Out-of-level derrick base on a temporary pad
A small derrick is set up on a temporary steel pad beside a bridge abutment. The pad was levelled with packed timber, and the base is supported on four outrigger-style feet. The operator notices the mast is leaning about 1.5 degrees off vertical, which is within the manufacturer's 2 degree tolerance. The ground beneath one foot has begun to settle, and a crack has appeared in the packed timber under that foot. The load chart assumes the crane is level within 1 degree. The next lift is a 1.8 t generator at a 14 m radius; the chart at 14 m gives 2.2 t. The dogger says the lean is within tolerance and the chart has margin, so the lift can go ahead while the pad is monitored.
1. Is the lift acceptable? Address the lean, the chart assumption, and the settling.
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No. There are two separate problems. First, the chart assumes the crane is level within 1 degree, but the mast is at 1.5 degrees. The tolerance for erection is not the same as the tolerance for the load chart. At a lean, the load radius increases on the low side and the derrick's stability is reduced, so the 2.2 t chart value no longer applies to the actual geometry. Second, the settling foot and cracked timber show the support is failing. A support that is moving is not a support, and the lean will increase as it settles further.
The lift must stop. The crane should be lowered and the pad rebuilt so all feet bear on a stable, level base, with the load spread over adequate area. The base should then be checked for level within the chart tolerance before any lift. Monitoring during a lift is not a control; by the time movement is seen, the crane may already be past the point of recovery.
2. What would you record and communicate before returning the derrick to service?
Read the worked answer
Record the observed lean, the settling, and the cracked timber, with the time and the lift that was stopped. Record the re-levelling work: the pad design, the bearing material, the load-spreading arrangement, and the measured level after the rebuild. Record the competent person's verification that the base is level within the chart tolerance and that the support can carry the design reactions. Communicate the stop and the return-to-service decision to the supervisor, the rigging crew, and the next operator, so the history is not lost at shift change. Note any restriction on the crane, such as a reduced radius or load, imposed by the base condition. The documentation should allow a later reader to see why the crane was stopped and why it was safe to restart.