Shipyard gantry cranes operate under some of the most demanding structural and environmental conditions in the lifting industry. Large spans, heavy and asymmetrical loads, long travel distances, outdoor exposure, wind loads, and rail foundation tolerances all place exceptional demands on crane structural design. Among the most critical design elements influencing performance, safety, and service life is the leg structure configuration, particularly the choice between rigid legs and flexible (hinged) legs.
In modern shipyard gantry crane engineering, rigid leg and flexible leg designs are not optional variations but carefully calculated structural solutions that enable large-scale cranes to operate safely over long rail tracks and uneven foundations. This article provides an in-depth explanation of what rigid and flexible legs are, why they are necessary, how they function structurally, and how to select the right configuration for shipyard gantry crane applications.
1. Overview of Gantry Crane Leg Structures in Shipyards
A shipyard gantry crane typically consists of:
Main girders (single or double)
Two supporting legs (often of different structural behavior)
End carriages and wheel assemblies
Hoisting and trolley systems
Rail and foundation system
Unlike indoor bridge cranes, shipyard gantry cranes operate outdoors over long rail tracks, often exceeding hundreds of meters. Rail foundations are subject to settlement, temperature expansion, concrete shrinkage, and long-term deformation. If both legs were completely rigidly fixed to the ground, excessive internal stresses would quickly develop in the crane structure.
To address this, shipyard gantry cranes commonly adopt a rigid leg + flexible leg configuration, rather than two fully rigid legs.
2. What Is a Rigid Leg in a Shipyard Gantry Crane?
2.1 Definition and Structural Characteristics
A rigid leg is a crane support leg that is structurally fixed to the main girder and end carriage without rotational freedom at its connection points. It provides:
High lateral stiffness
Strong resistance to horizontal forces
Stable support against wind, braking, and skewing loads
In most designs, the rigid leg is constructed as a box-type welded steel structure, directly connected to the main girder through rigid joints.
2.2 Structural Function of the Rigid Leg
The rigid leg serves as the primary positioning and load-resisting element of the gantry crane. Its main responsibilities include:
Resisting longitudinal forces during crane travel
Absorbing horizontal wind loads
Maintaining crane alignment along the rail
Transferring braking and acceleration forces to the foundation
In shipyard environments where cranes are exposed to strong coastal winds, the rigid leg plays a critical role in ensuring overall crane stability.
3. What Is a Flexible (Hinged) Leg?
3.1 Definition and Structural Features
A flexible leg, also referred to as a hinged leg or articulated leg, is designed to allow controlled rotation or displacement at specific connection points—usually at the interface between the leg and the main girder or between the leg and the end carriage.
This flexibility is not weakness; it is a deliberate engineering solution.
3.2 Why Flexibility Is Necessary
Shipyard gantry cranes travel over long distances where:
Rail elevation differences are unavoidable
Foundation settlement occurs over time
Temperature causes rail expansion and contraction
Minor track misalignment is inevitable
If both legs were rigid, these factors would induce:
Large secondary stresses in the main girder
Wheel unloading or overloading
Increased risk of rail climbing or derailment
Fatigue cracking in welded joints
The flexible leg allows the crane structure to accommodate rail and foundation imperfections without transferring excessive stress into the steel structure.
4. Rigid Leg vs. Flexible Leg: Structural Interaction
4.1 Load Distribution Mechanism
In a typical shipyard gantry crane:
Vertical loads (self-weight + lifted load) are shared by both legs
Horizontal loads (wind, braking, skewing) are primarily resisted by the rigid leg
Displacement and rotation caused by track irregularities are absorbed by the flexible leg
This division of roles ensures that the crane structure remains statically determinate or near-determinate, reducing internal stress concentration.
4.2 Influence on Main Girder Stress
Without a flexible leg, rail unevenness would force the main girder to deform excessively, leading to:
High bending stress
Torsional deformation
Reduced fatigue life
The flexible leg acts as a stress-relief mechanism, significantly improving the long-term durability of shipyard gantry cranes.
5. Common Flexible Leg Design Solutions in Shipyard Gantry Cranes
5.1 Hinged Upper Connection
One of the most common solutions is a pinned or hinged joint between the flexible leg and the main girder, allowing limited rotation in the vertical plane.
5.2 Articulated End Carriage
Some designs allow rotational freedom at the end carriage connection, enabling the wheel set to adapt to track unevenness.
5.3 Slotted or Sliding Connections
In large-span gantry cranes, sliding or slotted connections may be used to accommodate thermal expansion and contraction.
Each solution is carefully calculated to balance flexibility with sufficient structural stiffness.
6. Application Scenarios in Shipyards
6.1 Large Shipbuilding Gantry Cranes
For ultra-large shipbuilding gantry cranes with spans exceeding 80–120 meters, flexible legs are essential. Rail alignment over such distances can never be perfectly maintained, making flexibility a structural necessity rather than a design preference.
6.2 Ship Repair and Block Assembly Areas
In ship repair yards where foundation conditions may be older or uneven, flexible legs help cranes operate safely without requiring excessive civil foundation upgrades.
6.3 Coastal and Offshore-Exposed Shipyards
Shipyards near the sea experience strong wind loads and temperature variations. The rigid leg ensures wind stability, while the flexible leg protects the structure from thermal-induced stresses.
7. Impact on Safety and Service Life
7.1 Reduced Risk of Derailment
By maintaining consistent wheel contact and load distribution, flexible leg designs reduce wheel unloading, which is a common cause of derailment in rigid-only structures.
7.2 Improved Fatigue Performance
Shipyard gantry cranes are long-life assets, often designed for 25–30 years of service. Flexible leg designs significantly reduce fatigue damage in critical welded joints.
7.3 Easier Maintenance and Alignment
Flexible leg structures tolerate minor rail deviations, reducing the frequency and cost of rail realignment and crane structural adjustments.
8. Design Standards and Engineering Considerations
Rigid and flexible leg designs must comply with international standards such as:
FEM (Europe)
EN 13001
ISO 8686
GB/T standards (China)
CMAA (selected applications)
Engineering analysis typically includes:
Finite element analysis (FEA)
Wind load simulation
Seismic load assessment (if applicable)
Rail tolerance evaluation
Fatigue life calculation
Improper leg configuration selection can lead to excessive structural stress, even if all components meet strength requirements.
9. Choosing the Right Configuration for Your Shipyard Gantry Crane
When selecting a rigid and flexible leg configuration, key factors include:
Crane span and lifting capacity
Rail length and foundation quality
Wind load conditions
Travel speed and duty class
Long-term maintenance strategy
For most medium to large shipyard gantry cranes, a one rigid leg + one flexible leg design provides the optimal balance of stability, adaptability, and structural efficiency.
10. Conclusion
Rigid leg and flexible leg design is a fundamental engineering principle in shipyard gantry crane construction. Rather than being a secondary detail, leg configuration directly determines how the crane responds to real-world conditions such as rail unevenness, wind loads, thermal expansion, and long-term foundation movement.
By combining the stability of a rigid leg with the adaptability of a flexible leg, shipyard gantry cranes achieve superior safety, durability, and operational reliability. For shipyards investing in large-scale lifting equipment, understanding this structural concept is essential for making informed, long-term decisions that protect both assets and operations.