In recent years, the urbanization process is getting faster and faster, and the prefabricated steel structure building industry has achieved unprecedented development. People have higher and higher requirements for the practicability and safety of buildings. In modern construction engineering, steel structure design has certain advantages, and its application in construction is becoming more and more extensive. Combined with years of work experience, K-home summed up 8 professional basic knowledge about steel structure, the content is long, please read it patiently:
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The material selection principle of steel structure is to ensure the bearing capacity of the load-bearing structure and prevent brittle failure under certain conditions. It is comprehensively considered according to the importance of the structure, the load characteristics, the structural form, the stress state, the connection method, the thickness of the steel and the working environment. of.
The four steel types proposed in &#;Code for Design of Steel Structures&#; GB- are the &#;appropriate&#; types and are the first choice when conditions permit. The use of other types is not prohibited, as long as the steel used meets the requirements of the specification.
Fourth, the main technical content of steel structure:
(a) High-rise steel structure technology. According to the building height and design requirements, the frame, frame support, cylinder and giant frame structure are respectively used, and the components can be steel, stiff reinforced concrete or steel tube concrete. The steel members are light and ductile, and can be welded or rolled, which is suitable for super high-rise buildings; stiff reinforced concrete members have high rigidity and good fire resistance, and are suitable for medium and high-rise buildings or bottom structures; steel pipe concrete is easy to construct, For column structures only.
(b) Space steel structure technology. The space steel structure has the advantages of being lightweight, high rigidity, beautiful appearance and fast construction speed. The ball joint flat grid, multi-layer variable-section grid and reticulated shell with a steel pipe as the rod are the structural types with the largest amount of space steel structure in my country. It has the advantages of large space rigidity and low steel consumption and can provide complete CAD in the design, construction and inspection procedures. In addition to the grid structure, there are also large-span suspension cable structures and cable-membrane structures in space structures.
(c) Light steel structure technology. A new structural form consisting of walls and roof envelopes is made with light-colored steel plates. 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, round steel made of flexible support systems and high-strength bolt connections. 30m or more, the height can reach more than ten meters, and light cranes can be set up. The amount of steel used is 20-30kg/m2. Now there are standardized design procedures and specialized production enterprises, with good product quality, fast installation speed, lightweight, low investment, and construction is not limited by seasons, suitable for all kinds of light industrial plants.
(d) Steel-concrete composite structure technology. The beam and column load-bearing structure composed of section steel or steel management and concrete components is a steel-concrete composite structure, and its application range has been expanding in recent years. The composite structure has the advantages of both steel and concrete, with high overall strength, good rigidity, and good seismic performance. When the outer concrete structure is used, it has better fire resistance and corrosion resistance. Combined structural members can generally reduce the amount of steel by 15 to 20%. The composite floor and concrete-filled steel tubular components also have the advantages of less or no formwork, convenient and fast construction, and great promotion potential. It is suitable for frame beams, columns and floors of multi-story or high-rise buildings with large loads, industrial building columns and floors, etc.
(e) High-strength bolt connection and welding technology. 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, flexible dismantling, high bearing capacity, good fatigue resistance and self-locking, and high safety. It has replaced riveting and partial welding in the project and has become the main connection method in the production and installation of steel structures. For steel components and thick plates made in the workshop, automatic multi-wire arc submerged welding should be used, and the box-column clapboard should use melting nozzle electro slag welding and other technologies. In the field installation and construction, semi-automatic welding technology, gas-shielded flux-cored welding wire and self-shielded flux-cored welding wire technology should be used.
(f) Steel structure protection technology. The protection of steel structures includes fire prevention, anti-corrosion and rust prevention. Generally, it is not necessary to do anti-rust treatment after the fire-retardant coating treatment, but it still needs to be anti-corrosion treatment in buildings with corrosive gas. There are many types of domestic fire retardant coatings, such as TN series, MC-10, etc. Among them, MC-10 fire retardant coatings include alkyd enamel paint, chlorinated rubber paint, fluororubber paint and chlorosulfonated paint. In the construction, the appropriate coating and coating thickness should be selected according to the steel structure type, fire resistance grade requirements and environmental requirements.
Steel structure engineering involves a wide range of technical difficulties and must follow national and industry standards in its promotion and application. Local construction administrative departments should pay attention to the construction of the specialization stage of steel structure engineering, organize the training of quality inspection teams, and summarize 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 mature technology of steel structure CAD. The mass academic group 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 can be rewarded in the near future.
There are three types of connection methods for steel structures: weld connection, bolt connection and rivet connection.
(a), Welding Seam Connection
The welding seam connection is to partially melt the electrode and the weldment by the heat generated by the arc, and then condense into a weld after cooling, so as to connect the weldment as a whole.
Advantages: no weakening of component section, saving of steel, simple structure, convenient manufacturing, high connection rigidity, good sealing performance, easy to use automatic operation under certain conditions, and high production efficiency.
Disadvantages: The heat-affected zone of the steel near the weld due to the high temperature of welding may be brittle in some parts; during the welding process, the steel is subjected to unevenly distributed high temperature and cooling, resulting in welding residual stress and residual deformation of the structure. Bearing capacity, stiffness and performance have a certain impact; due to the high stiffness of the welded structure, local cracks are easy to expand to the whole once they occur, especially at low temperatures. Defects may occur that reduce fatigue strength.
(b), Bolt Connection
Bolted connection is to connect the connectors into one body through bolts, such as fasteners. There are two types of bolted connections: ordinary bolted connections and high-strength bolted connections.
Advantages: simple construction process and convenient installation, especially suitable for site installation and connection, and easy to disassemble, suitable for structures that require assembly and disassembly and temporary connections.
Disadvantages: It is necessary to open holes on the plate and align the holes when assembling, which increases the manufacturing workload and requires high manufacturing accuracy; 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. Plate (or angle steel), so the structure is more complicated and it costs more steel.
(c), Rivet Connection
The rivet connection is a rivet with a semi-circular prefabricated head at one end, and the nail rod is quickly inserted into the nail hole of the connecting piece after burning red, and then the other end is riveted into a nail head with a rivet gun to make the connection tight. solid.
Advantages: riveted force transmission is reliable, plasticity and toughness are good, quality is easy to check and guarantee, and it can be used for heavy and directly bearing dynamic load structures.
Disadvantages: The riveting process is complicated, the manufacturing cost is labor and material, and the labor intensity is high, so it has been basically replaced by welding and high-strength bolt connections.
Welding is the most important connection mode in steel structures at present. It has the advantages of not weakening component sections, good rigidity, simple structure, convenient construction and automatic operation&#;.
(a) 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, with simple equipment and 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 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, the internal defects of the weld are less, the plasticity is good, and the impact toughness is good, which 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 wire and flux suitable for the main metal, the welding wire should meet the requirements 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 protective medium of the arc to isolate the molten metal from the air to keep the welding process stable. The arc heating of gas shielded welding is concentrated, the welding speed is fast, and the penetration depth is large, so the strength of the weld is higher than that of manual welding. And good plasticity and corrosion resistance, suitable for the welding of thick steel plates.
(b), The Form Of The Weld
The welding seam connection form can be divided into four forms: butt joint, lap joint, T-shaped joint and fillet joint according to the mutual position of the connected components. The welds used for these connections 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.
(c) Weld Structure
1. Buttweld
Butt welds transmit force directly, smoothly, and have no significant stress concentration, so they have good mechanical performance and are suitable for the connection of components bearing static and dynamic loads. However, due to the high-quality requirements of butt welds, the welding gap between weldments is strict, and it is generally used in factory-made connections.
2. Fillet weld
The form of fillet welds: fillet welds can be divided into side fillet welds parallel to the force acting direction and front fillet welds perpendicular to the force acting direction and obliquely intersecting the force acting direction according to their length direction and the direction of external force action. slanted fillet welds and surrounding welds.
The cross-sectional form of fillet weld is divided into ordinary type, flat slope type and deep penetration type. The hf in the figure is called the fillet size of the fillet weld. The ratio of the leg side of the ordinary section is 1:1, which is similar to an isosceles right triangle, and the force transmission line is bent more violently, so the stress concentration is serious. For the structure directly bearing the dynamic load, in order to make the force transmission smooth, the front fillet weld should adopt the flat slope type with the size ratio of the two fillet edges 1:1.5 (the long side should follow the direction of the internal force), and the side fillet weld should adopt the ratio of 1. : 1 deep penetration.
(a). The Structure Of Ordinary Bolt Connection
The Form And Specification Of Ordinary Bolts
The common form used by the steel structure 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 represented by their performance grades, such as &#;grade 4.6&#;, &#;grade 8.8&#; 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 numbers after the decimal point (0.6, 0.8) indicate the yield ratio of the bolt material, that is, the ratio of the yield point to the minimum 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, turned by machine tools, with smooth surfaces and 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 is accurate). Because of its high machining accuracy, close contact with the hole wall, small connection deformation, and good mechanical performance, it can be used for connections with large shear and tensile forces. However, it is more labor-intensive and costly to manufacture and install, so it is less used in steel structures.
Grade C bolts (rough bolts) are made of grade 4.6 or 4.8 steel, rough processing, and the size is not accurate enough. Only type II holes are required (that is, the bolt holes are punched on a single part at one time or drilled without a drill. Generally, the hole diameter is larger than that of bolts. The rod diameter is 1~2mm larger). When the shear force is transmitted, the connection deformation is large, but the performance of transmitting the tensile force is still good, the operation does not require special equipment, and the cost is low. Commonly used for bolted connections in tension and secondary shear connections in structures that are statically or indirectly dynamically loaded.
Arrangement Of Ordinary Bolted Connections
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The arrangement of bolts should be simple, uniform and compact, to meet the force requirements, and the structure should be reasonable and easy to install. There are two types of arrangement: side-by-side and staggered (as shown in the figure). The parallel is simpler, and staggered is more compact.
(B). The Stress Characteristics Of Ordinary Bolted Connections
(C). The Stress Characteristics Of High-Strength Bolts
High-strength bolted connections can be divided into friction type and pressure type according to design and force requirements. When the frictional connection is subjected to shearing, the maximum frictional resistance may occur between the plates when the external shear force reaches the limit state; when the relative slip occurs between the plates, it is considered that the connection has failed and is damaged. When the pressure-bearing connection is sheared, 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 ultimate failure of the screw shearing or the hole wall bearing pressure is the limit state.
Henan Steel Structure Engineering Technology Co., Ltd. specializes in the construction of steel structure workshops, warehouses, workshops and other projects, and can provide quotations, renderings, installation drawings and other services according to the budget. For more questions, please consult our professional team.
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This is a guest post kindly submitted by Brad Pritts, Technical Director of Unibond, a longtime reader of our blog QualityInspection.org, who often contributes to conversations around the topic of my articles. A big thanks to Brad for this content&#;
Unibond has been producing brake shoes in the USA since . In , we expanded production in China, and, to this day, produce brakes in both countries. We produce high volume models in our US plant, and medium to lower volume models in China. This allows us to capitalize on more complex equipment and tooling for the high volume models, while using simpler, but more labor-intensive techniques for the models we build in China. Some models are run in both locations, allowing us to shift back and forth as market conditions change. Using the right steel is &#;mission-critical&#;, so we take our steel purchasing seriously on both continents. In this article, I will tackle the question of whether Chinese steel products &#;can be trusted?&#;
Not surprisingly, China is the biggest steel producer in the world. In recent years they have produced over 1,000 million tons per year! But only a small portion is exported &#; less than 10% in recent years. China&#;s steel producers have been kept busy making steel for China&#;s own consumption, including megaprojects such as their well known high-speed rail system.
The US produces &#;only&#; about 75 million tons of steel annually and imports another 25 million tons. The US&#; biggest steel suppliers are Canada, Brazil and Mexico, accounting for about half of the imports. Prior to the section 232 trade sanctions imposed by the Trump Administration, China furnished less than 3% of the steel imported by the USA. In , for example, China was the #10 source for the US and fell off the top 10 in with the imposition of tariffs. Even Italy sells us more steel than China!
Customers using these products might well ask about their quality and safety. Before addressing Chinese steel products specifically, let&#;s look at a few facts about steel generally.Customers using these products might well ask about their quality and safety. Before addressing Chinese steel products specifically, let&#;s look at a few facts about steel generally.
To the outsider, &#;steel&#; might seem simple. Inside the industry, it is a different story. Making and using steel is a complicated business. The questions buyers may ask include:
Even our &#;simple&#; heavy-duty brake shoes use different grades of steel for their components and a dozen different grades for our brake hardware.
Over the long term, the steel market is global and &#;efficient&#;, meaning that the price of steel products is highly competitive worldwide. When significant changes occur, it may take time for the market to adapt, but historically it has done so. So once quality issues and shipping costs are taken into account, costs are generally very similar worldwide. Of course, when price changes are driven by tariffs or other legal changes, the market can be distorted&#; as it is right now, with tariffs providing US steel producers &#;cover&#; to raise prices above world market level. In , the combination of the tariffs and increased demand brought steel prices to all-time highs&#; but that&#;s a topic for a different article. (When the tariffs were originally imposed in March , US prices started climbing the day before the public announcement&#; and they increased 20% to match the tariff nearly overnight.) Recent negotiations with the EU and Japan promise to end the US tariffs for most of the US steel trading partners, allowing domestic prices to improve.
Here, the answer is that like many products in China, you can get whatever level of quality you are willing to pay for! The best Chinese steel mills achieve world-class quality levels&#; at the worst, quality can be terrible. Global quality leaders like Apple and Audi source material in China, and they&#;re well known for being demanding. Perhaps the biggest risk is using the wrong grade of steel, whether due to honest mistakes, fraud or anything in between. If the wrong grade is used, even the best quality in the world won&#;t make it right. Of course, human nature being what it is, these problems can occur anywhere in the world, as they did at Japan&#;s respected steel producer Kobe in . We work closely with part suppliers and steel suppliers to ensure we are using optimal grades.
In producing brake shoes and other brake parts, using the right grade of steel is &#;mission-critical&#;, and we treat it that way. Our OEM customers consider the correct steel material a &#;safety-critical&#; quality characteristic. That&#;s because the strength of the part itself &#; especially the welds &#; depends on the grade of steel.
Some of the measures quality-conscious importers use include:
We also monitor quality closely in those operations where the steel quality or grade can make a difference such as in welding or heat treatment.
While this has been alleged, it&#;s not the reason for the current steel tariffs. The author isn&#;t an attorney, much less an international trade law expert, so this article doesn&#;t claim to be the last word on this question. Having purchased Chinese steel products for 20 years, to me the picture is a mixed bag. On one hand, the Chinese steel producers are state-owned, and so enjoy politically controlled financing and government support. On the other hand, the most objectionable examples of government support such as export tax subsidies have been eliminated for sales of steel and similar materials. (These subsidies still exist for many steel products including brake parts&#; ouch!).
While enjoying support, Chinese steel producers also face obstacles created by the government. The steel industry is subject to increasing environmental controls. (At times of bad weather conditions, mills have been shut down for weeks or even months.) With the Olympics coming, similar shutdowns are likely so as to produce clear blue skies for the events.
Many of us have a simplistic view of the power of &#;command and control&#; of a state-owned economy, but in practice, it&#;s more complicated. For example, the Chinese steel industry is troubled by overcapacity. While the national government knows of the problem, individual provinces (think of state governments) are slow to shut down their local plants, keeping many less competitive steel producers alive despite the efforts of the central government. To date, little has been accomplished in this goal, despite years of pressure from the national leadership. Tellingly, the relatively few areas affected have been geographically close to Beijing. This calls to mind an ancient Chinese proverb: &#;The Mountains are high, and the Emperor is far away.&#;
Unibond has been producing brake shoes and hardware in China for over twenty years. From the start, we&#;ve developed and strictly enforced high quality expectations for our steel. We are proud of the work done by our Chinese supply chain, and proudly stand behind it!
About the author:
Brad Pritts is Unibond&#;s Technical Director. Among other responsibilities, he has been the primary manager of Unibond&#;s Asian supply base since . Earlier, he served as quality leader for an aftermarket auto body parts supply network in Taiwan and China, from to . Pritts has been in the automotive industry since , primarily in the quality field. He holds a B.S. (Ohio State University, ) and M.B.A. (University of Michigan, ).
About Unibond:
Unibond produces brake shoes for the heavy-duty truck market in both the US and Asia; additionally, they import a full line of brake hardware. Unibond is based in Ferndale, Michigan and has been in business since . Unibond is a wholly-owned subsidiary of the Deshler Group, a large privately-held manufacturer of auto parts as well as participating in several related industries.
If you&#;re importing Chinese steel products, these resources could help you, too:
If you want to learn more, please visit our website China Steel Structure Manufacturer.