2024-11-12
Viewpoint
(1) Some books on steel structures do suggest that high-strength bolts refer to bolts with a strength exceeding grade 8.8. For this view, first of all, the British and American standards do not support this view, and there is no definition of “strong” and “weak” for a specific strength grade. Secondly, it does not conform to the “high-strength bolts” mentioned in our work.
(2) For the sake of comparison, the stress conditions of complex bolt groups are not considered here.
(3) The pressure-bearing stress of the screw is also considered in the design of pressure-bearing high-strength bolts, which will be introduced in detail in the “Comparison of pressure-bearing and friction-bearing high-strength bolts” later.
How much do you know about high-strength bolts?
The full name of high-strength bolts in production is high-strength bolt connection pairs, which are generally not referred to as high-strength bolts.
According to the installation characteristics, they are divided into: large hexagon head bolts and torsion shear bolts. Among them, the torsion shear type is only used in grade 10.9.
According to the performance grade of high-strength bolts, they are divided into: grade 8.8 and grade 10.9. Among them, only large hexagonal high-strength bolts are available in grade 8.8. 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 yield strength ratio, that is, the ratio of the actual measured value of the yield strength to the actual measured value of the ultimate tensile strength. Grade 8.8 means that the tensile strength of the bolt rod is not less than 800MPa, and the yield strength ratio is 0.8; grade 10.9 means that the tensile strength of the bolt rod is not less than 1000MPa, and the yield strength ratio is 0.9.
In structural design, the diameters of high-strength bolts are 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.
In shear design, high-strength bolts are divided into: high-strength bolt pressure type and high-strength bolt friction type according to design requirements.
The bearing capacity of the friction type depends on the anti-slip coefficient of the force-transmitting friction surface and the number of friction surfaces. The friction coefficient of red rust after sandblasting (shot) is the highest, but from the actual operation point of view, it is greatly affected by the construction level. Many supervision units have proposed whether the standards can be lowered to ensure the quality of the project.
The bearing capacity of the pressure type depends on the minimum value of the shear capacity of the bolt and the pressure bearing capacity of the bolt rod. In the case of only one connection surface, the shear bearing capacity of the M16 friction type is 21.6~45.0kN, while the shear bearing capacity of the M16 pressure type is 39.2~48.6 kN, which is better than the friction type.
In terms of installation, the pressure type process is simpler, and the connection surface only needs to be cleaned of oil and floating rust. The tensile bearing capacity along the axis direction is very interesting in the steel structure specification. The design value of the friction type is equal to 0.8 times the pre-tension force, and the design value of the pressure type is equal to the effective area of the screw multiplied by the design value of the tensile strength of the material. It seems that there is a big difference, but in fact the two values are basically the same.
When bearing shear force and axial tension at the same time, the friction type requirement is that the ratio of the shear force borne by the bolt to the shear bearing capacity plus the ratio of the axial force borne by the screw to the tensile bearing capacity is less than 1.0, and the pressure type requirement is that the square of the ratio of the shear force borne by the bolt to the shear bearing capacity plus the square of the ratio of the axial force borne by the screw to the tensile bearing capacity is less than 1.0. That is to say, under the same load combination, the safety reserve of the pressure-type high-strength bolts of the same diameter in design is higher than that of the friction-type high-strength bolts.
Considering that the friction surface of the connection may fail under repeated strong earthquakes, the shear bearing capacity at this time still depends on the shear resistance of the bolts and the pressure bearing capacity of the plate. Therefore, the seismic code stipulates the calculation formula for the ultimate shear bearing capacity of high-strength bolts.
Although the pressure-bearing type has an advantage in design values, it belongs to the shear-compression failure type. The bolt hole is a pore-type bolt hole similar to that of ordinary bolts. The deformation under load is much greater than that of the friction type. Therefore, the high-strength bolt pressure-bearing type is mainly used for non-seismic component connections, non-dynamic load component connections, and non-repeated component connections.
The normal use limit states of these two types are also different:
Friction-type connection refers to the relative slippage of the connection friction surface under the basic load combination;
Pressure-bearing connection refers to the relative slippage between the connecting parts under the standard load combination;
Ordinary bolts
1. Ordinary bolts are divided into three types: A, B, and C. The first two are refined bolts and are rarely used. Ordinary bolts generally refer to C-level ordinary bolts.
2. C-level ordinary bolts are often used in some temporary connections and connections that need to be disassembled. Common ordinary bolts commonly used in building structures are M16, M20, and M24. Some rough bolts in the machinery industry may have a relatively large diameter and special uses.
High-strength bolts
3. The material of high-strength bolts is different from that of ordinary bolts. High-strength bolts are generally used for permanent connections. Commonly used ones are M16~M30. The performance of oversized high-strength bolts is unstable and should be used with caution.
4. The bolt connection of the main components of the building structure is generally made of high-strength bolts.
5. The high-strength bolts shipped from the factory are not divided into pressure-bearing type and friction type.
6. Is it a friction-type high-strength bolt or a pressure-bearing high-strength bolt? In fact, there is a difference in the design and calculation method:
(1) Friction-type high-strength bolts use sliding between the plate layers as the ultimate state of bearing capacity.
(2) Pressure-bearing high-strength bolts use sliding between the plate layers as the limit state of normal use, and connection failure as the ultimate state of bearing capacity.
7. Friction-type high-strength bolts cannot fully utilize the potential of the bolts. In practical applications, for very important structures or structures that bear dynamic loads, especially when the load causes reverse stress, friction-type high-strength bolts should be used. At this time, the unused potential of the bolts can be used as a safety reserve. In addition, pressure-bearing high-strength bolts should be used to connect in order to reduce the cost.
Differences between ordinary bolts and high-strength bolts
8. Ordinary bolts can be reused, but high-strength bolts cannot be reused.
9. High-strength bolts are generally made of high-strength steel (45 steel (8.8s), 20MmTiB (10.9S), and are prestressed bolts. The friction type uses a torque wrench to apply the specified prestress, and the pressure-bearing type unscrews the plum head. Ordinary bolts are generally made of ordinary steel (Q235), and only need to be tightened.
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 common.
11. The screw hole of an ordinary bolt is not necessarily larger than that of a high-strength bolt. In fact, the screw hole of an ordinary bolt is relatively small.
12. Ordinary bolts A, B Grade A screw holes are generally only 0.3~0.5mm larger than bolts. Grade C screw holes are generally 1.0~1.5mm larger than bolts.
13. Friction-type high-strength bolts transmit loads by friction, so the difference between the screw and the screw hole can reach 1.5~2.0mm.
14. The force transmission characteristics of pressure-type high-strength bolts are to ensure that the shear force does not exceed the friction force under normal use, which is the same as friction-type high-strength bolts. When the load increases further, relative slip will occur between the connecting plates, and the connection relies on the shear resistance of the screw and the pressure of the hole wall to transmit force, which is the same as ordinary bolts, so the difference between the screw and the screw hole is slightly smaller, 1.0~1.5mm.
Column foot anchor bolts
15. There is no grade for anchor bolts, only the material difference: Q235 and Q345. The most commonly used anchor bolts in building structures are column anchor bolts.
16. Column anchor bolts are neither ordinary bolts nor high-strength bolts. Strictly speaking, they are not bolts. Column anchor bolts generally use M20 or M24.
17. The manufacturing standard of column anchor bolts should be the same as that of ordinary bolts. The embedded length of column anchor bolts should be related to the friction between it and concrete, as well as the form of anchor bolts.
Expansion bolts and chemical bolts
18. Whether it is expansion anchor bolts or chemical anchor bolts, they are not the connection forms specified in the national standard specifications. Such connections should be avoided, especially in important connections. Pre-embedded parts should be used.
19. Expansion anchor bolts mainly rely on the friction between the expansion tube and concrete to resist pullout. The magnitude of the pullout resistance is closely related to the construction process, and the human factor is large. It is useless to conduct tensile tests for random inspections.
20. Chemical anchor bolts are formed by punching holes with a punching machine, and then chemical slurry is poured in and the bolt rod is placed in order to achieve anchoring.
21. Expansion bolts and chemical bolts are actually both anchor bolts. In some cases, expansion bolts or chemical anchor bolts are needed because they are not pre-buried. But this situation should be avoided in the design. Because anchor bolts should be pre-buried. For example, column foot anchor bolts. Because only in this way can the best bonding and force be guaranteed. Moreover, drilling holes afterwards often causes damage to the stress-bearing steel bars in the concrete and the concrete itself.
22. In the concrete specification, components pre-buried in concrete are called pre-buried parts. According to the documents of the Ministry of Construction, expansion bolts shall not be used for curtain walls. In general new construction projects, expansion anchor bolts are strictly prohibited and should be pre-buried.
