Antiquated Structural Systems Series, Part 6

7/24/2019 Antiquated Structural Systems Series, Part 6

http://slidepdf.com/reader/full/antiquated-structural-systems-series-part-6 1/4STRUCTURE magazine November 2008

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For this series of articles, “antiquat-ed” has been dened as meaningoutmoded or discarded for reasons

of age. In reality, however, most if not allof the systems that have been and willbe discussed are no longer in use simplybecause they have been replaced by moreinnovative or more economical methodsof construction.Most of the antiquated systems discussed

so far have been out of popular use for aconsiderable number of years, with somedating back to the rst part of the last cen-tury. However, the subject of this articledeals with a system that was still in use lessthan 20 years ago.The purpose of this series is to compile

and disseminate a resource of informationto structural engineers for projects thatinvolve the repair, restoration, or adaptive reuse of olderbuildings for which no drawings exist. It is hoped that this

will enable structural engineers to share their knowledge ofexisting structural systems that may no longer be in use, butare capable of being adapted or reanalyzed for safe reuse inthe marketplace of today and the future.

53

Antiquated Structural Systems SeriesStructural Steel Composite Stub-Girder Construction – Part 6 By D. Matthew Stuart, P.E., S.E., F. ASCE, SECB

system went onto be used in alarge number of

high-rise buildings in North Americaup through the 1980s. However, thesystem was eventually abandoned becauseof increased labor cost associated with

both fabrication and the need for shoringuntil the eld-cast concrete slab attainedsufcient strength. Advantages of the stub-girder systemthat led to its use during the time periodin which it was popular included:

1) Reduction in steel tonnage by asmuch as 25% over conventionalcomposite oor framing due to:a) Improved structural efciency

as a result of the greater depthof the stub-girder compared toa conventional system; and

b) Improved structural efciencydue to the ability of transverseoor framing members to actas continuous beams throughopenings between stubs.

2) Reduction of overall depth of thestructural oor framing system byas much as 6 to 10 inches over aconventionally framed compositeoor system, which allowed for areduced oor-to-oor height andoverall height of the building andassociated cladding.

TestingPrior to use of the stub-girder system,

a load test was performed at GrancoSteel Products Company in St. Louis.The test specimen included a W14x48continuous bottom beam, W16x26 stubsand oor beams, and a 5-foot-wide, 3¼-inch-deep, lightweight concrete slab overa 2-inch metal deck ange, which wasattached to stubs via shear connectors(Figure 2, page 54 ).The test specimen was loaded beyond

calculated design load, with initialfailure occurring at the exterior endof the outermost stub at one end of

the stub-girder. The method of failureincluded web crippling and delamina-tion of the web from the ange. Ap-plication of additional load resulted incrushing of the slab at the inside edgeof the same stub. However, separationbetween the bottom of the slab and thetop of the stubs did not occur, whichindicated that composite behavior wasmaintained up to the point of local-ized crushing of the concrete slab.

Web stiffeners added to the failed stuballowed the system to achieve a nal

failure load that was 2.2 times greaterthan the calculated design load.The methods of design used to deter-

mine capacity of the section includeda non-prismatic beam analysis, aVierendeel girder/truss analysis, and anite element analysis. For the Vierendeelanalysis, stubs and transverse oor beamsact as verticals and the concrete slab andcontinuous beam act as chords. SeeFigure 3 (page 55) for a comparison of atypical Vierendeel truss and stub-girdercomponents. All three of these methods

The Stub-GirderComposite System

A stub-girder is a composite system

constructed with a continuous structuralsteel beam and a reinforced concrete slabseparated by a series of short, typically

wide, ange sections called stubs. Stubsare welded to the top of a continuousbeam and attached to the concrete slabby shear connectors. Spaces betweenends of stubs are used for installationof mechanical ducts and other utilitysystems and for placement of transverseoor beams that span between stub-girders. Ideally, the depth of stubs andoor beams are identical to allow for

transverse framing to support a concreteslab deck, which spans parallel to thestub-girder, and to facilitate compositeaction between the oor beam and slab(Figure 1).Stub-girder construction was rst used

in 1971 at the 34-story One Allen Cen-ter ofce building in Houston, Texas.The system was developed by Joseph P.Colaco, Ellisor Engineers, Inc., to fa-cilitate integration of mechanical ductsinto steel oor framing of repetitive,multistory high-rise construction. This

Figure 1: FloorFraming Systems.Courtesy AISC.

CORE

w BEAMS1

FLOOR PLAN

± 10’ - 0”

± 4 ’ - 0 ”

DUCT

CEILINGSECTION 1

CONVENTIONAL FLOOR FRAMING SYSTEM(a)

II

I I

I

CORE

2

FLOOR PLAN

I II

I ISECTION 2

STUB GIRDER SYSTEM(b)

± 10’ - 0”

± 3 ’ - 6 ”

CEILING

DUCT

W14 STUB GIRDER



of analysis provided a close representation ofactual behavior of the stub-girder; however,the Vierendeel and nite element methodsmore closely identied secondary momenteffects on each side of the openings. TheVierendeel method of analysis also provideda more accurate representation of actualsteel stress, while the nite element methodprovided a more accurate representation ofstress in the concrete slab, including highstresses that resulted in crushing of concreteat the inside edge of the rst exterior stub asobserved in the test specimen. Additional tests of stub-girders were per-formed in the late 1970s in Canada. Theprimary purpose of these tests was to deter-mine effects of changes in spacing and depthof stubs, and to establish failure modes of astub-girder. Results conrmed that behaviorof a stub-girder was similar to a Vierendeelgirder/truss. Supplementary conclusions ofthese tests included:

1) The stiffness of the girder increases asthe length of the open panel betweenthe stubs decreases.

2) Shear distortions at open panels (as aresult of the Vierendeel action) were animportant parameter in determiningelastic deection of the stub-girder, butdid not inuence rotation of solid endsections of the overall girder.

3) Tensile cracking of the concrete slabat the ends of open panels occurred atrelatively low loads, but did not have asignicant impact on elastic stiffnessof the girder.

4) Further extensive cracking of theconcrete slab at the ends of openpanels occurred in the inelastic rangeof the girder. It was further determinedthat ultimate strength and ductility ofthe girder could be improved throughuse of internal reinforcement withinthe slab that was placed to resist theobserved cracking.

5) Precision of the Vierendeel methodof analysis was dependent on accuracyof distribution of shear forces betweenthe concrete slab and the continuouslower beam across open panels andassumptions made relative to locationof points of contraexure withinopen panels.

6) Failure of shear connectors resultedas a combination of shearing andprying effects.

7) To prevent premature failure dueto web crippling at stubs, stiffenersshould be provided.

8) Five different failure mechanisms wereidentied: buckling of the stub web,concrete failure in the vicinity of shearconnectors, diagonal tension failureof the concrete slab, shearing off ofheaded stud connectors, and combinedyielding of the steel beam and crushingof the concrete slab at the ends of openpanels due to cumulative effectsof primary and secondary(Vierendeel) moments.

Further research in Canada revealed aditional insights into behavior, design, aneconomical construction of stub-girders. Thresearch indicated that only partial end plastiffeners, rather than traditional tted stifeners, were required to reinforce stub webFurthermore, web stiffeners were not alwarequired at interior stubs. In addition, a continuous perimeter weld between the base the stub and the top of the continuous beam

was not required. Tests also conrmed throlled wide ange shapes were more conducto stub-girder construction than split T (WTor rectangular hollow tube (HSS) sections. Additional conclusions of these laterCanadian tests revealed that:

1) Deection computations using theVierendeel method of analysis weretypically conservative for service loaand unconservative forultimate loading conditions.

2) The amount of internal slabreinforcement, particularly in thedirection transverse to the stub-girder span, was established based onCanadian Standard Association (CSAcriteria available at the time of testing

3) The conventional method ofcalculating number of shear studsrequired and application of standardmethods of composite design toanalysis of stub-girders appeared toprovide satisfactory results; however,caution was recommended whenspecifying closely spaced studs,particularly at the end stub.

9’-7” 9’-7”58’-9”

9’-7” 10’-0”2’-1” 5’-0” 2’-6” 5’-0” 2’-6”2’-6”3’-0”3’-3½” 3’-3½” 3’-0”3’-3½” 3’-3½”

14 SHEAR CONN.(2 ROWS)

14 SHEAR CONN.(2 ROWS)

6 SH.CONN.(2 ROWS)

6 SH.CONN.(2 ROWS)

STRAINGAGE

LOADPT. LOAD

PT.LOADPT.A

A

W16x26 W16x26 W16x26

W16x26 W16x26 STUB(BOTH SIDES)WELDED TOW14

W16x26W16x26

W14x48 (36,000 PSI YIELD)

W16x26

6¾¨

B

BW14 x 103

4x4 - 4 /4 WWFDRAPED

W16 x 26W16 x 26

W14 x 48W14 x 48

SHEARCONNECTORS

SLAB WIDTH5’-0”

16 GA. C12” x 1½”CONTINUOUS

5/16” FILLET WELD

22GA. C2 COFAR

5 ’ 3 ”

L T . W T .

C O N C R E T E

C2 SHEAR CONN.EVERY CORRUGATION

5’4” LT. WT. CONCRETEOVER CORRUGATION

W16 x 26 STUB(BOTH SIDES)WELDED TOW10

W10 x 123-7/8” BOLTS

2L - 4” x 5½” x 5/8”

2L - 4” x 5½” x 5/8”w/2 - 7/8” & BOLTS/LEG

- 11” x 1” x 0’-11”WELDED TO W

STRAINGAGE

DEFLECTIONGAGE

- 15” x 1” x 1’-3”WELDED TO W14

3/163/163/163/163/16

SECTION A-A SECTION B-B

ELEVATIONPL

PL

Figure 2: Stub Girder Load Test. Courtesy of AISC.



Design GuidelinesFurther recommendations and guidelines

emerged throughout the 1980s for the stub-girder system. In fact, the American Instituteof Steel Construction (AISC) had plansto develop a design guide for stub-girderconstruction; however, because deeper wideange sections became more readily availableand guidelines for design of reinforced andunreinforced web openings became more

established (see AISC Steel Design GuideSeries 2;Steel and Composite Beams with WebOpenings ; 1990), it was never published. Inorder to document some of the nal designguidelines that were established for stub-girder construction, the following list ofcriteria is provided:

1) Economical spans for stub-girdersrange from 30 to 50 feet, with theideal span range being 35 to 45 feet.

2) Transverse oor beams should bespaced at 8 to 12 feet on center.

3) Stubs do not necessarily have to

be placed symmetrically about thecenterline of the stub-girder span.4) Use of 3 to 5 stubs per span is the

most common arrangement.5) The stub located nearest the end of

the stub-girder (and the surrounding,adjacent truss/girder elements) is themost critical member, as it directlycontrols behavior of the overallstub-girder. In addition, the end stubmay be placed at the very end of thecontinuous bottom beam, directlyadjacent to the support point.

6) Performance of a stub-girder isnot particularly sensitive tolength of stubs, as long as lengthof stubs are maintained within thefollowing limits:

a) Exterior stubs shouldbe 5 to 7 feet long.

b) Interior stubs shouldbe 3 to 5 feet long.

However, increasing the length of theopen panel between stubs will reducestiffness of the stub-girder.

7) Stub-girders must be constructed asshored composite construction inorder to take full advantage of theconcrete slab top chord. Further,because of the additional dead loadimposed by shoring from upper oorsin multi-story construction, the needfor shoring the non-composite sectionbecomes even more critical.

8) Stub-girders should be fabricatedor shored to provide a camber thatis equal to dead load deectionof the member.

9) Overall strength of a stub-girder is notcontrolled by compressive strengthof the concrete slab, therefore use ofhigh-strength concrete mixes providesno advantage.

10) It is typical for ribs of a metal oordeck to run parallel to the span of astub-girder. This orientation of ribstherefore increases the area of the topchord slab and also makes it possibleto arrange a continuous rib or troughdirectly above the stubs, which inturn improves composite interaction ofthe slab with the stub-girder.

11) Welds between the bottom of stubsand the top of the continuous bottombeam should be concentrated at theends of the stubs

where forces betweenthese two elements aregreatest.

12) Internal longitudinalslab reinforcement toadd strength, ductilityand stiffness to thestub-girder shouldbe provided in twolayers, one just belowand one just above theheads of shear studs.

13) Internal transverseslab reinforcementshould be providedto add shear strengthand ductility.Placing transversereinforcement in aherring bone pattern– i.e., diagonal to thedirection of the stub-girder span – willalso increase the effective width of theconcrete ange/top chord.

14) Flexural stiffness of the top chord slabof a stub-girder should be based on theconventional effective width allowedby standard composite beam designcriteria, except that the transformedsection should include contributionof both the metal deck and internallongitudinal reinforcement.

15) It is not proper to include the topange of stubs in the calculation ofmoment of inertia of the top chordslab element.

In conclusion, the stub-girder methodconstruction was and still is an innovsolution to multi-story, framed steel construction. However, as deeper wange sections became more availabthe marketplace and design engineerscame more accustomed to analyzing holes in wide ange beams, the use of sgirder construction waned. In additibecause of extra labor costs associated fabrication of stub-girders and the neceto construct stub-girders as shored comp

construction, the system priced itself outhe industry. ▪

D. Matthew Stuart, P.E., S.E., F. ASCE, SECB is licensed in 20 states. Matt currently was a Senior Project Manager at the main ofce of CMX located in New Jersey, and alsoas Adjunct Professor for the Masters of Structural Engineering Program at Lehigh UnivBethlehem, PA. Mr. Stuart may be contacted [email protected] .

(CONTINUOUS CONCRETE SLAB)TOP CHORD

VERTICAL WEB

(STUB &TRANSVERSEBEAMS)

BOTTOM CHORD

(CONTINUOUS BOTTOM BEAM)

TYPICAL WIDE FLANGE VIERENDEEL TRUSS(EQUIVALENT STUB GIRDER COMPONENTS)

Figure 3.

16) Modeling of stubs as verticals ofthe Vierendeel truss/girder involvesdividing the stubs up into verticalelements equal to one-foot lengthsof the section, spaced at one foot oncenter from one end of the stub to tother. Vertical stub elements shouldbe modeled as xed at the top andbottom, at the top chord (concreteslab) and bottom chord (continuousbeam) of the truss/girder, respectiv

17) Transverse oor beams should bemodeled as single vertical webmembers/elements of the truss/girdThe top and bottom of the membershould be modeled as pinned at topand bottom chords.

The online version of this article contains references.Please visitwww.STRUCTUREmag.org .



References A Stub-Girder System for High-Rise Buildings Joseph P. Colaco AISC National Engineering ConferenceNew York, NY; May 1972

An Experimental Investigation of Stub-Girders Y.W. Lam, T. Rezansoff and M.U. HosainUniversity of Saskatoon, Canada Behavior of Building Systems and Building Components ConferenceVanderbilt University Nashville, TN; March 1979

Some Aspects of Stub-Girder DesignReidar Bjorhovde and T.J. ZimmermanUniversity of Alberta and the CISCCanadian Structural Engineering ConferenceMontreal, Quebec; February 1980Reprinted in the AISC Engineering Journal; Third Quarter 1980

Structural Engineering Handbook Chapter 18; Stud Girder Floor SystemsReidar BjorhovdeUniversity of PittsburghCRC Press LLC 1999

Documents

Antiquated Structural Systems Series, Part 6