1 Characteristics of steel structure
(1) The construction period is short, and the production and installation are highly mechanized. The main components of the steel structure are the finished products of the steel plant, the welding workload is small during the construction process, the processing is simple and fast, and the material performance is guaranteed by the steel plant, and the project quality is high. It is suitable for mass production, from the factory processing of components to on-site hoisting, the degree of industrialization is much higher than the labor-intensive cast-in-place reinforced concrete structure.
(2) The steel structure is light in weight, has good mechanical properties and can withstand larger loads. Therefore, the cross-sectional size of the steel structure is small. When the same span and the same load are used, the weight of the steel roof truss is at most 1/4 or 1 of the reinforced concrete roof truss /3, the weight of the steel structure is small, which is convenient for transportation and hoisting and reduces transportation costs.
(3) The steel structure is more consistent with the assumptions of the mechanical calculation. The internal structure of the steel is relatively uniform, close to an isotropic body, and is almost completely elastic within a certain stress range. Therefore, the steel structure can more clearly reflect the force properties of the structure, the force transmission is clearer and clearer, and can accurately reflect the design intent.
(4) The steel structure is easy to disassemble and assemble. The steel structure is produced by a specialized metal component factory (beams, roof truss, columns, etc.), which are connected by electric welding or bolts (or high-strength bolts) on the construction site. The construction and installation are simple and rapid. Moreover, the bolted steel structure is convenient for transformation and demolition. It is widely used in various temporary construction, booth layout, and cargo space layout. The construction speed is fast, the labor cost is saved, the construction cycle is reduced, and the economic benefits are obvious.
(5) The flexibility of steel structure in plane layout.
(6) Suitable for large-span structures. Large span is almost impossible for reinforced concrete, but steel structure can solve the problem very well. Now it is widely used in airports, bridges, stadiums, etc.
(7) The steel structure has poor fire resistance and high temperature resistance. Although steel is a non-combustible material, under the action of high temperature, its mechanical properties such as yield strength and elastic modulus will decrease with the increase of temperature. At about 550°C, the decrease is more obvious, generally around 15min. Lost weight and collapsed. On September 11, 2001, the World Trade Center in the center of New York, USA was attacked by terrorists. After being hit by two civil airliners, the skyscraper collapsed and caused thousands of damages only 20 minutes later. Heavy casualties. The main reason is that under the action of high temperature, steel components quickly lose strength and bearing capacity. As far as the fire resistance of steel is concerned, it is far lower than that of masonry and steel concrete structures. Therefore, the fire protection problem of steel structures is becoming increasingly prominent.
2 Fire protection measures for industrial steel structure workshops
2.1 Fire zoning of industrial steel structure workshop
The factory buildings required by modern industry are often large-space, large-span, and transparent. In order to effectively control the fire within a small area, the "Code for Fire Protection Design of Buildings" requires the division of fire compartments in buildings, and clearly stipulates the maximum allowable area of fire compartments at all levels. The existing light steel workshops usually occupy a large area. For example, the building area of the main workshop of the rolling mill generally exceeds 5000 m2, and the area allowed by the specification is 3000 m2 (the production category is Category C. After the fire protection coating is adopted, the fire resistance rating is Three-level consideration), so fire compartments should be separated. Due to the process requirements of the complete equipment production line, the steel rolling workshop is usually several kilometers long. It is impossible to divide the workshop in half with a firewall and cut off the coherent production line. From the production point of view, such a scheme cannot meet the production requirements and will certainly not be used by the owner. agree. Install automatic sprinklers throughout the workshop to double the allowable fire zone area to meet the requirements of the specification. But pay attention to the following two issues: (1) According to Article 4.3.2 of the "Specification for Automatic Sprinkler System Design", the installation of closed sprinklers does not have much effect on large space buildings exceeding 8m. The height of industrial steel structure workshops mostly exceeds 8m. (2) Some Category C three-level single-layer steel structure workshops have an area of 9000 m2 and need to be divided into 3 fire compartments. If the whole workshop is installed with self-spraying, the allowable area of the fire compartment is doubled.
2.2 Spraying fireproof coating method
The spraying fireproof coating method is to spray the fireproof coating directly on the surface of the component with a spraying machine to form a protective layer. Steel structure fireproof coatings can be divided into organic fireproof coatings and inorganic fireproof coatings according to the different adhesives used. According to the thickness of the coating, they are divided into two types: thin coating and thick coating. Thin coatings have thinner coatings and are decorated Effect, the construction method can be sprayed or brushed, and the coating expands and thickens to protect the steel component at high temperature. With light weight and simple construction, it is suitable for concealed structures, exposed steel beams, diagonal braces and other components. The thickness of the thin-coated steel structure coating is usually 1~6mm, and the fire resistance limit can reach 0.5~1.5h. This kind of coating is also called steel structure expansion fireproof coating. Thick-coated steel structure fireproof coating is also called steel structure fireproof and heat insulation coating. The thickness is generally 7-25mm, with a granular surface, low thermal conductivity, low density, and a fire resistance limit of 0.5-3.5h.
2.3 Shielding
The steel structure is arranged in a wall or ceiling composed of refractory materials, or the components are hidden in the gap between two walls, and the purpose of fire prevention can be achieved by adding a little refractory material or not. This is an economical method of fire prevention.
2.4 Pouring concrete or masonry brick method
The critical temperature at which the steel structure loses its stability is 540°C. Under the action of high temperature, the heat will be quickly transferred to the inside and the temperature rises quickly. Scientific experiments show that the fire resistance time of bare steel structures is only 15 minutes. When the structure may be damaged by hot molten metal, it should be made of refractory bricks or heat-resistant materials. The heat insulation layer is used to protect; when the structure surface is exposed to radiant heat above 150℃ for a long time or may be exposed to flame in a short time, heat insulation layer or water jacket should be adopted. When cast-in-place concrete is used as the outer layer, it can be directly cast on the steel structure to form, the construction is simple and the effect is obvious.
2.5 Maintenance of the selection of structural materials
The selection of maintenance structure materials is very researched, and many fire cases are caused by unqualified materials or non-compliance with usage requirements. Among them, unqualified insulation materials account for a large proportion of fires. Therefore, the selection of building materials is very important. The competent unit must strictly control the circulation and use of unqualified products.
2.6 Construction requirements
In recent years, fires in a cold rolling mill and waste storage tanks have caused great economic losses and social impacts. The reason is all due to the construction party's brutal construction or poor consideration. Therefore, it must be carefully organized, scientifically constructed, and strictly abide by the "Fire Protection Code".
2.7 Other
Fire prevention methods are now continuously developed. Steel and concrete composite structures can not only exert the mechanical properties of steel but also the fire prevention effect of concrete, and are currently widely used. In addition, sprinklers and independent water curtains are installed in the workshop. Although the cost is high, the fire prevention effect is also very significant.
3 Selection of main steel structure
Steel structural components should provide high working reliability. In addition to the strength of steel, they should also have other working indicators and process technical indicators, such as welding performance, brittleness and fatigue resistance, ultimate plasticity and crack growth resistance, etc. . The effect of material on brittle failure is mainly reflected in the plasticity and toughness of the steel itself, which in turn depends on the chemical composition, crystal structure and smelting method of the steel. Studies have shown that the cold brittle resistance of low-alloy steel is higher than that of low-carbon steel, while the cold brittle resistance of low-carbon steel, stabilized steel, semi-stabilized steel and rimmed steel, decreases in order. Studies have shown that the Inner Mongolia Plateau has long winters and short summers. In addition, most of the surface temperature in winter is below 0℃. In some high-altitude areas, the annual average temperature is around -2～-4℃, and the average temperature in winter is around -24～-26℃. Therefore, to prevent the brittle failure of steel under low temperature environment, it is very important to choose the steel for structure correctly. The portal frame structure is an important load-bearing structure that bears static load or indirect dynamic load. The steel selection should be selected according to the specific working environment temperature. When the ambient temperature is >-20℃, Q235B•Z, Q345A (or B), Q390A (or B) should be used; when the ambient temperature is ≤-20℃, the steel can be Q235B•Z, Q345A, Q390A Wait.
4 Stability of steel structure
Stability is a prominent problem often encountered in the design of steel structures. Many structural collapses are not caused by insufficient strength, but caused by overall instability. If the stability problem is not handled properly, it will cause undue losses. (1) The problem of local instability. In order to effectively solve the problem of local instability, it is recommended to install horizontal stiffeners and longitudinal stiffeners, and at the same time, strictly limit the aspect ratio. (2) The problem of overall instability. When the beam has a plank (steel plate or concrete slab) and is firmly connected to the compression flange of the beam, the overall stability of the beam may not be considered. Consider the overall stability when the ratio of free length to width exceeds a certain limit. (3) Instability problems during operation. Failure to follow the regulations or improper use will cause local stress concentration, or cause the most unfavorable load distribution, and then cause instability.
5-node design
5.1 Beam-Column Connection Node
5.2 Column and foundation connection node selection
The connection node between the column foot and the foundation is designed to be hinged or rigid as required. The hinged node directly generates a bending moment on the foundation. The main factors that determine the foundation section are vertical force and shear force. Therefore, the size of the foundation section is small, and the size of the steel structure column Larger; rigidly connected nodes produce bending moments on the foundation, and the vertical force and shear force do not play a significant role. The section size of the foundation is larger, and the section of the steel column is smaller.
6 Setting of anti-shear key
Under the action of an earthquake, the vertical and horizontal horizontal forces of the plant are very large. Exceeding the bearing capacity of the stud bolt, at this time, the shear force of the stud bottom plate is not satisfied, and a shear connector is required. "Technical Specification for Steel Structures of Portal Frame Light Buildings" Column anchor bolts should not be used to withstand the horizontal shear force at the bottom of the column feet. This horizontal shear force can be borne by the friction between the bottom plate and the concrete foundation (coefficient of friction can be 0.4) or by setting shear keys. Specification: The horizontal force is borne by friction or by the shear key.
Post time: Dec-11-2020