Structural Steel Fundamentals

Aug 04,2023


First, the characteristics of steel structure:

1. The self-weight of the steel structure is light

2. The reliability of steel structure work is high

3. Steel has good vibration (seismo) resistance and impact resistance

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

5. The steel structure can be assembled accurately and quickly

6. Easy to make a sealed structure

7. The steel structure is easy to corrode

8. Poor fire resistance of steel structure

Second, the grade and performance of commonly used steel structure steel

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

Third, the principle of material selection of steel structure

The principle of material selection of steel structure is to ensure the bearing capacity of the load-bearing structure and prevent brittle failure under certain conditions, according to the importance of the structure, load characteristics, structural form, stress state, connection method, steel thickness and working environment and other factors are comprehensively considered.

The four steel models proposed in the "Steel Structure Design Code" GB50017-2003 are "appropriate" models, which are the first choice when conditions permit, and do not prohibit the use of other models, as long as the steel used meets the requirements of the specification.

Fourth, the main steel structure technical content:

(1) High-rise steel structure technology. According to the height of the building and the design requirements, frame, frame support, cylinder and giant frame structure are used, and its components can be steel, reinforced concrete or steel tube concrete. The steel components have good light weight and ductility, and can use welded section steel or rolled section steel, which is suitable for ultra-high building buildings; Rigid reinforced concrete components have large rigidity and good fire performance, which are suitable for medium and high-rise buildings or bottom structures; Steel tube concrete is easy to apply and is only used for column structures.

(2) Space steel structure technology. The space steel structure has light weight, large rigidity, beautiful shape and fast construction speed. The ball node plate grid, multi-layer variable section grid frame and grid shell with steel pipe as the member are the structural types with the largest amount of space steel structure in China. It has the advantages of large space stiffness and low amount of steel, and can provide complete CAD in the design, construction and inspection procedures. In addition to the grid structure, the space structure also has a long-span suspension cable structure, cable membrane structure, etc.

(3) Light steel structure technology. Accompanied by a new structural form consisting of a wall and roof envelope made of lightweight colored steel plates. 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, round steel made of flexible support system and high-strength bolt connection, column spacing can be from 6m to 9m, span up to 30m or more, height up to more than ten meters, and light lifting four. The amount of steel used is 20~30kg/m2. Now there are standardized design procedures and specialized production enterprises, good product quality, fast installation speed, light weight, less investment, construction is not limited by the season, suitable for all kinds of light industrial plants.

(4) Steel-concrete composite structure technology. The load-bearing structure of beams and columns composed of section steel or steel management and concrete components is a steel-concrete composite structure, and the application range has been expanding in recent years. The combined structure has the advantages of both steel and concrete, with high overall strength, good rigidity, good seismic performance, and good fire resistance and corrosion resistance when the outsourced concrete structure is adopted. Composite structural components can generally reduce the amount of steel used by 15~20%. Composite building covers and steel pipe concrete components also have the advantages of less mold support or no mold support, convenient and fast construction, and have great promotion potential. It is suitable for frame beams, columns and building covers of multi-storey or high-rise buildings with large loads, columns and building covers of industrial buildings, etc.

(5) High-strength bolting and welding technology. High-strength bolts are used to transmit stress through friction force and are composed of three parts: bolt, nut and washer. High-strength bolt connection has the advantages of simple construction, flexible dismantling, high bearing capacity, fatigue resistance and good self-locking, high safety, etc., and has replaced riveting and partial welding in the project, and has become the main connection means in the production and installation of steel structure. For steel components made in the workshop, the thick plate should adopt automatic multi-wire arc buried welding, and the box-shaped column separator should adopt molten nozzle electroslag welding and other technologies. In the on-site installation and construction, semi-automatic welding technology and gas shielded flux cored wire and self-protecting flux cored wire technology should be adopted.

(6) Steel structure protection technology. Steel structure protection includes fire prevention, anti-corrosion, anti-rust, generally used in the fireproof coating treatment does not need to be anti-rust treatment, but in the building with corrosive gas still need anti-corrosion treatment. There are many types of domestic fireproof coatings, such as TN series, MC-10, etc., among which MC-10 fireproof coatings include alkyd enamel, chlorinated rubber paint, fluoroelastomer coating and chlorosulfonated coating. In the construction, appropriate coatings and coating thicknesses should be selected according to the steel structure type, fire resistance grade requirements and environmental requirements.

V. Objectives and measures of steel structure:

Steel structure engineering involves a wide range of technical difficulties, and must follow national and industry standards in the promotion and application. Local construction administrative departments should attach importance to the construction of the professional stage of steel structure engineering, organize the training of quality inspection teams, and timely summarize work practice and new technology application. Colleges and universities, design departments and construction enterprises should accelerate the training of steel structure engineering and technical personnel and promote mature steel structure CAD. Mass academic groups should cooperate with the development of steel structure technology, widely carry out academic exchanges and training activities at home and abroad, and actively improve the overall level of steel structure design, production and construction and installation technology in the near future.

6. Connection method of steel structure

There are three connection methods of steel structure: weld connection, bolt connection and rivet connection.

(1) Weld connection

The weld connection is the local melting of the electrode and the weldment through the heat generated by the arc, and the weldment is condensed into a weld by cooling, so as to connect the weldment into a whole.

Advantages: does not weaken the cross-section of the component, saves steel, simple structure, convenient manufacturing, large connection rigidity, good sealing performance, easy to adopt automated operation under certain conditions, high production efficiency.

Disadvantages: The heat-affected zone formed by the high temperature of welding of the steel near the weld may be brittle in some parts; During the welding process, the steel is subjected to uneven distribution of high temperature and cooling, which causes welding residual stress and residual deformation of the structure, which has a certain impact on the bearing capacity, stiffness and performance of the structure; Due to the large rigidity of the welded structure, the local crack is easy to extend to the whole once it occurs, especially at low temperature, it is easy to cause brittle fracture; The plasticity and toughness of the weld connection are poor, and defects may occur during welding, which reduces the fatigue strength.

(2) Bolted connection

Bolted connection is a fastener that connects the connecting parts into a single piece. Bolted connections are divided into two types: ordinary bolted connections and high-strength bolted connections.

Advantages: simple construction process, easy installation, especially suitable for site installation and connection, but also easy to disassemble, suitable for the need to assemble and disassemble the structure and temporary connection.

Disadvantages: It is necessary to open holes and assemble holes on the plates, which increases the manufacturing workload and requires high manufacturing accuracy; The bolt hole also weakens the cross-section of the component, and the connected parts often need to be lapped with each other or add auxiliary connecting plates (or angle steel), so the structure is more complex and expensive steel.

(3) Rivet connection

Rivet connection is a rivet with a semicircular prefabricated nail head at one end, the nail rod is burned red and 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 achieve fastening.

Advantages: riveting transmission is reliable, plasticity, toughness are good, quality is easy to check and guarantee, can be used for heavy and directly bearing dynamic load structures.

 

Disadvantages: The riveting process is complicated, the manufacturing is labor-intensive, and the labor intensity is high, so it has been basically replaced by welding and high-strength bolt connections.

Seven, welding connection

(1) Welding method

The commonly used welding methods for steel structures are 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, with simple equipment, flexible and convenient operation. However, the labor conditions are poor, the production efficiency is lower than that of automatic or semi-automatic welding, and the weld quality is highly variable, which depends on the technical level of the welder to a certain extent.

The weld quality of automatic welding is stable, the internal defects of the weld are few, the plasticity is good, and the impact toughness is good. It is suitable for welding long 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 flux suitable for the main metal. The welding wires should comply with the provisions of national standards. The flux should be determined according to the welding process requirements.

Gas shielded welding uses inert gas (or CO2) gas as the protective medium of the arc to isolate the molten metal from the air to keep the welding process stable. Gas shielded welding arc heating is concentrated, the welding speed is fast, and the penetration depth is large, so the weld strength is higher than that of manual welding. And good plasticity and corrosion resistance, suitable for the welding of thick steel plates.

(2) Weld form

The weld connection form can be divided into four forms: butt joint, lap joint, T-shaped connection and corner joint according to the mutual position of the connected components. The welds used in these connections have two basic forms: butt weld and fillet weld. In specific applications, it should be selected according to the force of the connection, combined with manufacturing, installation and welding conditions.

(3) Weld structure

1. Butt weld

The force transmission of the butt weld is direct, smooth, and there is no significant stress concentration, so the force 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 the butt weld, the welding gap between the welds is strictly required, and it is generally used in factory-made connections.

2. Fillet weld

The form of fillet weld: according to its length direction and the direction of external force action, the fillet weld can be divided into a side fillet weld parallel to the direction of force action, a front fillet weld perpendicular to the direction of force action and a diagonal fillet weld oblique to the direction of force action. Weld and circumferential weld.

The cross-section form of the fillet weld is further divided into ordinary type, flat slope type and deep penetration type. In the figure, hf is called the size of the welding foot of the fillet weld. The ratio of the welding foot edge of the ordinary cross-section is 1:1, which is similar to the isosceles right triangle. The force transmission line is bent violently, so the stress concentration is serious. For the structure that directly bears the dynamic load, in order to make the force transmission smooth, the front fillet weld should adopt a flat slope type with a size ratio of 1:1.5 of the two fillet edges (the long side is in the direction of the internal force), and the side fillet weld should adopt a deep penetration type with a ratio of 1:1.

Eight, bolt connection

(1) Structure of ordinary bolt connection

1. Forms and specifications of ordinary bolts

The common form used in steel structures is the large hexagonal head type, and its code name is represented by the letter M, the nominal name and the diameter (mm). M18, M20, M22, M24 are commonly used in engineering. According to international standards, bolts are uniformly represented by the performance grade of bolts, such as "4.6", "8.8", etc. The number before the decimal point indicates the minimum tensile strength of the bolt material. For example, "4" means 400N/mm2, and "8" means 800N/mm2. The number after the decimal point (0.6, 0.8) indicates the yield ratio of the bolt material, that 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.

Grade A and B bolts (refined bolts) are made of grade 8.8 steel, machined by machine tool turning, with smooth surface and accurate size, and are equipped with Class I holes (that is, bolt holes are drilled or expanded on assembled components, the hole wall is smooth, and the hole is accurate). Due to its high machining accuracy, close contact with the hole wall, small connection deformation and good mechanical performance, it can be used for connections that withstand large shear and tensile forces. However, manufacturing and installation are labor-intensive and costly, so it is rarely used in steel structures.

Class C bolts (rough bolts) are made of grade 4.6 or 4.8 steel, rough processing, and the size is not accurate enough. Only Class II holes are required (that is, bolt holes are punched on a single part at one time or drilled without a drilling die. Generally, the hole diameter is 1~ 2mm larger than the diameter of the bolt rod). When transmitting shear force, the connection deformation is large, but the performance of transmitting tensile force is still good, the operation does not require special equipment, and the cost is low. Often used for bolt connections that bear tensile force and secondary shear connections in structures that bear static loads or indirectly bear dynamic loads.

2. Arrangement of ordinary bolt connections

The arrangement of the bolts should be simple, unified and compact, meet the force requirements, and have a reasonable structure and easy installation. There are two arrangements of juxtaposition and staggered arrangement (as shown in the figure). The juxtaposition is simpler and the staggered arrangement is more compact.

(2) Mechanical characteristics of ordinary bolt connections

1. Shear bolt connection

2. Tension bolt connection

3. Tension and shear bolt connection

(3) Force characteristics of high-strength bolts

High-strength bolt connections can be divided into two types: friction type and pressure-bearing type according to design and force requirements. When the friction-type connection is subjected to shearing, the maximum friction resistance that may occur between the plates is the limit state; when it exceeds the relative slip between the plates, it is considered that the connection has failed and is damaged. When the pressure-bearing connection is shearing, the friction force is allowed to be overcome and the relative slip between the plates occurs, and then the external force can continue to increase, and the final failure of screw shearing or hole wall pressure that occurs later is the limit state.