8 fundamental basics of steel structures – essential for beginners!

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8 fundamental basics of steel structures – essential for beginners! I. Characteristics of steel structures
1. Steel structures have a low self-weight.
2. They exhibit high reliability in operation.
3. Steel possesses good vibration and shock resistance.
4. The manufacturing process for steel structures is highly industrialized.
5. Steel structures can be assembled accurately and quickly.
6. It is easy to create sealed structures using them.
7. Steel structures are prone to corrosion.
8. Their fire resistance is poor.

II. Grades and properties of steels commonly used in steel structures
1. Carbon structural steels: Q195, Q215, Q235, Q255, Q275, etc.
2. Low-alloy high-strength structural steels.
3. High-quality carbon structural steels and alloy structural steels.
4. Steels for specialized purposes.

III. Principles for selecting materials for steel structures
The principles for selecting materials for steel structures aim to ensure the load-bearing capacity of the structure and prevent brittle failure under certain conditions. These principles take into account factors such as the importance of the structure, load characteristics, structural design, stress conditions, connection methods, thickness of the steel, and the working environment. The four steel grades specified in the \"Code for Design of Steel Structures\" GB50017-2003 are those that are recommended for use; they should be the first choice when conditions permit. The use of other grades is not prohibited, as long as the steel used meets the requirements of the code. IV. Main technical aspects of steel structures (1) High-rise steel structure technology. Depending on the building height and design requirements, frame, frame-supported, tube, and mega-frame structures are used, with their components being made of steel, reinforced concrete, or concrete filled with steel tubes. Steel components are lightweight and have good ductility; welded or rolled steel sections can be used, making them suitable for super-high-rise buildings ； Reinforced concrete members with high strength exhibit good fire resistance, making them suitable for mid-to-high-rise buildings or their base structures ； Concrete-filled steel tubes are easy to construct and are used only in column structures. (2) Space steel structure technology. Space steel structures have a low self-weight, high stiffness, attractive appearance, and fast construction speed. Spherical node plate grids, multi-story variable cross-section grids, and reticulated shells using steel pipes as members are the structural types with the highest usage of spatial steel structures in China. It features high spatial stiffness and low steel consumption, along with established design, construction, and inspection procedures, as well as comprehensive CAD support. In addition to grid structures, spatial structures also include long-span cable-stayed structures, cable-membrane structures, etc. (3) Light steel structure technology. This is accompanied by a new structural format consisting of walls and roof enclosures made from lightweight colored steel plates. Light steel structure systems consisting of large-section thin-walled H-shaped steel wall beams and roof purlins welded or rolled from steel plates over 5mm thick, flexible support systems made of round steel, and high-strength bolt connections; the column spacing can range from 6m to 9m, the span can reach 30m or more, the height can be in the tens of meters, and light hoists can also be installed. Steel consumption is 20–30 kg/m2. There are now standardized design procedures and specialized manufacturing enterprises; the product quality is good, installation is fast, the weight is low, the investment required is minimal, construction is not restricted by seasons, and it is suitable for various light industrial buildings. (4) Steel-concrete composite structure technology. Beam and column load-bearing structures composed of steel sections or steel members together with concrete elements are known as steel-concrete composite structures, and their application scope has been expanding in recent years. Composite structures combine the advantages of both steel and concrete, featuring high overall strength, good stiffness, and excellent seismic performance. When a concrete overlay is used, they also exhibit improved fire resistance and corrosion resistance. Composite structural members can generally reduce steel usage by 15–20%. Composite floor systems and concrete-filled steel tube members also offer the advantages of reduced or no formwork requirements, as well as easy and fast construction, making them highly promising for wider adoption. Suitable for frame beams, columns, and floor slabs in multi-story or high-rise buildings subjected to large loads, as well as for columns and floor slabs in industrial buildings. (5) High-strength bolt connection and welding techniques. High-strength bolts transmit stress through friction, and consist of three parts: the bolt, the nut, and the washer. High-strength bolt connections offer numerous advantages, such as ease of construction, flexibility in disassembly, high load-bearing capacity, good fatigue resistance and self-locking properties, as well as high safety. In engineering applications, they have replaced riveting and partial welding, becoming the primary joining method in the fabrication and installation of steel structures. For steel components manufactured in the workshop, automatic multi-wire arc submerged welding should be used for thick plates, while techniques such as electrode slag welding should be employed for box column diaphragms. During on-site installation and construction, semi-automatic welding techniques, as well as gas-shielded cored wire and self-shielded cored wire technologies, should be employed. (6) Protection technologies for steel structures. The protection of steel structures includes fire resistance, corrosion resistance, and rust prevention. Generally, no further rust prevention treatment is needed after applying fire-resistant coatings; however, corrosion resistance treatment is still required in buildings exposed to corrosive gases. There are many types of fire-resistant coatings available in China, such as the TN series and MC-10, among which MC-10 fire-resistant 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 type of steel structure, fire resistance requirements, and environmental conditions. V. Objectives and measures for steel structures: Steel structure projects involve a wide range of aspects and present significant technical challenges; therefore, it is necessary to adhere to relevant standards and industry regulations when implementing them. Construction administrative departments in various regions should pay attention to the professional phase of steel structure projects, organize proper training for quality inspection teams, and promptly summarize work practices and the application of new technologies. Colleges and universities, design departments, and construction companies should accelerate the training of technical personnel for steel structure projects and promote the technically mature CAD for steel structures. Academic organizations should keep pace with the development of steel structure technology by carrying out extensive academic exchanges and training activities both domestically and internationally, and strive to improve the overall level of design, fabrication, and construction techniques for steel structures in a timely manner. VI. Connection methods for steel structures: The connection methods for steel structures include weld bonding, bolt bonding, and rivet bonding. (1) Weld joint formation: Weld joint formation involves using the heat generated by an arc to locally melt the welding rod and the workpiece; upon cooling, this molten material solidifies to form a weld, thereby joining the workpieces together. Advantages: it does not reduce the cross-sectional area of the components, saves steel, has a simple structure, is easy to manufacture, offers high connection stiffness and good sealing performance; it can be easily automated under certain conditions, resulting in high production efficiency. Disadvantage: The heat-affected zone formed in the steel near the weld due to the high temperatures associated with welding may cause the material in certain areas to become brittle ； During the welding process, the steel is subjected to uneven heating and cooling, which results in welding residual stresses and deformations in the structure; these effects have a certain impact on the structure’s load-bearing capacity, stiffness, and service performance ； Due to their high stiffness, local cracks in welded structures can easily propagate throughout the entire structure; brittle fracture is particularly likely to occur at low temperatures ； Welded joints have poor plasticity and toughness, and defects may occur during welding, reducing their fatigue strength. (II) Bolt connection: A bolt connection is one in which the connected components are joined together using bolts as fasteners. Bolt connections are divided into ordinary bolt connections and high-strength bolt connections. Advantages: The construction process is simple and installation is easy; it is particularly suitable for on-site installation and connection, and it is also easy to disassemble, making it appropriate for structures that require assembly and disassembly as well as temporary connections. Disadvantages: It requires drilling holes in the panels and aligning those holes during assembly, which increases the manufacturing workload and demands high precision in production ； Bolt holes also weaken the cross-section of the components, and the connected parts often need to overlap or require additional connecting plates (or angle steel), resulting in a more complex structure and higher material consumption. (III) Rivet connection: In this connection method, a rivet with a semi-circular pre-formed head at one end is used; the shank of the rivet is heated red-hot and then quickly inserted into the hole in the component to be connected. Afterwards, a rivet gun is used to form a head on the other end as well, thereby securing the connection. Advantages: Riveting provides reliable force transmission, it has good plasticity and toughness, the quality is easy to inspect and ensure, and it can be used in heavy structures and those subjected to dynamic loads. Disadvantages: The riveting process is complex, requires significant labor and materials, and involves high physical strain; as a result, it has been largely replaced by welding and high-strength bolt connections. VII. Welding Joints (I) Welding Methods The commonly used welding methods for steel structures are arc welding, which includes manual arc welding, automatic or semi-automatic arc welding, as well as gas shielded welding, among others. Manual arc welding is the most commonly used welding method in steel structures, as it features simple equipment and flexible, convenient operation. However, the working conditions are poor, the production efficiency is lower than that of automatic or semi-automatic welding, and the variability in weld quality is high, depending to a certain extent on the skill level of the welder. Automatic welding produces welds with stable quality, few internal defects, good plasticity, and high impact toughness, making it suitable for welding longer continuous welds. Due to manual operation, semi-automatic welding is suitable for weld curves or welds of any shape. For automatic and semi-automatic welding, welding wires and fluxes that are suitable for the base metal should be used. The welding wires must comply with the requirements of relevant standards, while the flux should be selected based on the requirements of the welding process. Gas shielded welding uses an inert gas (or CO2) as a shielding medium for the arc, thereby isolating the molten metal from air and maintaining stability during the welding process. Gas shielded welding features concentrated arc heating, fast welding speed, and deep penetration; therefore, the strength of the weld is higher than that of manual welding. It also has good plasticity and corrosion resistance, making it suitable for welding thick steel plates. (II) Weld form: The weld connection forms can be classified into four types—butt joint, lap joint, T-joint, and corner joint—based on the relative positions of the components being connected. The welds used for these connections come in two basic forms: butt welds and fillet welds. In practical applications, the choice should be made based on the stress conditions of the connection, taking into account the manufacturing, installation, and welding conditions. (III) Weld structure 1. Butt welds Butt welds transmit forces in a direct and smooth manner, without significant stress concentration; as a result, they have good load-bearing properties and are suitable for joining components subjected to static or dynamic loads. However, due to the high quality requirements for the butt welds and the strict requirements regarding the welding gap between the welded parts, it is generally used in connections manufactured in factories. 2. Fillet welds: Types of fillet welds: Depending on their length direction and the direction of the applied force, fillet welds can be classified into side fillet welds that are parallel to the direction of the force, face fillet welds that are perpendicular to that direction, diagonal fillet welds that intersect the force direction at an angle, as well as circumferential welds. The cross-sectional forms of fillet welds are further divided into regular type, flat slope type, and full penetration type. In the figure, hf refers to the leg size of the fillet weld. For the standard cross-section, the ratio of the leg lengths is 1:1, resulting in a shape similar to an isosceles right triangle. The load transfer path bends sharply, leading to severe stress concentration. For structures subjected to dynamic loads, in order to ensure smooth force transmission, the fillet welds on the front face should adopt a flat-bevel design with a side length ratio of 1:1.5 (the longer side aligned in the direction of the internal forces), while the fillet welds on the side faces should use a full-penetration design with a ratio of 1:1. VIII. Bolt Connections (I) Structure of Ordinary Bolt Connections 1. Forms and specifications of ordinary bolts: The common form used in steel structures is the large hex head type, which is denoted by the letter M along with the nominal diameter in millimeters. M18, M20, M22, and M24 are commonly used in engineering. According to international standards, bolts are uniformly designated by their performance grades, such as \"Grade 4.6\" and \"Grade 8.8\", etc. The digit before the decimal point indicates the minimum tensile strength of the bolt material; for example, “4” means 400 N/mm2, and “8” means 800 N/mm2. The digits after the decimal point (0.6, 0.8) represent the yield strength ratio of the bolt material, that is, the ratio of the yield strength to the minimum tensile strength. Based on the machining precision of the bolts, ordinary bolts are further divided into three grades: A, B, and C. Grade A and B bolts (precision bolts) are made from 8.8 grade steel, manufactured through machine turning; they have a smooth surface and accurate dimensions, and are used with Class I holes (that is, the bolt holes are drilled or reamed in the assembled components, with smooth hole walls and precise alignment). Due to its high precision in processing, tight contact with the hole wall, minimal deformation during connection, and excellent load-bearing properties, it can be used for connections that are subject to high shear and tensile forces. However, manufacturing and installation are labor-intensive and costly, so it is less commonly used in steel structures. Class C bolts (roughly forged bolts) are made of 4.6 or 4.8 grade steel; they are roughly machined and their dimensions are not very accurate. They require Class II holes, meaning that the bolt holes are formed in a single operation on the part, without the use of drilling dies. Generally, the hole diameter is 1–2 mm larger than the bolt shank diameter.) When transmitting shear forces, the connection deforms significantly, but it performs well in transmitting tensile forces; no special equipment is required for operation, and the cost is low. Commonly used for bolted connections subjected to tensile forces, as well as for secondary shear connections in structures subjected to static loads or indirectly to dynamic loads. 2. Arrangement of ordinary bolt connections: The arrangement of bolts should be simple, uniform, and compact; it must meet load-bearing requirements, feature a reasonable structure, and facilitate installation. There are two arrangements: side-by-side and staggered (as shown in the figure). Side-by-side is simpler, while staggered is more compact. (II) Mechanical properties of ordinary bolt connections: 1. Bolt connections subjected to shear forces; 2. Bolt connections subjected to tensile forces; 3. Bolt connections subjected to both tensile and shear forces. (III) Mechanical properties of high-strength bolts: High-strength bolt connections can be classified into friction-type and compression-type based on design and mechanical requirements. In shear loading, frictional joints operate under the limit condition in which the external shear force equals the maximum frictional resistance that can occur between the plates ； When this limit is exceeded, relative sliding occurs between the plates, and the connection is considered to have failed and broken. In shear, a compressive-type connection allows the friction force to be overcome and relative sliding between the plates to occur; thereafter, the external force can continue to increase, with the ultimate failure mode being screw shear or compression of the hole walls.