Ανάλυση ρωγμών ιστού δοκού εναέριας γερανογέφυρας: Αιτίες και λύσεις

Web cracking at the variable cross-section at the end of the main girder is a common and serious problem in overhead cranes during long-term service. Understanding the causes of crack formation is essential for prevention and resolution. The following is an analysis of the causes of web cracking at the variable cross-section at the end of an overhead crane main girder.

1. Root Causes of Overhead Crane Girder Web Cracking

1.1 Stress Concentration

To meet dimensional requirements as well as strength and stiffness needs under various working conditions, the end of the main girder is often designed with a variable cross-section. However, the geometric discontinuity at the section change creates stress concentration, meaning the stress at the variable cross-section is significantly higher than in other areas. Stress concentration zones are prone to crack initiation and propagation. For example, a mechanical analysis of an overhead crane shows that the equivalent stress contour exhibits noticeably higher stress at the variable cross-section than elsewhere.

The product analyzed above features a sloped transition at the variable cross-section, whereas the structure in this case uses a right-angle transition, which creates an even more severe stress concentration condition.

1.2 Welding Quality

Welding quality directly affects structural safety and service life. Improper welding operations can readily produce defects such as porosity, slag inclusions, incomplete penetration, and weld cracks. These defects act as stress concentration points and easily become crack initiation sites under external loads. Welding quality issues are more likely to occur at variable cross-sections due to the complexity of the weld geometry. In this case, weld quality is difficult to guarantee at the right-angle corner of the variable cross-section weld.

1.3 Material Defects

Internal defects such as non-metallic inclusions, porosity, and micro-cracks may be present in steel due to process issues during production. These defects tend to propagate under stress and develop into visible cracks. In addition, variations in material properties between different batches can cause localized stress concentration, potentially initiating cracks. Material information is unavailable for this case, so material defects cannot be ruled out as a contributing factor.

1.4 Fatigue Loading

Overhead cranes are frequently subjected to repeated dynamic loads during operation. The variable cross-section of the main girder experiences cyclic stress variations, particularly when lifting loads. This repeated stress cycling leads to fatigue failure of the material — fatigue cracks can develop after prolonged service even when stress levels remain below the material’s yield strength. The variable cross-section is more susceptible to cracking due to the combined effects of stress concentration and material fatigue.

1.5 Design Deficiencies

Design factors that increase the risk of cracking include abrupt cross-section transitions without smooth contouring, failure to adequately account for stress concentration effects, and neglect of fatigue design considerations. Omitting reinforcement at the variable cross-section during the design phase can also lead to cracking in long-term service. Proper design therefore plays an important role in reducing crack formation.

1.6 Overloading and Improper Use

Although cranes are designed with a certain safety margin according to applicable standards, overloading and skew pulling or diagonal lifting are not uncommon in practice. Frequent overloading and improper lifting practices subject the main girder to stresses beyond the design values, leading to localized stress concentration, accelerated material fatigue, and ultimately crack formation. Overloading can also cause structural deformation, further intensifying stress concentration and crack propagation.

1.7 Vibration and Impact

Cranes are inevitably subjected to vibration and impact during operation. Excessive rail joint gaps can generate substantial impact loads as the crane travels across rail joints. Long-term vibration and impact cause fatigue damage and promote the propagation of micro-cracks. Cracks are more likely to initiate and propagate at variable cross-sections where stress concentration is already present.

2. Solutions for Overhead Crane Girder Web Cracking

A comprehensive set of measures should be taken to prevent and reduce web cracking at the variable cross-section at the end of an overhead crane main girder:

2.1 Improve Structural Design

Optimize the variable cross-section design by using smooth, gradual transitions to reduce stress concentration. Incorporate fatigue design to address long-term fatigue concerns. Recommended remedial reinforcement measures include:

Add triangular gusset plates at the connection, as shown in the figure below, to improve local stiffness.

Another approach is reinforcement by patching, as shown in the figure below.

However, because the original structure already has a reinforcing plate on the lower part of the web, any new reinforcing plate must be positioned to avoid the existing one. This creates a certain stress concentration issue at the adjacent welds.

2.2 Improve Welding Quality

Strictly control the welding procedure to ensure defect-free welds. Use advanced welding techniques and inspection methods such as ultrasonic testing (UT) and radiographic testing (RT) to promptly detect and repair welding defects.

2.3 Ensure Material Quality

Use high-quality materials free of initial defects. Perform regular material testing to confirm that material properties meet requirements.

2.4 Operate Equipment Properly

Provide necessary training for operators to prevent overloading. Operate and maintain the equipment in accordance with design specifications to extend its service life.

2.5 Perform Regular Maintenance and Inspection

Conduct regular maintenance inspections on overhead cranes, with particular attention to monitoring variable cross-section areas, to detect and repair cracks promptly.

Σύναψη

The measures outlined in this overhead crane girder web crack analysis can effectively prevent and reduce web cracking at the variable cross-section at the end of an overhead crane main girder, ensuring safe equipment operation.

Η Σίντι

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