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What is Steel Structure

Steel structure is an engineering structure formed by processing, connecting, and installing steel plates, round steel, pipes, cables, and sections. It can withstand the influence of various natural and human factors, has high reliability and significant social and economic benefits, and is an essential structural form widely used in various engineering constructions.

Because steel can be recycled and reused, steel structures are energy-saving and environmentally-protecting and meet the economic needs of sustainable development.
The steel structure is widely used in high-rise buildings, large factories, large-span space structures, light steel structures, and residential buildings. In addition, steel structure also occupies an important position in many industries, such as highway and railway bridges, thermal power main plant and boiler steel frames, transmission, and transformation towers, radio and television communication towers, offshore oil platforms, nuclear power plants, wind power generation, water conservancy construction, underground foundation steel sheet piles, etc.

In urban construction, steel structures have been widely used in projects such as subways, urban light rail, overpasses, environmentally friendly buildings, and public facilities. At the same time, steel structures are also commonly used in small light structures such as supermarket shelves, scaffolding, square sketches, sculptures, and temporary exhibition halls.

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:

Long-span structure
As the span of the structure increases, the proportion of the structure’s deadweight in the total load also increases. Reducing deadweight can significantly improve economic benefits. For long-span structures, the lightweight advantage of steel structures is particularly prominent.

For example, the gymnasium in Shanghai, which can accommodate 80,000 people, adopts a spatial steel roof structure that combines radial cantilever lattices and circumferential trusses, with a long axis of 288.4 meters, a short axis of 274.4 meters, and a maximum cantilever span of 73.5 meters.

In addition, the main bridge of the Runyang Yangtze River Bridge, built in 2005, adopts a single-hole double-hinged steel box girder suspension bridge with a main span of 1,490 meters, setting a record of first in China and third in the world.

High-rise buildings
High-rise buildings have become one of the symbols of modern cities. The high strength of steel and the light weight of steel structures are significant for high-rise buildings. High strength makes the cross-sectional size of components smaller, increasing the effective use area; light weight can significantly reduce the load borne by the structure, foundation, and foundation and reduce the cost of foundation engineering.

Steel and steel-concrete hybrid structures account for over 80% of the world’s 50 tallest buildings. The New York Sears Tower (now known as the Wills Tower) is an all-steel structure building with 110 floors and a total height of 443 meters.

In recent years, China’s high-rise steel structures have sprung up like mushrooms after rain. In 1997, the Shanghai Jinmao Tower was completed with 88 floors and a total height of 420.5 meters; in the same year, the Shanghai World Financial Center started construction with 98 floors and a total height of 460 meters, indicating that my country can build super-high-rise steel structures independently.

Industrial buildings
Part or entire steel structures are often used for industrial buildings with large spans or wide column spacing, especially those that need to carry large-tonnage cranes. Ordinary industrial buildings in China have also adopted steel structures in recent years to shorten the construction period and realize the return on investment as soon as possible.

Lightweight structure
Lightweight structure refers to a structure with a small load or a small span. The main load of this type of structure comes from its weight, and it is often made of cold-bent, thin-walled steel or small steel.

High-rise structures
High-rise structures such as towers and masts have a large height and a small cross-section. Wind and seismic loads are usually the main forces, and the deadweight significantly impacts the structure, so steel structures are often used.

Movable structures
For example, hydraulic steel gates and ship lifts can give full play to lightweight steel structures, thereby reducing the cost of opening and closing equipment and reducing the power consumption required for operation.

Detachable or movable structures
Such as temporary buildings for construction, steel trestles, mobile exhibition halls and mobile platforms, these structures can give full play to the advantages of light weight, easy transportation and convenient installation of steel structures.

Containers and large-diameter pipelines
Steel structures are widely used in liquid (gas) storage tanks, oil and gas pipelines, hydraulic pressure pipelines, and other fields. For example, in the Three Gorges Water Conservancy Project, the inner diameter of the pressure steel pipe used by the generator set reaches 12.4 meters.

Structures with high seismic requirements
In earthquake-prone areas, steel structures are often used in buildings and facilities that meet high seismic requirements due to their good toughness and seismic resistance.

Projects that need to be delivered as soon as possible
This type of project can make full use of the characteristics of steel structure, such as short construction period, lightweight, and easy transportation. It can be completed and put into use quickly.

Steel structures have the following advantages and disadvantages:

Advantages:

High strength and lightweight: Steel has high strength and elastic modulus, so steel structure components are small and light. Steels of different strength grades can meet various needs. Even steels with lower strength usually have a lower density-to-strength ratio than concrete and wood, which makes steel structures lighter under the same stress conditions and can be designed with larger spans. Because the components are smaller and take up less space, they are more convenient to transport and install.

Uniform material and high reliability: The organization of steel is uniform, close to isotropic homogeneous bodies, and it is produced by steel mills with strict quality control and good stability. The actual working performance of steel structures usually conforms to the theoretical calculation results, so their reliability is high.

Good plasticity and toughness: The tensile strength and compressive strength of steel are close, with good plasticity and toughness, can withstand impact and dynamic loads, and show good seismic performance.

Easy to mechanize: Steel structures are produced in factories from rolled profiles and steel plates. The manufacturing process is easy to mechanize, with high production efficiency, speed, precision, and stable quality. It is an engineering structure with a high degree of industrialization.

Easy installation and short construction period: The installation of steel structures is relatively simple, the construction period is short, and they can be put into use quickly, thus quickly generating economic benefits.

Good sealing: Steel structures have good sealing and are suitable for manufacturing atmospheric and high-pressure container structures and large-diameter pipelines with high sealing performance requirements.

Good heat resistance: When the surface temperature of the steel structure is within 200℃, the strength of the steel changes little, so it is suitable for higher temperature environments. However, thermal insulation protection measures should be taken if the structure is subjected to radiant heat for a long time (such as reaching 150℃).

Disadvantages:

Poor fire resistance: The fire resistance of steel structures is poor. When the surface temperature of the steel reaches 300-400℃, the strength and elastic modulus will drop significantly and almost fall to zero when the temperature reaches 600℃. For applications with higher fire resistance requirements, protective measures need to be taken, such as enclosing the steel structure with concrete or other fireproof materials or applying fireproof coatings to improve fire resistance.

Poor corrosion resistance: Steel is prone to rust in a humid or corrosive environment, so the corrosion resistance of steel structures is inadequate, and regular maintenance is required, which increases maintenance costs.

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.