Connection method and characteristics of steel structure

The connection of steel structure is to combine section steel or steel plates into components, and assemble each component into nodes and key components of the entire structure. The way of connection and its quality directly affect the working performance of the steel structure, therefore, in the design of the connection, it is necessary to follow the principles of safety and reliability, clear force transmission, simple structure, convenient manufacturing and steel saving.

The connection methods of steel structures usually include weld connection, rivet connection and bolt connection, and the latter two are commonly known as fastener connections (Figure 31).

Weld seam connection

Weld connection is the most important connection method of modern steel structure. Its advantages are: (1) simple structure, strong adaptability to geometric shapes, any form of components can be directly connected; (2) Do not weaken the cross-section, save labor and materials; (3) Easy production and processing, automatic operation, high ergonomics and reliable quality; (4) The connection has good tightness and large rigidity of the structure.

The disadvantages of weld connection are (1) in the heat-affected zone near the weld, the metallographic structure of the steel changes, resulting in local material deterioration and brittleness; (2) The residual stress and residual deformation of welding reduce the bearing capacity of the compression member; (3) The welding structure is very sensitive to cracks, and once local cracks occur, it is easy to extend to the whole, and the problem of low temperature and cold brittleness is more prominent; (4) High requirements for materials, strict welding procedures, and large quality inspection workload.

Rivet connection

Rivet connections are manufactured in two ways: hot riveting and cold riveting. Hot riveting is made by inserting a red-hot nail blank into the nail hole of the member and riveting it with a rivet gun or pressure riveting machine. Cold riveting is made by riveting at room temperature. Hot riveting is generally used in building steel structures.

The

material of the rivet should have good plasticity, usually made of special steel BL2 mouth and BL3 steel.

After the rivet is riveted, the nail rod shrinks

from high temperature and gradually cools, but it is stopped by the steel plate between the nail heads, so the shrinkage tensile stress is generated in the nail rod, and the compressive force is generated on the steel plate, making the connection very tight. When the component is subjected to shear force, a great friction force is generated on the contact surface of the steel plate, which greatly improves the working performance of the connection.

The

quality and force performance of rivet connections are closely related to the production method of nail holes. The production methods of nail holes are divided into two categories: I. and II. Class I holes are drilled with drilling dies, or punched into smaller holes first, and then expanded and drilled during assembly, and the quality is better. Class II holes are punched or drilled without drilling dies, although the preparation method is simple, but the nail holes are not easy to align when the components are assembled, and the quality is poor. Important structures should be made of Class I holes.

Compared with the weld connection, the steel structure connected by rivets has good plasticity and toughness, easy quality inspection, reliable connection, good dynamic load resistance, and low material requirements for the main steel. However, the structure of rivet connection is complex, the hole making and riveting are steel-intensive, and the nail hole weakens the main material section, so in addition to some heavy and directly bearing the dynamic load structure still has applications, the general steel structure has rarely been used.

Bolted connection

Bolted connections are divided into two types: ordinary bolted connections and high-strength bolted connections

1. Ordinary bolt connection

Ordinary bolts are divided into three levels: A, B and C. Class A and B are refined bolts, and Class C is rough bolts. Class C bolts

The material performance grade is 4.6 or 4.8. The number before the decimal point indicates that the tensile strength of the finished bolt is not less than

400N/m㎡, the decimal point and the number after the decimal point indicate that the yield ratio is 0.6 or 0.8. Class A and B bolt materials

The performance grades are divided into 5.6 and 8.8 grades, and their tensile strength is not less than 500N/mm2 and 800N/mm2, respectively, and the yield ratio is 0.6 and 0.8, respectively.

Class A and B refined bolts are made of blanks after cutting and refining on lathes. The surface is smooth, the size is accurate, the screw diameter is the same as the bolt hole diameter, but the screw diameter is only allowed to have negative tolerances, and the bolt hole diameter is only allowed to be positive tolerances, which has high requirements for the quality of the hole. Due to the high precision, the shear performance is good. However, the production and installation are complex, the price is high, and it has rarely been used in steel structures.

Class C bolts are pressed from unmachined round bars. Due to the rough surface of the bolt, holes with the design diameter (Class II holes) are generally punched on a single part or drilled without drilling. The diameter of the bolt hole is 1.5~3mm larger than the diameter of the bolt rod (see Table 3.1 for details). For the connection using C-class bolts, due to the large gap between the bolt rod and the bolt hole, when subjected to shear force, a large shear slip will be generated, and the deformation of the connection will be large. However, it is easy to install and can effectively inherit the tensile force, so it can generally be used in the connection of axial tension along the bolt rod, as well as the shear connection of the secondary structure or temporary fixing during installation.

(2) High-strength bolted connection

High-strength bolts are generally made of 45 steel 40B steel and 20MnTiB steel, after heat treatment, the tensile strength of the bolt should not be less than 800N/mm2 and 1000N/mm2, respectively, and the yield ratio is 0.8 and 0.9, that is, the performance level of the former is 8.8, and the performance level of the latter is 10.9.

High-strength bolts can be divided into two types: large hexagonal head type [fig. 32(a)] and twist-shear type [fig. 32(d] according to their shape. When installed, the nut is tightened with a large torque by means of a special wrench, so that the screw generates a large pre-tension force and clamps the connected parts. When the external force is transmitted by the friction force of the contact surface of the part, it is called a high-strength bolt friction type connection. When the contact surface is allowed to slip like ordinary bolts, relying on bolt shear and hole wall pressure to transmit external force, it is called high-strength bolt pressure-bearing connection.

The hole diameter of friction-type connection high-strength bolts is 1.5~2.0mm larger than the nominal diameter d of the

bolt, and the hole diameter of pressure-bearing type connecting high-strength bolts is 1.0~1.5mm larger than that of the bolt nominal diameter d. The advantages of friction connection are convenient construction, less weakening of components, removable and replaceable, small shear deformation of bolts, can withstand dynamic load, fatigue resistance, good toughness and plasticity, including the respective advantages of ordinary bolts and rivet connections, and has become an excellent connection form instead of rivet connections, especially suitable for structures bearing dynamic loads. The bearing capacity of the pressure-bearing connection is higher than that of the friction connection, but the integrity and stiffness are poor, the shear deformation is large, and the strength reserve is relatively low, so it cannot be used in the structure bearing the dynamic load.

The above is the most commonly used connection method in steel structure engineering, each method has its own characteristics, suitable for different forms of steel structure buildings. The choice of specific solutions also needs to be judged according to the actual situation.