What is rotational capacity (ROCAP) testing?

Per ASTM A325 section 6.3.1, the rotational capacity test is defined as a test, “that is intended to evaluate the presence of a lubricant, the efficiency of the lubricant, and the compatibility of assemblies as represented by the components selected for testing.” In a 1970 study referenced by the Research Council on Structural Connections (RCSC), it was shown that galvanizing increases the friction between the bolt and nut threads as well as the variability of the torque-induced pretension. A lower required torque value and more consistent results are obtained when lubricated nuts are used as part of the bolt assembly. Rotational capacity testing must show that the galvanized lubricated nut may be rotated from the snug tight condition well in excess of the rotation required for pretensioned installation without stripping. In layman’s terms, the test is required to show that the nut will not gall on the bolt and that the assembly will develop the pretension load desired.

Since, this test requires that each material lot of bolt and nut be tested individually, contractors will typically administer this test on the job site. However, upon request Portland Bolt can perform this test in our facility prior to shipping.


This video is a quick example of a few steps required when performing a ROCAP test.

Procedure

1. Standard A325 Rotational Capacity Procedure

The bolt, nut, washer assembly shall be assembled in to a steel joint so that 3-5 threads are located between the bearing surfaces of the bolt head and nut. The assembly shall be tightened to no less than 10% of the specified proof load. After initial tightening, the nut position shall be marked relative to the bolt and the specified rotation shall be applied. The specified rotation shall be as follows:

  • 2/3 rotation (240deg), for bolt lengths that are 4 times the diameter or less.
  • 1 rotation (360deg), for bolt lengths that are over 4 times diameter but no more than 8x.
  • 1-1/6 rotation (420deg), for bolt lengths that are greater than 8 times diameter.
  • For lengths over 12 times the diameter, the test is not applicable.

After the tightening rotation has been applied, the assembly shall be taken apart and examined for compliance. The assembly fails to pass if any of the following occur:

  • Inability to install the assembly to the nut rotation specified.
  • Inability to remove the nut after installing the the specified rotation
  • Shear failure of the threads as determined by visual examination following removal.
  • Torsional or tension failure of the bolt. Elongation of the bolt, between the nut and bolt head is to be expected and should not be classified as a failure.

2. Department of Transportation (DOT)/AASHTO Rotational Capacity Procedure

This Rotational Capacity Test follows the same general procedures as the standard test, but with a few more measurements and details. The bolt assembly is tightened to a specified initial tension (example: 3/4″ = 3kips) and the nut position is marked relative to the bolt head. The assembly is then tightened to a minimum specified tension (3/4″ = 28kips) and the torque is measured. The recorded torque value must not exceed the stated maximum (3/4″= 438ft-lbs). The assembly is then further tightened, to the following rotation:
  • 2/3 rotation (240deg), for bolt lengths that are 4 times the diameter or less.
  • 1 rotation (360deg), for bolt lengths that are over 4 times diameter but no more than 8x.
  • 1-1/3 rotation (480deg), for bolt lengths that are greater than 8 times diameter.
The tension after this final tightening is recorded, and shall equal or exceed 1.15x the minimum installation tension above (3/4″=32kips).

After the tightening rotation has been applied, the assembly shall be taken apart and examined for compliance. The assembly fails to pass if any of the following occur:

  • Exceeding the maximum allowable torque in the torque/tension comparison.
  • Failure to achieve the required rotation.
  • Failure to achieve the required tension at the required rotation.
  • Thread failure. The nut shall turn, with your fingers, on the bolt threads to the position it was in during the test. If you cannot turn the nut with your fingers it is considered thread failure.
  • Torsional or tension failure of the bolt. Elongation of the bolt, between the nut and bolt head is to be expected and should not be classified as a failure.

Examples

  1. Bridge Rotational Capacity
  2. Standard
  3. Federal Highway Administration

Referencing Specifications

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12 comments

    It would appear that the rotational capacity test is specifically designed for galvanized nuts and bolts, which are lubricated as part of the manufacturing process. But what about plain black bolts? Our office is tasked with doing this on A325 Class 3 bolts, but since all they have is the water soluble cutting fluid from the manufacturing process, many of these fail due to exceeding the maximum torque values that are allowed.

    @David- You are correct, normal, black bolts and nuts are not normally lubricated, other than the residual oily cutting fluid. There are two ways that the lack of lubricant can be overcome. First, and easiest, is by adding some sort of stick wax in the field. This is commonly done with galvanized bolts as well, and I think most state DOT’s are fine with this practice. The second option is to specify supplement S1 in A563. This requires that the nuts, regardless of finish, be lubricated. This method, although possible, is likely going to add considerable cost and time since it would be a special run.

    @Jason- Yes, rotational capacity testing can be performed on mechanically galvanized A325s. We can perform that test upon request.

    My bolt suppplier cannot perform ROCAP test on A325 type 1 bolt, but they can perform “TEST ASSEMBLY LOAD & TORSIONAL’. Is it the same term with ROCAP test.

    Thankyou

    @Bambang- It sounds very similar, but without seeing specifics of the test they perform, I couldn’t be certain. Both ASTM and AASHTO have very specific requirements for their respective rocap tests in regards to torque, tension and configuration.

    If the bolts are only required to be installed to a snug tight condition, is the ROCAP test still a requirement? I see no indication in any specification that this test is waived for snug tight installations, but I see no value in performing the test in this situation.

    @Mark – Neither ASTM nor AASHTO have any provisions for waiving the rocap test in situations like these, but I would assume the the engineer of record would be able to waive them, as the ultimate responsibility would be his.

    Mr. Mckinnon,
    For a bolt lot that has been tested for Rotational Capacity, is it required to be tested for “Pre-installation Verification” at site of installation, per RSCS Section 7.2?
    Regards,

    @Hattem – Yes, I believe that the bolts need to be pre-installation verified at the job site prior to installation, regardless of whether or not they were ro-capped at the supplier.

    Hello, A325 states that the rotational capacity testing shall be performed by the responsible party prior to shipment. The responsible party is defined as the organization that supplies the fastener to the purchaser. Doesn’t this mean that the test should be done every time by the bolt manufacturer?

    @Joel – In theory, yes. However, the test is only valid with a matched set of nut and washer. If the bolt manufacturer is selling the bolts without nuts or washers, as is common, then the test is invalid. Additionally, there is a considerable cost to perform the test, so virtually all manufacturers only perform it when requested and when selling the bolts/nuts/washers as a complete assembly. The ASTM committee is working on making the rotational capacity test a supplemental requirement, but the revision is still in committee for now.

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