1、 Characteristics of Steel Structures
1. The steel structure has a relatively light self weight
2. High reliability of steel structure operation
3. The steel has good vibration (shock) resistance and impact resistance
4. The industrialization level of steel structure production is relatively high
5 steel structures can be assembled accurately and quickly
6. Simply made into a sealed structure
7. Steel structures are prone to corrosion
8. Poor fire resistance of steel structures
2、 The grades and properties of commonly used steel structures
Carbon structural steel: Q195, Q215, Q235, etc
2 Low alloy high-strength structural steel
3. High quality carbon structural steel and alloy structural steel
4 Special Purpose Steel
3、 Principles for material selection of steel structures
The principle of material selection for steel structures is to ensure the load-bearing capacity of the load-bearing structure and avoid brittle failure under certain conditions, taking into account factors such as the importance of the structure, load characteristics, structural mode, stress state, connection method, steel thickness, and operating environment.
The four types of steel models proposed in the "Code for Planning of Steel Structures" GB50017-2003 are "suitable" for use, and are the first choice when conditions permit. The use of other models is not prohibited, as long as the steel used meets the requirements of the code.
4、 Main Steel Structure Skills Content
1. High rise steel structure skills
According to the building height and planning requirements, the structure, structural support, tube and Megastructure can be selected respectively, and its components can be steel, reinforced concrete or concrete filled steel tubes. Steel components are lightweight and have good ductility, and can be welded or rolled sections, suitable for ultra high rise buildings; Stiffened reinforced concrete components have high stiffness and good fire resistance, making them suitable for mid to high-rise buildings or bottom structures; Steel tube concrete construction is simple and only used for column structures.
2. Space steel structure skills
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 be used in planning, construction, and inspection procedures, and can provide complete CAD. In addition to grid structures, spatial structures also include large-span suspension structures, cable membrane structures, etc.
3. Skills in light steel structure
Accompanied by a new structural approach consisting of lightweight colored steel plates for wall and roof enclosure structures. 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. There are now standardized planning procedures and specialized production enterprises, with good product quality, fast installation speed, light weight, low investment, and construction not subject to seasonal constraints, suitable for various light industrial factories.
4. Skills in steel concrete composite structures
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 advantages of both steel and concrete, with high overall strength, good rigidity, and good seismic performance. When using an external concrete structure, it has better 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 structural beams, columns, and floors of multi-story or high-rise buildings with large loads, as well as industrial building columns and floors.
5. High strength bolt connection and welding skills
High strength bolts transmit stress through friction and are composed of three parts: bolts, nuts, and washers. The high-strength bolt connection has the advantages of simple construction, sensitive removal, high bearing capacity, good fatigue resistance and self-locking performance, and high safety. It has replaced riveting and partial welding in engineering and has become the main connection method in steel structure production and installation. Automatic multi wire arc submerged welding shall be used for thick plates of steel members manufactured in the workshop, and melting tip Electroslag welding welding shall be used for box column diaphragms. In the construction of on-site devices, semi-automatic welding skills, gas shielded flux cored welding wire, and self-protection flux cored welding wire skills should be selected.
6. Steel structure protection skills
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 fireproof coatings, but in buildings with corrosive gases, anti-corrosion treatment is required. 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、 Policy and Measures for Steel Structures
The steel Structural engineering involves a wide range of areas and is difficult in skills. It must follow the national and industrial standards and specifications in its promotion and application. Local construction administrative departments should attach importance to the construction of the professional stage of steel Structural engineering, organize the training work of quality inspection teams, and summarize the operation practice and new skill application in time. Colleges and universities, planning departments and construction enterprises should speed up the cultivation of skilled personnel in steel Structural engineering and promote steel structure CAD with mature skills. Mass academic organizations should cooperate with the development of steel structure skills, conduct extensive academic exchanges and training activities at home and abroad, and actively improve the overall level of steel structure planning, manufacturing, and construction equipment skills, in order to achieve rewarding progress in the near future.
6、 Connection method of steel structure
There are three types of connection methods for steel structures: weld connection, bolt connection, and rivet connection.
1. Weld joint connection
The welding seam connection is achieved by the heat generated by the arc, which partially melts the welding rod and the weldment, and then cools and condenses into the welding seam, thereby connecting the weldment into one body.
Advantages:
It does not weaken the cross-section of components, saves steel, has a simple structure, convenient production, high connection stiffness, good sealing performance, and is easy to use for automated operations under certain conditions, with high production efficiency.
Disadvantages:
The heat affected zone formed by the high temperature effect of welding on the steel adjacent to the weld seam may result in some parts of the material becoming 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 operational performance of the structure; Due to the high stiffness of welded structures, local cracks can easily propagate to the entire structure, especially at low temperatures where brittle fracture occurs; The plasticity and toughness of the weld joint are poor, and defects may occur during welding, resulting in 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 to disassemble. It is suitable for structures that need to be installed and dismantled and temporary connections.
Disadvantages:
It is necessary to drill holes on the board and align them during assembly, increasing the production workload and requiring high precision in production; 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 steels), resulting in more complex construction 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 toughness, and easy quality inspection and assurance. It can be used for heavy-duty and directly loaded structures.
Disadvantages:
The riveting process is complex, requires labor and materials, and has high labor intensity, 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 Shielded metal arc welding, automatic or semi-automatic Arc welding and gas shielded welding.
Shielded metal arc welding is the most commonly used welding method in steel structure. Its equipment is simple, and its operation is sensitive and convenient. But the labor conditions are poor, the production efficiency is lower than that of automatic or semi-automatic welding, and the variability of weld quality is large, which to some extent depends on the skill level of the welder.
The welding seam quality of automatic welding is stable, with fewer internal defects, good plasticity, and good impact toughness, making it suitable for welding longer direct welds. Semi automatic welding is suitable for welding curves or welds of any shape due to manual operation. Automatic and semi-automatic welding should use welding wires and fluxes that are suitable for the main metal. The welding wires should comply with national standards, and the flux should be determined according to the welding process requirements.
Gas shielded welding uses inert gas (or CO2) gas as the protective medium for the arc, isolating the molten metal from air and maintaining the stability of the welding process. Gas shielded welding has a convergence of 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 method
The welding seam connection methods can be divided into four types based on the mutual positions of the connected components: butt joint, lap joint, T-joint, and corner joint. There are two basic types of welds used for these connections: butt welds and fillet welds. In specific applications, the selection should be based on the stress situation of the connection, combined with the production, installation, and welding conditions.
3. Weld structure
Butt welds:
The force transmission of the butt weld is direct, smooth, and there is no significant stress concentration phenomenon, so the stress performance is good, and it is 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 production.
Fillet welds:
The method of fillet welding: According to its length direction and the direction of external force action, fillet welding can be divided into side fillet welding seams parallel to the direction of force action, front fillet welding seams perpendicular to the direction of force action, oblique fillet welding seams that intersect with the direction of force action, and circumferential fillet welding seams.
The section method of fillet welds can be divided into general type, flat slope type, and deep penetration type. 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. General bolt methods and standards
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 performance level, such as "4.6 level", "8.8 level", 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):
Made of grade 8.8 steel and processed by machine tool turning, it has a smooth surface and precise dimensions, and is equipped with Class I holes (i.e. bolt holes are drilled or expanded on assembled components, with smooth hole walls and accurate hole alignment). Due to its high machining accuracy, close contact with the hole wall, small joint deformation, and good stress performance, it can be used for joints that accept large shear and tensile forces. However, production and installation are labor-intensive and costly, so they are rarely used in steel structures.
C-grade bolts (rough bolts):
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 diameter). When transmitting shear force, the connection deformation is large, but the performance 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. Placement of general bolt connections
The placement of bolts should be simple, unified, and compact, meeting the stress requirements, with a reasonable structure and easy installation. There are two placement methods: parallel and staggered. The juxtaposition is relatively simple, and the staggered columns are more compact.
The stress characteristics of general bolt connections:
Shear bolt connection
Tension bolt connection
Connection of tension and shear bolts
The stress characteristics of high-strength bolts:
The high-strength bolt connection can be divided into two types according to planning 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 the limit, it is considered that the connection has failed and is damaged. When the pressure bearing joint is subjected to shear, it allows the friction force to be overcome and relative slip between the plates occurs. Then, the external force can continue to increase, and the ultimate failure of the screw shear or hole wall pressure that occurs after this is the limit state.
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