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FindArticles / Business / ACI Structural Journal / Sep/Oct 2005 Headed Studs in Concrete: State of the Art by Ghali, Amin, Youakim, Samer A Comments Tweet The use of headed studs is proposed for several practical applications instead of conventional reinforcing bars anchored by hooks and bends. The main advantages are: simpler installation and less congestion of reinforcement and more effective anchorage. Experiments simulating the applications are discussed. Design recommendations are given. The paper discusses applications of headed studs in slabs and footings, beams with thin webs, crossties in columns and walls, precast beams, deep beams and pile caps, and in beam-column joints. Use of a headed bar, as opposed to a bar with a hook, is advantageous in applications where there is demand for the yield strength at a section of the bar close to its end. Keywords: anchorage; development length; stirrups; studs; reinforcement. (ProQuest Information and Learning: ... denotes formulae omitted.) INTRODUCTION Headed studs are increasingly used to replace conventional reinforcement (Fig. 1). Headed studs are smooth or deformed bars, commonly short relative to the lengths of concrete members, and provided with forged or welded heads for anchorage at one or both ends. In this paper, the terms headed stud and headed bar have the same meaning. In many applications, the studs run in the transverse direction of the member. Projects in which headed studs have been used include offshore structures, bridges, and thousands of flat plates in Europe, Australia, East Asia, and North America. Headed studs can also be used advantageously to reduce congestion in beam-column joints and in zones of lap splices. Requirements for anchorage can create detailing problems due to the long development length or the presence of hooks and bends. The present paper reviews practical applications in which headed studs can be used to replace conventional reinforcing bars. Experimental research at the University of Calgary and at other research institutions, are reviewed. The specimens in these experiments represent practical 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 of beams.15,16 Experiments17 have shown that an anchor head area equal to 9 or 10 times the cross- sectional area of the stem can provide secure mechanical anchorage with negligible slip and develop the full yield force for studs of yield stress [function of]^sub y^ up to 500 MPa. With this type of stud, the full yield strength of the studs can be employed immediately adjacent to the anchor head. A tapered head (Fig. 1(b)) with a maximum thickness at the stem [congruent with] 0.6 the diameter of the stem d^sub b^ is sufficient for strength. Page 1 of 4 Headed Studs in Concrete: State of the Art | ACI Structural Journal | Find Articles at ... 8/12/2011 http://findarticles.com/p/articles/mi_qa5310/is_200509/ai_n21383574/?tag=mantle_ski ... Minimizing the volume of the stud head simplifies its production by forging and reduces the congestion of the reinforcement in the concrete forms. Research at The University of Texas at Austin18-20 has shown that studs with smaller anchor heads and deformed stem can also be used in some applications, considering in their design that the full yield strength is developed at a specified development length away from the head. SIGNIFICANCE OF THE REVIEW OF STATE OF THE ART The ACI 318 Code21 allows the use of mechanical anchorages that are "capable of developing the strength of the reinforcement without damage to concrete." Designers, increasingly using the headed studs, cannot take full advantage of the superiority of anchorage when adhering to code's requirements. This is because the code does not allow the use of smaller amounts of reinforcement or larger spacing when headed bars are used. The present review of extensive research that shows many uses of headed bars and gives design recommendations should be of help to designers and writers of codes or technical reports. ANCHORAGE OF BARS Anchorage of reinforcing bars is often achieved by the use of 90-, 135-, or 180-degree hooks. If the tensile force and the stress developed in the hook are T and ^sub s^, respectively, a radial force T/R per unit length is exerted by the bar on the concrete inside the bend; where R is the inner radius of the bend. The average bearing stress on the concrete is T/(Rd^sub b^); where d^sub b^ is the diameter of the bar. The ACI 318 Code21 requires that R 2d^sub b^ for d^sub b^ 5/8 in. (16 mm). With this radius, the average bearing stress on the concrete is ( ^sub s^ d^sub b^^sup 2^/4)/(2d^sub b^^sup 2^) = 0.4 ^sub s^. When ^sub s^ approaches the yield strength [function of]^sub y^ of the bar, the bearing stress can damage (split or crush) the concrete inside the bend and result in bend slip; thus, the hook cannot develop the stress [function of]^sub y^ in the bar. For this reason, building codes such as ACI 318-0521 require minimum values for the inner radius R and in many applications require that the bend engage a heavier bar, running perpendicular to the plane of the bend (Fig. 1(a)). Even when this requirement is satisfied, the slip that occurs at the hooks causes the full yield strength of the bars to be developed only at some distance away from the bends. Leonhardt and Walther22 measured the slip that occurs at the bends of 90-, 135-, and 180- degree hooks, when engaging heavier bars lodged inside the bends. At stress level of ^sub s^ = 400 MPa (60 ksi), with a concrete strength of [function of]'^sub c^ = 25 MPa (3600 psi), the measured slip varied between 0.1 and 0.25 mm (0.004 and 0.010 in.) and increased rapidly with the increase of ^sub s^, reaching between 0.2 and 0.9 mm at ^sub s^ = 500 MPa (0.008 and 0.035 in. at 70 ksi). With the headed studs in Fig. 1(b), Eligehausen17 measured slip varying between 0.013 and 0.033 mm at ^sub s^ = 400 MPa and between 0.023 and 0.045 mm at ^sub s^ = 500 MPa (0.5 × 10-3 and 1.3 × 10^sup -3^ in. at 60 ksi and between 0.9 and 1.8 × 10^sup -3^ in. at ^sub s^ = 70 ksi), with [function of]'^sub c^ = 25 MPa (3600 psi). The lower bearing stress and the smaller slip make studs with a head at each end more effective than conventional stirrups in controlling concrete cracks that intersect the stems at any location between the heads (for example, cracks due to shear or splitting forces). ADVANTAGES OF HEADED STUDS When headed studs are used, the congestion and the time of installation can be reduced by the use of a smaller number of studs of larger diameter. For speedy and accurate Page 2 of 4 Headed Studs in Concrete: State of the Art | ACI Structural Journal | Find Articles at ... 8/12/2011 http://findarticles.com/p/articles/mi_qa5310/is_200509/ai_n21383574/?tag=mantle_ski ...
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Page 1: Headed Stubs in Concrete

FindArticles / Business / ACI Structural Journal / Sep/Oct 2005

Headed Studs in Concrete: State of theArt

by Ghali, Amin, Youakim, Samer A

CommentsTweet

The use of headed studs is proposed for several practical applications instead ofconventional reinforcing bars anchored by hooks and bends. The main advantages are:simpler installation and less congestion of reinforcement and more effective anchorage.Experiments simulating the applications are discussed. Design recommendations are given.The paper discusses applications of headed studs in slabs and footings, beams with thinwebs, crossties in columns and walls, precast beams, deep beams and pile caps, and inbeam-column joints. Use of a headed bar, as opposed to a bar with a hook, is advantageousin applications where there is demand for the yield strength at a section of the bar close toits end.

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

(ProQuest Information and Learning: ... denotes formulae omitted.)

INTRODUCTION

Headed studs are increasingly used to replace conventional reinforcement (Fig. 1). Headedstuds are smooth or deformed bars, commonly short relative to the lengths of concretemembers, and provided with forged or welded heads for anchorage at one or both ends. Inthis paper, the terms headed stud and headed bar have the same meaning. In manyapplications, the studs run in the transverse direction of the member. Projects in whichheaded studs have been used include offshore structures, bridges, and thousands of flatplates in Europe, Australia, East Asia, and North America.

Headed studs can also be used advantageously to reduce congestion in beam-column jointsand in zones of lap splices. Requirements for anchorage can create detailing problems dueto the long development length or the presence of hooks and bends. The present paperreviews practical applications in which headed studs can be used to replace conventionalreinforcing bars. Experimental research at the University of Calgary and at other researchinstitutions, are reviewed. The specimens in these experiments represent practicalapplications of headed studs in slabs,1-4 beams,5,6 columns,7-9 walls,10 structuraldiaphragms (shearwalls),11 corbels,12 beam-column joints,13,14 and dapped ends ofbeams.15,16

Experiments17 have shown that an anchor head area equal to 9 or 10 times the cross-sectional area of the stem can provide secure mechanical anchorage with negligible slip anddevelop the full yield force for studs of yield stress [function of]^sub y^ up to 500 MPa.With this type of stud, the full yield strength of the studs can be employed immediatelyadjacent to the anchor head. A tapered head (Fig. 1(b)) with a maximum thickness at thestem [congruent with] 0.6 the diameter of the stem d^sub b^ is sufficient for strength.

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Minimizing the volume of the stud head simplifies its production by forging and reduces thecongestion of the reinforcement in the concrete forms. Research at The University of Texasat Austin18-20 has shown that studs with smaller anchor heads and deformed stem can alsobe used in some applications, considering in their design that the full yield strength isdeveloped at a specified development length away from the head.

SIGNIFICANCE OF THE REVIEW OF STATE OF THE ART

The ACI 318 Code21 allows the use of mechanical anchorages that are "capable ofdeveloping the strength of the reinforcement without damage to concrete." Designers,increasingly using the headed studs, cannot take full advantage of the superiority ofanchorage when adhering to code's requirements. This is because the code does not allowthe use of smaller amounts of reinforcement or larger spacing when headed bars are used.The present review of extensive research that shows many uses of headed bars and givesdesign recommendations should be of help to designers and writers of codes or technicalreports.

ANCHORAGE OF BARS

Anchorage of reinforcing bars is often achieved by the use of 90-, 135-, or 180-degree hooks.If the tensile force and the stress developed in the hook are T and ^sub s^, respectively, aradial force T/R per unit length is exerted by the bar on the concrete inside the bend; whereR is the inner radius of the bend. The average bearing stress on the concrete is T/(Rd^subb^); where d^sub b^ is the diameter of the bar. The ACI 318 Code21 requires that R2d^sub b^ for d^sub b^ 5/8 in. (16 mm). With this radius, the average bearing stress onthe concrete is ( ^sub s^ d^sub b^^sup 2^/4)/(2d^sub b^^sup 2^) = 0.4 ^sub s^.When ^sub s^ approaches the yield strength [function of]^sub y^ of the bar, the bearingstress can damage (split or crush) the concrete inside the bend and result in bend slip; thus,the hook cannot develop the stress [function of]^sub y^ in the bar. For this reason, buildingcodes such as ACI 318-0521 require minimum values for the inner radius R and in manyapplications require that the bend engage a heavier bar, running perpendicular to the planeof the bend (Fig. 1(a)). Even when this requirement is satisfied, the slip that occurs at thehooks causes the full yield strength of the bars to be developed only at some distance awayfrom the bends.

Leonhardt and Walther22 measured the slip that occurs at the bends of 90-, 135-, and 180-degree hooks, when engaging heavier bars lodged inside the bends. At stress level of ^subs^ = 400 MPa (60 ksi), with a concrete strength of [function of]'^sub c^ = 25 MPa (3600psi), the measured slip varied between 0.1 and 0.25 mm (0.004 and 0.010 in.) andincreased rapidly with the increase of ^sub s^, reaching between 0.2 and 0.9 mm at ^subs^ = 500 MPa (0.008 and 0.035 in. at 70 ksi). With the headed studs in Fig. 1(b),Eligehausen17 measured slip varying between 0.013 and 0.033 mm at ^sub s^ = 400 MPaand between 0.023 and 0.045 mm at ^sub s^ = 500 MPa (0.5 × 10-3 and 1.3 × 10^sup -3^in. at 60 ksi and between 0.9 and 1.8 × 10^sup -3^ in. at ^sub s^ = 70 ksi), with [functionof]'^sub c^ = 25 MPa (3600 psi). The lower bearing stress and the smaller slip make studswith a head at each end more effective than conventional stirrups in controlling concretecracks that intersect the stems at any location between the heads (for example, cracks dueto shear or splitting forces).

ADVANTAGES OF HEADED STUDS

When headed studs are used, the congestion and the time of installation can be reduced bythe use of a smaller number of studs of larger diameter. For speedy and accurate

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Page 2: Headed Stubs in Concrete

installation, sets of double-headed studs can be fitted at specified spacing in nonstructuralsheet 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 theanchorage. This mechanical participation to the anchorage, however, can be partly lostwhen, because of imprecise workmanship, the heavier bar is not in contact with the innerface of the hook. With studs, the head provides positive anchorage, without the need forenhancement.

A stud is longer than the vertical effective part of a stirrup (compare Fig. 1(a) and (b)) andthus can intersect more shear cracks. A crack approaching a stirrup leg near a bend tends tofollow the bend, rather than intersecting the leg and controlling the width of the crack. Thecover to the longitudinal bars has to be greater than the specified minimum plus thediameter of the stirrups (Fig. 1(a)); thus, when stirrups are used in lieu of studs, thedistance d between the centroid of the tensile reinforcement and the extreme compressionfiber will have to be smaller by an amount equal to the diameter of the stirrups. Thereduction in flexural and shear strength of the member, caused by the smaller d, has to becompensated for by the provision of a greater amount of flexural and shear reinforcements;the added amount can be significant in thin slabs.

APPLICATIONS

Punching shear of slabs and footings

Figure 2(a) and (b) show two types of stud shear reinforcement (SSR) widely used in slabsand footings in many countries. The studs in Fig. 2(a) have forged heads at one end; at theother end, the studs are welded to a rail (steel strip) that serves for anchorage and holdingthe studs vertically at the appropriate spacing. The studs in Fig. 2(b) have forged heads ateach end; the heads at the lower end snugly fit in a sheet metal trough (or in othernonstructural elements) that serves as a spacer. Typical arrangement of the studs in plan toresist punching shear at an interior column in slabs or footings is shown in Fig. 2(c). Theshear-reinforced zone should extend outwards from the column to the vicinity of a criticalsection at which the shear stress due to the transfer of factored shear force combined withfactored unbalanced moment does not exceed [straight phi]v^sub c^; where [straight phi]is the strength reduction factor and v^sub c^ is the nominal shear strength of concrete.(According to ACI 318-05, [straight phi] = 0.7 and

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Page 3: Headed Stubs in Concrete

FindArticles / Business / ACI Structural Journal / Sep/Oct 2005

Headed Studs in Concrete: State of theArt

by Ghali, Amin, Youakim, Samer A

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...

The vertical section in Fig. 2(c), in the shear-reinforced zone of a slab, shows the position ofthe SSR relative to other reinforcements. For best performance, ACI 421.1R-992recommends an optimum height of the stud equal to the thickness of the slab or footingminus the sum of the minimum specified covers with a tolerance equal to minus one-halfthe diameter of the flexural reinforcing bars. In slabs, the SSR are commonly fastened withtheir rail or trough on wood forms, before the installation of other slab reinforcement.Alternatively, particularly in footings, the SSR can be supported by the top reinforcement inan inverted position (with the rail or trough at the top).

ACI 318-05 considers (in most cases) that the nominal shear strength at the critical sectionat d/2 from the column face is equal to

...

when studs are used as shear reinforcement, ACI 421.1R-99 recommends that v^sub n^ beless than or equal to

...

It also recommends that within the shear-reinforced zone v^sub c^ be equal to

...

When stirrups are used, lower stresses are permitted by ACI 318-05

...

...

This is because the studs are more efficient than stirrups in concrete confinement. Inaddition, ACI 421.1R-99 allows the spacing between studs to be 0.75d compared with 0.5dfor stirrups. These differences in design rules permit thinner slabs or require less amountsof shear reinforcement when studs are used.

Beams with thin webs

The thickness of the web of precast beams is often governed by constructability rather than

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by strength requirement. For ease in installation of the reinforcement and in castingconcrete, the web should be wide enough to accommodate the two legs of closed stirrups,the minimum side covers, and a sufficient space in between for casting and vibrating theconcrete. Additional width is required to accommodate draped pretensioned strands orducts of post-tensioned tendons. The web thickness can be reduced by replacing the legs ofconventional stirrups by double-headed studs (Fig. 3(a)). Draped external post-tensionedtendons can be located adjacent to the two sides of the web.

Modern precast pretensioned girders,23 widely used in bridge decks, are made continuousby post-tensioned strands inserted in sheet metal ducts located in the midsurface of theweb. For ease in construction, the thickness of the web cannot be much less than 175 mm (7in.). Figure 3(b) shows an alternative design24 using external post-tensioned tendons anddouble-headed studs in midsurface of a web of thickness 100 mm (4 in.).

ACI 318-05 permits shear reinforcement spacing not exceeding d/2 or 3h/4 fornonprestressed or prestressed beams, respectively; h is the overall thickness of the member.Fabrication and accommodation of hooks of bars of diameter 16 mm (5/8 in.) is relativelydifficult. For this reason, the spacing between stirrups in bridge I-girders is controlled bythe practical bar diameter rather than code requirements. In these cases, one double-headed stud of diameter 25 to 30 mm (1 to 1-1/4 in.) can be used to replace several stirruplegs. The advantage is saving in the labor cost of installation of reinforcement.

Crossties in columns and walls

Double-headed studs are used in Fig. 4 as crossties in columns and walls. Each stud is asubstitute for one or more single-leg stirrup(s) (Fig. 1(a)). In columns, the conventionalclosed stirrup following the perimeter of the cross sections should be maintained, with thestuds 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 in Fig. 4(c). For ease in installation ofreinforcement, the heads of studs may be placed adjacent to the vertical bars in columns, asshown in Fig. 4(a) and (b); but this is not a requirement to enhance the anchorage of thestuds. Experiments on concrete columns under concentric compression loading7 and undersimulated-seismic loading8 have shown that placing the vertical bars behind the heads issufficient to prevent premature buckling of the vertical bars after spalling of concrete cover.Columns with headed studs as crossties have exhibited improved ductility and equal orgreater strength than companion columns with conventional tie reinforcement.

Shearwalls

Reinforced concrete structural diaphragms (shearwalls) resisting lateral forces in buildingsare subjected to compressive axial forces due to gravity loads combined with reversiblebending moments. This combination causes concentration of normal stresses, often resistedby boundary elements containing high reinforcement ratios of vertical bars andconfinement ties. ACI 318-05 specifies the volumetric ratio and the spacing of ties.Accordingly, the boundary elements of structural diaphragms are in many cases, especiallyin earthquake zones, congested with heavy vertical bars, closed stirrups, and crossties.

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Page 4: Headed Stubs in Concrete

FindArticles / Business / ACI Structural Journal / Sep/Oct 2005

Headed Studs in Concrete: State of theArt

by Ghali, Amin, Youakim, Samer A

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Figure 5 shows horizontal sections of tested11 shearwalls having 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 in Fig. 5(b). Almost the samestrength and improved ductility were observed for the walls with the studs.

Corbels

A corbel is a short cantilever often supporting a precast beam on a bearing plate, exertingfactored vertical and horizontal forces V^sub u^ and N^sub u^ (Fig. 6(a)). The strut-and-tie model presented in the figure has been shown12 to be an accurate design tool forcorbels. The distance between the bearing plate and the tip of the corbel is not sufficient todevelop the tensile force T in the top reinforcement. Welded cross bars or plates orhorizontal loops are often used to enhance the anchorage. Headed studs resisting the tensileforce T offer anchorage without congestion of conventional reinforcement. Figure 6(b)represents details of a tested corbel specimen and also represents a reduced model of acorbel supporting two precast girders.

Precast beams

Frequently, over a short length at the ends, the depth of precast beams is drasticallyreduced (refer to Fig. 7(a) representing a dapped end). The beams are commonly simplysupported on bearing plates. Similar to corbels, dapped beam ends must be designed tocarry factored forces V^sub u^ and N^sub u^. Again, the strut-and-tie modeling is avaluable design tool. The model shown in Fig. 7(a) and the reinforcement arrangement inFig. 7(b), using headed studs, have been proposed for design.15 Recent tests at theUniversity of Calgary verified the proposed reinforcing system and its capacity using boththe strut-and-tie model and the shear friction method.16, 25

Splitting forces

Headed studs can be used to control cracking due to splitting forces caused by concentratedloading at prestressing anchors and at support bearings. The main advantage is thereduction of congestion of reinforcement shown in Fig. 8.

Figure 9(a) shows the distribution of vertical tensile stresses at the anchorage ofprestressing tendons in a concrete slab. The potential splitting of the slab in a horizontalplane near the middle surface is indicated. The anchor zone of a band of single-strandprestressing tendons is shown in Fig. 9(b). The congestion of reinforcement in Fig. 8 ispartly caused by hair-pin stirrups. In modern construction, the hairpin stirrups are replacedby vertical headed studs (Fig. 10(a) to (c)).

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Deep beams and pile caps

Figure 11(a) represents a strut-and-tie model for the design of deep beam or a two-pile cap.A free-body diagram of Node A is shown. Often, the size of the node and the dimensions ofthe cap are not sufficient to anchor the tie by bond. In Fig. 11(a), the tie consists of plain(non-deformed) studs with heads located outside the node. With this arrangement, theanchorage of the stud relies solely on the bearing stress at the head. With head area 9 to 10times the area of the stem, the three faces of the concrete prism representing node A can beconsidered subjected to compressive stress (C-C-C node); the anchor heads of the studscreate the compression on the vertical face of the prism. ACI 318-05 permits higher stressfor a C-C-C node, compared to the C-C-T node that will exist when the tie is anchored bybond within the node.

Beam-column joints

Two connections of beams to columns are shown in Fig. 11(b) and (c). Single-headed studsare used for anchorage of the longitudinal bars of the beams and the columns to avoidcongestion within the joint. Away from the joints, lap splices relying on bond or other typesof splices can be used to extend the studs longitudinally in the beams or the columns.Tests13,14 subjecting the connections to the transfer of reversible moments have verifiedthe suitability of single-headed studs for use in seismic zones. Headed studs are widely usedin California in the connections of bridge piers to their superstructures.

Other applications

It is advantageous to use a bar anchored mechanically by a head, as opposed to a hook,when there is demand for the yield strength at a section of the bar close to its end. Theprevious example applications do not cover all uses of headed bars.

EXPERIMENTAL VERIFICATIONS

Results of some experiments that study the behavior of structures reinforced by headedstuds are reviewed in the following.

Slab punching shear3

The specimen in the inset of Fig. 12(a) (representing the connection of a reinforced concreteslab to an edge column extending above and below the slab) was simply supported on threeedges. The column transferred to the slab a constant shearing force V representing gravityload and a reversible unbalanced moment M representing the effect of an earthquake.Shearing force V and unbalanced moment M were gradually increased, with M/V =constant, until a target serviceability shearing force V^sub u^ was reached. Then cyclicdisplacements of increasing amplitude were imposed at the ends of the columns to producethe unbalanced moment. Cyclic moment transfer was continued after the peak momentM^sub u^ until the loss of 25% of M^sub u^.

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Page 5: Headed Stubs in Concrete

FindArticles / Business / ACI Structural Journal / Sep/Oct 2005

Headed Studs in Concrete: State of theArt

by Ghali, Amin, Youakim, Samer A

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Figure 12(a) and (b) compare the graphs3 of M versus the drift ratio for a slab without shearreinforcement and a slab having SSR (stud-rails having an area equal to nine times thecross-sectional area of the stem) arranged as shown in the inset of Fig. 12(b). In thecompared tests, the value of V^sub u^ = 0.6V^sub c^, with V^sub c^ being the nominalshear strength, without moment transfer or shear reinforcement according to ACI 318-05.Provision of SSR reduced significantly the rate of stiffness degradation (the slope of theascending parts of the loops) due to the cyclic moment reversals. At 1.5% drift ratio, thestiffness of connection without SSR was approximately 50% of the stiffness with SSR. At 3%drift ratio, the stiffness of the connection without SSR was almost lost. The drift ratio withSSR reached approximately 4% without appreciable loss of strength. This is higher thanwhat is commonly expected in a major earthquake. After the cyclic loading described above,the slab with SSR was subjected to V combined with M, at a constant M/V ratio, in loadcontrol to examine its residual strength. It was concluded that with SSR, the shearresistance to gravity load is maintained after severe earthquake. The highest drift ratiopermitted by IBC26 for concrete flat plate supported directly on columns is 2.5%; such astructure must have shearwalls or other bracing systems that limit the drift ratio to thevalue 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 from300 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 in Fig. 3(a). The stirrups had 90-and 135-degree hooks at the ends; the studs were made of straight bars cut from the samestock used for fabricating the stirrups and were welded to circular heads of area nine timesthe cross-sectional area of the stem. All the beams failed in shear, as planned. In terms ofstrength and ductility, the performance of the beams with studs was equal to or slightlybetter than the beams with stirrups. The advantages of studs are ease of installation andcontrol.

Berner and Hoff6 presented results of tests on three large-scale specimens representing astrip of approximate width and depth 1.0 x 0.7 m (40 x 28 in.) of a wall for offshoreplatforms. The specimens were restrained at the ends to behave as horizontal continuousdeep beams of "nominal" center-to-center spans of 2.8 m (110 in.). The three beams hadidentical longitudinal top and bottom reinforcement. Vertical headed studs were used asshear reinforcement, of ratio 1, 1.5, and 2%. The ultimate central load, over a length equal tohalf the length of the clear span, was more than twice the value permitted by ACI 318 andthe measured strengths increased with an increase in the shear reinforcement ratio. Theauthors concluded that the codes needed to be changed to reflect the superior behavior ofthe headed studs; in particular, ACI 318 unnecessarily limited [function of]^sub y^ for the

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shear reinforcement to 413 MPa (60 ksi) and its contribution to the shear strength to(2/3) ....

Columns7

Figure 13(a) to (d) show the cross sections of five specimens representing short columnstested in axial compression. The objective was to compare the confinement effect of double-headed studs with that of single-leg stirrups with 90- and 180-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 reinforcement without vertical bars;Specimens 4 and 5, respectively, had stirrups and studs as confinement reinforcement inaddition to vertical bars. Closed stirrups, following the perimeter of the cross section, wereprovided in Specimens 2, 3, 4, and 5. The concrete strength f ' for the five specimens was 20MPa (3000 psi). The stirrups and the studs were made from bars of diameter 5.7 mm (0.20in.) and yield strength of 595 MPa (86 ksi). The diameter of the stud heads was 18 mm (0.7in.).

Graphs for the load versus the axial strain in the five specimens are shown in Fig. 13(e). Thefailure load varied between 1580 and 2100 kN (356 and 472 kips), corresponding,respectively, to Specimen 1 (unconfined) and Specimen 5 (with vertical bars and studs). Atfailure of Specimen 3 (with studs but no vertical bars), spalling of the cover and horizontalcracking occurred, while the core remained intact. This is contrary to Specimen 2 (withstirrups but no vertical bars) where diagonal cracks traversed the thickness of the specimen.Spalling of the cover of Specimen 4 (with stirrups and vertical bars) occurred at the 90-degree hooks, where their ends popped out of the cover and protruded from the columnface. Similar observations were reported by other researchers.27,28 The strainmeasurements indicated yielding of the studs; the maximum strain in the single-leg stirrupswas well below the yield strain.

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Page 6: Headed Stubs in Concrete

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Headed Studs in Concrete: State of theArt

by Ghali, Amin, Youakim, Samer A

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From Fig. 13(e) and the test observations, it was concluded that double-headed studs ascrossties, while not requiring vertical bars behind the heads to enhance anchorage, exhibitlarge strain beyond yield at failure load of the column. Anchorage of crossties by 180- and90-degree hooks engaging heavier bars is not sufficient to develop the yield stress in theties. Columns exhibit better ductile behavior and greater ultimate strength when double-headed studs replace conventional crossties. Because of the superior performance of thestuds, codes should allow a reduced volumetric ratio and/or larger spacing when studs areused as crossties in lieu of stirrups.

Cyclic lateral loading of columns8,9

Nine column specimens, reinforced with either double-headed studs or conventionalcrossties, were tested under seismic loading.8,9 The columns had a cross section of 250 ×500 mm (10 × 20 in.) and a total height of 1500 mm (59 in.) and were laterally loaded tobend about their weak axis. The columns were subjected to a constant axial load Pcorresponding to either 20 or 30% of their nominal axial capacity Po combined withincrementally increasing lateral-displacement reversals. The columns were made from 25MPa (3600 psi) concrete and had a longitudinal reinforcement ratio of 1.3%. Weldingcircular plates with a diameter three times that of the stem produced the double-headedstuds. The same stock of bars was used for the stem of the studs and the stirrups. It wasshown that, while columns with either type of lateral reinforcement attained the samestrength, columns with double-headed studs exhibited superior behavior in terms ofductility and energy dissipation. All column cross sections had a closed peripheral stirrup.Several columns had two single-leg stirrups as crossties. In Column SD-6, shown in Fig. 4(d), a single double-headed stud, as shown in Fig. 4(d), replaced the two crossties. AlthoughColumn SD-6 contained almost half the volumetric ratio of ties of columns with single-legstirrups and half the minimum amount required by ACI 318-05 for seismic design, theultimate capacity and ductility of Column SD-6 were similar to the other column specimens.This shows that ACI 318 requirements for confinement reinforcement are overlyconservative for columns subjected to axial loads levels less than 30% of their nominalcapacity.

Walls10

Sixteen wall elements were tested at the University of Toronto10 under monotonic in-planevertical compression, with some of the elements being subjected to vertical compressioncombined with horizontal in-plane tension. In addition to the reinforcement layers runningparallel to the surfaces, eight walls were confined with double-headed studs running in thedirection normal to the wall surfaces; the other eight walls did not contain confining studs.The stud heads enclosed the reinforcement layers and had an area approximately equal to

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nine times the cross-sectional area of the stem. It was concluded that the double-headedstuds increased both the strength and the ductility of the wall elements. Based on theexperimental results, an analytical model was developed to predict the compressivestrength of wall elements confined with headed studs.

Repair and rehabilitation29,30

A series of circular columns29 representing bridge piers were severely damaged undersimulated-earthquake loading, and then repaired and tested again. One of the repairtechniques involved placement of a strong jacket along the damaged region so futureflexural hinging would be forced to occur just above the jacket. To ensure that flexuralyielding of longitudinal reinforcement would not occur at the column base, the jacketedregion was reinforced with headed studs to avoid congestion of reinforcement. Subsequenttesting of the repaired columns showed that their stiffness and strength were comparable tothose of the original ones.

Six pier walls were loaded in the weak direction under cyclic loading to near failure.30 Fiveof the damaged pier walls were repaired with conventional crossties with 90- and 135-degree hooks; one wall was repaired with double-headed studs as crossties. The area of theheads was 13 times the area of the stem. The six repaired pier walls were retested under thesame loading conditions to compare their performance. Due to the additional confinementprovided by the heads, the wall repaired with studs performed better than similar wallsrepaired with conventional crossties. It was also found that the heads provided sufficientanchorage without the need to engage the longitudinal bars of the walls.

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Page 7: Headed Stubs in Concrete

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Headed Studs in Concrete: State of theArt

by Ghali, Amin, Youakim, Samer A

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Cyclic lateral loading on shearwalls11

A sustained 1000 kN axial load was applied on the shear-walls shown in Fig. 5, while at 3.3m above the base, the walls were pushed back and forth to produce imposed reversals ofhorizontal top displacement of increasing amplitude. The stud heads had an area equal tonine times the cross-sectional area of the stem. The walls, having the same volumetric ratioof transverse reinforcement, attained almost the same ultimate lateral strength anddisplacement; the envelope curves of the lateral force-displacement relationship for bothwalls were quite similar. The wall with double-headed studs, however, displayed betterenergy dissipation capacity (determined by the summation of the areas enclosed by thelateral force-displacement hysteresis loops). This research confirmed that double-headedstuds could be a substitute for single-leg stirrups as crossties in the boundary elements ofshearwalls.

Vertical and horizontal forces on corbels12

Six corbels with the dimensions shown in Fig. 6(b) (with studs having head area equal tonine times the cross-sectional area of the stud) were subjected to vertical forces V^sub u^combined with horizontal forces N^sub u^ = V^sub u^/5. The magnitudes of V^sub u^and N^sub u^ were monotonically increased up to failure. The horizontal force representedthe reaction component that can develop due to shrinkage or temperature drop of a precastbeam supported by the corbel. The corbels were designed by the strut-and-tie model (Fig. 6(a)) to fail by the yielding of the tie or by crushing of the concrete at Node B. Plain ordeformed studs of 20 mm (0.8 in.) diameter with forged heads of 60 mm (2.4 in.) diameterwere used for the tie. The conclusion from this research was that both plain and deformeddouble-headed studs can be used as main tension reinforcement in corbels. Double-headedstuds placed in the compression zone in the direction normal to the corbel faces cansignificantly increase the ductility; this was confirmed by experiments at The University ofTexas at Austin31,32 on the overhangs of bridge piers.

Slab splitting at anchors of prestressing tendons4

Tests were conducted at The University of Texas at Austin33 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 were duplicated at the University ofCalgary,4 replacing the hairpin stirrups by headed studs (Fig. 10(b)). Seven 9.5 mmdiameter hairpin stirrups used in the Texas tests were replaced by the same number ofheaded studs of the same diameter, welded to a rail. The studs had forged heads of area tentimes the cross-sectional area of the stem. The edges of the specimens in Texas and inCalgary were subjected to compressive forces through a special adapter to closely simulate

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the anchors of a band of six strands.

The ultimate loads in the tests in Calgary were higher than those of the Texas tests. Severalstuds reached yielding before failure, indicating the effectiveness of the anchors. Due to theconfinement of concrete by the stud heads, the bearing stress under the anchor plates in thetests in Calgary could reach more than two times the compressive strength of the concrete.The conclusion was that headed studs are effective in the control of splitting cracks in theanchor zones of prestressed slabs. In addition, the studs provide confinement of theconcrete in the anchor zone. Equation (1) is suggested to give the cross-sectional area ofheaded studs A^sub sv^ required to control the splitting crack due to prestressing.

... (1)

where a is the vertical dimension of the anchor(s) of the prestressing tendon(s) (in Fig. 10(b)); h is the slab thickness; [function of]^sub y^ is the yield strength of the studs; and[function of]^sub pu^ and A^sub ps^ are the ultimate strength and cross-sectional area ofthe tendon. The maximum prestressing force applied to the anchor is assumed equal to 0.7[function of]^sub pu^A^sub ps^ according to the Post-Tensioning Manual.34 The studsshould be arranged at a distance 0.40h 0.55h from the anchor plate.

Beam-column joints13,14

Four interior bridge beam-column joints with either conventional or headed reinforcementwere tested under seismic loading13 to evaluate the current design requirements of theCalifornia Department of Transportation (Caltrans35). Headed studs were used within thejoints to resist shear stresses and to confine the concrete and were also used for longitudinalbars of the column. The head had a diameter equal to 3.2 times the diameter of the stem. Itwas concluded that headed studs produced comparable behavior to that of theconventionally reinforced joints; but the constructability of the joints was improved due tothe use of fewer bars with larger diameters and the elimination of the hooks. Similar testswere conducted at the University of California at San Diego on bridge column-beam kneejoints36 and pile-foundation connections37 with almost all the reinforcement consisting ofheaded studs. Again, the results demonstrated the effectiveness of headed studs in bridgejoints under seismic loads.

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Page 8: Headed Stubs in Concrete

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Headed Studs in Concrete: State of theArt

by Ghali, Amin, Youakim, Samer A

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Five beam-column corner joints and two exterior beam-column joints were tested underseismic loading14 to evaluate the potential of using headed studs as longitudinal beam andcolumn reinforcement within the joints. The area of the stud heads varied from 4 to 11 timesthe cross-sectional area of the stems. It was concluded that the behavior of joints withheaded reinforcement performed as good as or better than similar joints with 90 degreehooks.

Thirty-two tests on simulated beam-column joints were conducted by Bashandy19 toinvestigate the behavior of beam-column joints with headed stud anchorage. The tests weresimilar to earlier tests38,39 performed on hooked bar anchorage in beam-column joints. Itwas found that the anchorage performance of the headed studs was equivalent to or betterthan bars with conventional hooks.

ANCHORAGE BY COMBINATION OF BOND AND BEARING

In the research mentioned above, the studs were made of plain or deformed bars andmostly had head areas equal to nine or 10 times the area of the stem (Fig. 1(b)) and the ratio([function of]^sub y^/[function of]'^sub c^) was as high as 25; where [function of]^sub y^is the yield strength of the studs and [function of]'^sub c^ is the concrete strength. Indesign using such studs, nominal yield strength up to 500 MPa (72 ksi) can be consideredavailable at any section of the stem with [function of]'^sub c^ 20 MPa (2900 psi). Whenstuds of this type have nominal yield strength [function of]^sub y^ MPa (72 ksi), the full[function of]^sub y^ value may be considered available at any section of the stem, providedthat the ratio ([function of]^sub y^/[function of]'^sub c^) does not exceed 25; however,more tests are needed to verify this statement; note that in some of the tests mentionedabove, [function of]^sub y^ has exceeded 500 MPa (72 ksi).

Studs made of deformed bars and heads of smaller areas have been used in research at TheUniversity of Texas at Austin.18-20 An empirical equation was developed for the bondlength between the head and the section at which the nominal yield strength can beconsidered available. The equation is given in the following section.

SPLITTING OF COVER

A headed stud running parallel to an exterior surface of a concrete member is shown in Fig.14. For protection against corrosion or fire, the distance c between the centerline of the studand the surface must be greater than the radius of the head plus the specified clear coverc^sub c^. For example, when c^sub c^ = 20 mm (0.8 in.) and the diameter of the stud andits head are 20 and 60 mm (0.8 and 2.4 in.), respectively, c 50 mm (2 in.). When c issmall, the bearing stress behind the anchor head can cause splitting (side blow-out or

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spalling) of the cover and c needs to be greater than the required minimum for protection.Alternatively, spalling can be prevented by the use of closed stirrups in the planeperpendicular to the stud. The stirrups can be designed to resist a resultant splitting force of0.3T^sub y^, where T^sub y^ is the yield force of the stud. This empirical recommendationis based partly on analysis of the results of tests12 and partly on Eq. (1), assuming that thehead diameter is 3d^sub b^, c = 3d^sub b^, ae = 0.75(3d^sub b^) and the yield strength ofthe stud is developed by bearing at the head; Eq. (1) can be used, although it was notdeveloped for this application. The stirrups are to be arranged so that the resultant of theirforces4 is approximately at a distance c from the stud head, as shown in Fig. 14.

For a stud having a head of area equal to 9 or 10 times the area of the stem, splitting of thecover need not be of a concern when c 3.5d^sub b^; where d^sub b^ = stud diameter.Furthermore, the bar stress developed by the head can be assumed equal to [function of]^sub y^, provided that ([function of]^sub y^/[function of]'^sub c^) and the stud is close tono more than one exterior surface. This empirical recommendation is supported by anequation resulting from extensive testing by Thompson;20 the equation, given below, willshow that when (A^sub nh^/A^sub b^) 9 and c 3.5d^sub b^, the bar stress [functionof]^sub s head^ developed by the head can be equal to the yield strength [function of]^suby^ when ([function of]^sub y^/[function of]'^sub c^) 29; where A^sub nh^ = the netarea of the head (head area minus bar area A^sub b^). For the steel and concrete used inmost countries, ([function of]^sub y^/[function of]'^sub c^) is normally less than 29.

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Page 9: Headed Stubs in Concrete

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Headed Studs in Concrete: State of theArt

by Ghali, Amin, Youakim, Samer A

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Thompson20 conducted tests on 46 specimens representing a C-C-T node in a strut-and-tiedesign model. Headed deformed studs were used as tie reinforcement; the head area wasten times the cross-sectional area of the stud. Thompson also conducted tests on 27 lapsplices using deformed studs of 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 and parallel to one exterior surfaceor two orthogonal exterior surfaces of the concrete member. Based on the experiments,Thompson proposed that the stress developed by the bearing of the head [function of]^subs head^ can be computed by Eq. (2), dependent upon the cover or the side covers of the bar.

... (2)

... (3)

where c^sub 1^ = the minimum distance between the centerline of the bar and the surfaceof the member; c^sub 2^ = the minimum distance between the centerline of the bar and anexterior surface perpendicular to c^sub 1^; the distance c^sub 2^ is greater than c1. Whenthe bar runs parallel and close to only one exterior surface, set = 2.0 and replace c^sub1^ by c in Eq. (2); where c is the distance between the centerline of the bar and the exteriorsurface.

Consider a headed stud of yield strength [function of]^sub y^ = 500 MPa (72 ksi) used in aconcrete member with compressive strength [function of]'^sub c^ = 20 MPa (2900 psi);assume that the stud head area is equal to 10 times the bar area (A^sub nh^ = 9A^sub b^).Assume further that the yield stress of the stud is entirely developed by bearing; thus,[function of]^sub s head^ = [function of]^sub y^. To prevent spalling of the concrete cover,Eq. (2) gives c 3d^sub b^ when the stud runs parallel to one exterior surface. Thisexample indicates that the empirical limit c 3d^sub b^, recommended above, isconservative. When the stud runs parallel to an edge at the intersection of two orthogonalexterior surfaces, spalling need not be of concern when c^sub 1^ = c^sub 2^ 6d^sub b^.

When [function of]^sub s head^

... (4)

where l^sub d^ is the development length of a nonheaded deformed bar of the samediameter, ACI 318-05 gives equations for l^sub d^. The coefficient (1/0.3) is included in Eq.(4) because the tests show that a portion of the force in the bar developed by bond drops asthe portion developed by the head approaches the value (A^sub b^[function of]^sub s^head). The validity of Eq. (4) is limited for l^sub a^ between 6d^sub b^ and l^sub d^;Thompson has set the lower limit of this range for the validity of his empirical equation. The

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upper limit is set because the development length of a headed stud la cannot exceed itsdevelopment length in the absence of the head. Substitution of the upper limit l^sub a^ =l^sub d^ in Eq. (4) gives ([function of]^sub s head^/[function of]^sub y^) = 0.7. Thismeans that when ([function of]^sub s head^/[function of]^sub y^) 0.7, the developmentlength l^sub a^ should be taken equal to l^sub d^.

As an example of the results that Eq. (4) gives, calculate (l^sub a^/l^sub d^) by varying([function of]^sub s head^/[function of]^sub y^). The results in Table 1 indicate that with ahead that develops 85% of the yield strength, the development length can be taken equal tohalf that of a nonheaded bar.

SUMMARY AND CONCLUSIONS

Several practical applications of headed studs in concrete structures have been proposedand some results of supporting experimental research have been presented. For theseapplications, the studs are made of plain or deformed bars, and have head areas equal tonine or 10 times the cross-sectional area of the stem. With this head area, the anchorage bybearing is sufficient to develop the yield strength of the stud, with negligible slip. In design,nominal yield strength [function of]^sub y^ 25[function of]'^sub c^ can be consideredavailable at the stem section adjacent to the head. The thickness of the anchor head must besufficient so that the bearing pressure does not cause yielding by bending or shear of thehead before the tensile stress in the stem reaches yield. The anchor heads are produced byforging or by welding a plate to the bar end. Forged heads are commonly tapered; themaximum thickness of the head at the perimeter of the stem needs not be more thanapproximately 0.6d^sub b^; where d^sub b^ is the diameter of the stud.

Experimental research has also shown that, in some applications, deformed studs can beused with heads of areas smaller than nine to 10 times the cross-sectional area of the stem.In this case, anchorage relies on the bearing stress at the head combined with the bondstress over a development length l^sub a^ shorter than that the development length l^subd^ for a deformed bar in tension required by ACI 318-05 having no bend or hook.

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Page 10: Headed Stubs in Concrete

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Headed Studs in Concrete: State of theArt

by Ghali, Amin, Youakim, Samer A

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The main advantages of using headed studs are more efficient anchorage, simplerinstallation, less congestion of reinforcement, and improved confinement. ACI 421.1R-99recognizes these advantages and recommends rules for design for punching shear thatpermit thinner slabs and/or less shear reinforcement when studs are used instead ofstirrups.

REFERENCES

1. Ghali, A., and Dilger, W. H., "Anchoring with Double-Head Studs," ConcreteInternational, 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-Slab Connections with StudShear 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., "Headed Studs in AnchorZones 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 Reinforcement 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 Disturbed Strain Regions ofConcrete 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 Concrete Walls andColumns," Concrete International, V. 19, No. 6, June 1997, pp. 59-66.

8. Youakim, S. A., and Ghali, A., "Ductility of Concrete Columns with Double-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 with Double-Head StudsUnder Earthquake Loading: Parametric Study," ACI Structural Journal, V. 100, No. 6, Nov.-Dec. 2003, pp. 795-803.

10. Kuchma, D. A., and Collins, M. P., "The Influence of T-Headed Bars on the Strength andDuctility of Reinforced Concrete Wall Elements," ACI Spring Convention, Seattle, Wash.,Apr. 1997, 30 pp.

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

12. Birkle, G.; Ghali, A.; and Schäfer, K., "Double-Head Studs Improve CorbelReinforcement," Concrete International, V. 24, No. 9, Sept. 2002, pp. 77-84.

13. Naito, C. J.; Moehle, J. P.; and Mosalam, K. M., "Evaluation of Bridge Beam-ColumnJoints under Simulated Seismic Loading," ACI Structural 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 of HeadedReinforcement in Beam-Column Joints Subjected to Earthquake Loads," ACI StructuralJournal, V. 95, No. 5, Sept.-Oct. 1998, pp. 590-606.

15. Herzinger, R. M., and Elbadry, M. M., "Stud Reinforcement in Dapped Ends of PrecastBeams," PCI Journal. (in press)

16. Herzinger, R. M. and Elbadry, M. M., "Stud Reinforcement in Dapped Ends of BridgeGirders," Proceedings of the 2004 Bridge Conference, 2004CBC, Prestressed ConcreteInstitute, Charlotte, N.C., May 17-18, 2004, 18 pp. (CD-ROM).

17. Eligehausen, R., "Report on Pull Tests on Deha Anchor Bolts," Report No. DE003/01-96/32, Institut fur Werkstoffe in Bauwesen, University of Stuttgart, Sept. 1996 (Researchcarried out on behalf of Deha Ankersysteme GMBH &Co. KG, Gross-Gerau).

18. DeVries, R. A., "Anchorage of Headed Reinforcement in Concrete," PhD dissertation,The University of Texas at Austin, Austin, Tex., 1996, 294 pp.

19. Bashandy, T. R., "Application of Headed Bars in Concrete Members," PhD dissertation,The University of Texas at Austin, Austin, Tex., 1996, 303 pp.

20. Thompson, M. K., "The Anchorage Behavior of Headed Reinforcement in CCT Nodesand Lap Splices," PhD dissertation, The University of Texas at Austin, Austin, Tex., 2002,503 pp.

21. ACI Committee 318, "Building Code Requirements for Structural Concrete (ACI 318-05)and Commentary (318R-05)," American Concrete Institute, Farmington Hills, Mich., 2005,430 pp.

22. Leonhardt, F., and Walther, R., "Welded Wire Mesh as Stirrup Reinforcements-ShearTests on T-Beams and Anchorage Tests," Bautechnik, V. 42, Oct. 1965. (in German).

23. Green, K. L., and Tadros, M. K., "The NU Precast/Prestressed Concrete Bridge I-GirderSeries," PCI Structural Journal, V. 39, No. 3, May-June 1994, pp. 26-39.

24. Ariyawardena, N., and Ghali, A., "Design of Precast Prestressed Concrete MembersUsing External Prestressing," PCI Structural Journal, V. 47, No. 2, Mar.-Apr. 2002, pp. 84-94.

25. Loov, R. E., "Review of A23.3-94 Simplified Method of Shear Design and Comparisonwith Results Using Shear Friction," Canadian Journal of Civil Engineering, V. 25, No. 3,June 1998, pp. 437-450.

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Headed Studs in Concrete: State of theArt

by Ghali, Amin, Youakim, Samer A

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

27. Tanaka, H.; Park, R.; and McNamee, B., "Anchorage of Transverse Reinforcement inRectangular Reinforced Concrete Columns in Seismic Design," New Zealand NationalSociety for Earthquake Engineering, V. 18, No. 2, June 1985, pp. 165-190.

28. Sheikh, S. A., and Yeh, C.-C., "Tied Concrete Columns Under Axial Load and Flexure,"Journal of Structural Engineering, ASCE, V. 116, No. 10, 1990, pp. 2780-2800.

29. Lehman, D. E.; Gookin, S. E.; Nacamuli, A. M.; and Moehle, J. P., "Repair ofEarthquake-Damaged Bridge Columns," ACI Structural Journal, V. 98, No. 2, Mar.-Apr.2001, pp. 233-242.

30. Haroun, M.; Pardoen, G.; Bhatia, H.; Shahi, S.; and Kazanjy, R., "Structural Behavior ofRepaired 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.; and Kreger, M. E., "Behaviorand Design of Large Structural Concrete Bridge Pier Overhangs," Center for TransportationResearch Report CTR-1364-1, Austin, Tex., 1997.

32. Wood, B. A.; Kreger, M. E.; and Breen, J. E., "Experimental Investigation of DesignMethods for Large Cantilever Bridge Bents," Center for Transportation Research ReportCTR-1364-3F, Austin, Tex., 1997.

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

34. Post-Tensioning Institute, "Anchorage Zone Design," Post-Tensioning Manual, 6thEdition, 2002, 51 pp.

35. Caltrans, "Seismic Design Criteria Version 1.1," California Department ofTransportation, Division of Structures, Sacramento, Calif., 1999.

36. Ingham, J. M.; Priestley, M. J. N.; and Seible, F., "Seismic Performance of a Bridge KneeJoint Reinforced with Headed Reinforcement," Report No. SSRP-96/06, University ofCalifornia, San Diego, Structural Systems Project, La Jolla, Calif., Sept. 1996, 113 pp.

37. Sritharan, S., and Priestley, M. J. N., "Seismic Testing of a Full-Scale Pile-DeckConnection Utilizing Headed Reinforcement," Report No. TR-98/14, University of

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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 Anchorages in Beam-ColumnJoints," ACI JOURNAL, Proceedings V. 72, No. 5, May 1975, pp. 198-209.

39. Minor, J., and Jirsa, J. O., "Behavior of Bent Bar Anchorages," ACI JOURNAL,Proceedings V. 72, No. 4, Apr. 1975, pp. 141-149.

Amin Ghali, FACI, is Professor Emeritus, Department of Civil Engineering at the Universityof Calgary, Calgary, Alberta, Canada. He is a member of ACI Committee 435, Deflection ofConcrete Building Structures; and Joint ACI-ASCE Committees 343, Concrete BridgeDesign; and 421, Design of Reinforced Concrete Slabs; and is a consulting member of ACI318-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 of Calgaryin 2002. His research interests include behavior of concrete structures under earthquakeloading, finite element analysis, and serviceability of concrete structures.

Copyright American Concrete Institute Sep/Oct 2005Provided by ProQuest Information and Learning Company. All rights Reserved

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ACI Structural Journal

Articles in Sep/Oct 2005 issue of ACI Structural Journal

Long-Term Performance of Corrosion-Damaged Reinforced Concrete Beamsby Maaddawy, Tamer El; Soudki, Khaled; Topper, TimothyCorrosion-Induced Cracking: Experimental Data and Predictive Modelsby Vu, Kim; Stewart, Mark G; Mullard, JohnRepair of Bridge Girders with Composites: Experimental and Analytical Validationby Di Ludovico, Marco; Nanni, Antonio; Prota, Andrea; Cosenza, EdoardoBehavior of Reinforced Concrete Columns Under Variable Axial Loads: Analysisby Esmaeily, Asad; Xiao, YanHeaded Studs in Concrete: State of the Artby Ghali, Amin; Youakim, Samer ATime-Dependent Risk Assessment of Structural Deterioration Caused byReinforcement Corrosionby Li, Chun Qing; Melchers, Robert EReliability Analysis for Eccentrically Loaded Columnsby Szerszen, Maria M; Szwed, Aleksander; Nowak, Andrzej SDynamic Responses of Flat Plate Systems with Shear Reinforcementby Kang, Thomas H -K; Wallace, John WHigh-Performance Fiber-Reinforced Cement Composites: An Alternative for SeismicDesign of Structuresby Parra-Montesinos, Gustavo JAnalytical Model to Evaluate Failure Behavior of Plated Reinforced Concrete BeamsStrengthened for Shearby Colotti, Vincenzo; Spadea, Giuseppe; Swamy, R Narayan; de Souza Sánchez Filho,Emil; Et alLongitudinal Steel Stresses in Beams Due to Shear and Torsion in AASHTO-LRFDSpecificationsby Rahal, Khaldoun NSeismic Resistance of Square Concrete Columns Retrofitted with Glass Fiber-Reinforced Polymerby Memon, Muhammad S; Sheikh, Shamim ASteel-Free Composite Slabs Made of Reactive Powder Materials and Fiber-ReinforcedConcreteby Hassan, Ammar; Kawakami, Makoto

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Behavior of Concrete Bridge Deck Slabs Reinforced with Fiber-Reinforced PolymerBars Under Concentrated Loadsby El-Gamal, Sherif; El-Salakawy, Ehab; Benmokrane, BrahimSeismic Retrofit of Octagonal Columns with Pedestal and One-Way Hinge at Baseby Johnson, Nathan; Saiidi, M Saiid; Itani, Ahmad; Ladkany, SamaanPerformance of Glass Fiber-Reinforced Polymer Reinforcing Bars in TropicalEnvironments-Part I: Structural Scale Testsby Mukherjee, Abhijit; Arwikar, S J

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