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F ROM THEFO U N D A T I O N

Revisions Due inSpecification for Cold-Formed

Steel Structural Members

Will Affect Lightweight Steel Framing Usage

Introduction: This article, the eighth in

the Foundations series on lightweight steel

framing systems, was prepared by the Steel

Products News Bureau (420 Lexington

Avenue, New York, New York) based on

information gathered from the American

Iron and Steel Institute.

Structural engineers, architects,specifiers and contractors who

employ the studs and joists of light-weight steel framing in buildingconstruction or renovation should beaware of upcoming revisions in thespecifications.

Changes have been proposed in thecurrent (1986) edition of the AmericanIron and Steel Institute (AISI) Specifica-tion for the Design of Cold-Formed SteelStructural Members. This Specificationcovers framing members cold formed

from carbon or low-alloy steel sheet,strip and plate, a process performed onpress-brake or roll-forming machines.When adopted by all major buildingcodes, the Specifications revisions willlead to improvements in designing andspecifying lightweight steel framingmembers.

A growing segment of structuralsteel usage, lightweight steel framing(hereafter called LWSF) is used forinterior partitions, floor joists, andcurtain walls that carry the exteriorfinish of the building. Steel stud load-

bearing walls have also been employedin buildings from a single floor to fivestories high. Among the advantagesresponsible for LWSFs popularity arethese: close dimensional tolerances; highstrength-to-weight ratio; durability;

non-combustibility; speed of erection;and panelized shipment of completewall systems.

Proposed RevisionsBecause of LWSFs increasing

importance, those who formulate theAISI Specification are determined tokeep it up to date technologically andresponsive to the needs of LWSF users.

...proposed revisions reflect

the findings of continuing

research on the behavior of cold-formed steels.

Accordingly, proposed revisions reflectthe findings of continuing research onthe behavior of cold-formed steels. Theyalso seek to simplify the Specificationsorganization and content. The followingare a few of the recommended revi-sions.

1. Shaped Web Openings. LWSF ismanufactured with regularly-spacedholes in the webs of studs, joists and

rafters. These serve as raceways forelectrical wiring and plumbing lines aswell as for snap-in bracing members.Electricians and plumbers dont have todrill or cut openings, thereby minimiz-ing construction time, cost and the

possibility of damage to the LWSFmembers.

Previous versions of the Specificationdealt only with circular holes whereas,increasingly, holes of a variety ofconfigurations are being usedovals,keystones, rectangles, etc. Manufactur-ers of the cold-formed steel framinghave already conducted extensive teststo determine sectional strengths ofmembers with such web holes.

Now, the Specification must arrive ata formula to accurately calculate theeffective cross-sectional area and, thus,the load-carrying capacity of members

with these varied openings. Codeauthorities have been insistent on thispoint. A task force has been formed toaccomplish it, with load testing andanalyses already done. The work isexpected to be finalized in a futurerevision to the Specification.

2. Through-Fastened Flanges. Forbeams having the tension flangeattached to deck or sheathing and thecompression flange unbraceda roofpurlin or wall girt subjected to windsuctionthe bending capacity is lessthan a fully-braced member, but greater

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than an unbraced member. This partialrestraint is a function of the rotationalstiffness provided by the panel-to-purlinconnection, primarily a function of theLWSF member thickness, sheet thick-ness, fastener type and fastener location.To ensure adequate rotational stiffnessof the roof and wall systems, theSpecification revision provides aformula to calculate strength, instead ofhaving to run costly tests. It alsoexplicitly states the acceptable panel andfastener types.

3. Applicable Steels. The Specifica-tion requires the use of steel of struc-tural quality, such as those defined byspecifications of the American Societyfor Testing and Materials (ASTM).Although 14 such steels are listed, thethree most used for LWSF members are:A446/A446M (Grades A, B, C, D and F),zinc-coated (galvanized) by the hot-dipprocess; A570/A570M, a hot-rolledcarbon steel; and A792, aluminum-zincalloy-coated by the hot-dip process.Others include high strength/low alloy

steels and steels with improved forma-bility or improved atmospheric corro-sion resistance.

4. Diaphragm Construction. Steeldeck, when properly attached to astructural frame, becomes a diaphragmcapable of resisting in-plane shearforces. The Specification revisions willpermit the in-plane structural perform-ance (load-carrying capacity) of floor,roof or wall steel diaphragm construc-tion to be established by a givencalculation formula or by test.

The factors of safety required arebased on statistical studies from full-

scale tests. For example, the studiesconcluded that the quality of mechanicalconnectors is easier to control thanwelded connections, and the variation inthe strength of mechanical connectors issmaller than for welded corrections.Thus, their performance is morepredictable and a smaller factor of safetyis justified. The factors of safety forearthquake loading are slightly largerthan those for wind, due to seismicductility demands.

5. Wall Stud Strength. With therevisions, the safe load-carryingcapacity of a stud may be computed on

the shell alone (without sheathing) or onthe basis that sheathing (attached to oneor both sides of the stud) furnishesadequate lateral and rotational supportto the stud in the plane of the wall.

Both ends of the stud must be bracedto restrain rotation about the longitudi-nal stud axis and horizontal displace-ment perpendicular to the stud axis. Thesheathing shall be connected to the topand bottom members of the wallassembly to enhance the restraintprovided to the stud and stabilize theoverall assembly. When sheathing is

utilized for stability of the wall studs, itshall retain adequate strength andstiffness for the expected service life ofthe wall. Additional bracing shall beprovided, as required, for adequatestructural integrity during constructionand in the completed structure.

6. Lateral Buckling Strength. Deter-mining the nominal moment strength oflaterally-unbraced segments of flexuralmembers has been restructured.

Application of the equations for singly-,doubly- and point-symmetric sectionshas been clarified.

7. Bibliography. In its bibliography,the commentary on the Specification

adds a total of 15 technical articles on thesubject, ranging from The StructuralPerformance of Cold-Formed SteelMembers with Perforated Elements andSteel Deck Institute Diaphragm DesignManual, Second Edition to Develop-ments and Future Needs in WeldingCold-Formed Steel and Behavior of C-and Z-Purlins Under Uplift.

Research on cold-formed steelconstruction started under sponsorship

of the AISI at Cornell University in 1939.In 1946, the first edition of the Specifica-tion was published. It has been revisedsix times, with the current version dated1986.

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The Revision ProcessAISIs Advisory Group on the

Specification proposes revisions,develops them on a consensus basis,and brings out a new Specificationabout every six years. The AdvisoryGroup is comprised of consultingengineers, researchers, specialists fromthe steel industry, and designers fromcompanies manufacturing cold-formedsteel members, components and

assemblies. It works through anextensive group of subcommittees suchas those on corrections, channel and Z-sections, compression members,diaphragms, flexural members, materi-als, stud design, test procedures, andwelding.

before publishing a new Specifica-tion, revisions are disseminated togroups directly involved with cold-formed steelarchitects, engineers,LWSF fabricators, building codeorganizations, and othersto get theirreactions to proposed changes. Therevisions described earlier in this article

should be finalized by year-end tobecome part of the new Specificationissued by AISIs Committee on Con-struction Codes and Standards.

After that, it is presented for adop-tion by the major code bodies-BuildingOfficials and Code AdministratorsInternational (BOCA), and the Southern

Building Code Congress International(SBCCI). When the Specificationbecomes part of these codes, it is thenincorporated into law upon adoption bycities, counties and states as part of theirbuilding codes.

Future RevisionsAs mentioned previously, many past

revisions in the Specification derivedfrom research on cold-formed struc-

tures. Future revisions may well resultfrom some of these ongoing researchprojects: strut purlins, bracing of purlinsfor standing seam roofs, web crippling,effects of web holes in complete wallassemblies, web penetrations in beams,X-bracing against wind load, anddetermination of stability in C- and Z-sections.

A Technical Center for Cold-FormedSteel Structures was established at theUniversity of Missouri-Rolla to provideacademic and industrial organizationswith an integrated approach for han-dling design standards, engineeringeducation, technical services, andresearch For further information on theCenter, contact Dr. Wei-Wen Yu,Department of Civil Engineering,University of Missouri-Rolla, Rolla, MO

65401-0249.To obtain a copy of the upcoming

new Specification, including revisionsfrom the current edition, contact:American Iron and Steel Institute, 113315th St. NW, Washington, DC 20005.

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