Four-Fall, Two-Fall, and a Sling Angle Nobody Checked
Your 12 t maximum-capacity hammerhead crane has a 60 m jib. The manufacturer's chart for this jib gives, at 40 m radius, 3.6 t on two-fall reeving and 6.2 t on four-fall; both figures are gross and already include the hook block. You must land a steel equipment skid at the far edge of a slab 40 m from the mast, but the skid is sitting on a truck 30 m from the mast, so the trolley has to travel out with the load on. The skid weighs 4,150 kg. The rigging you intend to use is a 4-leg chain sling weighing 62 kg, four shackles at 3.5 kg each, and a 180 kg spreader beam. Each sling leg is rated 2.0 t as a single vertical leg. As rigged, each leg will make 30 degrees with the horizontal. Wind is a steady 5 m/s.
1. Calculate the load the crane will actually carry and decide whether two-fall reeving works at 40 m.
Read the worked answer
Add everything hanging from the hook. The chart figures are gross and already include the hook block, so the block drops out of the sum - but the rigging does not:
4,150 + 62 (sling) + 4 x 3.5 = 14 (shackles) + 180 (spreader) = 4,406 kg, call it 4.41 t.
Two-fall at 40 m is 3.6 t. 4.41 / 3.6 = 1.22, so the crane would be asked for 122 per cent of rated capacity. That is not a margin to argue away in the cab; it is the overload limiter's territory, and if the limiter is working the pick simply will not happen.
Four-fall at 40 m is 6.2 t. 4.41 / 6.2 = 71 per cent, which works. Before using it, check three things: that four-fall reeving is permitted on this jib length and that the column being read really is the four-fall column rather than an interpolation; that the reeving change is done by a competent person with the hook landed and the crane locked out; and that the longer hook block does not cost you the headroom needed to land the skid on the slab.
Finally, confirm the 40 m column is the correct one. The skid is picked at 30 m but placed at 40 m, and the governing radius is the one where the crane feels the most load moment - 40 m.
2. Check the sling assembly against the load. Is it adequate, and if not, what do you change?
Read the worked answer
No. Use the standard method for a multi-leg sling: with four legs you design on three carrying the load, because the load is never shared perfectly, and you apply the angle from the horizontal.
Capacity = 3 x WLL per leg x sin of the angle to the horizontal = 3 x 2.0 x sin 30 degrees = 3 x 2.0 x 0.5 = 3.0 t.
The hook load is 4.41 t, and the sling itself must carry the skid plus the spreader, 4.33 t - either way it is over. 3.0 t against 4.41 t means the assembly is overloaded by roughly 47 per cent before anything leaves the truck.
The fix is geometry, not optimism. At 45 degrees: 3 x 2.0 x 0.707 = 4.24 t, still short. At 60 degrees: 3 x 2.0 x 0.866 = 5.2 t, which covers 4.41 t with about 18 per cent in hand. So use longer slings, or a wider spreader that keeps the legs steeper, or legs of higher WLL - and check the shackles and master link against the same figure.
Then check the skid's own lifting lugs, because the load path does not stop at the sling. And measure the angle with the load just clear of the ground, since the legs take up and the angle changes as the load comes on.
3. At what radius do you verify capacity, and what would you do if the placement radius were 44 m?
Read the worked answer
At the maximum radius at which the load will be handled - here 40 m, at the far edge of the slab - not at the pick radius. The chart gives the lowest capacity at the greatest radius, so if 40 m is inside the chart the 30 m pick is automatically inside it. The reverse is not true, and that assumption is how cranes go over.
If the placement radius were 44 m, go back into the chart at 44 m on the same jib and the same reeving and read a fresh figure. It will be lower than 3.6 t on two-fall and lower than 6.2 t on four-fall. If 44 m lies beyond the published radius for that jib and reeving, there is no capacity there - not a small one - and the lift does not happen from that position.
The options are then to reposition the crane, use a different jib configuration only where the chart covers it, or break the load down into smaller pieces. Nothing else is legitimate.
Finally, re-check the chart's assumptions: level crane on its specified foundation, wind inside the manufacturer's limit, no ice, and the correct reeving. Any one of those not met invalidates the number just read, however carefully it was read.