8/12/2019 Bolt Spec Review.pdf

1/2October 2004 Modern Steel Construction

The Research Council on Struc-tural Connections (RCSC)recently voted to include sev-eral changes in the Specification

for Structural Joints Using ASTMA325 or A490 Bolts (also known as thebolt spec). RCSC will issue a reviseddocument dated June 30, 2004, and AISCplans to incorporate the revised specifi-cation in the next Manual of Steel Con-struction when it is published in 2005.While the amendments will not createmajor changes to bolted connections,they will clarify a few key points. Onechange simply emphasizes a currentrequirement, and several others provideoptions to fabricators and erectors when5/16 ASTM F436 washers, or hardenedplates/bars are not readily available.

Slip Critical ConnectionClarification

RCSC felt it was necessary to clarifythe requirement that slip critical connec-tions be designed to meet the strengthrequirements as shear/bearing joints(Sections 5.1, 5.2 and 5.3). These require-ments are in addition to meeting the slipresistance requirements in Section 5.4.The requirement is not a new one, but itseemed to require additional emphasis.The shear/bearing limits are checked inslip critical joints because the joints mayslip at or near the factored load. In somecases with class B or C faying surfaces,

the shear/bearing strength may be lessthan the slip-critical strength. Underthese conditions, the connection must bedesigned for the shear/bearing strength.Meeting these requirements provides thestrength limit reliability for slip critical

joints.

Revised Washer Requirements inTable 6.1

For many years, the bolt spec hasrequired washers that are 5/16-thick andhardened in accordance with ASTM F436where pretensioned ASTM A490 bolts

greater than 1 diameter are used inoversized holes or slots in the outer ply.Similarly, bolted connections with longslots in the outer ply required 5/16-thickhardened plate washers or hardened con-tinuous bars. These requirements were inplace because research showed normalwashers deformed under the high loadsgenerated by tightening ASTM A490

bolts. This, in turn, made achieving therequisite pretension difficult. The foot-notes to the table clearly prohibited theuse of multiple thicknesses of standardthickness ASTM F436 washers.

While the provision has been in placefor many years, conditions demandingthem are not frequent. Therefore, thehardened, thick washers and hardenedplates/bars have not always been readilyavailable. Recognizing the difficulty ofobtaining these washers, the RCSC hasadded alternatives to 5/16 hardened

washers, hardened plate washers andhardened bars. In connections with long-slotted holes in the outer ply and usingASTM A490 bolts, the revised bolt specwill permit an ASTM F436 washer of

standard thickness with a 3/8 structuralgrade plate washer. It will also permit anASTM F436 washer with a structuralgrade 3/8-thick continuous bar. Connec-tions with oversized or short-slottedholes in the outer ply using ASTM A490

bolts can now use either ASTM F436hardened washers 5/16-thick, as before,or a 3/8-thick plate washer and an ordi-nary thickness ASTM F436 washer. Aform of the revised table appears below.

Where ASTM A490 bolts are preten-sioned in connections and the connectedmaterial has yield strength less than 40ksi (usually ASTM A36), the existing boltspec requires washers under the headand nut to prevent galling of the con-nected material. The requirement for thewasher under the head is waived whentwist-off bolts with bearing diametersunder the head equal to or greater thanthe diameter of an ASTM F436 washer

are used. Recent research indicates thatthis exception is also valid for all twist-off bolts with a bearing circle diametermeeting the requirements of ASTMF1852. (Schnupp, K.O., and T.M. Murray,

Bolt Spec Previewby Tom Schlafly

RCSCfocus on

Changes in the next edition of RCSCs bolting specification clarify some finer points of washer usage.

ASTM DesignationNominal BoltDiameter, d

b,

in.

Hole Type in Outer Ply

Oversized Short-Slotted Long-Slotted

A325 or F1852 1

ASTM F436a

5/16-thick plate

washer or con-tinuous barb, c

A490

1

> 1 ASTM F436 with 5/16 thicknessb, d

ASTM F436washer with

either a 3/8-thickstructural gradeplate washer orcontinuous barb

a This requirement shall not apply to heads of round head tension-control bolt assemblies meeting the

requirements in Section 2.7 and providing a bearing circle diameter meeting the requirements of ASTM

F1852.b Multiple washers with a combined thickness of 5/16 or larger do not satisfy this requirement.c The plate washer or bar shall be of structural-grade steel material but need not be hardened.d Alternatively, a 3/8-thick plate washer and an ordinary thickness F436 washer may be used.The plate

washer need not be hardened.

Table 6.1 Washer Requirements for Bolted Joints with

Oversized and Slotted Holes in the Outer Ply

8/12/2019 Bolt Spec Review.pdf

2/2

Effects of Head Size on the Performanceof Twist-Off Bolts, Virginia PolytechnicInstitute and State University, CC/VTI-ST 03/09, July 2003.) In other words,twist-off bolts with the smaller headdiameter may be used without a washerunder the head.

Further changes are under considera-

tion by RCSC, particularly with regard toslip critical design strength, the use ofbolts with welds, the treatment of thickfills in slip-critical connections and thenominal strength of a bolt in shear withthreads included in the shear plane.RCSCs current revisions to the bolt spec,however, will not change the number orsize of bolts required in a connection if itwas designed properly using the provi-sions of the 2000 edition. Nevertheless,the exception to the washer requirementsmay make life a little easier for fabrica-tors and erectors.

Tom Schlafly is director of research for theAmerican Institute of Steel Construction,Inc., and is also a member of the RCSC.

October 2004 Modern Steel Construction