The use and requirements of high-strength bolts

The use and requirements of high-strength bolts

High strength bolts are commonly referred to as high-strength bolt connection pairs in production. Each connecting pair includes a bolt, a nut, and two washers, all produced in the same batch and processed in the same heat treatment process. According to installation characteristics, it is divided into large hexagonal head bolts and torsional shear type bolts.

Level: 8.8 and 10.9

Strength: 400MPa level

Bolt performance

According to the performance level of high-strength bolts, they are divided into 8.8 and 10.9 levels, with torsion shear type only used in 10.9 levels. In terms of marking method, the number before the decimal point represents the tensile strength after heat treatment, and the number after the decimal point represents the yield strength ratio, which is the ratio of the measured yield strength value to the measured ultimate tensile strength value. Grade 8.8 indicates that the tensile strength of the bolt rod is not less than 800MPa, and the yield strength ratio is 0.8; Grade 10.9 indicates 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 diameter of high-strength bolts is generally M16/M20/M22/M24/M27/M30, but M22/M27 is the second choice series. Normally, M16/M20/M24/M30 is the main choice.

When assembling high-strength bolt connections, the side of the nut with a circular countertop should face the chamfered side of the washer. For the assembly of high-strength bolts with large hexagonal heads, the chamfered side of the washer under the bolt head should face the bolt head. The large hexagonal high-strength bolt is a pressure bearing type.

Grade strength

The performance grades of bolts for steel structural join can be divided into more than 10 grades, including 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9 and 12.9. Among them, 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 label consists of two parts of numbers, representing the nominal tensile strength value and yield strength ratio of the bolt material. For example, bolts with a performance level of 4.6 mean:

1. The nominal tensile strength of bolt material reaches 400MPa level;

2. The yield strength ratio of bolt material is 0.6; The nominal yield strength of bolt material reaches 400 × 0.6=240MPa level.

Performance grade 10.9 high-strength bolts, after heat treatment, can achieve:

1. The nominal tensile strength of bolt material reaches 1000MPa level;

2. The yield strength ratio of bolt material is 0.9; The nominal yield strength of bolt material reaches 1000 × 0.9=900MPa level.

The meaning of bolt performance grade is an internationally recognized standard. Bolts with the same performance grade, regardless of their material and origin, have the same performance, and only the performance grade can be selected for design.

Anti slip coefficient test

The Code for Acceptance of Steel Structures GB50205 clearly stipulates that the production and installation units shall conduct anti slip tests and re inspections respectively (Article 6.3.1 of the strong clause). The anti slip coefficient must be greater than or equal to the design value.

When making specimens on site, they should be made of the same material and batch as the steel structural components they represent, using the same friction surface treatment process and having the same surface condition. They should also be connected by high-strength bolts of the same performance level in the same batch and stored in the same environment for anti slip testing. If the component is a finished product leaving the factory, in addition to the anti slip coefficient test report required in the factory, the manufacturer should also provide three sets of test pieces for each batch to inspect whether the anti slip coefficient meets the requirements on site after the component enters the site.

In the inspection of engineering data, many construction sites only have anti slip coefficient test reports from the manufacturing plant in the factory, and there is a lack of retest reports on the anti slip coefficient of specimens provided to the construction site.

The anti slip coefficient inspection should be based on the steel structure manufacturing batch, with each 2000t of a single project as a manufacturing batch, and those less than 2000t should be considered as a batch. When two or more surface treatment processes are used for the friction surface of components in a single project, each surface treatment process needs to be inspected. Three sets of specimens per batch.

The conventional treatment methods for friction surfaces mentioned in GB50017-2003 include sandblasting (shot blasting), applying inorganic zinc rich paint after sandblasting (shot blasting), and generating red rust after sandblasting (shot blasting).

It should be pointed out that many engineering projects have high anti slip coefficients for inspection, and the actual friction surface treatment of components is poor, completely unable to meet the design requirements for anti slip coefficients; Alternatively, the friction surface required by the design is coated with inorganic zinc rich paint after sandblasting (shot), while the one submitted for inspection is unpainted (with reduced friction coefficient after painting). For projects with large values of anti slip coefficient design, if such situations are found by on-site supervision, they must be ordered to rectify and retest the friction surface after treatment, otherwise serious quality hazards may be left behind.

Acceptance code GB50205 requires that the torque coefficient of the large hexagonal high-strength bolt connection pair needs to be retested before use, while the torsion shear type high-strength bolt connection pair needs to be retested for pre tension, and can only be used after passing the test. The retest batch size is 8 samples per 3000 sets For high-strength bolt connection pairs with the same strength grade and diameter but different bolt lengths, if it cannot be proven to be of the same batch, they should be considered as different specifications and should be sampled and tested separately. The GB/T3632-2008 Torsional Shear High Strength Bolt and GB/T1231-2006 Large Hexagon High Strength Bolt specifications stipulate that under the same batch, if the bolt length is ≤ 100mm, the length difference is ≤ 15mm; or if the bolt length is ≥ 100mm, the length difference is ≤ 20mm, it can be considered as the same Length). The tightening of high-strength bolts should be divided into initial tightening and final tightening. For large nodes, they should be divided into initial tightening, secondary 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. To prevent omissions, high-strength bolts after initial or re tightening should be marked with a color on the nut. Mark the high-strength bolts after final tightening with another color on the nuts. Gas cutting and hole expansion are strictly prohibited during on-site installation of high-strength bolts. High strength bolts are generally required to be exposed with no less than 2-3 buckles, and 10% of them are allowed to be exposed with 1 or 4 buckles.

During the initial tightening, re tightening, and final tightening of high-strength bolts, the bolts at the connection should be tightened in a certain sequence, generally from the center of the bolt group to the outside. The initial, secondary, and final tightening of high-strength bolts should be completed on the same day and cannot be completed after the next day.

Construction torque calculation

According to the GB50205-2001 specification, the calculation formula for initial tightening torque is as follows:

Torsional shear type T0=0.065Pc * d

Large hexagonal T0=0.05Tc

During final tightening, the torque shear type high-strength bolts are tightened with a plum blossom head as the tightening symbol. For those who cannot use tools to unscrew the plum blossom head due to structural reasons, the number of plum blossom heads that cannot be removed during final tightening should not exceed 5% of the total number of bolts at that node, and they should be marked using torque method or other methods according to the specifications, and the final tightening torque should be checked.