What are steel structures? Steel structures are made mainly of steel and are one of the main types of building structures nowadays. Steel’s characteristics are high strength, lightweight, good overall rigidity, and strong deformation ability, so it is particularly suitable for building large-span, super-high, and super-heavy buildings.
A steel structure consists of steel beams, columns, trusses, and other components of steel sections and plates connected by welds, bolts, or rivets.
Common steel grades and properties of steel structure:
1. Carbon structural steel: Q195, Q215, Q235, Q355, etc.
2. Low alloy, high-strength structural steel.
3. High-quality carbon structural steel and alloy structural steel.
4. Special purpose steel.
Methods of connecting steel structures
Three methods of connecting steel structures are 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 and condense them into a weld after cooling, thereby connecting the weldment into one.
Advantages: No weakening of the cross-section of the component, saving steel, simple structure, easy manufacturing, high connection stiffness, good sealing performance, easy-to-use automated operation under certain conditions, and high production efficiency.
Disadvantages: The heat-affected zone formed by the high temperature of welding near the weld may cause the material of some parts to become brittle.
During the welding process, the steel is subjected to unevenly distributed high temperature and cooling, which causes welding residual stress and residual deformation in the structure, which has a specific impact on the bearing capacity, stiffness, and performance of the structure;
Due to the high stiffness of the welded structure, local cracks can easily extend 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 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 and convenient installation. It is especially suitable for installation and connection on-site. It is also easy to disassemble. It is ideal for structures that need installation and disassembling and temporary connections.
Disadvantages: Drilling holes in the plate and aligning the holes during assembly increases the manufacturing workload and requires high manufacturing precision. Bolt holes also weaken the component’s cross-section, and the connected parts often need to overlap each other or add auxiliary connecting plates (or angle steels), making the structure more complicated and consuming more steel.
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 is challenging. It is easy to check and ensure quality and can be used for heavy and directly dynamic load structures.
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.
Steel structure welding
Arc welding is the most commonly used welding method for steel structures. It includes manual arc welding, automatic or semi-automatic arc welding, and gas-shielded welding.
Manual welding:
Manual arc welding is the most commonly used welding method in steel structures. Its equipment is simple, and its operation is flexible and convenient. However, the working conditions are poor, the production efficiency is lower than that of automatic or semi-automatic welding, and the variability of weld quality is large, which depends on the technical level of the welder to a certain extent.
Automatic welding:
The weld quality of automatic welding is stable; the weld has fewer internal defects, good plasticity, and good impact toughness. It is suitable for welding longer direct welds.
Semi-automatic welding:
Due to manual operation, semi-automatic welding is suitable for welding curves or welds of any shape.
Automatic and semi-automatic welding should use wire and flux, which is ideal for the primary metal. The welding wire should comply with the provisions of national standards, and the flux should be determined according to the welding process requirements.
Gas shielded welding:
Gas-shielded welding uses inert gas (or CO2) gas as the arc protection medium to isolate the molten metal from the air and keep the welding process stable. The arc heating of gas-shielded welding is concentrated, the welding speed is fast, and the penetration depth is significant, so the weld strength is higher than that of manual welding. It also has good plasticity and corrosion resistance, which makes it suitable for welding thick steel plates.
Steel structure welding form
Steel structure welding forms can be divided into four forms according to the relative positions between the connected components, namely butt joint, lap joint, T-shaped connection, and fillet joint. There are two primary forms of welds used in these connections: butt weld and fillet weld. In specific applications, the selection should be made according to the stress conditions of the connection, combined with the manufacturing, installation, and welding conditions.
Welding seam structure:
1. Butt welds
Butt welds transmit force directly and smoothly, without significant stress concentration, so they have good stress performance and apply to the connection of components that bear static and dynamic loads. However, due to the high-quality requirements of butt welds, the welding gap requirements between weldments are strict, and they are generally used in factory-made connections.
2. Fillet welds
The form of fillet welds: fillet welds can be divided into side fillet welds parallel to the direction of force, front fillet welds perpendicular to the direction of force, oblique fillet welds oblique to the direction of force, and circumferential welds according to their length direction and the direction of external force. The cross-sectional forms of fillet welds are divided into ordinary, flat slope, and deep penetration types.
The development history of steel structure in China
China had outstanding achievements in iron structure in the early days. Still, due to the constraints of the feudal system and underdeveloped science, my country has been stuck at the level of iron buildings for more than 2,000 years. It was not until the end of the 19th century that my country began to adopt modern steel structures.
After the founding of New China, the application of steel structures made significant progress, far exceeding the past in both quantity and quality. After 2000, my country’s economy grew significantly, national strength was enhanced considerably, and steel production became a world power.
Around 2008, driven by the Olympic Games, there was a boom in steel structure construction. Strong market demand promoted the rapid development of this industry, and many steel structure venues, airports, stations, and high-rise buildings were built.
Some steel structures and buildings, such as the Olympic National Stadium and others, have world-class production and installation technology. We have reached a high level in design, manufacturing, and installation technology and mastered various complex building designs and construction techniques.
Characteristics of steel structure
1. Steel has toughness, plasticity, uniform material, and high structural reliability.
It is suitable for bearing impact and dynamic loads and has good seismic resistance. Steel’s internal structure is uniform, close to an isotropic homogeneous body.
The actual working performance of the steel structure is more in line with the calculation theory, which makes it highly reliable.
2. Steel is a strong and light material.
It has high strength and a high elastic modulus. Compared with concrete and wood, its density-to-yield strength ratio is relatively low.
Therefore, under the same stress conditions, the steel structure has a small cross-section and is light, which makes it easy to transport and install. It suits structures with large spans, high heights, and heavy loads.
3. The steel structure has good sealing performance.
Since the welded structure can be completely sealed, it can be made into high-pressure containers, large oil pools, pressure pipelines, etc., with good air and water tightness.
4. The steel structure is heat-resistant but not fire-resistant.
When the temperature is below 150℃, the properties of the steel change very little.
Therefore, steel structures are suitable for hot workshops, but when the structure’s surface is subjected to heat radiation of about 150°C, it must be protected by heat insulation boards.
When the temperature is 300°C-400°C, steel’s strength and elastic modulus drop significantly, and at a temperature of about 600°C, the strength of steel tends to zero. Refractory materials must protect steel structures to improve the fire resistance level in buildings with special fire protection requirements.
5. Steel structures have poor corrosion resistance, especially in humid and corrosive environments, and are prone to rust.
Generally, steel structures must be rusted, galvanized, or painted and maintained regularly. For offshore platform structures in seawater, special measures such as “zinc block anode protection” are required to prevent corrosion.
6. Steel structure manufacturing and installation are highly mechanized, and steel structure components are easy to manufacture in factories and assemble on-site.
The finished products of mechanized steel structure components manufactured in factories have high precision, high production efficiency, fast on-site assembly speed, and a short construction period. Steel is a structure with the highest degree of industrialization.
