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ACI Structural Journal/September-October 2004 699

ACI Structural Journal, V. 101, No. 5, September-October 2004.MS No. 03-198 received June 12, 2003, and reviewed under Institute publication

policies. Copyright 2004, American Concrete Institute. All rights reserved, includingthe making of copies unless permission is obtained from the copyright proprietors.Pertinent discussion including authors closure, if any, will be published in the July-August2005ACI Structural Journal if the discussion is received by March 1, 2005.

ACI STRUCTURAL JOURNAL TECHNICAL PAPER

Seismic regions are characterized by reinforced concrete (RC)structures that have been designed without seismic provisions. Asthe structural upgrade of such structures becomes necessary, oneof the approaches that recent design guidelines suggest is that ofthe hierarchy of strength. By increasing the strength and ductilityof critical components, their brittle and catastrophic failure is

prevented and the occurrence of more desirable mechanisms ispromoted. This approach allows improving the global behavior ofthe structure. The key issue of strengthening design of RC frames isrepresented by the beam-column connection. This paper presents a

technique based on fiber-reinforced polymer (FRP) composites forthe seismic upgrade of RC beam-column connections that wasvalidated with tests on 11 initially underdesigned specimens. Theresults of the experimental investigation are presented, and adiscussion on how different parameters influenced the behavior ofthe samples in terms of strength or ductility, or both, is offered.

Keywords: beams; columns; ductility; fibers; polymers; reinforcement.

INTRODUCTIONIn seismic areas, the strengthening of underdesigned reinforced

concrete (RC) structures represents a crucial issue involvingtechnical and social aspects. Because such structures wereoriginally designed to carry only gravity loads, they lack the

ductility and the hierarchy of strength that induce a globalfailure mechanism appropriate for seismic conditions.Typically, columns have minimum cross-sectional dimensionsand their longitudinal steel reinforcement is inadequate tosatisfy flexural and shear demand generated during anearthquake. This results in a weak column-strong beamconstruction that, under seismic loads, may lead to theformation of local hinges in the column. The associatedfailure mode represents the lower bound of the hierarchy ofstrength and is characterized by a brittle and catastrophicstructural failure (Bracci, Reinhorn, and Mander 1992).Furthermore, the lack of appropriate size and spacing ofcolumn ties increases the risk of brittle and local failuremechanisms such as the collapse of the column end,

resulting in crushing of the unconfined concrete, instabilityof the longitudinal steel reinforcing bars in compression, andpullout of those in tension when spliced.

The terms connection and subassemblage are hereinequally adopted to indicate the entire substructure extractedfrom a frame (Bonacci and Wight 1996) and given by columns,beams, and their intersection zone. The term joint is restrictedto the portion defined by the beam-column intersection.

RESEARCH SIGNIFICANCEThe present research focuses on the seismic strengthening

of underdesigned RC frames, which nowadays represents a

strong technical and social need in many parts of the world.The innovative aspect or the proposed technique is thecombined use of FRP laminates and bars. This combinationcreates a synergism that was not attained previously. Anexperimental campaign was conducted on scaled beam-column connections. It aimed at validating different upgradeschemes and demonstrating that such a technique has thepotential to become a sound and effective solution for thestrengthening of RC frames located in seismic areas.

PHILOSOPHY OF SEISMIC UPGRADE

Different alternatives could be selected for the seismicupgrade of deficient RC structures: the required seismicperformances can be attained by increasing the strength, orthe ductility, or both. Seismic guidelines such as FEMA 273(1997) underline that the objective can be reached by localmodification of components, removal of irregularities,structural stiffening or strengthening, mass reduction,seismic isolation, or energy dissipation. Within thesepossible approaches, the work herein presented deals withupgrade solutions based on the local strengthening ofstructural components. The driving criterion is the hierarchy

of strength: by boosting the strength of those memberswhose failure is not desirable, it is possible to attain a globalperformance characterized by the failure of more ductile andenergy-dissipating components. Such an upgrade, eventhough dealing with local strength issues, enables to achievea more ductile global performance of the system. In terms ofthe behavior of the underdesigned frame, the lower boundpertains to the column failure. The upgrade of columns, byproviding them with higher strength by confinement and/ormore flexural reinforcement, could move the failure to occurin the joint. Calvi, Magenes, and Pampanin (2001) underlinedthat, in the case of interior connections, moving the failurefrom the column to the joint can improve the global behaviorof the frame, reducing the displacement demand on the

column. The shear failure of the joint is brittle, however, andits influence on the global performance needs to be evaluatedto understand the increase or reduction it provides in termsof energy dissipation of the entire frame.

To move up along the hierarchy of strength, the jointshould be strengthened next. The upgrade of both columnand joint could allow movement from the intermediate level

Title no. 101-S69

Selective Upgrade of Underdesigned Reinforced

Concrete Beam-Column Joints Using Carbon Fiber-

Reinforced Polymersby Andrea Prota, Antonio Nanni, Gaetano Manfredi, and Edoardo Cosenza

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ACI Structural Journal/September-October 2004700

of the hierarchy of strength (that is, shear failure of the joint)to its upper bound (that is, beam failure). Inducing suchfailure would be the best result of seismic upgrade. Formation ofplastic beam hinges would mean that a ductile and veryeffective energy dissipating mechanism be achieved,maintaining the integrity of the global structure.

PROPOSED SOLUTION FOR UPGRADEOF BEAM-COLUMN CONNECTIONS

For upgrading RC beam-column connections, theproposed technique is based on the combined use of externallybonded fiber-reinforced polymer (FRP) laminates and near-surface-mounted (NSM) FRP bars. The externally bondedlaminates may be used to confine the column and to improveductility and enhance performance of the compressedconcrete. Laminates can also be used in the joint zone andbeams to improve the shear capacity. The NSM bars are usedin the column to improve flexural capacity by providing

additional tension reinforcement. The simultaneous presence ofFRP confinement by the laminates prevents the NSMreinforcement from becoming ineffective as a result ofload reversals. A selective upgrade results from choosingdifferent combinations and locations of externallybonded laminates and bars to obtain different structuralsubassemblage performances.

To assess the validity of this strengthening methodology,an experimental program was carried out. Its objectives wereto investigate the effects of FRP reinforcement on the

behavior of the beam-columnconnection, its failure mechanism,and its ductility. The aim was to demonstrate that, by meansof a targeted upgrade, it is possible to establish a hierarchy ofstrength in the subassemblage.

Extensive literature is available on installation andperformance of externally bonded FRP laminates using wetmanual layup. In particular, the reader is referred to a recentACI publication (ACI Committee 440 2002) that deals withall aspects of this concrete strengthening method. Eventhough less developed, the complementary strengtheningmethod of NSM reinforcement has been well documented(Nanni and Faza 2002; Prota, Parretti, and Nanni 2003). Thisstrengthening method consists of cutting shallow grooves inconcrete and embedding the FRP bars in an epoxy or

cementitious paste. NSM reinforcement installation isrelatively simple in the case of flexural strengthening ofbeams and slabs, particularly in the case of negative momentregions. For the case of interest in this paper, a continuousbar has to be placed along the side faces of two consecutivecolumns and through the beam. As a result, the fieldinstallation would entail a combination of concrete cuttingand drilling; particular attention should be paid during thegrooving process to avoid cutting steel.

Within the presented research, the procedure of drillingholes through the joint and making grooves on the columnfaces was eased and sped up by nailing wood strips directlyto the forms prior to casting (Prota et al. 2000). Interviewswith contractors and observations of other projects high-

lighted that field installation is possible with conventionaltools. The authors believe that once the upgrade methodologyhas been fully validated in terms of structural performance,further studies may be necessary on the technological side tooptimize the installation procedure of NSM rods and toimprove constructibility.

TEST PROGRAM AND SETUPThe experimental program consisted of 11 tests on interior

RC beam-column connections (two beams only). Theinvestigated parameters were: the constant axial load levelon the column, P; the type of FRP reinforcement (laminatesand bars); and the amount of FRP reinforcement applied. Atone of the column ends away from the joint, a constant axialload P was applied by means of a hydraulic jack (that is,Location (1) in Fig. 1) independently operated. At the end ofthe other column, a load cell was placed to record the totalreaction provided by the testing frames. To simulate pinconnections, two steel rollers were used at both ends of thecolumn. Two additional loads were applied on the ends ofeach beam with independently operated jacks (that is,Locations (2) and (3) in Fig. 1). A load cell on each jackconnected to the beam recorded the applied force. As thespecimen rested on the laboratory structural floor, a greasedplywood sheet between the specimen and the floor limitedfriction and allowed for the free movement of the specimens.

Andrea Prota is an assistant professor of structural engineering at University of

Naples Federico II, Naples, Italy. He received his Master of Science in civil engineering at

the University of Missouri-Rolla, Rolla, Mo., and his PhD in structural engineering at the

University of Naples Federico II. His research interests include the seismic behavior

of reinforced concrete and masonry structures, the use of advanced materials for new

construction, and retrofit of existing structures using innovative techniques.

Antonio Nanni, FACI, is the V&M Jones Professor of Civil Engineering at the

University of Missouri-Rolla. He is a member of the Concrete Research Council, ACICommittees 437, Strength Evaluation of Existing Concrete Structures (current Chair);

440, Fiber Reinforced Polymer Reinforcement (founding Chair); 544, Fiber Reinforced

Concrete; 549, Thin Reinforced Cementitious Products and Ferrocement; and JointACI-ASCE-TMS Committee 530, Masonry Standards Joint Committee. His research

interests include the performance of concrete-based structures.

Gaetano Manfredi is a professor of structural engineering at the University of Naples

Federico II, where he received his PhD in structural engineering. His research interests

include seismic engineering and the use of advanced composites in civil structures.

Edoardo Cosenza is a professor of structural engineering at the University of Naples

Federico II, where he received his PhD in structural engineering. His research interests

include seismic engineering, steel-concrete composite structures, and composite materials

for construction.

Fig. 1Test setup.

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First, forces representing gravity loads were applied on thecolumn and both beams (Fig. 2(a)); the direction of suchloads is north-to-south in Fig. 1. Then, the two forces (shearforces) on the beams changed cyclically to simulate aseismic action inducing shear and bending moment on thecolumns while the sum of gravity loads (that is, axial load onthe top column and beam shear forces) remained constant.Figure 2(b) and (c) depict the load pattern before and after

moment inversion on beams, respectively; they allow tounderline that the entity of seismic action on columns can becalculated based on equilibrium considerations for any givencombination of shear forces applied on the beams. Dashedarrows in Fig. 2(b) and (c) indicate the reversal forces (that is,reversal cycles).

To select the axial load P, similar tests performed oninterior and exterior subassemblages, and reported in theliterature, were considered. Three different axial load ratios(that is, axial load P divided by the product of the grossconcrete area times its compressive strength) equal to 0.1,0.2, and 0.3 were selected for the column; in this paper, theyare denoted with codes L, H, and M, respectively.

TEST SPECIMENSTypical frame and geometrical ratios of underdesigned

structures were taken into account in choosing the dimensionsof tested connections. Laboratory constraints required to adoptsome scaling to maintain the specimen size and weight to amanageable level. The objective was to design beam-columnconnections typical of gravity load frames built during the1960s without seismic provisions. For this, specimen designwas carried out following the recommendations of the ACI318-63 Building Code, predating the current code by 39 years.

The limited number of planned tests imposed to eliminatethree of the seven typical construction details (Beres et al.1996) that have been found to be critical during a seismicevent, namely, lapped splices of column reinforcement,discontinuities of beam reinforcement, and constructionjoints. The amount and spacing of longitudinal reinforcement aswell as tie size and spacing were determined according toACI 318-63 recommendations for 10 of the specimens. Thelack of proper confinement was analyzed with the remaininglast specimen by omitting some ties in the bottom column.The percentage of longitudinal reinforcement in the columnswas equal to 1.92% of the concrete column, which was stillwithin the typical range of 1 to 2%. The specimens weremade without transverse reinforcement in the joint regionand with dimensions yielding a weak-column strong-beam construction.

A square column with side equal to 200 mm and a rectangularbeam with a 200 x 355 mm cross section was selected. Basedon a design compressive strength of concrete equal to 30 MPa,the selected axial load ratios were achieved by applying axialloads of 124.5, 249.0, and 373.5 kN to the top column ofspecimen types L, H, and M, respectively, as reported inTable 1. They correspond to average stresses equal to 3, 6,

Fig. 2Loading arrangement: (a) gravity loads; (b) seismic loads before moment inversion onbeams; and (c) seismic loads after moment inversion on beams.

Fig. 3Geometry and internal reinforcement of specimen.

Table 1Test matrix

SpecimenAxial

load, kNNSM

bar type

Column Joint/beam

Laminatewrapping

NSMbars

|| tobeamaxis

tobeamaxis

NSM ||to beam

axis

L1 124.5

L2 124.5 x

L3 124.5 1 x x

L4 124.5 2 x x x x

H1 249

H2 249 x

H2U 249 x

H3 249 1 x x

H4 249 2 x x x x

H5 249 1 x x x x

M3 373.5 2 x x

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702 ACI Structural Journal/September-October 2004

and 9 MPa, respectively. The cross-shaped specimen was2.64 m long in the column direction and 3.05 m wide in thebeam direction, respectively. Four 16 bars were placed asthe longitudinal column reinforcement and 10 ties, spacedat 200 mm on center, were used as transverse reinforcement.According to the ACI recommendations, the first tie of eachcolumn was placed at 100 mm from the face of the beam.Three22 and two18 bars were used as negative and positivelongitudinal reinforcement of the beams, respectively. U-shaped10 stirrups, spaced at 100 mm on center, were used as

transverse reinforcement of the beams. In all columns andbeams, the concrete cover was equal to 38 mm. The geometry ofa typical specimen is depicted in Fig. 3.

MATERIAL PROPERTIESSpecimens were fabricated in four different placementshaving concrete cylinder compressive strengthsfc as shownin Table 2. Tensile tests were performed in accordance withASTM A 370 on three coupon specimens for each differentdiameter of steel bar. The yield strength was calculatedaveraging the results of each set of three samples. For 16,18, and 22 bars, the average yield strength was 449, 559,and 511 MPa, respectively.

Carbon FRP (CFRP) was selected for both bars andlaminates. Two different types of CFRP bars were used.Type 1 bars (Fig. 4), with diameter equal to 9.5 mm, showedan average tensile strength equal to 2155 MPa, elasticmodulus equal to 113.9 GPa, and an ultimate strain equal to

1.89%. These average values were obtained by performingthree tensile tests as described in Micelli and Nanni (2001).For Type 2 bars (Fig. 5), with diameters equal to 8 mm, theaverage tensile strength was equal to 2014 MPa, elasticmodulus equal to 108.3 GPa, and an ultimate strain equal to1.86%. Figure 4 and 5 allow observation of the differentsurface properties of Type 1 and 2 bars. The former (Fig. 4)has lugs similar to a steel bar; the sequence of four samplesdepicted in Fig. 4 shows how surface configuration variesalong the circumference. Type 2 bars are characterized by asmooth and sandblasted surface (Fig. 5); the four samples ofFig. 5 show the helicoidal winding of fibers that confine thecore of the bar.

Unidirectional CFRP laminates were adopted with the

following nominal properties: ultimate tensile strength equalto 4323 MPa, modulus of elasticity equal to 264.0 GPa, andthickness equal to 0.165 mm (Yang et al. 2002).

UPGRADE SCHEMESTable 1 summarizes upgrade schemes of tested subas-

semblages. Specimens L1 and H1 were used as controls.Subassemblages L2, H2, and H2U represent the first level ofupgrade aimed at moving the failure from the column to thejoint. For this, wrapping the end of each column (close to thejoint) for a length of 380 mm was carried out using two pliesof CFRP on each column end (Fig. 6). Specimen H2U was

Table 2Summary of experimental results

Specimen

fc ,

MPa Failure mode

Ultimatecolumn

shear, kN

Story drift angle

At cracking,%

Ulti-mate,

%

L1 38.9Compression failure

of columns41.18 0.30 3.11

L2 39.8 Tension failure of columns 44.21 0.27 2.76

L3 38.9 Shear failure of joint 57.24 0.56 3.30

L4 36.5 Column-panel interface 56.60 N/A 5.38

H1 31.7 Compression failureof columns 38.45 0.30 2.82

H2 36.5 Combined column-joint 49.70 0.35 3.50

H2U 36.5 Combined column-joint 51.19 0.35 3.53

H3 31.7 Shear failure of joint 62.35 0.36 2.42

H4 39.8 Column-joint interface 70.42 N/A 4.27

M3 39.8 Shear failure of joint 56.17 0.62 3.27

Note: N/A = not available.

Fig. 4Type 1 CFRP bars.

Fig. 5Type 2 CFRP bars.

Fig. 6Upgrade schemes: Scheme 2 (Specimens L2, H2,

and H2U) and Scheme 3 (Specimens L3, M3, and M3).

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equal to H2 in terms of upgrade solution; however, it wascharacterized by a confinement defect generated by omittingtwo ties in the bottom column. For Specimens L3, H3, andM3, NSM bars were installed on the columns prior towrapping them with CFRP laminates. The application ofsuch FRP bars, continuous through the beam, providesadditional reinforcement fully anchored and effective inthe maximum moment region of the column (Fig. 6)(Prota et al. 2001b).

In Specimens L4 and H4, the joint was also strengthenedalong with the columns. Because the major concern for the joint

is constituted by the presence of shear stresses, a diagonalreinforcement should ideally be used. Considerations based onthe practical installation of CFRP reinforcement suggestedstrengthening the joint zone in both directions parallel andperpendicular to the beam axis. In the parallel direction,NSM bars were used, while in the perpendicular direction,CFRP laminates were applied (Fig. 7). Three CFRP bars,each 1520 mm long, were installed; the CFRP laminate(one ply) covered only the area of the joint withoutextending to the columns to simulate field conditions (thatis, the presence of slab would limit access to top column).To anchor the NSM bars, CFRP single-ply U-wrapping(covering the bottom surface) of the beams for a width of510 mm from the beam-column interface was also used. The

Upgrade Level 5 is equivalent to Level 4 except for thepresence of laminates in the direction parallel to the beamaxis that substituted the CFRP bars of Scheme 4 (Fig. 7)(Prota et al. 2001b). The four FRP bars per side installedinto the column of Schemes 3, 4, and 5 increased the reinforce-ment ratio from 1.92 to 2.28% (that is, L3, H3, and H5) and2.16% (that is, L4, H4, M3). depending on the type of FRPbar used in each specimen (Table 1).

The design of the upgrade schemes was checked byperforming a strength assessment of the columns; theobjective was to verify the validity of the designed strengtheningconfigurations prior to proceeding with FRP installation.The bottom column was analyzed because it represents theworst situation as compared with the top column. Calculationswere run assuming a design compressive strength ofconcrete equal to 30 MPa and considering the load conditioncorresponding to the axial load ratio of 0.2 (that is, typeSubassemblages H). Summing the gravity load applied onthe beams (that is, 40 kN on each side) to that acting on thetop column (Table 1), an axial load Pu of 329 kN wasobtained for the analysis performed according to the numericalprocedure developed by Realfonzo et al. (2002). Threecolumns were considered: control (Scheme 1), wrapped(Scheme 2), strengthened with Type 1 NSM bars andwrapped (Scheme 3); the contribution of NSM in compressionwas disregarded. The calculated nominal moment Mn and

the shear force Vm thatMn would induce in each column arereported in Table 3. The nominal shear capacity Vn for eachscheme was also computed. The comparison between Vmand Vn highlights that the member fails for flexure in allcases; the expected failure mode is reported in Table 3. Theinteraction diagrams were also computed for the threeconsidered columns and are depicted in Fig. 8; they underlinehow the effect of CFRP wrapping is significant only abovethe balanced condition (Scheme 2). The addition of NSMbars (Scheme 3), whose contribution in compression wasdisregarded, provided a strong benefit below the balanced

Table 3Theoretical analysis on column strengthand failure mode

Column

Designfc ,

MPa Pn, kN Mn, kNm

Vm (from

Mn), kN Vn, kNExpected

failure

Control 30 329 40.25 36.59 66.3 F* - CC

Wrapped 30 329 42.22 38.38 139.1 F* - CC

Wrapped+ NSM

30 329 73.42 66.74 139.1 F - NSM

*Flexural failure.Concrete crushing.

Breaking of NSM bars.

Fig. 7Upgrade schemes: Scheme 4 (Specimens L4 andH4) and Scheme 5 (Specimen H5).

Fig. 8Interaction diagrams for unstrengthened andstrengthened columns.

Fig. 9Concrete crushing in column of Specimen L1.

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condition and does not affect the behavior of the column forhigh axial load as compared with the only-wrapped. For theaxial load ratios of 0.1 and 0.2 (that is, axial load on bottomcolumn equal to 204 and 329 kN, respectively), the wrappingyields a moment increase equal to 7.44 and 109.37%,respectively; due to the presence of NSM bars and wrapping, anincrease of column flexural capacity equal to 4.89 and82.42%, respectively, is expected. Such values confirmthat a small strength increase is provided by the wrappingbelow the balanced point; the NSM bars are very effective inthat region and their contribution decreases as the axialload increases.

OBSERVED FAILURE MODES

Experimental outcomes are summarized in Table 2. Bothcontrol connections (L1 and H1) showed column failure dueto concrete crushing (Fig. 9). Wrapping of the column endsmoved the failure from the compression to the tension sidefor the low axial load (L2): wide cracks at the column-jointinterface were observed, indicating an advanced deformation ofthe yielded steel, while light damage of the joint occurred asshown by shear cracks (Fig. 10). In the case of higher axialload on the columns (H2 and H2U), a combined column jointfailure was experienced: still-wide cracks at the column-jointinterface denoted a high level of steel deformation, but thedamage on the joint was more pronounced than for connectionL2 (Fig. 11). The installation of NSM bars as flexural

reinforcement for the column along with wrapping (L3,H3, and M3) allowed for the movement of the failurefrom the column to the joint (that is, shear failure) (Fig. 12).

Regardless of the axial load level, no damage wasobserved on the tension side of the column at the interface withthe joint for these three specimens. Strengthening of the joint(L4, H4, and H5) induced the failure to occur at thecolumn joint interface (Fig. 13). This type of failure isrelated to the layout of the FRP reinforcement; namely, inthe direction perpendicular to the beam axis, the laminate wasterminated at the column joint interface to account for thepresence of the floor system. The computer filecontaining the recorded data for Specimen H5 was notsaved; for this reason, values are not shown in Table 2.This specimen provided only additional informationconcerning the failure mode.

STRENGTH AND DUCTILITY PERFORMANCETable 2 reports values of column shear corresponding to

the failure of the subassemblage, illustrating how strengthincreases with the level of upgrade. For the lowest axial loadratio of 0.1, connection Schemes 3 and 4 showed almost thesame strength, increasing the performances of the controlspecimen by approximately 39%. An increase in strengthequal to approximately 7% was achieved by only wrappingthe columns (Specimen L2).

In the series with an axial load ratio equal to 0.2, columnwrapping allowed a gain in strength of approximately 30%and compensated for the induced lack of confinement of

Fig. 10Tensile column failure in Specimen L2.

Fig. 11Combined failure of Specimen H2U: joint (left)and column (right).

Fig. 12View of failed joint in Specimen L3: front (left) andlateral (right).

Fig. 13Failed Specimen H4 from top (left) and bottom(right) of beam.

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Subassemblage H2U, whose behavior was very similar toH2. A difference of approximately 13% was observedbetween Connections H3 and H4, whose strength wasapproximately 83% larger than that shown by Connection H1.For Type 3 specimens, the experimental evidence providedvalues of ultimate column shear independent of the axial loadratio.

Measurements of linear variable displacement transducers(LVDTs) placed on the beams allowed for the calculation ofthe story drift angle of the connection. Within a performance-based approach to earthquake at-risk buildings (ERBs), thestory drift angle is the key acceptance criterion (FEMA 3021998) as it has been recognized that the deficiency of seismicperformance is mainly related to the lack of ductility (Priestley1997). Increasing story drift angle and energy dissipationcapacity is then the main objective in the seismic upgrade of at-risk structures. Because the tested subassemblage represents anextracted portion of the frame, values of story drift angles are

representative of the behavior of the real structure (Bonacciand Wight 1996). Their analysis is then crucial to understandhow different levels of upgrade influence the global seismicperformance of strengthened RC frames.

For the low axial load ratio, column wrapping alonecaused a loss of ductility equal to approximately 11%, whilefurther column strengthening with FRP bars (that is, L3) andjoint strengthening (that is, L4) improved the ductility byapproximately 6 and 73%, respectively, compared with thecontrol subassemblage. In the case of Type H connections(highest axial load), the column wrapping boosted ductilityby approximately 24%, while their strengthening with bothFRP laminates and bars (that is, H3) lowered it by approxi-mately 14%. The upgrade of the joint region (that is, H4)increased the story drift angle by approximately 51%compared with the control specimen. In the results evaluation,the differences in concrete strength surely affected Series Hmore than L, as Table 2 shows. Finally, the ultimate story

Fig. 14Column shear versus story drift angle for Specimens: (a) L1; (b) L2; (c) L3;and (d) L4.

Fig. 15Column shear versus story drift angle for Specimens: (a) H1; (b)H2; (c) H3; and (d) H4.

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drift angle was almost the same for Connections L3 and M3,while a reduced value was provided by H3: the lowerconcrete strength seems to affect the performance of thesubassemblage in terms of ductility. Figure 14 and 15 depicttrends of column shear versus story drift angle for connec-tions of Series L and H, respectively; a similar diagram

characterized Subassemblage M3.Values of the story drift angle corresponding to joint crack

initiation were also computed and summarized in Table 2;seismic guidelines (such as ATC40 [1996]) consider such astage as a first level of degradation of the frame. Because thejoint region was not instrumented, crack initiation wasdetected based on the appearance of first diagonal cracks inthe nodal zone. Such visual methodology could not beapplied in the case of Connections H4 and L4, where thejoint was covered by FRP. For subassemblage Schemes 1and 2, the story drift when the joint started cracking was veryclose to 10% of the value of connection failure. Such valuesincreased for Scheme 3 specimens, ranging between 15 to19% of the ultimate drift angles; this is due to the contribution

provided by NSM bars to the tensile strength of the joint(Prota et al. 2001a).

COMPARISON BETWEEN UPGRADE SCHEMESThe previous analysis of test results underlined that,

depending on the axial load ratio and concrete strength, theupgrade of a component (that is, column or joint) has differenteffects on strength and ductility of the subassemblage. Based onthe experimental outcome, the fuses defined by similarspecimens in terms of column shear versus story drift anglewere analyzed. This was done at both first cracking of thejoint and failure of the subassemblage. Because the back ofeach specimen was hidden during tests, measurementsprovided by strain gages installed on the side facing the floorwere used to verify the symmetry of strain and crackdistribution; this was also checked postmortem when thespecimen was removed from the floor. For each level ofupgrade (that is, 1, 2, 3, and 4), areas having experimentalpoints as vertexes are depicted in Fig. 16.

At joint crack initiation, both control connections reachthe same story drift for the same cracking shear in thecolumn (that is, point indicated by the arrow in Fig. 16). Thecolumn wrapping does not imply a significant change both interms of cracking force and ductility (Scheme 2), while thecombination of FRP laminates and bars allows a gain ofapproximately 60% in both column shear and story drift at

joint cracking (Scheme 3). First cracking of joints reinforcedwith FRP (Scheme 4) is not discussed as the visual method(that is, based on observation) used for detecting crackinginitiation was not applicable in these cases.

In terms of ultimate subassemblage performances, theupgrade Scheme 2 determines a gain in strength rangingbetween 10 and 26%, while Scheme 3 generates an increasebetween 43 and 55%. Both schemes do not allow for asignificant improvement in terms of ductility, which can beeven reduced by the wrapping of columns with low axial

load ratio or by a combined application of FRP laminates andbars to columns with low concrete strength. In these cases,the presence of FRP increases the sectional ductility of thecolumn (in terms of curvature of its cross section), butreduces its deformability as a member and also provides astiffening effect to the entire subassemblage. Because thestrengthening of the joint causes a considerable improvement ofthe performances of the connection (that is, gain between 44and 75% in strength and between 50 and 75% in story driftangle), it is expected that such an upgrade will performsatisfactorily under seismic loads.

CONCLUSIONS

The combination of externally bonded FRP laminates andNSM bars was experimentally validated for the seismic upgradeof underdesigned beam-column connections. Experimentaloutcomes confirmed that varying the external reinforcementamount (number of plies and bars), the location (column orcolumn plus joint), and reinforcement type (laminates, bars ortheir combination) could allow the engineer to decide the level ofthe hierarchy of strength and failure mode that the connectionshould attain. Laboratory findings highlighted that the jointregion needs to be strengthened to achieve a significant gain bothin strength and ductility. For different axial load levels, shearfailure of the joint occurred at very similar values of columnshear; previous experimental outcomes evidenced that the axialload ratio does not significantly influence shear failure of the

joint (Murakami et al. 2000). Prota et al. (2001a) proved that thisoccurs due to a particular combination of principal compressionand tension stresses for which the joint approaches critical valuesof its nominal shear stress (Paulay and Priestley 1992). Theexperiments pointed out the following:

Along with the amount and location of FRP, axial loadand material properties can play an important role onthe global performances of upgraded subassemblages;

Strengthening the column can improve the behavior ofthe subassemblage, but, due to the brittle failure of thejoint, it does not provide much in terms of ductility. Theupgrade of the joint zone increases its deformabilityand also provides a significant contribution to the ductility

of the system; The termination of the laminate in the direction perpen-

dicular to the beam axis that is required by actual fieldconditions could determine a shear failure at the columnjoint interface; and

Performed tests highlighted the influence of axial loadlevel and concrete strength on the global behavior. Thissuggests that a reliable assessment of the conditions ofthe original structure, particularly with respect toapplied loads, material properties, and actual hierarchyof strength, could represent a crucial step towards asuccessful upgrade.

Fig. 16Column shear versus story drift angle at cracking ofjoint (left-hand side) and at failure of subassemblage (right-hand side).

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2004 cfrp

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