First, the characteristics of steel structure
1, the steel structure is lighter in weight
2, the reliability of steel structure work is higher
3. The steel is resistant to vibration (shock) and impact resistance.
4. The degree of industrialization of steel structure manufacturing is relatively high.
5, steel structure can be assembled accurately and quickly
6, easy to make a sealed structure
7, the steel structure is easy to corrode
8, steel structure is poor fire resistance
Second, the grade and performance of steel for common 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
Third, the principle of material selection of steel structure
The material selection principle of the steel structure is to ensure the bearing capacity of the load-bearing structure and prevent the occurrence of 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, etc. of.
The four steel models proposed in the "Code for Design of Steel Structures" GB50017-2003 are "suitable" 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. Frames, frame supports, cylinders and mega frame structures are used according to the height and design requirements of the building. The components can be steel, stiff reinforced concrete or concrete filled steel tube. The steel member has good ductility and can be welded steel or rolled steel. It is suitable for super high-rise buildings. The stiff reinforced concrete members have high rigidity and good fireproof performance. They are suitable for medium and high-rise buildings or bottom structures. Used only for column structures.
(2) Space steel structure technology. The space steel structure has light weight, high rigidity, beautiful appearance and fast construction speed. The ball joint plate net frame, multi-layer variable cross-section net frame and net shell with steel pipe as the rod are the structural types with the largest amount of space steel structure in China. It has the advantages of large space rigidity, low steel consumption, design, construction and inspection procedures, and can provide complete CAD. In addition to the grid structure, the space structure still has a large span suspension structure and a cable membrane structure.
(3) Light steel structure technology. A new structural form consisting of a wall and a roof envelope with a light colored steel plate. Large-section thin-walled H-shaped steel wall beam and roofing rafter welded or rolled by steel plate of 5mm or more, made of flexible support system and high-strength bolted steel structure made of round steel. The column spacing can be from 6m to 9m, and the span can reach 30m or more, the height can reach more than ten meters, and the light crane can be set. The amount of steel used is 20 to 30 kg/m2. Now we have standardized design procedures and specialized production enterprises. The products are of good quality, fast installation speed, light weight, low investment, and construction is not subject to seasonal restrictions. It is suitable for all kinds of light industrial plants.
(4) Steel concrete composite structure technology. The beam and column load-bearing structure composed of steel or steel management and concrete members is a steel-mixed composite structure, and its application range has been expanded in recent years. The combined structure has the advantages of both steel and concrete, and has high overall strength, good rigidity and good seismic performance. When using the outer concrete structure, it has better fire resistance and corrosion resistance. The composite structural member can generally reduce the amount of steel used by 15 to 20%. The combination of the floor and the concrete-filled steel tube members also has the advantages of less or no support, and convenient and rapid construction, and has a great promotion potential. Suitable for frame beams, columns and floor coverings, industrial building columns and floor coverings for multi-storey or high-rise buildings with large loads.
(5) High-strength bolting and welding technology. High-strength bolts transmit stress through frictional forces and consist of bolts, nuts and washers. The high-strength bolt connection has the advantages of simple construction, flexible removal, high bearing capacity, good fatigue resistance, self-locking property and high safety. The riveting and partial welding have been replaced in the engineering, which has become the main connecting means in the manufacture and installation of steel structures. For steel members made in the workshop, thick plates should be welded by automatic multi-wire arc, and box-shaped column partitions should be welded with electric slag welding. Semi-automatic welding technology and gas-shielded flux-cored wire and self-shielded flux-cored wire technology should be used in on-site installation and construction.
(6) Steel structure protection technology. Steel structure protection includes fireproofing, anti-corrosion and anti-rust. Generally, it is not required to be rust-proof after treatment with fire-retardant coatings, but it is still required to be treated in corrosion in buildings with corrosive gases. There are many types of domestic fireproof coatings, such as TN series, MC-10, etc. Among them, MC-10 fireproof coatings include alkyd enamel, chlorinated rubber paint, fluororubber coating and chlorosulfonated coating. Appropriate coatings and coating thicknesses should be selected during construction according to steel structure type, fire rating requirements and environmental requirements.
V. Objectives and measures of steel structure
Steel structure engineering involves a wide range of areas and is technically difficult. It must comply with national and industry standards in the promotion and application. The construction administrative departments of all localities should pay attention to the construction of the special stage of steel structure engineering, organize the training of quality inspection teams, and summarize the work practices and new technology applications in a timely manner. Colleges and universities, design departments and construction enterprises should accelerate the training of steel structure engineering technicians and promote the technically mature steel structure CAD. The mass academic groups should cooperate with the development of steel structure technology, extensively carry out academic exchanges and training activities at home and abroad, and actively promote the overall level of steel structure design, production and construction and installation technology in the near future.
Sixth, the connection method of steel structure
There are three types of connection methods for steel structures: weld connection, bolt connection and rivet connection.
(1), weld connection
The weld joint connects the electrode and the weldment partially by the heat generated by the arc, and is cooled and condensed into a weld to join the weldment into one body.
Advantages: no weakening of the section of the component, saving steel, simple structure, convenient manufacturing, high connection rigidity, good sealing performance, easy to adopt automatic operation under certain conditions, high production efficiency.
Disadvantages: the heat affected zone formed by the high temperature of the weld near the weld may be brittle in some parts; the steel is subjected to uneven distribution of high temperature and cooling during the welding process, causing the structure to produce residual stress and residual deformation of the structure. Bearing capacity, stiffness and performance have certain influences; due to the high rigidity of the welded structure, local cracks can easily spread to the whole as soon as they occur, especially at low temperatures; brittle fracture is easy to occur; weld joints have poor plasticity and toughness. Defects may occur, resulting in reduced fatigue strength.
(two), bolted
The bolted connection connects the connectors together by means of a fastener such as a bolt. The bolt connection is divided into two types: ordinary bolt connection and high strength bolt connection.
Advantages: The construction process is simple and easy to install. It is especially suitable for installation and connection on the construction site, and is also easy to disassemble. It is suitable for installation and disassembly structures and temporary connections.
Disadvantages: It is necessary to open holes in the plate and assemble the holes, increase the manufacturing workload, and require high precision in manufacturing; the bolt holes also weaken the cross-section of the components, and the connected parts often need to overlap each other or add auxiliary connections. The plate (or angle steel) is therefore more complicated and more expensive.
(three), rivet connection
The rivet connection is a rivet with a semi-circular prefabricated nail head at one end. After the red rod is burned red, it is quickly inserted into the nail hole of the connecting piece, 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 is reliable, plasticity and toughness are good, quality is easy to check and guarantee, and it can be used for heavy and direct dynamic load bearing structures.
Disadvantages: The riveting process is complicated, the manufacturing cost is labor, and the labor intensity is high, so it has been basically replaced by welding and high-strength bolt connection.
Seven, welded connection
(1) Welding method
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, and its equipment is simple and flexible. However, the labor conditions are poor, the production efficiency is lower than the automatic or semi-automatic welding, and the variability of the weld quality is large, which depends to some extent on the technical level of the welder.
The weld quality of automatic welding is stable, the internal defects of the weld are less, the plasticity is good, the impact toughness is good, and 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 shall be carried out with welding wire and flux compatible with the main metal. The welding wire shall comply with the national standards and the flux shall be determined according to the welding process requirements.
Gas shielded welding uses inert gas (or CO2) gas as a protective medium for the arc to isolate the molten metal from the air to keep the welding process stable. Gas shielded 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. It has good plasticity and corrosion resistance and is suitable for welding thick steel plates.
(two), weld form
The weld joint form can be divided into four forms, such as butt joint, overlap, T-joint and corner joint, according to the mutual position between the connected members. The welds used for these joints are in two basic forms: butt welds and fillet welds. In the specific application, it should be selected according to the force of the connection, combined with the manufacturing, installation and welding conditions.
(3) Weld construction
1, butt weld
The butt weld transmission force is direct, smooth, and has no significant stress concentration, so the force performance is good, and it is suitable for the connection of components subjected to static and dynamic loads. However, due to the high quality requirements of the butt welds, the welding gap between the weldments is strict, and it is generally used in factory-made joints.
2, fillet weld
The form of fillet weld: the fillet weld can be divided into a side fillet weld parallel to the direction of force action and a front fillet weld perpendicular to the direction of force action, oblique to the direction of force action, depending on the length direction and the direction of action of the external force. Diagonal fillet welds and perimeter welds.
The fillet weld cross-section is divided into ordinary, flat and deep-melt. In the figure, hf is called the fillet size of the fillet weld. The proportion of the common-type welding foot is 1:1, which is similar to the isosceles right-angled triangle. The transmission line is bent more severely, so the stress concentration is serious. For the structure directly bearing the dynamic load, in order to make the transmission force smooth, the front fillet weld should adopt a flat slope type with a ratio of two weld angles of 1:1.5 (long side and inner force direction), and the side fillet weld should adopt a ratio of 1 :1 deep-melting type.
Eight, bolted connection
(1) Structure of ordinary bolt connection
1. Forms and specifications of ordinary bolts
The common form of the steel structure is a large hexagonal head type, the code number of which 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 level of bolts, such as "4.6 grade", "8.8 grade" and so on. The number before the decimal point indicates the minimum tensile strength of the bolt material, such as "4" for 400N/mm2 and "8" for 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 lowest tensile strength.
According to the machining accuracy of bolts, ordinary bolts are divided into three levels: A, B, and C.
A and B grade bolts (refined bolts) are made of 8.8 grade steel, machined by machine tool, smooth surface, accurate size, and equipped with class I holes (ie bolt holes are drilled or expanded into the assembled components). The hole wall is smooth and accurate to the hole). Due to its high processing precision and close contact with the hole wall, the connection deformation is small, the force performance is good, and it can be used for the connection with large shear force and tension. However, manufacturing and installation are labor-intensive and costly, so they are less used in steel structures.
C-class bolts (coarse bolts) are made of 4.6 or 4.8 grade steel. They are rough and inaccurate in size. Only type II holes are required (ie bolt holes are punched on a single part or drilled without a drill. General bore ratio bolts) The rod diameter is 1~2mm). When the shear force is transmitted, the connection deformation is large, but the performance of transmitting the tension is good, the operation does not require special equipment, and the cost is low. Commonly used for bolting connections that are subject to tensile forces and secondary shear connections that are subjected to static loads or indirectly subjected to dynamic load structures.
2, the arrangement of ordinary bolts
The arrangement of the bolts should be simple, uniform and compact, meeting the requirements of the force, and the structure is reasonable and easy to install. There are two arrangements of alignment and staggered. The juxtaposition is simpler and the misalignment is more compact.
(2) Stress characteristics of ordinary bolt connection
1. Sheared bolt connection
2, pull bolt connection
3, pull the bolt connection
(3) Stress characteristics of high-strength bolts
High-strength bolted joints can be divided into friction type and pressure type according to design and force requirements. When the friction type connection is subjected to shearing, the external frictional force reaches the maximum frictional resistance that may occur between the plates. When the relative slip occurs between the plates, the connection is considered to be broken and destroyed. When the pressure-type connection is sheared, the frictional 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 resulting screw shear or the ultimate damage of the wall pressure is the limit state.