Torque is an invalid measure for a pre-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 Therefore, recognition of these standard torque values has long been withdrawn.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. 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. The 2020 edition of the RCSC Specification added the prohibition against the calibrated wrench method where the head of the bolt is the turned element.

No, ASTM A307 bolts cannot be fully-tensioned. 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.

Provisions in the RCSC Specification Section 8.2 include five methods for high-strength bolt pre-tensioning. The use of these procedures is governed by the provisions listed below: turn-of-nut method, calibrated wrench method, twist-off tension-control bolt method, direct tension indicator method, and the combined method. (The combined method uses a combination of initial torque and a specified rotation beyond that. It was added in the 2020 edition of the RCSC specification and is used often with F3148 fixed spline drive bolts.) RCSC also allows the use of alternative-design components, assemblies, and methods within limits of Section 2.12. 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 RCSC Specification 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.

There are five approved methods to pretension bolts: Turn of Nut, Calibrated Wrench, Twist-off Tension Control, Direct Tension Indicator, and Combined methods. The specific requirements of each method are described in RCSC sections 8.2.1 through 8.2.5. The requirements in these sections will not be paraphrased in these FAQs. But there are provisions worthy of note:

All of the methods require starting in the snug tight condition. While the snug tight condition was developed to be less demanding than the pretensioned condition, it does have essential requirements such as some tension (full effort of an ironworker or a few impacts of a wrench) and ‘firm contact’ of the connected elements. The method requires snugging from them most rigid point outward. And may require a repeat of the process.

Turn of Nut is reliable. RCSC encourages matchmarking to improve that reliability and facilitate inspection.

Calibrated Wrench is very dependent on the condition of the components and connected parts. Because of that, the RCSC Specification requires that the required torque be established every day. RCSC understands that this is a burden but feels that it is needed to assure the required pretension is achieved. The 2020 RCSC Specification added a prohibition against using the calibrated wrench method when the bolt head is the turned element.

Twist-off Tension method, as with the other methods, depends on snugging prior to pretensioning. These assemblies are sensitive to lubrication. Incorrect lubrication can lead to insufficient pretension or to tension high enough to break the bolt. That is why these bolts are not permitted to be relubricated in the field.

Direct Tension Indicators provide an extra measure of pretension. The method does require a preinstallation verification that includes a demonstration that the device does not indicate required pretension before it should. The orientation and location of the DTI is important, protrusions are to be compressed, not ground off. And as with the other methods, DTIs have to be tightened in a systematic pattern. DTIs deform inelastically so if the bolt has been tightened and is later relaxed by the tightening of a neighboring bolt, the indicator may not reveal this.

The combined method includes an initial tension and additional rotation. The additional rotation is less than that required for the Turn of Nut method because the initial tension is greater than that resulting from the effort of an ironworker. This means the initial tension is dependent on the condition of the assembly, therefore it has to be confirmed weekly.

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.

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.

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).

RCSC Table 8.1 footnote a provides a rotation tolerance of -0+60 degrees. As stated in RCSC Specification Sections 9.2.1 through 9.2.45, 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 high-strength 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 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.

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 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 to resolve the issues that led to the problem.