Basic Knowledge Of Steel Structure 2

Welding method

The welding method commonly used for steel structure is arc welding, including manual arc welding, automatic or semi-automatic arc welding and gas shielded welding.

Manual arc welding is the most common welding method in steel structure, its equipment is simple, flexible and convenient operation. But the working conditions are poor, the production efficiency is lower than automatic or semi-automatic welding, and the variability of weld quality is large, which depends on the technical level of welders to a certain extent.

The weld quality of automatic welding is stable, with fewer internal defects, good plasticity and good impact toughness. It is suitable for welding long direct welds. Semi-automatic welding for manual operation, suitable for welding curve or arbitrary shape of weld. Automatic and semi-automatic welding should use welding wire and flux suitable for the main metal, welding wire should meet the provisions of national standards, flux should be determined according to the requirements of welding process.

Gas shielded welding uses inert gas (or CO2) gas as a protective medium for the arc to isolate the molten metal from the air to keep the welding process stable. Gas shielded welding arc heating concentration, welding speed, welding depth is large, so the weld strength is higher than that of manual welding. And good plasticity and corrosion resistance, suitable for thick steel plate welding.

Weld form

Weld joints can be divided into butt joints, lap joints, T-joints and corner joints according to the mutual position of the connected members. The welds used for these joints have two basic forms, butt welds and fillet welds. In the specific application, it should be selected according to the force of the connection, combined with the manufacturing, installation and welding conditions.

Welding bead structure

1.Butt weld

The force transfer of butt weld is direct, smooth, and there is no significant stress concentration phenomenon, so the mechanical performance is good, which is applicable to the connection of members bearing static and dynamic loads. However, due to the high quality requirements of butt weld, the gap between welds is strictly required, which is generally used in factory-made connections.

2.Fillet weld

The form of fillet weld: fillet weld is different by its length direction and external force action direction, can be divided into parallel to the side fillet weld of force action direction, perpendicular to the front fillet weld of force action direction and oblique fillet weld of force action direction and girth weld.

Fillet weld section form is divided into ordinary, flat slope and deep melting type. The ratio of welding toe edge of ordinary section is 1:1, which is similar to isosceles right triangle. The bending of force transmission line is more severe, so the stress concentration is serious. For the structure directly under dynamic load, in order to make the force transmission smooth, the front fillet weld should adopt the flat slope type with the size ratio of two welding corners of 1:1.5 (along the direction of internal force along the long side), and the side fillet weld should adopt the ratio of 1:1 deep melting type.

bolted connection

Common bolted joint construction

1.Common bolt form and specification

The common form used for steel structures is the large hexagonal head type, which is denoted by the letter M and nominal and diameter (mm). M18, M20, M22, M24 are commonly used in engineering. Press international standard, the bolt is unified with the performance grade of bolt to express, be like "4.6 class", "8.8 class" etc. The number before the decimal point indicates the minimum tensile strength of the bolt material. For example, 4 indicates 400N/mm2 and 8 indicates 800N/mm2. The numbers after the decimal point (0.6, 0.8) indicate the yield ratio of the bolt material, that is, the ratio of the yield point to the minimum tensile strength.

According to the processing accuracy of bolts, ordinary bolts are divided into A, B, C three

Grade A and B bolts (refined bolts) are made of grade 8.8 steel, turned by machine tools, with smooth surface and accurate size, and equipped with class ⅰ holes (that is, the bolt holes are drilled or expanded on the assembled components, and the hole wall is smooth and accurate). Due to its high machining accuracy, close contact with the hole wall, its connection deformation is small, good mechanical performance, can be used to bear large shear and tensile force of the connection. However, manufacturing and installation are more laborious, high cost, so it is seldom used in steel structure.

Class C bolts (rough bolts) are made of 4.6 or 4.8 steel, rough processing, size is not accurate, only requires class II holes (that is, bolt holes are punched in a single part or drilled without a drill die. The general aperture is 1~2mm larger than the bolt diameter). In the transmission of shear force, the connection deformation is large, but the performance of the transmission force is good, the operation without special equipment, low cost. It is commonly used for bolt connections bearing tension and secondary shear connections in structures bearing static loads or indirect dynamic loads.

2.Arrangement of common bolt connections

The arrangement of bolts should be simple, unified and compact, meet the stress requirements, reasonable structure and easy to install. There are two kinds of arrangement: juxtaposition and dislocation. Juxtaposition is simple and dislocation is compact.

Stress characteristics of common bolt connection

1.Sheared bolt connection

2.Tension bolt connection

3.Tensile shear bolt connection

Stress characteristics of high strength bolt

High strength bolt connection can be divided into friction type and pressure type according to design and stress requirements. When the friction connection is subjected to shear, the maximum frictional resistance of external shear force between plates is the limit state. When over the relative slip occurs between the plates, that is, the connection is considered to have failed and destroyed. When the bearing connection is shear, friction is allowed to be overcome and relative slip between plates occurs, and then the external force can continue to increase, with the ultimate failure of screw shear or bore wall bearing occurring later as the limit state.