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What is Steel Structure

A steel structure is made of prefabricated materials (steel), processed into components in the factory, and then assembled on site. Therefore, the connection nodes between components are essential to forming the steel structure and ensuring its safe and regular operation.

• Structurally, steel is used as a material.
• Structural steel is used as the main load-bearing component, not for decoration or other purposes.
• When the substructure or local components of the engineering structure use structural steel as the main load-bearing component, it can also be called a steel structure. For example, the stadium stands are concrete structures, while the cantilevered canopy is steel.

Types of Structure Steel

1. Portal frame;

2. Frame structure – pure frame, central support frame, eccentric support frame, frame tube (dense column frame);

3. Grid structure – grid, grid shell;

4. Cable membrane structure – cable suspension and membrane structure, including tensioned, skeleton, and inflatable.

Steel structures are widely used in buildings, mainly in the following aspects:

1. High-rise buildings

Structure Steel has the advantages of high strength, lightweight, and earthquake resistance, so they are widely used in high-rise buildings. Compared with concrete structures, steel structures can reduce self-weight and foundation costs. Many super high-rise buildings worldwide, such as Burj Khalifa and Shanghai Tower, use steel structures or steel-concrete hybrid structures.

2. Bridge

Bridges with rigid frames are the primary load-bearing structures. The beams and legs or piers (platforms) form a rigid connection. Due to the consolidation of the piers and beams, the beams and piers are subjected to force as a whole. The piers bear the vertical pressure caused by the load on the beams, the bending moment, and horizontal thrust. The famous Golden Gate Bridge and Tower Bridge in London use steel structures.

3. Industrial buildings

Structure steel, such as plants and warehouses, is mainly used in industrial buildings. They have a short construction period and a large span. A steel frame can be 36 meters without a central column, which meets the layout of indoor production equipment and improves space utilization.

4. Steel structure housing

Steel housing is mainly used in countries and regions with frequent earthquakes. It provides better safety protection and offers simple construction and environmental protection advantages compared with traditional brick-concrete structures.

Advantages of steel structures:

Lightweight and high strength, high strength per unit mass of materials
Good plasticity and toughness, seismic performance, large deformation without fracture, light structure, and material toughness.
Good material uniformity, clear constitutive relationship, close to the ideal isotropic elastic-plastic body, consistent with the assumptions of mechanical calculations.
The high degree of industrialization and a short construction period.
Good sealing.
With good heat resistance below 250℃, the strength and stiffness of the steel structure change very little.

Disadvantages of steel structure:

Material price is higher than traditional concrete
Poor corrosion resistance
Poor fire resistance and the use of anti-corrosion and fire-retardant coatings increase the cost of steel structures.
Prominent stability issues, such as axial compression members, bending members, and compression bending members, are prone to overall instability and local instability.

Common problems in steel structure design:

Steel structure design directly affects the safety and stability of the structure. Many factors will affect the structural design, so it is necessary to start from multiple angles to do an excellent job in steel structure design.

1. After getting the building drawings, do not blindly model and calculate. First, conduct a comprehensive analysis, communicate with the architect, and fully understand the various situations of the project.
2. Preprocessing work before modeling and calculation. For example, loads should be calculated accurately and cannot be estimated. Enter it entirely following construction practice or use requirements.
3. When modeling the building structure, you must understand the meaning of each parameter, not modify the parameters blindly, and have a basis for modification.
4. The calculation should fully consider the economy under technical conditions. The component’s cross-section shall not be increased at will.

Property of steel structure:

The properties of steel structures are significant in construction. They mainly involve strength, plasticity, cold bending performance, impact toughness, welding, and durability. Steel’s chemical composition directly affects these properties.

The following are the specific effects of common elements on the property of steel structures:

1. Strength

The strength of steel is one of the most essential properties in steel structure design. It is usually measured by yield strength (σy) and tensile strength (σu). Steel structure design is based on the yield strength of steel. High yield strength can reduce the dead weight of the structure, save steel, and reduce construction costs.

As steel’s carbon content increases, the yield point and tensile strength increase, but the plasticity and impact resistance decrease.

When the carbon content exceeds 0.23%, steel’s welding performance deteriorates. Therefore, the carbon content of low-alloy structural steel used for welding generally does not exceed 0.20%. High carbon content will also reduce the steel’s resistance to atmospheric corrosion. High-carbon steel in open-air material yards is prone to rust; carbon can also increase steel’s cold brittleness and aging sensitivity.

2. Plasticity

Steel’s plasticity generally refers to its property of having significant plastic deformation without breaking after stress exceeds the yield point. The leading indicators for measuring steel’s plastic deformation capacity are elongation δ and cross-sectional shrinkage ψ.

3. Cold bending performance

The cold bending performance of steel measures its resistance to cracking when bent at room temperature to produce plastic deformation. Cold bending tests are used to test the bending deformation performance of steel under a specified bending degree.

4. Impact toughness

The impact toughness of steel refers to its ability to absorb mechanical kinetic energy during the fracture process under impact load. It is a mechanical property that measures the resistance of steel to impact load, which may cause brittle fracture due to low temperature and stress concentration. The impact toughness index of steel is generally obtained through impact tests of standard specimens.

Aluminum, titanium, vanadium, niobium. Aluminum, titanium, vanadium, niobium, and other elements are beneficial elements in steel. They are all strong deoxidizers during steelmaking and are also commonly used alloying elements. Adding these elements in appropriate amounts can improve the structure of steel, refine the grains, significantly increase its strength, and improve its toughness.

5. Welding performance

Steel’s welding performance refers to its ability to obtain a good welding joint under certain welding process conditions. Welding performance can be divided into welding performance during welding and welding performance in terms of use.

6. Durability

Many factors affect steel’s durability. The first is its poor corrosion resistance; protective measures must be taken to prevent it from corroding and rusting.

Effects of chemical elements on steel structure property

1. Carbon (C)
Carbon is the second-most important element after iron, and it directly affects steel’s strength, plasticity, toughness, and welding performance.

2. Silicon (Si)
Silicon is a deoxidizer, and its deoxidation effect is more robust than manganese. It is a beneficial element in steel. When the silicon content is low, it can improve the strength of steel but has no apparent effect on plasticity and toughness.

3. Manganese (Mn)
Manganese has a positive effect on carbon steel’s mechanical properties. It can improve steel’s hardness, strength, and wear resistance. Manganese content of less than 0.8% can significantly improve carbon steel’s yield and strength limits while maintaining its original plasticity and impact toughness.

4. Phosphorus (P)
Phosphorus can improve cutting performance and corrosion resistance so that the phosphorus content can be appropriately increased in free-cutting or weathering steel.

5. Aluminum (Al)
Aluminum has anti-oxidation and anti-corrosion properties. Aluminum, chromium, and silicon can significantly improve steel’s high-temperature non-scaling performance and corrosion resistance.

6. Titanium (Ti)
Titanium is a potent deoxidizer. It can make steel’s internal structure dense and refine the grains, reduce aging sensitivity and cold brittleness, and improve welding performance.

Connection methods of steel structure

Steel structures have three connection methods: weld, bolt, and rivet.

Weld connection

Weld connection involves using the heat generated by the arc to partially melt the welding rod and the weldment, cool them, and condense them into a weld, thereby connecting the weldments into one.

Advantages: It does not weaken the component’s cross-section, saves steel, has a simple structure, is easy to manufacture, has high connection stiffness, good sealing performance, is easy to use automated operations under certain conditions, and has high production efficiency.

Disadvantages: The heat-affected zone formed by the high welding temperature near the weld may make the material brittle in some parts.

During the welding process, the steel is subjected to unevenly distributed high temperatures and cooling, which causes welding residual stress and residual deformation in the structure. This specifically impacts the bearing capacity, stiffness, and performance.

Due to the high stiffness of the welded structure, local cracks can quickly expand to the whole structure once they occur, especially at low temperatures, which are prone to brittle fracture; the plasticity and toughness of the weld connection are poor, and defects may occur during welding, which reduces the fatigue strength.

Bolt connection

Bolt connections connect the connecting parts using fasteners such as bolts. They are divided into ordinary bolt connections and high-strength bolt connections.

Advantages: simple construction process, easy installation, especially suitable for installation and connection on-site, easy to disassemble, suitable for structures that need to be assembled and disassembled and temporary connections.

Disadvantages: holes need to be drilled on the plate and aligned during assembly, which increases the manufacturing workload and requires high manufacturing precision; bolt holes also weaken the cross-section of the component, and the connected parts often need to overlap each other or add auxiliary connecting plates (or angle steels), so the structure is more complicated and more steel is consumed.

Rivet connection

A rivet connection involves quickly inserting a rivet with a semicircular prefabricated nail head at one end into the nail hole of the connecting part after the nail rod is burned red and then riveting the other end into a nail head with a rivet gun to make the connection tight.

Advantages: Riveting is reliable for force transmission, has good plasticity and toughness, is easy to check and ensure quality, and can be used for heavy structures and structures that directly bear dynamic loads.

Disadvantages: The riveting process is complicated, and the manufacturing cost is labor-intensive, so it has been replaced by welding and high-strength bolt connections.