Slip-Critical Connections and Turn-of-Nut Pretensioning: Practical Specs for Bridge Applications


October 26, 2015

Category: Design  -  Published by: Yannick Martin

As explained in our article published on July 23, 2015, entitled Slip-Critical Connections: Use of the Turn-of-Nut Method, this pretensioning method is based on the relative rotation between the nut and bolt head in order to develop the necessary preload tension force in the bolt. This preloading generates a normal force between the faying surfaces and as a result, the required frictional resistance in the slip-critical connection is achieved.

While the principle is simple, several details come into play when it is put into practice. This article presents some of the factors that must be taken into consideration when designing, executing or inspecting slip-critical bolted connections that have been tightened using the Turn-of-Nut method.

Tightening Using the Bolt Head or Nut: Differences and Restrictions

When pretensioning bolts using the Turn-of-Nut method, a part of the bolt (the head or the nut) is stationary while the other is mobile and rotates on its axis. The choice of the stationary and the mobile component is decided on a case-by-case basis. The accessibility of the bolt head or nut with the tightening instrument is usually the deciding factor. For example, a spud wrench will be used when the extremity of the bolt is in a restrained or hard-to-access area. On the other hand, larger tightening equipment can be employed in areas where there is sufficient clearance.

Spud wrench

Even though the method itself is called Turn-of-Nut, the bolt head can equally be the mobile component during the tightening process. In fact, in cases where it’s impossible to turn the nut, the rotation of the bolt head is acceptable. Although article A10.1.6.5 of the Canadian Highway Bridge Design Code (CAN/CSA S6) does not require A325 bolts installed in standard holes to be tightened with washers, certain government agencies can request that hardened washers be systematically placed under the mobile portion of the bolt (such is the case in Quebec, where this criteria is specified in article a) of the Cahier des charges et devis généraux (CCDG)).

Attention: when A490 bolts are installed in standard holes, CAN/CSA S6 (article A10.1.6.5) specifies the use of washers under the rotating component (head or nut).

Number of Threads Protruding Beyond the Nut

Contrary to popular belief, it isn’t necessary to have a minimum of three threads protruding from the outer face of the nut to develop full slip-resistant capacity. In fact, according to CAN/CSA S6-14 (article A10.1.6.7), the tip of the bolt need only to be flush with or outside the face of the nut to be deemed acceptable.

As was the case for the mandatory use of hardened washers in Quebec, government agencies can also have specific requirements regarding bolt length. The Quebec Transportation Ministry’s CCDG equally specifies that the threaded extremity of bolts must exceed the nut by at least three (3) millimeters, i.e. one-eighth (1/8) of an inch.

Bolting Sequence in Large Assemblies

In large assemblies, the stiffest areas are bolted first, gradually progressing towards the less rigid areas. Bolts are snug-tightened and then fully tightened, using the Turn-of-Nut method for example, also in the above order (most rigid to less rigid).

Steelworkers often use fitting-up bolts to ensure that faying surfaces are firmly in contact with one another when snug-tightening is performed. A number of these bolts are placed throughout the assembly before snug-tightening is carried out. The CCDG (article specifies that these bolts must be marked with red paint before work begins so that they can be easily identified. The pretension force in fitting-up bolts can exceed that normally developed using the Turn-of-Nut method, ensuring that components are firmly joined together. Snug-tightening and the final tightening of bolts are then carried out as described above. The fitting-up bolts are subsequently removed and the assembly is completed using the remaining connection bolts.

Tool used to perform the final tightening
Air impact wrench used for snug-tightening

Special Criteria for Galvanized Bolts

Bolts can be galvanized to obtain increased protection against corrosion. The bolt’s mechanical properties, such as elastic limit and strength limit state, are not affected by the galvanization process. However, certain particular details must be taken into account while they are being fabricated. For example, CAN/CSA S6-14 (article A10.1.3.2) provides specifications regarding the thread size of galvanized nuts as well as their lubrication.

It should be noted that rust protection using galvanization is only applicable to Type 1 ASTM A325 bolts. A490 bolts must never be galvanized (CAN/CSA S6-14, article 10.4.5) as this can render them susceptible to hydrogen embrittlement3.

Reuse of Pretensioned Bolts

The stress induced in pretensioned bolts can cause permanent deformations in the threaded portion. During testing1,2, it was observed that the tensile strength of black ASTM A325 bolts tended to decrease after three (3) cycles of tightening and untightening while the tensile strength of ASTM A490 bolts began decreasing from the very first cycle. This explains why the Canadian Bridge Design Code excludes A490 bolts for reuse but allows black A325 bolts to be reused (article A10.1.6.10). It should be mentioned that CAN/CSA S6-14 only allows the reuse of black A325 bolts and indicates that galvanized bolts cannot be reused4. Moreover, the code goes on to specify that snug-tightened bolts can sometimes require touch-ups or adjustments in order to allow faying surfaces to come into full contact with one another, and that generally speaking, such procedures are not considered as reuse3.


1 R. J. Christopher, G. L. Kulak, and J. W. Fisher, “Calibration of Alloy Steel Bolts”, Journal of the Structural Division, ASCE, Vol. 92, ST2. April 1966.

2 J. L. Rumpf and J. W. Fisher, “Calibration of A325 Bolts”, Journal of the Structural Division, ASCE, Vol. 89, ST6, December 1963.

3 G.L. Kulak, “High Strength Bolting for Canadian Engineers”, Canadian Institute of Steel Construction, Toronto, 2005.

4 CAN/CSA S6-14. 2014. “Canadian Highway Bridge Design Code”, Canadian Standards Association, Mississauga, Canada.

Should you have any questions regarding this article, contact us.


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