When using a 4-leg sling, some factors usually lead the load to be shared only by the 2 shortest legs, that are diagonally opposed.
When the load weight is considerable (more then 1 tonne), one is not able to check by hands if one or two legs are loose, leading to a false impression that all legs share equally the load.
In these cases, we can check what is the force in each leg using load cells.
Check out our experiment:
- We used 04 load cells Eilon Engineering Ron StageMaster 6000G5, 3 t, positioning one load cell per leg (https://www.eilon-engineering.com/).
- We used a 4-leg sling chain, all legs connect to a shackle. It caused length to the lifting points to be slightly different. This simulates unavoidable difference among legs, due to large length tolerances allowed.
- Sling legs were adjusted to have the same length.
- We connected the slings to a very stiff load, weighting about 700 kgf.
- We lifted the load.
- We checked legs with hands and found that two of them were loose.
- We checked the values on load cell software, which showed 348 kgf, 303 kgf, 23 kgf and 18 kgf, confirming that only two legs were carrying all the load.
FACTORS THAT AFFECT FORCES IN THE LEGS
- Structural stiffness of the load – the stiffer the load, the more force will be concentrated in the 2 shorter legs.
- Difference in length between the legs of the slings – the greater the difference, the less equitable the distribution of forces.
- Dynamic forces of the load – noticeable slack in 2 legs during offshore lifting operations.
HOW TO DESING 4-LEG SLINGS
- Most conservative approach: consider 2 legs loaded and the others only balancing the load.
- Simplification of technical standards and codes: consider 3 legs loaded (be cautious!).
- Optimized approach: perform non-linear structural analysis taking into account the load and sling stiffness.
- Consider forces in all 4 legs by monitoring with load cells and adjusting leg lengths with turnbuckles, chain hoist, lever hoists etc.
All pictures by Leonardo Roncetti.
