Capacity Design for Synchronized Lifting with Two RTG Cranes

In heavy lifting operations, particularly in ports, shipyards, precast concrete plants, and large-scale industrial sites, the need for precise, safe, and efficient lifting of extremely heavy loads has grown significantly. Rubber Tyred Gantry (RTG) cranes, known for their mobility, flexibility, and high lifting capacity, are often employed in such operations. In many cases, a single RTG crane may not provide the required lifting capacity, prompting the use of synchronized lifting with two cranes. Proper capacity design for these operations is crucial to ensure safety, structural integrity, and operational efficiency. This article explores the considerations, methodologies, and best practices for designing synchronized lifting operations with two RTG cranes.

Understanding Synchronized Lifting

1. Load Sharing: The load must be divided appropriately between the two cranes. Unequal load distribution can lead to overloading of one crane, posing serious safety risks.
2. Crane Alignment: The cranes must maintain precise alignment and movement synchronization to prevent swaying, twisting, or tilting of the load.
3. Structural Considerations: The lifting points on the load and the spreader bars must be designed to withstand the forces applied by both cranes.

Without careful planning, synchronized lifting can result in catastrophic accidents, equipment damage, or structural failure. Therefore, a meticulous approach to capacity design is essential.

Factors Affecting Capacity Design

Several key factors must be considered when designing synchronized lifting operations with two RTG cranes:

1. Crane Load Capacity

Each RTG crane has a rated lifting capacity based on its design, structural strength, and mechanical systems. In tandem operations, the combined lifting capacity is not simply the sum of the two individual capacities. Designers must consider:

• Dynamic Load Effects: RTG cranes are subject to dynamic forces due to movement, acceleration, and wind.
• Safety Factors: Standard engineering safety factors (typically 1.25 to 1.5) must be applied to each crane’s rated capacity.
• Unequal Load Distribution: Variations in cable tension, boom flexibility, or ground conditions can result in one crane bearing more load than the other.

A conservative approach is usually taken: the load should not exceed 80% of the combined rated capacities to provide a safety margin for potential imbalances.

2. Load Geometry and Weight Distribution

The shape, weight, and center of gravity of the load significantly influence tandem lifting design. Long, asymmetrical, or irregular loads can generate torsional moments that both cranes must resist. Engineers often perform a load analysis using computer simulations or structural calculations to determine:

• Optimal lifting points to minimize twisting and bending
• The required spreader bar length to balance the load
• Center of gravity adjustments to ensure even load sharing

For example, lifting a 100-ton precast concrete beam requires calculating not only the vertical forces but also horizontal moments that may induce crane tipping or uneven stress on the load.

3. Ground and Mobility Conditions

RTG cranes move on rubber tires, giving them flexibility but also introducing mobility-dependent factors:

• Surface Load Bearing: The ground must support the combined static and dynamic loads of both cranes. Soft or uneven surfaces can lead to uneven crane settlement, altering load distribution.
• Crane Maneuverability: Precise steering and braking control are needed to synchronize movement. Any lag or misalignment can create dangerous load swing.

4. Structural Integrity of Cranes and Load Handling Gear

Both the cranes and lifting accessories—wire ropes, slings, spreader bars, shackles—must be designed for tandem lifting. Key considerations include:

• Spreader Bar Design: The bar must be rigid enough to distribute the load evenly without excessive bending.
• Rope Tension Synchronization: Using load sensors and synchronization systems ensures that each crane carries its intended share of the load.
• Rigging Angle: Slings and cables must be arranged to avoid excessive angles that increase tension beyond safe limits.

Methods for Achieving Safe Synchronized Lifting

Modern synchronized lifting relies on a combination of careful planning, structural analysis, and technological assistance.

1. Load Monitoring and Control Systems

Advanced RTG cranes are equipped with load monitoring systems that continuously measure cable tension, crane tilt, and weight distribution. For tandem lifting, these systems:

• Alert operators if load imbalance exceeds safe limits
• Enable automatic adjustment of hoist speeds to maintain synchronization
• Record data for post-operation verification

This approach reduces human error and increases lifting accuracy.

2. Pre-Lift Simulation

Computer-aided engineering (CAE) software allows engineers to simulate tandem lifting scenarios before executing them in the field. Simulations consider:

• Crane positioning and span
• Load geometry and lifting points
• Environmental factors such as wind and ground conditions

By predicting potential problems, simulation helps optimize lifting points, crane spacing, and load distribution strategies.

3. Operator Coordination

Even with advanced systems, operator coordination remains critical. Standard operating procedures for tandem RTG lifting include:

• Clear communication protocols, often using radios or intercom systems
• Pre-lift meetings to define crane roles, signals, and emergency procedures
• Step-by-step synchronized lifting with slow, controlled movements

4. Structural and Safety Checks

Before lifting, engineers perform a detailed inspection of both cranes, the rigging gear, and the load itself. Checks include:

• Verifying crane maintenance and mechanical integrity
• Confirming load weight and distribution calculations
• Inspecting spreader bars, slings, and shackles for wear or deformation

Case Study: Lifting a 200-Ton Precast Module

Consider a scenario where two 120-ton RTG cranes are required to lift a 200-ton precast concrete module at a construction site. The capacity design process would involve:

1. Load Analysis: Engineers determine the center of gravity, calculate moments, and select optimal lifting points.
2. Spreader Bar Design: A reinforced steel spreader bar of 12 meters is designed to ensure equal load distribution.
3. Crane Positioning: The mobile gantry cranes are spaced 15 meters apart with synchronized alignment on reinforced ground pads.
4. Safety Margin: Load is limited to 80% of combined capacity, giving a safety margin of 32 tons per crane.
5. Monitoring Systems: Load sensors and a tandem lifting control system ensure balanced lifting during the entire operation.

The result is a safe, efficient lift completed without load swing, structural damage, or operator stress.

Best Practices for Capacity Design

1. Conservative Load Planning: Never exceed 80–85% of combined crane capacities.
2. Professional Engineering: Employ structural engineers for rigging analysis and crane placement.
3. Technology Integration: Use load monitoring, synchronization controls, and simulation software.
4. Operator Training: Skilled operators with tandem lifting experience are essential.
5. Regular Inspections: Conduct pre-lift inspections and routine maintenance for all lifting equipment.

Conclusion

Synchronized lifting with two rubber tyred gantry cranes provides an effective solution for handling heavy, oversized, or irregular loads that exceed the capacity of a single crane. However, achieving safe and efficient operations requires careful capacity design, taking into account crane load limits, load geometry, ground conditions, and rigging integrity. By combining advanced technology, engineering analysis, and skilled operation, tandem RTG lifting can be executed safely and reliably, maximizing operational efficiency while minimizing risk.

Investing time and resources in proper capacity planning not only protects personnel and equipment but also ensures the success of complex lifting projects, making synchronized RTG operations a vital capability in modern heavy-lifting applications.