Eight Basic Knowledge of Steel Structures

1、 Characteristics of steel structure:

1. The steel structure has a relatively light self weight

2. High reliability of steel structure work

3. The steel has good vibration (shock) resistance and impact resistance

4. The industrialization level of steel structure manufacturing is relatively high

5. Steel structures can be assembled accurately and quickly

6. Easy to make into a sealed structure

7. Steel structures are prone to corrosion

8. Poor fire resistance of steel structures

2、 The grades and functions of commonly used steel structures

1. Carbon structural steel: Q195, Q215, Q235, Q255, Q275, etc

2. Low alloy high-strength structural steel

3. High quality carbon structural steel and alloy structural steel

4. Specialized steel

3、 Material selection criteria for steel structures

The material selection criteria for steel structures are to ensure the load-bearing capacity of the load-bearing structure and prevent brittle failure under certain conditions, taking into account factors such as the importance of the structure, load characteristics, structural methods, stress conditions, connection methods, steel thickness, and working environment.

The four types of steel models proposed in the "Code for Design of Steel Structures" GB50017-2003 are "suitable" for use, and are selected first when conditions permit. The use of other models is not prohibited, as long as the steel used meets the requirements of the specification.

4、 Main technical content of steel structure:

(1) High rise steel structure technology. According to the construction height and design requirements, frames, frame supports, tubes, and mega frame structures can be selected, and their components can be steel, reinforced concrete, or steel tube concrete. The steel components are lightweight and have good ductility, and can be welded or rolled sections, suitable for construction of ultra-high floors; Stiffened reinforced concrete components have high stiffness and good fire resistance, making them suitable for mid to high rise construction or bottom structures; Steel tube concrete construction is simple and only used for column structures.

(2) Space steel structure technology. The spatial steel structure has light weight, high stiffness, beautiful appearance, and fast construction speed. The spherical node flat grid, multi-layer variable cross-section grid, and grid shell with steel pipes as members are the most commonly used structural types of spatial steel structures in China. It has the advantages of high spatial stiffness and low steel consumption, and can provide comprehensive CAD in design, construction, and inspection procedures. In addition to grid structures, spatial structures also include large-span suspension structures, cable membrane structures, etc.

(3) Light steel structure technology. A new structural method consisting of walls and roof enclosure structures made of lightweight colored steel plates. A lightweight steel structural system consisting of large cross-section thin-walled H-shaped steel wall beams and roof purlins welded or rolled with steel plates of 5mm or more, flexible support systems made of round steel, and high-strength bolts. The column spacing can range from 6m to 9m, the span can reach 30m or more, and the height can reach more than ten meters. It can also be equipped with a lightweight crane. The amount of steel used is 20-30kg/m2. Now there are standardized Engineering design process and specialized manufacturers, with good product quality, fast installation speed, light weight, low investment, and construction free from seasonal constraints, which are suitable for various light industrial plants.

(4) Steel concrete composite structure technology. The load-bearing structure of beams and columns composed of shaped steel or steel processing and concrete components is a steel-concrete composite structure, and its application scope has been expanding in recent years. The composite structure combines the strengths of both steel and concrete, with high overall strength, good rigidity, and outstanding seismic performance. When using an external concrete structure, it has outstanding fire resistance and corrosion resistance. Composite structural components can generally reduce the amount of steel used by 15-20%. Composite floors and steel tube concrete components also have the advantages of less or no formwork support, convenient and fast construction, and have great potential for promotion. Suitable for frame beams, columns, and floors built with large loads on multiple or high-rise buildings, as well as industrial construction columns and floors.

(5) High strength bolt connection and welding technology. High strength bolts transmit stress through friction and are composed of three parts: bolts, nuts, and washers. High strength bolts have the advantages of simple construction, flexible removal, high bearing capacity, good fatigue resistance and self-locking, and high safety. They have replaced riveting and partial welding in engineering and become the main connecting means in steel structure production and installation. For steel members manufactured in the workshop, thick plates shall be subject to active multi wire arc submerged welding, and box column diaphragms shall be subject to Electroslag welding welding and other technologies. During on-site device construction, semi-active welding technology, gas maintenance flux cored welding wire, and self maintenance flux cored welding wire technology should be selected.

(6) Steel structure protection technology. Steel structure protection includes fire prevention, corrosion prevention, and rust prevention. Generally, it is not necessary to undergo rust prevention treatment after being treated with fire-resistant coatings, but it is still necessary to undergo corrosion prevention treatment in construction with corrosive gases. There are many types of fireproof coatings in China, such as TN series, MC-10, etc. Among them, MC-10 fireproof coatings include alkyd enamel, chlorinated rubber paint, fluororubber paint, and chlorosulfonated paint. During construction, appropriate coatings and coating thicknesses should be selected based on the steel structure type, fire resistance rating requirements, and environmental requirements.

5、 Objectives and Measures for Steel Structures:

The steel Structural engineering has a wide range of contacts and great technical difficulties. It is necessary to follow the national and industrial standards and specifications in the promotion and application. The local construction administrative department should pay attention to the construction in the professional stage of the steel Structural engineering, organize the training of the quality inspection team, and timely summarize the work practice and new technology application. Colleges and universities, design departments and construction enterprises should speed up the training of technical personnel for steel Structural engineering and promote steel structure CAD with mature technology. Mass academic organizations should cooperate with the development of steel structure technology, widely carry out academic exchanges and training activities at home and abroad, actively improve the overall level of steel structure design, production, and construction equipment technology, and can have rewards in the near future.

6、 Connection methods for steel structures

There are three types of connection methods for steel structures: weld connection, bolt connection, and rivet connection.

（1） Welding seam connection

The welding seam connection is achieved by melting the welding rod and the weldment partially through the heat generated by the arc, and then cooling and condensing them to form a welding seam, thereby connecting the weldment into a whole.

Advantages: It does not weaken the cross-sectional area of components, saves steel, has a simple structure, is easy to manufacture, has a high connection stiffness, has good sealing function, and is easy to choose active operation under certain conditions, with high output power.

Disadvantages: The heat affected zone formed by the high temperature effect of welding in the steel near the weld seam may cause some parts of the material to become brittle; During the welding process, the steel is subjected to uneven distribution of high-temperature cooling, resulting in residual welding stress and deformation in the structure, which has a certain impact on the bearing capacity, stiffness, and functional use of the structure; Due to the high stiffness of welded structures, some cracks can easily propagate to the entire structure, especially at low temperatures where brittle fracture occurs; The plasticity and durability of weld joints are poor, and defects may occur during welding, leading to a decrease in fatigue strength.

（2） Bolt connection

Bolted connection is the process of connecting the connecting parts into a whole through bolts, which are fasteners. There are two types of bolt connection: general bolt connection and high-strength bolt connection.

Advantages: The construction process is simple and the installation is convenient, especially suitable for the connection of on-site devices and easy disassembly. It is suitable for structures that need to be installed and dismantled and temporary connections.

Disadvantages: Holes need to be drilled and aligned during assembly on the board, resulting in increased manufacturing workload and high requirements for manufacturing accuracy; Bolt holes also weaken the cross-section of the component, and the connected parts often need to overlap with each other or add auxiliary connecting plates (or angle steel), resulting in a more complex structure and steel consumption.

（3） Rivet connection

Rivet connection refers to the process of quickly inserting a semi circular prefabricated rivet head at one end into the hole of the connecting piece after the rivet rod is burned red, and then using a rivet gun to rivet the other end into a rivet head to achieve a secure connection.

Advantages: Riveting has reliable force transmission, good plasticity and durability, and is easy to inspect and ensure quality. It can be used for heavy-duty and structures that directly accept dynamic loads.

Disadvantages: The riveting process is complex, manufacturing requires labor and materials, and labor intensity is high, so it has been basically replaced by welding and high-strength bolt connection.

7、 Welding connection

（1） Welding method

The common welding method of steel structure is Arc welding, including craft Arc welding, active or semi-active Arc welding and gas maintenance welding.

Craft Arc welding is the most commonly used welding method in steel structures. Its equipment is simple, and its operation is flexible and convenient. But the labor conditions are poor, the output power is lower than that of active or semi-active welding, and the variability of weld quality is large, which to some extent depends on the technical level of the welder.

Active welding has stable weld quality, fewer internal defects, good plasticity, and good impact resistance, making it suitable for welding longer direct welds. Semi active welding is suitable for welding curves or welds of any shape due to manual operation. Active and semi-active welding should use welding wires and fluxes that are suitable for the main metal. The welding wires should comply with the rules of national standards, and the flux should be determined according to the welding process requirements.

Gas maintenance welding is the use of inert gas (or CO2) gas as the maintenance medium for the arc, isolating the molten metal from the air to ensure stable welding process. Gas maintenance welding has centralized arc heating, fast welding speed, and large melting depth, so the weld strength is higher than that of manual welding. And it has good plasticity and corrosion resistance, suitable for welding thick steel plates.

（2） Welding seam method

The welding seam connection methods can be divided into four methods based on the mutual positions of the connected components: butt joint, lap joint, T-joint, and corner joint. There are two basic methods of welding seams used for these connections: butt welds and fillet welds. In detailed application, selection should be based on the stress conditions of the connection, combined with manufacturing, installation, and welding conditions.

（3） Weld structure

1. Butt weld

The force transmission of butt welds is direct, smooth, and without significant stress concentration, resulting in outstanding stress bearing function, suitable for the connection of components under static and dynamic loads. However, due to the high quality requirements of butt welds and the strict requirements for welding gaps between weldments, they are generally used in the connection of factory manufacturing.

2. Fillet weld

Method of fillet welding: According to the difference in length direction and external force effect direction, fillet welding can be divided into side fillet welds parallel to the force effect direction, front fillet welds perpendicular to the force effect direction and oblique fillet welds intersecting the force effect direction, and surrounding welds.

The section method of fillet welds can be divided into general type, flat slope type, and deep penetration type. The hf in the figure is referred to as the weld leg size of the fillet weld. The ratio of weld leg edge of general section is 1:1, which is similar to isosceles Right triangle. The bending of force transmission line is severe, so the stress concentration is serious. For structures that directly receive dynamic loads, in order to ensure smooth force transmission, it is recommended to use a flat slope type with a ratio of 1:1.5 for the front fillet weld (the long side follows the direction of internal force), and a deep penetration type with a ratio of 1:1 for the side fillet weld.

8、 Bolt connection

（1） Structure of general bolt connection

1. General bolt methods and specifications

The general method for selecting steel structures is the large hexagonal head type, which is indicated by the letter M and the nominal and diameter (mm). M18, M20, M22, and M24 are commonly used in engineering. According to international standards, bolts are uniformly indicated by their functional grade, such as "Grade 4.6", "Grade 8.8", etc. The number before the Decimal separator indicates the minimum tensile strength of the bolt material. For example, "4" indicates 400N/mm2, and "8" indicates 800N/mm2. The figures after the Decimal separator (0.6, 0.8) indicate the yield ratio of the bolt material, that is, the ratio of the yield point to the lowest tensile strength.

According to the machining accuracy of bolts, they are generally divided into three levels: A, B, and C.

A. B-grade bolts (refined bolts) are made of 8.8 grade steel and machined by machine tools. The surface is smooth and the dimensions are accurate, and they are equipped with Class I holes (i.e. bolt holes are drilled or expanded on assembled components, with smooth hole walls and precise hole alignment). Due to its high machining accuracy, tight contact with the hole wall, small joint deformation, and good stress function, it can be used for joints that accept large shear and tensile forces. However, manufacturing and installation are more labor-intensive and costly, so they are less commonly used in steel structures.

Grade C bolts (rough bolts) are made of grade 4.6 or 4.8 steel, with rough machining and imprecise dimensions. Only Class II holes are required (i.e. bolt holes are punched on a single part in one go or drilled without a drilling jig. Generally, the hole diameter is 1-2mm larger than the bolt rod diameter). When transmitting shear force, the connection deformation is large, but the function of transmitting tensile force is still good, and the operation does not require special equipment, resulting in low cost. Commonly used for bolt connections that accept tension and secondary shear connections in structures that accept static or indirect dynamic loads.

2. Arrangement of general bolt connections

The arrangement of bolts should be simple, uniform, and compact, meeting the stress requirements, with a reasonable structure and easy installation. There are two types of arrangement methods: parallel and staggered. The juxtaposition is relatively simple, and the staggered columns are more compact.

（2） The stress characteristics of general bolt connections

1. Shear bolt connection

2. Tension bolt connection

3. Connection of tension and shear bolts

（3） The stress characteristics of high-strength bolts

The high-strength bolt connection can be divided into two types according to design and stress requirements: friction type and pressure type.

The maximum frictional resistance that may occur between the plates when the external shear force reaches the limit state when the friction type connection is subjected to shear; When relative slip occurs between the plates when it exceeds, it is considered that the connection has failed and is damaged.

When the pressure bearing joint is subjected to shear, it allows the frictional force to be overcome and generates relative slip between the plates. Then, external forces can continue to be added, and the ultimate failure of the screw shear or hole wall pressure generated after this is the limit state.