Boltinggalvanizedsteel GAA Ch4[1]

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CHAPTER 4 Bolting Galvanized Steel

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contents

Bolting steel structures

Zinc coatings for

fastenersInfluence of thegalvanized coating ondesign

Structural bolts andbolting techniques

Galvanized commercialgrade bolts

Galvanized tower bolts

Galvanized high strength structural bolts

Modes of force transferin bolted joints

Bolting category system

Design for boltedstructural joints

Tightening proceduresfor high strengthstructural bolts

Inspection of highstrength bolted joints

Flush spliced structuraljoints in galvanized steel


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Chapter 4 Bolting galvanized steel

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Zi pla i gZinc plating on fasteners producesrelatively light, uniform coatings ofexcellent appearance which are generallyunsuitable for outdoor exposure withoutadditional protection.

There is in general an economic upper limitto the coating mass which can be appliedby plating, although certain specialisedroofing fasteners are provided with zincplated coatings up to 35 to 40 m thick.Where heavy coatings are requiredgalvanizing is usually a more economicalternative.

Zinc plated bolts having a tensile strengthabove 1000 MPa must be baked for therelief of hydrogen embrittlement.

Zinc plated high strength bolts and nutsmust be also provided with asupplementary lubricant coating to provide

for satisfactory bolt tightening.See t rqu /i du d i r la ii igh i g .

Australian standards for zinc plating requirethat one of a range of chromate conversioncoatings be applied in accordance withAustralian Standards 1791 Chromateconversion coating on zinc and cadmiumelectrodeposits. Clear, bleached, iridescentor opaque films may be produced,depending on the level of resistance to wetstorage staining required.

Australian Standard 1897 Electroplatedcoatings on threaded components (metriccoarse series) specifies plating thicknesseswhich can be accommodated on externalthreads to required tolerances.

sh rardi i gSherardising produces a matt grey zinc-ironalloy coating. The process impregnatessteel surfaces with zinc by rumbling smallcomponents and zinc powder in drumsheated to a temperature of about 370oC.The least known of the various processesfor zinc coating steel, sherardising is not

used in Australia. The process ischaracterised by its ability to produce avery uniform coating on small articles.

The thickness of sherardised coatings isgenerally of the order of 15m but can varydepending on cycle time from 7.5 to 30m.Sherardised coatings therefore fall betweenzinc plated and galvanized coatings inthickness and life.

Although sherardising is an impregnationprocess there is some build up indimensions. British Standard 729 Zinccoatings on iron and steel articles, Part 2:Sherardised coatings recommends anoversize tapping allowance of 0.25mm onnuts to ensure easy assembly withsherardised bolts.

M ha i al (p ) pla i gMechanical or peen plating offersadvantages in the zinc coating of fasteners.Coatings are uniform, and because theprocess is electroless there is no possibilityof hydrogen embrittlement. High strengthfasteners not susceptible to embrittlementneed not be baked after coating. Lubricant

coatings must be applied to ensuresatisfactory tightening.


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The presence of either galvanized coatingsor zinc plating on high strength bolts, andgalvanized coatings on structural membersmay need to be taken into account indesign. The following factors should beconsidered:

1. Slip factors of mating surfaces2. Fatigue behaviour of bolted galvanized

joints3. Bolt relaxation4. Effect of galvanized coating on nut

stripping strength5. Torque/induced tension relation in bolt

tightening

1. slip fa r aff i gma i g urfa

In a friction type bolted joint all loads in theplane of the joint are transferred by thefriction developed between the matingsurfaces. The load which can betransmitted by a friction type joint isdependant on the clamping force applied bythe bolts and the slip factor of the matingsurfaces.

Australian Standard 4100 Steel structuresassumes a slip factor of 0.35 for cleanas-rolled steel surfaces with tight mill scalefree from oil, paint, marking inks and otherapplied finishes. AS 4100 permits the use

of applied finishes such as galvanizing infriction type joints, but requires that the slipfactor used in design calculations be basedon test evidence in accordance with theprocedures specified in Appendix J of thestandard. Tests on at least three specimensare required, but five is preferred as thepractical minimum.

Bearing type joints are not affected by thepresence of applied coatings on the jointfaces, so galvanizing may be used withoutaffecting design strength considerations.

slip fa r f galva iz d a i gResearch conducted in Australia andoverseas shows mean slip factors forconventional galvanized coatings over alarge number of tests to be in the range

0.14 to 0.19, as compared to 0.35 for cleanas-rolled steel.

Design values take these lower slip factorsinto account, and galvanized steel is usedwidely in high strength friction type joints.

Work by Professor WH Munse* forInternational Lead Zinc ResearchOrganization, and others, shows that theslip factors of galvanized surfaces can besubstantially improved by treatments suchas wire brushing, light brush off gritblasting, and disc abrading. In each casethe treatment must be controlled to providethe requisite scoring or roughening toexpose the alloy layers of the coating. Caremust be taken to ensure that excessivecoating thickness is not removed.

The following table shows the results ofslip factor testing various galvanizedsurfaces in four-bolt joints.

surfa tr a m n . f Av rag Mi Max

As received 15 0.14 0.11 0.18

Weathered 3 0.20 0.15 0.26

Wire brushed 4 0.31 0.27 0.33Grit blasted 2 0.31 0.28 0.34

* WH Munse High strength bolting of galvanized connections presented to the symposium Bolting galvanizedconnections and new steel design specifications, Australian Institute of Steel Construction and Australian ZincDevelopment Association, 1968

It is important to recognise more recentdevelopments in galvanizing technologywhich produce harder final layers of zinc.Testing has been undertaken to establishhigher slip factors for structural steelproduced in the modern galvanizingfacilities. Designers should check with thegalvanizer before assuming a slip factor forslip critical joints in a structure.

Fully alloyed grey galvanized coatingswhich can result from the galvanizing ofsilicon steels have also been shown todevelop higher slip factors.

Slip factors given here are indicative only,and designs must be based on proven slipfactors established by testing in accordancewith the requirements of AS4100,Appendix J.

Influence of the galvanized coating on design


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2. Fa igu b havi ur f b l dgalva iz d j iWhile the galvanized coating behavesinitially as a lubricant it has been shown infatigue tests carried out by Munse thatafter the first few cycles galvanized matingsurfaces tend to lock up and further slipunder alternating stress is negligible. Thefigure below taken from work by Munseillustrates this effect. Note the rapidlydecreasing amplitude of slip from first tosecond and then to fifth stress cycle.

Further indications of lock up behaviourbecame apparent when joints weredisassembled, galling of the galvanizedcoating being observed in regions wherethere had been high contact pressure.Where no initial slip can be tolerated areduced slip factor must be used in design.The slip factor of the galvanized coatingmay be improved by wire brushing or brushoff grit blasting as discussed above, butslip factors for galvanized surfaces posttreated in this way must be verified inaccordance with Appendix J of AS 4100.

3. B l r laxa iThe possible effect of bolt relaxationcaused by the relatively soft outer zinc layerof the galvanized coating on the membermust also be considered. If the zinc coatingflowed under the high clamping pressure itcould allow loss of bolt extension andhence tension. This factor was also studied

by Munse. He found a loss of bolt load of6.5 percent for galvanized plates and boltsdue to relaxation, as against 2.5 percent foruncoated bolts and members. This loss ofbolt load occurred in 5 days and littlefurther loss is recorded. This loss can beallowed for in design and is readilyaccommodated.

Stress versus slip for fatigue specimen subjected toalternating stress of 23.2 kips/in2 Galvanized membersand bolts. First, second and fifth stress cycles. WHMunse Structural Behaviour of hot galvanized boltedconnections. Proceedings 6th International Conferenceon Hot Dip Galvanizing 1968.

4. eff f galva iz da i g u rippi gr g h

Galvanizing affects bolt-nut assemblystrength primarily because the nut must betapped oversize to accommodate thethickness of the zinc coating on the boltthread. The oversize tapped thread reducesthe stripping strength of the nut whentested on a standard size threaded mandrel.

In high strength bolting correct tightening isessential and Australian Standard 1252High strength steel bolts with associatednuts and washers makes no exceptions foroversized tapped galvanized nuts andspecifies that all high strength nuts mustmeet the full stripping load when tested ona standard-size hardened mandrel. To meetthis requirement galvanized high strengthnuts must have a higher specified hardnessin accordance with AS 1252. For this reasonnormal high strength nuts must not begalvanized and tapped oversize for use inhigh strength bolted joints.


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5. t rqu /i du d ir la i i igh i gThe relationship between torque andinduced tension in tightening is dependenton bolt and nut thread surface finish,thread surface coatings, and conditionsof lubrication.

Galvanized coatings and zinc plated coatingson threads both increase friction between thebolt and nut threads, and make the torque/induced tension relation much more variable.

The effect of lubricants on galvanized orzinc plated threads is significant. Thetorque/tension relationship shows muchreduced variability, and it becomes possibleto tighten in excess of the minimum tensionwithout danger of bolt fracture.

Torque/induced tension-relation for M2O high strengthstructural bolts, galvanized and lubricant coated, andas-galvanized.

The diagram shows the torque/inducedtension relation for as-galvanized, andlubricant coated galvanized M20 highstrength structural bolts. With theas-galvanized assemblies there is a widescatter in induced tension at any one torquelevel and torque could not be used toprovide a reliable method for gauging therequired minimum bolt tension specified inAS 4100 before bolt fracture occurred. Boltfailures in torsion resulted from the highfriction between the as-galvanized bolt andnut threads. Accordingly, AS 4100 does notrecognise the use of the torque controlmethod for tensioning galvanized or zincplated bolts, as discussed under Full igh i g .

Lubri a d a i g hr adBecause of the poor torque/induced tensionrelationship of galvanized or zinc platedhigh strength bolt/nut assemblies AS 1252specifies that supplementary lubricationmust be provided. Lubricants should bepre-applied by the manufacturer.

Effectiveness of lubricants is checked by anassembly test which requires the bolt towithstand a minimum of between 180o and420o from a snug position, depending onbolt length, before bolt fracture occurs.

Even when lubricant coated, galvanized andzinc plated high strength bolt/nutassemblies produce a wide scatter ininduced tension for a given level of torqueduring tightening. Therefore only part-turntightening or direct tension indicatortightening methods may be used asdiscussed under Par - ur igh i g and Dir i i di a r igh i g .


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Three main types of metric bolt are used instructural engineering in Australia: c mm r ial b l to AS 1111,

strength grade 4.6 M dium r g h r w r b l to

AS 1559, strength grade 5.6 High r g h ru ural b l to AS

1252, strength grade 8.8

Design provisions for structural bolts arecontained in Australian Standard 4100-1998Steel structures. This standard, in limitstates design format, supersedes AS 1250which was in a working stress format. AS4100 also incorporates the design andinstallation clauses of high strength boltsfrom AS 1511 which it also supersedes.

R l va Au ralia s a dardRelevant material standards referenced byAustralian Standard 4100 are the currenteditions of:

As 1110 ISO metric hexagon precisionbolts and screws

As 1111 ISO metric hexagoncommercial bolts and screws

As 1112 ISO metric hexagon nuts,including thin nuts, slottednuts and castle nuts.

As 1252 High strength steel bolts withassociated nuts and washersfor structural engineering

As 1275 Metric screw threads forfasteners

As 1559 Fasteners bolts, nuts andwashers for towerconstruction

s r g h d ig a i , m rib lThe strength of metric structural bolts isspecified in terms of the tensile strength ofthe threaded fastener and definedaccording to the ISO strength grade systemwhich consists of two numbers separatedby a point, for example 4.6. The firstnumber of the designation represents onehundredth of the nominal tensile strength(MPa) and the number following the pointrepresents the ratio between nominal yieldstress and nominal tensile strength.

For example a Property Class 4.6 bolt has:

Tensile strength of 4 x 100 = 400 MPaYield stress of 0.6 x 400 = 240 MPa

Structural bolts and bolting techniques

Galvanizedcommercialgrade boltsMetric commercial grade low carbon steelbolts used in the structural steel industryare manufactured to Australian Standard1111 ISO metric hexagon commercial boltsand screws which calls for a tensilestrength of 400 MPa minimum, with theProperty Class designation 4.6. Designstresses are specified in AS 4100.

Id ifi a i f mm r ial b lCommercial bolts Property Class 4.6 carrythe makers name and the metric M on thebolt head. Nuts generally are supplied toStrength Grade 5 and carry no markings.

Galvanized tower bolts

Transmission towers are designed ascritically stressed structures and the verylarge number of towers used provided the

incentive to reduce weight and cost byapplication of the plastic theory basis fordesign. This design concept calls for ahigher strength bolt than the standardcommercial 4.6 bolt. The medium strengthtower bolt to Australian Standard 1559Fasteners bolts, nuts and washers fortower construction was developed to meetthis need. Property Class is 5.6 andgalvanizing is the standard finish to providecorrosion protection matched to thestructure.

As maximum shear strength values arerequired the thread is kept out of the shearplane. Transmission towers are oftenerected in high snow country and it is alsonecessary to have a bolt with good lowtemperature notch toughness. Short threadlengths and specified notch ductility meetthese requirements.

AS 1559 calls for the following properties:

Tensile strength minimum 480 MPa

Yield stress, minimum 340 MPa

Stress under proof load 320 MPa

Charpy V-notch impact at 0oC:Average of 3 tests, minimum 27JIndividual test, minimum 20J


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nu l ki g f w r b lTransmission towers are constructed fromgalvanized structural sections using singlebolted joints, and positive prevention of nutloosening is necessary in critical situations.This requirement is met by effective initialtightening and some additional measure toensure nut locking, such as punching and

distortion of the bolt thread at the outer nutface after tightening, or the use ofgalvanized prevailing torque type lock nuts.

Id ifi a i f w r b lGalvanized metric tower bolts carry themetric M on the bolt head together with theletter T for Tower, and the makers name.Property Class 5 nuts are normally used,without markings.

Galvanized high strength structural boltsThe use of high strength structural bolts toAS 1252 in appropriate structural designsprovides improved economy and efficiencyin the fabrication of galvanized structuresby permitting:

1. Smaller bolts of higher strength2. Fewer bolts and bolt holes, resulting in:3. Lower fabrication cost for members4. Faster erection and reduced erection cost

AS 1252 calls for the following properties

Tensile strength, minimum 830 MPa

Yield stress, minimum 660 MPa

Stress under proof load 600 MPa

Minimum breaking load:M20 nominal diameter 203 kNM24 nominal diameter 293 kN

In structural applications galvanized highstrength structural bolts are commonly usedin M20 and M24 metric diameter in bothflexible and rigid connections. M30diameter is less used in structuralapplications, particularly when fulltightening is required to AS 4100, becauseof the difficulty of on-site tensioning toachieve specified minimum bolt tensions.M36 should never be specified if fulltensioning to AS 4100 is required.

Galva iz d high r g h uNut threads are tapped oversize aftergalvanizing to allow for the increased threaddiameter of the galvanized bolt. To ensure thatnut stripping strength is adequate afteroversize tapping, galvanized high strength

nuts are manufactured from steel with ahigher specified hardness than other highstrength nuts, as discussed under ov r iz appi g all wa f r galva iz d u .

As1252 p ifi h f ll wi g m ha i al pr p r i

nu yp Pr f l adr MPa

R kw ll hard HRc HRB Vi k r hard HV

Max Mi Mi Max Mi

Galvanized 1165 36 24 - 353 260

All others 1075 36 - 89 353 188

Id ifi a i f high r g h b l ,u a d wa h rGalvanized high strength bolts to AS 1252Property Class 8.8 can be identified by threeradial lines on the bolt head, with the makersname and the metric M. Nuts to PropertyClass 8 for use with structural bolts can beidentified by three circumferential lines on theface of the nut. Relative to nominal threadsize, high strength structural bolt heads andnuts are visibly larger than commercial boltsand nuts. Flat round washers for use with highstrength structural bolts can be identified bythree circumferential nibs.


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In the design of individual bolts in boltedstructural connections, there are threefundamental modes of force transfer:

1. sh ar/b ari g m d . Forces areperpendicular to the bolt axis and aretransferred by shear and bearing on theconnecting plies bolting categories4.6/S, 8.8/S and 8.8/TB described below.

2. Fri i m d . Forces to betransferred are perpendicular to the boltaxis as in shear/bearing mode, but loadcarrying depends on the frictionalresistance of mating surfaces bolting

category 8.8/TF

3. Axial i m d . Forces to betransferred are parallel to the bolt axis may apply in combination with otherbolting categories.

Modes of force transfer in bolted joints

Bolting category systemThe following bolting category identificationsystem is based on that used in AS4100:

ca g ry 4.6/s Commercial bolts usedsnug tight

ca g ry 8.8/s High strength structuralbolts used snug tight

ca g ry 8.8/tF High strength structuralbolts fully tightened infriction type joints

ca g ry 8.8/tB High strength structuralbolts fully tightened inbearing type joints

This category designation system is derivedfrom the Strength Grade designation of thebolt, for example 8.8, and the bolting designprocedure which is based on the followingsupplementary letters:

s represents snug tight

tF represents fully tensioned, frictiontype joint

tB represents fully tensioned, bearingtype joint

ca g ry 4.6/s refers to commercial boltsof Strength Grade 4.6 tightened snug tight asdescribed undertigh i g pr dur f r high r g h ru ural b l . (Snug tight is the final mode of tighteningfor bolting categories 4.6/S and 8.8/S, andthe first step in full tensioning for boltingcategories 8.8/TF and 8.8/TB).

ca g ry 8.8/s refers to high strengthstructural bolts of Strength Grade 8.8 usedsnug tight.

High strength structural bolts in the snugtight condition my be used in flexible jointswhere their extra capacity can make themmore economic than commercial bolts. Thelevel of tightening achieved is adequate forjoint designs where developed bolt tension isnot significant. Behaviour of the bolt underapplied loads is well known and accepted.

ca g ry 8.8t refers to both categories8.8/TF and 8.8/TB

ca g ry 8.8/tF refers to high strengthstructural bolts Strength Grade 8.8 used infriction type joints, fully tensioned in a

controlled manner to the requirements ofAS 4100.

AS 4100 specifies that friction type jointsmust be used where no slip is acceptable.They should also be used in applicationswhere joints are subject to severe stressreversals or fluctuations as in dynamicallyloaded structures such as bridges, except inspecial circumstances as determined by theengineer. Where the choice is optional,bearing type joints are more economic thanfriction type.

ca g ry 8.8/tB refers to high strengthstructural bolts Strength Grade 8.8 used in

bearing type joints, fully tensioned in acontrolled manner to the requirements ofAS 4100.


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Varia i i d ig valuwi h b l r g h a d j id igDesign values vary with joint design, bolttype and level of bolt tightening. The tablebelow indicates the range of design valuesin shear which apply to bolts of the samenominal diameter (M20) in varying strengthgrades, used in various joint designs, instandard size holes (Kh=1), in accordancewith AS 4100

A summary of structural design procedureto AS 4100 has been produced by theAustralian Steel Institute.

D ig f r high r g hb l i gAS 4100 specifies conditions for theapplication of high strength structural boltsin both friction type and bearing type joints.Bolts are tightened to the same minimuminduced tension in both types of joint.

t i yp j iFor joints in which the only force is anapplied tensile force in the direction of thebolt axes, the tensile force on any boltshould not exceed the following table.

Maximum p rmi ibl b l i , knNominal diameter ofbolt, mm Static load:

M16 104

M20 163

M24 234

M30 373

Design for bolted structural joints

B l yp a d b l i g a g ri

B l i ga g ry

B l r g hgrad

Mi imum il r g h

(MPa)

Mi imumyi ld r g h

(MPa)

nam Au ralias a dard

M h d f i i g/r mark

4.6/S 4.6 400 240 Commercial AS 1111 Use snug tight.Least costly and most commonlyavailable 4.6 grade bolt.

8.8/S 8.8 830 660 High strengthstructural

AS 1252 Bolts used are snug tight.The high strength structural bolt has alarge bolt head and nut because it isdesigned to withstand full tensioning.It can also be used in a snug tightcondition.

8.8/TF 8.8 830 660 High strength structuralbolt, fully tensionedfriction type joint

AS 1252 For categories 8.8/TF and 8.8/TBbolts are fully tensioned to therequirements as AS 4100.Cost of tensioning is an importantconsideration in the use of thesebolting categories.

8.8/TB 8.8 830 660 High strength structuralbolt, fully tensionedbearing type joint

AS 1252

Bolt and jointdesignation

Design value in shear, kN

Threads included in shear plane Threads excluded from shear plane4.6/S 44.6 62.38.8/S 92.6 1298.8/TF 35.5 35.58.8/TB 92.6 129


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Fri i yp j i ubj h ar, a d mbi d h ara d i .High strength hexagon head bolts are usedas described under Galva iz d highr g h ru ural b l .

sh ar j iIn joints subject to shear only in the planeof the friction faces the number of highstrength bolts and their disposition shouldbe such that the resulting load at any boltposition does not exceed the value:

Slip factor* x number of x minimumeffective boltinterfaces tension

*Slip factor is the coefficient of friction on the matingsurfaces and can be defined as the ratio of the shearforce between two plies required to produce slip, tothe force clamping the plies together.

AS 4100 provides that the slip factor forclean as-rolled steel surfaces shall be takenas 0.35. When protective coatings arepresent on mating surfaces, AS 4100specifies that the slip factor applied indesign must be that of the protectivecoatings, based on test evidence asdiscussed under slip fa r fgalva iz d a i g .

J i ubj x r al i iaddi i h arAn externally applied tension in the

direction of the bolt axis reduces theeffective clamping action of the bolt. Toallow for this effect, the InteractionEquation of AS 4100 (Rule 9.3.3.3)

V*sf + N*tf < 1.0 Vsf Ntf

Wh r :

V*sf = design shear force on the bolt inthe plane of the interfaces

N*tf = design tensile force on the bolt

= capacity factor

Vsf = nominal shear capacity of the bolt

Ntf = nominal tensile capacity of the bolt

B ari g yp j i ubj h ar a d mbi d h ara d iIn bearing type joints, design followsconventional practice based on allowabletension, shear and bearing values asspecified in AS 4100. Design of a joint asbearing type infers that some slip intobearing may take place.

AS 4100 specifies that shear or momentconnections subject to stress reversal, orwhere slip would not be acceptable shallbe designed as friction type joints. Bearingtype joints must be designed in accordancewith AS 4100 using the allowable forcesdetailed in the table below. Provided jointsurfaces are free from oil, dirt, loose scale,loose rust, burrs or defects which wouldprevent solid seating, AS 4100 permits theuse of applied coatings without change indesign values.

J i ubj h ar f r lyBearing type joints subject to shear forceonly, and which are less than 500 mm longin the direction of the applied shear force,shall be proportioned so that the shearforce on any bolt does not exceed themaximum permissible shear force,permitted by the table.

For joints greater than 500 mm long refer toclause 9.3.2.1 of AS 4100.

J i ubj h ar a d ilf rBearing type joints subject to shear andtensile forces shall be proportioned so thatthe tensile force on any bolt does notexceed that permitted by the ParabolicInteraction Equation of AS 4100 (Rule9.3.2.3)

V*f 2 + N*tf 2

< 1.0 Vf Ntf

Wh r :

= capacity factor

Vf = nominal bolt shear capacity

Ntf = nominal tensile capacity of the bolt

( () )

( () )

Maximum p rmi ibl appli d f r u i g m ri b l As 1252

Diam rf b l ,mm

Maximum p rmi ibl i : Fri i yp a db ari g yp j i

Maximum p rmi ibl appli d f rb ari g yp j i , kn

sh ar (1) B ari g pr j dar a

thr ad dp r i

U hr ad dp r i

16 104 59 83 Refer Clause9.3.2.4 of AS410020 163 93 129

24 234 133 186

30 373 214 291n (1) Threaded portion based on core are Ac defined in AS 1275.

Unthreaded portion based on area of shank (nominal diameter)


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The installation and tightening of a highstrength structural bolt/nut assembly is atleast as costly as the bolt/nut assemblyitself, and the selection of bolt type andbolt tightening procedure is an important

consideration in the economics of highstrength bolted structures.

s ug igh i gSnug tight is defined in AS 4100 as the fulleffort of a man on a standard podgerspanner, or the point at which there is achange in note or speed of rotation when apneumatic impact wrench begins impactingsolidly. Podger spanners are graded inlength in relation to bolt size and strength,and are, for example, of the order of450mm long for M20 high strengthstructural bolts, and 600mm long for M24

high strength structural bolts.Snug tightening is applied in the followingsituations;

1. The final level of bolt tightening ingeneral structural bolting usingcommercial bolts ca g ry 4.6/s

2. A final level of bolt tightening usinghigh strength structural bolts ca g ry 8.8/s . Different designvalues must be applied than forprocedures 8.8/TF and 8.8/TB using thesame bolts, as discussed inVaria i

i d ig valu wi h b l r g ha d j i d ig .3. An intermediate level of bolt tension

applied as the first stage in fulltightening ca g ri 8.8/tF a d8.8/tB.

The growing popularity of high strengthstructural bolts to AS 1252 used in a snug tightcondition leads to the situation where boltsmay require full tightening to AS 4100 in oneapplication and only snug tightening in another.t pr v fu i a d ur rr igh i g h d ig r mu i di a

l arly h l v l f igh i g r quir d, ib h drawi g a d p ifi a i .Steps must be taken to ensure that thisinformation is conveyed to all those involvedin installation, tightening and inspection.

s ug igh i gWhen snug tightening is used as the firststage for full tightening in procedures 8.8/TF and 8.8/TB, the intention is to bring theplies into snug contact ready for finaltightening. The clamping force applied bysnug tightening is highly variable asillustrated below, but is not significantwhen bolts are subsequently fully tightened since the bolt tension/bolt elongationcurve is relatively flat, variations in thesnug tight condition result in only smallvariations in final bolt tension.

Bolt tension/bolt elongation curve for a typical highstrength structural bolt.

The range of final bolt tensions after part turntightening exceeds minimum specified bolt tensiondespite variability in snug tightening.

Full igh i g (mi imum b l i )For joints designed in accordance with AS4100, either as 8.8/TF friction type or 8.8/TB bearing type, bolts must be fullytightened to the following minimumtensions:

Nominal bolt Minimum boltdiameter tension, kN

M16 95

M20 145

M24 210

M30 335

M36* 490* If M36 bolts are specified the part turn method oftightening should be used only after specialinvestigationinto the capacity of the available equipment.

To attain these bolt tensions AS 4100permits galvanized or zinc plated bolts to betightened by either the part turn of nutmethod, or by the direct tension indicatormethod. Torque control tightening ofgalvanized or zinc plated bolts and nuts isprohibited in AS 4100 because of thevariable torque/induced tension relationshipof zinc coatings even when lubricantcoated.

Tightening procedures for high strength structural bolts


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nu r a i fr m h ug- igh di i As 4100

Di p i i f u r fa f b l d parn 1, 2, 3 a d 4

Bolt length (underside ofhead to end of bolt)

Both faces normalto bolt axis

One face normal to boltaxis and other sloped

Both facessloped

Up to and including 4diameters

1/3 turn 1/2 turn 2/3 turn

Over 4 diameters but notexceeding 8 diameters

1/2 turn 2/3 turn 5/6 turn

Over 8 diameters but notexceeding 12 diameters(see Note 5)

2/3 turn 5/6 turn 1 turn

n :1. Tolerance on rotation: for 1/2 turn or less, one-twelfth of a turn (30o) over and nil under

tolerance; for 2/3 turn or more, one-eighth of a turn (45o) over and nil under tolerance.2. The bolt tension achieved with the amount of nut rotation specified above will be at

least equal to the specified minimum bolt tension.3. Nut rotation is the rotation relative to the bolt, regardless of the component turned.4. Nut rotations specified are only applicable to connections in which all material within

the grip of the bolt is steel.5. No research has been performed to establish the turn-of-nut procedure for bolt lengths

exceeding 12 diameters. Therefore, the required rotation should be determined byactual test in a suitable tension measuring device which simulates conditions of solidlyfitted steel.

Par ur igh i g1. Line up holes with drift pins to maintain

dimensions and plumbness of thestructure

2. Fit bolts in remaining holes. Use taperwashers if surface slope exceeds 3o anduse flat washers under the rotatingcomponent.

3. Tighten all bolts to snug tight position,progressing systematically from themost rigid part of the joint to the freeedges.

4. On large joints take a second run tocheck all bolts are snug tight.

5. Match mark installed nuts and boltsusing a punch to show that snugtightening is complete. These marks canthen be used for final tightening andinspection.

Part turn tightening. When tightening by hand or forpermanent indication of tightening bolts and nutsshould be match marked.

6. Complete tightening using the part turnmethod according to the table on page56. Tightening should proceedsystematically from the most rigid partof the joint to its free edges. Wrenchsockets should be marked at position180o apart to guide the operator intightening.

7. Knock out drift pins, replace with bolts

8. Bring these bolts to snug tight positionas in step 3, match mark as in step 5and complete tightening as in step 6.

9. Mark joint to indicate tightening hasbeen completed. One method is to drawlines with crayon between each bolthead forming a squared pattern.

Dir i i di a r igh i gSeveral direct tension indicating devices

have been developed to provide a simplemethod of checking that minimum bolttension has been developed. The mostcommonly used in Australia is the loadindicator washer.

The load indicator is similar in size to anormal circular washer, with four to sevenprotrusions depending on size, on one face.It is assembled under the bolt head so thatthe protrusions bear on the underside of thehead. As the bolt is tightened theprotrusions are flattened, and reduction ofthe gap by a specified amount indicates

that minimum bolt tension has beenreached. For use with galvanized structuralbolts load indicator washers are suppliedwith a galvanized finish.

Load indicating washers fitted under bolt head. Notegap which is reduced as nut is tightened.

tigh i g pr dur wi h l adi di a r wa h r1. Ensure that the bolts are high strength

bolts to AS 12522. Place load indicator on the bolt with

protrusions abutting the underside ofthe bolt head or abutting a structuralflat washer if the bolt head is to beturned in tightening.

3. Fit the bolt into place and assemblewith nut and standard hardenedwasher. If a taper washer is required itis preferable that this be fitted underthe nut but alternatively it may beplaced between the load indicator andthe structural steel.


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4. Carry out a preliminary tightening tosnug tight position, using a podgerspanner or pneumatic impact wrench.It is important to begin tightening at themost rigid part of the joint progressingsystematically to the free edges.On large joints take a second run overbolts to check that all are snug tight.

5. Carry out final tightening by reducingthe gap between bolt head and loadindicator to approximately 0.25 mm forgalvanized bolts. In aggressiveexposure conditions the gap may befully closed to exclude moisture.

Should a nut be slackened after being fullytightened a new load indicator must befitted before the second tightening.

Fi i g l ad i di a r u d r uIn applications where it is necessary torotate the bolt head rather than the nut, theload indicator can be fitted under nut usinga special nut face washer which is heattreated to the same hardness as the bolt.Care must be taken that the nut facewasher is fitted concentric with the nut andthe correct way up, otherwise it may turnrelative to the load indicator resulting ininaccurate load indication due to damage tothe protrusions.

Experience has shown that on medium tolarge projects the extra cost of loadindicators is offset by major savings ininstallation, supervision, and inspection ofhigh strength joints.

Inspection of high strengthbolted joints

Flush spliced structural joints ingalvanized steel

Because of the increasing use of highstrength structural bolts in the snug tightcondition the designer must clearly indicatethe level of tightening required in drawingsand specifications, and he must ensure thatthis information is conveyed to all thoseinvolved in installation, including theinspector.

In structural joints using either 4.6/S or8.8/S procedures the site inspector needonly be concerned that the correct bolt typeand number of bolts have been used in thejoint. Since the level of tightening requiredis snug tight, this would have beenachieved during erection.

In joints using galvanized bolts and 8.8/TF

or 8.8/TB procedures, only visual inspection

is necessary. The inspector should checkthat the correct fasteners and washershave been used and correctly installed, andthat none show physical damage whichmight indicate they have been driven intomis-aligned holes.

Galvanized bolts which have been tightenedby the part turn of nut method can bechecked by their match markings. Whereload indicating washers have been used forfinal tightening, inspection is greatlysimplified.

Tightening of bolts by the torque controlmethod has been deleted from AS 4100. Forguidance on the use of a torque wrench forinspection refer to AS 4100 Supplement

1-1999, Appendix CK.

The increasing popularity and used of hotdip galvanizing as a stand alone or barefinish for structural steel members meansthat a consistency in the overall finish isdesirable. This can be affected by the typeof connections used. Welding, whilepractical, requires coating touch up whichmay spoil the visual continuity of thegalvanized coating in some applications.Conventional bolted connections, areversatile and economic as is the method offlush splice connections.

A method of flush-splicing structural steelmembers was conceived by Arthur Firkins,formerly Director of Technical services,Australian Institute of Steel Construction.

The connection uses flat-head countersunkUnbrako high strength socket screws throughbeam flanges into threaded holes in the flange

and web connecting members. The result is aflush finish to beam flange surfaces withoutprotruding bolt heads or nuts, in a joint withthe performance characteristics needed instructural applications.

s ru ural p rf rmaIn order to investigate joint behaviour, a testspecimen was subjected to tensile testing atthe University of Sydney to determine theflange force transfer capacity of a typicalsplice. Test results showed that the splice

conformed to the requirements of AustralianStandard 4100 Steel Structures. The testresults also confirmed the designed capacityof the flange beam calculated in accordancewith AS 4100. As a result of this testing,structural engineers can now incorporateunobtrusive flush-spliced structuralconnections, confident that their design willmeet the requirements of AS 4100.

Fa r a d hr adThe fasteners employed are Unbrako highstrength flat-head socket screws, ISOmetric series, mechanically zinc plated to acoating thickness of 25m to give adequatecorrosion protection.


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The specification for these bolts is:

Material: Unbrakohigh-gradealloy steel*

Hardness: Re36-44

Ultimate tensile strength: 1100MPa

0.2% yield stress: 990MPa

Thread class 4g* In the international method of designating boltstrength these bolts would be classified as Grade 10.9

M12, M16 and M20 screw sizes are used.

D ig f flu h-b l d pliDimensional criteria for connections incommonly used beams are given in thetable below. These criteria apply to bothfully-bolted splices (Drawing A) and bolted/welded splices (Drawing B). This systemwill allow relatively large flange forcetransfer in members of all types and sizes.

Splice plates should be at least equal toflange or web thickness and not less thanscrew diameter.

I alla i pr durProcedures for the installation of Unbrakosocket head screws is contained in theproduct manual published by Unbrako.

Dim i al ri ria

M mb r Fla g pla W b pla B l *Fla g W b

siz Fla g f

W b w

Wid hmm

thi kmm

Wid hmm

thi kmm

siz siz

UB s i

200 UB 30 9.6 6.3 50 20 150 6 M16 M12

250 UB 37 10.9 6.4 50 20 150 6 M16 M12

310 UB 40 10.2 6.1 50 20 150 6 M16 M12

360 UB 51 11.5 7.3 50 20 150 6 M16 M12

410 UB 54 10.9 7.6 50 20 150 8 M16 M12

460 UB 67 12.7 8.5 50 20 150 8 M20 M16

530 UB 82 13.2 9.6 75 20 150 10 M20 M16Uc s i

250 UC 73 14.2 8.6 100 20 150 8 M20 M16

200 UC 46 11.0 7.3 75 20 150 8 M20 M12* Unbrako flat head socket screws Grade 10.9

1. Suggested criteria in the table should be verified for specific design load cases.2. For serviceability state, Ply in bearing (beam flange) will usually govern design (AS 4100 9.3.2.4(2)).3. Ultimate failure in the test was the flange plate component failing in tension.4. Flange plate component thickness should be greater than flange thickness and equal to or greater than bolt diameter.5. Web plate component thickness should be greater than web thickness.6. n = number of rows of bolts in flange or web as required by design see Drawing (A). Note: Bolt shear strength (10.9) will

rarely govern.7. Bolts should be specified as Unbrako flat-head socket screws Grade 10.9, mechanically zinc/tin plated to a coating thickness of 25m.8. Holes in flange plates should be tapped 0.1mm oversize to allow for the coating thickness on screw threads.9. Tapped threads should be plugged during the galvanizing process using bolts of appropriate diameter (Grade 4.6 hex head uncoated).

Fully-bolted splice, drawing A, and bolted/weldedsplice, drawing B.

Documents

Boltinggalvanizedsteel GAA Ch4[1]