High-strength bolts are typically categorized into two main types based on their stress behavior: friction-type connections and bearing-type connections. In friction-type connections, the primary goal is to create sufficient friction between the connected plates, which is achieved by tightening the nuts to a specified pre-tension force during construction. This is commonly done using the torque method.
To ensure that the required pre-tension is achieved, it's essential to use calibrated wrenches before and after each construction session. The tightening process is usually carried out in two stages: initial tightening and final tightening. The initial tightening torque should be at least 30% of the final tightening torque. The final torque value must be calculated according to established standards. During the tightening process, only the nut should be turned, and the sequence should follow a strict pattern—starting from the center and moving outward. All tightening operations should be completed within a single working day to maintain consistency and accuracy.
Currently, high-strength bolts are mainly divided into two types: torque-controlled bolts and hexagonal head bolts. For torque-controlled bolts, the final tightening is controlled by the removal of the plum blossom head, making quality control straightforward. However, for hexagonal head bolts, it's more challenging to verify the quality of the final tightening. This often leads to inconsistencies in quality inspection for steel structure bolted connections.
For example, in one steel structure project, the final tightening of the hexagonal high-strength bolts was not properly checked by the construction or supervisory team. Some inspectors used a torque wrench directly on the bolt after tightening, which could lead to over-tightening. Others applied the torque value from the end of construction rather than the calculated check torque as per specifications. This can alter the mechanical properties of the bolts and even cause them to break.
To prevent such issues, qualified torque wrenches are sealed and inspected before and after each shift. Any wrench that shows discrepancies is sent to a professional calibration service for repair. Only after passing the test is it allowed back into use. This ensures consistent and accurate application of torque, maintaining the integrity and safety of the structural connections throughout the construction process.
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