What is the turn of the nut method, and does it apply to anchor bolts?
The turn of the nut method is a very popular and reasonably reliable method for tensioning (F3125) A325 and A490 structural bolts without the need for expensive torque wrenches or tension measuring devices. It is a simple guide that tells the user to rotate the nut a specified amount depending on the bolt length and slope of the items being bolted together.
The chart below, summarized from the RCSC standard, is for rotation after the bolt has been snug tightened, meaning that the plies are in firm contact with one another, and the assembly has been tightened sufficiently so that the nut cannot be removed without the use of a wrench. This is only applicable to joints where all the material within the grip is steel.
|Bolt Length||Both faces normal to bolt axis||One face sloped not more than 1:20, other face normal to bolt axis||Both faces sloped not more than 1:20 from normal to bolt axis|
|Not more than 4d||½ turn (180deg)||½ turn (180deg)||2/3 turn (240deg)|
|More than 4d but not more than 8d||½ turn||2/3 turn||5/6 turn (300deg)|
|More than 8d but not more 12d||2/3 turn||5/6 turn||1 turn (360deg)|
|*When bolt length exceeds 12d, nut rotation shall be determined by testing using a tension calibrator.|
How does this apply to anchor bolts?
Normally, it doesn’t. Per the AISC Steel Construction Manual (page 14-10, 14th Edition), the majority of anchorage applications do not require pretensioning. They simply recommend that the nuts be snug tightened. However, in some special applications, there are procedures for tensioning anchor bolts using a modified turn of the nut methodology. The American Association of State Highway and Transportation Officials (AASHTO), in LRFD Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals, details turn of the nut methods for the top nut in double nut moment connections. As above, these rotation recommendations are for after the nut is snug tight.
|Diameter||F1554g36||F1554g55, g105, A449, A615 and A706g60|
|1-1/2” and below||1/6 turn (60deg)||1/3 turn (120deg)|
|Above 1-1/2||1/12 turn (30deg)||1/6 turn|
|*applicable only to the top nut in double nut moment connections|