July 23, 2015
Bolted connections in bridge structures are almost all installed using a torque-controlled tightening method. The use of controlled tightening enables the development of friction force between the plates of the bolted connections; they are therefore slip-critical connections. The resulting friction force allows for an enhanced performance of connections in fatigue conditions, reduces deflections in the structure and prevents bolts from loosening when subjected to impact loads and vibrations.
The Turn-of-Nut method is widely used to obtain the minimum preload tension force specified for bolts in slip-critical connections. This method consists in first snug-tightening the bolts. This first step is achieved with a few impacts of a pneumatic impact wrench or the full effort of a person using a spud wrench to bring the connected surfaces into firm contact with each another1.
The final tightening is then performed by applying torque to the nut (or the head) in order to achieve an additional turn with respect to the bolt axis according to the specifications in the table below, which is taken from CAN/CSA-S6-14:
The final tightening is usually controlled by first match-marking the bolt and nut in order to ensure that the specified amount of rotation is achieved (1/3 turn from snug position, 1/2 turn from snug position, etc.)
The following video presents a demonstration of the Turn-of-Nut method.
In the most recent edition of the Canadian Highway Bridge Design Code, CAN/CSA-S6-14, the cs and ks coefficients that are used to calculate the slip-resistance of bolted connections have been modified, resulting in a superior slip-resistance calculation with the Turn-of-Nut method than that obtained with the coefficients in the earlier edition. For example, sand-blasted connections (Class B) secured with ASTM A325 bolts will have a 20% greater slip-resistance when calculated with the values indicated the 2014 edition than with those indicated in the 2010 edition.
Stay informed! Another article presenting additional specifics of the Turn-of-Nut method will be published this fall.
For any questions regarding this article, contact us.
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