AISC - Inspection of Welded and Bolted Joints

Inspection ofWelded and Bolted

Joints

AuthorRobert E. Shaw, Jr., PE is presi-dent of the Steel Structures Tech-nology Center, Inc., Novi,Michigan. A graduate civil en-gineer from Rose-Hulman Instituteof Technology, he began his careerin the steel construction industry in1973.

The SSTC focuses on thedevelopment and presentation oftechnical training seminars relatedto the design, fabrication, erectionand inspection of steel-framedstructures, and also provides con-sulting services in these areas. Mr.Shaw is the principal instructor forthe Federal HighwayAdministration's two-day seminaron High-Strength Bolts for Bridges.He also conducts two-day trainingcourses on the inspection of steel-framed structures for the Interna-tional Conference of BuildingOfficials. Mr. Shaw is a consultantto the Lincoln Arc Welding Founda-tion, serving as editor to update theclassic textbook on welding designentitled, Design of Welded Struc-tures, by Omer Blodgett. Mr. Shawalso serves as editor for theSSTC's bi-monthly newsletterSteel Inspection News, whichfocuses on the quality and inspec-tion of steel-framed buildings andbridges.

Before funding the SSTC in1990, Mr. Shaw was employed bythe American Institute of Steel Con-struction as associate director ofeducation from 1986 to 1990 andwas a regional engineer from 1981to 1986. Before joining AISC hewas a sales engineer for the Missis-sippi Valley Structural SteelDivision of Bristol Steel and IronWorks, Inc. His duties includedsales and project management forsteel fabrication and erection con-tracts in the upper Midwest.

SummaryEngineers and fabricators shouldbe aware of the inspection require-ments for each of the four bolt in-stallation methods approved by theResearch Council on StructuralConnection (RCSC). These re-quirements will discussed alongwith the meaning of the RCSC andAISC inspection rules. This dis-cussion will include sampling andfrequency and the proper use of abolt tension calibrator.

The American WeldingSociety's inspection requirementsfor various types of welds and theirapplications will also be discussed.The rules will be explained alongwith practical suggestions on howacceptable welds can be achievedin real buildings. Cases will bepresented.

29-1

© 2003 by American Institute of Steel Construction, Inc. All rights reserved.This publication or any part thereof must not be reproduced in any form without permission of the publisher.

Inspection of Bolted Joints

Robert E. Shaw, Jr., PEPresident

Steel Structures Technology Center, Inc.Novi, Michigan

Over the past 40 years or so, since the advent of the use ofhigh-strength bolts, there has been much trial and tribulationover the inspection of installed bolts. Several attempts havebeen made to refine and clarify the codes and methodologies, withthe most recent changes coming with the 1985 Allowable StressDesign Specification for Structural Joints Using ASTM A325 andA490 Bolts and its 1988 companion, commonly referred to as the"Bolt Spec."

Unfortunately, the momentum of 40 years of practice in theindustry, as well as our industry's tendency to adopt changeslowly and cautiously, has meant that there is continuing needfor presentations and discussions on the topic of high-strengthbolting. Confusion continues to reign over joint types,installation requirements and inspection requirements.

The principal obligation for proper specification and inspectionof high strength bolts lies with the engineer. The engineer hasthe responsibility to design a project that provides bothstructural integrity and serviceability, without unnecessaryexpense to the owner. Unfortunately, there are dozens of ways towaste the owner's money by improperly specifying or over-specifying the requirements for bolted joints and theirinspection. The purpose of this paper is to discuss the possiblepitfalls and recommend suitable practices.

We will look at proper specifications, including a few designconsiderations, material concerns, assembly concerns,installation techniques and good inspection sequences. A good jobrequires the inter-related work of engineer, supplier,fabricator, erector and inspector.

29-3


Selection of Joint Type

The first step is the determination by the structural engineer of thetype of each joint - shear/bearing, slip critical or direct tension.If shear/bearing, a second decision must be made - snug tightinstallation or pretensioned installation. These four options give twodifferent installation requirements and four different inspectionlevels. The specifics of the installation and inspection requirementsfor each will be covered in a latter section.

Shear/Bearing Joints

The vast majority of joints inbuilding structures areshear/bearing and need betightened only to the snug-tightcondition. The principle of theshear/bearing joint is that theload is transferred from plane toplane through shear in the bolt,the load being transferred to thebolt by bearing against itsshank. In essence, we have aseries of pins to carry the load.The weight of the members andother dead loads duringconstruction is usually enough tocreate the direct bearing ofsteel against bolt, hence nofurther movement of the jointwould be expected under live load.

Some shear/bearing joints are of enough structural importance that theAISC Specification for Structural Steel Buildings, section J1.12(ASD), J1.11 (LRFD), requires that the bolts be pretensioned. Thesejoints are as follows:

1. Column splices in all tier structures 200 ft. or more inheight.

2. Column splices in tier structures 100 to 200 ft. in height, ifthe least horizontal dimension is less than 40% of theheight.

3. Column splices in tier structures less than 100 ft. in height,if the least horizontal dimension is less than 25% of theheight.

4. Connections of all beams and girders to columns and any otherbeams and girders on which the bracing of the columns isdependent, in structures over 125 ft. in height.

5. In all structures carrying cranes of over five-ton capacity:roof truss splices and connections of trusses to columns,column splices, column bracing, knee braces, and cranesupports.

6. Connections for supports of running machinery, or of any otherlive loads which produce impact or reversal of stress.

7. Any other connections stipulated on the design plans.

29-4


For all other cases, A307 bolts or snug-tight high-strength boltscould be used. However, slip-critical joints and direct tension jointsmust also be considered, although not mentioned in this section, asrequiring high-strength bolts.

Tightening bolts in column splices in tier buildings and other similarstructures with a narrow aspect ratio provides better splice stiffnessand helps maintain column and frame alignment. Tightening bolts atmembers that brace the columns helps maintain frame alignment, as wellas the lateral support of the columns.

For those members of crane buildings that directly participate incarrying crane loads, both vertical and lateral, their connections arepretensioned to provide added stiffness to the joints and frame, aswell as handle vibrations from the crane movement that could, in somepeople's opinion, cause the nuts too loosen.

For reciprocating machinery and other machinery that inducesconsiderable vibration into a joint, the tightening of the bolts notonly stiffens the joint and frame, it reduces the chance that thevibration would cause the nut to loosen. It is the opinion of mostfastening experts that vibration alone will not cause the nut toloosen, if the bolt is tightened, because of the friction between thenut face and the washer or steel and also between the bolt threads andnut threads. However, if the joint is permitted to slip with asufficient number of reversals of slip, the nut will indeed back offthe threads. For this reason, these joints are treated as slip-critical, as we will see later.

The last provision is to include those special situations where thestructural engineer determines that the pretensioning would indeed beimportant to joint performance, but the joint may slip without seriousconsequence to the joint or structure.

It has been estimated that as high as 98% of all building joints couldbe considered as shear/bearing joints that need tightened only to thesnug-tight condition. The above cases are relatively rare in commonbuilding construction, as are the slip-critical and direct tensioncases yet to be discussed.

Slip-Critical Joints

A slip-critical joint is assumedto behave in an entirelydifferent manner than a shear-bearing joint. In a shear-bearingjoint, one steel element bearsagainst the bolt, transferringload, the load is carried throughthe bolt in shear to the nextsteel element, then unloaded tothe next element by bearingagainst the bolt hole. In a slip-critical joint, however, thetransfer mechanism is totallyreliant upon friction forcesbetween the two steel elements.The friction is a result of thecompressive forces between the

29-5


parts, and the compression is the result of the tension in the bolts.The only other variable in the strength of a slip-critical joint isthe coefficient of friction between the parts. Therefore, achieving atleast the required tension is critical to the performance of a boltedjoint.

The slip-critical joint is fairly rare joint in building applications.The specific situations a joint must be treated as slip-critical arelisted in the RCSC Bolt Specification, section 5(a), as follows:

1. Joints subject to fatigue loading.2. Joints with bolts installed in oversized holes.3. Except where the Engineer intends otherwise and so indicates

in the contract documents, joints with bolts installed inslotted holes where the force on the joint is in adirection other than normal (between approximately 80 and100 degrees) to the axis of the slot.

4. Joints subject to significant load reversal.5. Joints in which welds and bolts share in transmitting load at

a common faying surface.6. Joints in which, in the judgement of the Engineer, any slip

would be critical to the performance of the joint or thestructure and so designated on the contract plans andspecifications.

Few building joints are subjected to true fatigue loading. To meet theAISC definition of fatigue, one needs at least 20,000 cycles of a highenough stress level to fall into the fatigue category. Seismic jointsare generally not considered in fatigue categories because the numberof cycles applied is so small. Seismic joints are designed forductility, rather than fatigue performance. Joints resisting windmoments see many cycles, but the stresses are rarely of significantmagnitude where fatigue enters into the design. Most bridge joints,however, would be categorized as subject to fatigue, and thereforewould be slip-critical.

There are, indeed, certain types of joints in seismic and non-seismicdesign where most engineers would want to avoid joint slip. Criticalmoment connections where slip would allow significant framedeformation, or void the stress transfer mechanisms of the joint, maybe considered as slip-critical. Certainly, typical floor framingjoints do not fall into this category, even though occasionalreversals of stress may occur during seismic events.

Oversized holes are not common except in very thick materials that arefabricated without pre-assembly, or for cases when connecting toexisting connections with less than exact conditions. Slotted holesoriented in the loaded direction are also rarely used. If used, themagnitude of slip would exceed the 1/16" commonly accepted forshear/bearing joints, and therefore slip is to be avoided. Slots thatare perpendicular to the direction of applied load are common foradjustment and tolerances, but these cases may be treated asshear/bearing joints unless other conditions dictate.

The definition of "significant" in "significant load reversal" is onethat is missing from both specification and commentary. Most commonly,this section is considered for bracing elements. There is typicallyload reversal in many bracing systems, but is it significant? One mustconsider several factors when considering whether or not to make abracing joint slip-critical. How much movement would be expected in

29-6


the joint, figuring bolt diameter, hole diameter, accuracy of holeplacement through punching or drilling, and the random nature in whichbolts are located within the holes upon completion of the snugging andtightening (if any)? If the joint were to slip, would the amount ofmovement possible cause the joint or the structure to perform in adifferent manner? Would it have a significant effect upon buildingdrift or lateral stability? How much would joint movement contributeto lateral drift when compared to drift from other sources - memberdeformation, bending and elongation? How often is it expected thatwind or seismic loading (or other lateral loading) will be significantenough to overcome the slip resistance generated by just snugging thejoint? Is the significance and frequency of these loadings enough thatthe additional slip in the joint would even be noticeable whencompared to other building movements and behavior? Would there be acumulative effect if all or several of the bracing joints were toslip?

When welds and bolts share load, there can be no movement allowed inthe bolted joint. Because of the inherent stiffness of a weld, theweld tends to take all the load. The bolts should be tightened firstto develop the proper tension and clamping force, then the weldsshould be completed. Welding first could distort the joint so thatproper mating of the surfaces necessary to develop the frictionalresistance would be difficult or impossible. Research by Jarosch andBowman ("Tension Butt Joints with Welds and Bolts in Combination",Engineering Journal. 1st Quarter 1986) has added cautions when usingthis type of detail with transverse welds.

The final clause is to permit the engineer to use good judgement forcases that may not be adequately addressed by specification. Examplescould be movement-intolerant supports for critical laboratoryequipment or machinery.

Direct Tension Joint

The third type of joint - thedirect tension joint - was addedto the specification in 1985.Prior to 1985, all high-strengthbolts were pretensioned.Beginning in 1985, most couldremain snug tight. Theassumptions used in designinghanger-type details, particularlythose where prying action couldbe a factor, was based upon apretensioned bolt. Themethodology became ineffectivewith snug tight bolts. Secondly,there is concern that, if thebolts were only snug tight, theload applied to the fitting wouldcause separation of the parts,hence allowing movement in the joint and supported elements. For thesereasons, direct tension joints must have the bolts pretensioned.

29-7


Purchasing and Materials

For the fabricator, purchasing the right materials is the first step.Review the project specifications to determine if there are anyspecific requirements for the job. Certain projects may have buy-America requirements. Some may call for the rotational-capacitytesting required under AASHTO and FHWA specifications, even though theproject may be a building. Some engineers have specifically excludedGrade C, C3, D and 2 nuts from their projects because of their risk ofstripping. Others may handle the stripping problem by specifying aminimum hardness for the nuts. Metric bolts are being specified forsome government projects.

The inspector is not the one to check purchase orders, but theinspector should check the containers to make sure the specifiedmaterials have been provided. The inspector should also look forsupporting documentation of bolt quality in the form of test reportsand certifications.

Under ASTM A325, the following items must be provided when requestedby the purchaser:

(1) heat number and heat analysis of the steel from which thebolts are made,

(2) numerical results of hardness, tensile and proof load tests,(3) results of rotational-capacity tests (required for zinc-

coated assemblies, optional for black assemblies),(4) zinc coating thickness measurements,(5) results of visual burst inspection,(6) statement of compliance of dimensional and thread fit

requirements,(7) lot number assigned to fasteners,(8) title and signature of individual responsible for

certification, and(9) mailing address of responsible party.

Similar requirements apply to nuts and washers. ASTM A490 simplyrequires copies of the inspection reports when requested by thepurchaser. Testing of A490 bolts may still, unfortunately, be done bythe shipping lot method.

The Fastener Quality Assurance Act has placed a great deal of pressureupon the fastener manufacturers to provide accurate and truthful testreports and certifications. Most bolt distributors are also beingdiligent about their certifications, but some should be watchedcarefully. Verify that the certification paper identification agreeswith the container labeling. If the containers are unmarked, rejectthe lot.

Many times the bolts, nuts and washers are produced by differentmanufacturers. They may be supplied by different distributors.Individually, each fastener component may meet the productspecification, but when the bolt, washer and nut are assembled,problems may arise.

29-8


Fastener Identification

Reports about the use of unmarked bolts, nuts and washers have,fortunately, died off from the furor of the late 80's. The FastenerQuality Assurance Act passed by Congress certainly caught theattention of manufacturers and distributors. Unmarked fastenersare/were typically imported fasteners from questionable sourcespurchased by a distributor. Those trying to save a buck may have beentempted to sell or use such product.

Obviously, if the fasteners have not been marked properly according toASTM requirements, the manufacturer or distributor has probably beenskirting or ignoring other significant portions of the ASTMspecifications. Strength, ductility and thread fit are critical tobolt performance. Were these unmarked fasteners satisfactory in theseareas? Probably not.

The first thing to look for is the manufacturer's mark. Nearly everysingle component - bolt, nut and washer - must have the manufacturer'smark either stamped into it or in raised form. Obviously, washers mustuse stamping only. For bolts, the head marks are typically raised. Allbolts, even the lowly A307 bolt, must bear the manufacturer's mark.

Bolts

A325 bolts must clearly say"A325" on them. A Type 1 bolt mayhave three radial lines at 120°,but this is optional to themanufacturer. A Type 2 bolt wouldbe identified by three radiallines at 60°. Type 2 bolts havenot been manufactureddomestically for several years,and if found, would be worthy ofadditional investigationregarding source and quality.Type 3 bolts, used for weatheringsteel applications, have the A325underlined. A490 bolts must say"A490". The Type 1 has noadditional markings. The Type 2,still available, has six radiallines at 30°. The Type 3 bolt for weathering steel applications willhave the A490 underlined.

A449 bolts, used only for bolts over 1-1/2" in diameter (the maximumdiameter of A325 and A490 bolts), will not say "A449", but will havethree radial lines at 120°. Only the Type 1 bolt is acceptable. TheType 2 A449 bolt carries only three radial lines at 60° (similar to theA325 Type 2), but these bolts go only to 1" diameter and are not to beused.

A307 bolts will have "307A" or "307B" on them, depending upon grade.Either grade is acceptable. The A307 bolt, until about five years ago,was not required to carry grade identification, so there are probablysome legitimate A307 bolts in the market with no markings. Let thebuyer and user beware.

29-9


A354 bolts may be used, per AISC specification, only for anchor boltsand as threaded rod. They should not be used for a steel-to-steeljoint. A325 or 490 bolts should be used in these cases. Grade BC isthe lower strength of the two and is marked with a "BC" on the head.Grade BD, with the equivalent strength of an A490, must be marked withsix radial lines 60° apart and may have an optional "BD" on the head.Grade BD bolts over 2-1/2" must have the additional "BD" marking. A354studs and threaded product must carry "BC" or "BD" on one end of theproduct.

Nuts

All nuts must also carry themanufacturer's mark. Nuts andwashers, as commodity items, aremore at risk of being unmarked.The A563 grade C nut isidentified by threecircumferential arcs spaced at120° about the face of the nut.For C3 nuts, a "3" is added tothe face of the nut. For D, DHand DH3 nuts, "D", "DH" or "DH3"will appear on the face. A194nuts of grade 2 and 2H will carrya "2" or "2H" on the face.

Acceptable nuts for A325 and A490bolts are chosen from ASTM A563,grades C, C3, D, DH and DH3, andA194 grades 2 and 2H. Dependingupon the grade of bolt, andwhether it is black orgalvanized, only certaincombinations are acceptable.These acceptable combinations arenoted by a circle in the adjacentchart.

For A307 grade A and B bolts,A563 nuts of Grades A or B can beused, depending upon diameter ofbolt. It is not required to haveeither the manufacturer's mark orgrade identification on grades Aor B, per ASTM. Manufacturers are required to mark the grade with aletter "A" or "B" only when required by the purchaser. Grades C, C3,D, DH and DH3 are also suitable nuts, and it would be advisable toselect these grades of nuts for use, rather than use unmarked nuts.

A449 type 1 bolts may also use an A563 grade A heavy hex nut, butstronger A563 nuts suitable for A325 bolts would make a better choice.

29-10


In the above examples, suitable nuts were based upon the use of blackbolts. When the bolts are galvanized, stronger heat-treated nuts arerequired. ASTM A563 Appendix Table X1.1 provides recommendations forsuitable nuts for various diameters of both black and galvanizedbolts. No such table exists in ASTM A194. Nuts with higher strengthlevels than that of the specified or recommended nut can usually besubstituted.

Washers

F436 washers complete the fastener assembly. There is another washerspecification, ASTM F844 for "Washers, Steel, Plain (Flat), Unhardenedfor General Use," but these washers have no strength or hardnesscharacteristics required. Any such requirements must be part of thepurchase order. These F844 washers do not need marked with either agrade or manufacturer.

F436 washers, on the other hand, are the only washers listed by AISC.There are minimum hardness and testing requirements. F436 washers musthave the manufacturer's mark stamped into them on one face. If thewasher is for a weathering steel application, a "3" must also bestamped into the washer face. Some manufacturers place "F436" on thewasher, but this is at their option. Clipped washers may sometimeshave their manufacturer's mark removed, and this is acceptable.

Manufacturer's Marks

To tie the fastener back to the manufacturer's certifications,especially in cases where the fasteners are not in their originalshipping container with the lot number labels, it is helpful to beable to correlate the manufacturer mark back to the manufacturer.

The Industrial Fastener Institute publishes a "North AmericanManufacturer Identification Markings for Fasteners," publication IFI-122, for $15.00 that relates manufacturer's mark to a company name andaddress. The latest regular edition was published in 1988, but it isconstantly updated. Also available is a "Fastener QualityInternational Guide" (IFI-122A) for $20.00, but it is a few years outof date. To order, contact the Industrial Fastener Institute inCleveland at (216) 241-1482.

The Department of Defense publishes a similar document ofinternational markings, Military Handbook "Listing of FastenerManufacturers' Identification Symbols," identified as MIL-HDBK-57B(IS)and dated 30 November 1990. Order from Defense Publications by faxonly at (215) 697-1462. For problems, call 215-697-2179. Thispublication is free.

29-11


Pre-Testing Fastener Assemblies Prior to Installation

One of the most important areas of high-strength bolt installation isactually performed prior to bolt installation. The 1985 ASD and 1988LRFD Bolt Specifications give specific instructions regarding pre-testing of fasteners prior to installation. The 1988 Spec is betterwritten, with some clarifications and additions over the 1985 version,but the 1985 version is still the one referenced in all three modelbuilding codes.

Pre-testing is required in some form for all slip-critical, directtension and pretensioned shear/bearing joints. Joints that need to betightened only to snug tight are exempted from these provisions.

The purpose of these provisions is:1. to verify the quality of the materials, both bolt and nut and

washer, if used, and their ability to work together withoutnut stripping,

2. to verify the method selected for pretensioning will actuallyprovide the required pretension, and

3. to verify the installation crew's understanding of the method.

All pre-installation testing should be done in a bolt calibrationdevice such as a Skidmore-Wilhelm. Install the bolt in the Skidmore,with spacer plates and washers as needed for the bolt length, snug thebolt, then tighten the bolt using the procedure that will be used forthe work. Verify that the achieved bolt tension is at least 5% higherthan that required by the specification.

If the turn-of-the-nut method is selected, pretesting is performed atthe start of the work. Under the 1985 provisions, a representativesample of 3 bolts and nuts of each diameter, length and grade arepretested. Under the 1988 provisions, the sample is changed to threebolt and nut assemblies of each diameter, length, grade and lot. Thisprovides additional verification of thread mating between the bolt andnut assemblies that are actually going to be used. Under the 1985provisions, you could choose three bolts at random and three nuts atrandom and try them out. No additional checks were necessary, even ifbolt manufacturers or nut manufacturers were different from thosetested. Variations occur within bolts and nuts, even from the samesupplier.

If the calibrated wrench method is selected, calibration is requiredeach day on three bolts of each diameter, length and grade of boltbeing tested, treated as assemblies. The 1988 Spec clarified andexpanded this to state that the assemblies are made of three bolts ofeach diameter, length, grade and lot with nuts of each diameter, gradeand lot. Recalibration is required for changes in bolt or nut threadcondition, installation conditions, etc.

For alternate design bolts such as twist-off bolts or lock pin andcollar fasteners, the pretesting is required only at the start of thework and includes three bolts of each diameter, length and grade. Nomention of nuts is made here, but these items are typically deliveredpre-assembled. Regardless, the nut statement should be added. Neitherthe 1985 or 1988 Spec calls for testing by lot, but they should.

29-12


For direct tension indicators, the 1985 Spec calls for testing, at thestart of the work, three dti's for each diameter and grade offastener. The 1988 provisions added the words "with three typicalbolts." There is no mention of testing by lot in either version, butit is suggested that dti's should be tested by lot. Second, there isno real test of the bolts or nuts required, just the direct tensionindicator. Each bolt and nut assembly to be used should be tested as aunit for suitable performance.

Testing as assemblies creates more sorting, storage and record-keepingwork for the field, ending the random "mix and match" mode withoutpretesting each possible combination of bolt lot and nut lot. It does,however, increase the probability of getting good pretensioned bolts.

Testing by lot is being performed for high-strength bolts at themanufacturer's level or at the supplier's level under the provisionsof recently revised ASTM's. However, there are still many bolts incirculation manufactured and sold under the old ASTM shipping lotmethod, so it may take some time before on-site assembly testingprocedures can be modified to reflect the pretesting performed priorto delivery to the jobsite.

Additional testing may be appropriate, such as the FHWA procedures forrotational capacity tests for assemblies. Not a part of the RCSC/AISCBolt Specs or the current ASTM Specs, FHWA's methods provideadditional testing for bolt ductility, strength, assembly thread fitand the adequacy of the lubricant.

The effort required to install the bolt should not be extraordinary,which would be indicative of inadequate lubrication. No specifictorque values are dictated by specification except under the FHWA andAASHTO specifications. In these two specifications, the torque (infoot-pounds) must not exceed 0.25 times the bolt diameter (in feet)times the bolt tension (in pounds). This torque reading can be takenat any point when the bolt is above the required tension.

A thread stripping check is performed under the FHWA and AASHTO specsat a deliberately over-tensioned level. The fasteners are tighteneduntil twice the rotation required under turn-of-the-nut tightening isachieved. In other words, if a short bolt requires one-third of aturn, the bolt is tightened two-thirds of a turn past snug tight, thenthe assembly is disassembled and checked for stripping. Any threadshearing or stripping of threads would indicate a thread fit problemor too soft a nut. By design and specification, the bolt hardness andthread design is such that the bolt should break before the boltthreads strip.

Another aspect of the FHWA / AASHTO test at this point (havingachieved twice the required turns) is that bolt ductility is verified.The bolt is able to stretch more than required. Along with this, thebolt at this point (at twice the required turns) must have at least15% more pretension than that required by specification, indicatingthat the bolt was neither too soft nor stretched too easily.

All the above tests are described as being conducted in a boltcalibration device such as a Skidmore-Wilhelm. Often, however, boltsare too short to be installed in a Skidmore and an alternative methodmust be used for pretesting. This is typically done with the use ofdirect tension indicators. The dti's are placed under the bolt head

29-13


using any available bolt long enough to fit in the Skidmore. The boltis then tightened to the minimum required tension (plus 5%, asrequired by Spec), and the average gap is determined for the dti atthat tension. This step is repeated two more times, and the average ofthe averages is determined. This becomes known as the "calibratedgap."

For calibrated wrench installation of short bolts, the wrench is setto compress the dti below the calibrated gap when the bolt is testedin a steel plate. For short twist-off bolts, the dti must becompressed below the calibrated gap when the spline shears off. It maybe necessary to restrain the twist-off bolt head from rotation duringtesting because of the dti presence. This procedure cannot be used,however, on short bolts that will be installed using the turn-of-the-nut method because much of the rotation provided will be used tocompress the dti, not elongate the bolt. For short bolts using theturn-of-the-nut method, there is no valid method of pre-installationtesting and we rely upon forty years of satisfactory experience andtesting.

Snugging the Joint

If the joint is not properly snugged, no pretensioning method willwork correctly. Snugging the joint has often been overlooked, and manyan argument has arisen over "loose" bolts caused by improperly snuggedjoints.

First, we must understand that not all bolts must be pretensioned. Themajority of bolts in buildings must be tightened only to the "snug-tight condition." Certain shear/bearing joints must be pretensioned,as well as slip-critical joints and direct tension joints.

The current RCSC Allowable Stress Design Bolt Specification (11/13/85)defines "snug tight" in section 8(c) as "the tightness that existswhen all plies in a joint are in firm contact."

This leaves the door open for bolts being only "finger-tight" when theparts are solidly seated and no gaps are present. Indeed, the boltedjoint would probably perform quite adequately in this "finger-tight"mode, as shear/bearing behavior requires no bolt tension, even snug.

The sentence that follows the definition of "snug tight" states that"This may be attained by a few hits of an impact wrench or the fulleffort of a man using an ordinary spud wrench." In the LRFD Bolt Spec(6/8/88), this sentence was moved to the Commentary.

The latest definition of "snug tight" has been provided by the AISC'sLRFD Specification for Structural Steel Buildings (12/1/93). Thisdefinition combines both the condition and the tightening method used.The new definition states "The snug-tight condition is defined as thetightness attained by either a few impacts of an impact wrench or thefull effort of a worker with an ordinary spud wrench that brings theconnected plies into firm contact." By using this definition, the"finger tight" possibility is removed.

The only fear of the above definition is an inspector who decides to

29-14


verify that full effort was used on the installer's spud wrench, thenproceeds to apply an inspection spud wrench to the bolts to see ifthey turn. What a travesty that would be, because we are reallyconcerned only with the steel being in contact.

Some people have mistakenly adopted the "few hits" or "full effort"provisions to mean snug tight. If the result of these efforts is ajoint in which a gap still remains, the joint is not snug and thepretensioning procedure will not work properly.

Effect of Improper Snugging

What is the effect of having a joint that is not properly snuggedbefore pretensioning? How serious are the consequences? These answersdepend upon the pretensioning method used, but there are some commoneffects.

First, if the joint is not in solid contact for all the bolts, thepretensioning method employed on the first bolt will only serve tofurther draw down the gap between the steel elements. The installerassumes the first bolt is tight, but it's not. The next bolt tightenedfurther draws down any remaining gap, and the initial bolt becomeslooser still. This can become a compounding series in some joints.

If the turn-of-the-nut technique is used for pretensioning, a gap ofjust 1/16" is the equivalent of 5/8 turn on a 3/4" bolt (10 threadsper inch), or 1/2 turn on a 1" bolt (8 threads per inch). A gap ofjust 1/64" still requires nearly 60° of rotation on a 3/4" bolt just toremove the gap, far exceeding the under-tightening tolerance of turn-of-the-nut. On a 1" bolt, the 1/64" gap would need 45° additionalrotation to remove the gap. Obviously, if matchmarking is performedwhile gaps are still present, the provided turns will be substantiallyinadequate to induce the required pretensions.

If the calibrated wrench technique is used, the installer stands abetter chance of getting the bolt properly pretensioned. This isbecause this method requires "touching up" all the bolts in the jointuntil they fail to further impact. This is the only method thatrequires "touching up." This touching up will be a time-consumingeffort if the joint is not properly snugged in the first place. Thisis not to say that the calibrated wrench technique is the best method,or that snugging is not critical to the technique. Wrench settings andthe equipment used could be such that the wrench fails to impactbecause of high torques needed to initiate rotation, and the boltscould still be inadequately pretensioned.

If twist-off bolts are used, the joint requires a proper snug of theentire joint prior to twisting off the bolt spline. If the splineshears off during the snugging operation, the installation method isno longer valid and the pretension required will not be achieved.Visually, however, the installer and inspector will assume the bolt tobe tight because the spline has been sheared off.

If direct tension indicators are used, the dti may be compressedduring snugging to a point that the feeler gage used to check theremaining gap will be refused. When adjacent bolts are tightened, theprior bolt "tightened" will loosen, but the dti gap will not berestored because the dti is inelastic. Checking these bolts later witha feeler gage would indicate that the bolt had the proper tension at

29-15


one time, but would not indicate if inadequate snugging was performedprior to installation.

Inspecting for Snug

It is truly imperative that the "snug tight" condition be verifiedprior to pretensioning bolts. This may be a part of the quality jobperformed by the ironworkers, with no outside inspection. If the BoltSpecification is followed, however, the inspector is to verify snug.Section 9 (a) of the 1988 LRFD Bolt Spec clearly states "Allconnections shall be inspected to insure that the plies of theconnected elements have been brought into firm contact." The 1985 ASDBolt Spec says essentially the same thing. This requirement applieswhether the bolts remain snug tight only, or are pretensionedfollowing snugging.

The installation crew may be permitted to stuff and snug jointswithout the observation of the inspector, as long as the proper boltmaterials and lengths are used and no unfair reaming or flame-cuttingof holes is performed. The inspector is then called upon to verifythat the joints are snug prior to any pretensioning work. If the turn-of-the-nut method is to be used, matchmarking of the joints should bedone while the inspector is there. This eliminates the "matchmarkingof the bolts after impacting," which occurs from time to time.

The installation crew and inspector must understand where contact inthe joint is important and where it is not. Contact is necessary atthe point where the bolt goes through the hole. Contact is notnecessary at the edges of the joint, or even within the joint awayfrom the bolt holes. Research has indicated that the slip resistanceis provided immediately around the bolt holes, as illustrated by themasking provisions of Figure C3 of the Bolt Spec. The "circular areaaround all holes" (one inch but not less than the bolt diameter) isthe area that is active in transferring stress through friction. Inshear/bearing joints that do not need friction, the gaps must still beremoved around the bolt holes to ensure proper pretensioning. In "snugtight only" bolted joints, gaps between the steel where the bolts gothrough the joint could lead to excessive deformation of the connectedparts and undesirable movement of the joint or connected members. Gapsat the bolt holes could also contribute to hidden corrosion of thebolts themselves if water can collect in the holes. For additionalcommentary on gaps at the joint, see the AISC's Quality Criteria andInspection Standards, 3rd Edition.

In many joints, one can look at the joint from the side and judgewhether the joint has been properly snugged. In large joints withseveral rows of bolts, this is not possible. Judgement must be usedbased upon part thicknesses, whether an impact wrench was used forsnugging, if the nuts show some peening of the corners, and the caredemonstrated by the crew. If it is suspected that the joint has notbeen properly snugged, with gaps between the parts in the interiorarea of the joint, remove a bolt from the suspect area and insert atape measure with a thin hook on the end, or some other form of bentfeeler gage, and feel is there a gap between the parts. If the hook orfeeler does not drop between the parts, pass the joint as snug. Dothis in as many areas as is necessary, based upon judgement.

29-16


Most black bolts can be re-installed after removal, provided the nutcan be run up the full length of the bolt threads by hand. Ifgalvanized bolts or A490 bolts were used in the joint, it is safest tosimply replace them. However, if the bolts were given only nominaltension during snugging (no impact wrench was used), these bolts canprobably be reinstalled without a problem.

In summary, the extra time and effort to ensure the proper snugcondition will pay off in the long run when the bolts are pretensioned(if required). Many "loose" bolts come not from improper tighteningtechnique, but from improper snugging. If you can't be assured off goodsnugging, then no installation method is reliable. You'll be stuckwith arbitration inspection using a hand torque wrench, one of thecostliest and least effective methods of bolt inspection that can beemployed.

The Turn-of-the-Nut Installation Method

The turn-of-the-nut method has been around for many years. In theearly Fifties, the railroad industry developed a turn-of-the-nutmethod for installing bolts. Essentially, one full turn beyond handtight was provided. In subsequent years, further research was providedand turns were measured from the "snug tight" condition, rather thanhand tight. The "turns tables" have been modified occasionally, butthe current table has been in use since 1978.

The principle behind the turn-of-the-nut method is the controlledelongation of the bolt. Because of the pitch of the threads, turningthe nut a prescribed rotation elongates the bolt a certain amount. Theelongation has a direct correlation to the bolt tension. As boltsbecome larger in diameter, the number of threads per inch decreasesaccordingly, and therefore the same number of turns will provide atleast the required amount of pretension for a given length-diameterratio.

The turn-of-the-nut table iswell-known. For bolts less thanor equal to four diameters inlength, say a 3/4" by 3" bolt,1/3 turn must be provided. A 3/4"by 5" bolt would receive 1/2turn. A 3/4" by 6-1/2" bolt wouldreceive 2/3 turn.

For bolts over 12D in length,there is too much variation toprovide tabular values. It isrequired that the installer use abolt tension calibrator, such asa Skidmore-Wilhelm, and determinethe number of turns required toprovide the required boltpretension. It may well be that2/3 turn will be adequate, but it must be proven by test.

29-17


Rotation Tolerance

There is a tolerance to the amount of applied rotation. For turns of1/2 turn or less, the nut may be under-rotated by no more than 30°. Forturns of 2/3 turn or more, the nut may be under-rotated by 45°. If thenut does not receive sufficient rotation, the desired pretension maynot be achieved.

The second tolerance applies to over-rotation. The same numericalvalues apply. For turns of 1/2 turn or less, over-rotation is limitedto 30°. For 2/3 turn or more, 45°. The problem of over-rotation is thatthe bolt may be stretched to the point of breaking, or perhaps to thepoint where nut stripping will occur.

In reality, the ductility of fasteners is such that such limited over-rotation is quite restrictive. Physical testing indicates that boltsof 4D in length generally can sustain an additional full turn beyondthe required turns before breaking. Bolts of 8D in length and more cangenerally withstand 1-1/2 to 2 full turns beyond the required turnsbefore breaking. Such ductility is not always guaranteed, however, andmay be adversely affected by poor lubrication or by having just one ortwo bolt threads in the grip of the joint. Also, A490 bolts exhibitreduced ductility when compared to A325 bolts.

Because of the inherent ductility of high-strength bolts, there israrely concern about cases where the nuts have been over-rotated. Themost common mode of failure when extreme over-rotation occurs is whenthe bolt breaks in a combination of torsion (from tightening) andtension. If the bolt does not break during this worst-case stresscondition, it should not break in service. Normally, when a boltbreaks during installation, the sound is distinctly noticeable and theresult is easily visible as a loose bolt.

A second risk of over-rotation is nut stripping. Bolt threads arerarely at risk of stripping because of the thread design. However,nuts that are not heat-treated, A563 grade C, C3, D and A194 grade 2,are at risk of stripping if their hardness is below 89 Rockwell B or180 Brinell. If heat treated nuts are used (A563 grade DH or DH3 orA194 grade 2H), the risk of stripping before bolt fracture isnegligible.

Sloping Surfaces

The second component of the"turns table" is when there is asloping surface below either thebolt head or nut, or both. Whenthis case exists, additionalturns may be warranted, per theadjacent table.

The sloping surfaces provisionsapply when the slope of thesurface exceeds 1:20, or about 3°.Bolts are tested by themanufacturer using a wedgetensile test of either 6° or 10°under the bolt head, so the boltcan take this slope. However,

29-18


there will be extra rotation needed to overcome the loss caused by thebending at the head, and therefore 1/6 turn is added for each slopingsurface over what would be required for parallel surfaces.

If the sloping surfaces are caused by the 16-2/3% (10°) bevel used forchannel and S-section flanges, then a beveled washer must be used. Therequired turns increase for the sloping surface is not required,because the beveled washer has returned the head or nut to theparallel condition.

Pre-Installation Verification

Prior to stuffing bolts in the steel, the Bolt Spec section 8(d)(1)requires that three sample bolts of each diameter, length and grade beinstalled in a bolt calibration device (Skidmore-Wilhelm orequivalent). The bolts are to be snugged by the crew using the methodthey would use on the steel, then given the required rotation for thatlength and diameter of bolt. The tension read on the Skidmore must beat least 5% above the required pretension. The 1988 Bolt Spec addedthat this test must be performed for each diameter, length, grade andlot.

When the pre-installation verification test is performed, there may beinstances where the required turns do not quite develop the requiredtension plus 5%. The Skidmore is a hydraulic device, and as such, issofter than the steel in which the bolt will be installed. Thehydraulic fluid between the bushing and body moves and compresses, andtherefore some of the rotation is actually taken by this fluid action.When the bolt is installed in a solid steel joint, there is nodeformation between bolt and nut and all the rotation is used toelongate the bolt. For a given rotation, the tension in a solid steelplate will be higher than the tension read in the Skidmore. This couldbe further proven, even to the point of developing a calibrationcurve, by using identical fastener assemblies and using measuredtorques vs. tensions read in the Skidmore.

This condition would typically be found with longer bolts (just under4D, 8D or 12D) that are installed in the Skidmore with a maximumnumber of threads in the grip (minimum stickout). The longer boltlength and the full number of available threads to elongate wouldreduce the achieved tension for a given rotation. However, theprobability of having actual undertensioning in installed bolts, evenunder these conditions, is very small, and the amount ofundertensioning would be negligible.

Installation and Inspection

The installation sequence should start with snugging the joint. Thereis no need for an inspector to witness such snugging, provided thereis no flame cutting or unfair reaming of holes beyond specified BoltSpec tolerances. When snugging is complete, the inspector should becalled to verify the snug condition. The inspector verifies that thejoint is solidly seated, with no improper gaps, and that the propergrade and length of bolt was used (by verifying the stickoutcondition), with proper washer placement (if required) and proper nutmaterial. The installation crew is then permitted to matchmark the endof the bolt and a corner of the nut. At this point, the inspector maycontinue to other duties. After the required turns have been applied,the inspector returns to the joint to visually verify that the

29-19


required turns were applied, checking for the proper rotation betweenbolt and nut.

The inspector should also monitor the crew upon occasion to verifythat the proper technique is routinely applied during thepretensioning. This is not an important item if matchmarking is used,but crucial if the crew insists upon using the "watch the chuck turn"method. If the crew does not matchmark, regular observation isnecessary to ensure that, the crew consistently watches the wrenchchuck turn during pretensioning and that the bolt head (unturnedelement) is prevented from turning. It is not necessary to observeeach individual bolt or joint, just verify that the crew uses goodpractice.

If turn-of-the-nut has been properly applied, verified by matchmarkingafter snugging and checked after pretensioning, there is no validreason to submit the completed joint to any additional testing.Unfortunately, torque testing is still required for turn-of-the-nutinstallation under the AASHTO Bridge Specifications, but hopefullythis requirement will be removed when the above inspection techniquesare employed, especially with matchmarked joints. If we have goodmaterials, snug properly, and use the proper technique, and inspectfor these along the way, why would we want to use a highly variabletorque value to test them? The torque is not as reliable as the turn-of-the-nut method.

The Calibrated Wrench Installation Method

The calibrated wrench method uses a special type impact wrench totighten the bolts. Rather than impact until the wrench operatorreleases the trigger, the wrench is adjusted to automatically stopimpacting when a certain resistance is felt by the wrench. The wrenchis adjusted so that it stops impacting when the bolt has achieved atleast the required tension, as determined using a bolt calibrationdevice, but does not over-rotate the fastener beyond the turn-of-the-nut tables.

The procedure has been used for many years, but fell out of favor withmany because installers were failing to calibrate their wrenches asrequired for the procedure. The RCSC Bolt Spec withdrew permission touse the calibrated wrench procedure in 1980, but reinstated it in 1985with additional provisions intended to clarify and highlight thecalibration requirements.

In addition to the pneumatic impact wrenches typically found onjobsites, hydraulic wrenches have also been used for installing bolts.In these units, the hydraulic pressure required to tighten a fastenerto at least the required tension is determined by using a boltcalibration device. The tool is activated until the required pressureis reached. Actually, this same tooling could be used to install boltsusing turn-of-the-nut techniques by simply watching the wrench chuckrotation or by match-marking, then shutting off the unit when therequired turns had been applied.

Pneumatic impact wrenches depend upon an internal cam unit forcontrol. When the desired resistance, actually torque, is reached, thecam unit shifts and the wrench stalls out. If the air pressure or air

29-20


volume is inadequate, however, the control mechanism will not functionproperly and will continue to impact the fastener, although at aslower, weaker level. For this reason, the calibrated impact wrenchmust be calibrated with a given air supply condition. The wrenchshould be calibrated using the same compressor and pressure settings,air hose and air hose length that will be used on the work. If anadditional wrench is to be driven off the compressor, the wrenchcalibration should be checked with both wrenches in operationsimultaneously as well as individually. If a significant length ofhose from compressor to wrench is either added or removed, then thewrench should be recalibrated. In essence, the bolt calibrator shouldbe mounted on the steel in the immediate vicinity of the bolts beinginstalled, not on the ground next to the air compressor.

The Bolt Spec calls for calibration every day, before installationbegins, with three fastener assemblies of each diameter, length andgrade (and lot, per 1988 Spec). An assembly, by specification, wouldbe comprised of a bolt from a specific production lot with a nut froma specific production lot. If a certain lot of, say 7/8" x 3" bolts isused with 7/8" nuts from three different lots, all mixed, then thewrench must be calibrated so that it properly installs the bolts usingany of the three lots of nuts. Lot control becomes essential whenusing the calibrated wrench method.

Washers representative of those being used in the work must beincluded in the test, but lot control is not mandated except underAASHTO / FHWA requirements for bridge work.

A third factor (air supply and lot control being the first two) isinvolved in the calibration of the wrench. If there is a significantdifference in the quality of fastener lubrication, then the wrenchmust be calibrated for the varying lubrication conditions. A well-oiled bolt, washer and nut assembly will require considerably lesstorque than one that is nearly dry or one that exhibits someindications of rust. Hence, if the wrench is calibrated using well-oiled bolts, then used on a poorly-lubricated bolt, the resultanttension will be less. The same concerns apply if the bolt, nut orwasher surfaces contain dirt, grit or sand.

Efficient calibration becomes key to the use of the calibrated wrenchmethod of installation. Some projects have used a separate calibratedwrench for each given diameter, length and grade being installed,changing wrenches when a different bolt group is being installed.Every wrench is calibrated each morning. It is possible to simplifyoperations by calibrating the wrench to properly install a wider rangeof bolts, perhaps the entire range of a given diameter and grade. Thiswould be accomplished by setting the wrench to install at a tensionwell above the required pretension, but yet not high enough to exceedthe turns tables. This same wrench setting could be tested on anotherlength of bolts of the same diameter and grade, and then another. Withluck, perhaps one wrench setting could be used throughout the day forall bolts of the same diameter and grade being installed, or perhapsonly two wrenches would be needed instead of several.

Snugging the joint can be done with either the calibrated wrench(actually in the uncalibrated condition, releasing the trigger whensnug is achieved) or with an impact wrench, or with a hand wrench forlighter framing. After snugging, the joint should be inspected toverify snug, then approved for pretensioning of the bolts. After the

29-21


wrenches are calibrated, pretensioning can begin. The wrench operatorshould tighten the bolts using a systematic pattern, observing thechuck rotation as tightening proceeds. If the rotation of the nutexceeds the turns table for turn-of-the-nut, the wrench calibrationshould be rechecked. After tightening all the bolts in the pattern,the operator should return to "touch up" each bolt in the pattern.Only the calibrated wrench method calls for such "touching up."

The inspector has the responsibility during calibrated wrenchtightening operations to observe the installation technique. Theinspector should observe the calibration of the wrench prior toinstallation, then observe the technique to verify that the crewproperly uses the wrench, does not adjust the wrench calibrationwithout use of the bolt calibration device, does not use the wrench onfasteners from a different group that requires a different wrench,does not use fastener assemblies with significantly differentlubrication characteristics than those bolts used for calibration, anddoes tighten in a systematic manner, observing the wrench chuckturning to verify that over-rotation is not being demanded by thewrench, and finally touches up each bolt after tightening.

The Direct Tension Indicator Installation Method

The direct tension indicator, ordti, for short, is a special loadcell device used as proof thatthe required tension has beenprovided in the assembly. Theeffectiveness of the dti,however, is only as good as thetechniques used in installing thefastener. The manufacturing andtesting of the dti itself isgoverned by ASTM F959.

The principle behind the dti isthat the protrusions (bumps)formed into the device will becompressed by the tension in thebolt. Through manufacturingcontrols, the gap remainingbetween the dti face and the fastener element against which it isplaced will not close to, below a specified gap until after thefastener has reached the required fastener tension. Therefore, whenthe gap in the dti is below the specified gap, the bolt's tension willbe higher than that required.

The gap is determined by the use of a feeler gauge inserted betweenthe protrusions (bumps) in the dti face. The preferred position of thedti is with the bumps facing outward directly underneath the bolthead. During installation, the bolt head must be held from turning toprevent abrasion of the bumps, thereby rendering the dti measurementinvalid. For building applications, this condition would call for a0.015" feeler gauge to be refused entry in half or more of the gaps ofthe dti.

29-22


Other positions may be used for the dti. If the bolt head will beturned, a hardened F436 washer must be placed between the bolt headand dti. If the dti is placed at the nut end of the assembly, an F436washer must be used between the dti and nut, whether or not the nut isallowed to turn. In each of these cases, a 0.005" feeler gauge is usedto check the gaps.

For all bridge applications, the 0.005" feeler gauge is used to checkthe gaps. This is used to ensure that the gap between the dti and thefastener itself can be bridged by the applied coating system andcrevice corrosion will be avoided.

Standard F436 washers are needed behind the dti when the dti is usedover an outer ply containing an oversized or slotted hole. Thisprevents the dti from cupping into the hole and voiding the gapmeasurement technique. In no case are the dti bumps permitted to faceinward toward the steel joint. It must face outward toward either thebolt head or the nut. Otherwise, the dti will cup outward, opening thegaps larger and voiding the measurement technique.

It is necessary to test the dti at the jobsite prior to installationto meet the provisions of the Bolt Spec, section 8(d)(4). Three dti'sare tested in a bolt calibration device. The bolts are tightened to 5%above the required bolt tension, then the gaps are checked. Half ormore of the gaps in the dti must not refuse the feeler gauge. Thismeans that when the bolt is tightened in the field, half or more gapsrefusing the feeler gauge means that the bolt tension will be higherthan that required. If the dti is used in other positions (say underthe nut instead of under the bolt head), then the dti should also bepretested in this position.

After the quality of the dti has been verified in this manner, thebolt installers can proceed to their installation and snuggingoperations. The dti's should be installed in the same position astested. The joint is snugged using a systematic technique, thenchecked to ensure that the dti's have not been over-compressed by thesnugging operation. The installer should check that at least half thegaps do not refuse the feeler gauge. This is done to ensure that thebolt did not reach its required tension during snugging, thensubsequently loosen when adjacent bolts were snugged. Since the dti isinelastic, it will not rebound to increase the gap when the preload isreleased. Therefore, a bolt "over-tightened" during snugging, thensubsequently loosened, will still appear to be properly tensioned bythe having an adequate number of refusals for the post-installationcheck.

The snugging check of the dti should be performed by the boltinstaller, not the inspector, as this is a step in the installationprocess. The inspector should verify the snug condition of the jointvisually, and also observe that the installer conducts the snug checkwith a feeler gauge.

After the entire joint has been deemed snug by the inspector, withoutpre-compressed dti's, the installation crew can then proceed tofurther tighten the bolts until at least half the dti bumps refuseentry of the feeler gauge. The crew may use a trial and error system,a calibrated wrench method or a visual judgement to establish when tostop impacting. The feeler gauge is then reinserted into the dti gaps.If the feeler gauge is refused in at least half the gaps, the bolt is

29-23


assumed to be properly tensioned. The inspector should observe thatthe feeler gauge is used consistently and properly by the installationcrew, and may wish to spot check dti's at random using a feeler gauge.

AASHTO has recently added a provision that no torque testing of thecompleted joint is required if the dti method is used.

The Twist-Off Bolt Installation Method

The twist-off bolt is a speciallydesigned bolt that has a splineat the end that is used by theinstallation tool. The spline isdesigned to shear off from thetorque generated by the wrench.This torque is the result ofwrench efforts to turn the nut inthe clockwise (tightening)direction, as well as counter-acting efforts to turn the boltshank in the counter-clockwisedirection. When the twist-offbolt's head grabs the steel, thebolt shank itself will not turn,and all rotation takes place withthe nut.

The spline has been designed so that it will not shear off until thebolt is above the required tension, but will not be too strong tocause over-tightening of the bolt beyond reasonable limits. When thespline shears off, the wrench no longer functions.

The twist-off bolt is completely dependent upon the torque tensionrelationship, which can vary greatly depending upon the quality andtype of lubrication of the assembly. The manufacturers of twist-offbolts generally produce a very consistent and durable lubricant thatresists water, mild solvents and rust for some time.

Because of the interdependence of the bolt, nut and washer upon thetorque used for installation, the twist-off bolt unit is pre-assembledby the manufacturer. Substitutions of other nuts or washers mayadversely affect performance and cause bolt tensions to be too high ortoo low.

Prior to installation, the twist-off bolts must be tested at the sitefor performance. This is as required by section 8(d)(3) of the BoltSpec. Three samples of each diameter, length and grade of twist-offbolt must be tested in a bolt calibration device (Skidmore) prior tobeing installed in the steel. When the installation tool snaps off thespline, the resultant tension shown in the Skidmore must be at least5% higher than the required tension for that bolt. Although the BoltSpec does not call out specifically for testing by lot, it issuggested that this testing be done by production lot as received fromthe manufacturer or distributor.

29-24


For cases where the twist-off bolt is too short to fit into aSkidmore, then pre-installation testing must be done using calibrateddti's as described earlier. The head of the bolt must be restrainedfrom rotation, as the dti bumps under the bolt head will providelittle frictional resistance to force the bolt to stop turning andforce the wrench to turn the nut. If the bolt is allowed to turn, thedti bumps will become abraded and the measurement of the dti bumpswill be inaccurate. Secondly, the spline will typically shear offsooner if the nut is not rotating and the bolt shank is rotating, notrepresentative of the conditions of installation typically found onthe steel.

When pre-installation testing is complete, the bolts must be installedand the joints snugged using a systematic method, as with allinstallation procedures previously discussed. Care must be used tomake sure that the splines are not twisted off during the snuggingoperation. Any bolts that twist off during snugging must be replaced.In some cases, deep sockets are used on conventional impact wrenchesto snug the joints, therefore protecting the splines. Upon completionof snugging, the inspector should verify the snug condition visuallyto make sure no improper gaps exist. The inspector should also verifythe presence of all splines. Upon acceptance of the snugged joint bythe inspector, the installation crew can proceed to tighten the twist-off bolts with the installation wrench until the splines shear off.Again, a systematic pattern should be used for this step. Theinspector should observe that the crews used the installation wrenchproperly and that all splines have been sheared off by the wrench.AASHTO has recently added that no torque testing of the completedjoint is required if twist-off bolts are used.

Inspection Procedures

Up until this point, no mention has been made of any torque testing ofany completed joints, except to say when torque testing is notrequired by AASHTO. Yet, each method described above has contained theentire inspection procedure from start to finish. Under the provisionsof the RCSC Bolt Specification, no torque testing of any completedjoint is required unless a dispute arises.

All the inspection procedures above carry a common focus:1. Check the materials and certifications.2. Check the assembly in a bolt calibration device for material

quality.3. Check the validity of the installation technique for that

group of fasteners.4. Check the knowledge of the installation crew in applying the

proper technique.5. Visually check after snugging to verify that the snug

condition has been achieved. For dti's, also verify thatthe installers check for an adequate number of remaininggaps in their dti's. For twist-off bolts, check that thesplines are intact.

6. Observe the installation crew for proper technique. This doesnot mean that the installation of each individual bolt isobserved, but to verify that the crew understands andfollows the proper techniques (and the crew understandsthat the inspector is around to make sure they followthem.)

29-25


Torque testing is applicable only when there is a dispute, per section9(b) of the Bolt Spec. The methodology listed there is not intended asan inspection method, only as a last-ditch effort to resolve claimsthat the crew may not have followed the proper techniques for aparticular joint. The Spec clearly states that this section is to beused only when inspection per 9(a) has been performed - the visualobservation of the pre-installation testing, checking for snug, andobservation of the installation technique of the crew.

For AASHTO and FHWA bridge work, torque testing is still required for10% of the bolts in each connection, minimum two per connection, forbolts installed using turn-of-the-nut or calibrated wrench methods. Itis hoped that these torque testing provisions will be removed fromAASHTO shortly, at least for the turn-of-the-nut method when match-marking is employed. The torque-testing procedures of AASHTO aresimilar to the Bolt Spec section 9(b), except that only three boltsare used to determine the inspection torque, not five.

For additional consideration of the value of torque testing, Bolt Specsection 9(c) places severe limitations on its application andvalidity. The procedure is to be used "at the time of tensioning thejoint," not the following week, month or at the completion of theproject. AASHTO states that torque testing must be done within 24hours after completion of pretensioning of the bolts to be tested.Torque values have also been known to vary as much as 30% when tryingto relate test torque to actual installed tension, meaning that allthe torque testing, performed at great expense, has provided verylimited information on actual bolt tension.

Summary

For most buildings, all joints will be shear/bearing joints that needto be installed only to the snug-tight condition. For these joints,the inspector should verify the quality of materials and verify thatthe joints have been properly snugged. No flame-cutting of holes,slotting of holes or unfair reaming should be allowed withoutinspector presence and engineer notification. No pre-installationtesting or installation observation is required. No torque testing isrequired. Inspection expenses are minimal when this type of joint isspecified.

For shear/bearing joints that must be pretensioned, materials must bechecked, the pre-installation verification of materials and proceduremust be performed, the snug condition must be verified, and theobservation of technique must be conducted. In previous versions ofthe Bolt Spec Commentary, it was suggested that, since the actualpretension of these joints was not critical to performance, adequateinspection would be to look for peening of the turned element. Thiswould indicate that it had received impact wrench tightening to somedegree. Until this statement returns to the Spec, however, it would beinappropriate to follow such guidelines.

For slip-critical joints, inspection of materials, observation of thepre-installation testing, checking for the snug condition andobservation of the installation technique is necessary. In addition,inspection of the quality of the faying surface is also required.

29-26


For direct tension joints, the same inspection as required for slip-critical joints should be performed, except there is no inspection ofthe faying surfaces.

The proper inspection of bolted joints starts with the structuralengineer. The engineer must first select the type of joint(s) found inthe structure, then convey this information to the fabricator, erectorand inspector. The project specifications should include a detailedlist of the operations to be performed by the inspector, includingtiming of such inspections, for each type of joint.

The engineer should not accept torque testing as a substitute for theobservation testing as prescribed by the Bolt Spec. To meet any of thethree model building codes, the Bolt Spec must be followed, andtherefore the observation techniques as prescribed are paramount.

29-27


Documents

AISC - Inspection of Welded and Bolted Joints