AISC


6.6 Methods for Fully Tensioned Installation

6.6.1. What torque is required to fully tension a highstrength bolt?

Torque is an invalid measure for fully tensioned installation, unless it is calibrated. In 1951, the first RCSC Specification incorporated a table of standard torque values for the installation of fully tensioned high-strength bolts. However, depending upon the condition of the threads, it was demonstrated that the resulting installed tension varied by as much as plus or minus 40 percent. It is now known that clean, well lubricated threads result in tensions that are higher than required (and probably a few broken bolts), whereas, rusted, dirty, or poorly lubricated threads result in tensions that are below the minimum required. Therefore, recognition of these standard torque values has long been withdrawn. Accepted procedures for fully tensioning high-strength bolts can be found in the RCSC Specification Section 8.2 (see also 6.6.3.). If torque is to be used as in the calibrated wrench method as described in the RCSC Specification Section 8.2.2, it must be calibrated on a daily basis for the lot, diameter, and condition of bolts being installed.

6.6.2. Can an ASTM A307 bolt be fully tensioned?

No, ASTM A307 is the bolting strength equivalent of the ASTM A36 steel specification. As such, it is a mild steel material that is suitable only for use in snug-tight bearing connections. Note that ASTM A307 bolts are seldom used in structural connections today, except perhaps for the end connections of purlins and girts, incidental sub-framing, and as anchor rods.

6.6.3. What are the accepted procedures for fully tensioning (pre-tensioning) high-strength bolts?

Provisions in the RCSC Specification Section 8.2 include four methods for the pre-tensioning of high-strength bolts. The use of these procedures is governed by the provisions listed below: turn-of-nut pre-tensioning, calibrated wrench pre-tensioning, twist-off-type tension-control bolt pre-tensioning, and direct tension indicator pre-tensioning. RCSC also allows the use of alternative-design fasteners and alternative washer-type indicating devices. When used properly, each method can produce properly tensioned high-strength bolts.

Regardless of the method used, pre-installation verification must be performed, the snug-tight condition must be achieved prior to pre-tensioning, washers must be positioned as required in Section 6.2, and installation should commence at the tightest part of the joint and progress toward the free edges. Several cycles may be needed to achieve a snug tight condition

6.6.4. How should the turn-of-nut pre-tensioning be used?

The RCSC Specification covers the turn-of-nut method in Section 8.2.1.

6.6.5. How should the calibrated wrench pre-tensioning be used?

The RCSC Specification covers the calibrated wrench method in Section 8.2.2. Hardened washers must be used under the element to be turned in tightening.

6.6.6. How should twist-off-type tension-control (TC) bolt pre-tensioning method be used?

The 2004 RCSC Specification covers the use of twist-off-type tension-control (TC) pre-tensioning in Section 8.2.3.

It should be noted that the sheared-off splined end of an individual bolt indicates only that, at the time the splined end was broken, enough torque had been applied to the bolt to fracture the break-neck. Proper tension is assured for all bolts in a connection only if the bolts have been systematically snugtightened and subsequently fully tensioned as specified.

Note that specific and proper lubrication of “tension-control” or twist-off bolts is essential to the reliable installation of these fasteners.

6.6.7. How should the direct-tension-indicator pre-tensioning be used?

Direct-tension-indicator pre-tensioning is covered in Section 8.2.4 of the RCSC Specification.

Strict adherence to the manufacturer’s installation instructions is required with direct-tension indicators (DTI).

6.6.8. What is the upper limit on the installed tension of high-strength bolts?

As stated in RCSC Specification Sections 9.2.1 through 9.2.4, installed tensions in excess of those given in RCSC Specification Table 8.1 shall not be cause for rejection. Accordingly, there is no specified upper limit on the installed tension of highstrength bolts. This supports the long-standing rule of thumb that as long as the bolt is not broken during installation, the bolt is adequate for service.

This general rule applies because the bolt is subjected to combined stress (tension and torque) during installation. Once installed, however, the torque is relaxed and the bolt is essentially subject only to a tensile stress that is always less than the combined stress. Thus, even if the bolt were on the verge of failure during installation, it would be subject to a less demanding state of stress (simple pretension) during service

6.6.9. A bolt has broken during installation. Is this cause for significant concern?

A bolt may break during installation for several reasons. When trying to snug-tighten joints involving very thick plies the force required to deform the plates and achieve firm contact may be high enough to rupture the bolt. In fact, repeated attempts to further compact a joint that each result in broken bolts may actually signal that firm contact has been achieved and that the pre-tensioning process can proceed.

There is a longstanding rule of thumb that as long as the bolt is not broken during installation, the bolt is adequate for service. This general rule applies because the bolt is subjected to combined stress (tension and torque) during installation. Once installed, however, the torque is relaxed and the bolt is essentially subject only to a tensile stress that is always less than the combined stress. Thus, even if the bolt were on the verge of failure during installation, it would be subject to a less demanding state of stress (simple pretension) during service. The occasional breaking of a bolt should not be cause for concern, though the cause should be sought so that the issues that led to the problem can be resolved.