What should be considered in the construction of high-rise steel structures? (2)

May 05, 2019

Third, the basic requirements for the selection of high-rise steel structural materials

 

(I) Principles of Steel Selection The principle of steel selection is to make the structure safe and reliable and meet the requirements of use, and to maximize the possibility of saving steel and reducing the cost. Different use conditions should have different quality requirements. In general, it is of course not advisable to use high-quality steel easily, and in the main structure, it is not possible to blindly select steel of poor quality. In terms of the mechanical properties of steel, the indexes such as yield point, tensile strength, elongation, cold bending performance and impact toughness are indicators for measuring the quality of steel from different aspects. When designing steel structures, According to the characteristics of the structure, choose the appropriate steel. The suitability of steel selection is not only an economic issue, but also related to the safety and service life of the structure. The following structural characteristics should be considered when selecting steel: 1. Type and importance of structure Due to the different conditions of use and the location of the structure, the structure can be divided into three categories: important, general and minor. For example, civil large-span trusses and heavy-duty working crane beams are important; roof trusses and columns of ordinary factories are general; ladders, railings, platforms, etc. are secondary, and steels should be selected differently according to different situations. The grade.

 

2. Properties of the load According to the nature of the load, the structure can be divided into two types: the static load and the dynamic load. In structures or components that are subjected to dynamic loads, there are differences between frequent full load and infrequent full load. Therefore, different grades should be used if the load properties are different. For example, for heavy duty crane beams, steels with good impact toughness and fatigue properties, such as Q345C or Q235C, should be used. For structures or components that are generally subjected to static loads, such as ordinary welded roof trusses and columns (under normal temperature conditions), Q235BF can be used.

 

3. Connection method Different connection methods have different quality requirements for steel. For example, welded steel, due to welding stress, welding deformation and welding defects inevitably in the welding process, in the case of changes in the force properties and temperature changes, it is easy to cause notch sensitivity, resulting in cracks in the components, even brittle fracture Therefore, the welded steel structure has high requirements on the chemical composition, mechanical properties and weldability of the steel. For example, the content of carbon, sulfur and phosphorus in steel is low, the index of plasticity and toughness is high, and the weldability is good. However, for non-welded structures (such as structures joined by high-strength bolts), these requirements can be relaxed.

(2) Basic requirements for the selection of high-rise steel structural materials 1. Steel grade, steel number, strength design value, selection principle and required mechanical properties, chemical composition limits, etc. of the steel used for high-rise steel structures, and welding materials used for connection The requirements of materials such as bolt fasteners, except for the cold bending performance of steel as the basic guarantee condition, are basically the same as the general steel structure. The design value of the strength of thick steel plate should be in accordance with the design and construction of high-rise building steel structure. The provisions of the Regulations are implemented. In addition, Q235A grade ordinary carbon structural steel should only be used as a secondary non-welded component. 2. The steel properties of seismic high-rise building steel structures should also meet the following requirements: (1) the steel yield ratio is not less than 1.2, and the structure of seismic fortification of 8 degrees and 8 degrees or more is not less than 1.5; (2) Obvious yield step, elongation greater than 20%, and good ductility to maintain ductility; (3) steel and steel grade B high-rise building steel structure yield point should not exceed 10% of its standard value.

 

3. When the load-bearing structure is in exposed and low-temperature environment, the steel should also consider the requirements of resistance to atmospheric corrosion and avoiding cold and brittleness.

 

4. In the range of beam-column joints with welded joints, when the joints are strong, the thickness is greater than 50mm, and the tensile force along the thickness direction is required, the elongation in the thickness direction should be additionally required (more than 20%). ~25%) to prevent lamellar tearing.

 

5. In the same high-rise structure, different steel grades and strength grades of steel may be selected according to the location of the components and the force conditions.

 

6. The profiled steel plate used for the envelope structure and the composite panel should be Q235 steel.

 

7. The anti-shear welding nail used in the high-rise steel structure, the material such as the magnetic rod of the nail rod should meet the requirements of the relevant standards, and the special welding machine welding tool should be used for the welding to ensure the shear bearing capacity.

 

8. The concrete markings, steel bars and design strengths of the composite slabs and composite members of the high-rise building steel structure, as well as the lightweight aggregate concrete materials, shall comply with the requirements of the current national regulations.