1. Construction of high-strength bolts
The high-strength bolts are made of heat-treated high-strength steel and are classified into 8.8 grades and 10.9 grades in terms of performance grades.
High-strength bolts can be divided into two types: friction type connection and pressure type connection. The high-strength bolt friction type connection relies solely on the frictional resistance between the connected members to transmit the shear force, and the shear force is equal to the frictional force as the limit state of the load-bearing energy. The connection is tight, the force is good, the fatigue is resistant, and the dynamic load is suitable, but the joint surface needs to be treated as a friction surface, such as sand blasting and sand blasting, and then applying inorganic zinc paint.
The force transmission characteristic of the high-strength bolt pressure-type connection is that when the shear force exceeds the friction force, the relative slip between the components occurs, and the bolt shaft begins to be sheared and the wall of the hole is pressed. However, on the other hand, the frictional force gradually decreases as the external force continues to increase, and the shear force is entirely carried by the shaft body when the connection is nearly broken. The pressure-type high-strength bolts are broken by bolts or steel plates as the ultimate state of load carrying capacity, and the possible damage forms are the same as ordinary bolts. This bolted connection should also be in a limit state where normal use is not occurred. The high-strength bolt pressure-type connection surface only needs to be rust-proof.
According to different bolt construction and construction methods, high-strength bolts can be divided into two types: large hexagonal high-strength bolts and torsion-shear high-strength bolts (Fig. 1). Large hexagonal heads are the same as ordinary hexagonal head thick bolts. The threaded end is similar to the rivet, but with a Phillips chuck and an annular groove that controls the tightening torque at its threaded end, see (1) b).
Dimensions and specifications (see table below) Class 8.8 is only for large hexagonal high-strength bolts, class 10.9 for torsion-cut high-strength bolts and large hexagon head high-strength bolts.
A high-strength bolt connection consists of bolts, nuts, washers, mechanical properties and effective area of ​​high-strength bolted joints (see table below).
2, the performance of high-strength bolts
(1) Pre-tension of high-strength bolts
The pre-tension of the high-strength bolt is achieved by tightening the nut. The pre-tension is generally controlled by the torque method, the corner method or the twisting of the tail of the bolt (tear-shear type).
1) Torque method A special wrench that can directly display the torque is used to apply torque according to the relationship between the previously measured torque and the bolt tension, and the necessary over-tension value is included.
2) The corner method is divided into two steps: initial screwing and final screwing. The initial screwing is to use the ordinary wrench to make the connected members close to each other. The final screwing is based on the initial tightening position. The final tightening angle determined by the bolt diameter and the thickness of the stack is rotated by a powerful wrench. When the nut is screwed to a predetermined angle value, the tension of the bolt reaches the required pre-tension value.
3) The torsional shear type torsion shear type high-strength bolt has the same force characteristics as the general high-strength bolt. However, the method of applying the pre-tension is to use the cross section of the torn bolt to cut the head, see (Fig. 1b) to control the pre-tension value. This type of bolt exerts a pre-tensioning force and is accurate.
(2) Anti-slip coefficient of friction surface of high-strength bolted joint
The friction type high-strength bolt connection relies entirely on the frictional force transmission force between the connected members, and the friction assisting force has the anti-slip coefficient of the friction surface determined by the surface treatment of the material to be joined and the contact surface thereof in addition to the bolt pre-tensioning force. related.
Note: 1. The direction of derusting of the wire brush should be perpendicular to the direction of the force.
2. When the connecting member adopts different steel grades, the value is taken according to the corresponding lower strength.
3. When using other methods, the treatment process and anti-slip coefficient values ​​must be determined by experiments.
(3) Principles of bolt selection
Common bolts used in construction steel structures should generally adopt Class C bolts, and the wells should meet the requirements of the national standard "Hexagon head bolts - Class C" and "Hexagon head bolts". Class C bolts are suitable for tensioned connections, less sheared connections and mounting connections that are not directly subjected to dynamic loads. A and B bolts are also available.
When selecting high-strength bolts, only the bolt level (10.9 or 8.8) and the category (friction type or pressure type) are required, and the steel grade and steel type requirements are not required. Friction type connections are generally used in steel structure engineering. High-strength bolts shall comply with the national standard "High-strength large hex head bolts for steel structures", "High-strength large hex head bolts for steel structures", "Technical conditions for large hex nuts and washers", "Twisted shear type high strength for steel structures" The provisions of the screw joints.
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