Tightening of Structural Joints - Blacks Fasteners

1 31 FASTENERSF asteners your guarantee of quality industrial fastenersTightening of Structural JointsThe design, fabrication,assembly and inspection ofsteel structures using metrichigh strength structuralbolts and nuts to AS 1252are covered in AS 4100 - SAAS teel Structures Code whichshould be referred to formore detailed requirements for boltingCategories are inessence the same as thosepreviously given in AS 1511 1984. The ASAA High StrengthStructural bolting Code which waswithdrawn on 26/10 following are abstracts fromAS 4100.(1) AssemblyEach bolt and nut shall beassembled with at least one washerand where only one washer isused it shall be placed under therotating component. Tightening ofthe bolts shall proceed from thestiffest part of the joint toward thefree no circumstances shall boltswhich have been fully tightened bereused in another joint orstructure. They may be retightenedonce in the same hole.

2 (2) Methods of TighteningTightening methods permitted canbe either part turn method or useof direct tension indicators (CoronetRLoad Indicators).a) Part Turn Tightening MethodOn assembly all bolts and nuts inthe joint are first tightened to asnug tight condition. Snug tight isdefined as the tightness attained bythe full effort of a man using astandard podger spanner or by afew impacts of an impact marks are thenestablished to mark the relativeposition of the bolt and nut. Thebolts are then finally tightened bythe amount shown in Table ) Direct Tension Indicators1 Tightening of bolts and nuts shallbe in accordance with themanufacturer s instructions andthe following procedure2. Onassembly all bolts and nuts in thejoint are first tightened to the snugtight condition. Then the bolt andnut are tightened to provide theminimum tension specified inTable ) This method of Tightening canbe carried out with CoronetRLoadIndicators.

3 Refer page ) AS 4100 requires that thesuitability of the device shall bedemonstrated by testing at leastthree specimens for each diameterand grade in a calibration devicecapable of indicating bolt tensionand proving that the deviceindicates a tension at least 105% ofthe specified minimum.(3) InspectionBolts and nuts that show on visualinspection any evidence ofphysical defects shall be removedand replaced by new ones. Thefollowing methods shall be used tocheck that all bolts are fullytightened. For part turn Tightening , by ensuring that thecorrect part turn from the snugposition can be measured orobserved. For direct tensionindicator Tightening , by ensuringBolting Categories Blacks Bolt body 3/9/06 2:10 PM Page 3132 Fasteners your guarantee of quality industrial fastenersTightening of Structural JointsFASTENERSB olting Categories the manufacturer s specifiedtightening procedure has beenfollowed and that the developmentof the minimum bolt tension isindicated by the tension ) Direct Tension IndicatorsInspect according to themanufacturer s the event that Blacks FastenersCoronetRLoad Indicators havebeen used, these recommendationsare set out on page 51.

4 (4) Inspection of BoltTension using a TorqueWrencha) In the event that the specifiedprocedure for part-turn tighteningie. method verification andapplication of match marking forlater inspection, was not followedand direct tension indicators werenot installed some method forsubsequent checking of bolttension is sometimes required bythe inspection that Tightening by torquecontrol was found to be reliable inpractice, not least because fewerectors purchased the equipmentnecessary to perform theprocedure for calibration of thebolts/wrench combinations whichare to be used in the structure, andwas deleted from the SAA HighStrength bolting Code. Logically, itis also not reliable for inspection ofthe correct tension in bolts procedure given in thefollowing is suitable for detectinggross under-tension, eg. boltswhich have been snugged only,but cannot be relied upon todistinguish bolts which althoughDisposition of outer face of bolted parts (See notes 1, 2, 3, 4)Bolt LengthBoth FacesOne Face normalBoth Faces(Underside of headnormal to axis to bolt axis andslopedto end of bolt)other slopedUp to and including 4 diameters1/3turn1/2turn2/3turnOver 4 diameters but notexceeding 8 diameters1/2turn2/3turn5/6turnOver 8 diameters but notexceeding 12 diameters(see note 5)2/3turn5/6turn1 turnTable 24 AS 4100 - 1990 Nut Rotation from the Snug-Tight on rotation: for 1/2turn or less, one twelfth of a turn (30 ) over andnil under tolerance.

5 For 2/3turn or more, one eighth of a turn (45 ) over and nilunder bolt tension achieved with the amount of nut rotation specified in Table24 will be at least equal to the minimum bolt tension specified in Table rotation is the rotation relative to the bolt, regardless of the rotations specified are only applicable to connections in which allmaterial within the grip of bolt is research has been performed to establish the turn-of-nut procedure forbolt lengths exceeding 12 diameters. Therefore, the required rotation shouldbe determined by actual test in a suitable tension measuring device whichsimulates conditions of solidly fitted Blacks Bolt body 3/9/06 2:10 PM Page 3233 Tightening of Structural JointsFASTENERSF asteners your guarantee of quality industrial fastenerstightened well beyond snug maynot have been fully :The principal factors which limitthe reliability of the method are:-a) the equivalence of thread andbearing face surface conditionand lubrication of thecalibration samples and ) the occurrence of gallingduring ) the time lapse betweentensioning and inspectionespecially as regards corrosionwhich may have is emphasised that correcttensioning can only be assuredby 1.

6 Using the correct bolts andnuts ( Blacks AS 1252 HighStrength Structural )2. Verifying proper snugging ofall bolts in the joint . (Thisshould be the time of firstinspection - joint should besolid)3. Applying match marks -desirably permanent, orverifying about 1-2mm gap atCoronet load indicator. Theload indicator inherentlyprovides a permanent witnessof correct Witnessing that the toolingavailable can easily achieve therequired part-turn or crushthe load indicator to thespecified average TensionInformation forSetting InspectionWrenches( ) CalibrationInspection wrench may be either ahand-operated or adjustablepower-operated wrench. It shouldbe calibrated at least once per shiftor more frequently if the need toclosely simulate the conditions ofthe bolts in the structure sodemands. The torque value determinedduring the calibration may not betransferred to another point being that there is no inspection torque for each sizeof bolt!

7 Each lot of bolts and each toolto be deployed must beindividually calibrated at thetime of inspection with atorque wrench is virtuallyimpossible because it is practicallyimpossible to obtain samples forthe calibration procedure whichtruly represent the bolts to beinspected. This is illustrated by which shows the torque-tension calibration of three M24galvanised bolt assembliessubmitted from a site by a partyrequired to apply least three bolts,desirably of the same size(minimum length may have to beselected to suit the calibrationdevice) and conditions as thoseBolting Categories Blacks Bolt body 3/9/06 2:10 PM Page 3334 Fasteners your guarantee of quality industrial fastenersFASTENERST ightening of Structural Jointsunder inspection should be placedindividually in a calibration devicecapable of indicating bolt : Without thiscalibrating device torquewrench inspection to the code isnot possible!

8 A hardened washer should beplaced under the part calibration specimen shouldbe tensioned in the calibratingdevice by any convenient meansto the minimum tension shownfor that diameter in Table 25. Theinspection wrench then should beapplied to the tensioned bolt andthe torque necessary to turn thenut or bolt head 5 degrees(approximately 25mm at 300mmradius) in the tensioning directionshould be determined. The averagetorque measured in the tests of atleast three bolts should be taken asthe job inspection torque.( ) InspectionBolts represented by the sampleprescribed in Paragraph B2 whichhave been tensioned in thestructure should be inspected byapplying, in the tensioningdirection, the inspection wrenchand its job inspection torque tosuch proportion of the bolts in thestructure as the supervisingengineer For guidance it is suggestedthat a suitable sample size wouldbe 10 percent of the bolts but notless than two bolts in eachconnection are to be inspected.

9 ( ) ActionWhere no nut or bolt is turned bythe job inspection torque, theconnection should be accepted asproperly tensioned. Where any nutor bolt head is turned by theapplication of the job inspectiontorque, this torque should then beapplied to all other bolts in theconnection and all bolts whosenut or head is turned by the jobinspection torque should betensioned and , the fabricator orerector at is option, may retentionall of the bolts in the connectionand then resubmit the connectionfor Categories 25 Bolt Tension Information forSetting Inspection WrenchesBolt TensionNominalboltMinimumdiameterkNKipst on Blacks Bolt body 3/9/06 2:10 PM Page 3435 FASTENERSF asteners your guarantee of quality industrial fastenersTightening of Structural JointsBolting Categories 15 This data, established on specimens returned from a site where inspection wasrequired by the responsible Engineer, illustrates the difficulty of applying torqueinspection to establish the correct tensioning of bolting Categories plotted points show tension against the more consistent dynamic friction (nutin motion) torque rather than the torque to overcome static friction of a stationarynut as in the procedure in the Australian Structural Steel Code.

10 Either way thecalibration torque is determined on freshly tensioned assemblies which may ormay not be what is to be first point for the M24 x 100 removed from the structure is plotted twice as thewrench ran out of travel before reaching the 270 Nm set point the first Blacks Bolt body 3/9/06 2:10 PM Page 35