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Eight basic knowledge of steel structure
1. Characteristics of steel structure
1. Steel structure has light weight
2. The reliability of steel structure work is high
3. Steel has good vibration (shock) resistance and impact resistance.
4. Steel structure manufacturing has a high degree of industrialization
5. Steel structures can be assembled accurately and quickly
6. Easy to make a sealed structure
7. Steel structures are prone to corrosion
8. Steel structure has poor fire resistance
2. Grades and properties 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. Special purpose steel
3. Principles of material selection for steel structures
The principle of material selection for steel structures is to ensure the load-bearing capacity of the load-bearing structure and prevent brittle failure under certain conditions, based on comprehensive considerations such as the importance of the structure, load characteristics, structural form, stress state, connection method, steel thickness, and working environment. of.
The four steel models proposed in the "Steel Structure Design Code" GB50017-2003 are the models that are "suitable" to be used. They 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 technical contents
(1) High-rise steel structure technology.
According to the building height and design requirements, frame, frame support, cylinder and giant frame structures are adopted respectively, and its components can be made of steel, stiff reinforced concrete or steel tube concrete. Steel components are lightweight and have good ductility. Welded steel or rolled steel can be used, which is suitable for ultra-high-rise buildings. Stiff reinforced concrete components have high stiffness and good fire resistance, and are suitable for mid-to-high-rise buildings or bottom structures. Concrete steel tubes are easy to construct. For column structures only.
(2) Space steel structure technology.
The space steel structure has light weight, high stiffness, beautiful appearance and fast construction speed. Ball-node flat-plate grids with steel pipes as rods, multi-layer variable-section grids and grid shells are the structural types with the largest consumption of space steel structures in my country. It has the advantages of large spatial rigidity and low steel consumption. It has complete CAD procedures for design, construction and inspection. In addition to grid structures, space structures also include long-span suspension cable structures, cable-membrane structures, etc.
(3) Light steel structure technology.
It is accompanied by a new structural form consisting of walls and roof envelopes made of lightweight colored steel panels. It is a light steel structure system composed of large-section thin-walled H-shaped steel wall beams and roof purlins welded or rolled by steel plates above 5mm, flexible support systems made of round steel and high-strength bolt connections. The column spacing can be from 6m to 9m, and the span can be up to 30m or more, the height can reach more than ten meters, and light cranes can be installed. The amount of steel used is 20~30kg/m2. There are now standardized design procedures and specialized production enterprises. The products have good quality, fast installation speed, light weight, low investment, and construction is not subject to seasonal restrictions. It is suitable for various light industrial plants.
(4) Steel-concrete composite structure technology.
The beam and column load-bearing structure composed of shaped steel or steel management and concrete components is a steel-concrete composite structure, and its application scope has been increasingly expanded in recent years. The composite structure has the advantages of both steel and concrete. It has high overall strength, good rigidity, and good seismic resistance. When it is constructed with outer concrete, it has better fire resistance and corrosion resistance. Combined structural components can generally reduce the amount of steel used by 15 to 20%. Combined floor slabs and steel tube concrete components also have the advantages of less or no formwork, convenient and fast construction, and have great potential for promotion. It is suitable for frame beams, columns and floor coverings of multi-story or high-rise buildings with larger loads, and industrial building columns and floor coverings.
(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 bolted connections have the advantages of simple construction, flexible disassembly, high bearing capacity, good fatigue resistance and self-locking properties, and high safety. They have replaced riveting and partial welding in engineering and become the main connection method in the production and installation of steel structures. For steel components made in the workshop, thick plates should use automatic multi-wire arc submerged welding, and box column partitions should use fusion nozzle electroslag welding and other technologies. During on-site installation and construction, semi-automatic welding technology, gas shielded welding flux-cored wire and self-shielded flux-cored wire technology should be used.
(6) Steel structure protection technology and steel structure protection include fire protection, anti-corrosion, and anti-rust. Generally, anti-rust treatment is not required after fire-retardant coating treatment. However, anti-corrosion treatment is still required in buildings with corrosive gases.
There are many types of fire retardant coatings in China, such as TN series, MC-10, etc. Among them, MC-10 fire retardant coatings include alkyd enamel paint, chlorinated rubber paint, fluorine rubber paint, and chlorosulfonated paint. During construction, appropriate coatings and coating thickness should be selected based on the steel structure type, fire resistance grade requirements and environmental requirements.
5. Objectives and measures of steel structures
Steel structure engineering involves a wide range of areas and is technically difficult. National and industry standards must be followed during promotion and application. Construction administrative departments in various regions should pay attention to the construction of the professional stage of steel structure projects, organize quality inspection team training, and promptly summarize work practices and new technology applications. Colleges and universities, design departments and construction companies should accelerate the training of steel structure engineering technical personnel and promote technically mature steel structure CAD. Mass academic groups should cooperate with the development of steel structure technology, carry out extensive academic exchanges and training activities at home and abroad, and actively improve the overall level of steel structure design, production, construction and installation technology, and reward them in the near future.
6. Connection method of steel structure
There are three connection methods for steel structures: weld connection, bolt connection and rivet connection.
(1) Weld connection
The welding seam connection uses the heat generated by the arc to partially melt the welding rod and the weldment, and then condenses into a weld after cooling, thereby connecting the weldment into one body.
advantage:
It does not weaken the component section, saves steel, has a simple structure, is easy to manufacture, has high connection stiffness, good sealing performance, can be easily automated under certain conditions, and has high production efficiency.
Disadvantages: The heat-affected zone formed by the high temperature of welding on the steel near the weld may cause the material to become brittle in some parts; during the welding process, the steel is subjected to unevenly distributed high temperatures and cooling, causing the structure to produce welding residual stress and residual deformation, which has a negative impact on the structure. The bearing capacity, stiffness and service performance have a certain impact; due to the high stiffness of the welded structure, local cracks can easily expand to the whole once it occurs, especially brittle fracture at low temperatures; the plasticity and toughness of the weld connection are poor, so when welding Defects may occur, reducing fatigue strength.
(2) Bolt connection
Bolt connection is to connect connecting parts into one body through fasteners such as bolts. Bolt connections are divided into two types: ordinary bolt connections and high-strength bolt connections.
Advantages: The construction process is simple and easy to install. It is especially suitable for installation and connection on the construction site. It is also easy to disassemble. It is suitable for structures that need to be assembled and disassembled and temporary connections.
Disadvantages: It is necessary to open holes in the panels and align the holes during assembly, which increases the manufacturing workload and requires high manufacturing accuracy; the bolt holes also weaken the component section, and the connected parts often need to overlap each other or add auxiliary connections. Plate (or angle steel), so the structure is complicated and consumes more steel.
(3) Rivet connection
Rivet connection is to use a rivet with a semicircular prefabricated nail head on one end. After the nail rod is heated red, it is quickly inserted into the nail hole of the connector, and then the other end is also riveted into a nail head with a rivet gun to make the connection tight. solid.
Advantages: riveting force transmission is reliable, plasticity and toughness are good, quality is easy to check and guarantee, and can be used for heavy-duty and structures that directly bear dynamic loads.
Disadvantages: The riveting process is complex, manufacturing is labor-intensive and labor-intensive, so it has been basically replaced by welding and high-strength bolt connections.
7. Welding connection
(1) Welding method
The commonly used welding method for steel structures is arc welding, including manual arc welding, automatic or semi-automatic arc welding, and gas shielded welding.
Manual arc welding is the most commonly used welding method in steel structures. It has simple equipment and flexible and convenient operation. However, the working 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 depends to a certain extent on the technical level of the welder.
The weld quality of automatic welding is stable, with fewer internal defects, good plasticity and good impact toughness, and is suitable for welding long direct welds. Semi-automatic welding is manual operation and is suitable for welding curves or welds of any shape. Automatic and semi-automatic welding should use welding wire and flux that are suitable for the main metal. The welding wire should comply with the provisions of 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 arc's protective medium to isolate the molten metal from the air to maintain the stability of the welding process. The arc heating of gas shielded welding is concentrated, the welding speed is fast, and the penetration depth is large, so the weld strength is higher than that of manual welding. It has good plasticity and corrosion resistance and is suitable for welding thick steel plates.
(2) Welding seam form
Weld connection forms can be divided into four forms: butt joint, lap joint, T-shaped connection and corner joint according to the mutual positions of the connected components. 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 of the connection and the manufacturing, installation and welding conditions.
(3) Weld structure
1. Butt weld
Butt welds transmit force directly and smoothly without significant stress concentration, so they have good mechanical properties and are suitable for connecting components that bear both static and dynamic loads. However, due to the high quality requirements of butt welds and the strict welding gap requirements between weldments, they are generally used in factory-made connections.
2. Fillet weld
Form of fillet weld: Depending on its length direction and the direction of external force action, fillet welds can be divided into side fillet welds parallel to the direction of force action, front fillet welds perpendicular to the direction of force action and oblique to the direction of force action. oblique fillet welds and perimeter welds.
The cross-section forms of fillet welds are divided into ordinary type, flat slope type and deep penetration type. hf in the figure is called the leg size of the fillet weld.
The ratio of the leg side of the ordinary cross-section is 1:1, which is similar to an isosceles right triangle. The force transmission line bends violently, so the stress concentration is serious. For structures that directly bear dynamic loads, in order to ensure smooth force transmission, the front fillet weld should be flat-sloped with a ratio of the two weld fillet sides of 1:1.5 (the long side is in the direction of the internal force), and the side fillet weld should be made with a ratio of 1:1. : 1 deep melt type.
8. Bolt connection
(1) Structure of ordinary bolt connection
1. Forms and specifications of ordinary bolts
The common form used by steel structures is the large hexagonal head type, and its code is represented by the letter M and the nominal and diameter (mm). M18, M20, M22, M24 are commonly used in engineering. According to international standards, bolts are uniformly expressed by the performance grade of the bolt, such as "Grade 4.6", "Grade 8.8", etc. The number before the decimal point indicates the minimum tensile strength of the bolt material, such as "4" indicating 400N/mm2, and "8" indicating 800N/mm2. The numbers after the decimal point (0.6, 0.8) represent the yield ratio of the bolt material, which is the ratio of the yield point to the minimum tensile strength.
According to the processing accuracy of bolts, ordinary bolts are divided into three levels: A, B, and C.
Class A and B bolts (refined bolts) are made of grade 8.8 steel and processed by machine tools. They have smooth surfaces, accurate dimensions, and are equipped with Class I holes (that is, the bolt holes are drilled or expanded on the assembled components). , the hole wall is smooth and the hole alignment is accurate). Due to its high processing precision and close contact with the hole wall, its connection deformation is small and its mechanical performance is good, so it can be used for connections that can withstand large shear forces and tension forces. However, manufacturing and installation are labor-intensive and costly, so they are rarely used in steel structures.
Grade C bolts (rough bolts) are made of grade 4.6 or 4.8 steel. The processing is rough and the dimensions are not accurate enough. They only require Class II holes (that is, the bolt holes are punched in a single part at one time or drilled without a drilling die. Generally, the hole diameter is larger than that of bolts. The rod diameter is 1~2mm larger). When transmitting shear force, the connection deforms greatly, but the performance of transmitting tensile force is good, no special equipment is required for operation, and the cost is low. It is often used for bolted connections that bear tension and secondary shear connections in structures that bear static loads or indirectly bear dynamic loads.
2. Arrangement of ordinary bolt connections
The arrangement of bolts should be simple, uniform and compact, meet the stress requirements, have a reasonable structure and be easy to install. There are two arrangements: parallel and staggered (as shown in the picture). Parallel arrangement is simpler, staggered arrangement is more compact.
(2) Stress characteristics of ordinary bolt connections
1. Shear bolt connection
2. Tension bolt connection
3. Tension shear bolt connection
(3) Stress characteristics of high-strength bolts
High-strength bolt connections can be divided into friction type and pressure-bearing type according to design and stress requirements. When the friction type connection is subjected to shear, the external shear force reaches the maximum frictional resistance that may occur between the plates as the limit state; when it exceeds the relative slippage between the plates, the connection is considered to have failed and is destroyed. When the pressure-bearing connection is in shear, the friction force is allowed to be overcome and relative slippage occurs between the plates. Then the external force can continue to increase, and the subsequent shearing of the screw or the final destruction of the hole wall pressure is the ultimate state.
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