Co-lifting: when two cranes lift the same load

Some loads are too long, too heavy or too skewed for a single crane to lift them alone and keep them stable. The solution can be tandem lifting – two (or more) cranes or winches lift and move the same load at the same time. It sounds like a simple extension of a normal lift, but the rules treat it as a lift with markedly tightened requirements: tandem lifting requires a crane operator certificate regardless of the crane's size (BEK 1978/2021 § 4(7)) and a fixed work procedure (BEK 428/2022 § 55), because the risk of the cranes pulling against each other or loading the load unevenly is real.

Co-lifting is typically used for long elements such as bridge elements, masts, large machinery parts or steel structures where the centre of gravity is far from both ends, or where the load's own weight exceeds what a single crane on site can handle. You don't choose co-lifting because it's easy — it's used when it's necessary and because it makes significantly greater demands on planning than an ordinary single lift.

A tandem lift must be planned carefully. The executive order requires a work procedure to be laid down and used so that the crane operators are coordinated (BEK 428/2022 § 55). For a complicated tandem lift, written instructions must be drawn up (BFA Bygge & Anlæg). Everyone taking part — crane operators, slingers and others — must be carefully instructed in how this particular lift is to be carried out, who leads it, and what to do if something deviates from the plan.

The weight distribution for each crane or hoist for the lift and transport must be calculated in advance. It is not enough to estimate how much of the load each crane bears — the calculation must be ready before the lift begins, and it is checked against the 75 percent limit each crane must comply with.

The cranes used for tandem lifting should be of the same type and ideally have the same system for speed control. If the cranes do not fit well together — for example because one hoists significantly faster or more jerkily than the other — it becomes difficult to keep the load horizontal and smooth throughout the lift and the risk that one crane suddenly takes more of the load increases.

For combined lifting, if necessary, special lift gear must be constructed such as a spreader bar or similar which is arranged so the weight is distributed correctly and so only a vertical lift of the load occurs with each crane throughout the process. Standard lift gear designed for a single lift cannot be reused for this task without further work because the requirements for distribution and vertical pull are stricter.

Co-lifting may only be performed by certified crane operators and communication between the involved crane operators and the competent leader must be agreed and stable throughout — typically via radio where everyone hears the same commands simultaneously. If communication is lost or doubt arises about how the load is hanging, lifting is halted until conditions are clarified.