Load Chart Selection for a Fly Jib Pick at Height
You are asked to lift a 3.8 t steel beam onto a plant room roof 24 m above the crane. The crane is a 60 t slew crane with 40 m of main boom and a 10 m fly jib offset at 30 degrees. The beam must be placed at a radius of 26 m from the slew centre. The load chart for the main boom alone at 26 m and 40 m boom shows 4.5 t. The fly jib chart at the same radius shows 3.2 t. The rigger says the load is under 4.5 t so the main boom chart applies. The beam has a 600 mm wide flange and will be lifted with a two-leg sling at 60 degrees included angle. Wind is calm.
1. Which load chart applies, and does the lift comply?
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The fly jib chart applies, because the crane is configured with the fly jib fitted and offset at 30 degrees. The main boom chart is irrelevant once the jib is in use: the jib reduces the crane's capacity because of the extra moment and the jib's own structural limits. The rigger's reasoning is wrong.
On the fly jib chart the capacity at 26 m radius is 3.2 t. The beam is 3.8 t, so the lift is over capacity by 0.6 t, about 19%. It does not comply and must not proceed.
Options: reduce the radius by repositioning the crane; remove the fly jib and use the main boom if the geometry allows the beam to reach the roof; use a larger crane; or break the beam into smaller lifts. The load chart in use must be the one matching the actual configuration, and the operator should verify the configuration against the chart before every lift.
2. How does the sling angle affect the tension in each leg, and what is the tension at 60 degrees included angle?
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The included angle is the angle between the two legs. At 60 degrees included, each leg is 30 degrees from vertical. The tension in each leg is half the load divided by the cosine of the angle from vertical: 1.9 t divided by cos 30 degrees, which is 1.9/0.866, about 2.19 t per leg.
As the included angle widens, the leg tension rises sharply. At 90 degrees included the factor is 0.707, giving about 2.69 t per leg, and at 120 degrees it is 0.5, giving 3.8 t per leg, equal to the whole load. This is why sling angles are limited and why the rigger must not shorten the sling to gain height without recalculating.
The sling assembly, shackles and beam lifting points must all be rated for at least the calculated leg tension with the appropriate factor of safety, and the beam's flange must be checked for the lateral force the sling imposes.
3. What would you check about the beam and rigging before accepting the lift, even if the capacity were adequate?
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I would check the beam's mass against its documentation, not the rigger's estimate, and confirm the centre of gravity so the beam hangs level. The lifting points must be suitable for the beam and rated for the load, and the sling must be protected from sharp edges on the 600 mm flange.
The sling assembly must be inspected for damage, tagged and rated for the leg tension, and the shackles matched to the lifting points. The beam's flange may need a spreader to prevent the sling crushing it or pulling the section out of shape.
I would also check the landing area on the roof, the roof's capacity to take the beam, the tag lines and the communication with the roof crew. Even with adequate capacity, an uncontrolled or damaged load is still a dropped-load hazard.