Retrofitting crane beams

Since the 1980s, carbon fiber fabric concrete repair technology, as an advanced structural repair method with high efficiency and low cost, has been attached great importance to and widely studied and applied in civil engineering. At present, the technology of fiber reinforced concrete is relatively mature. However, due to the essential difference between the properties of steel and concrete, the technology of strengthening steel structure and concrete is very different. The failure mode of steel structure strengthened by CFRP is mostly interface failure. At present, the research on carbon fiber fabric strengthening steel structure is still in the preliminary exploratory stage at home and abroad, and the real application in engineering has not been carried out on a large scale.

Characteristics of crane beams strengthened with carbon fiber fabric

In industrial plant, the normal operation of crane girder has a direct impact on the normal production, especially for steelmaking plant, crane tonnage is usually large or even overloaded, and frequent operation, making crane girder damage and cracking phenomenon occur from time to time. The failure of crane girder system of steel structure in industrial plant is mostly caused by fatigue, and most of the crane girders are destroyed before they reach the design service life because of the high indoor temperature and other factors. The traditional reinforcement method has certain reinforcement effect, such as welding steel plate or section steel. But at the same time, it also increases the weight of crane beam and the decrease of space utilization ratio, resulting in the redistribution of stress and stiffness, usually causing stress concentration in weld seam, fatigue cracks under the action of fatigue load, resulting in secondary fatigue damage of steel crane beam. Because CFRP has high tensile strength, strong formability (such as good bonding with steel members at the corner of the steel structure), good corrosion resistance and fatigue resistance, the reinforcement effect of steel crane girder is much better than that of traditional methods. Different strengthening methods are adopted according to different diseases, such as sticking CFRP on the tensile surface of the component can improve the flexural capacity of the component. In the fatigue part of steel structure, the residual fatigue life can be improved by sticking carbon fiber fabric. The technology effect of strengthening steel members with carbon fiber fabric is remarkable, and it has become a hot research topic of experts and scholars.

Analysis and measures of engineering difficulties

Lack of standard specifications

In view of the limited application of carbon fiber reinforced steel structure technology in practical engineering, there are no ready-made standards and specifications to follow. It is a new technology to bond carbon fiber fabric to reinforce and repair steel structure. It is very different from carbon fiber fabric to reinforce concrete. They are different in bonding mode, failure interface and mode. Therefore, in the course of the project, combining with the previous engineering experience, adopting the process of edge test, edge validation, edge optimization and edge construction, the optimum reinforcement design scheme is designed according to the material selection, bonding performance, process performance, fatigue performance test of cruciform joints, crane beam model test, structural calculation and analysis.

Bad engineering environment

In steel structure industrial buildings, steel crane girder is a very important part, and its environment is complex. For example, steel-making plant, if the stoppage of reinforcement will cause serious economic losses, so in the normal operation of the plant to reinforce the construction, at this time there are high temperature and dust in the working environment of the structure, the construction environment is bad, the physical strength and endurance of workers is a great test. Moreover, the ambient temperature of the steelmaking workshop is very high, and the bonding time and working temperature of the previous bonding materials can not meet the requirements. In the aspect of material selection, Ma Mingshan of China Metallurgical Architectural Research Institute Co., Ltd. completed 11 batches of 200 groups of material performance tests. The mechanical properties of several bonding materials, tensile and shear properties of steel to steel, normal tensile bonding properties of steel to carbon fiber fabric, fatigue properties, wettability, curing time, external disturbance and overlap properties of steel to cloth were systematically investigated. Finally, the bonding material and carbon fiber fabric suitable for strengthening the industrial workshop of steel structure are selected.

Short construction period

Considering the particularity of the CFRP strengthening project of the steel crane girder in industrial buildings, the construction period is short, and one day's shutdown will cause huge economic losses. Therefore, it is a great challenge to the requirement of construction technology and the coordination of personnel. Therefore, it is suggested to adopt quick-drying binder and matching construction technology in reinforcement construction, which can greatly shorten the shutdown time.

Construction inspection and acceptance

Material inspection

For carbon fiber fabric, structural adhesives and other reinforcement materials, it is necessary to enter the field once in accordance with the dosage of the project. When entering the venue, the product qualification certificate, product quality inspection report and packaging integrity shall be inspected jointly with the supervision unit. At the same time, we should witness the sampling inspection for the safety performance of the products.

Process quality inspection and control

When the steel structure is strengthened and repaired with carbon fiber cloth and matching resin bonding materials, the construction of the next process can be carried out only after the previous process has passed the inspection. When construction quality fails to meet the requirements, remedial measures or rework should be taken immediately. During construction, special personnel should be responsible for quality inspection and detailed records.

Dimensional deviation and bond quality inspection

When strengthening crane girder of steel structure with CFRP sheet, it is necessary to stick and reinforce the girder strictly according to the design size, and the relative deviation of sticking position is less than 10 mm. The total effective area should not be less than 95% of the total bond area. If a single hollowing area is less than 2500mm2, it is recommended to use needle syringe to repair the hollowing part. If the area of a single empty drum is more than 2500 mm 2, the carbon fiber cloth at the empty drum should be cut off, and the carbon fiber cloth should be re-overlapped and pasted with the same amount of carbon fiber cloth. When re pasted, the lap length of each end should not be less than 200mm.

Epilogue

As a new type of reinforcement method, carbon fiber fabric(CFRP) reinforced steel structure will be widely used in the field of steel structure with the improvement of standard specification and construction technology.