Application and requirement of high strength bolt

The full name of high-strength bolt in production is high-strength bolt connection pair, generally referred to as high-strength bolt. Each connection pair includes a bolt, a nut and two washers, which are all produced in the same batch and processed in the same heat treatment process. According to the installation characteristics, it can be divided into large hexagon head bolt and torsional shear bolt.
Level: 8.8 and 10.9
Strength: 400MPa
Bolt performance
According to the performance grade of high-strength bolt, it is divided into 8.8 and 10.9 grades, among which the torsional shear type is only used in 10.9 grades. In the marking method, the number before the decimal point indicates the tensile strength after heat treatment, and the number after the decimal point indicates the yield strength ratio, that is, the ratio of the measured value of yield strength to the measured value of ultimate tensile strength. Grade 8.8 indicates that the tensile strength of bolt rod is not less than 800MPa, and the yield strength ratio is 0.8; grade 10.9 indicates that the tensile strength of bolt rod is not less than 1000MPa, and the yield strength ratio is 0.9. In the structural design, the diameter of high-strength bolt is generally M16 / M20 / M22 / M24 / M27 / M30, but M22 / M27 is the second choice series, and M16 / M20 / M24 / M30 is the main choice under normal circumstances.
During the assembly of high-strength bolt connection pair, the side with round table of nut shall face the chamfered side of washer. For the assembly of large hexagon head high strength bolt connection pair, the chamfered side of the washer under the bolt head shall face the bolt head. Large hexagon high strength bolt is pressure bearing type.
Grade strength
The performance grades of bolts for steel structure connection are more than 10 grades, including 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9 and 12.9. The bolts of grade 8.8 and above are made of low-carbon alloy steel or medium carbon steel and heat treated (quenched and tempered), which are generally called high-strength bolts and the rest are generally called ordinary bolts. The bolt performance grade mark consists of two parts of numbers, which respectively represent the nominal tensile strength value and yield strength ratio of bolt material. For example, a bolt of property class 4.6 means:
1. The nominal tensile strength of bolt material reaches 400MPa;
2. The yield strength ratio of bolt material is 0.6; the nominal yield strength of bolt material is 400 × 0.6 = 240mpa.
The performance grade is 10.9 high-strength bolt, and the material can reach:
1. The nominal tensile strength of bolt material is up to 1000MPa;
2. The yield strength ratio of bolt material is 0.9; the nominal yield strength of bolt material is 1000 × 0.9 = 900MPa.
The meaning of bolt performance grade is the international general standard. For bolts with the same performance grade, regardless of the difference in material and place of origin, their performance is the same, and only performance grade can be selected in design.

Anti sliding coefficient test
The code for acceptance of steel structures (gb50205) clearly stipulates that the fabrication and installation units shall carry out anti sliding test and re inspection respectively (clause 6.3.1 of the strong clause). The anti sliding coefficient must be greater than or equal to the design value.
When making the test piece on site, the test piece and the representative steel structure members shall be made of the same material and batch, the same friction surface treatment process and the same surface state shall be adopted, and the same batch of high-strength bolt connection pairs of the same performance grade shall be adopted, and stored in the same environment for anti sliding test. If the components are finished products, in addition to the anti sliding coefficient test report in the factory, the manufacturer shall also provide three groups of test pieces for each batch to inspect whether the anti sliding coefficient meets the requirements on site after the components enter the site.
In the inspection of engineering data, many construction sites only have the anti slip coefficient test report of the manufacturer in the factory, and lack the re test report of the anti slip coefficient of the test pieces provided to the construction site.
The inspection of anti sliding coefficient shall be based on the manufacturing batch of steel structure, and each 2000t of single project shall be regarded as a manufacturing batch, and those less than 2000t shall be regarded as a batch. When two or more surface treatment processes are selected for the friction surface of components in a single project, each surface treatment process shall be inspected. Three sets of test pieces for each batch.
According to GB50017-2003, the conventional treatment methods of friction surface include sand blasting (shot), inorganic zinc rich paint after sand blasting (shot), red rust after sand blasting (shot), etc.
It should be specially pointed out that the anti slip coefficient of many projects submitted for inspection is very high, and the actual friction surface treatment of components is poor, which can not meet the anti slip coefficient required by the design; or the friction surface required by the design is coated with inorganic zinc rich paint after sandblasting (shot blasting), while the submitted one is not painted (the friction coefficient decreases after painting). For the project with large design value of anti sliding coefficient, if such situation is found by on-site supervision, rectification shall be ordered and the friction surface after treatment shall be retested, otherwise serious quality risk may be left.
The acceptance code gb50205 requires that the torque coefficient of large hexagon high-strength bolt connection pair shall be retested before use, and the pre tension of torsional shear type high-strength bolt connection pair shall be retested before use. The retest batch is 8 sets of random inspection for every 3000 sets (for the high-strength bolt connection pairs with the same strength grade, the same diameter but different bolt length, if it can not be proved to be the same batch, it is regarded as different specifications, and shall be sampled and tested separately. GB / t3632-2008 torsional shear type high strength bolt and GB / t1231-2006 large hexagon high strength bolt specification stipulates: under the premise of the same batch, but when the bolt length is ≤ 100mm, the length difference is ≤ 15mm; or when the bolt length is ≥ 100mm, the length difference is ≤ 20mm, which can be regarded as the same length). The tightening of high-strength bolts shall be divided into initial tightening and final tightening. For large-scale joints, they should be divided into initial tightening, re tightening and final tightening. The initial tightening torque is about 50% of the construction torque, and the re tightening torque is equal to the initial tightening torque. In order to prevent omission, the high-strength bolt after initial or re tightening shall be marked on the nut with color. For the high-strength bolt after final tightening, mark the nut with another color. Gas cutting and reaming are strictly prohibited in the field installation of high-strength bolts. Generally, the exposure of high-strength bolts shall not be less than 2-3 buckles, and 10% of them are allowed to be exposed for 1 or 4 buckles.
When the high-strength bolts are initially tightened, re tightened and finally tightened, the bolts at the joints shall be tightened in a certain order, generally from the center of the bolt group to the outside. The initial tightening, re tightening and final tightening of high-strength bolts shall be completed on the same day, and the final tightening shall not be completed after the next day.
Calculation of construction torque
According to GB50205-2001, the calculation formula of initial tightening torque is as follows:
Torsional shear t0 = 0.065pc * D
Large hexagon t0 = 0.05tc
At the final tightening, the high-strength bolt of the torsion shear type is marked by the screw off of the plum head. For the quincunx head that cannot be screwed off with tools except for the structural reasons, the quincunx head that cannot be screwed off in the final tightening shall not exceed 5% of the total number of bolts in this node, and shall be marked with torque method and other methods according to the specification requirements, and the final tightening torque inspection shall be carried out.