In general, when properly installed, the high-strength bolt-nut assembly will not loosen. When snug-tight bolts are used, the loading will be such that the loosening of a nut will not occur. When fully-tensioned bolts are required, as for slip-critical connections subjected to vibratory or fatigue loading, the installed tension and the attendant friction on the threads will prevent the nut from loosening.

In some other cases, such as nuts on anchor rods (for which full-tensioning is generally inappropriate), further consideration may be required. In this case, an additional jamb-nut or second nut may be provided. Alternatively, the threads can be spiked or marred or the nut can be tack-welded to the base metal to prevent it from turning. Note that the latter two solutions are permanent actions. There also exist proprietary nut devices with locking features to prevent the nut from backing off.

The RCSC Specification defines a snug-tightened joint as a joint in which the bolts have been installed in accordance with Section 8.1. Note that no specific level of installed tension is required to achieve this condition, which is commonly attained after a few impacts of an impact wrench or the full effort of an ironworker with an ordinary spud wrench. Note that no specific level of installed tension is required to achieve this condition. The plies should be in firm contact, a condition that means the plies are solidly seated against each other, but not necessarily in continuous contact. There is no upper limit to the pretension that can be present in a snug-tightened joint. Twist-off-type tension-control bolts can be used in snug-tightened joints, even if the splined ends are severed during installation.

It is a simple analogy to say that a snug-tight bolt is installed in much the same manner as the lug nut on the wheel of a car; each nut is turned to refusal and the pattern is cycled and repeated so that all fasteners are snug. Essentially, snug-tight bolts utilize the higher shear/bearing strength of high-strength bolts with installation procedures similar to those used for ASTM A307 common bolts, which are never fully tensioned (see FAQ 6.6.2).

Bolts installed in the snug-tight condition are permitted in most applications. They are permitted when used in:

1. Bearing-type connections, except as stipulated in Section E6 of the AISC Specification
2. Tension or combined shear and tension applications, for Group 120 bolts only, where loosening or fatigue due to vibration or load fluctuations are not design considerations (AISC J3.2)

unless pretensioned or slip-critical bolts are required. (see FAQ 6.5.3).

First, there are differences between pretensioned and slip-critical connections. Slip-critical connections are more expensive due to faying surface requirements, and both are more expensive than bolts in the snug-tight condition. Both the AISC Specification and RCSC Specification have provisions regarding what bolts need to be pretensioned. The AISC Specification Section J3.1 indicates that the following conditions require pretensioned bolts:

1. As required by the RCSC Specification
2. Connections subjected to vibratory loads where bolt loosening is a consideration
3. End connections of built-up members composed of two shapes either interconnected by bolts, or with at least one open side interconnected by perforated cover plates or lacing with tie plates, as required in Section E6.1

RCSC Specification Section 4.2 indicates that the following conditions require pretensioned bolts:

1. Joints in which bolt pretension is required in the specification or code that invokes this Specification;
2. Joints that are subject to significant load reversal;
3. Joints that are subject to fatigue load with no reversal of the loading direction;
4. Joints with Group 120 bolting assemblies that are subject to tensile fatigue; and
5. Joints with Group 144 or Group 150 bolting assemblies that are subject to tension or combined shear and tension, with or without fatigue.

As with pretensioning, both AISC and RCSC have provisions regarding conditions that require slip-critical connections.

The AISC Specification Section J3.1 indicates that the following conditions require slip-critical joints:

1. As required by the RCSC Specification
2. The extended portion of bolted, partial-length cover plates, as required in Section F13.3

RCSC Specification Section 4.3 indicates that the following conditions require slip-critical joints:

1. Joints in which bolt pretension is required in the specification or code that invokes this Specification;
2. Joints that are subject to significant load reversal;
3. Joints that are subject to fatigue load with no reversal of the loading direction;
4. Joints with Group 120 bolting assemblies that are subject to tensile fatigue; and
5. Joints with Group 144 or Group 150 bolting assemblies that are subject to tension or combined shear and tension, with or without fatigue.
   1. Joints that are subject to fatigue load with reversal of the loading direction;
   2. Joints that utilize oversized holes;
   3. Joints that utilize slotted holes, except those with applied load approximately normal (within 80 to 100 degrees) to the direction of the long dimension of the slot; and
   4. Joints in which slip at the faying surfaces would be detrimental to the performance of the structure.

Slip-in bolted connections are not a structural concern for the majority of connections in steel building structures. The commentary to RCSC Specification Section 4.1 states that “The maximum amount of slip that can occur in a joint is, theoretically, equal to twice the hole clearance. In practical terms, it is observed in laboratory and field experience to be much less; usually about one-half the hole clearance. Acceptable inaccuracies in the location of holes within a pattern of bolts usually cause one or more bolts to be in bearing in the initial, unloaded condition. Furthermore, even with perfectly positioned holes, the usual method of erection causes the weight of the connected elements to put some of the bolts into direct bearing at the time the member is supported on loose bolts and the lifting crane is unhooked. Additional loading in the same direction would not cause additional joint slip of any significance.”

No, when a bolt is installed in the vertical position, the head of the bolt does not need to point upward. There is no requirement governing the entering direction of the bolt. Some people feel that bolts should be installed with the head up so that a loosened bolt will not fall from the hole. However, a falling nut is nearly as dangerous as a falling bolt, and a bolt without a nut should not be relied on to carry the load.

Neither the AISC nor the RCSC Specification has provisions regarding the orientation of the nut. It is unlikely that nuts would be systematically installed with the markings to the inside, so it is likely that at least some of the markings will be visible during inspection. Manufacturers have the option of making the nuts with either a double chamfer or with one washer face, but for both configurations, either orientation is allowed during installation.