Eight basic knowledge of Metal Structure
1. The characteristics of Metal Structure
(1)Metal Structure self weight is light
(2)The working reliability of the metal structure is higher
(3)Anti-seismic and anti- shock of metal structure is good
(4)Metal Structure manufacturing is highly industrialized
(6)Easy to do seal structure
(7)Metal Structures are prone to corrosion
(8)Poor fire resistance
2. Commonly used Metal Structure materials grades and performance
(1)Carbon structural steel：Q195、Q215、Q235、Q255、Q275 ect.
(2)High strength low alloy structural steel
(3)High quality carbon structural steel and alloy steel
(4)Special purpose steel
3. Principles of material selection for Metal Structure
Material selection of Metal Structure principle is the guarantee of the bearing capacity of the bearing structure and prevent appear brittle failure under certain condition, according to the importance of the structure, load characteristics, structure styles, stress state, the connection method, the thickness of the steel and the work environment factors to make consideration.
“Specification for design of steel structure” GB50017-2003 put forward four kinds of steel model used is “appropriate” model, is the first choice when conditions permit, does not prohibit to use other types, as long as the use of steel to meet the requirements of the specification.
4. The main Metal Structure technology content
(1)High-rise steel structure technology. According to design requirements and building height to apply frame, frame support, barrel and mega frame structure, its components can use steel, steel concrete or steel tube concrete. Steel structure is with good ductility, can use welding steel or rolling steel, used in high-rise construction; Steel concrete member stiffness is big, fire prevention performance is good, suitable for high-level building or bottom part; Steel tube concrete construction is simple, only used for column structure.
(2)Space Metal Structure technology. Space steel structure is with light weight, large rigidity, beautiful shape, fast construction speed. Ball joint plate truss with steel pipe as rod, Multi-layer variable cross-section mesh frame and reticulated shell are the most important structural types of steel structure in China. With the advantages of large space stiffness and low steel volume, it can provide complete CAD in design, construction and inspection procedures. In addition to the network structure, there are large span suspension cable structure, cable film structure and so on.
(3) Light Metal Structure technology. The light color steel plate made into the wall and roof structure to the new structure. By more than 5 mm steel plate welding or rolling the thin-wall h-beam and roofing purline, round steel made of flexible support system and high strength bolt connection of light steel structure system, column distance from 6 m to 9 m, span up to 30 m or more, up to a dozen meters high. Use steel volume 20 ~ 30kg/m2. Now has a standardized design program and professional production enterprises, products of good quality and fast installation, light weight, less investment, construction season without limit, suitable for all kinds of light industrial workshop.
(4)Steel-concrete composite structure technology. In recent years, the beam and column load-bearing structure composed of section steel or steel tube and concrete components is adopted as steel composite structure. Steel composite structure has both of the advantages of steel and concrete, the overall strength, good rigidity and good seismic performance. When using outsourcing concrete structure, a good fire and corrosion resistance. Composite structural members can generally reduce the amount of steel used by 15-20%. The composite roof cover and concrete-filled steel tube members also have the advantages of less or no dies, convenient and fast construction, and have great promotion potential. Suitable for frame beams, columns and roof overs of multi-storey or high-rise buildings with large loads, industrial building columns and floor covers, etc.
(5) High strength bolt connection and welding technology. High strength bolts are composed of bolts, nuts and washers, to transfer the stress by friction. High strength bolt connection construction is simple, dismantle process is flexible, high bearing capacity, good fatigue resistance and self-locking, high security, etc. The project has replaced the riveting and part welding, become the main connection for steel structure production and installation.
(6)Steel structure protection technology. Steel structure protection including fire protection, anti-corrosion, anti-rust, is generally used in the fire retardant coating, no need to do antirust processing, but in the construction with gas corrosion were still need to make the anti-corrosion process. Domestic fire-retardant coating sort is more, such as series of TN, MC – 10 etc. In construction, it should be based on steel structure, fire resistance rating requirements and environmental requirements to choose the right coating and coating thickness.
5.Goals and measures of Metal Structure
Metal Structure projects cover a wide range and have great technical difficulties. Local construction administrative departments should pay attention to the construction of steel structure projects in the specialized stage, organize the quality inspection team training, and timely summarize the work practice and new technology application. Colleges and universities, design departments and construction enterprises should accelerate the training of technical personnel of steel structure engineering and promote the technology of mature CAD of steel structure. Public 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 installation technology.
6. Connection method of Metal Structure
The connection methods of steel structure include welding connect, bolt connection and rivet connection.
(1) welding connect
The welding connection is to make the welding rod and the welding part partially melt by the heat generated by the arc, and then solidify into the welding seam by cooling, so as to connect the welding part into a whole.
Advantages: do not weaken component section, save steel, simple construction, convenient manufacturing, high joint rigidity, good sealing performance, easy to use automation under certain conditions, high production efficiency.
Disadvantages: the steel around the weld may be brittle in some parts due to the thermal effect of welding high temperature. In the process of welding, the steel is subjected to unevenly distributed high temperature and cooling, which causes the structure to generate welding residual stress and residual deformation, which has certain influence on the bearing capacity, rigidity and service performance of the structure. Due to the high stiffness of the welded structure, local cracks are easy to spread to the whole, especially brittle fracture at low temperature. The joint has poor plasticity and toughness, which may lead to defects and reduce fatigue strength.
(2) Bolt connection
Bolt connection are fastened the connect parts together with bolts. Bolt connection is divided into ordinary bolt connection and high strength bolt connection.
Advantages: simple construction process, convenient installation, especially suitable for site installation connection, easy disassembly, suitable for installation and disassembly structure and temporary connection.
Disadvantages: it is necessary to open holes on board parts, increase the manufacturing workload, and require high precision of manufacturing. The bolt hole also weakens the cross section of the component, and the connected parts often need to lap each other or add auxiliary connecting plate (or Angle steel), so the structure is more complex and more expensive steel.
(3) Rivet connection
Rivet connection is putting one rivet with a semicircular prefabricated screw at end to insert the nail rod quickly after burn red fitting nail holes, then the other end with a rivet gun also play riveting head, to make the connection to tighten.
Advantages: reliable riveting force, good plasticity and toughness, easy quality inspection and guarantee, can be used for heavy and direct load bearing structures.
Disadvantages: the riveting process is complex, the manufacture take time and material, and the labor intensity is high, so it has been replaced by welding and high strength bolt connection.
7. Welding connection
(1) welding method
The welding methods commonly used in Metal Structure is arc welding, including manual arc welding, automatic or semi-automatic arc welding and gas protection welding.
Manual arc welding is the most common welding method in steel structure. The equipment is simple, process is flexible and convenient but the labor conditions are poor, the production efficiency is lower than automatic or semi-automatic welding, and the quality of the weld is highly variable, which depends to a certain extent on the welder’s technical level.
The automatic welding seam has stable quality, less internal defects, good plasticity, good impact toughness, suitable for welding long direct welding seam. Semi-automatic welding is suitable for welding curves or any shape seams due to manual operation. Automatic and semi-automatic welding shall adopt the welding wire and flux suitable for the main metal, the welding wire shall conform to the national standard, and the flux shall be determined according to the welding process requirements.
Gas shielded welding uses inert gas (or CO2) as arc protective medium to insulate molten metal from air so as to keep the welding process stable. The arc heating concentrated, the welding speed is fast and the welding depth is large, so the weld strength is higher than that of manual welding. And it is with good plasticity and corrosion resistance, which is suitable for thick steel plate welding.
(2) Welding Seam styles
According to the mutual position of the connected components, the weld joint can be divided into four types: butt joint, lap joint, t-joint and corner joint. The welding seams used for these connections are butt weld and corner weld. In the specific application, the choice should be made according to the force condition of the connection, combined with the conditions of manufacture, installation and welding.
(3) Welding Seam structure
A. Butt weld
Direct, smooth and no significant stress concentration phenomenon exists in the butt weld, so the mechanical performance is good, which is applicable to the component connection under static and dynamic loads. However, due to the high quality requirement of butt weld, the gap between welding parts is strict, which is usually used in the connection of factory manufacture.
B. Corner joint
Corner joint form: corner joint according to the length direction and the direction of the external force, can be divided into the side corner joint which is parallel to the force direction, front corner joint which is perpendicular to the force direction , slant corner joint and round corner joint.
8 Bolt connection
Common bolt connection structure
(1) Common bolt specification
The common form of the bolt is the large hexagon head, whose code name is denoted by the letters M and nominal and diameter (mm). M18, M20, M22, M24 are commonly used in engineering. According to international standards, bolts are expressed in terms of bolt performance grade, such as “4.6” and “8.8”. The number before the decimal point represents the minimum tensile strength of the bolt material, such as “4” for 400N/mm2 and “8” for 800N/mm2. The Numbers (0.6, 0.8) after the decimal point represent the buckling ratio of the bolt material, namely the ratio of the yield point to the lowest tensile strength.
According to the processing precision of bolts, ordinary bolts are divided into A, B and C levels.
A and B grade bolts (refined) made of 8.8 grade steel by the machine tool turning processing, the surface is smooth, precise size, and with Ⅰ class hole. Because of its high machining precision and close contact with the hole wall, it has small joint deformation and good mechanical performance, which can be used to withstand large shear force and tension. However, manufacturing and installation are more labor-intensive and costly, so they are rarely used in steel structures.
C grade bolts is made of 4.6 or 4.6 grade steel. Rough machining, size is not accurate, only ask Ⅱ class hole (i.e. last rushed into the bolt hole in a single parts or don’t have to drill into the diamond. Generally, the hole diameter is 1-2mm larger than the bolt diameter. )When the shear force is transferred, the connection deformation is large, but the performance of the transmission tension is good, the operation requires no special equipment, and the cost is low. Usually used for bolted connection under tension and secondary shear connection in structures under static load or indirect dynamic load.
(2) Arrangement of ordinary bolted connections
The arrangement of bolts should be simple, uniform and compact, meet the requirements of force, reasonable structure and easy installation. There are two kinds of arrangement: juxtaposition and misalignment. Juxtaposition is simpler, misalignment is more compact.
1)Mechanical characteristics of high strength bolts
High strength bolt connection can be divided into two types: friction type and pressure type according to design and force requirement. When the friction-type connection is subjected to shear, the maximum frictional resistance may occur when the external shear force reaches the limit state. When the relative slip occurs between the plates when the connection is exceeded, it is considered that the connection is damaged due to failure. Pressure type connection when shear, allows friction be overcome and the relative slip between panel, and then the external force can continue to increase, and then the shear or screw hole wall pressure eventually damage for the limit state.