Crane runway girders supporting cranes are a very important element in manufactory buildings. In case of welded crane runway girders supporting large cranes, in particular, the fatigue fractures caused by heavy cyclic loading may lead to the destruction of a building and there are many foreign repor...
Crane runway girders supporting cranes are a very important element in manufactory buildings. In case of welded crane runway girders supporting large cranes, in particular, the fatigue fractures caused by heavy cyclic loading may lead to the destruction of a building and there are many foreign reports on such cases. Also in Korea, there often occur accidents caused by the fatigue fractures of welded crane runway girders in manufactory buildings, which eventually causes operation stoppage and threatens building safety in many ways.
Fatigue cracks which primarily appear in welded crane runway girders become gradually wider as cyclic stresses are continuously applied and in the end lead to the destruction of an entire building. In other words, fatigue fractures are time-reliant and appear even in a stress condition lower than yield load and once the development of fatigue cracks reaches to a certain critical durability, the building begins to fall down rapidly.
In particular, welded crane runway girders for steel mills discussed in the study which receive large dynamic loads and complex stresses created by welded overhead crane runway girders and back girders in combination have unique fatigue cracks and damages when compared with those in other buildings. In this context, the study analyzes cases studies on the fatigue cracks of welded crane runway girders in manufactory buildings, which were carried out on-site to find out their causes, and maps out measures for them.
In addition, the study introduces examples of reinforcement works performed based on the results of the study and at the same time, provides basic data on maintenance and management to prevent fatigue fractures probable to occur in the future.
The study draws up conclusions as follows:
1) According to the damage types of welded crane runway girders as analyzed through the case studies in Chapter Ⅲ, the major cause for fatigue fractures in welded crane runway girders is the fatigue from crane travelling but other than this, stress from the bending moment of cranes and freights, axial stress caused by the eccentricity of the rails on which crane wheels run and loads created by the vibration of crane runway girders when braking cranes. The types of damages are the joint connecting flanges to webs, where local stresses concentrate, welded parts of vertical and horizontal stiffeners, and welded parts of gusset plates connected to back girders.
2) The fatigue cracks that primarily appear in welded crane runway girders become wider gradually as time goes by and when they reach a certain critical durability, the building begins to fall down rapidly. Therefore, it is necessary to keep the safety of buildings through regular checking and proper maintenance and reinforcement works.
3) As welded crane runway girders are usually plate girders, they receive a large vertical load and have many parts to be welded such as plate, rib and stiffener. Therefore, it is necessary to take into consideration welding parts vulnerable to fatigues in the designing stage and to carry out thorough inspections of welding parts for the prevention of fatigue cracks.
4) Because it is very likely that a high tensile stress may be created in the joints of welded crane runway girders by the cyclic moment from the eccentricity of rails, it is necessary to check the variations of crane rails at any time and if defects are found, they should be corrected immediately.
5) The damage assessment of welded crane runway girders is essential for determining how to rectify the defects. Therefore, damage assessment should be made in a comprehensive manner by taking into account impacts to the safety of buildings such as strength, load-carrying capacity, durability, etc. Also, the kind, condition and importance of damaged parts should be examined before damage assessment. In other words, the replacement of an entire crane runway girder, or the immediate maintenance or reinforcement of cracks should be determined according to the rational damage assessment of welded crane runway girders.
6) In order to prevent fatigue fractures resulting from fatigue cracks, regular safety checking is very important. In particular, crane operations in manufactory buildings, which have large eccentric loads should be done with the daily checking and cyclic safety diagnosis of crane runway girders.
Crane runway girders supporting cranes are a very important element in manufactory buildings. In case of welded crane runway girders supporting large cranes, in particular, the fatigue fractures caused by heavy cyclic loading may lead to the destruction of a building and there are many foreign reports on such cases. Also in Korea, there often occur accidents caused by the fatigue fractures of welded crane runway girders in manufactory buildings, which eventually causes operation stoppage and threatens building safety in many ways.
Fatigue cracks which primarily appear in welded crane runway girders become gradually wider as cyclic stresses are continuously applied and in the end lead to the destruction of an entire building. In other words, fatigue fractures are time-reliant and appear even in a stress condition lower than yield load and once the development of fatigue cracks reaches to a certain critical durability, the building begins to fall down rapidly.
In particular, welded crane runway girders for steel mills discussed in the study which receive large dynamic loads and complex stresses created by welded overhead crane runway girders and back girders in combination have unique fatigue cracks and damages when compared with those in other buildings. In this context, the study analyzes cases studies on the fatigue cracks of welded crane runway girders in manufactory buildings, which were carried out on-site to find out their causes, and maps out measures for them.
In addition, the study introduces examples of reinforcement works performed based on the results of the study and at the same time, provides basic data on maintenance and management to prevent fatigue fractures probable to occur in the future.
The study draws up conclusions as follows:
1) According to the damage types of welded crane runway girders as analyzed through the case studies in Chapter Ⅲ, the major cause for fatigue fractures in welded crane runway girders is the fatigue from crane travelling but other than this, stress from the bending moment of cranes and freights, axial stress caused by the eccentricity of the rails on which crane wheels run and loads created by the vibration of crane runway girders when braking cranes. The types of damages are the joint connecting flanges to webs, where local stresses concentrate, welded parts of vertical and horizontal stiffeners, and welded parts of gusset plates connected to back girders.
2) The fatigue cracks that primarily appear in welded crane runway girders become wider gradually as time goes by and when they reach a certain critical durability, the building begins to fall down rapidly. Therefore, it is necessary to keep the safety of buildings through regular checking and proper maintenance and reinforcement works.
3) As welded crane runway girders are usually plate girders, they receive a large vertical load and have many parts to be welded such as plate, rib and stiffener. Therefore, it is necessary to take into consideration welding parts vulnerable to fatigues in the designing stage and to carry out thorough inspections of welding parts for the prevention of fatigue cracks.
4) Because it is very likely that a high tensile stress may be created in the joints of welded crane runway girders by the cyclic moment from the eccentricity of rails, it is necessary to check the variations of crane rails at any time and if defects are found, they should be corrected immediately.
5) The damage assessment of welded crane runway girders is essential for determining how to rectify the defects. Therefore, damage assessment should be made in a comprehensive manner by taking into account impacts to the safety of buildings such as strength, load-carrying capacity, durability, etc. Also, the kind, condition and importance of damaged parts should be examined before damage assessment. In other words, the replacement of an entire crane runway girder, or the immediate maintenance or reinforcement of cracks should be determined according to the rational damage assessment of welded crane runway girders.
6) In order to prevent fatigue fractures resulting from fatigue cracks, regular safety checking is very important. In particular, crane operations in manufactory buildings, which have large eccentric loads should be done with the daily checking and cyclic safety diagnosis of crane runway girders.
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