Bolt tightening methods depends on types of bolts and connections. After aligning the holes in a joint, sufficient bolts shall be placed and brought to a SNUG – TIGHT condition to ensure that all parts of the joints are brought to full contact with each other.
“SNUG-TIGHT” as defined is the effort of a person using SPUD WRENCH or tightness attained by a few impacts of an IMPACT WRENCH. When slip resistant connections are not required, high strength bolts are tightened to a 'snug-tight' using an ordinary spud wrench.
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When slip resistant connections are desired, following three methods are used?
- Turn-of-the-nut tightening.
- Direct tension indicator tightening
- Calibrated wrench tightening (torque control method).
Turn-of-the-nut tightening
It is also called as Part turn method. Cheap, more reliable, and common method used in steel structure.
After all the bolts have reached the snug tight condition, it shall be further tightened by the amount of rotation as specified in the table below.
In this case, full efforts of a man using an ordinary spud wrench would bring the bolt to a snug tight position. No wrench extension or long wrenches should be used to avoid tensioning; the normal spud wrench is usually 12 inches (300mm) long.
Direct tension indicator tightening
Another way to try to ensure proper pretensioning of a bolt is through the use of direct tension indicators (DTIs). These washers have protrusions that must bear against the unturned element. As the bolt is tightened the clamping force flattens the protrusions and reduces the gap.
The gap is measured with a feeler gage. When the gap reaches the specified size, the bolt is properly pretensioned. A standard impact wrench is used to tighten the bolt with the load indicator washer inserted.
DTIs are completely independent of torque resistance of the bolt assembly, and because the compression of the DTI bumps can be visually observed (Silicone), bolt installers tend NOT to leave the bolts with insufficiently compressed DTI's.
Installation of ASTM959 direct tension indicators
It is essential that direct tension indicators be properly oriented in the assembly.
b) The bolt head is stationary while the nut is turned – DTI under nut (washer required).
c) The nut is stationary while the bolt head is turned – DTI under bolt head (washer required).
d) The nut is stationary while the bolt head is turned – DTI under nut.
Calibrated wrench tightening method.
The calibrated wrench tightening method is performed by applying a calculated average torque value to fasteners based on daily testing representative fastener assembly samples using the exact configuration of installation tools.
Except for projects having very few bolt combinations that will be pretensioned in a minimal number of days, calibrated wrench is the most expensive method to pretension bolts.
A Skidmore-Wilhelm calibration device is used to calibrate the impact wrench to achieve the specified tension.
How ASTM F1852 and F2280 are Installed
F1852 and F2280 bolts are twist-off-type tension-control bolts. These bolts must be pretensioned with a twist-off-type tension-control bolt installation wrench that has two coaxial chucks.
The inner chuck engages the splined end of the bolt. The outer chuck engages the nut. The two chucks turn opposite to one another to tighten the bolt. The splined end of the F1852 and F2280 bolt shears off at a specified tension.
Conclusion
In this article, we have explored the bolt tightening methods such as sung-tight, turn of the nut, DTI tightening, calibrated wrench and twist-off bolts (ASTM F1852 and F2280) installation in steel structures.