift is rescheduled, require a written lift plan with two taglines and tagline handlers positioned outside the fall zone, and stop the lift if gusts approach the manual limit.