Key Load Shape Factors That Influence RTG Crane Lifting Performance

Rubber tyred gantry (RTG) cranes are widely used in container terminals, logistics yards, manufacturing facilities, and intermodal operations to handle loads efficiently and safely. While many operators and planners focus on load weight, lifting height, and yard layout when evaluating RTG crane performance, one factor is often underestimated: load shape. Load geometry plays a decisive role in stability, lifting safety, rigging configuration, spreader operation, crane travel speed, and even the cycle time of a lifting process.

Understanding how load shape affects RTG crane lifting performance is essential for ensuring operational efficiency, reducing wear and tear on equipment, and preventing accidents caused by instability or uncontrolled load movements. This article explores the key load shape factors that influence RTG crane operations and offers practical insights for selecting the right equipment setup and lifting strategy.

1. Load Symmetry and Asymmetry

One of the most critical shape factors that impacts RTG crane lifting performance is whether the load is symmetrical or asymmetrical.

Symmetrical Loads

Symmetrical loads—such as ISO containers, evenly stacked pallets, or modular concrete blocks—allow the RTG spreader or rigging system to grasp the load in a uniform and balanced manner. Benefits include:

• Even load distribution across the spreader twistlocks or rigging points

• Predictable center of gravity

• Minimal risk of sway caused by uneven mass

• Higher lifting speed due to stable geometry

For RTG cranes, symmetrical loads represent the ideal lifting scenario because the crane can operate near its optimal performance limits.

Asymmetrical Loads

Irregular-shaped equipment, long structures, oversized machinery, or loads with uneven protrusions present significantly more challenges. Asymmetry affects:

• Load balance

• Center of gravity detection

• Spreader positioning

• Rigging point selection

Even small deviations in symmetry can generate eccentric loading, which causes:

• Uneven stress on the hoist ropes

• Twist and rotation during hoisting

• Increased sway amplitude

• Reduced lifting speed due to safety constraints

• Higher risks when traveling with suspended loads

RTG operators often must use special rigging (slings, adjustable beams, or custom frames) to compensate for asymmetrical geometry.

2. Load Height-to-Width Ratio

The proportion or aspect ratio of a load is a major determinant of lifting behavior.

Tall and Narrow Loads

High center-of-gravity (CoG) loads such as fabricated steel columns, machinery casings, or stacked crates are prone to:

• Excessive pendulum motion

• Tipping risk during lifting and travel

• Higher susceptibility to wind loads

These loads require:

• Reduced hoist and trolley speeds

• Auxiliary tag lines

• More precise operator control

Wide and Flat Loads

Flat, wide loads (e.g., steel plates, concrete panels) impact:

• Spreader coverage area

• Rigging spread requirements

• Maneuverability within yard lanes

Wide loads often challenge the crane’s ability to move through container rows or narrow pathways. Operators may need to adjust:

• Crane travel alignment

• Container stack spacing

• Lifting height clearance

For flat loads, airflow and wind resistance also become important. Even moderate winds can cause tilting or spinning.

3. Load Length and Overhang

Loads that extend beyond typical container dimensions present unique difficulties for RTG cranes.

Long Loads

Pipes, beams, wind turbine components, and long machinery skids produce:

• Higher bending moments

• Increased rigging span requirements

• Alignment challenges during pick and place

Long loads increase the crane's dynamic load variability when traveling, especially during acceleration or deceleration.

Overhanging Features

Bulk machinery with protrusions, attachments, or extensions may:

• Interfere with adjacent containers

• Block the spreader from achieving full twistlock engagement

• Require custom lifting points

Overhang also introduces risks of collisions when traveling under limited-clearance structures in the yard.

4. Irregular or Complex Load Geometry

Many modern RTG operations now involve non-containerized cargo, particularly in multipurpose terminals or manufacturing facilities. Irregular loads include:

• Scrap bundles

• Machinery with non-uniform frames

• Precast concrete elements

• Structural assemblies

These loads lack predictable lines or dimensions, influencing:

• Load stability

• Rigging balance

• Sling angle calculations

• Rotation behavior during lifting

Crane operators often need additional tools such as:

• Rotating lifting beams

• Non-standard lifting frames

• Manually adjustable rigging points

Such loads require reduced lifting speeds and enhanced operator training to maintain control.

5. Load Center of Gravity Position

The shape of a load directly determines where its center of gravity (CoG) lies. A CoG that is high, offset, or difficult to determine can lead to:

• Sudden tilting

• Increased sway

• Higher stress on rigging equipment

Common CoG-related issues include:

• Loads with one side heavier than the other due to dense components

• Multi-section equipment with internal weight concentration

• Tall loads with a naturally elevated CoG

For RTG cranes, misjudging CoG placement can compromise the spreader’s ability to keep the load level. This may require:

• Multi-point lifting

• Adjustable slings

• Load balancing before lifting

6. Surface Shape and Contact Points

The load’s surface shape influences how the crane interacts with the load.

Flat Surfaces

Flat surfaces accommodate:

• Spreaders

• Lifting beams

• Magnets or vacuum lifters (if applicable)

Such loads provide consistent contact areas and stable rigging.

Curved or Uneven Surfaces

Cylindrical, spherical, or irregularly contoured loads introduce:

• Rolling risks

• Sling compression issues

• Difficulty in stabilizing during lift-off

• Increased rotational movement

Cylindrical loads like pipe bundles may require:

• Choker slings

• Wedge blocks

• Pipe cradles

7. Load Rigidity vs. Flexibility

The internal structure of the load affects how it behaves during lifting.

Rigid Loads

Rigid loads maintain shape under stress, offering:

• Predictable movement

• Stable rigging performance

Containers, metal blocks, or solid machinery belong to this category.

Flexible or Semi-Rigid Loads

Bundles of rods, scrap, timber, or similar materials may deform under tension. Their geometry changes during lifting, causing:

• Shifting center of gravity

• Internal movement

• Sling compression or misalignment

This requires additional reinforcement or mechanical bundling before lifting.

8. Aerodynamic Shape and Wind Sensitivity

Wind is a critical external factor for RTG crane operation, and the load shape determines how wind forces act on the suspended load.

Loads that increase aerodynamic drag include:

• Flat panels

• Tall items

• Lightweight bulky materials

Wind impact may cause:

• Load spinning

• Lateral drift

• Increased sway amplitude

Yards must follow stricter wind-speed lifting limits for loads with sensitive shapes.

9. Compatibility With Standard Spreader Frames

RTG cranes primarily handle containers with ISO-standard dimensions. Any load shape that deviates from container geometry may require:

• Manual spreader adjustment

• Special lifting beams

• Multi-sling configurations

If the load shape is incompatible with automated spreader systems, RTG crane cycle time increases significantly.

Conclusion

Load shape is a fundamental - yet often overlooked - factor affecting RTG crane lifting performance. Symmetry, aspect ratio, overhangs, irregular geometry, CoG characteristics, aerodynamic properties, structural rigidity, and surface shape all directly influence how safely and efficiently a load can be lifted and transported.

By understanding these shape-related factors, operators and planners can select the appropriate rigging methods, adjust crane operating parameters, prevent instability, and optimize lifting efficiency. For equipment suppliers like Aicrane, considering load geometry ensures that customers receive the safest and most reliable RTG crane solutions tailored to their operational needs.