seismic retrofit of reinforced concrete slab-column

514 ACI Structural Journal/July-August 2009

ACI Structural Journal, V. 106, No. 4, July-August 2009.MS No. S-2008-021.R2 received January 28, 2008, and reviewed under Institute

publication policies. Copyright © 2009, American Concrete Institute. All rights reserved,including the making of copies unless permission is obtained from the copyright proprietors.Pertinent discussion including author’s closure, if any, will be published in the May-June2010 ACI Structural Journal if the discussion is received by January 1, 2010.

ACI STRUCTURAL JOURNAL TECHNICAL PAPER

This paper describes an experimental investigation on thebehavior of specimens of reinforced concrete slab-column connec-tions subjected to increasing cyclic lateral drift and constantgravity loading. The five tested specimens were retrofitted usingshear bolts. Shear bolts, as transverse shear reinforcement, wereinstalled externally in three specimens in holes drilled through theslabs’ thickness in the vicinity of the columns. The test results showthat shear bolts increase lateral load-resisting capacity, lateraldrift capacity at peak and ultimate loads, and ductility of the slab-column connections. Shear bolts also change the failure mode ofthe slab-column connections from brittle to ductile and increasethe energy dissipation capacity.

Keywords: columns; ductility; punching shear; seismic loads; shear bolts;slabs.

INTRODUCTIONFlat reinforced concrete slabs supported on columns are

easy to construct. These two-way slabs must be able tosustain bending and shear actions around the column area,which cause complex three-dimensional stress and strainstates and result in principal tension stresses being inclinedwith respect to the slab’s plane. Therefore, flexural reinforcementalone cannot provide adequate ductility of these connectionsespecially when deformations are large, for example, duringseismic events. Adding shear reinforcement at the columnarea of these slabs can substantially increase punching shearcapacity and ductility, which has been shown by severalresearchers (Hawkins 1974; Dilger and Ghali 1981; Megallyand Ghali 2000; El-Salakawy et al. 2000; Robertson et al.2002). However, in many practical cases, especially in buildingsdesigned using older codes, these shear reinforcements were notprovided during construction.

If only strength requirements under gravity loads areconsidered, shear reinforcement is often not necessary.Concrete alone can sustain imposed shear stresses, and flexuralreinforcement will carry flexural tension. Designs, according toevery design code, ensure that the connection should fail inflexure before reaching its punching shear strength.However, even under gravity loads alone, after reaching itsflexural capacity and yielding, the slab-column connectionswill deform and the shear strength of the surroundingcracked concrete will diminish, leading eventually topunching shear failure at large deformations, which can havea negative effect on the robustness of the entire structural system.

The need for punching shear retrofit of existing slabs canbe due to change in the type and intensity of loads imposedon the structure, corrosion of reinforcement, a need toconstruct openings next to the column, and design orconstruction errors. Other researchers—for example, Ghaliet al. (1974), Ebead and Marzouk (2002), and Stark et al.(2005)—have proposed strengthening methods for existing

slab-column connections. Shear bolts, which were developed atthe University of Waterloo, consist of a stem with a head onone end and a washer with nut at the other threaded end. Themethod is conceptually simple and aesthetically appealing.Figure 1(a) shows a shear bolt, and Fig. 1(b) shows schematicsof the installation of shear bolts in a concrete slab.

Shear bolts retrofit technique has been previously testedby El-Salakawy et al. (2003) and Adetifa and Polak (2005)on slab-column connections under static loads, where theslab-column connections strengthened with shear boltsshowed a substantial increase in punching shear capacity andductility. This paper reports new research on the applicationof shear bolts to reinforced concrete slab-column connectionssubjected to both gravity and seismic loads. Seismic loadsare simulated by reversed cyclic horizontal displacementswith increasing intensity. Five connections were tested: twowithout shear reinforcement and three with shear reinforcementin a form of shear bolts.

RESEARCH SIGNIFICANCEImproving punching shear capacity and ductility of slab-

column structures at large lateral deformations is an importantissue, especially in seismic zones. Shear bolts are a newpunching shear retrofit method for existing flat slab structures.In this research, slab-column connections strengthened withvarying shear bolt layouts were tested under constant gravityloading and increasing cyclic lateral displacements. Theresults show that the retrofitted slabs perform better thanoriginal slabs in terms of lateral load capacity, drift ratios,and ductility.

Title no. 106-S49

Seismic Retrofit of Reinforced Concrete Slab-Column Connections Using Shear Boltsby Wensheng Bu and Maria Anna Polak

Fig. 1—Shear bolt and its installation in concrete slab.

515ACI Structural Journal/July-August 2009

EXPERIMENTAL INVESTIGATIONTest program

This study focuses on how shear bolts can improve slabpunching shear capacity and increase ductility of the slab-column connections in seismic zones based on experimentalevidence. Five full-scale specimens (SW1 to SW5) weretested, three of which were strengthened by shear bolts,while the other two were without shear reinforcements.Figure 2 shows all five specimens. Specimens SW1, SW2,and SW3 are in Group 1. These specimens were subjected toconstant gravity load: V = 110 kN (24.75 kips), and were castfrom concrete with a compressive strength of fc′ = 35 MPa(5075 psi). This resulted in a gravity shear ratio V/Vn = 0.54,where Vn = 0.33 b0d (MPa) (Vn = 4 b0d [psi]); b0 isthe perimeter length of the critical section; and d is the effectivethickness of the slab taken as 90 mm (3.5 in.). Group 2 includesSpecimens SW4 and SW5, with the average concretestrength fc′ = 46 MPa (6670 psi), and with applied constantgravity load V = 160 kN (36 kips). This resulted in gravity shearratio V/Vn = 0.68.

Test specimensThe specimens can be regarded as taken from a prototype

structure in which the flat slab spans 3.75 m (148 in.)between columns (Adetifa and Polak 2005). All specimenshave slab dimensions of 1800 x 1800 x 120 mm (71 x 71 x4.7 in.) with top and bottom column stubs (200 x 200 mm)(7.9 x 7.9 in.) extending out 700 mm (27.6 in.) away from theslab top and bottom faces (Fig. 3). Horizontal loads wereapplied at the column stubs 565 mm (22.3 in.) from the slabtop and bottom faces. The specimens were simply supportedat the 1500 x1500 mm (59.1 x 59.1 in.) perimeter on thebottom face of the slab. On the slab top face, two sidesnormal to the direction of the applied lateral load were alsorestrained to resist the edges of the slab from lifting. Thedimensions of the slabs were chosen to represent the locationsof contraflexure lines for the case of gravity loads. In case ofgravity plus horizontal cycling loads (as in the case of the

fc′ fc′present tests), the locations of contraflexure lines normal tothe horizontal loading direction change depending on thedirection of the horizontal loading. Therefore, because in thesetup the location of supports remain the same (between theactual locations of the lines of contraflexure), thick neoprenepads were provided on top and bottom of the slab to allowrotations. The neoprene pads were approximately 25 mm(1 in.) thick and 50 mm (2 in.) wide installed along thesupporting lines, as shown in Fig. 3. For this project, the toparea of the slab adjacent to the column was subjected tocompression, which was opposite to a real slab-columnsystem subjected to gravity loads. This was due to the waythe gravity load was applied, which was through the columnfrom the vertical actuator (Fig. 4(b)). Table 1 shows thespecimens details, including slab names, dimensions,number of bolt peripheral rows, and gravity load applied toeach specimen. Figure 3 also provides information ondimensions and loading for the specimens.

Figures 5(a) and (b) show slab bottom (tension) and topreinforcing mat, respectively. On the bottom of the slab, theflexural reinforcement ratio was 1.05% for the outer bars(10M at 100 mm [4 in.]), and 1.3% for the inner bars (10Mat 90 mm [3.5 in.]), to ensure the same moment capacities inboth orthogonal directions. On the slab top face, thereinforcement ratio was 0.58% (10M at 200 mm [8 in.])in both directions. The reinforcement of the columns

Wensheng Bu is a Structural Engineer with WarleyParsons Colt Engineering,Edmonton, AB, Canada. He received his PhD from the Department of Civil andEnvironmental Engineering, University of Waterloo, Waterloo, ON, Canada. Hisresearch interests include experimental testing, theoretical analysis, and seismicretrofit methods for reinforced concrete structures.

Maria Anna Polak, FACI, is a Professor in the Department of Civil and EnvironmentalEngineering, University of Waterloo. She received her BSc and MSc in Civil Engineeringfrom Cracow University of Technology, Poland, and her PhD from the University ofToronto, Toronto, ON, Canada. She is a member of ACI Committee 435, Deflection ofConcrete Building Structures, and Joint ACI-ASCE Committee 445, Shear andTorsion. Her research interests include experimental and numerical work related toshear and torsion in concrete members, nonlinear finite element analysis, andmaterial modeling.

Fig. 2—Five specimens and shear bolt layout. (Note: 1 kN =0.225 kip.)

Fig. 3—Specimen dimensions, loading, and support conditions.(Note: 1 mm = 0.0394 in.)

Table 1—Slab dimensions, concrete properties, and vertical load

Slab name

Slabdimensions,

mm Columns, mm

Concrete compres-

sion strength,

MPa

Concrete tension

strength, MPa

Number of bolt rows

Vertical constant load V, (kN)

SW1 1800 x1800 x120

Simplysupported

along1500 x1500

200 x 200 x 700

Displacements applied at 565 mm from the slab surface.

35 2.9 0 110

SW2 35 2.9 4 110

SW3 35 2.9 6 110

SW4 46 3.1 6 160

SW5 46 3.1 0 160

Note: 1 MPa = 145 psi; 1 kN = 0.225 kips; and 1 mm = 0.0394 in.


consisted of three 25M bars on two opposite faces with 10Mat 100 mm (4 in.) closed ties. The columns were designed totransfer shear force and cyclic moments to the slab.

Material propertiesThe specimens were cast using ready mixed concrete,

supplied in two batches. Concrete cylinders (100 x 150 mm[4 x 6 in.]) were prepared and tested for its compressive andtensile strength when the tests were carried out. Yield stressand ultimate tensile strength for reinforcing bars and shearbolts were obtained by testing both the test coupons andactual bars. Table 1 provides the concrete strength for all fivespecimens. Table 2 shows reinforcing bars and shear boltsyield stress, tensile strength, and percentage elongations.The nominal values of yield stress and tensile strengthshould be adopted in any theoretical strength calculations.

Transverse shear reinforcement—shear boltsThe shear bolt was developed for retrofitting existing

slabs. They are installed after drilling holes in an existingslab structure. Each steel shear bolt is made of a stem with ahead at one end, and a washer with a nut at the other threaded

end (Fig. 1). No adhesive is required between bolts and aconcrete slab. For the current tests, the bolts were 9.5 mm(3/8 in.) in diameter. The holes were drilled through the slabsusing a 12.7 mm (1/2 in.) diameter drill bit. The bolts weretightened against the slab by a standard wrench to a torquecausing approximately 10 to 15% of bolt yield strain. Fourperipheral rows of shear bolts were installed in Specimen SW2.Six rows of bolts were installed in Specimens SW3 and SW4.There were no bolts installed in Specimens SW1 and SW5.Each peripheral row of bolts around the column consisted ofeight bolts placed orthogonally in two directions (lines)extending from all four sides of the column (Fig. 2). Peripheralbolt rows were spaced, starting from the column face, forSpecimen SW2 at 45, 55, 70, and 80 mm (1.8, 2.2, 2.8, and3.2 in.), and for SW3 and SW4: 45, 55, 70, 80, 60, and 90 mm(1.8, 2.2, 2.8, 3.2, 2.4, and 3.5 in.). The bolts were spaced atuneven intervals to avoid drilling through the flexuralreinforcements.

Experiment setupThe test setup, located at the University of Waterloo

structural laboratory, is shown in Fig. 4(a) and (b). Thetesting frame consists of four W-shaped steel columns and adouble-channel crosshead. Two horizontal hydraulic actuatorsare installed on each side of the frame to apply cyclic lateralload. A vertical actuator is installed in the middle of thecrosshead to apply a constant vertical load at the top of theconcrete column. During testing, a steel pan with rollers isplaced between a vertical actuator and the top of upperconcrete column to free horizontal movement of the columnduring lateral cycling loading while maintaining the verticalload. The concrete slab is supported by a closed square ringsteel beam of W-shape, which in turn is supported by foursteel columns. Four short steel bracing beams are connectedbetween the ring beam and the testing frame to restrainlateral movement of the ring beam. In addition, four short,adjustable steel stoppers with 25.4 mm (1 in.) thick neoprenepads are installed horizontally between the slab edges andthe steel test frame to resist horizontal movement of slab

Fig. 4—Testing setup.

Fig. 5—Specimen details: (a) tension reinforcement; and (b)compression reinforcement. (Note: 1 mm = 0.0394 in.)

ACI Structural Journal/July-August 2009 517

from possible unequal forces from the two opposing horizontalactuators, while at the same time allowing rotation at the slabedges. The concrete slab is supported on its bottom surfaceon four sides during testing. On the slab top face, two edgesnormal to the horizontal loads are restrained by reaction steelbeams to prevent lifting of the slab. Each end of the reactionbeam is held in position by two steel rods attached to theground base. The steel test frame was designed to ensureadequate strength and stiffness under the testing load of theconcrete specimens (Bu 2008).

InstrumentationString pots, linear variable displacement transducers

(LVDTs), and potentiometers were used to measure thedisplacements. Two string pots were horizontally connectedto top and bottom column ends to measure the column lateraldrifts. To record the possible slab horizontal shifting, twostring pots were installed in the horizontal loading direction.At the bottom concrete column end of the specimen, a stringpot was connected vertically to record the column enddisplacement relative to the fixed ground point. A series ofdisplacement transducers were installed on top and bottomfaces along two orthogonal directions. At four locations, thetransducers were aligned vertically and installed on both topand bottom slab surfaces to measure the vertical displacementdifference, which is used to estimate shear crack width. Allthe string pots and displacement transducers were fixed onto arigid steel frame attached separately to the floor of the laboratory.

Strains were measured on both flexural and shearreinforcements. Strain gauges (5 mm [0.2 in.] length)were attached in locations shown in Fig. 5(a) and (b). Shearbolts were instrumented in lines extending in the orthogonaldirections with strain gauges attached along the bolts’ stems.

Test procedureEach specimen was loaded in two stages. In the first stage,

the controller was switched to load control. A vertical loadwas applied on the top of the upper column of the specimenat the loading rate of approximately 20 kN/min. (4.5 kip/min.).Slabs SW1, SW2, and SW3 were loaded up to 110 kN(24.75 kips), vertical load, while SW4 and SW5 wereloaded up to 160 kN (36 kips). The vertical loads were thenkept constant at this value and the two horizontal actuatorswere then started to apply horizontal drift to the top andbottom column ends following the loading path shown in Fig. 6.

Horizontal loading pathThe loading path was designed to show stiffness degradation

using multiple load cycles. The load cycles at the same driftratio were repeated three times (Fig. 6). A cycle of small driftratio (approximately 0.5%) was used between each group ofthree cycles of same drift ratio. It was done to monitor thebehavior of slab-column connections at small deformationsand the reduction in stiffness after larger drift ratio events.

After 3.0% drift, the loading path was applied in a monotonicallyincreasing way at successive cycles without repeating. It isimportant to note that the recorded lateral drift ratios (realdrift ratio) on column ends, by independent string pots, wereslightly different from the ones applied by the controllers.This was due to various factors such as slab horizontalmovement, slab rotation, and deformation of the steeltesting frame. Drift ratios used in this paper are all calculatedusing the recorded independent lateral displacements.

EXPERIMENTAL RESULTS AND DISCUSSIONMoment-versus-drift ratio

Unbalanced moments applied to the specimens werecalculated from two lateral forces multiplied by theirdistance to the slab’s center: 0.625 m (24.6 in.). The loadsand lateral drifts are presented for positive and negativedirections. Positive direction is defined when the top columnmoves from left to right side of the top column of thespecimens (Fig. 3(b)).

Moment versus horizontal drift ratios for Specimens SW1,SW2, and SW3 (Group 1) are shown in Fig. 7, and for SW4and SW5 (Group 2) in Fig. 8. Specimens SW1, SW2, andSW3 (concrete strength: 35 MPa [5075 psi]) had an appliedvertical load of 110 kN (24.75 kips) on the top column end,and SW4 and SW5 (concrete strength: 46 MPa [6670 psi])had a vertical load of 160 kN (36 kips). Table 3 shows thelateral loads and corresponding drift ratios for both directionsof loading. Comparing with the control specimen, SW1, thepeak load of Specimen SW2 increased 21%, and correspondingdrift ratio increased 115%. Specimen SW3 showed an increaseof 24% in peak load and a 67% increase in correspondingdrift. In Group 2, Specimen SW4 showed a 37% increase inlateral peak load capacity and a 100.0% increase incorresponding lateral drift compared to Specimen SW5.Specimens without bolts, SW1 and SW5, failed shortly afterattaining maximum lateral moment. Specimen with bolts—SW2, SW3, and SW4—continued to deform with little lossof lateral load (moment) carrying capacity. Their maximumlateral drifts were over 7%, at which time the tests had to be

Table 2—Reinforcement material propertiesFlexural reinforcement Shear bolts

Coupon testReinforcing bar test(nominal strength) Coupon test

Shear bolt tests(nominal strength)

Yield stress, MPa

Tensile strength, MPa

Maximum elongation, %

Yield stress, MPa


Yield stress, MPa

Tensile strength, MPa Elongation, %

Yield stress, MPa


520 750 20.8 470 650 378 510 11.5 370 500

Notes: All coupons were machined of 1/4 in. (6 mm) diameter, 2 in. (50 mm) gauge length. Nominal strengths were calculated using normal reinforcing bar section area [100 mm2

(0.155 in.2)]. 1 MPa =145 psi.

Fig. 6—Loading path.


terminated due to excessive displacements of the columnsand inability to apply vertical loads to them.

Figure 9 shows the five backbone curves of hysteresiscurves of moment versus lateral drift. These backbonecurves were formed by connecting peak points at the firstcycle of each same-drift-cycles group. They clearly showinitiation of punching load failure for SW1 and SW5 (nobolts); the post-peak ductility of Specimens SW2, SW3, andSW4 (with bolts) and the increase of peak load capacity; andthe corresponding drift ratio of the specimens strengthenedwith shear bolts.

Connection stiffnessPeak-to-peak stiffness of each lateral loading cycle was

calculated lateral force/lateral drift ratio (Fig. 10). Theconnection stiffness decreased rapidly (to 40 to 50% originalstiffness) during the repeated cycles up to 1.0% drift ratio. Itshould be noted that the stiffness decreased after eachrepeated cycle; in every three successive same-drift cycles,the stiffness decreased more in the second cycle than it did in

the third one. The stiffness decrease between the first and thesecond cycle was more than twice of that between the secondand third cycles. Low drift cycles, between higher driftgroups, also showed stiffness degradation.

Shear bolts had some effect in increasing the connectionstiffness, but this effect is not significant. Shear bolts had aneffect on the specimen’s stiffness at large lateral deformation,when Specimens SW1 and SW5, without shear bolts, failedby punching, whereas Specimens SW2, SW3, and SW4,strengthened with shear bolts, could undergo far moredeformation without abruptly losing stiffness.

Drift ductilityDuctility is defined by a ratio of μ% = δ%/δy , where δy is

the displacement corresponding to flexural yielding of theslab, and δ% is the observed displacement corresponding toa certain load (percent of the maximum load in the post-peak

Fig. 7—Moment versus horizontal drift ratio for Group 1:(a) Specimen SW1; (b) Specimen SW2; and (c) Specimen SW3.

Fig. 8—Moment versus horizontal drift ratio for Group 2: (a)Specimen SW4; and (b) Specimen SW5.

Fig. 9—Backbone curves of moment versus horizontal driftratio.

ACI Structural Journal/July-August 2009 519

region). In this paper, δy is defined by two approaches: basedon the method by Pan and Moehle (1989) (designated asMethod 1) and by using the experimental value at yield(designated as Method 2). In Method 1, yield displacementis defined by points corresponding to (2/3)Pmax and Pmax inthe backbone curve. A line between the origin, the point of(2/3)Pmax, and crossing the horizontal line corresponding toPmax defines the assumed yield displacement δy. The peaklateral load, gravity load ratio V/Vn, and ductility at peakloads and at 95% of peak load (in the post-peak region) areshown in Table 3. Slabs strengthened with shear bolts havehigher ductility. In Group 1, the peak ductility increased33% and 26% for SW2 and SW3 (compared to SW1 with nobolts), respectively, based on Method 1. For Group 2, thepeak ductility increased by 28% for Specimen SW4 whencompared to SW5, based on Method 1.

From the experimental results, flexural reinforcing barsyielded at very similar drift ratios (approximately 1%) for allspecimens. The peak load ductilities increased by 100 to250%, based on Method 2 (Table 3).

Strains in shear boltsFor Specimens SW3 and SW4 (with six-row bolts), a total

of 12 strain data were measured for shear bolts, including sixbolts in lateral loading direction (Direction 1) and six intransverse direction (Direction 2). Similarly, for SpecimenSW2 strengthened with four-row bolts, four strains in boltsin Direction 1 and four in Direction 2 were measured. Shearbolts with strain gauges in Direction 1 are denoted as Bolts 1to 6, and in Direction 2 as Bolts 1a to 6a. The backbone(envelope) curves of lateral drift ratio versus bolt strains areshown in Fig. 11.

For all specimens, in Direction 1, the first two bolts (Bolt 1and Bolt 2) close to the columns experienced significantstrains. The third bolt (Bolt 3) had small strain and the fourthbolt (Bolt 4) remained inactive throughout the entire loadinghistory. Bolts 1 and 2 were activated apparently only afterthe drift reached at least 1%. This drift corresponds to thelateral load of approximately 35 kN (7.9 kips), which isaround 50% of the maximum lateral load attained by thespecimen. In Direction 1, only Bolt 1 in SW3 yielded.

Bolts in Direction 2 experienced larger strains, at the samedrift ratios, than their counterparts in Direction 1. Strains inBolt 1a of SW3 and SW4 reached 2.44 × 10–3 and 1.95 × 10–3,respectively, exceeding yield strain (εy = 1.9 × 10–3). Strain inBolt 1a of SW2 reached 1.72 × 10–3, which is close to yielding.

Flexural reinforcement strainsA total of 16 strain gages were attached to reinforcing bars

and embedded in the slab and their locations are shown inFig. 5. The drift ratios and locations at the first yielding in thenumbered reinforcing bars for all specimens are summarized

in Table 4. In the five specimens, SW1 to SW5, the bottomreinforcing bar (#1) and the top reinforcing bar (#3) goingthrough the column in Direction 1 yielded first. It is interestingto note that yielding of flexural bars occurred at approximatelythe same drift ratio regardless of the presence of shearreinforcement. It can be assumed that yielding happened atapproximately 1% drift; differences in the strain gauge readingsshould be attributed to different locations of flexural cracks withrespect to the location of the strain gauges in the specimens.

Vertical crack widthAt four locations, the LVDTs were set on both top and

bottom surfaces. The displacement difference was used as anestimation of opening width of inclined crack through slabthickness. For Specimens SW1 and SW5 (no bolts), therewas an abrupt crack width increase at lateral drift of justabove 3.0%. The two specimens had reached their peak loadat around 3% drift. Specimens SW2, SW3, and SW4,

Table 3—Peak lateral load and drift ductility for each specimen

Slab name V/VnPeak lateral

load, kN

Drift ratio at peak lateral

load, %

Ductility based on test results on bar yielding, %(Method 2)

Ductility based on Pan and Moehle (1989), %(Method 1)

Drift ratio at first yield

Drift ductility at peak lateral load

Drift ductility at 95% post-peak

Drift ratio at first yield

Drift ductility at peak lateral load

Drift ductility at 95% post-peak

Gr.1

SW1 0.54 54 2.7 1.33 2.1 2.4 1.6 1.8 2.0

SW2 0.54 65 5.8 0.91 6.5 7.9 2.3 2.5 3.1

SW3 0.54 67 4.5 0.68 6.6 9.6 1.8 2.6 3.7

Gr. 2SW4 0.68 77 5.2 0.96 5.4 6.9 2.3 2.3 2.9

SW5 0.68 56 2.6 1.04 2.6 3.1 1.4 1.8 2.2

Note: 1 kN = 0.225 kip.

Fig. 10—Peak-to-peak stiffness versus drift ratio of: (a)SW1, SW2, and SW3; and (b) SW4 and SW5.


strengthened with shear bolts, lasted many more cycleswithout sudden crack expansion. The crack width at 1.5, 2,and 3% drifts are summarized in Table 5.

Cracking and failure modes of specimensCracks on slab surfaces started from the corners of

columns on the tension bottom side, first at the application of

gravity loads. First crack on the top of the slabs was usuallyobserved at approximately 0.6 to 0.75% drift ratio. Onbottom surfaces, cracks first propagated toward all fouredges and corners, while on top surfaces, initial cracks developedfrom columns’ corners in the direction perpendicular to thelateral loading direction. The final crack patterns on top andbottom slab surfaces for Specimens SW1 and SW2 in Group 1are shown in Fig. 12.

Fig. 11—Lateral drift ratio versus strain in each bolt of the three specimens: (a)SW2; (b) SW3; and (c) SW4.

Table 4—Drift ratios at first yielding of reinforcing bars in the five specimens

Slab name

Drift ratio at first yielding, %

Reinforcing bar #1

Reinforcing bar #2

Reinforcing bar #3

Reinforcing bar #4

Reinforcing bar #5

SW1 1.3 at “d” 2.3 at “c” 1.5 at “d” (No yielding) 2.6 at “b”

SW2 1.1 at “c” 1.6 at “d” 1.0 at “d” 4.6 at “a” (No yielding)

SW3 0.7 at “c” 1.4 at “c” 1.7 at “b” 4.1 at “b” 1.3 at “b”

SW4 1.0 at “c” 1.3 at “c” 1.0 at “d” 5.5 at “b” 6.6 at “b”

SW5 1.2 at “d” 1.5 at “d” 1.0 at “c” (No yielding) 3.7 at “b”

Note: Location of strain gauges are shown in Fig. 5.

Table 5—Crack width at 1.5%, 2.0%, and 3.0% drift ratios for each specimen

Slab name

Crack width, mm

at 1.5% drift ratio at 2.0% drift ratio at 3.0% drift ratio

SW1 0.18 0.32 0.70

SW2 0.21 0.33 0.74

SW3 0.26 0.44 0.71

SW4 0.11 0.26 0.58

SW5 0.13 0.29 1.26

Note: 1 mm = 0.0394 in.

521ACI Structural Journal/July-August 2009

From the crack pattern and the hysteresis curves, it isobserved that SW1 and SW5 failed by punching shear mode;the other three (SW2, SW3, and SW4) were subjected toflexural failure mode. The three slabs attained the peaklateral load during testing, but afterward, with increasinglateral displacements, the load decreased only slightly.

Comparison with the ACI CodeBased on the ACI 318-05 Section 11.12 formulas for

punching shear design of two-way slab under gravity loadand connection moment, the following equation can be derived

(1)

where V is the applied vertical load; Vc , Vs are resistanceforces, from concrete and shear reinforcement, respectively;M is an unbalanced moment of the connection; b0 is theperimeter length of the critical section; d is the effectivethickness of the slab; and γv = 0.4. The moments calculatedusing Eq. (1) are displayed in Table 6. Also shown in Table 6are tested moments of each specimens at 1.0, 1.5, and 2.0%drift and peak load point (ultimate moment). For all specimens,the calculated moments using the ACI 318-05 formula aresmaller than the tested ultimate moments.

SUMMARY AND CONCLUSIONSThe experimental program was conducted to test the

effectiveness of shear bolt punching shear retrofit techniquein seismic zones. The results are presented in terms of

MJc

γvc------- 1

b0d-------- Vs Vc+( )

Vbod--------–=

Fig. 12—Final crack pattern on top and bottom surfaces of specimens in Group 1: (a)SW1; and (b) SW2.

Table 6—Moment capacity of each specimen

Slab name Theoretical moment

ACI 318-05, kNm

Test results

Peak moment, kNm

Drift ratio at peak moment, %

Moment at 0.5% drift ratio, kNm





SW1 22.4 68.7 2.85 13.4 26.3 37.0 49.3 58.7

SW2 47.9 88.9 5.97 30.0 36.6 43.4 55.3 64.3

SW3 47.9 89.3 5.25 24.3 33.7 41.9 54.4 65.0

SW4 46.6 93.2 5.72 39.1 47.0 52.9 63.6 72.8

SW5 17.3 77.9 2.69 40.0 46.5 53.0 64.2 72.0

Note: 1 kNm = 0.74 k-ft.


strains, deformations, and crack formation. The conclusionsand observations are made specifically for shear bolt retrofittedslabs; however, some of the findings can be helpful inenhancing understanding of the behavior of flat reinforcedconcrete slabs with and without shear reinforcements.

Based on the results of this experimental investigation, thefollowing conclusions are provided:

1. Shear bolts can change failure mode of the flat slab columnconnections. Slabs retrofitted with shear bolts failed by flexurewhile slabs without shear bolts failed by punching shear;

2. Peak lateral load capacity of slab-column connectionincreased when retrofitted with shear bolts;

3. Slab-column drift ratio at peak load of the slab-columnconnections increased by 66% up to 123% when retrofittedwith shear bolts;

4. By using shear bolts, the drift ductility of the slab-column connection at peak load point and post-peak can besubstantially increased (up to 400%);

5. The specimen retrofitted with shear bolts underwentmore lateral drift cycles at large deformation, showing anincrease in energy dissipation capacity;

6. The shear bolts located at a distance exceeding fourtimes the slabs effective thickness d have little effect on theslab’s behavior;

7. Strains on the bolts normal to the applied lateraldisplacements were generally larger than the strains in thedirection of loading; and

8. Vertical crack width remained in the range of approximately1 mm (0.039 in.) until the punching shear failure.

ACKNOWLEDGMENTSThis research was funded by a grant from the Natural Sciences and

Engineering Council (NSERC) of Canada. The shear bolts were manufacturedand donated by Decon Inc. Canada. The ready mixed concrete was donatedby Hogg Fuel and Supply Ltd. Ready Mix Division in Kitchener, ON,Canada. The authors wish to thank the technical staff of the Structural

Laboratory of Civil and Environmental Engineering, University ofWaterloo, ON, Canada for their support and assistance.

REFERENCESACI Committee 318, 2005, “Building Code Requirements for Structural

Concrete (ACI 318-05) and Commentary (318R-05),” American ConcreteInstitute, Farmington Hills, MI, 430 pp.

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seismic retrofit of reinforced concrete slab-column

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