Steel structures are a kind of structure, the main materials used to withstand their forces and transfer is steel. Joints used in steel structures are of weld, rivet or bolt and nut type, and depending on the type of joints of the parts and the corresponding controls are performed on them. Steel is currently one of the most important materials for the construction of building and bridge and other fixed structures. The strength of the steel (tensile stress) used in the range of 2400 to 7000 kg / cm2 is used for conventional buildings of steel with a resistance of 2400, which is referred to as “mild steel”.
In steel, generally about 3% of carbon and other impurities such as phosphorus, sulfur, oxygen and nitrogen, and other several materials are available. Steel making involves oxidizing and separating the additional and unnecessary available elements in the blast furnace product and adding the needed elements to produce the desired compound. Four different methods are used to make steel.
- Open hearth method
- Oxygen blowing method
- Electric furnace method
- Vacuum method
Steel as a material with unique specifications has been used long time in the construction of buildings. The ability to run precisely, the specific structural behavior, the ratio of resistance to the proper weight, along with the possibility of quick execution of steel structures with the details and delicacy of the architecture, has provided steel as a unique and inexpensive item in construction projects. So that if the limited weaknesses of this material, such as low resistance to corrosion and non-resistance to severe fires, are properly considered and controlled, they provide a wide range of possibilities to the designer that cannot be achieved in any other material.
Economic justification of steel structures
In the economic assessment of a steel building, only considering the cost of building materials and manpower is not enough and the rest of the factors affecting this issue should be considered. The following are effective in economy of a building:
– Land price: Due to the small size of the cross sections in steel buildings, less space is occupied by the structure of the frame skeleton and, compared to concrete structures; metal buildings in the plan are more effective. Therefore, the land cost per useful square meter of building will be lower in metal buildings.
– Available materials
– The final value of the building: The shorter the duration of a building construction, the lower the cost of the building. Due to different construction methods, we find that metal structures are less time consuming than other methods.
- The cost of the main frame skeleton of structure (carcassing)
- The effect of finishing
- The effect of installation of equipment and facilities
- How these factors affect the optimal utilization of the building
- The cost of making internal changes and bonification of the building
- The cost of destruction (in buildings with a short life span)
Investigation of steel consumption in steel structure buildings
In steel structure buildings, the cost is calculated according to the amount of steel used per square meter of floor area (horizontal image) or cubic meter of building. The cost of construction and steel consuming depends on the following factors:
- Number of floors
- Applied load to the floors ( dead and alive )
- Spans around the column
- The thickness of the ceiling
- Structural system (vertical and side load transmission system)
Load transfer in steel structures
The steel structure consists of a number of beams and columns in the form of a frame and also includes a number of amplifiers, in order to be more stable. Obviously, horizontal and vertical loads are transmitted through these components. In this way:
- The roof tolerates vertical loads, and transmits it through the beams to the beam abutments horizontally.
- The vertical bearing system (columns) transfers the loads from the two-head beam abutments to the foundation.
- Also, vertical and horizontal bracing systems transmit side-loads consequent of wind, earthquake, earth pressure, etc. to the foundations.
The nature of the load transfer through the beams to the abutments and the method of placing the beams (framework) depend on the following factors:
- Type of cross section usable according to architectural design
- The spacing of the abutments and the length of the span of the beam according to the design of the structures
- Load transfer method by load bearing components
- Selected abutment system (rigid, semi rigid, simple)
Fittings in steel structures:
The more ductile the structure is, the more energy it absorbs during the earthquake and the more favorable behavior it has. Mild steel is a suitable material due to its ductile nature and can absorb a large amount of energy. But experience has shown that in steel structures, if they do not use appropriate joints, their proper seismic performance will not be acceptable and will be damaged or destroyed by the earthquake.
Bracing or bracing systems in steel structures
According to the fact that the superconductors created or did not complete the triangulation, we divide into two groups:
- A) Frames without eccentricity bracing:
In these frames, the axis of all members is in a knot and has a vanishing point.
- B) Frames with eccentricity brackets:
With the eccentricity in steel structures, there are advantages in providing structural shaping and a better coefficient of confidence in the behavior of structures during an earthquake.
The first group frames are more effective than the second group in terms of strength and rigidity, but recent research has shown that areas where the high degree of ductility for the system in the periodic loads (earthquake mode) is being considered going back and forth, second category frames will have more advantage.
Most steel construction is made of steel type called mild steel. Mild steel is a very strong material. Take a circular bar of 1 inch / 25 mm steel diameter if you need to safely add this tape to your roof, you can hang up to 20,000 kilograms (20 tons) or any of the following.
This superb power has great benefits to buildings. Another important feature of the steel frame is its flexibility. It can be bent without cracking, which is another great advantage, as a steel building can be flexible when pushed by one side by catching, winding, or an earthquake. The third characteristic of steel is plastic or flexible. This means that when applied to a large force, it is not suddenly like a glass, but slowly bends from the shape. This feature allows steel buildings to become out of shape or to deform, resulting failure in steel frames is not suddenly collapse – the steel structure rarely collapses. In most cases, steel is much better in earthquakes due to these features.
Definition of metal column in steel structures
The column is a member which usually installed vertically in the building, and the floor loads consequent of the floors are transmitted by the beam and girder to it then transferred to the ground.
Column shapes in steel structures
The shape of the cross-section of the columns usually depends on the amount and condition of imposed load. All kinds of profiles and sheets are used to make metal columns. Generally, the columns are divided into two groups in terms of appearance:
- Rolling profile (profile) includes all types of beams and boxes: The best rolled profile for columns is wide beam, or square-shaped boxes because it works better than other sections for resistance. In most cases, the joints of the beams are easily applied to them.
- Composite sections: If the cross-section and characteristics of a profile (profile) are not enough to stand (imposed load tolerance and possible anchor) is not enough for a column, by multi-profile connection to each other, its proper column (composite sections) will be made.
How to create columns (compound sections):
Columns may be made according to the need for various combinations and connections of different types of profiles, but the most common connection for building columns is three types:
1- Connect two profiles to each other by way of doubling
First, stick two iron bars together and flat on each other. Then the two ends and middle of the column are welded and the column is returned and made welding like before. Then the reverse column is welded in the middle part. Do the same on the other side of the column and continue to weld in order to provide the required welding for column. This welding method is used to prevent the pivoting of the column due to the high temperature of the continuous welding. If there is no need for welding throughout the column, at least the welds should be executed in this way:
(A) The maximum distance between the weld lengths along the column lengths shall not exceed 60 cm non-continuously.
- B) The length of the initial and final welds of the column should be equal to the largest section width and should be done in altogether.
- C) The effective length of each piece of discontinuous welding should not be less than 4 times the size of the welding or 40 mm less.
- D) The contact between the bodies of the two profiles should not be more than a 5.1 mm gap, but less than 6 mm. Meanwhile, technical surveys show that there is not enough space to bearing. In that case, this gap should be filled with suitable filler materials including fixed-thickness steel blades.
2- Connection of two profiles with a universal sheet on the wings
In compound sections, the bonding plate is connected to the two profiles to form compound sections. The distance of the section welds (non-continuous) that connects the sheet to the profiles should not be greater than 30 centimeters. The maximum distance of the aforementioned distance in the case of ordinary steel is t22, t is the thickness of the sheet.
- Connection of two profiles with metal fasteners (belt): The most common type of column in Iran is the composite columns, which are arranged by two beams at a specified distance, and connect the horizontal or the left and right sides of the two profiles. Of course, the left and right fasteners that produce triangular shapes have better resistance to batten plates. In the case of such columns, in particular the column batten plate, the following should be observed:
- A) The horizontal fastener (splice) size of the column is less than the following values:
L: The length of the patch is at least distance of center to the center of the two profiles.
B: The width of the patch should not be less than 42 percent of its length.
T: The patch thickness should not be shorter than 35.1 of its length.
- B) Around the all patches and at the contact surface to the profiles flange, the welding action should be taken (the total length of the weld line on each side of the plate should not be less than the length of the plate).
- C) The distance between plates and its dimensions is determined on the basis of technical calculations.
- D) At the end of the column, be sure to use a sheet with a minimum length equal to the width of the column, in addition to the base reinforcement, a suitable place is available to attach the braces to the column.
- E) At the location of the beam or bridge to the column, it is necessary to have a reinforced sheet of sufficient dimensions on the flanges of the column already boiled.
Installation method of the angle on the columns floor (base plate) for the placement of the column
When calculating the dimensions of the flooring of the columns, at least the distance of the bollard from the edge of the column floor and the insertion point of the insert with the thickness of the weld required to hold the column, as well as the thickness of the end plate of the column and the dimensions of the column are carefully checked, then, according to the foregoing, The installation of the corners and the placement of the column was done in this way. On the bases of the plates we control the location of the column and the location of the ox. Welds the connection cores perpendicularly to the base plate, then install the column and install other cores and weld them to the base plate. Advantages of the perpendicularity of the two angles on the base plate In addition to the speed of operation and better positioning due to the direct contact of the pillar to the wing, the connection of the welding is done more correctly and more rigorously. It is clear that before welding, the pivots must be axially aligned and vertically aligned in two directions. After installing the columns, depending on the height of the column and the freedom of the column head, it is possible that, until the bridges are installed, the columns will be driven by wind speed and weight, which may have an unpleasant effect and cause weakness in the welding and fitting of the floor posts. For this reason, immediately after installation, the interlocking of the columns was made immediately by means of a rebar or a corner. Steel structures, Steel structures, Steel structures, Steel structures
Steel or Concrete Structures?
Nowadays most engineering buildings are made of robust construction materials, namely concrete and steel, or a combination of both of them. Further comparisons of these two materials are presented in determinant cases.
Advantages of the steel system
High strength: Steel excels in tolerance of any types of stresses such as bending, compression and tensile compared to reinforced concrete. This superiority of material strength causes the dimensions of the structural members to be reduced. Concrete is inherently weak in tolerance of tensile and bending, and therefore it requires reinforcement with steel.
Ductility and seismic performance: Ductility is inherent in steel materials and seismic performance of steel is better than concrete. This matter in resistance against extreme forces such as earthquakes, help absorb energy and maintain the stability of the structure. Of course, with the preservation of details and special measures, the ductility of the concrete members can be increased to the desired values. On the other hand, in general, the continuity and cohesion of concrete structures are more than steel structures, but with proper details consideration in the joints, desired coherence can also be achieved in steel structures.
Frame Skeleton volume and weight: The main advantage of the steel structure is that of a skeletal structure with less weight and less volume of its members. The large volume of concrete allows for the use of free space in the building and limits the need for further spacing between joints. Particularly in relatively high buildings, large dimensions of structural components, the use of spaces with problems, and in addition to the large dimensions of beams and columns, the weight of the structure and thereby the forces of the earthquake will significantly increase.
Improve long-term behavior: Because of the high elasticity modulus of steel and the lack of time effectiveness on structural shapes deformations, the control of deformations in these structures is simpler and more explicit than reinforced concrete structures. Deformation of concrete sections is under long-term flexural loads, and due to the heavy load of dead roofs in residential projects, these deformations are time-consuming and have adverse effects.
Terms and Conditions of Execution: The implementation of steel skeletons for Specialist and skilled personnel at the site needs less and allows the use of prefabricated pieces that are produced in the factory. This is very important in increasing the quality, speed, and uniformity of the run. While in the concrete skeleton frame, the work required at the site works more as a result of the need for precise planning and possible disruption of the way and time of the project.
Compensation payment and restoration of the structure: In cases of natural disasters such as earthquakes, in case of damage to the skeleton frame, in concrete skeleton frame the repair of damaged members is a complicated problem, while it is easier to repair and restoration of steel. It is also possible to make minor changes during work in steel structures than concrete structures.
Execution time: The execution time of the steel skeleton frame is considerably less because of the possibility of using pre-construction, light weight components and ease of provision of operational features to concrete skeleton frame. Concrete skeleton frame require more time to execute due to more stages of execution operations, such as framework, reinforcement, concreting, molding and processing time.
Restoration of cracks in metal structures
Steel parts and structures used on bridges may be cracked due to the passage of heavy trains, which require restoration due to the difficulty of replacing parts in case of damage. The crack starts from the focus of the stress concentration and grows that must stop. One of the methods used to stop cracks used in steel bridges is to use stopper holes. By doing this, the stress around the tip is reduced by 20 to 30 percent and is considered a good countermeasure. The diameter of the stopper holes in the steel bridges is 0-15 percent of the crack length and a maximum of 15 mm is proposed. Also, after the boring the end of the crack, the crack length can be restored using welding and filling materials.
Aluminum alloy constituents, galvanized steel and stainless steels. Therefore, the repair of cracks is different in terms of drilling. To stop cracks that appear in the frame door of the aircraft and helicopter, and their length is less than 2.5 centimeters; holes are created at the end of crack and restored by attaching the reinforcement plates.