Overturning Load and the Travelling Crane
You operate a rail-mounted tower crane that travels along the site on a prepared track. The track was laid on compacted fill, and it has rained heavily for two days. You are asked to travel with a 1.9 t load of scaffold tube in a stillage, trolley at 12 m, to the far end of the track 40 m away, then place the load. The chart for travelling gives 2.2 t at 12 m. As you begin to travel, you notice the crane's rail level looks slightly low near a joint, and the stillage swings gently as the crane moves. The supervisor says to keep going, it is 'only a short run'. There is also a 400 V site distribution board 3 m from the track at the far end, at 2.5 m height.
1. What are the stability issues with travelling this load, and what will you do?
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Travelling with a suspended load is the least stable mode of tower crane operation, and several things here point the wrong way.
- Track condition. Heavy rain on compacted fill can soften the subgrade and cause differential settlement at a joint. A low rail means the crane is no longer level; the load's moment then acts on a tilting base, and the rail bogies carry uneven load. This is a direct overturning precursor, not a cosmetic issue.
- Load behaviour. A stillage of scaffold tube has a high centre of gravity and loose contents. As the crane travels and the rail joint is crossed, the stillage swings; the swing adds a dynamic overturning moment that the static chart figure does not include.
- Chart margin. 1.9 t against 2.2 t travelling is a thin margin for a dynamic, swinging load on a suspect track. The chart assumes a level, sound track and a load under control.
- The board. A 400 V board 3 m from the track at 2.5 m height is inside the zone the swinging stillage could reach if control is lost.
Action: stop travel, lower the load to the ground clear of the track, and report the low rail joint. Do not travel further until the track is inspected and the crane levelled. Re-plan the route and the load: reduce the radius, use a lower-profile stillage or break the load down, and confirm the exclusion around the board.
2. How would you plan the travel and placement to control the risks once the track is cleared?
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Once the track is inspected, the joint corrected and the crane levelled and recorded, the travel is planned as follows:
- Reduce the moment. Travel with the trolley as close in as the chart and the load allow, so the load radius is minimised. Confirm the travelling chart for that radius and configuration.
- Control the load. Confirm the stillage is loaded so it cannot shift, is slung from its designated points, and has tag lines. A load that cannot swing is a load whose dynamic moment is predictable.
- Route and exclusion. Walk the route and mark the exclusion zone, including the 3 m clearance to the 400 V board at the far end. If the stillage could enter that zone, the board needs to be isolated or the route changed; the crane does not travel past it with a suspended load until that is resolved.
- Travel discipline. Travel at low speed with a spotter walking the track ahead, watching for further settlement and for obstructions. Stop at the joint and check level if anything looks wrong.
- Placement. Land the load in a designated laydown area, not under the board, and only then release the slings. Never leave a suspended load while the crane is unattended.
- Record. Log the track defect, the inspection and the revised plan, and report the supervisor's earlier instruction.