The method statement for bolt tightening describes the methodology and testing for torquing bolts in structural steel used in mechanical equipment provided to commercial buildings, plants, refineries, and their equipment structures.
Scope of Work
The scope of work includes bolt tightening requirements of all mechanical equipment structural joints and sections showing the bolt size to be used, field inspection, and implementing safety precautions during work execution to prevent accidents.
Work shall be performed according to project applicable standards and project specifications:
High Strength Bolt Tightening Inspection and Testing (ASTM A325/A490 Bolts)
Pre-Installation Procedure Verification Testing Turn-of-Nut Method (ASTM A325/ASTM A490 bolts)
Pre-Installation Procedure Verification Testing of Calibration Wrench (ASTM A325/ASTM A490 bolts)
Structural alignment inspection during erection
Correcting structural alignment errors
Pipe Racks, Steel Supports, and Miscellaneous Steel Structures
Structural Steel Foreman/Mechanical Foreman
Structural/Mechanical Skilled Workers
Tools and Equipment
Calibrated Torque Wrench
Procedure – Method Statement for Bolt Tightening
Preparation of site and materials
Unless the contract documents specify otherwise, determine which type of bolt materials will be used.
High-strength A325 or A490 bolts are required for all primary structural frames and some secondary framing.
Requisition of a specific number of bolts
Steel members to be joined must be free from oil, grease, or any loose adherent materials.
Any circumstances discovered by the contractor that have an impact on the progress, performance, or completion of work activities, such as discrepancies between drawings and delivered steel members to be jointed, incorrectly fabricated steel members, or incomplete or unacceptable material, must be reported in writing.
Before tightening the bolts, the contractor must double-check the connection points’ location, position, elevation, and quality.
To prevent injury while starting the work, the contractor must set up scaffolding, access, ladder, wood boards, or floor plates for the structural/mechanical workers who will be doing the task.
Stability of the Structure
The contractor is responsible for the availability and installation of any temporary bracing or guy cables needed to counteract the induced loads at all times. This responsibility also includes any temporary bracing required to keep partially constructed structural assemblies safe and stable.
Before final bolting connections are made, the structure must be plumbed, leveled, and braced.
Connections with Bolts
The materials, method of installation, tension controls, types of wrenches to be used, and inspection processes for structural joints formed using high-strength bolt assemblies must all comply with the AISC Specification for Structural Joints Using ASTM A325 or A490 Bolts.
When using a direct tension indicator washer, make sure you follow the manufacturer’s instructions.
All bolts must match the shop drawing, erection drawings, or other contract documents accurately.
After aligning the holes in the joint, adequate bolts should be installed and tightened to a Snug–Tight condition to guarantee that all portions of the joint are in complete contact with each other.
As defined, “Snug-Tight” refers to a person’s effort with a spud wrench or the tightness achieved with a few impacts of an impact wrench.
• Using the SPUD Wrench Method for manual operation
In this instance, a man using a standard spud wrench would be able to tighten the bolt to a snug fit. To avoid tension, no wrench extension or long wrenches should be used; a standard spud wrench is normally 12 inches (300mm) long.
• Using IMPACT Wrench Method
A few strikes with an impact wrench should be enough to tighten the nuts and bolts up snugly. This operation should not take more than 30 seconds if the impact wrench is powerful enough to apply to bolts that have already been tightened to full contact with the steel connection plates.
Once the snug-tight position is attained, a straight line on the bolt head should be marked with a metal marker or a pentel pen before going to the nut location. This will show the nut’s starting position before it is tightened to its final position.
The likelihood of a bolt loosening in a critical-slip connection where the joint is subjected to vibratory or fatigue loads is very high. In this scenario, an additional jamb-nut may be required after the bolts have been fully tensioned.
To prevent the nut from turning, the threads can be spiked or marred, or the nut can be tack-weld to the base metal.
• Direct Tension Indicator
Direct Tension Indicator tightening requires the use of a device that allows accurate measurement of bolt tension, such as a load indicator washer.
All bolts in the joints must be further tightened using either the turn-off-nut or calibrated wrench method after the initial tightening operation.
Once the bolt is snugly tight, the amount of rotation required should be applied to further tighten it. Refer to the table below for reference.
|Table||Disposition From Snug Tight Condition|
|Bolt length (underside head to the end of the bolt)||Bottom face normal to the bolt axis||One face normal to bolt axis and other sloped not more than 1:20 (beveled washer not used)||Both faces sloped not more than 1:20 from normal to the bolt axis (beveled washer not used)|
|Up to and diameter but not exceeding 4 diameters||1/3 turn||1/2 turn||2/3 turn|
|Over 4 diameters but not exceeding 8 diameters||1/2 turn||2/3 turn||5/6 turn|
|Over 8 diameters but not exceeding 12 diameters||2/3 turn||5/6 turn||1 turn|
Calibrated Wrench Method
Following the initial tightening required, a calibrated torque wrench must be used to tighten bolts to a tension that is at least 5% higher than the minimum bolt tension as per ASTM A325. See the table below for reference.
For each bolt diameter, length, and grade being fitted, the wrench must be calibrated at least once a year on a working day. Tightening in a device capable of indicating actual bolt tension is required for calibration. For each diameter of bolts being installed, three typical bolts shall be tightened.
|Nominal Bolt Size, inches||A-325 Bolts||A-490 Bolts|
Direct Tension Indicator Method
A standard impact wrench is used to tighten the bolt with the load indicator washer inserted.
Compressed air is the most common source of power. A sufficient amount of pressure must be applied to the tool, with a minimum of 100 psi for bolts with a diameter of 7/8″ or less. Pressure must be increased for larger bolts. Hose lines should be large enough to accommodate the number and size of wrenches that will be utilized.
|Load Indicator Gaps to Give Required Minimum Bolt Tension|
|Load Indicator Fitting||A325||A490|
|Under Bolt Head Black Finish Bolts||0.025 in.||0.015 in.|
|Under Nut with Hardened Flat Washer Black Finish||0.015 in.||0.010 in.|
|With the gaps shown above, required minimum bolt tensions will be induced as given in Table 2|
Prior to beginning the work, inspect the physical appearance and material specifications for bolting.
For bolt specification requirements, including diameters and manufacturer’s recommendations, make sure to refer to the IFC drawings. Always follow the standard compliance criteria.
Check, verify, and continuously monitor bolting work activities on the job site and in the laydown yard where assembly and bolt tightening are taking place.
For final acceptance, a Request for Inspection is to be submitted.
Ascertain that all workers are wearing the appropriate personal protective equipment (PPE).
To prevent injury to all personnel exposed by the operations, place warning devices such as warning tapes, signboards, and barriers around the work area.
Tools (both hand and machine-operated tools) and safety harnesses must be monitored and coded according to safety monthly monitoring guidelines to ensure that they are accepted and comply with daily site activities.
Attachments – Method Statement for Bolt Tightening
Job Safety Analysis
Inspection Request Form