Pre-tightening force of structural steel bolts and nuts depends on the yield of the material and the effective area of the bolt, taking into account a certain friction coefficient. The pre-tightening force of specialized steel structure bolts is established by tightening the nuts during construction. There are several methods for tightening nuts, as follows:
Based on the relationship between torque and pre-stress, first use an ordinary wrench to tighten the nut, and then use a visible torque. The value of this special wrench is tightened to the specified torque value.
Based on the close contact between the plates, the rotation angle of structural steel bolts and nuts is proportional to the pre-tightening force of the bolts. When tightening, use the short wrench to tighten the nut to a non-rotating position, and then use the long wrench to tighten the nut to the specified position. Achieve pre-tightening.
Used for torsion-shear type high-strength bolts. Structural steel bolts and nuts have special tails. When tightening, use a special wrench to hold the bolt and nut at the tail, one socket is rotated forward and the other socket is rotated in reverse. When the nut is tightened to a certain degree, the tail bolt will break. Because the depth of the groove on the tail of the bolt is determined by the relationship between the torsional torque and the pre-tightening force, the corresponding pre-tension value is reached when the bolt is twisted to the phase. In high-strength bolt connections, the size of the friction coefficient has a great influence on the bearing capacity. Experimental results show that the size of the friction coefficient is directly related to the quality of the structure material, the roughness of the contact surface, and the magnitude of the reverse force of the component material. The main factors and component materials of the contact surface and the reverse force should be used to increase the friction coefficient of the contact surface of the components in the connection area during construction.
The treatment methods include sandblasting, wire brush cleaning, etc. In the design, the friction coefficient should be used as much as possible according to the engineering conditions. Large processing methods should be clearly marked on the construction drawings.
In addition to the above treatment, there is also a method of using a portable electric grinding wheel to grind the contact surface. The grinding direction should be perpendicular to the force direction, and its anti-slip coefficient is equivalent to sandblasting treatment.
Structural steel bolts and nuts are a new type of connection developed in the 1950s. It has the advantages of simple structure, good mechanical performance, detachable, fatigue resistance, and no loosening under dynamic load. This is a promising connection method.