The deformation of metal structures is a rather common problem, and the main beam, as the primary load-bearing component of bridge-typegrúas, is inevitably affected. To minimize the occurrence of bridge frame deformation, the following research has been conducted on this issue:
Deformation Forms of the Bridge Frame
Reduction of the Camber of the Main Beam: The camber of the main beam, which is the upward curvature of the beam, decreases. This can affect the load-bearing capacity and stability of thegrúa. A reduced camber may lead to uneven stress distribution and increased deflection under load, potentially causing safety hazards.
Sidesway (Lateral Bending): The main beam may bend laterally, either towards the inside or outside. This lateral deformation can disrupt the normal operation of the crane, affecting the alignment of the lifting mechanism and the smooth movement of the trolley.
Wave Deformation of the Web Plate of the Main Beam: The web plate of the main beam shows a wavy pattern. This kind of deformation can reduce the structural integrity of the main beam and may also affect the performance of the crane’s lifting and moving functions.
Deformation of the End Beam: The end beam, which connects the two ends of the main beam, may also undergo deformation. This can lead to problems such as misalignment of the wheels and uneven load distribution on thegrúa‘s running mechanism.
Deviation of the Diagonal Dimensions: The diagonal dimensions of the bridge frame exceed the specified tolerance. This deviation can cause issues with the overall geometry of the bridge frame, affecting the stability and normal operation of the crane.
Causes of Deformation
Reasons for the Reduction of the Camber of the Main Beam
Influence of Internal Structural Stress: During the manufacturing process of the crane, internal stresses are inevitably generated in the metal structure due to factors such as welding, cooling, and forming. These internal stresses can gradually release over time, causing the main beam to deform and the camber to decrease. Por ejemplo, if the welding process is not carried out properly, uneven heating and cooling of the metal can lead to significant internal stresses, which may cause the main beam to sag over time.
Overloading and Unreasonable Use: When the crane is frequently operated beyond its rated load capacity or used in an improper manner, such as sudden starts, stops, and impacts, it can cause excessive stress on the main beam. This continuous overloading and improper operation will gradually reduce the camber of the main beam. Por ejemplo, in some construction sites, in order to catch up with the progress, the crane is often overloaded, which accelerates the deformation of the main beam.
Influence of the High-temperature Working Environment: If the crane operates in a high-temperature environment for a long time, the mechanical properties of the metal will change. The metal may become softer at high temperatures, reducing its strength and stiffness. Como resultado, the main beam is more likely to deform under the action of load, and the camber will decrease. Por ejemplo, in steel mills or foundries, the high-temperature environment poses a great challenge to the structural integrity of the crane.
Influence of Design and Manufacturing Processes: An inappropriate design may not fully consider the actual working conditions and load characteristics of the crane, resulting in insufficient strength and stiffness of the main beam. In addition, if the manufacturing process is not up to standard, such as inaccurate cutting, welding defects, and improper assembly, it will also affect the quality of the main beam and lead to a reduction in camber. Por ejemplo, if the welding seams are not properly processed, they may become weak points in the main beam, prone to deformation under load.
Unreasonable Hoisting, Storage and Installation of the Crane: During the hoisting, storage, and installation of the crane, if improper methods are used, it can cause damage to the main beam. Por ejemplo, if the crane is not properly supported during storage, the main beam may be subjected to uneven forces, leading to deformation. Similarmente, if the hoisting points are not selected correctly during installation, it can also cause the main beam to be deformed.
Unreasonable Repairs: When repairing the crane, if the repair methods are not appropriate, it may cause further damage to the main beam. Por ejemplo, if excessive heat is applied during welding repairs, it may cause the metal to expand and contract unevenly, resulting in a reduction in the camber of the main beam.
Reasons for the Occurrence of Sidesway
Horizontal Bending Generated during Use: During the operation of the crane, various forces act on the main beam, including horizontal forces generated by the movement of the trolley, wind forces, and the inertia force during braking. These horizontal forces can cause the main beam to bend horizontally over time. Por ejemplo, if the crane operates in a windy environment for a long time, the wind force may cause the main beam to gradually bend.
Precast Sidesway Required by the Manufacturing Process: In some cases, a certain amount of precast sidesway is intentionally designed in the manufacturing process to offset the deformation that may occur during use. Sin embargo, if the precast amount is not accurately controlled or the actual working conditions deviate from the design assumptions, it may lead to excessive or insufficient sidesway.
Horizontal Bending Caused by the Modification of Structural Components: When the structure of the crane is modified, such as adding or removing some components, it may change the force distribution of the main beam, resulting in horizontal bending. Por ejemplo, if additional equipment is installed on one side of the main beam, it will cause an imbalance of forces and lead to sidesway.
Bending of the Main Beam towards the Inside due to the Action of Horizontal Inertia Force during Use: When the crane is in operation, especially during sudden acceleration or deceleration, the horizontal inertia force generated by the moving parts can cause the main beam to bend towards the inside. This is because the inertia force tries to maintain the original state of motion of the parts, and when the crane changes its motion state, the main beam is affected by this force.
Reasons for the Wave Deformation of the Web Plate of the Main Beam
When the web plate is spliced, if the steel plate itself is not flat (and there is no leveling treatment before welding), under the action of the internal stress generated during welding, the web plate will produce wave deformation. The welding process involves rapid heating and cooling, which can cause uneven expansion and contraction of the steel plate. If the plate is not flat initially, these uneven deformations will accumulate and form a wavy pattern on the web plate.
Reasons for the Deformation of the End Beam
Outward Bending of the End Beam Caused by Welding Additional Plates or Angles: In order to enhance the connection rigidity between the main beam and the end beam, sometimes the user unit welds a steel plate (or a large angle steel) on the head of the main beam and the end beam. Sin embargo, this additional welding may introduce additional internal stress, causing the end beam to bend outward. The uneven distribution of the welding stress can cause the end beam to deform from its original shape.
Deformation of the End Beam Caused by Severe Rail Gnawing of the Cart: If the cart gnaws the rail severely, the lateral force generated during this process will act on the end beam. This continuous lateral force can gradually deform the end beam, affecting the running performance of the crane. Por ejemplo, if the wheels of the cart are not properly aligned or the rails are uneven, it will lead to rail gnawing and subsequent deformation of the end beam.
Deformation of the End Beam Caused by the Downward Deflection of the Main Beam during Use: As the main beam deflects downward over time due to various reasons, it will also affect the end beam. The change in the position and shape of the main beam will cause the end beam to bear additional forces, resulting in its deformation. The connection between the main beam and the end beam is such that the deformation of one will have an impact on the other.
Reasons for the Deformation of Diagonal Dimension Deviation
The deformation of the connection between the main beam and the end beam in the horizontal direction can lead to the deviation of the diagonal dimensions. This may be due to factors such as improper installation, uneven force distribution, or damage to the connection parts. If the bolts or welds connecting the main beam and the end beam are loose or damaged, it will cause the relative position between the two to change, resulting in the deviation of the diagonal dimensions.
Repair Methods of Bridge Frame Deformation
Prestressed Steel Bar Tensioning Method
This method involves installing prestressed steel bars on the main beam of the bridge frame. By tensioning these steel bars, a pre-compressive stress is applied to the main beam. This pre-compressive stress can offset the tensile stress generated during the operation of the crane, thereby reducing the deformation of the main beam. The prestressed steel bars are carefully designed and installed according to the specific situation of the bridge frame to ensure that they can effectively improve the structural strength and stability of the main beam. Por ejemplo, in the case of a main beam with a significant reduction in camber, the prestressed steel bar tensioning method can be used to restore its original shape and improve its load-bearing capacity.
Prestressed Steel Wire Rope Tensioning Method
Similar to the prestressed steel bar tensioning method, the prestressed steel wire rope tensioning method uses steel wire ropes as the prestressing elements. The steel wire ropes are installed on the main beam and tensioned to apply a preload to the beam. This method is often more flexible and suitable for some complex-shaped bridge frames. The steel wire ropes can be adjusted according to the actual deformation situation to achieve better repair effects. Por ejemplo, when the main beam has both camber reduction and sidesway problems, the prestressed steel wire rope tensioning method can be adjusted to address both issues simultaneously.
Flame Straightening Method
The flame straightening method is a common repair method for metal structure deformation. It uses the principle of thermal expansion and contraction of the metal. By heating specific areas of the deformed bridge frame with a flame, the metal in these areas expands. When the heated area cools down, it contracts, and by controlling the heating and cooling process, the deformation of the bridge frame can be corrected. Sin embargo, this method requires skilled operators to accurately control the heating temperature, area, and cooling rate. If not properly controlled, it may cause further damage to the bridge frame. Por ejemplo, when using the flame straightening method to repair the wave deformation of the web plate of the main beam, the operator needs to carefully heat the convex parts of the wave to make them contract and restore the flatness of the web plate.
En conclusión, understanding the deformation forms, causes, and repair methods of the crane bridge frame is crucial for ensuring the safe and reliable operation of the crane. By taking appropriate preventive measures and timely repair actions, the service life of the crane can be extended, and potential safety hazards can be effectively avoided. Regular inspection and maintenance of the crane bridge frame are also essential to detect and address deformation problems in a timely manner.
