Steps of steel structure manufacturing in the factory

Steps of construction of steel structure buildings

Assume we want to execute a steel structure project, the first step of execute contains a building proper foundation, almost same in all projects, but before explaining of building a foundation it should be noted first, we must set up foundation plans on ground and for setting up accurately, the necessary details must be specified in the plans. Including structure divided in the forms of a grid consisting of perpendicular axes and given location of related axes to other axes or specific points like road, neighbor buildings on ground and so on. (Normally axes of a line are marked with numbers like 1,2,3, … and the other line of axes are marked with the letters like A, B, C, … .  It should also be noted that columns and foundations have similar circumstances in terms of load, shown by similar marks: column with letter C and foundation with letter F are shown. Drawing sections and writing numbers of top of foundation, bottom of foundation, height of different parts of foundation, specification of lean concrete, specification of concrete, type and total diagrams needed to cut rebars must be shown on plans. Before setting up on the ground if the ground is uneven or there are plants and trees, the desired high alignment points must be removed and the area must be cleaned from trees and plants. Then we align the geographic north on the plan with the north geography direction in which the project is to be implemented (It’s called توجیه نقشه ) after that, implement one of the axis ( longitudinal or transverse axis ) location marked on plan, on the ground we land at least two nails at the beginning and end, which is called the base axis; now we implement other longitudinal and transverse axis ( with wooden or metal nails on ground ) with theodolite camera or for small projects with measure tape, work string, set square and plumb line . Now if we want to excavation foundation area, we need depth of excavation codes even the ground has a slight elevation, set a base point (B. m) around construction site (This point is made of concrete and rebar at a point away from a damage).

Tips of executive and technical excavation and زیر سازی foundation

Having basic information about the soil and soil type, such as: compressive strength of soil type, especially in terms of fall, groundwater status, glacial depth and other physical properties of the soil, which is determined by testing the soil, is very important. In the excavation of the foundation, during the execution of the underground, the wall may fall or the adjacent foundation may be emptied, which should be piled and the wall protected with different tools; In order to have sufficient resistance to the incoming loads, one of the solutions to prevent soil erosion and the foundation of the adjacent building is to perform the installation of the columns in the first place, and in the next stage, after gradual excavation, Other components of walling should be done.

Executive and technical tips of back filing and زیر سازی foundation

Abandoned wells are filled with suitable shafts and in case the place collides with the abandoned aqueduct, a composite or flat foundation should be used or the aqueduct should be covered with a protective concrete slab. Loam is not allowed for landfill. The thickness of the landfill crusts for density is 15 to 20 cm. To do compaction, we have to add some water to the soil and compact it with suitable rollers. Of course, landfill and compaction are only for landscaping and flooring, and landfill under the foundation is not allowed. In some cases, in order to protect the lean concrete substrate, we have to substructure the foundation, but the thickness of the substructure may be small (about 30 cm). In this case, it is possible to increase the thickness of the lean concrete. The height of the substructure can be done by observing the technical principles of rubble walling with cement mortar.

What is lean concrete?

It’s a concrete with low cement content under foundation also called cleaning concrete, it is usually poured to a thickness of 10 to 15 cm and 10fto 15 cm larger on each side than the concrete itself.

How is shuttering done?

Shuttering must be done with clear timber without knots with a thickness at least 2.5 cm or with flat metal sheets or pressed bricks (11 or 22 cm brick blade with cement plaster to prevent bleeding). It should be noted that ordinary foundations a plastic sheet (nylon) as form work can be used in the excavation wall.

NOTE: In reinforcement the distance between rebars and free surface of concrete in foundation it must not be less than 4 cm. Executive recommendations of rulebook in buildings with steel structures.

1. If the span length of truss or girder is more than 8 m, to compensate for the deformation due to dead loads, we must first give it a negative deflection (camber) or upward deformation. The value of deformation is determined by structural engineer.
2. To prevent corrosion of steel parts, the minimum thickness of structural components that are outdoors and exposed to atmospheric factors or other corrosive effects, should not be less than 6 mm. In dry environments and away from any corrosion effects, this value is reduced to 5 mm.
3. It is allowed to use local heating methods or mechanical deformation to create curvature or eliminate it (straightening the bend). The temperature of the heated locals should not exceed 565 ° C for special strong steels and 650 ° C for mild steels. It is not allowed to smooth the iron at high temperatures in such a way that the color of the place is under blue heat
4. 4- The edges that are cut by the flame (and in the future will be the place of entry of large tensile stresses) should be completely uniform and free of roughness of more than 5 mm. Roughness and scratches of more than 5 mm should be done by grinding and, if necessary, fix it by هموار welding; Also, flame-cut edges where the welding material will be placed should be as free of roughness and cuts as possible.
5. In  درز فشاریwhere the transmission of force through direct contact pressure forms part of the joint capacity, the surfaces of the parts in contact must be well prepared by turning, scraping, grinding and other suitable methods.
6. In lifting parts, especially long girders and trusses, special points that have been previously determined should be used with complete caution in order to prevent making high tension in the part.
7. In order to correct minor defects in construction, it is usually possible to use a low cut, impact or bend, but never use a cutting torch, especially to correct defects in the main parts that are usually under pressure. The use of a torch may be permitted in the repair of defects in the stringers which are not under pressure. In any case, after fixing the defect, cleaning the surfaces, especially the surfaces that are placed on top of each other, is necessary.
8. If deviations are observed in the final dimensions of the steel frame, if their values ​​are not more than the allowable deviations of the installation, the work done in the work row is considered good. In general, each installed component must be bent or aligned and positioned in the correct axis of diagnosis according to the plan, provided that its deviation is not more than 1/500.
9. There are always life hazards in the installation of metal parts; Therefore, all safety points, whether in terms of coverage or work, must be observed. Metal parts in the preliminary (temporary) installation must be connected with bolts or any possible means, so as to withstand the stresses of installation and maneuver of workers. Except in cases where adequate braces are permanently installed in the frame, temporary restraints and braces should always be provided until temporary safety is required and the steel frame has not reached its stability to avoid the risk of falling parts.

Advantages and disadvantages of metal buildings

The construction of the building has been done to meet the needs of human beings and engineers, architects are responsible for preparing the forms and proper execution of the building; The main axis of responsibility is:

A) Safety b) Beauty c) Economy

Considering that the buildings under construction in our country are mostly made of steel or concrete and unarmed buildings are built with special restrictions according to the 2800 Iranian Earthquake Regulations, familiarity with the advantages and disadvantages of buildings can play an essential role in the decision of owners and engineers.

Advantages of steel building:

High strength: The strength of metal parts is high and the weight-to-weight ratio is higher than concrete materials, so in large openings of sheds and high-rise buildings, buildings that are located on loose ground, is very important.

Uniform properties: Steel is produced in large factories under close supervision, its uniform properties can be assured and its properties, unlike concrete, are not affected by external factors. Atmosphere causes the consumption of materials.

Durability: The durability of steel is very good, metal buildings that are carefully maintained. They will be usable for a long time.

Elastic properties: The assumed elastic properties of steel with a very good approximation are practical. Steel follows Hooke's law well under great stress. For example, the moment of inertia of a steel section can be entered with confidence in the calculation. However, in the case of concrete section, the relevant figures are not very definite and reliable.

Ductility: One of the positive properties of steel materials is their ductility, which is able to withstand the stresses that actually cause the onset of failure and withstand dynamic and impact forces, while brittle concrete materials are extremely weak against these forces. One of the factors that informs the member during the breakdown and prevents sudden breakdown and its dangers.

Consistency of materials: Steel parts are continuous and homogeneous due to their constituent materials, but in concrete parts, damage in each earthquake causes damage to the concrete cover on the rebar weapon. Cracks that appear in the concrete cover are not controllable and probably" The building will be further weakened and destroyed in the next aftershock or earthquake.

Equilibrium strength of materials, strength: Steel materials are the same in tension and pressure and in shear are good and close to tension and pressure. They are small, at the predicted loads, they are subjected to torsion and consequently to the resulting shear. In reinforced concrete buildings, the concrete has good compressive strength, but low tensile or shear strength. So, if areas are likely to be under traction and unarmed, it will produce cracks and damage.

Explosion: In buildings with loads borne by the structure of the building, filler components such as blades and walls are not used. The destructive force of the explosion separates the retaining surfaces from the skeleton and the destructive energy is revealed, but the building as a whole will not be destroyed. In reinforced concrete buildings, the collapse of the walls will cause the destruction of the building.

Reinforcement and the possibility of reinforcement: Weak members of a steel building due to incorrect calculations, changes in rules and regulations, execution, etc. can be reinforced by welding or riveting or screwing new parts, or adding parts or openings.

Easy conditions of construction and installation: Preparation of steel parts in factories and installation in a location, different weather conditions with the necessary arrangements can be implemented.

Installation speed: The installation speed of metal parts takes less time than the execution of concrete parts.

Disposal of materials: Due to the supply of parts from factories, the disposal of materials is less than the preparation and use of concrete.

Low weight: * The average weight of a steel building can be estimated between 245 to 390 kg / m2 or between 80 to 128 kg / m3, while in reinforced concrete buildings these figures are between 480 to 780 kg / m2 or 160 to 250 Kg per cubic meter, respectively.

Occupancy of space: In two buildings equal in height and dimensions, columns and beams of metal buildings are smaller in terms of dimensions than concrete buildings, the level of occupation or dead space is more in concrete buildings.

Seismic force coefficient: The movement of the earth due to an earthquake causes the application of internal forces in the components of the building, in other words, the building on the ground that is vibrating randomly and in homogeneously, must be static and withstand the vibration of the earth. In reinforced concrete frames that weigh more, the coefficient of seismic force is higher than metal frames. Experience shows that the damage to short and rigid buildings built on solid ground is great. While in tall and flexible buildings, those built on soft ground suffered more earthquake damage. In other words, on soft ground where the ground vibration period is relatively large, short buildings give better results, and conversely on hard ground with a short period, tall buildings are less likely to fail.

The reaction of buildings to earthquake motion depends on the characteristics of the building itself in terms of rigidity or flexibility, and the most important characteristic of the building in its behavior against earthquakes is the natural period of vibration of the building.

Disadvantages of metal buildings:

Weakness at high temperatures: The strength of a metal building decreases with increasing temperature. If the steel frame deck reaches 500 to 600 ° C, the balance of the building is compromised.

Corrosion and decay of metal against external factors: Parts used in metal buildings are corroded against atmospheric factors and its dimensions are reduced and maintenance and protection costs are high.

Tendency of buckling parts to buckle: Due to the fact that steel parts are large and the dimensions used are usually small, the tendency of buckling in these parts is considered a weakness.

Improper welding: In steel buildings, the parts are connected to each other by welding, rivets, screws. The use of bolts and nuts, the manufacture of parts in factories is the most economical, most technical work that in our country for conventional buildings such facilities are not available. Welding connection is the biggest weakness due to lack of skills of welders, use of old machines, lack of precise control by supervising engineers, high cost of welding tests, etc.

Experience has shown that sheds built in factories do not have this defect if they comply with technical and standard specifications and have better structural resistance to incoming loads and earthquake force.

Sources:

1- Concrete and steel concrete, Dr. Shamsuddin Mojabi

2- Behavior and seismic design of reinforced concrete and steel buildings, Abbas Tasnimi

3- Static design and calculations - Arg Magardichian

4- Regulation 2800 and Iranian concrete

5- Steel structures, Shapoor Tahouni

6- Websites