Outrigger Pad Over a Vault
You are the signal person for a 120-ton all-terrain crane lifting an 18,000-pound rooftop unit at a 45-foot radius. The load chart for the boom length and radius allows 22,000 pounds, and the rigger says the load is balanced. One outrigger is set on a steel plate over a buried utility vault. As the operator takes slack, you see the plate settle about 1 inch and a crack open in the compacted fill beside it. The bubble level on the crane is still centered, and the operator says the chart has plenty of margin. Wind is 12 mph. The load is 6 inches off the deck. The general contractor is pressing to set the unit before a forecasted rain band arrives.
1. Identify the hazard you see and explain why the load chart alone does not make the lift safe.
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The hazard is ground bearing failure under the outrigger: the plate is settling and the fill is cracking over a vault. The load chart assumes the crane is level and the outriggers are on stable support; it does not account for a subsurface void or a pad that is sinking. The centered bubble level is not proof-the crane can be level now and still tip if one outrigger punches through. A 1-inch settlement under load is a warning, not a normal set. Stop the lift, lower the unit to the deck, and dog it.
2. What sequence do you follow, and who should make the decision to resume?
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Sequence: give stop, lower the load to the ground, dog it, and keep personnel out of the swing radius. Notify the operator and lift director. Do not signal another attempt until the outrigger support is evaluated-mats, an engineered pad, relocation, or a different setup. The decision to resume belongs to the lift director with the operator and a qualified person, not to the signal person alone. If the load cannot be lowered, stop all movement and clear the area. The schedule pressure and the 22,000-pound chart do not override a failing footing.
3. The rigger says the load is balanced and the operator says the chart is fine. How do you respond?
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Those statements address load weight and balance, not support. The signal person's stop authority is based on observed unsafe conditions. I would say: “I am stopping the lift because the outrigger is settling over a vault; the chart does not cover that.” Then lower the load. No signal to resume until the footing is corrected and the lift director authorizes the new setup.