When lifting heavy or oddly shaped objects, it is sometimes desirable to use two cranes in tandem rather than a single higher-capacity crane. Tandem lifts can be very useful, and they are often used to place structural beams for bridges, move large loads, and lift long components from horizontal to vertical positions. However, tandem lifts have the potential to become very dangerous if not handled properly.
The first step in safely executing a tandem crane lift is to prepare a rigging plan that will investigate all possibilities of overloading the cranes involved. OSHA 1926.1432 outlines a few requirements for tandem lifting plans. Tandem lifting plans must be developed by a qualified person, and if the qualified person determines that engineering expertise is required, the employer must ensure it is provided. The lift must also be directed by a person who meets the criteria for both a competent person and a qualified person, or by a competent person assisted by one or more qualified persons. Finally, the lift director must review the complete plan in a meeting with all workers who will be involved in the operation.
Lift Factors and Conditions
The goal of a detailed lifting plan is to ensure that the lift can be completed safely. For the lift to be safe, the cranes must not be overloaded or side loaded. When a lift involves more than one crane, there are many factors that could cause an overload situation to arise, and they must be accounted for when the plan is developed. Here are a few factors to consider when planning a tandem lift:
- Center of Gravity: It is necessary to know the center of gravity of a load to be able to balance the load correctly and distribute the load between cranes as desired. It is also necessary to account for any conditions that could cause the center of gravity to shift, such as wind blowing on the load or fluids in or on the load. If the center of gravity shifts or is not correctly aligned, the crane closer to the center of gravity could become overloaded, resulting in a failure.
- Capacities: There are two possibilities for loading during a tandem lift: equal distribution and unequal distribution. When the load is distributed equally between both cranes, it can be beneficial to use cranes of the same model or the same capacity range. Using two of the same or similar cranes will make it easier to synchronize movements between them, which will help reduce the chance of overload conditions. If the load is not intended to be distributed equally, cranes of different capacities can be used based on the portion of the load that will be carried by each crane. However, using cranes of the same model even when the load is distributed evenly can still make synchronizing movement easier. If the cranes have different movement speeds, the lift plan should define ways to keep the movements synchronized to avoid moving the center of gravity out of the position defined by the plan.
- Rigging: Many loads that are being moved with a tandem lift may require the use of additional rigging and lift equipment. Lifting devices, such as equalizer beams, can also help prevent the risk of unintended load distribution due to uncoordinated vertical movements. Equalizer beams can help keep the load distributed evenly in case crane movements become unsynchronized. Rigging equipment can also be used to ensure that the load is lifted above the center of gravity, rather than below it, for the most stability. Like the cranes, all rigging and lift equipment must have the necessary lifting capacities. Lift equipment can also add significant weight to the load, so the lifting plan must account for the additional weight of any equipment as well as the weight of the load itself.
- Side Loading: Side loading is one of the most dangerous risks of tandem lifting, and it can also happen because of uncoordinated movement. While crane booms can withstand some side loading forces, it is unsafe to allow horizontal loads or off-center lifts intentionally. The lifting plan should provide ways to avoid side loading, and cranes should be carefully positioned to minimize the side loading risk.
- Communication: The tandem lift plan should have a clearly and precisely defined system of communication to avoid any miscommunication and possible unintended movements. The qualified person directing the lift should be the person directing the personnel who are operating the machines. If necessary, relay persons should be used between the supervisor and operator. The operators and relay persons should know and understand all commands and communications. In case of an emergency, an emergency stop signal should be used by any person who observes an unsafe condition.
Tandem crane lifts have the potential to be dangerous, so every precaution must be taken to protect the safety of everyone involved. There are many additional factors to consider when planning a tandem lift that have not been described here. The qualified person planning and directing the lift is responsible for identifying all potential hazards and addressing them. The planners should also be aware of any national and local safety regulations, such as derating requirements, and ensure that the plan is compliant. Safety is the top priority when executing tandem lifts and all other critical lifts.