1. Construction steel has two possible damage forms

Plastic failure and brittle failure. The characteristics of both can be understood from three aspects: plastic deformation, nominal stress, and fracture form. Factors that affect brittle failure include harmful chemical elements, metallurgical defects, etc., but in general, the quality of the steel, stress concentration and low temperature have a greater impact. Preventing brittle failure requires the cooperation of reasonable design, correct manufacturing and correct use.

2. The σ-ε curve of steel is obtained under the following standard conditions:

Ⅰ) Standard specimen (no stress concentration); Ⅱ) Static load once stretched to failure ;Ⅲ) The test temperature is 20°C. According to the σ-ε curve of building steel, its work can be divided into four stages: elasticity, elastic-plasticity, plasticity and strengthening, and it is simplified into an ideal elastic-plastic body. Three basic performance indicators of steel are obtained from the tensile test: tensile strength fu, yield strength fy, and elongation δ5. Fu and fy are static strength indicators, and δ5 is the plastic performance indicator of steel under static load. All load-bearing structural steel materials should have the guarantee of passing these three indicators. For important components or components that require cold processing, the steel should also have the guarantee of passing the cold bending test.

3. Impact toughness Cv

Impact toughness Cv is an index indicating the ability of steel to resist brittle fracture under dynamic loads. Corresponding impact toughness should be proposed for structures that directly bear large dynamic loads. Require.

4. Stress

Under static load and unidirectional stress, the maximum stress of the section is required not to exceed the yield point; in complex stress states, the converted stress δeq is required not to exceed fy .

5. Understand the adverse effects of various factors on the performance of steel

It is necessary to distinguish between beneficial elements and harmful elements in the chemical composition, and special attention should be paid to the effects of carbon, sulfur, and phosphorus. Pay attention to the impact of stress concentration, which results in a local two-way or three-way tensile stress state, making the steel brittle. Stress concentrations should be avoided as much as possible through reasonable construction measures (such as smooth transitions).

6. Correctly select steel materials and propose reasonable index requirements

The specification recommends Q235, 16Mn, 16Mnq, 15MnV, and 15MnVq steel as load-bearing structural steel. Understand the expression of their grades. Metallurgical factory For items whose materials should be guaranteed and items that can be additionally guaranteed, master the method of correctly selecting steel materials and putting forward reasonable index requirements based on the specific conditions of the design structure.

Attachment: Steel structure grades

GB/T5613-1995 standard stipulates two grade expression methods for cast steel 1) Mainly based on the mechanical properties of yield strength and tensile strength Grade representation method, such as ZG200-400, etc. ZG is the symbol representing cast steel, and 200 and 400 are the minimum values ??of yield strength and tensile strength (MPa) respectively. 2) Grade representation based on chemical composition, such as ZG20Cr13, etc. Cr is the symbol of chromium element, 20 is the average carbon content (in parts per ten thousand), and 13 is the average chromium content (mass fraction) (%). Some letters and symbols are added to represent different meanings. For example, ZD345-570 is low alloy cast steel for general engineering and structural use; ZG200-400H is carbon cast steel useful for welded structures; ZGMn 13 is cast steel.

Expand

There are two main types of steel used for steel structures: low carbon structural steel and low alloy high-strength structural steel. The lowest grade of low-alloy high-strength structural steel starts with Q295, while the highest grade of carbon structural steel ends with Q275. Although the meaning of the nominal yield point is explained the same, the values ??are not repeated and are just connected. 1. The grades of carbon structural steel are: Q××× (A~D) + deoxidation method. Among them, Q××× represents the yield strength; A~D represents the impact toughness quality grade from bottom to high; during the pouring process, according to the different degrees of deoxidation, it is divided into killed steel〖TZ, Z〗, semi-killed steel〖b〗, and boiling steel〖 F〗. For Q235, the A and B levels can be (Z, b, F), the C level can only be (Z), and the D level can only be (TZ). 2. Low-alloy high-strength structural steel: A small amount of several alloying elements are added during the smelting process to significantly increase the strength of the steel. The alloy content is less than 5%, so it is called low-alloy high-strength structural steel; the grade indicates the same as carbon Structural steel is the same, and the quality grade is divided into 5 grades from A to E according to impact toughness. Grades A and B of low-alloy high-strength structural steel are killed steel, and grades C, D, and E are special killed steel. 3. Alloy structural steel: including high-quality steel, high-grade high-quality steel and special-grade high-quality steel; there are up to 77 grades of alloy structural steel, which are divided into 24 steel groups according to the main alloying elements.

Grade meaning: The grade of alloy structural steel consists of numbers and chemical symbols of alloy elements. The number indicates the average carbon content of the steel; the subsequent chemical symbol represents the added alloying element, and is suffixed with the representative number of the average content of this element (requirements for the average content code of each alloying element: when it is less than 1.5% Generally, it is not written, but when the grades are similar in this way, the one with high alloy element content must be marked with 1 to show the difference; when the content is 1.5%~2.49%, 2.50%~3.49%, 3.50%~4.49%, write them respectively. Marked by 2, 3, 4, if the content is higher, so on).

Quality grade mark: At the end of the steel grade, the metallurgical quality grade should be marked as required. High-quality steel should not be marked, high-quality steel should be marked with A, and special-grade high-quality steel should be marked with E.

When selecting steel materials, the structure must be safe and reliable, and the materials used must be economical and reasonable. Factors affecting the selection of steel: 1. Importance of the structure or component; 2. Load nature (static load or dynamic load); 3. Connection method (welding, riveting or bolt connection); 4. Working conditions (temperature or corrosive medium) For important structures, structures that directly bear dynamic loads, structures under low temperature conditions, and welded structures, higher quality steel should be selected.

What is the difference between light steel and heavy steel?

First of all, there is no name for light steel. Light steel is compared to ordinary steel.

Lightweight steel structure is a very vague concept with no strict definition. There is indeed no unified standard for determining whether a structure is ordinary steel or light steel. Many experienced designers or project managers often cannot fully explain it, but we can comprehensively consider and judge based on some data: 1. Lifting weight of the factory building : If it is greater than or equal to 25 tons, it can be considered as a common steel structure. 2. The amount of steel used per square meter: greater than or equal to 50KG/M2 can be considered a common steel structure. 3. Thickness of steel plate for main components: greater than or equal to 10MM, less used for light steel structures. In addition, there are some reference values: such as cost per square meter, maximum component weight, maximum span, structural form, eaves height, etc. The above can provide empirical data when judging whether a factory building is made of ordinary steel or light steel. Of course, many buildings now are Both light and common steel are available. But there are some that we can say with certainty that they are ordinary steel, such as: petrochemical plant facilities, power plant buildings, long-span sports venues, exhibition centers, high-rise or super high-rise steel structures. The scope of ordinary steel structures is very wide and can include various steel structures, regardless of load size, and even includes many contents of lightweight steel structures. The technical regulations for lightweight house steel structures only stipulate some more specific contents based on its "light" characteristics. , and the scope is limited to single-layer portal frames. Light steel structure is a very vague concept with no strict definition. Generally, there are two understandings. One is Chapter 11 "Light Steel Structure of Round Steel and Small Angle Steel" in the current "Steel Structure Design Code", which refers to a light steel structure made of round steel and angle steel smaller than L45×4 and L56×36×4. , which was mainly used for small structures that were not suitable for reinforced concrete structures in the era when steel was scarce. It is now basically not used, so this revision of the steel structure design code has basically tended to remove it. The other is a single-layer solid-web portal frame structure with a lightweight roof and lightweight exterior walls stipulated in the "Technical Regulations for Steel Structures of Portal Frame Lightweight Houses". The term "lightweight" here mainly means that the enclosure is made of lightweight materials. Material. It can be seen that the difference between light steel and ordinary steel is not the weight of the structure itself, but the weight of the surrounding materials it bears. The structural design concepts are still the same.

The difference between H-shaped steel and I-shaped steel

1. Whether I-shaped steel is ordinary or light, the cross-section size is relatively high and narrow, so the cross-section The moment of inertia of the two main sleeves is quite different. Therefore, it can generally only be used directly for members that are bent in the plane of their webs or formed into lattice-type stressed members. It is not suitable to use axial compression members or members that are bent perpendicular to the web plane, which greatly limits their application scope

2. H-shaped steel is an efficient and economical cutting Surface profiles (others include cold-formed thin-walled steel, profiled steel plates, etc.). Due to their reasonable cross-sectional shapes, they can make the steel more efficient and improve its cutting capacity. Different from the ordinary I-shaped steel, the wings of the H-shaped steel are widened, and the inner and outer surfaces are usually parallel, which makes it easy to connect with other components using high-strength screws. Its size constitutes a reasonable series with complete models, which is convenient for design and selection. The rolling of H-shaped steel is different from that of ordinary I-shaped steel, which only uses a set of horizontal rollers. Because its wings are wider and have no slope (or a very small slope), a set of vertical rollers must be added for simultaneous rolling. Therefore, its rolling process and equipment are more complex than ordinary rolling mills.

In short, H-shaped steel is a replacement product of I-beam, so the standard for selecting and using the two in practical applications is: try not to use I-beam.

The difference between hot rolling and cold rolling

1. Cold-rolled shaped steel allows local buckling of the section, so that the bearing capacity of the rod after buckling can be fully utilized; while hot-rolled shaped steel does not Allow local buckling of the section. 2. The causes of residual stress in hot-rolled steel and cold-rolled steel are different, so the distribution on the cross-section is also very different. The residual stress distribution on the cold-formed thin-walled steel section is curved, while the residual stress distribution on the hot-rolled steel or welded steel section is film-type. 3. The free torsional stiffness of hot-rolled steel is higher than that of cold-rolled steel, so the torsional performance of hot-rolled steel is better than that of cold-rolled steel.

Expand

Advantages of hot rolling The advantage of hot rolling is that it can destroy the casting structure of the steel ingot, refine the grains of the steel, and eliminate defects in the microstructure, thereby making the steel structure dense. , the mechanical properties are improved. This improvement is mainly reflected in the rolling direction, so that the steel is no longer isotropic to a certain extent; bubbles, cracks and looseness formed during pouring can also be welded under the action of high temperature and pressure.

Disadvantages of hot rolling

First, after hot rolling, the non-metallic inclusions (mainly sulfides, oxides, and silicates) inside the steel are pressed into thin sheets , delamination (interlayer) phenomenon occurs. Delamination greatly deteriorates the tensile properties of the steel along the thickness direction and may cause interlaminar tearing as the weld shrinks. The local strain induced by weld shrinkage often reaches several times the yield point strain, which is much larger than the strain caused by load. The second is the residual stress caused by uneven cooling. Residual stress is the internal self-balanced stress in the absence of external force. Hot-rolled steel sections of various sections have such residual stress. Generally, the larger the cross-section size of the section steel, the greater the residual stress. Although residual stress is self-balanced, it still has a certain impact on the performance of steel components under the action of external forces. For example, it may have adverse effects on deformation, stability, fatigue resistance, etc.

Cold rolling refers to processing steel plates or steel strips into various types of steel through cold drawing, cold bending, cold drawing and other cold processing at room temperature.

The advantages of cold rolling are fast forming speed, high output, and no damage to the coating. It can be made into a variety of cross-section forms to meet the needs of use conditions; cold rolling can make steel produce a large amount of plastic deformation, thereby increasing the yield point of the steel.

Disadvantages of cold rolling

The first disadvantage is that although there is no hot plastic compression during the forming process, there are still residual stresses in the section, which will inevitably affect the overall and local buckling characteristics of the steel. . Second, the cold-rolled steel section generally has an open section, which results in a low free torsional stiffness of the section. It is prone to torsion when subjected to bending and torsional buckling when subjected to pressure, and its torsional resistance is poor. Third, the wall thickness of cold-rolled formed steel is small, and there is no thickening at the corners where the plates are connected, so the ability to withstand localized concentrated loads is weak.

I believe that after the above introduction, everyone has a certain understanding of the summary of issues related to steel structure learning. Welcome to log in to Zhongda Consulting for more relevant information.

For more information about engineering/service/purchasing tender document writing and production to improve the bid winning rate, you can click on the official website customer service at the bottom for free consultation: /#/?source=bdzd