High-strength bolts, how much do you know?

     

High-strength bolts are called high-strength bolted joints in production, and are generally not simply referred to as high-strength bolts.
        According to the installation characteristics are divided into: large hexagon bolts and torsion shear bolts. The torsional shear type is only used in the 10.9 level.
        According to the performance grade of high-strength bolts, it is divided into: 8.8 and 10.9. Among them, the 8.8 grade has only large hexagonal high-strength bolts. In the marking method, the number before the decimal point indicates the tensile strength after heat treatment; the number after the decimal point indicates the ratio of the yield ratio to the measured value of the ultimate tensile strength and the ultimate tensile strength. . The meaning of 8.8 means that the tensile strength of the bolt rod is not less than 800 MPa and the yield ratio is 0.8; the level of 10.9 means that the tensile strength of the bolt rod is not less than 1000 MPa and the yield ratio is 0.9.
        In the structural design, the high-strength bolt diameter is generally M16/M20/M22/M24/M27/M30, but M22/M27 is the second choice series. Under normal circumstances, M16/M20/M24/M30 is mainly used.
        High-strength bolts are classified into shear design according to design requirements: high-strength bolt bearing type and high-strength bolt friction type. The bearing capacity of the friction type depends on the anti-slip coefficient and the number of friction surfaces of the frictional friction surface. The friction coefficient of the red rust after sandblasting (pill) is the highest, but it is greatly affected by the construction level from the actual operation. Many supervisory units It is suggested whether the standard can be lowered to ensure the quality of the project. The bearing capacity of the bearing type depends on the shear resistance of the bolt and the minimum value of the bolt bearing capacity. In the case of only one joint surface, the M16 friction type shear capacity is 21.6~45.0kN, while the M16 pressure type shear capacity is 39.2~48.6 kN, which is better than the friction type. In the installation, the pressure-bearing type process is simpler, and the joint surface only needs to remove oil and rust. The tensile bearing capacity along the shaft is very interesting in the steel structure specification. The friction design value is equal to 0.8 times the pre-tension. The bearing design value is equal to the screw effective area multiplied by the material tensile strength design value. It seems that There is a big difference. In fact, the two values ​​are basically the same. I have never understood why the specification should be written this way. Why do the same material use the two expressions to calculate the same value?
        When simultaneously bearing the shear force and the axial force of the rod axis, the friction type requirement is the ratio of the shearing force to the shear bearing capacity of the bolt and the sum of the axial force of the screw bearing and the tensile load bearing force is less than 1.0, and the bearing type is required. It is the square of the ratio of the shearing force to the shear bearing capacity of the bolt plus the square of the ratio of the axial force of the screw to the tensile bearing capacity is less than 1.0, that is to say, under the same load combination, the same diameter is supported. The high-strength bolts are designed to have a higher safety reserve than the friction-type high-strength bolts.
        Considering that under the repeated action of strong earthquakes, the joint friction surface may fail. At this time, the shear bearing capacity depends on the bolt shearing capacity and the bearing capacity of the plate. Therefore, the seismic specification stipulates that the high-strength bolt is sheared. Carrying capacity calculation formula.
        Although the pressure-bearing type has an advantage in the design value, because it belongs to the shear-destroy type, the bolt hole is a pore-type bolt hole similar to a normal bolt, and the deformation when subjected to the load is much larger than the friction type, so the high-strength bolt is under pressure. The type is mainly used for the connection of non-seismic members, the connection of non-bearing dynamic load members, and the connection of non-repetitive members.
        There are also differences in the normal use limit states of these two types:
        Friction type connection refers to the relative slippage of the joint friction surface under the basic combination of loads;
        Pressure-type connection refers to the relative slip between the joints under the combination of load standards;
        Weld and bolt knowledge
        Weld grade
        1. The weld grade is the construction acceptance level and there are three levels. The third level is the lowest, only visual inspection and dimensional inspection are required. The second-level requirements are for ultrasonic inspection. The highest level is required, and all inspections are required.
        2. For weld grades, the principle is that the tension level is higher than the pressure, and the power is higher than the static force.
        3. Butt welds are generally required to be non-destructively tested (or partially required). Therefore, the welding grade of the butt weld is generally two or one level, not less than two.
        4. It is not necessary for non-destructive testing of fillet welds. Therefore, if the fillet weld is one level, it does not make much sense. Generally fillet welds are secondary or tertiary.
        5. For the weld grade, see 7.1.1 of the steel gauge, and pay attention to the provisions of the article.
        Ordinary bolt
        1. Ordinary bolts are divided into three types: A, B, and C. The first two are refined bolts and are less used. Generally speaking, ordinary bolts refer to Class C ordinary bolts.
        2. Common C-class bolts are commonly used in some temporary connections and connections that need to be removed. Common bolts commonly used in building structures are M16, M20 and M24. Some mechanical industry rough bolts may be large in diameter and special in use.
        High-strength bolts
        3. The material of the high-strength bolt is different from the ordinary bolt. High-strength bolts are generally used for permanent connections. Commonly used are M16~M30. The oversized high-strength bolts are unstable and should be used with caution.
        4. The bolted joints of the main components of the building structure are generally connected by high-strength bolts.
        5. The factory's high-strength bolts are not divided into pressure-bearing type or friction type.
        6. Is it a friction type high strength bolt or a pressure type high strength bolt? In fact, there are differences in design calculation methods:
        (1) Friction-type high-strength bolts are slipped between the plates as the limit state of load carrying capacity.
        (2) The bearing type high-strength bolts are slipped between the plates as the normal use limit state, and the connection failure is taken as the limit state of the load capacity.
        7. Friction type high strength bolts do not fully exploit the potential of bolts. In practical applications, friction-type high-strength screws should be used for important structures or structures subjected to dynamic loads, especially when the load causes reverse stress. In this case, the unutilized bolt potential can be used as a safety reserve. In addition to this, high-strength bolted connections should be used to reduce the cost.
        The difference between ordinary bolts and high-strength bolts
        8. Ordinary bolts can be reused, and high-strength bolts cannot be reused.
        9. High-strength bolts are generally made of high-strength steel (45 gauge steel (8.8s), 20MmTiB (10.9S), which is a pre-stressed bolt. The friction type uses a torque wrench to apply the specified pre-stress, and the pressure type unscrews the plum head. Ordinary bolt Usually made of ordinary steel (Q235), just tighten it.
        10. Ordinary bolts are generally 4.4, 4.8, 5.6 and 8.8. High-strength bolts are generally 8.8 and 10.9, with 10.9 being the most.
        11. The screw holes of ordinary bolts are not necessarily larger than those of high-strength bolts. In fact, ordinary bolt holes are relatively small.
        12. Ordinary bolts A and B screw holes are generally only 0.3~0.5mm larger than bolts. Class C screw holes are generally 1.0 to 1.5 mm larger than bolts.
        13. Friction type high strength bolts transmit load by friction, so the difference between screw and screw hole can reach 1.5~2.0mm.
        14. The force transmission characteristic of the pressure-bearing high-strength bolt is to ensure that the shear force does not exceed the friction force under normal use conditions, which is the same as the friction type high-strength bolt. When the load is increased again, the relative slip will occur between the connecting plates. The connection relies on the shearing of the screw and the pressure of the hole wall to transmit the force, which is the same as the ordinary bolt, so the difference between the screw and the screw hole is slightly smaller, 1.0~1.5mm.
        15, column foot anchor
        Anchor bolts have no grade, only material points: Q235 and Q345. The most anchor bolts on the building structure are the anchor bolts.
        16. Column anchors are neither ordinary bolts nor high-strength bolts. Strictly speaking, it is not a bolt. Column anchors are generally M20 or M24.
        17. The manufacturing standards for column anchors should be the same as those for ordinary bolts. The length of the anchor bolts embedded should be related to the friction between them and the concrete, and also the form of the anchors.
        18. Expansion bolts and chemical bolts
        Whether it is an expansion anchor or a chemical anchor, it is not a connection form in the national standard, and the use of such connections, especially in important connections, should be avoided. All pre-embedded parts should be used.
        19. The expansion anchor is mainly pulled by the expansion of the expansion tube and the friction of the concrete. The relationship between the size of the pull-out force and the construction process is large, and the human factor is large. It is useless to perform the tensile test.
        20. The chemical anchor bolt is formed by punching with a punching machine, then poured into a chemical slurry, and the plug rod is placed to form an anchoring effect. Common brands such as Fischer and Heineken.
        21. Expansion bolts and chemical bolts are actually anchors. In some cases, expansion bolts or chemical anchors are required because they are not pre-buried. But this situation should be avoided in the design. Because the anchors should be pre-buried. For example, column anchors. Because this is the only way to ensure the best bonding and stress. And after the hole punching, it often causes damage to the steel bars and the shackles in the shackles.
        22. In the 砼 specification, for components pre-buried in concrete, they are called embedded parts. According to the Ministry of Construction documents, expansion bolts shall not be used for curtain walls. For new construction projects, expansion anchors are strictly forbidden and should be pre-buried.

Sodium Formate (English name Sodium formate; Formic Acid sodium salt), also known as sodium formate, contains two crystal waters in the crystal, so it is also called sodium formate dihydrate, sodium formate dihydrate, sodium formate dihydrate (Sodiumformatehydrate). Sodium formate is one of the simplest organic carboxylates. It is white crystals or powder with a slight Formic ACID odor. Slightly deliquescent and hygroscopic. Soluble in about 1.3 parts of water and glycerin, slightly soluble in ethanol and octanol, insoluble in ether. Its aqueous solution is alkaline. When heated, sodium formate decomposes into hydrogen and sodium oxalate, and then generates sodium carbonate.

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