What is rotational capacity (ROCAP) testing?

Per ASTM F3125 Annex section 2.1.1, the rotational capacity test is defined as a test, "intended to evaluate the presence of a lubricant, the efficiency of lubricant and the compatibility of assemblies." 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, Portland Bolt performs 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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26 comments

    This Rotational Capacity test can be applied to galvanized bolts of other grades different from A325? how about grade 5 or grade 8 bolts? the difference in head dimensions from heavy hex fasteners is something that variate the minimun tension, max torque values from this test?

    @Ricardo- A test can be performed on any grade you like, but the parameters and procedures would need to be modified to suit the grade being tested. The details of which we would leave up to the requestor.

    @Waqas- apologies, but we are not familiar with the specifics of testing metric fasteners and so are unsure of the answer to your question. Apologies.

    what is the percentage required for testing from each lot of assemblies ? for
    example if i am making 400pcs how many assemblies need to be sent out for testing ?

    @Les- The test sample quantity varies based on the number of bolts in the lot, and is not a linear value. For 400pcs, you’d need to test three assemblies.

    Good morning
    Have you found any issues with performing Rotational Capacity tests on A490 bolts that have been Geomet coated?

    @Pandiarajan- The rotational capacity test procedures are outlined in the appendix of the F3125 standard. DTI washers are not used in rotational capacity testing.

    FDOT specifications require type 1 A325 assemblies get Ro cap tested from snug tight condition of 5 Kips.Then turn the nut or bolt head to minimum of 51Kips at that tension the turned element does not reach 1/3 of turn, the QC then checks torque. The fastener assembly is then further tightened to additional 2/3 turn which is actually greater than 2/3 because 51Kips stop at least half of the 1/3 turn. My question is does the job torque value from 51Kips be used as a job torque value or is it the Pre-installation value from snug tight at 6KIPS then check the torque at the required turn of nut of 1/3 turn which is greater than the minimum of 51Kips.

    @Brad- We are a little confused by your description. For a 1″ diameter A325 bolt, the standard AASHTO/FHWA test, which most DOTs require, consists of turning from snug tight (5kips) to 51kips, at which you are not to exceed 1,062ft-lb of torque. You then rotate another 2/3 turn (more for longer bolts), after which the tension cannot be below 59 kips. Typical installation tension for 1″ A325s in pretensioned and slip critical joints is 51 kips, so the torque should relate to that value. If your DOT has different requirements, we suggest you confirm those with your DOT inspector.

    @Michael- No, but they are similar. A rocap test follows procedures to make sure that the bolt/nut assembly can achieve the needed tension. There are parameters around the tension and torque that the bolts need to fall between. A tension calibration, as far as we understand, is more to determine the necessary torque to achieve the required tension. It is less of a pass/fail and more of a procedure to gather information.

    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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