Discussion on the number of large interface structures driven by wear-and-hardness bolts

For the sake of comparison, we have converted the provisions of the above four specifications into different numbers of bolt joint head bolt force transmission ratio B1 and force unevenness coefficient A1, listed in. A1 refers to the ratio of the shear force transmitted by the head bolts to the average force of the rows of bolts. It can be seen that the regulations of rules 1 and 2 are relatively close, and 3 and 4 are also relatively close, but 1, 2 and 3, 4 are quite different. In order to investigate whether it is feasible to change B1 from 0.35 to 0.30 in the "New Bridge Regulations", the author was commissioned by the Editorial Committee of the "New Bridge Regulations" to study the issue.

At this stage, the bolt is only pulled and is not sheared, and the external shear force is completely resisted by the friction between the plates provided by the bolt tightening force (which causes the plate to be pressed). The original gap is always maintained between the bolt rod and the wall of the hole, and the deformation can be completely recovered after unloading, so this stage is also called the elastic stage. The load outside the stage does not exceed the maximum friction and the plate and bolt are in the elastic state, that is, point A in the figure. When P is greater than PA, the friction between the first and last rows of bolts is overcome by the external shear force, and the contact surface between the core plate and the cover plate begins to slip here. The increase of the relative displacement u starts to accelerate, and the joint enters the local slip. State, the redistribution of forces begins to occur between the bolts. As the load continues to increase, significant slippage occurs at the interface between the plates at all or most of the bolts in the joint.

The P~u curve state of the friction type high-strength bolt joint is the point B in the figure. If the gap between the bolt and the bolt hole is large, the relative displacement between the plates is already large, so that the rigidity of the structure is lowered and the deflection is increased. Therefore, in the actual design, the bearing pressure of the hole wall after the slip is not considered, and the load PA that overcomes the friction force is defined as the ultimate bearing capacity of the joint. This is a situation. On the other hand, if the gap between the bolt and the bolt hole is small or even no gap, the core plate and the cover plate are slightly slipped, and the bolt at the end is started to be sheared. If the strength of the bolt, the strength of the bolt and the plate member can be The resistance provided is greater than the maximum friction that the joint can provide, and the joint can resist the continuously increasing load, ie the BC section in the figure, and then the bolt or plate part of the area yields, and the slip increases rapidly until it reaches D point. The joint is broken. Point D is the limit state of the bearing capacity of the joint. This is the second case. In this case, the AB segment is shorter. Some documents only refer to the first type of joint described above as a friction type joint, and the second type as a shear type load bearing joint. But the BS5400 also classifies it as a friction-type high-strength bolt joint. The construction of such joints is difficult. At present, the first form of joints is mainly used in China.

Conclusion In summary, for the friction type high-strength bolt long joint, the second draft of the long-connected head row bolts in the second draft of the "Code for Design of Railway Bridges and Culverts" based on the reliability theory developed in China is based on the previous drafts. It is safe and reasonable to change 0.35 to 0.30.

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