aisc 1999

The American Instituteof Steel Construction(AISC) will welcomethe new millenniumwith the introduction

of the third edition of the Load and Resistance Factor Design Spec-ification for Structural Steel Build-ings (LRFD Specification) (1) tosupersede the 1993 LRFD Specifi-cation (2). The AISC Specificationhas come a long way since the firstone was introduced in 1923 with atotal length of nine pages. Origi-nally developed to create stan-dards of practice in the steelindustry, subsequent revisions ofthe specification have reflectedboth advances in research andchanges in design practice. Thecurrent state-of-the-art designmethod for steel buildings is Loadand Resistance Factor Design(LRFD), which was introduced byAISC in its 1986 Specification (3).The 1999 LRFD Specification willcontain several updated provisionsand some new ones as well. Aslight change in the overall formatwill also be apparent to regular

users. Some of the revisions dis-cussed were issued by AISC in theJanuary 30, 1998, Supplement No.1 (4).

The purpose of this paper is tohighlight the major changes andnew provisions of this 1999 AISCSpecification.

What’s New and DifferentAmong the new topics included

in the 1999 LRFD Specification arestability bracing and evaluation ofexisting structures (Chapter N).These stability bracing require-ments can be used in lieu of a sec-ond order analysis that takes intoaccount initial out-of-plumbness ofthe structure or out-of-straightnessof the member. The criteria pre-sented are based primarily on workin the late 50’s by George Winter atCornell University and more recentstudies by Joseph Yura at the Uni-versity of Texas. This section givesequations for minimum bracestrength and stiffness for frames,columns, and beams, necessary toensure member design strengthsare attained. The formulations are

Modern Steel Construction / May 2001

Cynthia J. Lanz

Cynthia J. Lanz

is Director of Specifications

at the American

Institute of Steel Construction

(AISC) in Chicago.

a function of the unbraced length,assuming K equal to one. In addi-tion to strength and stiffness equa-tions, other issues are addressedsuch as brace spacing and attach-ment. Two general types of bracingsystems are considered, relativeand nodal. Relative bracing is ap-plied relative to another bracing lo-cation, i.e. diagonal bracing orshear walls. Nodal bracing controlsdisplacement only at the particularbrace point, i.e. cross bracing be-tween two adjacent beams (see Fig-ure 1, which is Figure C-C3.1 fromthe Commentary (1)).

Another topic never before ad-dressed in the AISC Specification isthe evaluation and repair of exist-ing structures. This is in responseto the aging infrastructure and thedesire of owners to renovate in-stead of demolish. The new Chap-ter N presents procedures forevaluating the strength and stiff-ness of existing buildings by struc-tural analysis, gravity load testing,or a combination of the two, as re-quired by the Engineer of Recordor the contract documents. TheChapter is applicable only to struc-tures under static loading. Guide-lines for material propertiestesting, such as tensile properties,chemical composition, base metalnotch toughness, and filler metalare also included. The final sectionoutlines the format for an evalua-tion report.

Aside from these two com-pletely new design issues, the 1999LRFD Specification will includesome reorganized sections to pres-ent the information in a more con-cise manner as well as somerevisions to existing design provi-sions. Section D3, Pin-ConnectedMembers and Eyebars, and SectionE4, Built-Up Members, are nowpresented in outline form with de-tailing requirements listed sepa-rately from design strengthprovisions.

Two new structural steels arenow approved for use in the Speci-fication, ASTM A913 and A992. The

former is High-Strength Low-AlloySteel Shapes of Structural Quality,Produced by Quenching and Self-Tempering Process (QST). This ma-terial provides a specified CharpyV-notch value of 40 ft-lb. at 70° F.The ASTM A992 designation, enti-tled Steel for Structural Shapes forUse in Building Framing, wasgiven to the shape material de-scribed in AISC’s Technical Bulletin3 (A572 Gr. 50 with special require-ments). The A992 Grade 50 pro-vides for a better-defined structuralsteel through a maximum yieldstrength limit of 65 ksi, maximumyield to ultimate strength ratio of0.85 and carbon equivalent criteriafor weldability.

Chapter I, Composite Members,contains new criteria pertaining tothe application of shear connectorsin concrete-encased steel columnsand beams. AISC Steel DesignGuide Series 6, “Load and Resis-tance Factor Design of W-ShapesEncased in Concrete”, indicatesthat shear connectors are requiredto provide load transfer between asteel column and the surroundingconcrete. In response to this, the1999 LRFD Specification givesequations for the required force to

be transferred by the shear connec-tors for the case where the externalforce is applied directly to the steel,or directly to the concrete. An im-portant revision to note on this sub-ject is the increase in the resistancefactor for bearing, φB, from 0.60 to0.65. This is for consistency withAppendix C of the latest versionsof ACI 318 and ACI 318M. The de-sign strength of concrete-encasedbeams where shear connectors areprovided is given in a new para-graph where the design flexuralstrength is based on the plasticstress distribution of the compositesection.

Additional modifications in-cluded in Chapter I pertain to gen-eral composite column design andcomposite slabs when a singleshear connector is placed in a deckrib oriented perpendicular to thesteel beam. In this case, the newSpecification places an upper limiton the reduction factor on shearconnector strength to 0.75, insteadof 1.0. This is in response to recentand ongoing research that indicatesthe previous method of calculatingthe strength for a shear connector isunrealistic when there is a singleconnector in a rib. Because com-posite beam strength has a nonlin-ear dependency on shear forcetransfer, the flexural member ca-pacity will be much less affectedfor the higher ranges of compositeaction. In calculations for compos-ite column design strength, thespecified minimum yield stress ofthe steel components is now lim-ited to 60 ksi (formerly 55 ksi).

Some of the important new pro-visions appearing in Section J2,Welds, of the 1999 LRFD Specifica-tion are a new length reduction fac-tor for fillet welds, updated detailsfor fillet weld terminations, andfiller metal requirements. The newlength reduction factor applies to“end-loaded” fillet welds. The newterm “end-loaded” refers to longi-tudinal fillet welds parallel to theapplied load and designed to trans-mit load to the end of an axially

May 2001 / Modern Steel Construction

loaded member, i.e. longitudinallywelded lap joints at the end of axi-ally loaded members or welds at-taching bearing stiffeners. A lengthreduction factor, β, is applied toweld lengths greater than 100 timesthe weld size. The β equation isconsistent with the Eurocode 3standard, which is based on finiteelement studies and research per-formed on this subject over manyyears. The other revision appearingin Section J2 deals with fillet weldterminations. Welding detail char-acteristics are more clearly statedfor lap joints, connections and ele-ments undergoing cyclic forces,connections requiring flexibility ofoutstanding legs, fillet welds join-ing transverse stiffeners to plategirders, and fillet welds occurringon opposite sides of a commonplane. An expanded commentaryalso includes visual aids for furtherexplanation. Finally, filler metal re-quirements are given for certaincomplete-joint-penetration groovewelded T and corner joints andsplices of heavy shapes with ten-sion normal to the effective area. Inthe special cases given in new Sec-tion J2.6, the filler metal requires aspecified Charpy V-notch tough-ness of 20 ft-lbs at 40°F. Alterna-tively, the lower design strengthcondition for partial-joint-penetra-tion groove welds governs.

Most of the revisions to the boltprovisions of the new Section J3,Bolts and Threaded Parts, are con-sistent with the provisions in theResearch Council on StructuralConnections’ Load and ResistanceFactor Design Specification forStructural Joints Using ASTM A325or A490 Bolts (RCSC Specification)(5). Revisions include the followingtopics: snug-tight bolting, com-bined tension and shear of bearing-type connections, design ofslip-critical connections, and bear-ing strength. Previously limited toonly statically loaded shear con-nections, snug tightening is nowpermitted for ASTM A325 bolts in

tension or combined shear and ten-sion static applications. This re-flects recent research thatdemonstrated the ultimate strengthof ASTM A325 bolts under theseconditions is not affected by thelevel of pretension present. Forbearing-type connections loaded incombined tension and shear, thecoefficients in the design equationshave been modified slightly. Itshould be noted that the RCSCSpecification uses an elliptical solu-tion, while AISC has both astraight-line approximation of thisin the main Specification and theelliptical solution in an Appendix.Thus, the designer can select whichapproach to use. The designstrengths for slip resistance in slip-critical connections at factoredloads has been more convenientlyrelocated to the main body of theSpecification, exchanging placeswith the provisions for design atservice loads which are now in theAppendix. The mean slip coeffi-cient, µ, for Class C (galvanized)faying surfaces in slip-critical con-

nections has also been reduced to0.35. For slip-critical connectionssubject to combined tension andshear, the multiplier that accountsfor the effect of the tensile compo-nent on slip resistance has beenslightly modified to be consistentwith the RCSC Specification. Asubject affecting both bearing-typeand slip-critical connections isbearing strength. This section hasbeen updated based on recent re-search and the basis of the bearingcalculation has been modified tothe clear distance to an edge or be-tween fasteners, rather than on cen-ter-to-edge and center-to-centerdistances. The resulting designstrengths are largely unaffected bythis modification, except for verysmall edge distances and spacings.

In the 1999 LRFD Specification,Appendix K3, Design for CyclicLoading, replaces the old Appen-dix K3, Fatigue. The past method offatigue design relied upon multipletables consisting of cycles of load-ing, stress categories, design stressranges, and illustrative examples.

This has been replaced by a newformat where one table presentsthe situation description, the stresscategory, variables for the applica-ble equation, the potential crackinitiation point and pertinent illus-trative examples for each situation(see Figure 2 which is an excerptfrom Table A-K3.1 of the Specifica-tion (1)). One new detail includedis tension loaded plate elementsconnected at their end by trans-verse groove or fillet welds. Newcriteria are also included for fatigueresistance of non-pretensionedbolts subject to applied tension,such as used in hanger rods or an-chor rods. A similar format andconsistent criteria is being devel-oped for the AWS code.

One visible change throughoutthis version of the AISC LRFDSpecification is the conversion todual units format, U.S. Customaryunits and SI (metric) units. AISC in-troduced a metric conversion of the1993 LRFD Specification in 1994.The 1999 LRFD Specification willcombine the two by providing met-ric equivalents in parentheses fol-lowing the U.S. Customary unitvalue. The metric conversions arebased on ASTM E380, StandardPractice for Use of the InternationalSystem of Units (SI). Equations arealso presented in dual format.Where possible, they are non-di-mensionalized by factoring out ma-terial constants, such as E and G;otherwise the metric version islisted separately. This is in responseto an ongoing domestic movementto use the metric system of meas-urement, largely led by govern-ment agencies, as well as broaderinterest in relating and participat-ing in the international arena,which is almost exclusively basedon the SI system.

This is only a sampling of themajor changes you will find in theAISC 1999 LRFD Specification. Thereader is referred to the printeddocument for the exact and com-plete design requirements and

commentary. Work is underway onthe 3rd Edition Load and Resis-tance Factor Design Manual ofSteel Construction, which will beintroduced in late 2001.

References

1. Load and Resistance Factor De-sign Specification for StructuralSteel Buildings, AISC, Chicago,IL, 1999.



4. Supplement No. 1 to the Loadand Resistance Factor DesignSpecification for Structural SteelBuildings, AISC, Chicago, IL,1998.

5. Load and Resistance Factor De-sign Specification for StructuralJoints Using ASTM A325 orA490 Bolts, Research Council onStructural Connections, AISC,Chicago, IL, 2000.

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