Load and crane
Tandem lift: load share between two cranes or hooks
- where
- load on hook A and hook B (t)
- total load including lifting beam and rigging (t)
- distance from hook A to the centre of gravity (m)
- distance between the hooks (m)
- utilisation of each crane against its down-rated capacity (%)
- rated capacity of each crane at its radius (t)
- down-rating for the multiple-crane lift (%)
Explain this equation
Splits a load between two cranes or two hooks and checks each against a down-rated capacity. Use it when planning a tandem lift of a long beam, frame or vessel.
W is the total load hanging from both hooks, L is the distance between the hooks and a is the distance from hook A to the centre of gravity. R_A and R_B are the loads on each hook. C_A and C_B are each crane's rated capacity at its radius, and d is the percentage down-rating applied for the multiple-crane lift. U_A and U_B are the resulting utilisations.
Taking moments about hook A, hook B must carry W × a / L to balance the load, and hook A carries the rest. The hook nearer the centre of gravity takes more. The down-rating allows for the shares changing as the cranes move.
Sense check: with the centre of gravity in the middle, each hook takes half. With it 4 m from A on a 10 m spread, A takes 60% and B 40%.
Watch out: tilting the load, uneven hoisting or out-of-vertical hoist ropes shift the shares. BS 7121-1 calls for an appropriate down-rating of all cranes when the shares cannot be evaluated accurately; published guidance says this may need to be 20% or more.
Splits a load between two hooks by taking moments about the centre of gravity, then compares each share with the crane’s down-rated capacity.
Enter your values to see the result.
Good to know
Each hook carries the load in inverse proportion to its distance from the CoG: the hook nearer the CoG takes more. BS 7121-1 asks for an appropriate down-rating of all cranes where load sharing cannot be evaluated accurately; published commentary says it might have to be 20% or more. The share changes during the lift if the load tilts, the cranes slew or luff at different rates or the hooks are not vertical; the appointed person must allow for this.
Common mistake
Forgetting that a tilting load moves weight onto the lower hook, or that one crane’s motion can overload the other — a crane’s own load limiter cannot prevent that.
Limits
Static, hooks vertical, load level, rigid load. Rated capacity must be read from each crane’s chart for its actual radius and configuration, net of hook block and rigging above the load. A multiple-crane lift is a complex lift needing a specific plan. For planning only: this does not replace a lift plan prepared by a competent person under LOLER 1998 (reg. 8) and BS 7121, and the manufacturer’s WLL markings, load charts and data always govern over this result.
Sources
- BS 7121-1:2016 Code of practice for safe use of cranes. General (lift planning, appointed person, multiple-crane lifts, ground conditions, wind)
- Hoist Magazine, multiple-crane lifting under BS 7121-1 (down-rate all cranes where load sharing cannot be evaluated accurately; the down-rating might have to be 20% or more)
- The Lifting Operations and Lifting Equipment Regulations 1998 (SI 1998/2307), reg. 8: every lifting operation properly planned by a competent person, appropriately supervised and carried out safely
- HSE L113 Safe use of lifting equipment. Lifting Operations and Lifting Equipment Regulations 1998. Approved Code of Practice and guidance
Results are calculations from the published methods and figures shown, and depend on the data you enter. For planning and checking only. Scaffolds must be erected to NASC TG20 or a bespoke design by competent persons, and lifting operations planned under LOLER by a competent person. Never rely on these results instead of a design, a lift plan or manufacturer ratings.