Load Distribution in Single Girder vs Double Girder EOT Cranes

Electric Overhead Traveling (EOT) cranes are a critical component of material handling systems in manufacturing plants, warehouses, steel fabrication yards, and heavy industries. Among the most common types are single girder and double girder EOT cranes, each with distinct structural designs and load-handling capabilities. One of the most important aspects that differentiate the two is load distribution – how the crane structure distributes the load weight to the supporting components, such as end trucks, runway beams, columns, and the foundation.

Understanding how load is distributed in single and double girder EOT cranes not only informs crane selection but also plays a vital role in ensuring structural integrity, safety, and cost-efficiency.

What Is Load Distribution in EOT Cranes?

Load distribution refers to how the live load (the actual lifted weight) and the dead load (weight of the crane structure itself) are transferred through the crane components and eventually down to the support structure and ground. Proper load distribution is essential for:

• Preventing overstressing of structural components

• Ensuring smooth and balanced crane operation

• Prolonging the crane’s lifespan

• Reducing maintenance needs and costs

• Enhancing safety for operators and materials

Let’s examine how this distribution differs between single girder and double girder EOT cranes.

Load Distribution in Single Girder EOT Cranes

Structural Configuration

A single girder EOT crane consists of one main bridge girder, which supports the trolley and hoist. The trolley usually runs on the bottom flange of the girder (under-running design) or sometimes on top (top-running design in lighter duty cases).

Load Path

• The hoist and trolley load is concentrated directly on the single girder.

• The girder transmits the combined load (hoist + trolley + payload) to the end trucks, which are located at both ends of the girder.

• The end trucks then transfer the load to the runway beams and eventually to the supporting columns or building structure.

Load Distribution Characteristics

• Point Loading: The load is more concentrated since there’s only one girder.

• Lower Dead Load: The single girder crane structure is lighter, reducing the self-weight transmitted to the building.

• Uneven Stress: Since the hoist travels on one girder, there can be uneven stress distribution, especially under side pull or offset loads.

• Smaller Wheel Loads: Generally results in lower wheel loads on runway beams and foundations.

• Lower Cost Runway Beams: Because of lower wheel loads, the supporting structure can be more economical.

Design Considerations

• Limited lifting capacities (commonly up to 20 tons).

• Shorter spans (typically less than 25 meters).

• Ideal for light to medium-duty applications.

Load Distribution in Double Girder EOT Cranes

Structural Configuration

Double girder cranes feature two parallel bridge girders, with the trolley and hoist mounted on top of the girders (top-running design). This configuration allows for increased lifting capacity and better load handling.

Load Path

• The hoist sits atop the two girders, distributing the lifted load more evenly across both girders.

• The trolley load is transferred to both girders, which spread the load to the end trucks.

• End trucks transfer the loads to runway beams, and further to supporting structures.

Load Distribution Characteristics

• Balanced Load Sharing: Load is evenly shared between two girders, minimizing stress concentration.

• Higher Dead Load: The double girder system is heavier, increasing the double girder bridge crane self-weight.

• Higher Wheel Loads: Because of the heavier structure and higher capacities, wheel loads are greater.

• Increased Span Capacity: Supports longer spans, up to 40 meters or more.

• Increased Headroom: Trolley rides on top of the girders, allowing for better use of vertical space.

Design Considerations

• Suitable for heavier loads (25 to 500 tons or more).

• Required where higher lifting height, speed, or duty cycles are needed.

• Runway beams and columns must be designed for higher loads.

Key Differences in Load Distribution: Side-by-Side

Why Load Distribution Matters in Crane Selection

When choosing between a single and double girder EOT crane, understanding load distribution is critical for making informed decisions related to:

1. Structural Design of the Facility: Heavier wheel loads in double girder cranes require reinforced beams, columns, and foundations.

2. Cost of Ownership: While single girder cranes are cost-effective initially, double girder cranes offer better load distribution for heavy-duty applications, potentially reducing repair or structural reinforcement costs in the long term.

3. Safety Margins: Poorly distributed loads in single girder cranes (especially under offset loading) can cause premature wear, deflection, or even failure.

4. Crane Performance: Double girder cranes perform better under continuous operation and offer higher speeds and lifting heights due to even load distribution.

Use Cases Based on Load Distribution

Use Single Girder When:

• Lifting capacity is under 20 tons

• Span is relatively short

• Building height is limited

• Budget is tight

• Structural support can’t handle high wheel loads

Use Double Girder When:

• You need to lift more than 20 tons

• Long spans are required

• High-frequency or continuous lifting operations

• Lifting height and speed are key performance metrics

• Future upgrades in capacity are anticipated

Conclusion

Load distribution plays a foundational role in the engineering, performance, and operational reliability of both single girder and double girder EOT cranes. Single girder cranes offer a lighter, simpler load path suitable for lighter applications, while double girder cranes deliver superior load distribution, making them essential for heavy-duty operations.

Choosing the right type of crane involves not just looking at capacity in tons, but understanding how the load travels through the crane and the infrastructure it rests upon. A careful analysis of load distribution helps ensure that the overhead bridge crane system operates safely, efficiently, and cost-effectively throughout its service life.