Chart Reading, Reeving and Sling Angle on a Near-Capacity Pick
60 m jib tower crane. The cab chart page reads (capacities include the hook block and trolley, tonnes):
| Radius | 2-fall | 4-fall |
|---|---|---|
| 28 m | 5.6 | 6.9 |
| 32 m | 4.8 | 5.4 |
| 36 m | 4.1 | 4.4 |
Deduct the weight of everything below the hook. The pick is a 3.6 t machine base frame with a 0.22 t spreader beam and 0.06 t of slings and shackles, rigged 4-fall. It is picked from the laydown at 28 m radius, slewed, and landed on the level 6 slab at 36 m radius; the trolley passes through 32 m and 34 m on the way. The slinging plan uses two 2.5 t WLL synthetic slings in a bridle at 45° to the horizontal onto the frame's two lifting points. Wind at jib height is 12 m/s with gusts, and the frame has 3.1 m² of blank sail area.
1. Calculate the load to be lifted and determine the governing capacity. Is the lift inside the chart?
Read the worked answer
Load below the hook = 3.6 + 0.22 + 0.06 = 3.88 t.
The governing radius is the greatest radius the load reaches during the whole move, including landing, not the pick radius. The trolley goes out to 36 m, so use 36 m: 4-fall capacity 4.4 t.
Utilisation = 3.88 ÷ 4.4 = 88%.
So it is inside the chart, but only just. Checks before proceeding:
- The cab chart must match the crane as built: 60 m jib and 4-fall reeving. A 55 m chart or a 2-fall assumption invalidates the numbers (at 36 m, 2-fall is 4.1 t, which the load still passes, but the margin disappears).
- The LMI must be programmed for the same configuration and the same jib length.
- Confirm no part of the move, including a swing past the slab edge or any drift, takes the trolley beyond 36 m.
At 88% with wind and a sail area, the remaining margin is small; any change of geometry or configuration means re-checking before the lift, not after.
2. Check the slinging plan. Two 2.5 t WLL slings in a bridle at 45° to the horizontal.
Read the worked answer
Resolve the leg tension using the angle to the horizontal, not the included angle at the hook:
Leg tension = W ÷ (2 × sin 45°) = 3.6 ÷ (2 × 0.707) = 2.55 t per leg.
The frame weight is used here because the slings attach to the frame; the spreader beam is part of the hook load but sits above the slings.
2.55 t on a 2.5 t WLL sling is an overload before any design factor, dynamic allowance or tag-line side load is considered. Reject the plan.
Options:
- Steeper bridle: at 60° to horizontal, tension = 3.6 ÷ (2 × 0.866) = 2.08 t per leg, which fits.
- Four-leg slinging with load shared on all legs (only if the geometry genuinely shares load).
- Higher WLL slings and matching shackles.
Also verify the frame's lifting points are rated for the leg tension and direction, check the shackles for the same load, and confirm the included angle at the hook against the sling manufacturer's limit.
3. Wind at jib height is 12 m/s with gusts, and the frame has 3.1 m² of sail area. How does this affect the decision?
Read the worked answer
The load moment limiter knows nothing about wind. It measures load and radius; it cannot see the side force from a sail, the swing energy, or the extra dynamic amplification when the load is arrested.
What wind does to this pick:
- A 3.1 m² blank face at 12 m/s with gusts produces a significant horizontal force that swings the load and side-loads the jib and slinging.
- The 88% utilisation leaves little margin for the dynamic increase when the swinging load is stopped or landed.
- Tag lines become harder to control and the load is more likely to catch the slab edge on landing.
Decide against:
- The manufacturer's maximum wind speed for lifting, and any load sail-area restriction in the manual.
- The site lift plan limit and the actual gust readings at the jib, not at ground level.
If the pick proceeds, keep the radius as short as the landing allows, use long tag lines with competent handlers, land promptly without dragging, and stop if gusts rise or the load starts to sail. If the manual's limit for that sail area is approached, postpone the lift.