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Pipes are widely used for the installation of various metal structures. As a rule, these are round or square products. In order to make the correct calculation of the loads on structural elements, you need to know what is stronger — an ordinary round pipe, or a square profile pipe. Knowing the maximum load on a structural element, it is possible to very accurately determine which profile is more appropriate for use in the installation process.

## Strength indicators of round and square profiles

GOSTs for square and round pipes regulate a variety of processes:

- physical properties of surfaces.
- mechanical characteristics of the joints.
- the order in which the transformation is performed (bending, twisting, drawing, etc.).

Profile bending conditions may be different. Data on the outer diameter of stainless and carbon pipes also differ.

If bending is carried out by heating or filling the hollow space with bulk material, the outer diameter must be at least 3.5DN (nominal diameters). For pipe benders, this value is 4DN. Deviation from the requirements of GOSTs 494/90 and 617/90 is allowed if the decrease in the thickness of the bent wall is no more than 15%. In this case, you can reduce the nominal data to calculate the bending strength of the profile.

A part of technical mechanics that studies the resistance of various materials helps to compare the strength level of a hollow square and a circle of steel. When calculating the bending strength index, two formulas are used:

- Calculation of the length of the bent part.

L= 0.0175*r*α+Ι,

where

r – profile bending radius, mm.

α is the required bending angle, ^{about}.

I — distance 100/300 for equipment holding the workpiece.

- Calculation of the size of the bent section.

U = π*α/180*(r + DH/2),

where

α – bending angle, ^{about}.

r – bending radius, mm.

DH is the outside diameter.

The calculation of the profile pipe is completed after determining the magnitude of the stress, calculated in accordance with Hooke’s law:

H = M/W,

where

Μ is the degree of bending along the axis of the force.

W is the resistance to bending along the axis.

When carrying out calculations, it is necessary to take into account the measure of inertia of the body during rotation — the moment of inertia. With the same wall thickness of hollow pipes of different sections and specific gravity, the bending force for a square is 1.181 times greater than for a circle. But the radius of gyration of a round pipe is greater than that of a square one. Therefore, it is less durable.

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## Circle or square

Hollow metal (steel or aluminum) rods of square and round sections are widely used in the construction and production of metal structures. The hollow profile has a much lower weight compared to the bar. As a supporting element of a metal structure, a square is more convenient than a circle. It is easier to install. Two measured segments of a square profile can be connected by welding, unlike a circle. The flat surface of the square profile provides better contact with various hinged elements (polycarbonate, fiberboard, various types of pressed insulation, etc.).

And the presence of stiffeners allows you to successfully use the square in places where maximum loads are noted. If the bending force is directed at an angle of 90^{about} to the edge of the profile, the bending resistance is the same anywhere in the pipe. The bending load is strongest at the edges of the profile. Its central part is much more stable. The closed cross section also gives the profile a high torsional strength. This allows the use of square pipes in the installation of various complex structures:

- arch type vaults.
- roofs with steep slopes.
- ribbed domes.

The calculation of the profile pipe shows that the reliability of the structure is directly proportional to the material from which it is made. Concrete is much more reliable than wood, but significantly inferior to steel. The fact is that when using it, it is very difficult to fasten the runs. Anchor holes drilled in concrete will not be able to provide the necessary strength. The metal profile has a lot of advantages in comparison with other materials. And the square has the same number of advantages over the circle.

For example, when constructing restrictive structures (fences), the optimal pitch of the racks is 2.5 m. In case of frequent gusty winds, it should be reduced to 2 m. With a structure height of up to 1.5-2.0 m, the optimal size of a square with a wall thickness of 2 mm is 40×40 or 60×60 mm. The material used is a profile pipe for metal structures made of carbon or low alloy steel (GOST R 54157-2010). When installing larger structures, pipes of sizes from 50×50 to 200×200 mm can be used.

The area of use of square pipes is much wider than round ones. This is due to the fact that they have a higher degree of interaction with planes that have a symmetrical surface. Industries in which pipes of this section are not used are clearly defined:

- fabrication of frame structures.
- transportation of gaseous substances.
- water supply and sewerage.

A profiled pipe is not suitable for a pipeline due to the fact that, unlike a round pipe, it is designed to resist significant bearing loads. Due to the presence of corners, the speed of pumping gases and liquids inside a square pipe is much less than inside a round one.

## Making a square profile

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In theory, to make a square pipe, a rolling machine with the ability to form a profile of the required dimensions is sufficient. The circle is rolled in order to obtain a square. But from the point of view of product quality, this technology is completely unacceptable, since the mechanical characteristics of the pipes obtained differ significantly from those required by GOST for the worse. To produce a profile on an industrial scale, a complex of complex technological equipment is required.

Pipes, which are planned to be used for the installation of metal structures in the future, should be much stronger, and therefore be manufactured according to the full technological cycle. It includes five consecutive operations:

- Processing of steel tape (strip).
- Welding a round workpiece.
- Profiling.
- Quality control of finished products.
- Heat treatment.

Strip cutting is carried out on a special installation. The tape is wound on a rotating drum. Continuous rolling of the profile is carried out so that the winding machine does not stand idle. After welding, passing through the forming mill and emulsion processing, the workpiece takes on a normal round shape.

The next manufacturing step is profiling:

- rounding the workpiece along the entire length of the profile.
- four-sided crimp.

The resulting workpiece is subject to division into measured segments. During the entire machining process, it is necessary to cool the workpiece with water.

To ensure the strength of a welded square profile, it is critical to have an even and high-quality weld. It must be checked for leaks. In addition, to complete the square production process, it is necessary to carry out an eddy current flaw detection method. With its help, possible defects are detected:

- internal sinks.
- places of increased stress in the weld.

The final step in checking the quality of finished products is a visual inspection. Profiles with irregularities and other mechanical damage are subject to rejection. Reducing the number of defects contributes to the bulkhead pull rolls.

The steel profile production line includes the following units:

- cantilever decoiler for metal sheet processing.
- molding machine.
- welding unit and machine for cutting the edge of the seam.
- cooling area.
- profiling mill.
- cutting knife.

The line for the production of profiled pipes can be automatic and semi-automatic. Shafts on standard lines are rectangular.

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