General information:
The quality of steel used in the manufacture of metal structures is determined by:
1. mechanical properties: resistance to statistical effects (temporary resistances and yield strength under tension);
2. resistance to dynamic effects and brittle fracture (toughness at different temperatures);
3. indicators of plasticity (relative elongation);
4. resistance to stretching (bends in a cold state).
The value of these indicators is set by the state. standards. In addition, the quality of steel is determined by:
5. resistance to multiple loading ();
6. weldability, which is guaranteed by the appropriate chemical. composition of steel and its production technology.
7. Corrosion resistance.
Chemical properties of steel are divided into three groups:
1. ordinary strength (low carbon).
2. high strength.
3. high strength.
Construction steels and aluminum alloys.
Construction steel:
Depending on the chemical composition and mechanical properties, they are divided into two main groups:
1. Mild steel of ordinary quality, the mechanical properties of which depend on the carbon content.
2. Low carbon steel containing alloying additives that increase strength, toughness, as well as resistance to corrosion.
The main chemical components are iron and carbon. The usual carbon content by weight is 0.1-0.22%. With increased carbon content, steel becomes stronger, but it is more fragile and does not weld well. In addition, the steel contains basic and alloying additives and harmful impurities.
The main additives are manganese and silicon. Manganese increases the strength of steel, slightly reducing its ductility. Silicon increases strength, but deteriorates weldability and makes steel less resistant to corrosion. Carbon steel of ordinary quality manganese contains up to 0.7%, and silicon up to 0.35%. In low alloyed steels, manganese is a dopant and its content reaches 2%. Chromium, nickel, copper, molybdenum, and vanadium are also used as alloying additives.
Harmful impurities are phosphorus, sulfur, nitrogen and oxygen. Phosphorus makes the steel brittle at low temperatures and reduces its ductility when heated. Sulfur makes steel fractured at high temperatures. The content of phosphorus and sulfur in carbon steel of ordinary quality should not exceed 0.045 and 0.055%, respectively, and in low-alloy steels 0.035 and 0.04%.
According to the method of preparation, steel has become Martel and oxygen-converter. They are made boiling, calm and semi-calm. Boiling steel is immediately poured from the ladle into the molds; it contains a significant amount of dissolved gases. Calm steel is steel aged for a while in buckets. At the same time, deoxidizers (silicon, aluminum) are added to maintain the temperature and absorption of oxygen from steel (deoxidation of steel). Calm steels have a better composition and more uniform structure, but more expensive than boiling by 10-15%. Semi-quiescent steel is obtained with less deoxidation and occupies an intermediate position between calm and boiling.
The values of the main physical characteristics of steel:
Density ρ = 7850 kg / m3; modulus of longitudinal elasticity E = 2.1 * 105 M / Pa.
For building structures, the following steels are mainly used:
1. mild steel of ordinary quality, grade VST3; the letter B indicates that the steel is supplied at the same time by mechanical properties and chemical composition.
The degree of deoxidation is indicated (KP - boiling, PS - semi-calm, SP - calm), which are indicated after the steel grade, for example, HST 3 SP. With a higher content of manganese put the letter G, for example, TSA 3 Gps. Depending on the standardized indicators (chemical composition, mechanical properties and toughness), steel is divided into categories 1,2,3,4,5,6, which are indicated after the designation of steel,
for example, TSA 3 PS 5;
2. thermally treated steel grade ST T.
3. Low carbon steel grades 14G2, 10G2S1, 15HSND. The first two letters in the designation of low-alloy steels indicate carbon content in hundredths%, letters denote alloying elements (G - manganese, C - silicon, X - chromium, H - nickel, D - copper, A - nitrogen, F - vanadium), the number after the letter indicates the content of this alloying element in%, if it exceeds 1%.
The main steel is st. 3, most of the structures are made of it - floor beams, building trusses, columns of industrial and civil buildings. For structures operating at temperatures below 30 ° C and especially severe conditions and for .... Structures should be applied calm steel, and in other cases - semi-calm or boiling steel. Low-alloyed steels with increased strength are used for structures subjected to high forces, or directly affected by dynamic loads (for example, for subclasses of beams). The use of these steels provides significant savings in metal. Increased resistance to low alloy steels, which are more expensive than ordinary carbonaceous ones, should have a feasibility study.
Aluminum alloys:
The density of aluminum alloys is on average 2,700 kg / m3, i.e. almost three times less than that of steel, and the strength of the alloys differs little from the strength of steel. Deformations of structures made of aluminum alloys are significantly greater than deformations of steel structures. The following alloys are used for building structures: aluminum with magnesium, called magnals, grades AMG6-M and AMG61-M; aluminum with copper and magnesium - duralumin grades D1-T and D16-T; aluminum with magnesium and silicon - AVI T1, AD31-T1, and other types of aircraft. The letter “M” after designation of the alloy grade indicates the identified state of the metal, and the letter “T” is heat-treated.
Magnesiums have good weldability and high corrosion resistance. Duralumin badly welded and used in riveted designs; These alloys have high strength and least cost, but they are less resistant to corrosion than magnalium. The AB-T1 alloy, which contains copper in addition to magnesium and silicon, has a reduced resistance to corrosion. Duralumin and airplanes are used in a thermally compacted state; therefore, designs of them are designed mainly riveted, since Heating during welding significantly reduces the strength of the metal.
Corrosion of steel and aluminum alloys and measures to combat it:
Corrosion - the destruction of the surface of metals caused by electrochemical processes occurring in the material. As a result of corrosion, cross-sections and bearing capacity of structural elements are reduced. The corrosion rate is expressed by a decrease in the thickness of the structural elements in millimeters within one year. The corrosion rate depends on the degree of aggressiveness of the environment and on the shape of the cross sections of the structures. The accumulation of dust on the surface of the structure and its periodic wetting increase the corrosion rate. In the best conditions are designs that are well blown by air. Studies have shown that the I-section elements, which are located vertically, are more corrosive than tubular elements, and elements that are horizontal, are even more susceptible to corrosion.
The centers for the development of corrosion are the gaps between the elements of the package of sheets and shaped profiles. To protect against corrosion, steel structures are thoroughly cleaned and coated with oil paint.
The corrosion rate of aluminum alloys is 5-10 times less than steel. Alkaline solutions are the most dangerous for aluminum alloys. Designs are in the open air, slightly affected by corrosion. In the usual operating conditions of the construction of aluminum
Alloys do not need corrosion protection. Constructions that are in an environment of high aggressiveness are coated with enamels or varnishes. The places of contact between aluminum alloys and other materials (steel, concrete) are also of great danger. Therefore, such surfaces must be carefully insulated.
Advantages and disadvantages of metal structures:
Due to high mechanical characteristics and uniformity of the structure, steel is used in critical structures, with large spans and heights of buildings and structures, and with increased loads. However, due to the high cost and scarcity of the material, steel structures are used mainly in cases where they are economically much more profitable than iron-concrete.
Steel supporting structures are used:
in single-storey industrial buildings for roof structures with a span of 30 meters or more;
columns higher than 14.4 meters;
with a lifting capacity of cranes 500 kN or more, or with their heavy duty.
In single-storey buildings for various purposes (light lattice supporting structures with a grid of columns of at least 18 × 18 m). In high-rise buildings (steel frames of buildings with a regulatory long-term load of 3 to 1 MPa). In some engineering structures (cranes, racks) silos for storing materials that cannot be contained in iron-concrete tanks;
ground storage tanks for petroleum products;
bridges, power transmission towers.
In recent years, the use of lightweight steel building structures has been expanding. They are intended for workshops with light roofs and walls, i.e. such, where new, extremely lightweight insulators are used, for example, foam plastic with a mass of 100 kg / m3 and where steel consumption is significantly reduced.
The work of steel is shown in the stretching diagram:
Characteristic for carbon steel St3 are the proportionality limits δ ..., yield strength δm and temporary resistance δв.
Until the limit of proportionality is reached, steel works elastically and is deformations E and stresses δ. The existing linear dependence is expressed by Hooke's law E = δ / ε, where E is the modulus of longitudinal strain.
Beyond the proportional limit, the deformations grow faster than the stresses, the ductile work of steel begins, corresponding to the formation of a constant stress on the yield flow diagram. With further stretching, the stress increases again; and after reaching temporary resistance, destruction begins.
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Depending on the mechanical properties under tension, the steels used for steel building structures are divided into strength classes:
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etc.
The yield strength is called the standard resistance of steel to tension, compression and bending RH (MPa). Multiplying the value of the normative resistance by a transition coefficient of 0.6, one can obtain the normative resistance of the medium RHsr.
When calculating the structures, the calculated resistance is used, which is equal to the standard resistance divided by the safety factor for the materials Km /
R = RH / Km
Km = 1.1 .... 1.2
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