AISC


6.5. Bolt Installation

6.5.1. What can be done to prevent the nut from loosening?

In general, when properly installed, the high-strength boltnut assembly will not loosen. When snug-tight bolts are used, the loading will be such that 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 such cases, 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.

6.5.2. What is the definition of snug-tight bolt installation and when is it allowed?

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. 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 6.6.2)

6.5.3. When must bolts be fully tensioned?

Snug-tight high-strength bolts are permitted for all bearing joints except when fully tensioned (bearing or slip-critical) bolts are required per the AISC Specification Section J1.10 and the 2004 RCSC Specification Section 4.2. For example, highstrength bolts must be fully tensioned for:

1. Slip-critical connections (see 6.5.4 ). RCSC-2004, Sections 4.3.

2. Connections where the bolts are subject to direct tension loading.

3. Column splice connections in tier structures that are 200 ft or more in height, 100 to 200 ft in height if the least horizontal dimension is less than 40% of the height, or less than 100 ft in height if the least horizontal dimension is less than 25% of the height.

4. In structures over 125 ft in height, connections of beams and girders to columns and of any other beams and girders upon which the bracing of columns is dependent.

5. In structures carrying cranes with a capacity of over five tons, roof-truss splices and connections of trusses to columns, columns splices, column bracing, knee braces, and crane supports.

6. Connections for supports of running machinery or of other live loads that produce impact or stress reversal.

7. Other connections stipulated as fully tensioned on the design plans.

6.5.4. When should bolted connections be specified as slip-critical?

Slip in bolted connections is not a structural concern for the majority of connections in steel building structures. The RCSC Specification Commentary 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.” In some cases, slip resistance is required. The AISC and RCSC specifications list cases where connections must be designated by the structural engineer of record as slip-critical:

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°) to the direction of the long dimension of the slot

4. Joints in which slip at the faying surfaces would be detrimental to the performance of the structure

5. The extended portion of bolted, partial-length cover plates, as required in AISC Specification Section F13.3

6. Bolted connections with undeveloped fills, as required in Section 573 J5.2.(d)

One special case also exists. A nominal amount of slip resistance is required at the end connections of bolted built-up compression members so that the individual component will act as a unit in column buckling. As specified in the 2005 AISC Specification Section E6.2, “The end connection shall be welded or pre-tensioned bolted with Class A or B faying surface.”

6.5.5. When a bolt is installed in the vertical position must the head of the bolt point upward?

No. 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 load.

6.5.6 Must nuts be installed such that the markings are visible after installation?

Neither the AISC nor the RCSC Specification governs 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.