About the Author/WAYNE WALLACE

Wayne Wallace is the president of Applied Bolting Technology Products.The company provides bolting consulting services and manufactures directtension indicating washers from its base in Rockingham, Vermont. Wallaceis a member of the Research Council on Structural Connections, owner ofseveral patents, and author of numerous papers on the practical aspects ofquality assurance in structural bolting. He can be reached by phone at 800-552-1999, by fax at 802-460-3104, or e-mail at wwllace@sover.net.

RC Tests, Nut Factors,and the Europeans

I have never really understood the concept “rota-tional capacity” of bolts. After bolts are installed

(that is, tensioned correctly) is failure caused by somemysterious extra rotation? Does the external loadingof the bolt somehow “rotate” it a little more, and then,presto! It breaks? And does this mysterious externalrotational force only apply to galvanized A325 bolts,but not to black A325 bolts, to A490 bolts, to A194B7 studs, or to SAE J429 cap screws? I don’t thinkso.

ASTM Rotational Capacity TestsASTM A325 states that the final supplier of the gal-

vanized ASTM A325 bolt set, including bolt, nut, andflat washer, must provide evidence that three of theseassemblies have been tightened to double the turn-of-nut “turn-from-snug” angle in a solid joint without break-ing. And AASHTO (they’re the bridge people) calls for asimilar test of galvanized bolt sets with a few more con-ditions on maximum nut factor (read ‘lubrication’) andminimum preload (read ‘tension’) achieved in the test.(A325 RC test — Reference: ASTM A325 Specifica-tion, Published under the jurisdiction of the ASTM F16Committee on Fasteners, section 6.3 and clause 10.)

The ASTM RC test, as it is called, is intended totest the lubrication and thread fit of the high strengthassembly – the “nut factor”:

Without adequate lubrication on the nut or bolt threadthe torque resistance will defeat the preloading effortbefore the intended preload of 70% minimum ultimatetensile strength (UTS) is achieved. And, someone hasdecided, in the case of AASHTO, that this limit on “nutfactor” is 0.25. Without adequate control on nutovertapping, which is of course necessary on galva-nized bolt assemblies, the thread fit might be so loosethat the nut just slips off the thread at preload levelsfar below minimum. If overtapping is missed or under-sized the bolt will break in torsion below a preload of70% min UTS, guaranteed.

So it’s not really a test for the bolt’s “capacity” towithstand additional “rotation” in service that underliesthe so-called “rotational capacity” test. It’s really in-tended to be a last minute sort of “point-of-purchase”check on the integrity of the bolt/nut set, because thecomponents are usually made by different manufac-turers. But, it’s only two or three bolt sets from eachassembly lot, so it’s a pretty small sample.

And even though the bolt installation might be withDTIs, which actually measure the bolt preload, an RCtest is still demanded for the galvanized bolts. Puz-zling. When the bolts successfully compress a DTI itreally means that ALL the bolts have been tested to70% - 80% of minimum UTS. Not just a sample of twoor three. So, why bother with an RC test when DTIsare going to be used?

Are there any other tests that do the same thing asthe RC test? Yes. In North America, one such test iscalled the RCSC “Pre-Installation Test.”

RCSC Pre-Installation TestsThe RCSC, called the “Council,” is the Research

Council on Structural Connections. Their website iswww.boltcouncil.org. This is a non-profit volunteer or-ganization of about 75 individuals (structure design-ers, university professors, manufacturers, etc.) whohave some interest and knowledge of structural steelbolting. Meetings are open to the public, but officialmembership and voting status is by invitation only.The Council publishes the “Specification for StructuralJoints using A325 and A490 Bolts,” which theoreticallyis called into law for every steel structure in NorthAmerica, defines design parameters and installationrequirements, and is the basis for the AISC, CISC,AREA, and ASCE specifications. (RCSC Pre-Installa-tion Test — Reference: Research Council on Struc-tural Connections, “Specification for Structural Jointsusing ASTM A325 or A490 Bolts” 2004 edition, sec-tion 7.)

The “Specification”, as it is called, states that, onevery project site, a test be done in which three boltsfrom each lot are to be inserted into a bolt tension

Nut Factor: k =Torque Resistance

preload x diameter

calibrator (read Skidmore) and tightened by whatevermethod is going to be employed on the site. The boltpreload read by the Skidmore must be at least theminimum tension (and that comes from the RCSCSpecification too) plus 5% for good measure.

Why is the RCSC “pre-installation test” needed? Inthe opinion of the Council, it is imperative to see (a) ifthe bolt assembly will take that preload, and (b) thatthe workers and equipment can actually produce thedesired result.

But again, if DTI’s are used, why bother to do a testof (only three) bolt assemblies, when by compressingthe DTI’s it will be shown that 100% of the bolts haveachieved correct preload. Indeed, the Council is nowballoting a provision that, if passed, will allow certain“calibrated” lots of DTI’s to be usedinstead of Skidmores in the test.

And now the Europeans haveimported our Rotational Capacitythinking and immortalized it in theirREALLY complicated ISO/EN14399 Part 2 “Suitability test forpreloading.”

EN 14399-2 Suitability TestsEN 14399 (High-strength Struc-

tural Bolting Assemblies forPreloading) is to Europe whatASTM A490M and ANSI is to NorthAmerica, except that it is mostlyjust for Property Class 10/10.9bolts. It covers bolts up to M36only. EN 14399 is an importantspecification for North America toknow about because if we intendto participate in the European mar-ket this is the definitive finishedproduct specification. There areeight sections approved, with the9th (DTI’s) about to be approved. It is ONLY metric, ofcourse. Have you noticed? That’s the system of theworld. (ISO/EN 14399 CEN — Reference: EuropeanStandard EN 14399, “High Strength Bolting Assem-blies for Preloading,” section 2 - “Suitability Test forPreloading,” published by the European Committee forStandardization, Brussels, 2005.

Part 3 (called “System HR”) mimics the BS4395higher strength type of bolt and nut, with long threadsand deep nuts, that are intended to fail by fracture ofthe bolt shank. In Eurospeak, this is confirmed subtlyin the Scope section where it says “…to obtain ductil-ity predominately by plastic elongation of the bolt…”These are like our A490 bolts and 563 nuts.

Part 4 (called “System HV”) mimics the DIN 6914/15 bolt assembly, with short threads, shallow nuts,that are intended to fail by thread stripping. InEurospeak, this is confirmed subtly in the Scope sec-

tion where it says “…to obtain ductility predominantlyby plastic deformation of the engaged threads…”

Both EN14399 Parts 3 and 4 are preceded by Part2, “Suitability Test for Preloading,” in which it is ex-pected that a sample of bolts from an assembly lot betension tested in a solid joint (note – solid, not a hy-draulic cell such as a Skidmore) for nut factor, maxi-mum preload achieved, and for rotational angular ca-pacity to certain defined limits.

The graph which illustrates the angles and loads tobe measured can be seen in EN 14399. But the angu-lar values are actually found in the European equiva-lent of our RCSC Specification, called ENV 1090 “Ex-ecution of Steel Structures.” The latter specificationalso defines the “k class” of the assembly: either K2

for torque control tightening, K1for a combined method (really abedding torque plus part turn), andK0 for the DTI method. These Kfactors are just that - k factors, ornut factors. (Reference: EuropeanStandard prEN 1090-2, “Executionof Steel Structures, Part 2: Tech-nical Requirements… Stage 34,”published by the European Com-mittee for Standardization, Brus-sels, 2005 (draft).

K1 applies to a combinedtorque-plus-turn method, and issimilar to the DIN 18800 pt 7specification. I’ve heard thiscalled “14399 Suitability Test Mod-erately Heavy”. It reflects the Ger-man perspective on bolting, withshallow nuts, large nut lots, atorque/tension test with bolts fromone heat but different lengths, andbolts probably only tensioned intheir design to 70% of YIELD.

K2 applies to a pure torque control method. It de-rives from the French system, with torque/tension testson every batch of bolt/nut assemblies, and the K2 fac-tors are used to determine tightening torques. I hearsecond hand that some discretion and knowledge isneeded in the field to use the K2 factor. This is “14399Suitability Test Really Heavy”. Maybe you’ve noticedthat the French mostly use concrete anyway, and nowyou know why. Stated kindly, their bolting proceduresare involved.

K0 applies to tightening with EN 14399-9 DTI’s, andof the three makes the most sense to me becauseyou simply don’t need to measure torque and tension.You don’t need a K anything. This means they thinkthe nut factor is irrelevant, and to me that makes com-plete sense. Just squash the bumps, and if you suc-ceed, who cares what the k factor was. This is whatI’d call “14399 Suitability Test Light”.

Who Cares About K Factor Here, andthe Europeans Agree

Reprinted from Distributor’s Link Magazine, Winter 2009