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Welding is a process in which, with the help of energy, two separate objects are brought together by forming bonds between their atoms (molecules). The welding method can be used not only to join metal materials such as steel, aluminum, copper, titanium, etc., but also to join non-metals such as plastics and ceramics; it can even handle the joining of metals and non-metals together. All these applications are collectively referred to as engineering welding. Structures manufactured by welding are called welded structures, also known as engineering welded structures. Based on the object and application, they can be roughly divided into four categories: welding structures for buildings, welding structures for storage tanks and containers, welding structures for pipelines, and conductive welding structures. The building steel structures we refer to include all these four types of welding structures. The structural materials used are steel, with ordinary carbon steel and low-alloy structural steel being the most common types. The main welding methods include shielded metal arc welding, gas shielded welding, self-shielded arc welding, submerged arc welding, stud welding, spot welding, etc. (1) Manual arc welding: The method of welding that relies on the heat of an arc is called arc welding. Manual arc welding is a type of arc welding in which a weld rod is used manually to carry out the welding process; it is the most commonly used method for welding steel structures in construction. The principle of manual arc welding is shown in Figure 7-1: the electrode and the workpiece serve as the two electrodes through which an arc is generated; this arc produces a large amount of heat, which melts the electrode and the workpiece. The end of the welding rod melts to form droplets, which then transfer to the molten base metal of the workpiece and merge there, forming a pool where a series of complex physical and metallurgical reactions take place. As the arc moves, the liquid molten pool gradually cools and crystallizes to form a weld seam. Under high temperatures, the flux applied to the steel core of the welding electrode melts into slag, which covers the surface of the molten metal. This slag not only protects the molten metal from reacting chemically with harmful elements such as oxygen and nitrogen in the air, but it also participates in the chemical reactions taking place in the molten pool and can infiltrate the alloy, thereby forming a protective slag layer on the surface of the cooling and solidifying metal. (2) Gas shielded arc welding, also known as metal inert gas arc welding, uses the welding wire and the workpiece as the two electrodes. An arc is generated between these electrodes, producing heat that melts the welding wire and the base material of the workpiece. At the same time, a shielding gas is introduced into the welding area to isolate the arc, the melted welding wire, the molten pool, and the surrounding base material from the air. The welding wire is fed automatically and continues to melt under the action of the arc, merging with the melted base material to form the weld metal. This welding method is abbreviated as GMAW (Gas Metal Arc Welding). Depending on the shielding gas used, it can be further divided into CO2 gas shielded arc welding. It is the most widely used welding method today; its characteristics include the use of high currents and thin welding wires, which results in fast welding speeds, deep penetration, and high efficiency. (3) Self-shielded arc welding: Self-shielded arc welding was once known as gas-free arc welding. It has better wind resistance compared to gas shielded arc welding; even at a wind speed of 10 m/s, it is still possible to obtain welds without pores and with excellent mechanical properties. Due to automatic welding, the welding efficiency is extremely high. The welding gun is lightweight and does not require a gas cylinder, making it very convenient to use; however, the cost of the welding wire is higher than that in CO2 shielded welding. (4) Submerged arc welding: Submerged arc welding is a type of arc welding in which the arc burns under a cover of fusible granular flux. The bare welding wire that is continuously fed into the molten pool serves both as a metal electrode and as a filler material; the arc burns beneath the flux layer, melting the welding wire and the base material to form the molten pool. The molten flux turns into slag, which covers the surface of the liquid metal pool and separates the hot metal in the pool from air. In addition to providing protection, the flux forms slag, which also participates in metallurgical reactions with the molten metal, thereby affecting the chemical composition of the weld metal. (5) Narrow-gap welding: This method makes use of specialized techniques from existing gas-shielded welding processes. Its characteristic is that the cross-sectional area of the groove in the welded joint is smaller compared to that in grooves created by manual arc welding or gas-shielded welding. Narrow-gap welding can be performed in the flat, horizontal, and vertical positions; horizontal welding is suitable for column joints on construction sites, while flat and vertical welding are appropriate for corner joints of box columns in factories, as well as for welding columns to beams. (6) Stud welding: Stud welding is a method in which a welding current is passed between the stud and the base material, thereby heating and joining the areas in contact with each other. It is primarily used for welding shear connectors and concrete anchor bolts, and it is also widely employed in connectors for installing insulation and soundproofing materials. (7) Spot welding: Here, spot welding refers to a type of resistance welding that is different from the spot welding used in the assembly of building steel structures. It involves applying electricity directly to the welding area; the resistance generated heats up that area, thereby raising its temperature, and then the parts are joined under pressure. Spot welding is commonly used in the automotive industry and household appliances, and it is also employed for complex joints in steel structures.