Load Chart, Radius, and Ground Bearing on a Tight Site
A 50-ton rough-terrain crane is set up on a compacted gravel pad to lift a 14,000 lb pump skid from a truck at a 30 ft radius and set it at a 42 ft radius, 90 degrees around the swing. The boom is 70 ft long with no jib. Outriggers are fully extended on mats. The site is tight: a building wall is 20 ft from the swing path, and a trench runs along one side. The operator's load chart shows 16,500 lb capacity at 30 ft and 11,800 lb at 42 ft for the configured boom length. The ground under one outrigger mat is soft from recent rain. The crew wants to make one pick and swing without reconfiguring.
1. Evaluate the lift against the load chart and determine whether the plan is acceptable.
Read the worked answer
Capacity at the two radii:
- At 30 ft: 16,500 lb capacity vs 14,000 lb load, margin about 2,500 lb (about 18 percent).
- At 42 ft: 11,800 lb capacity vs 14,000 lb load. The load exceeds capacity by 2,200 lb. The lift is not acceptable at the set radius.
The swing problem: The load is picked at 30 ft and set at 42 ft. As the boom swings out, the radius increases and capacity falls. The load is over capacity before it reaches the set point, so the crane would be overloaded during the swing, not just at the end.
Other factors: The chart values assume level ground, fully extended outriggers, and the stated boom length. Soft ground under one outrigger invalidates those assumptions. The building wall 20 ft from the swing path and the trench are additional hazards.
Conclusion: The plan is not acceptable. Options: move the crane closer to the set point to reduce the set radius, use a larger crane, reduce the load (partial disassembly), or pick and set in two positions with a re-rig. Do not make the swing as planned.
2. What must be done about the soft ground under the outrigger, and how does it affect the load chart?
Read the worked answer
Effect on the chart: Load charts assume the crane is level and the outriggers are on stable ground that can support the reactions. Soft ground allows the outrigger to settle, which tilts the crane, shifts load to other outriggers, and can cause a tipping failure. The chart capacity no longer applies.
Required actions:
- Stop and assess the soil. Use the crane's outrigger reaction values from the manufacturer and compare them with the bearing capacity of the pad and soil.
- Improve the support: larger mats or crane pads to spread the load, or compacted crushed stone, based on the calculated reactions.
- Re-level the crane after the support is improved and verify level with a bubble level or inclinometer.
- Re-check the outriggers for settling during the lift; a spotter should watch them.
- Keep the outriggers fully extended and the mats in place; never block them partially.
Do not: add a single mat under one outrigger only, or rely on the outrigger float alone. The support must be designed for the actual reactions, and the lift must be re-evaluated after the ground is fixed.
3. Describe the lift plan elements and communication needed for this pick, including the swing path hazards.
Read the worked answer
Lift plan elements:
- Load weight and CG, rigging weight, and the calculated tension in each sling.
- Crane configuration, boom length, and the capacity at every radius in the swing path, not just the pick and set points.
- Ground support and outrigger reactions.
- Swing path and the exclusion zone, including the building wall and the trench.
- The set point and its support.
Hazards:
- The building wall limits the swing and could strike the load or boom.
- The trench is a collapse and fall hazard; keep personnel and outriggers away from its edge.
- Soft ground under one outrigger.
Communication:
- One designated signal person, with a backup, using agreed hand signals or a radio channel.
- The operator stops on any unclear signal.
- Tag lines on the load to control rotation, with personnel outside the fall zone.
- A pre-lift meeting with the crew, the signal person, and the operator covering the plan, the hazards, and the stop-work authority.
- A spotter watching the outriggers and the swing path.
Critical point: The plan must show capacity at the worst radius in the swing, which is the set radius here, and that radius exceeds capacity. The lift cannot proceed until the plan is changed.