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The most common methods of building construction around the world are three methods of building with masonry materials, building with steel frame and building with a concrete frame. Frameless structures are commonly used in low-rise buildings that are not of great importance. In the past, steel frame construction was more common in Iran, but today, the concrete frame has found a very good position in the construction industry. The choice of steel structure or concrete structure depends on many determining factors, the most important of which are mentioned below:

• The Geographical area of ​​the construction site of the structure
• The required strength of the structure
• The Time required for project delivery
• Budget required for the project

Steel structure vs concrete structure

Now we will examine different items in metal structures and concrete structure and express the advantages and disadvantages of metal structure and concrete structure:

Project construction site

One of the most important factors in choosing the type of structure is the construction site of the project. For example, in wet weather conditions and exposed to frequent rains, the metal structure is unsuitable because it is prone to rust and corrosion. In these areas, the concrete frame is usually more durable. On the other hand, in areas where sulfate ions are present, the bar in the concrete is exposed to attack. In addition, the condition of the faults in that area and the performance of the structure in the earthquake is also an effective factor in choosing the type of structure. Also, another important factor about the project site is the supply of materials required for the structure on site. For example, the proximity of the project site to the aggregate mines or concrete plant or the place of supply of iron tools, reduces the cost of transportation of materials and is economically effective in choosing the type of structure.

Structural strength

To choose a suitable frame for the building, we must first determine the type of operation of the building and its degree of importance. Comparing the strength of steel structures and concrete structures, one can not be described as more durable than the other because if the structures is properly designed and executed well, both frames must be able to withstand the forces. However, in the case of seismic force, the weight of the structure is one of the factors that aggravate this force, and since the weight of the concrete frame is much heavier than the weight of the steel frame, a stronger seismic force enters the concrete frame (The weight of the steel frame structure is about 250 to 390 kg/m² and the weight of the concrete frame structure is about 480 to 780 kg/m²). On the other hand, the high rigidity of the concrete causes no vibration in the concrete structure when the wind blows and strong shocks from the upper floors, and the residents of the concrete structure feel more relaxed than the steel structure.

In general, it can be said that the ratio of the strength of steel to its weight is larger compared to concrete. Another point is that the resistance of steel to tensile, compressive and shear forces is almost the same, and in reciprocating forces, tensile and compressive forces can be converted, and this makes the metal structure react well. But in concrete, resistance is only good in compressive forces, and concrete has poor performance in tensile and shear forces, and if it is not reinforced with bar, it can be easily cracked and destroyed. In addition, steel withstands dynamic and impact forces due to the homogeneity of materials and high ductility, while concrete is brittle due to its heterogeneous structure and in the face of these forces and explosions, the structure cracks and its performance is disrupted.

Another advantage of the steel structure vs concrete structure is that with increasing life, the strength in the steel structure does not decrease, but the concrete over time will suffer from creeping and cracking, which will damage the performance of the structure and reduce its strength. Also, in metal buildings, it is possible to develop, repair, and strengthen the members during construction and after construction. Of course, the disadvantage of steel in strength is the buckling of pressure components over time. Meanwhile, steel loses its resistance to high temperatures caused by fire and collapses at a temperature above 600 °C, while concrete at a temperature above 1000 °C shows the resistance about 1 to 6 hours.

Compare Project execution time in steel structure and concrete structure

If the speed of completion of the project is a priority for us, metal structure is a more suitable option than concrete structure because the concrete structure must be executed floor by floor and we must consider the time required for framing, piling and initial compacting of concrete. However, in the steel structure, it is possible to execute all the classes simultaneously with each other at the same time. Even if the precast steel frame is prepared from the factory and assembled at the project site, the execution speed will be higher than normal.

Also, one of the important advantages of steel structure vs concrete structure after operation in the future is the possibility of developing the structure, which means that if the employer or owners of the building structure want to add a floor to the building, steel structures has the important ability until by connecting columns to pre-executed columns by a screw or welded connection and connection of beams and so on developed the structure. Another case is the limitations on the execution of concrete structures in hot and cold weather conditions, which slows down the execution of work and even stops work in cases where the metal structure is an exception to these limitations.

Compare Project cost in steel structure and concrete structure

Since steel consumption is higher in steel frame vs concrete frame, the cost of construction of steel structure is much higher than the concrete structures and concrete frame is a more cost-effective option. According to the statistics and estimates, the construction cost of 1 m² concrete ceiling and columns in January 2018 was from 131 to 166 dollars, while the construction cost of 1 m² metal ceiling and columns was from 143 to 190 dollars. In addition to the cost of the bar, cement and molds should be gradually injected as the project progresses. But the cost of buying steel for a steel structure must be met early in the project, which puts more pressure on the employer in terms of economics and funding. Due to the fact that in our country, a home loan is granted to each floor after the completion of the ceiling, if the concrete structure is executed after the completion of each ceiling, the relevant loan can be received.

Another issue in comparing the cost of these steel structure and concrete structure is the issue of post-construction care and maintenance, where the concrete structure does not require special care and has a long service life, but a metal building needs additional costs to care for corrosion. However, if the steel is taken care of, its durability and service life of the concrete structure will increase. The positive points of the steel structure in the cost topic, one is the reduction of material waste (due to the manufacture of iron tools in the factory) and the consequent reduction of costs in the steel structure. Also, the remaining iron tools after the demolition of a steel structure have a relative value, while from the demolition of a concrete structure, only construction debris remains, and at the same time, the demolition of a concrete structure is more difficult and more expensive than a steel structure.

Space occupation and architectural interference

The concrete structure is heavier and bulkier than metal structure, and due to the large dimensions of the beams and columns in the concrete structure, we have more dead space in the building. In other words, in comparison with two identical concrete building and metal buildings, we will have less useful and usable space in the concrete structure, which causes architectural limitations, especially in the topic of providing parking lots. Also, in large openings, the height of the beams increases (due to the prevention of deformation and buckling), which reduces the useful height of the floors. Another point is that in metal structure there is no need to install a dropped ceiling, but in concrete structures we usually have to install a dropped ceiling. The positive point about concrete is the variety of concrete sections and the possibility of creating members with different sections.

Certainty, reliability and quality control

In the calculations of concrete structures, the numbers are obtained approximately by simplifying the equations, but in the metal structure, due to the homogeneity of the steel, the computational numbers have a high certainty compared to the practical values. Concrete is usually prepared at the project site and has less quality control, but steel is produced in the factory with high supervision and accuracy, and we consider a better reliability for steel, which saves materials.

The advantages and disadvantages of steel structur and concrete structure can be summarized as follows:

Advantages of concrete structures

• Lower construction and maintenance costs
• More resistance to humidity
• High resistance to fire and heat
• Ability to execute at different sections
• Availability and abundance of materials such as reinforced concrete, cement and bar
• It is easier to connect the beam and the ceiling due to the homogeneity of the materials
• Requiring less knowledge and expertise in design
• No need to weld or install bolts
• More suitable connections of columns and diaphragm to the ceiling
• Proper stability and durability of flexural frames

Disadvantages of concrete structures

• Need to spend more time reinforcement and concrete formwork
• Need soil laboratories to test concrete
• Increasing weight compared to metal structures
• Decreasing strength and stability of the structure over time
• Requiring dropped ceiling
• Limiting space
• More waste of materials
• More pressure during an earthquake due to the heavier weight of the structure
• The possibility of work being delayed or prolonged in the event of climate change such as rain
• The worthlessness of the debris from the demolition of the building

Advantages of steel structures

• The strength of the structure does not change over time and has a long life.
• The execution of floors can be done simultaneously and therefore the speed of execution of this type of structure is high.
• The presence of precast steel frames makes the construction of steel structure easier.
• Metal structures have high flexibility.
• Easier to repair and eliminate possible damage to the structural frame
• No need for specialized and highly experienced people
• Possibility of creating precast buildings and structures
• Less weight
• More efficient use of space
• More stability and resistance to severe earthquakes
• No need for a soil laboratory
• Ability to use to build sheds and other large or small buildings
• Easier demolition
• The initial calculations are closer to reality
• Ability to make changes or add new sections after creating floors
• Less construction time limitation during severe weather changes (snow and rain)
• Less waste of materials
• No need for a dropped ceiling
• Value of iron tools from building demolition

Disadvantages of steel structures

• There is a possibility of oxidation and rust of metal structures.
• They are not resistant to heat and deform in case of fire, which can be associated with irreparable damage (Like the Plasco building accident in 2016)
• More cost
• Possibility of corrosion of metal against various factors
• Requiring precision welding or installation of standard bolts
• Need to test for welding and how expensive it is
• In the absence of proper connections, they will suffer more damage in earthquake situations than concrete structures.
• The need for careful control and inspection of welding by the supervising engineer
• The tendency of the steel frame to bend naturally

Comparison of resistance of concrete structures and metal structures against earthquake

Earthquakes are among the natural disasters that cause many human and financial losses and cause huge damages every year. Buildings that do not have the necessary resistance to this accident are destroyed and endanger the lives of residents, so in the topic of construction, special attention is paid to the reinforcing of buildings, especially in earthquake-prone areas. Today’s buildings are made of steel and concrete and their strength against earthquakes is high. In the construction of metal buildings for beams and columns, steel profiles are used. Metal has properties such as long life, tensile strength and ideal compressive strength and plays an important role in increasing the strength of the building. Metal components are connected to each other by welding or bolts. Concrete buildings, as their name suggests, are made of reinforced concrete. These buildings also have sufficient resistance to earthquakes.

When building a home, whether it is a steel frame or concrete frame, you should consider the following points so that your building is safe against earthquakes and does not lead to human and financial risks. These points are as follows:

• Soil compaction should be checked before the foundation.
• After the excavation of the construction site, the allowable soil strength should be ensured before the foundation.
• In case of concrete structure, the roots of the columns should be executed before concreting and according to the map. The concrete used for this area must be compacted.
• The distance between the reinforcements and the molds must be observed.
• If, after concreting, the foundation reinforcement is visible, there is a defect in the work and it must be corrected.
• Do not do more than what is stated in the structural map to strengthen the beams and bridges because increasing their dimensions will have the opposite effect.
• During excavation, devices and equipment should not collide with adjacent buildings and should not be excavated in any way.
• Buildings with 4 floors or more should be spaced from adjacent buildings. This distance minimizes the damage caused by the impact of an adjacent building.
• The building should have a regular plan and no recesses or protrusions should be observed in it.
• The weight of the building should be placed on the lower levels as much as possible. For example, placing water sources on the upper floors of a building should be avoided.
• The components used for the facade of the building should not be detached during an earthquake and should have the necessary stability.

If the structure of metal and concrete buildings is built according to the required standards, it can withstand 7 magnitude earthquakes. Any concrete or metal system can be designed and executed with different resistances to be earthquake resistant and not damaged. Concrete structures can withstand more force due to their high weight. Of course, it should be noted that the lifespan of a concrete structure is 50 to 60 years, and these buildings will fluctuate over time and increase the lifespan, so they may be damaged by earthquakes. Unlike these structures, the strength of steel frame structures does not decrease with increasing life. Steel does not have high fire resistance and its resistance decreases if the temperature increases. In contrast, concrete structures have a high fire resistance. In addition, the execution of steel structures requires high accuracy and sensitivity due to the sensitivity of operations such as cutting, welding and transportation compared to concrete structures.

A strong earthquake puts vertical and lateral loads on a structure. Lateral forces shake the structure horizontally and cause damage to it. Therefore, if the structure does not have the necessary lateral resistance, it will collapse and suffer irreparable damage. It is very important that the different parts of the structure are connected in such a way that the kinetic force cannot affect it. In fact, this force must be transmitted to the ground in a continuous path. If there is no proper connection between the components, in the event of an earthquake, each of them will move separately, resulting in the collapse of the building. If only one or more components of the structure are strengthened, in case of damage to them, the whole structure will collapse because the necessary strength is not provided for the whole structure.

Ductility is also one of the structural features that include structural components. These components will not be damaged if the additional load is applied to them. In fact, the design of these components should be such that it has the necessary resistance to unexpected loads. The strength of mechanical, electrical, or other structures must be high to reduce damage in the event of a severe earthquake. Sturdy parts with proper connections strengthen the structure and make it ideal for severe earthquake vibration. Of course, if the foundation of the building can withstand the weight of the structure without settlement and has the capacity to transfer the forces to the ground, it can maintain its strength against earthquakes. A stable foundation does not move in the structures on which it relies and does not cause it to vibrate or fall.

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