Steel structure has the characteristics of high strength, light weight, good overall stiffness and strong deformation capacity, so it can be used to build long-span, ultra-high and ultra-heavy buildings.
1. The material has high strength and light weight. The steel has high strength and high elastic modulus. Compared with concrete and wood, the ratio of density to yield strength is relatively low.
Therefore, under the same stress conditions, the component section of steel structure is small, the self weight is light, it is convenient for transportation and installation, and it is suitable for structures with large span, high height and heavy load.
2. The steel has good toughness and plasticity, uniform material and high structural reliability. It is suitable for bearing impact and dynamic loads and has good seismic performance.
The internal structure of steel is uniform, close to isotropic homogeneous body. The actual working performance of steel structure is in line with the calculation theory. Therefore, the reliability of steel structure is high.
3. High degree of mechanization in steel structure manufacturing and installation. Steel structural members are easy to manufacture in the factory and assemble on the site.
The factory mechanized manufacturing of steel structure components has high precision, high production efficiency, fast site assembly speed and short construction period. Steel structure is one of the most industrialized structures.
4. The steel structure has good sealing performance.
Because the welded structure can be completely sealed, it can be made into high-pressure vessels with good air tightness and water tightness, large oil pools, pressure pipelines, etc.
5. Steel structure is heat-resistant but not fire-resistant. When the temperature is below 150 ℃, the change of steel properties is very small.
Therefore, the steel structure is suitable for thermal workshop, but when the structural surface is radiated by heat at about 150 ℃, it should be protected by heat insulation board.
When the temperature is 300 ℃ – 400 ℃ The strength and elastic modulus of steel decreased significantly. When the temperature was about 600 ℃, the strength of steel tended to zero.
In buildings with special fire protection needs, the steel structure must be protected with refractory materials to improve the fire resistance rating.
6. The corrosion resistance of steel structure is poor, especially in the environment of wet and corrosive medium, it is easy to rust.
General steel structures shall be derusted, galvanized or coated, and shall be maintained regularly. For offshore platform structures in seawater, special measures such as “zinc block anode protection” shall be adopted to prevent corrosion.
7. Low carbon, energy-saving, green and reusable. The demolition of steel structure buildings will hardly produce construction waste, and the steel can be recycled.
The strength index of steel is determined by elastic limit σ e. Yield limit σ y. And tensile limit σ u. The design is based on the yield strength of steel.
High yield strength can reduce the self weight of the structure, save steel and reduce the cost. tensile strength σ U is the maximum stress that the steel can bear before failure.
At this time, the structure loses its service performance due to large plastic deformation, but the structural deformation is large and does not collapse, which should meet the requirements of the structure to resist rare earthquakes.
The plasticity of steel generally means that after the stress exceeds the yield point, it has the property of significant plastic deformation without fracture.
The main index to measure the plastic deformation capacity of steel is elongation δ And reduction of area ψ.
The cold bending performance of steel is to measure the resistance of steel to cracks when plastic deformation occurs during bending at room temperature.
The cold bending performance of steel is to test the bending deformation performance of steel under the specified bending degree by cold bending experiment.
The impact toughness of steel refers to the ability of steel to absorb mechanical kinetic energy in the fracture process under impact load.
It is a mechanical property to measure the resistance of steel to impact load cutting, which may lead to brittle fracture due to low temperature and stress concentration.
Generally, the impact toughness index of steel is obtained through the impact test of standard specimens.
The welding performance of steel refers to the welding joint with good performance under certain welding process conditions.
Welding performance can be divided into welding performance in welding process and welding performance in service performance.
The welding performance in the welding process refers to the sensitivity that the weld and the metal near the weld do not produce thermal cracks or cooling shrinkage cracks during cooling.
Good welding performance means that there is no crack in the weld metal and nearby base metal under certain welding process conditions. The welding performance in service performance refers to the impact toughness at the weld and the ductility in the heat affected zone.
It is required that the mechanical properties of the weld and the steel in the heat affected zone shall not be lower than those of the base metal.
China adopts the welding performance test method of welding process and the welding performance test method of usability.
There are many factors affecting the durability of steel. First, the corrosion resistance of steel is poor, and protective measures must be taken to prevent corrosion and rust of steel.
Protective measures include: regularly maintain the steel paint, adopt galvanized steel, and adopt special protective measures under the conditions of strong corrosive media such as acid, alkali and salt.
For example, the offshore platform structure adopts “anode protection” measures to prevent the corrosion of the jacket, fix the zinc ingot on the jacket, and the seawater electrolyte will automatically corrode the zinc ingot first, so as to achieve the function of protecting the steel jacket.
Secondly, because the failure strength of steel under high temperature and long-term load is much lower than that under short-term load, the lasting strength of steel under long-term high temperature should be measured.
Steel will automatically harden and become brittle over time, that is, the phenomenon of “aging”. The impact toughness of steel under low temperature load shall be tested.