aci

ACI Structural Journal/September-October 2005 657

ACI Structural Journal, V. 102, No. 5, September-October 2005.MS No. 03-484 received December 11, 2003, and reviewed under Institute publication

policies. Copyright © 2005, American Concrete Institute. All rights reserved, including themaking of copies unless permission is obtained from the copyright proprietors. Pertinentdiscussion including author’s closure, if any, will be published in the July-August2006 ACI Structural Journal if the discussion is received by March 1, 2006.

ACI STRUCTURAL JOURNAL TECHNICAL PAPER

The use of headed studs is proposed for several practicalapplications instead of conventional reinforcing bars anchored byhooks and bends. The main advantages are: simpler installationand less congestion of reinforcement and more effective anchorage.Experiments simulating the applications are discussed. Designrecommendations are given. The paper discusses applications ofheaded studs in slabs and footings, beams with thin webs, crosstiesin columns and walls, precast beams, deep beams and pile caps,and in beam-column joints. Use of a headed bar, as opposed to abar with a hook, is advantageous in applications where there isdemand for the yield strength at a section of the bar close to its end.

Keywords: anchorage; development length; stirrups; studs; reinforcement.

INTRODUCTIONHeaded studs are increasingly used to replace conventional

reinforcement (Fig. 1). Headed studs are smooth or deformedbars, commonly short relative to the lengths of concretemembers, and provided with forged or welded heads foranchorage at one or both ends. In this paper, the termsheaded stud and headed bar have the same meaning. In manyapplications, the studs run in the transverse direction of themember. Projects in which headed studs have been usedinclude offshore structures, bridges, and thousands of flatplates in Europe, Australia, East Asia, and North America.

Headed studs can also be used advantageously to reducecongestion in beam-column joints and in zones of lapsplices. Requirements for anchorage can create detailingproblems due to the long development length or the presenceof hooks and bends. The present paper reviews practicalapplications in which headed studs can be used to replaceconventional reinforcing bars. Experimental research at theUniversity of Calgary and at other research institutions, arereviewed. The specimens in these experiments representpractical applications of headed studs in slabs,1-4 beams,5,6

columns,7-9 walls,10 structural diaphragms (shearwalls),11

corbels,12 beam-column joints,13,14 and dapped ends ofbeams.15,16

Experiments17 have shown that an anchor head area equalto 9 or 10 times the cross-sectional area of the stem canprovide secure mechanical anchorage with negligible slipand develop the full yield force for studs of yield stress fy upto 500 MPa. With this type of stud, the full yield strength ofthe studs can be employed immediately adjacent to theanchor head. A tapered head (Fig. 1(b)) with a maximumthickness at the stem ≅ 0.6 the diameter of the stem db issufficient for strength. Minimizing the volume of the studhead simplifies its production by forging and reduces thecongestion of the reinforcement in the concrete forms.Research at The University of Texas at Austin18-20 hasshown that studs with smaller anchor heads and deformedstem can also be used in some applications, considering intheir design that the full yield strength is developed at a speci-fied development length away from the head.

SIGNIFICANCE OF THE REVIEW OFSTATE OF THE ART

The ACI 318 Code21 allows the use of mechanical anchoragesthat are “capable of developing the strength of the reinforcementwithout damage to concrete.” Designers, increasingly usingthe headed studs, cannot take full advantage of the superiorityof anchorage when adhering to code’s requirements. This isbecause the code does not allow the use of smaller amountsof reinforcement or larger spacing when headed bars areused. The present review of extensive research that showsmany uses of headed bars and gives design recommenda-tions should be of help to designers and writers of codesor technical reports.

ANCHORAGE OF BARSAnchorage of reinforcing bars is often achieved by the use

of 90-, 135-, or 180-degree hooks. If the tensile force and thestress developed in the hook are T and σs, respectively, aradial force T/R per unit length is exerted by the bar on theconcrete inside the bend; where R is the inner radius of thebend. The average bearing stress on the concrete is T/(Rdb);where db is the diameter of the bar. The ACI 318 Code21

requires that R ≥ 2db for db ≤ 5/8 in. (16 mm). With thisradius, the average bearing stress on the concrete is (σs πdb

2/4)/(2db

2) = 0.4σs. When σs approaches the yield strength fy ofthe bar, the bearing stress can damage (split or crush) theconcrete inside the bend and result in bend slip; thus, thehook cannot develop the stress fy in the bar. For this reason,building codes such as ACI 318-0521 require minimumvalues for the inner radius R and in many applicationsrequire that the bend engage a heavier bar, running perpen-

Title no. 102-S67

Headed Studs in Concrete: State of the Artby Amin Ghali and Samer A. Youakim

Fig. 1—Conventional single-leg stirrup and headed stud:(a) hooks satisfying minimum requirements of ACI 318-0521

(1 in. = 25.4 mm); (b) stud with forged head at each end.

ACI Structural Journal/September-October 2005658

dicular to the plane of the bend (Fig. 1(a)). Even when thisrequirement is satisfied, the slip that occurs at the hookscauses the full yield strength of the bars to be developed onlyat some distance away from the bends.

Leonhardt and Walther22 measured the slip that occurs atthe bends of 90-, 135-, and 180-degree hooks, whenengaging heavier bars lodged inside the bends. At stresslevel of σs = 400 MPa (60 ksi), with a concrete strength of f ′c= 25 MPa (3600 psi), the measured slip varied between 0.1and 0.25 mm (0.004 and 0.010 in.) and increased rapidlywith the increase of σs, reaching between 0.2 and 0.9 mm atσs = 500 MPa (0.008 and 0.035 in. at 70 ksi). With theheaded studs in Fig. 1(b), Eligehausen17 measured slipvarying between 0.013 and 0.033 mm at σs = 400 MPa andbetween 0.023 and 0.045 mm at σs = 500 MPa (0.5 × 10–3

and 1.3 × 10–3 in. at 60 ksi and between 0.9 and 1.8 × 10–3 in.at σs = 70 ksi), with f ′c = 25 MPa (3600 psi). The lowerbearing stress and the smaller slip make studs with a head ateach end more effective than conventional stirrups incontrolling concrete cracks that intersect the stems at anylocation between the heads (for example, cracks due to shearor splitting forces).

ADVANTAGES OF HEADED STUDSWhen headed studs are used, the congestion and the time

of installation can be reduced by the use of a smaller numberof studs of larger diameter. For speedy and accurate installation,sets of double-headed studs can be fitted at specified spacingin nonstructural sheet metal troughs, as shown in Fig. 1(b).

A hook is required to engage a bar of larger diameter(Fig. 1(a)) that can enhance the anchorage. This mechanicalparticipation to the anchorage, however, can be partly lost when,because of imprecise workmanship, the heavier bar is not incontact with the inner face of the hook. With studs, the headprovides positive anchorage, without the need for enhancement.

A stud is longer than the vertical effective part of a stirrup(compare Fig. 1(a) and (b)) and thus can intersect more shearcracks. A crack approaching a stirrup leg near a bend tendsto follow the bend, rather than intersecting the leg andcontrolling the width of the crack. The cover to the longitu-dinal bars has to be greater than the specified minimum plusthe diameter of the stirrups (Fig. 1(a)); thus, when stirrupsare used in lieu of studs, the distance d between the centroidof the tensile reinforcement and the extreme compressionfiber will have to be smaller by an amount equal to the diam-eter of the stirrups. The reduction in flexural and shearstrength of the member, caused by the smaller d, has to becompensated for by the provision of a greater amount offlexural and shear reinforcements; the added amount can besignificant in thin slabs.

APPLICATIONSPunching shear of slabs and footings

Figure 2(a) and (b) show two types of stud shear reinforce-ment (SSR) widely used in slabs and footings in manycountries. The studs in Fig. 2(a) have forged heads at oneend; at the other end, the studs are welded to a rail (steelstrip) that serves for anchorage and holding the studsvertically at the appropriate spacing. The studs in Fig. 2(b)have forged heads at each end; the heads at the lower endsnugly fit in a sheet metal trough (or in other nonstructuralelements) that serves as a spacer. Typical arrangement of thestuds in plan to resist punching shear at an interior column inslabs or footings is shown in Fig. 2(c). The shear-reinforcedzone should extend outwards from the column to the vicinityof a critical section at which the shear stress due to thetransfer of factored shear force combined with factoredunbalanced moment does not exceed φvc; where φ is thestrength reduction factor and vc is the nominal shear strengthof concrete. (According to ACI 318-05, φ = 0.7 and

The vertical section in Fig. 2(c), in the shear-reinforcedzone of a slab, shows the position of the SSR relative to otherreinforcements. For best performance, ACI 421.1R-992

recommends an optimum height of the stud equal to thethickness of the slab or footing minus the sum of theminimum specified covers with a tolerance equal to minusone-half the diameter of the flexural reinforcing bars. Inslabs, the SSR are commonly fastened with their rail ortrough on wood forms, before the installation of other slabreinforcement. Alternatively, particularly in footings, theSSR can be supported by the top reinforcement in aninverted position (with the rail or trough at the top).

ACI 318-05 considers (in most cases) that the nominalshear strength at the critical section at d/2 from the columnface is equal to

vc 1 6⁄( ) fc′ MPa( ) 2= fc′ psi( )=

vn 1 3⁄( ) fc′ MPa( ) 4= fc′ psi( )=

Amin Ghali, FACI, is Professor Emeritus, Department of Civil Engineering at theUniversity of Calgary, Calgary, Alberta, Canada. He is a member of ACI Committee435, Deflection of Concrete Building Structures; and Joint ACI-ASCE Committees 343,Concrete Bridge Design; and 421, Design of Reinforced Concrete Slabs; and is aconsulting member of ACI 318-E, Shear and Torsion (Structural Concrete Building Code).

ACI member Samer A. Youakim is an assistant project scientist at the University ofCalifornia, San Diego, San Diego, Calif. He received his PhD from the University ofCalgary in 2002. His research interests include behavior of concrete structures underearthquake loading, finite element analysis, and serviceability of concrete structures.

Fig. 2—Stud shear reinforcement for slabs and footings: (a)stud-rail system; (b) double-headed studs held by nonstruc-tural sheet metal trough; and (c) top view and section showingtypical arrangement of studs in vicinity of interior column.


when studs are used as shear reinforcement, ACI 421.1R-99recommends that vn be less than or equal to

It also recommends that within the shear-reinforced zonevc be equal to

When stirrups are used, lower stresses are permitted byACI 318-05

and

This is because the studs are more efficient than stirrups inconcrete confinement. In addition, ACI 421.1R-99 allowsthe spacing between studs to be ≤ 0.75d compared with 0.5dfor stirrups. These differences in design rules permit thinnerslabs or require less amounts of shear reinforcement whenstuds are used.

Beams with thin websThe thickness of the web of precast beams is often

governed by constructability rather than by strengthrequirement. For ease in installation of the reinforcement and incasting concrete, the web should be wide enough toaccommodate the two legs of closed stirrups, the minimum sidecovers, and a sufficient space in between for casting andvibrating the concrete. Additional width is required to accommo-date draped pretensioned strands or ducts of post-tensionedtendons. The web thickness can be reduced by replacing thelegs of conventional stirrups by double-headed studs (Fig. 3(a)).Draped external post-tensioned tendons can be locatedadjacent to the two sides of the web.

Modern precast pretensioned girders,23 widely used inbridge decks, are made continuous by post-tensioned strands

inserted in sheet metal ducts located in the midsurface of theweb. For ease in construction, the thickness of the webcannot be much less than 175 mm (7 in.). Figure 3(b) showsan alternative design24 using external post-tensionedtendons and double-headed studs in midsurface of a web ofthickness 100 mm (4 in.).

ACI 318-05 permits shear reinforcement spacing notexceeding d/2 or 3h/4 for nonprestressed or prestressedbeams, respectively; h is the overall thickness of themember. Fabrication and accommodation of hooks of bars ofdiameter ≥ 16 mm (5/8 in.) is relatively difficult. For thisreason, the spacing between stirrups in bridge I-girders iscontrolled by the practical bar diameter rather than coderequirements. In these cases, one double-headed stud ofdiameter 25 to 30 mm (1 to 1-1/4 in.) can be used to replaceseveral stirrup legs. The advantage is saving in the labor costof installation of reinforcement.

Crossties in columns and wallsDouble-headed studs are used in Fig. 4 as crossties in

columns and walls. Each stud is a substitute for one or moresingle-leg stirrup(s) (Fig. 1(a)). In columns, the conventionalclosed stirrup following the perimeter of the cross sectionsshould be maintained, with the studs used only as crossties(Fig. 4(a) and (b)). Unlike the hooks in stirrups, the heads ofstuds do not need to engage a vertical bar, as shown inFig. 4(c). For ease in installation of reinforcement, the headsof studs may be placed adjacent to the vertical bars incolumns, as shown in Fig. 4(a) and (b); but this is not arequirement to enhance the anchorage of the studs. Experimentson concrete columns under concentric compression loading7

and under simulated-seismic loading8 have shown thatplacing the vertical bars behind the heads is sufficient toprevent premature buckling of the vertical bars after spallingof concrete cover. Columns with headed studs ascrossties have exhibited improved ductility and equal orgreater strength than companion columns with conventionaltie reinforcement.

ShearwallsReinforced concrete structural diaphragms (shearwalls)

resisting lateral forces in buildings are subjected to compressive

2 3⁄( ) fc′ MPa( ) 8= fc′ psi( )

1 4⁄( ) fc′ MPa( ) 3= fc′ psi( )

vn 1 2⁄( )≤ fc′ MPa( ) 6= fc′ psi( )

vc 1 6⁄( ) fc′ MPa( ) 2= fc′ psi( )=

Fig. 3—Studs as shear reinforcement in beams: (a) cross section of tested beams;5 and(b) thin-web girders.23 NU-girder stands for Nebraska University girder.22

660 ACI Structural Journal/September-October 2005

axial forces due to gravity loads combined with reversiblebending moments. This combination causes concentration ofnormal stresses, often resisted by boundary elementscontaining high reinforcement ratios of vertical bars andconfinement ties. ACI 318-05 specifies the volumetric ratioand the spacing of ties. Accordingly, the boundary elementsof structural diaphragms are in many cases, especially inearthquake zones, congested with heavy vertical bars, closedstirrups, and crossties.

Figure 5 shows horizontal sections of tested11 shearwallshaving boundary elements. Single-leg stirrups, detailed as in

Fig. 1(a), are used as crossties in Fig. 5(a), whereas double-headed studs, as shown in Fig. 1(b), are used as crossties inFig. 5(b). Almost the same strength and improved ductilitywere observed for the walls with the studs.

CorbelsA corbel is a short cantilever often supporting a precast

beam on a bearing plate, exerting factored vertical and hori-zontal forces Vu and Nu (Fig. 6(a)). The strut-and-tie modelpresented in the figure has been shown12 to be an accuratedesign tool for corbels. The distance between the bearingplate and the tip of the corbel is not sufficient to develop thetensile force T in the top reinforcement. Welded cross bars orplates or horizontal loops are often used to enhance theanchorage. Headed studs resisting the tensile force T offeranchorage without congestion of conventional reinforcement.Figure 6(b) represents details of a tested corbel specimen andalso represents a reduced model of a corbel supporting twoprecast girders.

Precast beamsFrequently, over a short length at the ends, the depth of

precast beams is drastically reduced (refer to Fig. 7(a) repre-senting a dapped end). The beams are commonly simplysupported on bearing plates. Similar to corbels, dapped beamends must be designed to carry factored forces Vu and Nu. Again,the strut-and-tie modeling is a valuable design tool. Themodel shown in Fig. 7(a) and the reinforcement arrangement

Fig. 4—Double-headed studs as crossties: (a) and (b)column cross sections; (c) horizontal section in wall;and (d) Specimen SD-6 having double-headed studreplacing two single-leg stirrups.9 15M and 20M bars havecross-sectional areas of 200 and 300 mm2, respectively.

Fig. 5—Horizontal sections in structural diaphragms withboundary elements having single-leg stirrups or double-headed studs as crossties. 10M and 15M bars have cross-sectional areas of 100 and 200 mm2, respectively.

Fig. 6—Double-headed studs in corbels: (a) forces oncorbel and strut-and-tie system for design; and (b) tested12

arrangement of reinforcement in corbels.

Fig. 7—Dapped end of precast beam:15 (a) strut-and-tiemodel; and (b) arrangement of headed studs.


in Fig. 7(b), using headed studs, have been proposed fordesign.15 Recent tests at the University of Calgary verified theproposed reinforcing system and its capacity using both the strut-and-tie model and the shear friction method.16, 25

Splitting forcesHeaded studs can be used to control cracking due to

splitting forces caused by concentrated loading atprestressing anchors and at support bearings. The mainadvantage is the reduction of congestion of reinforcementshown in Fig. 8.

Figure 9(a) shows the distribution of vertical tensilestresses at the anchorage of prestressing tendons in a concreteslab. The potential splitting of the slab in a horizontal plane nearthe middle surface is indicated. The anchor zone of a band ofsingle-strand prestressing tendons is shown in Fig. 9(b). The

congestion of reinforcement in Fig. 8 is partly caused byhair-pin stirrups. In modern construction, the hairpin stir-rups are replaced by vertical headed studs (Fig. 10(a) to (c)).

Deep beams and pile capsFigure 11(a) represents a strut-and-tie model for the design

of deep beam or a two-pile cap. A free-body diagram ofNode A is shown. Often, the size of the node and the dimen-sions of the cap are not sufficient to anchor the tie by bond. InFig. 11(a), the tie consists of plain (non-deformed) studs withheads located outside the node. With this arrangement, theanchorage of the stud relies solely on the bearing stress at thehead. With head area 9 to 10 times the area of the stem, thethree faces of the concrete prism representing node A can beconsidered subjected to compressive stress (C-C-C node);the anchor heads of the studs create the compression on thevertical face of the prism. ACI 318-05 permits higher stressfor a C-C-C node, compared to the C-C-T node that will existwhen the tie is anchored by bond within the node.

Fig. 8—Congestion of anchor zone of post-tensioningstrands at edge of slab with “hair-pin” stirrups used tocontrol splitting.

Fig. 9—Potential splitting of slab edge: (a) distribution ofvertical tensile stresses; and (b) anchorage of band of singleprestressing strands.4

Fig. 10—Control of splitting cracks:4 (a) use of hair-pinstirrups; (b) use of headed studs; and (c) headed studs(stud-rails) in post-tensioned bridge deck (Calgary,Canada). 10M bar has cross-sectional area 100 mm2.


Beam-column jointsTwo connections of beams to columns are shown in Fig. 11(b)

and (c). Single-headed studs are used for anchorage of thelongitudinal bars of the beams and the columns to avoidcongestion within the joint. Away from the joints, lap splicesrelying on bond or other types of splices can be used toextend the studs longitudinally in the beams or the columns.Tests13,14 subjecting the connections to the transfer ofreversible moments have verified the suitability of single-headed studs for use in seismic zones. Headed studs arewidely used in California in the connections of bridge piersto their superstructures.

Other applicationsIt is advantageous to use a bar anchored mechanically by

a head, as opposed to a hook, when there is demand for theyield strength at a section of the bar close to its end. Theprevious example applications do not cover all uses ofheaded bars.

EXPERIMENTAL VERIFICATIONSResults of some experiments that study the behavior of

structures reinforced by headed studs are reviewed in thefollowing.

Slab punching shear3

The specimen in the inset of Fig. 12(a) (representing theconnection of a reinforced concrete slab to an edge columnextending above and below the slab) was simply supportedon three edges. The column transferred to the slab a constantshearing force V representing gravity load and a reversibleunbalanced moment M representing the effect of an earthquake.Shearing force V and unbalanced moment M were graduallyincreased, with M /V = constant, until a target serviceabilityshearing force Vu was reached. Then cyclic displacements ofincreasing amplitude were imposed at the ends of the

columns to produce the unbalanced moment. Cyclic momenttransfer was continued after the peak moment Mu until theloss of 25% of Mu.

Figure 12(a) and (b) compare the graphs3 of M versus thedrift ratio for a slab without shear reinforcement and a slabhaving SSR (stud-rails having an area equal to nine times thecross-sectional area of the stem) arranged as shown in theinset of Fig. 12(b). In the compared tests, the value of Vu =0.6Vc, with Vc being the nominal shear strength, withoutmoment transfer or shear reinforcement according to ACI318-05. Provision of SSR reduced significantly the rate ofstiffness degradation (the slope of the ascending parts of theloops) due to the cyclic moment reversals. At 1.5% driftratio, the stiffness of connection without SSR was approxi-mately 50% of the stiffness with SSR. At 3% drift ratio, thestiffness of the connection without SSR was almost lost. Thedrift ratio with SSR reached approximately 4% withoutappreciable loss of strength. This is higher than what iscommonly expected in a major earthquake. After the cyclicloading described above, the slab with SSR was subjected toV combined with M, at a constant M/V ratio, in load controlto examine its residual strength. It was concluded that withSSR, the shear resistance to gravity load is maintained aftersevere earthquake. The highest drift ratio permitted by IBC26

Fig. 11—Headed studs in longitudinal directions ofmembers: (a) strut-and-tie model for two-pile cap; (b)exterior beam-column connection; and (c) corner beam-column connection.

Fig. 12—Punching shear tests on slab-column connections:3

(a) and (b) variation of unbalanced moment due to cyclicdisplacement versus drift ratio for specimens without andwith SSR, respectively (1 in. = 25.4 mm; 1 kip-in. = 113 N-m).


for concrete flat plate supported directly on columns is 2.5%;such a structure must have shearwalls or other bracing systemsthat limit the drift ratio to the value permitted by the code.

Shear reinforcement in concrete I-beams5

and deep beams6

A series of concrete I-beams with web thickness of 100 mm(4 in.) and depth varying from 300 to 600 mm (12 to 24 in.)were tested for shear strength.5 The shear reinforcementswere single-leg stirrups or double-headed studs, as shown inFig. 3(a). The stirrups had 90- and 135-degree hooks at theends; the studs were made of straight bars cut from the samestock used for fabricating the stirrups and were welded tocircular heads of area nine times the cross-sectional area ofthe stem. All the beams failed in shear, as planned. In termsof strength and ductility, the performance of the beams withstuds was equal to or slightly better than the beams with stirrups.The advantages of studs are ease of installation and control.

Berner and Hoff6 presented results of tests on three large-scale specimens representing a strip of approximate widthand depth 1.0 x 0.7 m (40 x 28 in.) of a wall for offshoreplatforms. The specimens were restrained at the ends tobehave as horizontal continuous deep beams of “nominal”center-to-center spans of 2.8 m (110 in.). The three beamshad identical longitudinal top and bottom reinforcement.Vertical headed studs were used as shear reinforcement, ofratio 1, 1.5, and 2%. The ultimate central load, over a lengthequal to half the length of the clear span, was more thantwice the value permitted by ACI 318 and the measuredstrengths increased with an increase in the shear reinforcementratio. The authors concluded that the codes needed to bechanged to reflect the superior behavior of the headed studs;in particular, ACI 318 unnecessarily limited fy for the shearreinforcement to 413 MPa (60 ksi) and its contribution to theshear strength to (2/3) .

Columns7

Figure 13(a) to (d) show the cross sections of five specimensrepresenting short columns tested in axial compression. Theobjective was to compare the confinement effect of double-headed studs with that of single-leg stirrups with 90- and180-degree hooks at the ends. Within the test zone, Specimen 1(not shown) had no reinforcement; Specimens 2 and 3,respectively, had stirrups and studs as confinement rein-forcement without vertical bars; Specimens 4 and 5, respec-tively, had stirrups and studs as confinement reinforcementin addition to vertical bars. Closed stirrups, following theperimeter of the cross section, were provided in Specimens2, 3, 4, and 5. The concrete strength f ′c for the five specimenswas 20 MPa (3000 psi). The stirrups and the studs were madefrom bars of diameter 5.7 mm (0.20 in.) and yield strengthof 595 MPa (86 ksi). The diameter of the stud heads was18 mm (0.7 in.).

Graphs for the load versus the axial strain in the fivespecimens are shown in Fig. 13(e). The failure load variedbetween 1580 and 2100 kN (356 and 472 kips), corresponding,respectively, to Specimen 1 (unconfined) and Specimen 5(with vertical bars and studs). At failure of Specimen 3 (withstuds but no vertical bars), spalling of the cover and horizontalcracking occurred, while the core remained intact. This iscontrary to Specimen 2 (with stirrups but no vertical bars)where diagonal cracks traversed the thickness of thespecimen. Spalling of the cover of Specimen 4 (with stirrupsand vertical bars) occurred at the 90-degree hooks, where

fc′ MPa( ) 8 fc′ psi( )( )

their ends popped out of the cover and protruded from thecolumn face. Similar observations were reported by otherresearchers.27,28 The strain measurements indicated yieldingof the studs; the maximum strain in the single-leg stirrupswas well below the yield strain.

From Fig. 13(e) and the test observations, it wasconcluded that double-headed studs as crossties, while notrequiring vertical bars behind the heads to enhanceanchorage, exhibit large strain beyond yield at failure load ofthe column. Anchorage of crossties by 180- and 90-degreehooks engaging heavier bars is not sufficient to develop theyield stress in the ties. Columns exhibit better ductilebehavior and greater ultimate strength when double-headedstuds replace conventional crossties. Because of the superiorperformance of the studs, codes should allow a reducedvolumetric ratio and/or larger spacing when studs are used ascrossties in lieu of stirrups.

Cyclic lateral loading of columns8,9

Nine column specimens, reinforced with either double-headed studs or conventional crossties, were tested underseismic loading.8,9 The columns had a cross section of 250 x500 mm (10 x 20 in.) and a total height of 1500 mm (59 in.)

Fig. 14—Bearing stress behind stud head (risk of spalling ofcover).

Fig. 13—Short columns tested under axial loads;7 cross-sectional dimensions 150 x 500 mm2: (a) and (b) Specimens 2and 3 have no vertical bars, but with stirrups and studs,respectively; (c) and (d) Specimens 4 and 5 have verticalbars with stirrups and studs, respectively; and (e) axial loadversus axial strain for Specimens 1 to 5. Specimen 1 had noreinforcement (unconfined). 10M bar has cross-sectionalarea of 100 mm2.

ACI Structural Journal/September-October 2005664

and were laterally loaded to bend about their weak axis.The columns were subjected to a constant axial load Pcorresponding to either 20 or 30% of their nominal axialcapacity Po combined with incrementally increasing lateral-displacement reversals. The columns were made from 25 MPa(3600 psi) concrete and had a longitudinal reinforcementratio of 1.3%. Welding circular plates with a diameter threetimes that of the stem produced the double-headed studs. Thesame stock of bars was used for the stem of the studs and thestirrups. It was shown that, while columns with either type oflateral reinforcement attained the same strength, columnswith double-headed studs exhibited superior behavior interms of ductility and energy dissipation. All column crosssections had a closed peripheral stirrup. Several columns hadtwo single-leg stirrups as crossties. In Column SD-6, shownin Fig. 4(d), a single double-headed stud, as shown in Fig. 4(d),replaced the two crossties. Although Column SD-6 containedalmost half the volumetric ratio of ties of columns withsingle-leg stirrups and half the minimum amount required byACI 318-05 for seismic design, the ultimate capacity andductility of Column SD-6 were similar to the other columnspecimens. This shows that ACI 318 requirements forconfinement reinforcement are overly conservative forcolumns subjected to axial loads levels less than 30% of theirnominal capacity.

Walls10

Sixteen wall elements were tested at the University ofToronto10 under monotonic in-plane vertical compression,with some of the elements being subjected to verticalcompression combined with horizontal in-plane tension. Inaddition to the reinforcement layers running parallel to thesurfaces, eight walls were confined with double-headedstuds running in the direction normal to the wall surfaces; theother eight walls did not contain confining studs. The studheads enclosed the reinforcement layers and had an areaapproximately equal to nine times the cross-sectional area ofthe stem. It was concluded that the double-headed studsincreased both the strength and the ductility of the wallelements. Based on the experimental results, an analyticalmodel was developed to predict the compressive strength ofwall elements confined with headed studs.

Repair and rehabilitation29,30

A series of circular columns29 representing bridge pierswere severely damaged under simulated-earthquake loading,and then repaired and tested again. One of the repair techniquesinvolved placement of a strong jacket along the damagedregion so future flexural hinging would be forced to occurjust above the jacket. To ensure that flexural yielding oflongitudinal reinforcement would not occur at the columnbase, the jacketed region was reinforced with headed studs toavoid congestion of reinforcement. Subsequent testing of therepaired columns showed that their stiffness and strengthwere comparable to those of the original ones.

Six pier walls were loaded in the weak direction undercyclic loading to near failure.30 Five of the damaged pierwalls were repaired with conventional crossties with 90- and135-degree hooks; one wall was repaired with double-headed studs as crossties. The area of the heads was 13 timesthe area of the stem. The six repaired pier walls were retestedunder the same loading conditions to compare theirperformance. Due to the additional confinement provided by

the heads, the wall repaired with studs performed better thansimilar walls repaired with conventional crossties. It wasalso found that the heads provided sufficient anchoragewithout the need to engage the longitudinal bars of the walls.

Cyclic lateral loading on shearwalls11

A sustained 1000 kN axial load was applied on the shear-walls shown in Fig. 5, while at 3.3 m above the base, the wallswere pushed back and forth to produce imposed reversals ofhorizontal top displacement of increasing amplitude. The studheads had an area equal to nine times the cross-sectional areaof the stem. The walls, having the same volumetric ratio oftransverse reinforcement, attained almost the same ultimatelateral strength and displacement; the envelope curves of thelateral force-displacement relationship for both walls werequite similar. The wall with double-headed studs, however,displayed better energy dissipation capacity (determined bythe summation of the areas enclosed by the lateral force-displacement hysteresis loops). This research confirmed thatdouble-headed studs could be a substitute for single-leg stir-rups as crossties in the boundary elements of shearwalls.

Vertical and horizontal forces on corbels12

Six corbels with the dimensions shown in Fig. 6(b) (withstuds having head area equal to nine times the cross-sectional area of the stud) were subjected to vertical forcesVu combined with horizontal forces Nu = Vu/5. The magni-tudes of Vu and Nu were monotonically increased up tofailure. The horizontal force represented the reaction compo-nent that can develop due to shrinkage or temperature dropof a precast beam supported by the corbel. The corbels weredesigned by the strut-and-tie model (Fig. 6(a)) to fail by theyielding of the tie or by crushing of the concrete at Node B.Plain or deformed studs of 20 mm (0.8 in.) diameter withforged heads of 60 mm (2.4 in.) diameter were used for thetie. The conclusion from this research was that both plain anddeformed double-headed studs can be used as main tensionreinforcement in corbels. Double-headed studs placed in thecompression zone in the direction normal to the corbelfaces can significantly increase the ductility; this wasconfirmed by experiments at The University of Texas atAustin31,32 on the overhangs of bridge piers.

Slab splitting at anchors of prestressing tendons4

Tests were conducted at The University of Texas atAustin33 on the use of hair-pin stirrups, as shown in Fig. 10(a),to control splitting of slabs at the anchor zone of a band of post-tensioned single strands (Fig. 9(b)). The Texas tests wereduplicated at the University of Calgary,4 replacing the hair-pin stirrups by headed studs (Fig. 10(b)). Seven 9.5 mmdiameter hairpin stirrups used in the Texas tests werereplaced by the same number of headed studs of the samediameter, welded to a rail. The studs had forged heads of areaten times the cross-sectional area of the stem. The edges ofthe specimens in Texas and in Calgary were subjected tocompressive forces through a special adapter to closelysimulate the anchors of a band of six strands.

The ultimate loads in the tests in Calgary were higher thanthose of the Texas tests. Several studs reached yieldingbefore failure, indicating the effectiveness of the anchors.Due to the confinement of concrete by the stud heads, thebearing stress under the anchor plates in the tests in Calgarycould reach more than two times the compressive strength of


the concrete. The conclusion was that headed studs areeffective in the control of splitting cracks in the anchor zonesof prestressed slabs. In addition, the studs provide confinementof the concrete in the anchor zone. Equation (1) is suggestedto give the cross-sectional area of headed studs Asv requiredto control the splitting crack due to prestressing.

(1)

where a is the vertical dimension of the anchor(s) of theprestressing tendon(s) (in Fig. 10(b)); h is the slab thickness;fy is the yield strength of the studs; and fpu and Aps are theultimate strength and cross-sectional area of the tendon. Themaximum prestressing force applied to the anchor isassumed equal to 0.7fpuAps according to the Post-TensioningManual.34 The studs should be arranged at a distance0.40h ~ 0.55h from the anchor plate.

Beam-column joints13,14

Four interior bridge beam-column joints with eitherconventional or headed reinforcement were tested underseismic loading13 to evaluate the current design requirementsof the California Department of Transportation (Caltrans35).Headed studs were used within the joints to resist shearstresses and to confine the concrete and were also used forlongitudinal bars of the column. The head had a diameterequal to 3.2 times the diameter of the stem. It was concludedthat headed studs produced comparable behavior to that of theconventionally reinforced joints; but the constructability of thejoints was improved due to the use of fewer bars with largerdiameters and the elimination of the hooks. Similar tests wereconducted at the University of California at San Diego onbridge column-beam knee joints36 and pile-foundationconnections37 with almost all the reinforcement consisting ofheaded studs. Again, the results demonstrated the effective-ness of headed studs in bridge joints under seismic loads.

Five beam-column corner joints and two exterior beam-column joints were tested under seismic loading14 to evaluatethe potential of using headed studs as longitudinal beam andcolumn reinforcement within the joints. The area of the studheads varied from 4 to 11 times the cross-sectional area ofthe stems. It was concluded that the behavior of joints withheaded reinforcement performed as good as or better thansimilar joints with 90 degree hooks.

Thirty-two tests on simulated beam-column joints wereconducted by Bashandy19 to investigate the behavior ofbeam-column joints with headed stud anchorage. The testswere similar to earlier tests38,39 performed on hooked baranchorage in beam-column joints. It was found that theanchorage performance of the headed studs was equivalentto or better than bars with conventional hooks.

ANCHORAGE BY COMBINATION OFBOND AND BEARING

In the research mentioned above, the studs were made ofplain or deformed bars and mostly had head areas equal tonine or 10 times the area of the stem (Fig. 1(b)) and theratio ( fy /f ′c ) was as high as 25; where fy is the yield strengthof the studs and f ′c is the concrete strength. In design usingsuch studs, nominal yield strength up to 500 MPa (72 ksi)can be considered available at any section of the stemwith f ′c ≥ 20 MPa (2900 psi). When studs of this type have

nominal yield strength fy MPa (72 ksi), the full fy value maybe considered available at any section of the stem, providedthat the ratio ( fy/f ′c ) does not exceed 25; however, more testsare needed to verify this statement; note that in some of thetests mentioned above, fy has exceeded 500 MPa (72 ksi).

Studs made of deformed bars and heads of smaller areashave been used in research at The University of Texas atAustin.18-20 An empirical equation was developed for thebond length between the head and the section at which thenominal yield strength can be considered available. Theequation is given in the following section.

SPLITTING OF COVERA headed stud running parallel to an exterior surface of a

concrete member is shown in Fig. 14. For protection againstcorrosion or fire, the distance c between the centerline of thestud and the surface must be greater than the radius of thehead plus the specified clear cover cc. For example, when cc =20 mm (0.8 in.) and the diameter of the stud and its head are20 and 60 mm (0.8 and 2.4 in.), respectively, c ≥ 50 mm (2in.). When c is small, the bearing stress behind the anchorhead can cause splitting (side blow-out or spalling) of thecover and c needs to be greater than the required minimumfor protection. Alternatively, spalling can be prevented bythe use of closed stirrups in the plane perpendicular to thestud. The stirrups can be designed to resist a resultant splittingforce of 0.3Ty, where Ty is the yield force of the stud. Thisempirical recommendation is based partly on analysis of theresults of tests12 and partly on Eq. (1), assuming that thehead diameter is 3db, c = 3db, ae = 0.75(3db) and the yieldstrength of the stud is developed by bearing at the head;Eq. (1) can be used, although it was not developed for thisapplication. The stirrups are to be arranged so that theresultant of their forces4 is approximately at a distance cfrom the stud head, as shown in Fig. 14.

For a stud having a head of area equal to 9 or 10 times thearea of the stem, splitting of the cover need not be of aconcern when c ≥ 3.5db; where db = stud diameter. Furthermore,the bar stress developed by the head can be assumed equal tofy, provided that ( fy/f ′c ) and the stud is close to no more thanone exterior surface. This empirical recommendation issupported by an equation resulting from extensive testing byThompson;20 the equation, given below, will show that when(Anh/Ab) ≥ 9 and c ≥ 3.5db, the bar stress fs head developed by thehead can be equal to the yield strength fy when ( fy/f ′c ) ≤ 29;where Anh = the net area of the head (head area minus bararea Ab). For the steel and concrete used in most countries,( fy/f ′c ) is normally less than 29.

Thompson20 conducted tests on 46 specimens representing aC-C-T node in a strut-and-tie design model. Headeddeformed studs were used as tie reinforcement; the head areawas ten times the cross-sectional area of the stud. Thompsonalso conducted tests on 27 lap splices using deformed studsof head area ranging from 2.2 to 5.7 times the cross-sectionalarea of the stud. In all these tests, the stud ran close by andparallel to one exterior surface or two orthogonal exteriorsurfaces of the concrete member. Based on the experiments,Thompson proposed that the stress developed by the bearingof the head fs head can be computed by Eq. (2), dependentupon the cover or the side covers of the bar.

(2)

Asv 0.3Apsfpu

fy

------ 1 2a3h------–

=

fs head 1.4Anh

Ab

--------c1

db

----- Ψf ′c fy≤=


(3)

where c1 = the minimum distance between the centerline of thebar and the surface of the member; c2 = the minimum distancebetween the centerline of the bar and an exterior surfaceperpendicular to c1; the distance c2 is greater than c1. When thebar runs parallel and close to only one exterior surface, set Ψ =2.0 and replace c1 by c in Eq. (2); where c is the distancebetween the centerline of the bar and the exterior surface.

Consider a headed stud of yield strength fy = 500 MPa(72 ksi) used in a concrete member with compressivestrength f ′c = 20 MPa (2900 psi); assume that the stud headarea is equal to 10 times the bar area (Anh = 9Ab). Assumefurther that the yield stress of the stud is entirely developedby bearing; thus, fs head = fy. To prevent spalling of theconcrete cover, Eq. (2) gives c ≥ 3db when the stud runsparallel to one exterior surface. This example indicates thatthe empirical limit c ≥ 3db, recommended above, is conser-vative. When the stud runs parallel to an edge at the intersectionof two orthogonal exterior surfaces, spalling need not be ofconcern when c1 = c2 ≥ 6db.

When fs head < fy, Thompson gives Eq. (4) for the distancela between the bearing face of the head and the section atwhich fy can be considered developed

(4)

where ld is the development length of a nonheaded deformedbar of the same diameter, ACI 318-05 gives equations forld. The coefficient (1/0.3) is included in Eq. (4) because thetests show that a portion of the force in the bar developed bybond drops as the portion developed by the head approachesthe value (Ab fs head). The validity of Eq. (4) is limited for labetween 6db and ld; Thompson has set the lower limit of thisrange for the validity of his empirical equation. The upperlimit is set because the development length of a headed studla cannot exceed its development length in the absence of thehead. Substitution of the upper limit la = ld in Eq. (4) gives( fs head/fy) = 0.7. This means that when ( fs head/fy) ≤ 0.7, thedevelopment length la should be taken equal to ld.

As an example of the results that Eq. (4) gives, calculate(la/ld) by varying ( fs head/fy). The results in Table 1 indicatethat with a head that develops 85% of the yield strength, thedevelopment length can be taken equal to half that of anonheaded bar.

SUMMARY AND CONCLUSIONSSeveral practical applications of headed studs in concrete

structures have been proposed and some results ofsupporting experimental research have been presented. Forthese applications, the studs are made of plain or deformedbars, and have head areas equal to nine or 10 times the cross-sectional area of the stem. With this head area, the anchorage

Ψ 0.6 0.4c2

c1

---- 2.0≤+=

la1

0.3-------

1fs head

fy

------------– ld but 6 db la ld≤ ≤;=

by bearing is sufficient to develop the yield strength of thestud, with negligible slip. In design, nominal yield strengthfy ≤ 25f ′c can be considered available at the stem sectionadjacent to the head. The thickness of the anchor head mustbe sufficient so that the bearing pressure does not causeyielding by bending or shear of the head before the tensilestress in the stem reaches yield. The anchor heads areproduced by forging or by welding a plate to the bar end.Forged heads are commonly tapered; the maximumthickness of the head at the perimeter of the stem needsnot be more than approximately 0.6db; where db is thediameter of the stud.

Experimental research has also shown that, in someapplications, deformed studs can be used with heads of areassmaller than nine to 10 times the cross-sectional area of thestem. In this case, anchorage relies on the bearing stress atthe head combined with the bond stress over a developmentlength la shorter than that the development length ld for adeformed bar in tension required by ACI 318-05 having nobend or hook.

The main advantages of using headed studs are moreefficient anchorage, simpler installation, less congestionof reinforcement, and improved confinement. ACI 421.1R-99recognizes these advantages and recommends rules fordesign for punching shear that permit thinner slabs and/orless shear reinforcement when studs are used instead ofstirrups.

REFERENCES1. Ghali, A., and Dilger, W. H., “Anchoring with Double-Head Studs,”

Concrete International, V. 20, No. 11, Nov. 1998, pp. 21-24.2. Joint ACI-ASCE Committee 421, “Shear Reinforcement for Slabs

(ACI 421.1R-99),” American Concrete Institute, Farmington Hills, Mich.,1999, 15 pp.

3. Megally, S., and Ghali, A., “Seismic Behavior of Edge Column-SlabConnections with Stud Shear Reinforcement,” ACI Structural Journal,V. 97, No. 1, Jan.-Feb. 2000, pp. 53-60.

4. Dilger, W. H.; Ghali, A.; Youakim, S. A.; and Hammill, N., “HeadedStuds in Anchor Zones of Post-Tensioned Slabs,” Concrete International,V. 27, No. 4, Apr. 2005, pp. 45-50.

5. Gayed, R. B., and Ghali, A., “Double-Head Studs as Shear Reinforce-ment in Concrete I-Beams,” ACI Structural Journal, V. 101, No. 4, July-Aug. 2004, pp. 549-557.

6. Berner, D. E., and Hoff, G. C., “Headed Reinforcement in DisturbedStrain Regions of Concrete Members,” Concrete International, V. 16,No. 1, Jan. 1994, pp. 48-52.

7. Dilger, W. H., and Ghali, A., “Double-Head Studs as Ties in ConcreteWalls and Columns,” Concrete International, V. 19, No. 6, June 1997,pp. 59-66.

8. Youakim, S. A., and Ghali, A., “Ductility of Concrete Columns withDouble-Head Studs,” ACI Structural Journal, V. 99, No. 4, July-Aug. 2002,pp. 480-487.

9. Youakim, S. A., and Ghali, A., “Behavior of Concrete Columns withDouble-Head Studs Under Earthquake Loading: Parametric Study,” ACIStructural Journal, V. 100, No. 6, Nov.-Dec. 2003, pp. 795-803.

10. Kuchma, D. A., and Collins, M. P., “The Influence of T-Headed Barson the Strength and Ductility of Reinforced Concrete Wall Elements,” ACISpring Convention, Seattle, Wash., Apr. 1997, 30 pp.

11. Mobeen, S.; Elwi, A.; and Ghali, A., “Double-Head Studs in Shear-walls,” Concrete International, V. 27, No. 3, Mar. 2005, pp. 59-63.

12. Birkle, G.; Ghali, A.; and Schäfer, K., “Double-Head Studs ImproveCorbel Reinforcement,” Concrete International, V. 24, No. 9, Sept. 2002,pp. 77-84.

13. Naito, C. J.; Moehle, J. P.; and Mosalam, K. M., “Evaluation ofBridge Beam-Column Joints under Simulated Seismic Loading,” ACIStructural Journal, V. 99, No. 1, Jan.-Feb. 2002, pp. 62-71.

14. Wallace, J. W.; McConnell, S. W.; Gupta, P.; and Cote, P. A., “Use ofHeaded Reinforcement in Beam-Column Joints Subjected to EarthquakeLoads,” ACI Structural Journal, V. 95, No. 5, Sept.-Oct. 1998, pp. 590-606.

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16. Herzinger, R. M. and Elbadry, M. M., “Stud Reinforcement in

Table 1—Anchorage of bars by combination of bond and bearing*

( fs head /fy) 0.95 0.90 0.85 0.80 0.75 ≥ 0.7

la/ld 1/6 1/3 1/2 2/3 5/6 1*Development length la expressed as fraction of development length ld of non-headed bars.


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18. DeVries, R. A., “Anchorage of Headed Reinforcement in Con-crete,” PhD dissertation, The University of Texas at Austin, Austin, Tex.,1996, 294 pp.

19. Bashandy, T. R., “Application of Headed Bars in Concrete Mem-bers,” PhD dissertation, The University of Texas at Austin, Austin, Tex.,1996, 303 pp.

20. Thompson, M. K., “The Anchorage Behavior of Headed Reinforcementin CCT Nodes and Lap Splices,” PhD dissertation, The University of Texasat Austin, Austin, Tex., 2002, 503 pp.

21. ACI Committee 318, “Building Code Requirements for StructuralConcrete (ACI 318-05) and Commentary (318R-05),” American ConcreteInstitute, Farmington Hills, Mich., 2005, 430 pp.

22. Leonhardt, F., and Walther, R., “Welded Wire Mesh as StirrupReinforcements—Shear Tests on T-Beams and Anchorage Tests,” Bautechnik,V. 42, Oct. 1965. (in German).

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26. IBC-03, “International Building Code,” International Code Council,Ill., 2003, 655 pp.

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30. Haroun, M.; Pardoen, G.; Bhatia, H.; Shahi, S.; and Kazanjy, R.,“Structural Behavior of Repaired Pier Walls,” ACI Structural Journal,V. 97, No. 2, Mar.-Apr. 2000, pp. 259-267.

31. Armstrong, S. D.; Salas, R. M.; Wood, B. A.; Breen, J. E.; andKreger, M. E., “Behavior and Design of Large Structural Concrete BridgePier Overhangs,” Center for Transportation Research Report CTR-1364-1,Austin, Tex., 1997.

32. Wood, B. A.; Kreger, M. E.; and Breen, J. E., “ExperimentalInvestigation of Design Methods for Large Cantilever Bridge Bents,” Centerfor Transportation Research Report CTR-1364-3F, Austin, Tex., 1997.

33. Sanders, D. H.; Breen, J. E.; and Duncan, R. R., “Strength andBehavior of Closely-Spaced Post-Tensioned Monostrand Anchorages,”Post-Tensioning Institute, Phoenix, Ariz., Oct. 1987, 49 pp.

34. Post-Tensioning Institute, “Anchorage Zone Design,” Post-TensioningManual, 6th Edition, 2002, 51 pp.

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36. Ingham, J. M.; Priestley, M. J. N.; and Seible, F., “Seismic Performanceof a Bridge Knee Joint Reinforced with Headed Reinforcement,” ReportNo. SSRP-96/06, University of California, San Diego, Structural SystemsProject, La Jolla, Calif., Sept. 1996, 113 pp.

37. Sritharan, S., and Priestley, M. J. N., “Seismic Testing of a Full-ScalePile-Deck Connection Utilizing Headed Reinforcement,” Report No. TR-98/14,University of California, San Diego, Structural Systems Project, La Jolla,Calif., Aug. 1998.

38. Marques, J. L. G., and Jirsa, J. O., “A Study of Hooked Bar Anchoragesin Beam-Column Joints,” ACI JOURNAL, Proceedings V. 72, No. 5, May1975, pp. 198-209.

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