Load Chart Selection with a Fly Jib and Reduced Counterweight
You must lift a 9.5 t steel vessel from a transport trailer to a foundation 30 m away. The only available crane is a 200 t crawler with a 60 m main boom and a 12 m fly jib offset at 20°. The crane is configured with a reduced counterweight to keep transport weights down. The chart for this configuration shows: main boom at 30 m radius, 11.2 t; with fly jib at the same radius, 8.4 t. The vessel's rigging (sling, shackles, hook block) weighs 0.9 t. Ground is firm clay with a verified bearing capacity of 150 kPa. The transport trailer cannot be moved closer than 30 m.
1. Assess whether the lift can be made with this configuration and identify the chart-reading traps.
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The gross load is 9.5 t plus 0.9 t rigging = 10.4 t. Against the main boom figure of 11.2 t that looks marginal but possible; against the fly jib figure of 8.4 t it fails outright. The fly jib is required because the vessel must be placed beyond the main boom's reach, so the governing chart is the fly jib at 8.4 t - 10.4 t exceeds it by 2 t. The lift cannot be made in this configuration.
Traps: reading the main boom line when the fly jib is fitted; ignoring rigging mass; ignoring the reduced-counterweight condition, which changes both capacity and stability and may impose a lower wind or radius limit; and assuming the 30 m radius is the maximum - the radius grows as the boom deflects under load and as the load swings, so the chart radius must be the worst-case operating radius, not the planned one. The correct action is to reconfigure: fit full counterweight if transport and ground allow, reduce the fly jib offset, reposition the crane, or use a larger crane. Do not 'just try it' - the chart is a limit, not a guide.
2. How would you verify the ground and set-up for the crawler if the crane were reconfigured to a compliant arrangement?
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A crawler spreads load through its tracks, but the pressure is not uniform: it rises under the boom side, during slewing, and when the load is at maximum radius. Use the manufacturer's ground-bearing pressure data for the actual configuration, including track load distribution, and compare against the verified 150 kPa. Firm clay at 150 kPa may be adequate for nominal track pressure but marginal under the peak track load; require a geotechnical confirmation for the specific crane, or place crane mats or a engineered pad to distribute load.
Check the travel path and set-up area for soft spots, services, excavations and slopes; the crane must be level within the manufacturer's tolerance, because out-of-level operation derates capacity and loads the tracks unevenly. Confirm the counterweight actually fitted matches the chart used, and record it. Barricade the slew zone, confirm the trailer position is within the planned radius, and recheck the radius at the moment of pick-up with the load just clear of the deck.