Jib Capacity, Missing Keepers, and a Defeated Anti-Two-Block
A 120 t crawler crane is assembled with a 42 m main boom and an 18 m jib offset at 30°. The lift is a 2.9 t air-handling unit to a roof at 36 m radius. The only load chart in the cab is for the main boom, and at 36 m radius with the 42 m boom it shows 4.1 t. The jib and its pendants add 1.8 t; the hook block weighs 0.5 t. The crew plans to use the main boom chart, subtract 1.8 t, and treat the remaining 2.3 t as capacity; the unit plus slings weighs 3.15 t. The boom is assembled with two missing boom-pin keepers, the boom stop is bent, and the crane is set on a 2.5% cross-slope with no mats. The LMI is configured for main boom only, and the anti-two-block weight is tied off to the boom tip because the jib was added. The site engineer says the roof is ready and the crane must be moved after the lift.
1. Evaluate the crew's capacity method. Is the lift permitted?
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The method is wrong. A jib is a different boom configuration: use the manufacturer's jib load chart for the 42 m main boom, 18 m jib, 30° offset, at 36 m radius and 360° swing. The main-boom chart does not become a jib chart by subtracting 1.8 t.
Even their arithmetic fails: they forgot the 0.5 t hook block. 4.1 - 1.8 - 0.5 = 1.8 t, not 2.3 t. The unit plus slings is 3.15 t, so it exceeds even their incorrect net figure by 1.35 t. A proper jib chart will usually be lower than the main-boom chart at the same radius, so there is no basis to proceed. Deduct the hook block and all rigging from the jib chart's rated capacity, and confirm the chart is for the actual counterweight and track configuration. If the resulting net capacity is below 3.15 t, the lift is not permitted with this configuration.
2. What do the missing boom-pin keepers, bent boom stop, cross-slope, and LMI configuration indicate? What must happen before assembly is accepted?
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All four findings are stop items:
- Missing boom-pin keepers: pins can walk out under load, releasing the boom. Replace keepers and inspect all pins.
- Bent boom stop: the boom stop limits overhoist; a bent stop may not engage. Repair or replace and function-test.
- 2.5% cross-slope, no mats: crawler cranes are typically limited to about 1% level; check the manual. 2.5% induces side load and swing drift. Level the crane and use mats.
- LMI configured for main boom only, anti-two-block tied off: the LMI will not know the jib geometry and the anti-two-block is defeated. Configure the LMI for the actual boom and jib combination, install and test the jib anti-two-block, and verify radius and angle against the chart.
Do not assemble or lift until a competent person inspects the boom, pins, stop, and LMI. A function test with the jib is required before the load.
3. Propose a safe alternative lift plan and the documentation needed.
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Alternative plan: use a larger crane or a configuration that gives net capacity above 3.15 t at 36 m. Options:
- Reduce radius by moving the crane closer, if the roof and ground allow, and recheck the jib chart.
- Use a longer main boom without jib if it can reach 36 m with adequate capacity.
- Set the unit with a larger crane, or hoist it in smaller pieces.
- If the jib is required, use the correct jib chart, correct counterweight, level ground, and LMI and anti-two-block setup.
Documentation: revise the lift plan with the correct chart and calculations, attach the chart page, record the LMI configuration and anti-two-block test, complete a pre-lift meeting, and have the competent person sign the setup inspection. After the lift, log the actual radius, load, and any defects.