Snug-tight vs pretensioned vs slip-critical joints (RCSC)

Who decides

For bolts in shear or combined shear and tension, the Engineer of Record must specify the joint type in the contract documents as snug-tightened, pretensioned or slip-critical. For slip-critical joints the slip resistance class is also specified. For bolts in tension only, the choice is snug-tightened or pretensioned ⁠¹. Design, installation and inspection all follow from that choice, so the drawings are where to look ⁠¹.

Scroll the table sideways to see every column.

Source: RCSC Table 4.1 ⁠¹.

Snug-tightened joints

Snug-tightened joints are allowed except where Sections 4.2 and 4.3 require otherwise ⁠¹. The snug-tight condition is defined as plies “brought into firm contact” with each bolt at least as tight as “a few impacts of an impact wrench, resistance to a suitable non-impacting wrench, or the full effort of an ironworker using an ordinary spud wrench” ⁠¹. The commentary adds an electric torque wrench run until it begins to slow, and says more than one pass through the bolt pattern may be needed ⁠¹.

There is no number. The tension in each bolt depends on the thickness, flatness and parallelism of the plies and the effort applied ⁠¹. In thick plates or plates with large burrs the faying surfaces may not touch at every bolt, and RCSC says this is generally not detrimental ⁠¹.

Installation per Section 8.1 ⁠¹:

1. Align the holes

   Bolts must go in without undue damage to the threads.

2. Fill every hole

   Place bolts in all holes, with washers where Section 6.1 requires them, and run the nuts on.

3. Compact systematically

   Work from the most rigid part of the joint.

4. Snug with enough thread

   The bolt end must be at least flush with the nut face.

Snug-tightened joints are used where slip into bearing does no harm. The ultimate strength of a connection does not depend on bolt pretension, and some joints are meant to slip or rotate ⁠¹. In practice slip is about half the hole clearance, and fabrication tolerances usually put some bolts in bearing before load is applied ⁠¹. Snug-tight is also the starting point for every pretensioning method, which is why RCSC restored the 2004 definition in 2014: the 2009 wording was not a suitable starting point for turn-of-nut ⁠¹.

Pretensioned joints

Required for ⁠¹:

• joints where the specification or code that invokes RCSC requires bolt pretension;
• joints subject to significant load reversal;
• joints subject to fatigue load with no reversal of loading direction;
• joints with Group 120 (A325, F1852) bolts subject to tensile fatigue;
• joints with Group 144 or 150 (A490, F2280) bolts in tension or combined shear and tension, with or without fatigue.

The commentary lists connections other specifications have required to be pretensioned: column splices in buildings with high height-to-width ratios, bracing connections for columns in tall buildings, various connections in buildings with cranes over 5 tons, and supports for running machinery or other sources of impact or stress reversal ⁠¹. Pretensioned joints still carry load by shear and bearing; pretension is required “for reasons other than slip resistance” ⁠¹. Faying surfaces need no special preparation ⁠¹.

Every bolt is first snugged, then pretensioned by one of the five methods to at least the Table 5.2 value ⁠¹. See A325 bolt torque and turn-of-nut method.

Slip-critical joints

Required, for shear or combined shear and tension, in ⁠¹:

• joints with fatigue load and reversal of loading direction;
• joints with oversized holes;
• joints with slotted holes, except where the load is within 80 to 100 degrees of normal to the slot’s long direction;
• joints where slip at the faying surfaces would harm the structure’s performance.

The bolts are installed exactly as in a pretensioned joint. The difference is the faying surfaces: they must be prepared per Section 3.2.2 to give a known slip coefficient, and inspection follows Section 9.3 ⁠¹. Joints with reversed fatigue load must be slip-critical because back-and-forth slip can speed up fatigue failure ⁠¹.

Why pretension does not raise ultimate strength

A pretensioned bolt clamps the plies, so friction carries the load first and the bolts see no shear until friction is overcome. Pretension is lost by the time the bolts fail in shear, so the ultimate limit state of a pretensioned joint is the same as for an identical joint with non-pretensioned bolts ⁠¹. Pretension controls slip and fatigue behavior, not ultimate shear capacity. That is why the choice between snug-tight and pretensioned is a design decision, and why RCSC leaves it to the Engineer of Record ⁠¹.

Sources

1. 1
   Research Council on Structural Connections, RCSC Specification for Structural Joints Using High-Strength Bolts, June 11, 2020Checked 21 Oct 2026

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