Crawler Crane Lift Near Live Line on Fill
During metro viaduct construction, a 90 t crawler crane must lift a 28 t precast segment at 22 m radius. The load chart gives 32 t capacity at that radius, so the lift is at 87.5% of capacity; the site lift plan limits lifts to 75%. The boom tip will be 7.5 m from an 11 kV overhead line. Wind is 18 km/h gusting to 28 km/h; the crane's limit is 32 km/h. The crane stands on compacted fill with an estimated bearing pressure of 180 kPa; the outrigger mats are 6 m × 2 m but only two mats are available. The segment must be placed before the evening concrete pour, and the line can be isolated only by a scheduled shutdown in four hours. The rigger says the load is 'light for this crane' and proposes to proceed while the electrician watches.
1. Should the lift proceed? Show the capacity, wind, electrical and ground checks.
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No, not as planned. The lift is 28/32 = 87.5% of the load chart, above the site's 75% limit; at 75% the maximum load at 22 m is 24 t, so the segment is 4 t too heavy for this radius and configuration. The gust is 28 km/h against a 32 km/h limit, leaving little margin for a 28 t segment with a large sail area. A boom tip 7.5 m from an 11 kV line must be checked against the site's electrical exclusion zone; an electrician watching is not a substitute for isolation or a confirmed safe clearance. Ground bearing pressure is 180 kPa on fill, and only two of four outriggers would be matted. Stop the lift, keep the load on the ground, and re-plan with a revised radius, a larger crane, line isolation, or all four mats.
2. What engineering controls would you require before rescheduling?
Read the worked answer
Obtain a line isolation permit, earth the 11 kV line, or relocate the crane so the boom and load remain outside the confirmed exclusion zone for the full slewing path. Use all four outrigger mats, verify fill compaction and bearing capacity, and increase mat area if 180 kPa is close to the allowable bearing pressure. Recalculate the lift with actual radius, boom length, rigging weight and segment weight, keeping the lift at or below 75% of the chart. Fit a wind monitor and stop at the lower of the manufacturer's limit and the site limit; treat 20 km/h gust as a review point. Use tag lines and a planned load path, brief the crew, and appoint a lift supervisor. Do not use an electrician as a human proximity alarm.
3. How would you handle the schedule pressure and the contractor's instruction to proceed?
Read the worked answer
Put the stop-work in writing, citing the specific breaches: 87.5% against a 75% limit, a 28 km/h gust against a 32 km/h limit, unconfirmed line clearance, and only two mats. Escalate to the site engineer and safety officer. Offer alternatives: use a smaller segment, bring in a larger crane at reduced radius, wait for the four-hour shutdown and isolate the line, or re-sequence the pour. Do not accept verbal assurance or a 'watching' electrician. If instructed to proceed, refuse and record the instruction. The schedule impact is a few hours; a dropped load or a line strike is far worse. After the lift, review the lift plan and add hold points for line isolation, mat verification and wind checks.