A tandem lift happens when two cranes lift the same load at the same time, such as a long steel beam or bridge section. This method is used when one crane cannot safely handle the load alone or when the load is too long to control from one hook point. It matters because the weight must be shared carefully, and a small change in angle or timing can overload one crane.
Engineers and crane operators plan the lift before any motion begins.
Key Facts
- Total load balance: W = F1 + F2 when the load is steady and vertical forces are the only support forces.
- For a level beam with crane hooks at distances d1 and d2 from the center of gravity: F1 d1 = F2 d2.
- If the center of gravity is closer to Crane 1, Crane 1 carries more of the load.
- Load rating charts limit how much a crane can lift at a given boom length, boom angle, and radius.
- Safety factor compares capacity to demand: safety factor = rated capacity / actual load.
- Side loading is dangerous because crane booms are designed mainly for compression and vertical lifting, not sideways force.
Vocabulary
- Tandem lift
- A lift in which two cranes support and move the same load at the same time.
- Center of gravity
- The point where the weight of an object can be treated as acting for balance calculations.
- Load radius
- The horizontal distance from the crane's center of rotation to the load's hook or center of gravity.
- Rated capacity
- The maximum load a crane is allowed to lift under specific conditions listed in its load chart.
- Rigging
- The system of slings, shackles, hooks, and spreader bars used to connect a load to a crane.
Common Mistakes to Avoid
- Assuming each crane automatically carries half the weight is wrong because the load share depends on the center of gravity and hook positions.
- Ignoring load radius is wrong because a crane's lifting capacity usually decreases as the load moves farther from the crane.
- Letting one crane move faster than the other is wrong because uneven motion can tilt the load and suddenly shift weight onto one crane.
- Forgetting the weight of rigging is wrong because slings, hooks, and spreader bars add to the total lifted load.
Practice Questions
- 1 A 12000 kg steel beam is lifted evenly by two cranes with the hooks placed symmetrically around the center of gravity. What mass load does each crane support, ignoring rigging weight?
- 2 A 9000 kg load is lifted by two cranes. Crane A supports 60 percent of the load and Crane B supports 40 percent. What mass load does each crane support?
- 3 A long beam begins to tilt during a tandem lift. Explain which crane may become overloaded and why the center of gravity and hook positions matter.