Installation and inspection of high strength bolts

I. Preface

The high-strength bolts for steel structure produced by domestic high strength bolts manufacturers mainly include A325 and A490 of American ASTM specification, f10t and s10t of Japanese JIS specification. Domestic high strength bolts manufacturers will produce bolts of Japanese and American specifications at the same time, because of the design specifications used in China, mainly referring to American AISC specifications; while the manufacturing and construction of steel structure factories mostly follow Japanese standards. Therefore, most of the steel structures of buildings use the torque control high-strength bolts of Japanese specifications, but the bolts used in bridge engineering are still mainly American specifications.

In practice, common designers directly use the relevant values of Japanese codes as the design basis, so mixing the two different codes is not correct in the overall safety concept of the structure. For bolts of different specifications, the calculated strength shall be converted according to the design concept of the same design specification before they can be used as the basis for design.

Contents:

II. Locking method of high strength bolt

Bolt locking methods include: torque control method, nut rotation method, direct tension indicator method, in which torque control method includes torque wrench method and broken tail bolt method.
1. Torque wrench method
The torque wrench method uses the torque to control the bolt pretension. The bolt locking method adopts the torque value obtained from the verification test on that day for the locking operation. After the bolt is locked, the torque wrench is also used to check whether the torque meets the standard.
2. Nut rotation method
Lock the nut until the joint surface is close to each other, and then rotate the nut with a manual wrench. The rotation of f10t bolt is 120 degrees except for M12 bolt, which is 60 degrees. When the bolt length exceeds 5 times the bolt diameter, the rotation amount shall be determined by the verification test. When the nut rotation method is used for construction, it is possible to visually check whether the locking is completed.
JIS specification in Japan is slightly different from ASTM specification in the United States, and table 1 shows the comparison between them.
Table 1 Comparison between Japanese specification and ASTM specification for nut rotation method

Species F10T、S10T ASTM A325 & A490 Remark
Bolt locking rotation angle 120 degrees (M12 60 degrees) below 5D (included) (1)Both sides are at right angles to the bolt axis: 1、D=Bolt diameter.
Less than 4D1 / 3 turn (120 degrees) 2、Each group of bolts shall be marked as shown in Figure 1.
4D~8D:1 / 2 turn (180 degrees)
8D~12D:2 / 3 turn (240 degrees)
(2)One side is perpendicular to the bolt axis, and the other side is 1:20 or less inclined to the bolt axis, without the use of inclined gasket:
Less than 4D: 1 / 2 turn (180 degrees)
4D ~ 8D: 2 / 3 turn (240 degrees)
8D ~ 12D: 5 / 6 turns (300 degrees)
(3)The slope between both sides and the vertical surface of bolt axis is less than 1:20, and inclined gasket is not used:
Less than 4D: 2 / 3 turn (240 degrees)
4D ~ 8D: 5 / 6 turns (300 degrees)
8D ~ 12D: 1 turn (360 degrees)
Angle + 30 degrees -30 degrees to + 60 degrees  
License difference (M12 +30 degree)

20191027205110 94501 - Installation and inspection of high strength bolts
Figure 1 Calculation of mark and rotation

3. Broken tail bolt method
The bolt end breaking method is a locking method which uses the torsional strength of the bolt end to control the pretension of the bolt. When the tail bolt method is used for construction, it can be visually checked whether the locking is completed.
The commonly used bolt is JSS s10t, while the bolt corresponding to ASTM A325 and A490 is F1852 and F2280 respectively, which are seldom used at present. When locking, the bolt group shall be locked to the snug tight state first, and then the end breaking torque control electric wrench shall be used to apply the pre torque. When the bolt reaches the pre tension value, the end will be twisted by the electric wrench; when the bolt group cannot be locked to the end breaking, the bolt group shall be replaced, and the end breaking tail shall not be cut off by itself.
4. Direct tension indicator method
Before the construction of direct tension indicator method, the gap between direct tension indicators (standard type) or resin extrusion status (resin indicator type) shall be tested by appropriate methods as the basis for construction and verification.
The standard type direct tension indicator is manufactured in accordance with ASTM f959 / f959m direct tension indicator specification of American Society for testing and materials. It is a washer with several half moon shaped raised bumps. Its material strength, shape, compression load, stamping, annealing treatment and coating during manufacturing, etc. shall comply with relevant regulations.
In the process of bolt group locking, the tension of bolt increases gradually, and the bump of washer is squeezed and flattened gradually. When the gap gauge cannot be placed into the gap between bolt head or nut and washer, it means that the bump of washer reaches the required deformation, that is to say, the bolt group reaches the design value of pretension.
For the bolt group installed by direct tension indicator method, the verification test shall be carried out before construction to determine the standard of visual inspection. Before the bolt is locked, it must be confirmed that the clearance gauge can extend a certain distance between the protrusions of the direct tension indicator. If the gap gauge cannot reach between the protrusions of the direct tension indicator, or if the distance between the protrusions exceeds a certain distance, the tension indicator shall be replaced.

III. Installation of high strength bolts

1. Except for the following situations, washers are not required for joints with high-strength bolts.

  • (1) When the inclination of the contact surface between the joint plate and the bolt head or nut to the vertical plane of the bolt axis is greater than 1:20, oblique washers shall be adopted to ensure that all the joint surfaces of the joint can be flat and tight.
  • (2) The torque control method shall be used for locking, and the rotating end shall be provided with gasket.
  • (3) When direct tension indicator is used to lock bolt connection:
  • A. when the direct tension indicator is on the bolt head side and the nut is turned to lock, the washer is placed on the nut.
  • B. when the direct tension indicator is on the nut side and the nut is turned to lock, the washer is placed between the nut and the direct tension indicator.
  • C. when the direct tension indicator is on the bolt head side and the bolt head is turned to lock, the washer is placed between the bolt head and the direct tension indicator.
  • D. when the direct tension indicator is on the nut side and the bolt head is turned to lock, the washer is placed on the bolt head.
  • (4) For bolted joint of the outermost steel plate, when using enlarged holes or slotted holes, washers shall be adopted. The size of washers shall be able to completely cover the bolt holes, and the washers shall have sufficient thickness and proper material (see Table 2).

When the bolts of ASTM A490, F10 or s10t are used for bolt joint and the steel with the falling strength less than 2.8tf/cm2 is used, hardened washers shall be used at the bolt head and nut.
Table 2 requirements of washers for bolt joints with enlarged or slotted holes

Bolt material Bolt nominal diameter Hole type of outermost steel plate bolt
Mm Enlarging hole Short slot Long slot
ASTM A325 13~38 Hardened washers are required 8 mm thick base plate (a, b)
ASTM A490 ≦25
JIS F10T
JSS S10T >25Hardened washer (a, c) with thickness of 8mm shall be adopted. Hardened washer plus base plate with thickness of 10 mm (a, b)
Remarks:
A. It is not allowed to take multiple washers and total their thickness.
B. The bolt base plate must be made of structural grade steel without hardening treatment.
C. It can also be replaced by hardened washer, plus base plate with thickness of 10 mm, which does not need hardening treatment.

2. Locking twice
Unless otherwise specified in the design drawing, the high-strength bolt shall be locked twice to the required pretension.

  • (1) For the first time, lock the adapter plate to the first locking torque (tight state), see Table 3 for details.
  • (2) Lock the second time to the specified pre tension range.

Table 3 first locking torque of f10t or s10t

Bolt nominal diameter (mm) Locking torque (kgf-m)
12 5
16 10
20 15
22 15
24 20
27 30
30 40

3. High strength bolt locking
During the installation of high-strength bolts, unless otherwise specified in the specification, the pre tension shall be applied by rotating nut method, direct tension indicator or torque wrench according to the specified procedure. In recent years, torque control bolts have been widely used in civil and construction engineering structures. According to AISC code for design of steel structures and Japanese code for design of steel structures, the conditions for application of pretension and calculation of pretension during installation of high-strength bolts, as well as the locking process of high-strength bolts are described as follows:
(1) Installation of bolts
The AISC code requires the use of A325 and A490 bolts to comply with the RCSC unless otherwise specified in the design code. As for the installation part of high-strength bolts, the working stress method design code requires that the bolt classified as friction type and the bolt bearing direct tension of the joint must be locked to the pre tension. For pressure type bolts without definite pre tension in design, they shall still be locked to “snug tight” or the first locking torque during installation. The limit design code (LRFD) requires the installation of high-strength bolts. In addition to the bolts at the joints, the bolts do not bear the tension and allow the relative sliding of the joint plate, and the design does not consider the fatigue effect caused by looseness and vibration, or repeated load, the bolts only need to be locked to the “snug tight state” according to the regulations, otherwise, the bolts after installation must reach the required pre tension. The purpose of the code is to make a certain amount of shear force can be transmitted by friction. The so-called tight state system uses an impact wrench to lock to several times of impact, or uses a general wrench to lock by manpower, so that the joint surface contacts closely. Generally speaking, there are some pretensions in the bolts in the tight state. In principle, the higher the pretension of the bolt, the better under the premise of not causing bolt damage or permanent deformation. The pre tension specified in the working stress design method and the limit design method is to make the effective stress of the bolt reach 70% of the tensile strength.
(2) Installation of JIS f10t and JSS s10t bolts
In Japanese steel structure design standard (ASD), the friction strength produced by pretension during the installation of high-strength bolts is used to resist the shear force caused by long-term or short-term load, while the LSD of steel structure requires high-strength bolts at the joints to provide enough breaking strength and friction strength to resist the load in the stress direction under the service limit. 。 The so-called breaking strength is to multiply the bolt pretension by the discount value, so as to ensure that the joint is still tight when it is under the load of the service limit. The design locking axial force calculation method of Japanese steel construction code is as follows:

Design pretension = 0.75988 8 2 - Installation and inspection of high strength boltsAe for F10T & S10T

Among988 8 2 - Installation and inspection of high strength boltsMinimum falling stress value of bolt test piece

Ae:Effective stress area of bolt

If AISC is compared with the specified pretension of 0.7988-8-3ae indicated by ultimate strength, f10t and s10t can be converted to 0.675988-8-3ae, which is slightly lower than the current AISC. When f10t and s10t are used for design, appropriate values shall be selected according to the specifications.
IV. Verification test before installation
In addition to the bolt joint without pretension specified in the contract drawing, the construction procedure and inspection verification test shall be carried out before the bolt installation to confirm that the bolt can reach its minimum pretension, and the test can only be installed and used after being approved by the supervision engineer. The implementation mode of the verification test shall be included in the installation plan and submitted to the supervision engineer for approval.
The verification test shall be carried out at the construction site and the axial force meter shall be used to confirm the pre tension. The axial force meter shall be calibrated annually. Before the test, it shall be confirmed that the lubrication state, locking equipment, locking method and procedure of the whole bolt set are the same as those of the actual construction. The test execution mode is divided according to the locking method as follows:
1. Torque wrench method

  • (1) The test shall be carried out before the installation of bolt locking work on that day.
  • (2) The test samples shall be classified according to the bolt groups of each diameter, strength and batch number, and at least three bolt groups shall be taken from each type of test body for test.
  • (3) The test results shall confirm that the locking method can develop the pretension required by the verification test, and establish the torsion standard during construction and construct according to it.
  • If the pretension obtained from any bolt group test fails to meet the pretension required by the verification test, the test must be carried out again after finding out the causes and solutions before the field installation, but the bolt group cannot be cleaned or lubricated arbitrarily before the retest.

2. Broken tail bolt method

  • (1) The test shall be carried out before the installation of bolts in each section (area).
  • (2) The test samples shall be classified according to the bolt groups of each diameter and strength, and at least 3 bolt groups shall be taken for each type of test body.
  • (3) the locking method shall be confirmed in the test results, which can develop the pre tension required for the verification test. If the pre tension cannot meet the requirements when the bolt is broken, the bolt groups with similar surface conditions shall not be used.

3. Nut rotation method

  • (1) The test shall be carried out before the installation of bolts in each section (area).
  • (2) The test samples shall be classified by bolt groups of each length, diameter and strength, and at least three bolt groups shall be taken for each type of test body for test.
  • (3) The test results shall confirm that the locking method can develop the pretension required for the verification test.
  • (4) The test shall include the establishment of the torsion standard during the construction, including the corresponding torsion value when the lock is close to the state and the lock is to the required rotation angle, and the construction shall be based on it.
  • (5) If the pretension obtained from the test of any bolt group fails to meet the pretension required by the verification test, the test must be carried out again after finding out the causes and solutions before the field installation.

4. Direct tension indicator method

  • (1) The test shall be carried out before the first bolt installation.
  • (2) For test sampling, bolt groups of each diameter, strength and direct tension indicator batch number shall be classified, and at least three bolt groups shall be taken for each type of test body for test.
  • (3) The test results shall confirm that the locking method can develop the pretension required by the verification test, and establish the torsion standard during construction and construct according to it.
  • (4) The test results shall include the inspection standards after the direct tension indicator is installed.
  • If the pretension obtained from any bolt group test fails to meet the pretension required by the verification test, the test must be carried out again after finding out the causes and solutions before field installation. For ASTM A325, A490, f1852 and f2280 bolts, the pre tension standard for the verification test before installation is in accordance with the provisions of “specification structural joints using high strength bolts” (2014) published by RCSC; for f10t and s10t bolts, the pre tension standard for the verification test is 1.05 times of the minimum pre tension.

V. Inspection after bolt locking

After locking, the surplus length of the bolt shall be inspected first, and the thread at the end of the bolt shall protrude from the nut by at least one complete thread, and not more than six threads.
1. Torque wrench method

  • (1) After completion of locking, visual inspection shall be conducted by the marked reference line after the first locking to check whether the washer rotates and the nut rotates abnormally.
  • (2) If the rotation of the bolt group is obviously insufficient in the visual inspection, the torque wrench can be used for locking again. After the locking is completed, confirm whether the torque value meets the standard.
  • (3) In addition to the visual inspection, the torque wrench shall be used to confirm whether the torque value of the bolt group after locking meets the requirements. If not, the bolt group shall be replaced. The torque value after locking shall not be less than the torque value corresponding to the specified minimum pretension according to the verification test results of that day.
  • (4) For each bolt group, if any bolt group needs to be replaced, after the bolt group is replaced and locked, all bolt groups of the group shall be rechecked and their torque value shall be confirmed.

2. Broken tail bolt method

  • (1) After the locking, visual inspection shall be carried out to confirm whether the end of the bolt has been broken, and the bolt must be kept straight, and the nut or gasket shall be close to the joint surface.
  • (2) If it is found through visual inspection that the bolt is not broken, it shall be reconfirmed that it is still straight and free of bending deformation, and then it shall be locked again. If it is still unable to be locked, the bolt set shall be replaced.
  • (3) In addition to the visual inspection, the torque wrench shall be used to confirm whether the torque value of the bolt group after locking meets the requirements. If not, the bolt group shall be replaced. The torque value after locking shall not be less than the torque value corresponding to the specified minimum pretension of the current section (area) verification test results.
  • (4) For each bolt group, if any bolt group needs to be replaced, after the bolt group is replaced and locked, all bolt groups of the group shall be rechecked and the torque value shall be confirmed.
  • (5) After locking, the end of the bolt protrudes from the nut, at least one complete screw tooth shall be provided, and no more than six screws shall be provided.

3. Nut rotation method

  • (1) After completion of locking, visual inspection shall be carried out by the marked reference line after the first locking to check whether the washer rotates and the nut rotates abnormally.
  • (2) After the bolt is locked, check whether the rotation amount of the nut meets the requirements. If the rotation amount is too large, replace the bolt group; if the rotation amount is insufficient, lock to the required rotation amount; if the rotation amount cannot be locked to the required rotation amount, replace the bolt group.
  • (3) After the bolt is locked, the thread at the end of the bolt shall protrude at least one complete thread of the nut, and not more than six threads.

4. Direct tension indicator method

  • (1) The maximum distance that the clearance gauge can enter between protrusions shall be determined according to the verification test before installation, and shall be inspected accordingly.
  • (2) After the bolt is locked, it must be confirmed that the clearance gauge cannot reach between the protrusions, or the distance of access is not greater than the distance of the verification test, otherwise it shall be locked again. If it still cannot meet the above requirements after locking again, the bolt set shall be replaced.
  • (3) After locking, the end of the bolt protrudes from the nut, at least one complete screw tooth shall be provided, and no more than six screws shall be provided.

5. The inspection methods of each locking method are summarized as follows

  • (1) When using the torque wrench method, in addition to the visual inspection, the torque confirmation inspection shall be conducted. The inspection frequency of the torque confirmation inspection shall be at least 10% of the number of bolt groups, and at least one spot inspection shall be conducted for every five beam column joints, and at least three bolt groups shall be inspected for each spot inspection. The above torque verification inspection shall be completed within three days after the bolt is locked and before the welding of the joint. If the inspection cannot be completed within three days after the bolt is locked due to force majeure, the inspection method and frequency shall be approved by the supervision engineer.
  • (2) In addition to the visual inspection, the tail bolt method shall be adopted. The inspection frequency of torque confirmation inspection shall be at least 2% of the number of bolt groups, and at least one spot inspection shall be conducted for every ten beam column joints, and at least two bolt groups shall be inspected for each spot. The above torque verification inspection shall be completed within three days after the bolt is locked and before the welding of the joint. If the inspection cannot be completed within three days after the bolt is locked due to force majeure, the inspection method and frequency shall be approved by the supervision engineer.
  • (3) When adopting the nut rotation method, visual inspection shall be carried out to confirm that the nut rotation meets the requirements, and the inspection frequency shall be at least 10% of the number of bolt groups, and at least one spot inspection shall be conducted for every five beam column joints, and at least three bolt groups shall be inspected for each place. The above inspection shall be completed before the joint is welded.
  • (4) When adopting the direct tension indicator method, the direct tension indicator shall conform to the provisions of ASTM f959, and shall be visually inspected to confirm that the gasket bulge reaches the required deformation. The inspection frequency shall be at least 2% of the number of bolt groups, and at least one spot inspection shall be conducted for every ten beam column joints, and at least two bolt groups shall be inspected for each spot. The above inspection shall be completed before the joint is welded.

Source: China High Strength Bolts Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@ugsteelmill.com

Summary
installation and inspection of high strength bolts - Installation and inspection of high strength bolts
Article Name
Installation and inspection of high strength bolts
Description
The high-strength bolts for steel structure produced by domestic manufacturers mainly include A325 and A490 of American ASTM specification, f10t and s10t of Japanese JIS specification.
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Publisher Name
www.ugsteelmill.com
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