Steel Structure QC Questions | Bolt Tightening | Skidmore Wilhelm Tension

This article is about Steel Structure QC Questions related to topic of Steel Structure Bolt Tightening, How Skidmore Wilhelm Tension Calibrator Works, Calibrated Wrench Pre Tensioning Method and Direct Tension Indicators (DTI’s) direct tension indicators are sometimes used instead of torque wrenches. In this article we summarize some international standards for CBT Test of steel structural QC Aramco.

Steel Structure QC Questions | Bolt Tightening | Skidmore Wilhelm Tension

Type of Bolt Joint

– Snug Tight
– Pre-tensioned
– Slip Critical

Methods of Bolt Tightening

– Turn-Off-Nut.
– Twist-Off-Bolt.
– Direct Tension Indicator (DTI).
– Calibrated Wrench.

Steel Structure Bolt Tightening / Pre Tensioning Procedure

1. In erection of steel structure, fastening materials such as bolts, nuts and washers shall be in accordance with ASTM A325, ASTM A 563, ASTM F436 and other applicable Code and Standard for primary connections.
2. Fastening materials for secondary connections shall be ASTM A307.
3. Fastening materials shall not be reused without prior approval.
4. Mill Certificates shall be verified as per ASTM Specifications.
5. Identify the conditions of connected plies, faying surfaces, bolt holes, burrs, and washer as per 12-SAMSS-008.
6. Check the fastening materials as per approved contract drawings and specifications.
7. All joint surfaces, including those adjacent to the bolt heads, nuts and washers shall be free of burrs, dirt and other foreign materials that would prevent solid seating of parts. (PIP STS05130 & 12-SAMSS-008).
8. Fasteners shall be inserted and aligned in the bolt hole and care to prevent undue damage of the thread joints.
9. Bolt assembly shall be brought to snug tight condition to ensure that it has a firm contact with the faying surfaces.
10. Bolt shall be tightened in sequence and systematic starting from the most rigid part of the joint to the free edges to minimized relaxation of the previously pre-tensioned bolts.
11. If impact wrenches to be used, it shall be of adequate capacity, sufficient supply of air to perform the required tightening.
12. All nuts shall be wax-dipped so as to reduce the friction during the installations.
13. Fasteners that are pre-tensioned and ready for inspection shall be color coded, die punch or otherwise mark the ends of the torque bolts.
14. Bolts shall have full thread engagement or penetration with the nut. The bolt length used shall be such that the end of the bolt extends beyond or at least flush with outer
    face of the nut. The procedure can be asked in CBT of Steel Structure QC Questions.
15. If Calibrated wrench to be used, it shall be calibrated in a daily basis.

SKIDMORE WILHELM TENSION CALIBRATOR

1. Use to perform the rotational capacity test
2. Suitability of the fastener assembly to satisfy the requirements
3. Calibration of the wrenches if applicable
4. Understanding and proper use by the bolting crew of the method of tensioning.
5. For bolts that are too long to fit in the Skidmore Wilhelm calibrator, the bolt, nut and washer assembly shall be assembled in the unit.
6. For bolt that are too short to test in the Skidmore Wilhelm Calibrator may tested in a steel joint. The maximum torque requirements, Torque ≤ 0.25 PD
   Torque = measured torque (foot-pounds)
   P = measured bolt tension, 1.15 times the fastener tension in the table (pounds)
   D= bolt diameter (feet)

How Skidmore Wilhelm Tension Calibrator Works

Is a hydraulic device that is designed for calibrating impact wrenches (and other types of installation tools) and testing shorter grip-length, high-strength fasteners. Each calibrator is a hydraulic load cell with a hole in the center for inserting a sample bolt, nut, and washer. As the bolt and nut are tightened, they compress the load cell, creating internal hydraulic pressure. A gauge measures the pressure and provide a dial readout calibrated in kilograms/pounds equivalent to the torque created in the fastener.

BASIC CALIBRATION SET-UP

1. Clamp the calibrator on a convenient beam or column using the two wing screws. Do not over tighten. Excessive force can damage the body.
2. Select a sample bolt for the wrench calibration test. Attached plate (item 5) appropriately sized for the bolt to the front of the calibrator using four cap screws (item 16). Anytime you change to a different size bolt for the test, you will need to change the plate as well.
3. Insert the bolt bushing (item 6), either round or hexagonal head, into the back of calibrator. The bushing fits over the back of two dowel pins and is held in place with the bushing retainer (item 15).
4. Insert the bolt from the back of the calibrator so that it fits into the bushing. From the front side of the calibrator, first place a hardened washer over the bolt, then tighten the nut so it is snug against the plate. The hardened washer prevent the nut from galling the plate.
5. The bolt can be tightened with an impact wrench, hand torque wrench, or other installation tool.
6. The procedure can be asked in CBT of Steel Structure QC Questions.

CALIBRATED WRENCH PRE TENSIONING METHOD

Calibrated wrench maybe used to achieve minimum bolt tension provided the wrenches are checked in daily basis in a tension measuring device for consistent tightening performance. Furthermore, a hardened washer must be used under the part of the fastener that turns during tightening.

Procedure:

1. Insert the bolt, hardened washer & nut as described in the basic calibration set-up.
2. Tighten the nut with the wrench to be calibrated.
3. Adjust the torque control mechanism or the air pressure or down so that the reading on the calibrator dial is at least 5% greater than the minimum tension required for the sized of the fastener. Minimum fastener tension requirements for A325 and A490 fasteners as specified by the Research Council on Structural Connections are printed on the gauge dial.

ALTERNATIVE DESIGN BOLT (TENSION-CONTROL BOLTS)

Tension control bolts are fasteners that undergo a physical change at installation such as the shearing off of a splined end. Although this bolts may require special installation tools, they can be tested in the calibrator in the same manner as standard Some tension control bolts have a round head instead of a hexagonal head. Special adapters are available from the factory for these bolts. Testing sample bolts will ensure that the installation tools and fasteners are performing to their manufacturers specs.

Procedure:

1. Install the special bushing available from the skidmore-Wilhelm for use with tension-control bolts. Use the same plate you would use for regular hexagonal bolts. Also install any special hardened flat washer used in the actual joint.
2. Tighten the tension control bolt, according to the manufacturers installation procedures, until twist-off. The reading on the calibrator dial should be at least 5% greater than the minimum tensioned required.

DIRECT TENSION INDICATORS (DTI’s)

Load indicating washer or DTI’s can be used in the calibrator and check to see that they are performing properly. Special bushing that allow a socket to be placed on the nut are available for those bolts where the washer is placed under the bolt head. These bushings replace the normal bushings used to prevent bolt head rotation.

Procedure:

1. Install the bolt with the load indicating washer under the bolt head and any special hardened flat washer used in the actual joint, through the front plate. Installing the bolt from the front allows a feeler gauge to be used to check the washer.

2. From the back of the calibrator, install the nut onto the bolt along with any special hardened flat washers used in the actual joints.
3. Tighten the fastener assembly, according to the device manufacturer installation procedures, until the washer indicates the proper tension. Tightening the assembly will require two (2) wrenches, one wrench to tighten the nut from the back and the second wrench to prevent the bolt head from rotating.
4. Compare the tension reading on the calibrator gauge to the minimum tension required. The reading should be at least 5% greater. The procedure can be asked in CBT of Steel Structure QC Questions answers.

TROUBLE SHOOTING SUGGESTIONS

1. If you are not satisfied with any test results (for example, if readings are widely scattered), check the following:

a. Make sure you are using hardened washer.
b. Make sure each bolt is properly marked A325 or A490.
c. Make sure that the bolts are not completely dry. On most bolts the rust preventive is sufficient lubrication. However, if bolt are completely dry there is a tendency for the tread to seize before the bolt gets up to tension. A small amount of oil on all bolts, both test and installation bolts, will correct this situation.

2. Use a new bolt, nut and washer for each test since used nuts and bolts give erratic calibration results. Also, the values marked on the gauge dial are minimum bolt tension as approved by the Research Council on Structural Connections and endorse by the AISC. It is recommended that the wrench be set to induce bolt tension 5% to 10% in excess of these values.
3. Daily rechecks or wrench calibration should be made. This should be done at the job site using the same length of hose and connections that will be used on the job. In this way, calibrating conditions will be identical with the job conditions.

ADDITIONAL INSTRUCTIONS FOR TENSION CONTROL BOLTS

Typically, the bolts, nuts, and washer are packaged as a calibrated assembly, So testing must be in strict accordance with the manufacturer’s specifications. Lubrication of the components is a key element of the specifications. Verify that correct lubrication has been used before testing. If bolts do not twist off according to specification, check the following:

1. Verify that the correct lubrication has been used.
2. Friction between the bolt head and the calibrator bushing should be great enough to prevent rotation of the bolt during tightening. If bolt turns, remove excessive oil or grease from the mating surface of the bushing.
3. If bolts are being tested above the AISC tension requirements, thicker plates may be needed. They are available from the factory. Refer to Bulletin 308-1 for more information.
4. Before starting pre-tensioning or torquing, inspector should check if the Torque Wrench has been calibrated as per SATR-M-2004. The calibration frequency, shall be in a daily basis. Torque Values are found in the Table Specification of Structural Joint Using ASTM A325 & ASTM F490 bolts.

Theoretical torque value (Tv) = Db × Tt × Coef. of friction

Where:
Tv = calculated torque value
Db = bolt diameter in ft.

Tt = tesion value in the table in kips

Coefficient of friction = 0.20

1 kip = 1000 lbs

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