Common problems of steel structure bolts

1. How to connect the steel structure?

The common connection methods of steel structure are welding, riveting, bolt connection (common bolt connection, and high strength bolt connection).

picture1- connecting methods diagram

2. The main classification of bolts.

According to the performance level, bolts are divided into 3.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, and other grades. The bolts of grade 8.8 and above are made of low-carbon alloy steel or medium-carbon steel with heat-treated (quenched and tempered), which are commonly known as high strength bolts. For those bolts bellow grade, 8.8 are generally called common bolts. The numbers before and after the decimal point are respectively expressed as the nominal tensile strength and yield ratio of bolt material.

picture2- performance comparison of bolts in different grade

• common bolts

Common bolts are commonly used hexagonal bolts, stud bolts, anchor bolts, etc., high-strength bolts are divided into torsion shear-type high-strength bolts and large hexagonal high-strength bolts.

According to the force transmission method, bolts are also divided into shearing bolts, tensile bolts, both shearing and tension bolts.

1. a. Anti-shear connection. Relying on screw pressure and anti-shear to transmit external force which is perpendicular to rod axis direction, there is a tendency of alternating motion for plates.
2. b. Tensile connection. Relying on the tensile strength of screw to transmit external force which is parallel to rod axis direction, the plates have a tendency to separate from each other.
3. c. The connection between tensile and shear joint action. Relying on the screw to transmit both the external force which is perpendicular to the rod axis and the external force that parallels to the rod axis. There are relative displacement and disengagement force between the plates.

• high-strength bolts

According to calculation and design methods, connection methods of high-strength bolts can be divided into friction-type connection and pressure-type connection.

1. a. Friction connection. The shearing force is transmitted through frictional resistance which is generated by squeezing. According to the design principle, the shearing force couldn’t exceed frictional resistance.
2. b. Pressure-bearing connection. The contact surface is allowed to slip. Take the ultimate bearing capacity that connection could reach as a design criterion. Its bearing capacity is higher than the friction type with a compact connection, but the shear deformation is greater than friction type.

3. Bolt arrangement and bolt spacing requirements.

Bolt arrangement concludes paralleled arrangement and staggered arrangement. The paralleled arrangement is simple, neat, and compact with the use of connecting plate in small size, but the component cross-section is largely weakened. The staggered arrangement is not compact with the use of connecting plate in large size, and the component cross-section is slightly weakened.