Blind Landing, Lost Signal and a Shock Load on the Jib
Concrete placing on a 14-storey frame with a 1.2 m³ skip (0.35 t empty, about 3.2 t full). The operator in the cab can see the hoist path but not the landing point on level 12, which is behind a stop-log. The designated signal person is on level 12 with a radio, in a position where the landing is visible but the hoist path is not. A relief signal person stands at ground level with a clear view of the whole path but no view of the landing. The landing is at 34 m radius where the chart gives 4.2 t. On the third cycle the skip's discharge gate catches a rebar cage, the load moment limiter flashes 96%, the operator hears a bang and sees the jib deflect, and a gust swings the skip toward the edge scaffold. After the skip is landed, the foreman wants 12 more skips to finish the pour.
1. State the operator's immediate actions from the moment of the catch and bang.
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In sequence:
- Stop hoisting and slewing the instant the load catches. Do not try to power through, and do not lower further into the obstruction.
- Hold the load steady, then take the strain off by hoisting or lowering slightly in the direction that frees the gate, guided by the signal person now that they can see the contact.
- Land the skip at the nearest clear, safe location, never over people, and take the weight off the jib.
- Report the event to the supervisor; do not resume lifting.
The bang and jib deflection are evidence of a shock load. Before any further use, a competent person must inspect the jib root and jib sections for cracked welds and deformed bolts, the hoist rope over its full length and especially at the sheave, the sheaves and drum, the trolley, the slew ring area and the LMI/limit functions. The skip gate and rigging are also part of the inspection. Any defect found removes the crane from service until repaired and function-tested. The 12 remaining skips are not a reason to skip this.
2. Evaluate the signalling arrangement for this blind landing and specify what must change before the pour continues.
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The arrangement is not adequate. The person responsible for landing must be able to see the load and the landing point; the person who can see the whole path cannot see the landing, so neither can direct the final placement. Relayed signals through a second person are a known error source, and the operator must never guess.
Requirements:
- One designated signal person in charge at any time. Only that person signals; everyone else stops if there is doubt.
- Reposition the level 12 signal person so they can see both the load and the landing, with a clear escape route and not standing under the load.
- A dedicated radio channel, agreed and unambiguous commands, and a pre-lift briefing covering the blind portion.
- If the signal is lost or unclear at any point, the operator stops and holds; a lost signal is a stop command.
- The ground person may assist with the travel portion only under an explicit, acknowledged handover.
If a position with sight of both cannot be found, change the method (camera, spotters at the landing with a shorter final landing, or a different landing point) rather than proceed on relayed signals.
3. The LMI read 96% during the catch, at a radius where the chart allows 4.2 t for a 3.2 t skip. What does this tell you, and can the pour continue at the same landing point?
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The static numbers looked comfortable: 3.2 t against 4.2 t is about 76%. The 96% reading shows what the LMI actually saw once dynamics were included: the snagged gate arrested the load, so the rope and jib took an impulse well above the static weight, and the gust added side load. The LMI reports; it does not prevent a shock load.
Therefore the pour cannot continue on the same basis:
- The geometry must change so the skip cannot drag across or catch the rebar cage: reposition the landing point closer in, use a rigger with a tag line to guide the gate, or provide a clear approach path.
- The radius must be re-checked at the new landing point, and the LMI limits confirmed.
- The crane must first pass the post-shock-load inspection and the limit/LMI function tests.
- Any repeat of a 90%+ indication with a snag is a stop-and-investigate event, not a normal cycle.
Continuing 12 identical cycles with the same catch point is a predictable repeat of the same overload.