Structural Design Plan for an 18-ton/5-ton 40-meter Type-A Double-Girder Gantry Crane

This design proposes a complete structural design plan for an 18-ton/5-ton 40-meter Type-A Double-Girder Gantry Crane to meet the loading and unloading operational needs of port freight yards. The equipment adopts a double main girder box-type structure, equipped with rigid and flexible legs. It features a large span, good stability, and a wide operational range, effectively improving port freight yard loading/unloading efficiency and reducing worker labor intensity. The following provides a detailed explanation from multiple aspects such as technical parameters, overall structure, main girder design, leg system, and strength calculations.

Design Overview and Technical Parameters

A Type-A Double-Girder Gantry Crane is a large-scale material handling equipment whose structural design directly affects load-bearing capacity and operational stability. The designed 18/5t-40m Type-A Double-Girder Gantry Crane is a medium-to-large lifting device. The main and auxiliary hook configuration meets the lifting needs of materials of different weights, and the 40-meter span design is suitable for large freight yard operations.

Main Technical Parameters:

• Lifting Capacity: Main hook 18 tons, Auxiliary hook 5 tons
• Span: 40 meters (distance between support centers at both ends of the main girder)
• Effective Cantilever Length: 8 meters on the rigid leg side, to be determined on the flexible leg side based on design
• Lifting Height: 14 meters (main hook)
• Hoisting Speed: 9.3 m/min (main hook)
• Gantry Travel Speed: 48 m/min
• Trolley Gauge: 2.5 meters
• Trolley Wheelbase: 2.0 meters
• Gantry Base: 10 meters
• Trolley Self-weight: 6 tons

Duty Class: Inferred as A5 duty class based on reference data for a 26-meter span crane in the web documents. This duty class indicates the crane is used under moderate load conditions with appropriate busy/idle levels, suitable for port freight yard loading/unloading operations.

Design Standards: Designed in accordance with the national standard GB/T14406-1993 “Gantry Crane” technical specifications to ensure structural safety and reliability. The manufacturing enterprise must hold a special equipment manufacturing license. Finite element analysis is used during the design process to optimize the main girder structure, controlling the maximum stress of metal components not to exceed allowable values. The accuracy of mid-span deflection directly affects equipment safety performance.

Overall Structural Design

The metal structure of a Type-A Double-Girder Gantry Crane mainly consists of double main girders, two rigid legs, two flexible legs, saddle structures, and upper/lower cross beams forming a portal frame structure. This structural form features high stiffness and good stability, especially suitable for large-span, high-tonnage lifting operations. The overall structural design must consider factors such as load distribution, wind load (for outdoor operation), impact load, etc., to ensure safe and reliable operation under various working conditions.

Structural Composition Framework:

• Main Load-Bearing Structure: Includes the bridge formed by two parallel main girders, constituting the main load-bearing frame of the equipment.
• Support System: The portal frame structure composed of rigid and flexible legs. The rigid legs are rigidly connected to the main girder, while the flexible legs are hinged to the main girder.
• Travel Mechanism: Bogie groups installed at the bottom of the legs, driving the entire machine to run along the tracks.
• Hoisting Mechanism: Consists of an electric hoist, pulley block, and hook, installed on the lifting trolley.
• Auxiliary Systems: Include electrical control systems, safety devices, operator’s cab, etc.

Structure Type Selection: This design adopts a box-type double-girder structure. Compared with truss structures, it offers advantages such as simpler manufacturing processes, easier maintenance, and a more aesthetically pleasing appearance. Although truss structures have the characteristics of light self-weight and good wind resistance, considering the disadvantages of high manufacturing labor and inconvenient maintenance, the box girder design is chosen. The main girder cross-section adopts an optimized box-type structure with necessary longitudinal stiffeners and transverse diaphragms internally to ensure local stability and overall load-bearing capacity.

Double Cantilever Design: This crane adopts a double cantilever structure. The cantilever length on both sides is determined according to actual operational requirements (effective cantilever length on the rigid leg side is 8 meters). The cantilever design can effectively expand the operating range, reduce track laying length, and improve site utilization. The ratio of the cantilever part length to the span needs to be reasonably controlled to avoid excessive internal forces in the main girder or uneven leg pressure due to excessive cantilever length.

Table: Main Components and Functions of the 18/5t-40m Type-A Double-Girder Gantry Crane

Component	Material	Functional Characteristics	Manufacturing Requirements
Double Main Girders	Q345B Steel Plate	Box section, main load-bearing body, track mounted on it	Weld deformation control, preset camber
Rigid Legs	Q345B Steel Plate	Rigidly connected to main girder, withstands bending moment and pressure	Stiffness matching, welding quality
Flexible Legs	Q345B Steel Plate	Hinged to main girder, primarily withstands pressure	Allows some deformation, hinge design
Saddle Structure	Q345B Cast Steel	Connects main girder and legs, transfers load	Casting quality, fatigue strength
Travel Bogies	ZG270-500	Drives entire machine to run along tracks	Wheel pressure calculation, drive matching

Main Girder Structural Design

The main girder, as the most important load-bearing component of the crane, its design directly relates to the safety, reliability, and performance of the entire machine. The 40-meter span double girders in this design must meet the lifting requirements of the 18-ton main hook and 5-ton auxiliary hook, while also considering the impact of the 6-ton trolley self-weight.

Main Girder Cross-Section Design: Adopts a box-section form, composed of upper/lower cover plates, left/right web plates, and internal stiffeners. Box girders have advantages such as high bending and torsional stiffness, good local stability, and mature manufacturing processes. Based on similar design experience, the main girder height is preliminarily determined to be about 2.2-2.5 meters (approximately 1/16-1/18 of the span), and the width about 0.8-1.0 meters. Specific dimensions need to be determined through detailed calculations to ensure strength and stiffness requirements are met.

Main Girder Internal Stiffening: Longitudinal stiffeners and transverse diaphragms are set inside the box girder:

• Longitudinal Stiffeners: Evenly arranged along the height direction of the web plates to prevent web plate buckling under shear force.
• Transverse Diaphragms: Set every 2.5-3.5 meters, must be set at positions corresponding to trolley wheels to ensure local compressive capacity.

Main Girder Camber Design: To compensate for deflection deformation under load, the main girder requires pre-camber. According to specification requirements, the pre-camber value is generally 1/800-1/1000 of the span. For a 40-meter span, the pre-camber value is approximately 40-50mm, using a parabolic pre-camber curve. The camber design must consider deflection deformation when the trolley is at mid-span, ensuring no significant sag under rated load.

Main Girder Connection Design: When designing main girder segments, transportation and installation conditions must be considered, setting site connection joints. Connection joints typically use high-strength bolt friction-type connections. Joint locations should avoid high-stress areas and have stiffening details. A saddle structure is set at the connection between the main girder and legs, using cast steel components for machining to ensure reliable connection.

Main Girder Calculation Key Points:

1. Strength Calculation: Includes bending stress under vertical loads, lateral bending stress under horizontal loads, and combined stress calculation.
2. Stiffness Calculation: Check mid-span deflection under rated load, controlled within 1/700 of the span.
3. Stability Calculation: Includes overall stability and local stability (stiffener spacing design).
4. Fatigue Check: Fatigue strength verification for key welds and connection areas.

Main girder material is selected as Q345B low-alloy high-strength steel. This material has advantages such as high strength, good weldability, and moderate cost, making it a common material for crane metal structures. For high-stress areas, Q390B material can be considered to improve load-bearing capacity.

Leg System Design

The leg system of the Type-A Double-Girder Gantry Crane adopts a one-rigid-one-flexible design scheme, i.e., one side is a rigid leg, the other a flexible leg. This design can effectively adapt to deformations caused by temperature changes, load effects, and other factors in large-span structures, preventing the structure from generating excessive additional stress.

Rigid Leg Design:

• Uses a rigid connection with the main girder, capable of withstanding bending moment, shear force, and axial force.
• The cross-section form adopts a box-type structure, transitioning harmoniously with the main girder cross-section.
• Multiple inspection platforms are set in the height direction for easy maintenance and repair.
• The bottom is connected to the travel bogie via a spherical hinge to adapt to track unevenness.

Flexible Leg Design:

• Uses a hinged connection with the main girder, primarily withstanding axial force and a small amount of shear force.
• The cross-section form is usually smaller than the rigid leg, allowing a certain amount of lateral deformation.
• A hinge device is set at the top, usually using pin connections.
• The bottom also uses a spherical hinge connection with the travel bogie.

Leg Height Determination: Comprehensively determined based on parameters such as lifting height (14 meters), trolley height, and spreader height to ensure sufficient lifting space. Leg height must also consider the influence of main girder pre-camber to ensure track gradient does not exceed specified values.

Saddle Structure Design: A saddle structure is set at the connection between the legs and the main girder, serving as the force transfer hub. The saddle is usually made of cast steel (e.g., ZG270-500), ensuring matching with the main girder and legs through precise machining. The saddle structure requires detailed stress analysis to avoid stress concentrations.

Leg Calculation Content:

1. Strength Calculation: Includes strength verification under pressure, bending moment, and shear force.
2. Stability Calculation: Especially the in-plane and out-of-plane stability of flexible legs.
3. Stiffness Calculation: Controlling leg deformation to ensure coordinated deformation with the main girder.
4. Connection Calculation: Strength verification of the connection joints between legs and the main girder.

The leg system design must also consider wind load effects, especially for a 40-meter span gantry crane where wind load may become one of the controlling loads. Calculate wind load based on the basic wind pressure of the usage area, considering height variation coefficients and shape coefficients.

Table: Comparison of Rigid Leg and Flexible Leg Characteristics

Characteristic	Rigid Leg	Flexible Leg
Connection with Main Girder	Rigid connection (welding or bolting)	Hinged connection (pin)
Main Forces Withstood	Bending moment, Shear force, Axial force	Primarily axial force
Cross-section Size	Larger, usually equal to main girder width	Smaller, allows some deformation
Manufacturing Difficulty	Higher, requires ensuring matching with main girder	Relatively simple
Temperature Deformation Adaptability	Poorer	Better

Mechanism Design and Selection

A complete gantry crane structural design includes not only the metal structure part but also considers the configuration and coordination of various working mechanisms. This design for the 18/5t-40m Type-A Double-Girder Gantry Crane includes three basic mechanisms: hoisting mechanism, trolley travel mechanism, and gantry travel mechanism.

Hoisting Mechanism Design:

• Main Hook Hoisting Mechanism: Rated lifting capacity 18 tons, hoisting speed 9.3 m/min.
• Auxiliary Hook Hoisting Mechanism: Rated lifting capacity 5 tons, hoisting speed to be determined (usually higher than main hook).
• Adopts the traditional arrangement form of electric motor – reducer – drum.
• Steel wire rope selects high-strength, rotation-resistant type, ensuring a safety factor of not less than 6.

Trolley Travel Mechanism Design:

• Trolley self-weight 6 tons, gauge 2.5 meters, wheelbase 2.0 meters.
• Adopts a four-corner drive form to ensure good adhesion.
• Travel speed to be determined (reference similar designs about 40-50 m/min).
• Wheels use forged alloy steel, bearings select spherical roller bearings to adapt to main girder deflection deformation.

Gantry Travel Mechanism Design:

• Gantry base 10 meters, travel speed 48 m/min.
• Adopts a diagonal drive method, each drive set includes electric motor, reducer, brake, and wheel group.
• Number of driving wheels not less than 1/2 of the total wheels to ensure sufficient driving force.
• Sets anti-wind and anti-skid devices to ensure safety in non-working states.

Electrical Control System:

• Adopts variable frequency speed regulation technology to achieve smooth starting and braking.
• Sets complete protection devices: overcurrent, overload, limit, overload, etc.
• Operator’s cab is suspended below the main girder, providing good visibility.

Safety Device Configuration:

1. Load Limiter: Prevents overload operation.
2. Height Limiter: Limits lifting height of the spreader.
3. Travel Limiter: Controls trolley and gantry travel range.
4. Buffer Device: Buffers set at track ends.
5. Wind Protection Devices: Rail clamps, anchor devices, etc.

Mechanism Selection Calculation:

1. Electric Motor Power Calculation: Comprehensively determined based on static power and dynamic power.
2. Brake Selection: Ensures sufficient braking torque.
3. Reducer Selection: Selected based on parameters such as speed ratio and torque.
4. Coupling Selection: Compensates for installation errors and deformation.

Mechanism design must consider duty class matching, ensuring the mechanism’s duty class is not lower than the entire machine’s duty class. For an A5 duty class crane, the mechanism’s duty class is usually M5 (medium usage frequency).

Structural Calculation and Finite Element Analysis

Crane metal structure design must undergo strict calculation verification to ensure strength, stiffness, and stability requirements are met. The structural calculation for this 18/5t-40m Type-A Double-Girder Gantry Crane is based on GB/T3811-2008 “Crane Design Code”, using a combination of traditional mechanical calculations and finite element analysis.

Load Combination Analysis:

1. Regular Loads: Include self-weight load, hoisting load, inertial load, etc.
2. Occasional Loads: Such as lateral force from skew running, collision load, etc.
3. Special Loads: Such as seismic load, test load, etc.

Calculation Case Selection:

• Main Case: Trolley at mid-span, full load 18 tons.
• Edge Case: Trolley at cantilever end, full load 18 tons.
• Dynamic Case: Considering effects of hoisting impact, travel inertia, etc.
• Wind Resistance Case: Working state and non-working state wind loads.

Main Girder Strength Calculation:

1. Bending Stress under Vertical Load: Calculate bending moment at mid-span and cantilever end.
2. Bending Stress under Horizontal Load: Consider inertial force and lateral force from skew running.
3. Combined Stress Calculation: Combine according to the fourth strength theory.

Stiffness Calculation:

• Vertical Static Stiffness: When the loaded trolley is at mid-span, deflection not exceeding L/700 (approx. 57mm).
• Dynamic Stiffness: Control vibration frequency to avoid resonance with mechanism frequency.
• Cantilever End Stiffness: Sag under full load not exceeding L1/350 (cantilever length L1).

Stability Calculation:

1. Overall Stability: In-plane and out-of-plane stability of the portal frame.
2. Local Stability: Control of width-to-thickness ratio of plate elements, stiffener design.
3. Buckling Analysis: Calculation of buckling critical stress for compression members.

Finite Element Analysis:
Use SolidWorks or similar software to build a 3D model for mesh division and boundary condition setting. Analysis content includes:

• Static Analysis: Determine stress distribution and deformation.
• Modal Analysis: Obtain natural frequencies and mode shapes of the structure.
• Fatigue Analysis: Evaluate fatigue life of key areas.

Connection Calculation:

1. High-Strength Bolt Connection Calculation: Determine bolt specifications and quantity.
2. Welded Connection Calculation: Weld size and form design.
3. Hinge Pin Calculation: Bending and shear stress verification.

Wind Resistance Calculation:

• Working State Wind Pressure: Selected according to crane design code.
• Non-Working State Wind Pressure: Based on local 50-year return period basic wind pressure.
• Wind Load Distribution: Consider structural windward area and shape coefficients.

Based on similar design experience, the maximum stress of metal components is controlled within 150.8 MPa. The control accuracy of mid-span deflection directly affects equipment safety performance. A safety factor of 1.1-1.3 must be considered during calculation to ensure structural reliability.

Manufacturing Process and Installation Plan

The manufacturing quality of a Type-A Double-Girder Gantry Crane directly affects its performance and safety reliability. The structural design of this 18/5t-40m crane must fully consider manufacturability, including various stages such as cutting, forming, welding, and assembly. At the same time, a reasonable installation plan must be formulated to ensure on-site installation quality.

Main Girder Manufacturing Process:

1. Plate Pre-treatment: Steel plates undergo shot blasting/rust removal, and shop primer coating.
2. Cutting Processing: Use CNC cutting to ensure dimensional accuracy.
3. Forming Process: Formed by bending machine, controlling springback effects.
4. Welding Process: Develop a reasonable welding sequence to control welding deformation.
5. Assembly Process: Use special assembly fixtures to ensure main girder camber and straightness.

Leg Manufacturing Process:

• Adopts a segmented manufacturing scheme for easy transportation and installation.
• Set machining datums to ensure alignment accuracy between segments.
• Symmetrical welding during the welding process to control angular deformation.

Welding Quality Control:

1. Welder Requirements: Certified, familiar with welding procedure specifications.
2. Weld Class: Main load-bearing welds are Class I, 100% non-destructive testing.
3. Welding Materials: Match base metal properties, low-hydrogen type electrodes.
4. Post-Weld Treatment: Stress relief annealing for important welds.

Anti-Corrosion Treatment:

• Surface Treatment: Sandblasting/rust removal to Sa2.5 grade.
• Coating System: Primer (Epoxy Zinc Rich) + Intermediate Coat (Epoxy Micaceous Iron Oxide) + Topcoat (Polyurethane).
• Total Dry Film Thickness not less than 200μm.

Installation Plan Design:

1. Site Preparation: Track installation acceptance qualified, foundation strength meets standards.
2. Segmentation Division: Main girder divided into 2-3 segments, legs into 2 segments for easy transportation.
3. Lifting Process:
   • First install leg segments, temporary fixing.
   • Lift main girder segments, connect in the air.
   • Finally install machinery house segment, complete overall installation.
4. Welding and Reinforcement: Weld connection interfaces, ensure installation accuracy and stability. During welding, take appropriate protective measures to avoid environmental pollution and harm to personnel. After welding, conduct quality inspection on welds to ensure they meet design requirements.
5. Precision Adjustment: Adjust precision indicators such as track parallelism, levelness, and verticality according to design requirements. Use professional measuring instruments for measurement and adopt appropriate adjustment methods to ensure track installation accuracy meets design requirements.
6. Inspection and Acceptance: After installation is complete, conduct comprehensive inspection and testing. Check if the installation of each component meets design requirements, if equipment operates normally, and if safety is ensured. Conduct necessary tests, such as load-bearing tests, stability tests, etc., to verify equipment performance and safety. Organize inspection and test results and submit them to relevant departments for acceptance. Only equipment that passes acceptance can be put into use.