1

Seismic Evaluation and Retrofit of Bridge Joints

Nicholas J. Ereckson 1 Pedro F. Silva 2 Genda Chen 3

ABSTRACT

A research program was initiated at the University of Missouri Rolla (UMR) to accomplish the following objectives: 1) investigate the failure of typical bridge bents in the state of Missouri; and 2) propose a retrofit scheme using fiber-reinforced polymers (FRP). A total of two specimens were tested under fully reversed cyclic lateral load. Prior to undergoing any strengthening, Unit 1 was tested up to onset of shear failure in the column. At this performance level, the column was strengthened using carbon FRP (CFRP) sheets to increase its shear and confinement capacity. After retrofit, testing proceeded up to onset of joint shear failure in the bent cap. At this performance level, the bent cap was strengthened with CFRP sheets to increase its joint shear capacity. Testing continued until failure of the test unit was observed. Unit 2 was strengthened according to the same retrofit scheme as in Unit 1, but strengthening was fully completed prior to testing. Unit 2 testing proceeded according to the same loading protocol as in Unit 1 for comparison reasons.

INTRODUCTION

Despite the fact that portions of the central United States are within zones of potential seismic activity, early design codes (circa 1965) contained little or no information dealing with the design structures to resist seismic loads. Today, design codes, including the AASHTO 1998 LRFD design code, contain rigorous design guidelines for seismic loads. Increases in design requirements have led to new structures designed to current seismic standards, however, it is also essential that existing structures be retrofitted to resist seismic loads in accordance with these new seismic design standards. As a structure is subjected to an earthquake, structural failure can be caused by failure of the supporting soils and foundation system, or by failure of the lateral force resisting elements of the structure itself. The moment resisting frame of a bridge’s column/bent cap provides one such lateral force resisting system. The purpose of this research project is to analyze and evaluate the potential column/bent cap connection performance levels under simulated lateral load, and to provide a potential retrofit hierarchy scheme for inadequately designed column/bent cap connections for bridges located in the central United States.

__________________ 1 Graduate Research Assistant, University of Missouri-Rolla 2 Assistant Professor, University of Missouri-Rolla 3 Associate Professor, University of Missouri-Rolla

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Motivation for Research

In 1811 an 8.0 magnitude earthquake was recorded near New Madrid Missouri. An earthquake of magnitude 6.0 in 1843, and a larger earthquake of magnitude 6.2 in 1895 followed the 1811 earthquake. These events originated from a fault located in the New Madrid earthquake zone, which runs through New Madrid County located in southeastern Missouri. Concern exists that this trend is likely to cause additional events in the future, with indications that, the probability for an earthquake of magnitude 6.0 or greater is significant in the near future, with a 50% chance by the year 2000 and a 90% chance by the year 2040 (Anderson et al., 2001). Unfortunately, it was not until the mid 1970’s that structures in the United States, and especially in the central United States, where designed with seismic design considerations (Priestley et al., 1995; Penzien, 2001).

Seismic design of bridges in the United States and many countries abroad is currently based on a capacity design approach. A fundamental basis of this seismic design method relies on carefully selecting and detailing regions where inelastic actions are to occur during seismic events. Furthermore, all other regions are designed to remain elastic under seismic loading according to a strength hierarchy sufficient to cope with potential strain hardening and uncertainties in material properties (Silva et al., 1999). Therefore, existing structures must be examined and retrofitted, where necessary, to ensure appropriate ductile behavior in the event of an earthquake (Priestley et al., 1994). This was the objective of this research program.

Research Objectives

The primary objective of this research was to design and verify a technique by which the connecting joint between a column and bent cap for a typical central United States bridge can be strengthened using carbon fiber reinforced polymers (CFRP). Two scaled models were constructed in the laboratory at UMR. The first scale model, which was designated as Unit 1, was tested under fully reversed cyclic loading in several phases as outlined below:

Phase I: The main objective of this phase was to substantiate findings from well established analytical models in the literature that shear failure of the column would develop at a displacement ductility of approximately 2 as a result of lack of transverse reinforcement in the column. In this phase, Unit 1 was tested in its un-strengthened state, and shear failure of the column was likely to be the first performance level observed. Consequently, cyclic load testing was carried out until it was apparent that column shear failure had indeed occurred. At this stage, testing was discontinued and the column was strengthened for shear using CFRP sheets.

Phase II: Similarly, but with the column strengthened for shear, the main objective of this phase was to substantiate findings from well established analytical models in the literature that the specimen would display a bent cap/joint shear failure at a displacement ductility of approximately 3.5. Therefore, cyclic testing resumed until this failure was observed. Once again, testing was discontinued, and the bent cap/joint was strengthened for joint shear.

Phase III: Again, cyclic testing of the specimen resumed until the designed displacement ductility of the column was achieved. At this point, the structure was considered to have reached its ultimate condition, and testing was discontinued.

Phase IV: The second scale model, which was designated as Unit 2, was strengthened according to the same retrofit scheme as Unit 1, but strengthening was fully completed prior to testing. Unit 2 testing proceeded according to the same loading protocol as in Unit 1 for comparison reasons.

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DESIGN OF THE UN-STRENGTHEN TEST UNITS

To test the specimen under simulated seismic/lateral loads, a testing configuration was designed to reproduce as close as possible the shear forces and bending moments that would be experienced in the prototype bridge. Criteria used to select the prototype bridge structure, details describing the test setup, and test unit analyses are described in this section.

Selection of the Prototype Bridge Structure

Selection of the prototype structure was accomplished with the main objective of selecting an existing bridge that was constructed with inadequate seismic design considerations. Consequently, plans from thirteen bridges were obtained from the Missouri Department of Transportation (MoDOT). These bridges were analyzed according to well-established seismic evaluation analysis tools in order to select a bridge that would depict a typical bridge in the state of Missouri in need of seismic retrofit.

Performance Levels for a Typical Bent Cap/Column-bent Connection

Possible performance levels and the ideal/objective performance level of a column/bent cap and its connection are discussed in detail. From Figure 1 it can be seen that there are three main locations where distress may develop in the structural system.

A-1) Ductile Flexural Response

A-2) Brittle Shear Response

A-3) Confinement of Plastic Hinge

A-4) Buckling of Longitudinal

Reinforcement

A) Column

B-1) Ductile Flexural Response

B-2) Brittle Shear Response

B) Bent Cap

C-1) Brittle Crushing of Diagonal Compression

Strut

C-2) Brittle Joint Shear Failure with

Reinforcement Pullout

C) Bent/Cap Joint

Figure 1 Performance Levels for a Typical Bent Cap/Column-bent Connection

Distresses main occur in the column, bent cap, and joint region. Likewise, there are a number of possible performance levels at each location. Of all of these performance levels, the desirable performance level is “A-1” because it represents the one with the highest energy dissipation capacity and least impact on post earthquake evaluation and retrofit of the structural system. In the event of an earthquake, it is desirable that the bridge column/bent cap system behaves in a ductile manner so that: A) there is sufficient warning prior to total failure of the system; B) the bridge is able to remain in service for use by emergency and other vehicles after

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the earthquake, and C) following the seismic event, it is feasible to repair any damage on the bridge due to inelastic deformations of the bridge elements. Based on this philosophy the column flexural response is the desirable performance level, because, in addition to dissipating energy released during an earthquake, the formation of plastic hinges is evident to bridge inspectors following a seismic event. Formation of plastic hinges occurs at the top, bottom, or both top and bottom of the bridge column. Plastic hinges are often visible due to spalling of the cover concrete. This approach provides a warning mechanism that the bridge in question requires some level of repair prior to a return to full service. This was the philosophy used to select the prototype bridge for retrofit such that the majority of the bridge elements remained elastic and only a few inelastic actions develop in selective locations.

Analysis was performed according to current state-of-the-art analytical tools to evaluate the seismic performance of the thirteen bridges selected for this initial seismic evaluation. Preliminary analyses were carried out for the selected bridges according to the performance levels depicted in Figure 1. Selection of the prototype structure was established according to the bridge structure that would experience the greatest damage level during a seismic event. Based on this criterion the prototype bridge selected for this research program was Bridge A-2428, as depicted in Table 1. This was selected as the prototype MoDOT bridge for this research/retrofit project. Bridge A-2428 substructure is depicted in Figure 2.

Table 1 Projected performance of analyzed bridges

Bent Cap Column Bridge # Flexural

Failure Joint Shear

Failure Column Shear

(#) (PASS/ FAIL) (PASS/ FAIL) (PASS/ FAIL) A-1466 FAIL PASS PASS A-1931 PASS MARGINAL PASS A-1938 PASS MARGINAL PASS A-2024 PASS MARGINAL FAIL A-2332 PASS FAIL MARGINAL A-2333 PASS MARGINAL MARGINAL A-2334 PASS MARGINAL MARGINAL A-2336 PASS FAIL MARGINAL A-2427 FAIL PASS PASS A-2429 PASS FAIL PASS A-2430 PASS FAIL PASS A-3478 FAIL FAIL MARGINAL A-2428 FAIL FAIL FAIL

Test Setup

The overall test setup is depicted in Figure 3. To test the specimen for its response to simulated seismic/lateral loads, a testing configuration was selected to reproduce the shear forces and bending moments as close as possible to those expected in the prototype bridge. The test units consisted of a T-Section column bent cap configuration. For ease of construction and testing, the testing configuration consists of placing the test specimen “upside-down”. In order to simulate the dead load that would be carried by the bent cap/column system, a 200 kip hydraulic

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jack was placed on the load stub. Above the jack was placed a short spreader beam that allowed for one #11 Dwyidag rod at each end to pass on the sides of the column and the cap beam.

Figure 2 Center Bent of Bridge A-2428 - Prototype

2'-1

0 1/

2"

17'

2'

1'-8

"

6'

Block A w/Pin Support

4'-6"

2‘-0

”5'

-7 1

/4"

Hydraulic Jack Used to Simulate Dead Load on Structure

Hydraulic Jack Used to Simulate Dead Load on Structure

Tie Downs

Load Cell

Block B w/Roller Support

Hydraulic ActuatorHydraulic Actuator

LaboratoryStrong Wall

LaboratoryStrong Floor

Self Reacting Loading Frame

Figure 3 Test Configuration-Profile View

The longitudinal rods were connected into two long spreader beams that were placed below and to the side of the bent cap. On top of these spreader beams were placed two beams (one at each end of the bent cap) that were used to simulate the load that would be applied to the bent cap by the girders. As the jack was extended, the load was transferred to the bent cap and the spreader beam self reacted at the base of the bent cap. In this manner the system was a self-contained system. As such the load was not transferred to either of the support blocks. This system is further discussed in the design portion of the bent cap joint region. Next, in order to prevent the overturning of the entire structure with the application of the lateral load, a 200 kip post tensioning force was applied to each end of the bent cap by the tie-downs. In addition, to duplicate as close as possible actions developed in the prototype structure, a pin connection was placed in Block A, and a roller was placed in Block B.

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Test Units Reinforcement Layout

The test specimens were designed and constructed as close as possible to the prototype bridge bent cap. Due to restraints imposed by laboratory floor space, and equipment, a full size model was not feasible; therefore, an 80% scale model of a portion of the prototype bridge bent was designed for testing in the lab. When designing the specimen, care was taken to reduce the area of reinforcement such that the reinforcement ratios were identical to the prototype. Specific reinforcement details for the columns and bent cap of the test units are shown in Figure 4, and longitudinal section of the test units is depicted in Figure 5. The column diameter was 24in, and the clear cover to the column longitudinal reinforcement was 2in. Longitudinal reinforcement of the column consisted of 14-#9 for a longitudinal reinforcement ratio of approximately 3%. The column transverse reinforcement consisted of #4-hoops spaced at 9 5/8in. for a volumetric reinforcement ratio of approximately 0.40%. The bent cap was 29in wide and 34.5in deep, and the clear cover to the longitudinal reinforcement was 1.5in all around. In the bent cap, the longitudinal reinforcement in the top (interface with the column) and bottom layer consisted, respectively, of 5-#8 and 10-#8, for a longitudinal reinforcement ratio of approximately 0.40% and 0.80%, respectively. The bent cap shear reinforcement consisted of #5 stirrups spaced at 7.25in.

2"2'

#4 @ 9 5/8" o/c14 - #9

2'-5"

1 - #5(Each Side)

#5 Stirrup@7 1/4" o/c

2'-1

0 1/

2"

10 - #8 Bars

5 - #8

4"

1 1/2“ (Typical)

(a) Column (b) Bent Cap

Figure 4 Bent Cap and Column – Reinforcement Layout

2' S

quar

eLo

ad S

tud

Bottom & TopBars BentAt Beam Ends

No StirrupsCenter of Bent Cap

1 1/8" ØA490Bolts

6"

5 - #8Top Reinf.

1 3/4" Ø PVCThrough LoadStud (As Shown)

2‘-0”

#4 Hoops @ 9 5/8" o/c

1 - #5(Each Side) #5 Stirrup

@7 1/4" o/c

10 - #8 BarsBottom Reinf.Arranged in 2 Layers

14 - #9

2"

2'-1

0 1/

2"

Figure 5 Longitudinal Section – Reinforcement Layout

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Construction of the Test Units

Throughout construction of the un-strengthen test specimens, care was taken to ensure that the test specimens were constructed in a similar manner to the prototype. A construction fabricator supplied the rebar bent to the required shape and dimensions per a set of bar bending diagrams. The first step in the construction consisted of tying the bent cap reinforcement cage as shown in Figure 6. Next the bent cap cage was placed inside wooden forms and the column was set in place, see Figure 7.Casting of the bent cap was next. The bent caps were poured prior to the columns, and were poured in three lifts of approximately twelve inches each. Each layer was thoroughly vibrated to prevent honeycombing of the concrete. The next step consisted of forming the column and load stub, as shown in Figure 8. Similarly, the columns were poured within one week of pouring the bent caps, and they also were poured in multiple lifts and were thoroughly vibrated. Finally, the wooden forms were stripped of the test units after four days of curing in place in ambient conditions, and the test unit was set in place. Figure 9 depicts Unit 1 fully instrumented during testing.

Figure 6 Bent Cap Reinforcement Cage Figure 7 Column Reinforcement Cage in Place

Figure 8 Test Unit During Column Casting Figure 9 Unit 1 During Testing

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Material Properties

Throughout construction, material samples of the concrete and reinforcing steel were taken so that the actual material properties could be used in the final analysis of the two specimens. Because there were two specimens and each specimen required two different days to pour, a total of four sets of concrete cylinders were taken. The strengths from these tests are given in Table 2. The reinforcing steel material properties were determined by performing a standard steel coupon tension test. Each bar size was tested to accurately determine the actual bar strengths (see Table 3). The type of carbon fiber that was used for strengthening was changed between the first and second specimens. However the material properties supplied by the manufacturer remained the same for both fibers as given in Table 4.

Table 2 Concrete Strengths

28 Day Strength

Strength at Time of Testing

(psi) (psi) Beam 5026 5342 Specimen #1 Column 3525 3697 Beam 4216 4609 Specimen #2 Column 4993 5419

Table 3 Rebar Coupon Strengths

Bar #

Yield Strength (ksi)

4 87 5 60 8 80 9 78

Table 4 Carbon fiber material properties

Specimen # 1 and 2 Ultimate Tensile Strength 550 ksi Ultimate Rupture Strain 1.67%

Tensile Modulus 33,000 ksi Fabric Width 24 in.

Nominal Thickness 0.0065 in/ply

Analyses of the Un-Strengthen Test Units

In this section the analysis of the units in their un-strengthen condition is covered step by step. It covers each of the performance levels outlined in Figure 1.

Column/Bent Cap Flexural Response Evaluation

Moment curvature analyses for the column and bent cap were carried out using the section properties depicted in Figure 4 and the material properties shown in Table 2 and Table 3. The moment-curvature analyses for the un-strengthen test units’ column with an axial load ratio of 5%, and for the bent cap considering no influence from axial loads were constructed using a moment curvature program. Lateral seismic loads applied to the column are directly transferred to the bent cap; consequently, flexural demands on the bent cap are a direct result of the moment capacity of the column. As such, in order to predict the expected response of the bent cap the moment-curvature capacity of the column must be converted to a moment-curvature demand on the bent cap. The interior joint the moment demand on the bent cap may be estimated as half of the column moment capacity. However, the study carried out in this project demanded a more

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precise evaluation of the flexural response of the bent cap for retrofit considerations. Combining the bending moment due to the axial load on the column and the bending moments due to the applied lateral loads; it can be shown that the maximum moment demand on the bent cap is given by the following expression:

( )4

dLP

2hL

LL

DL2

MM cP

bC

C

B

BCCD

B−⋅

+−

−⋅= Eq. 1

Since in the test unit the negative bending moment capacity is higher than the positive

bending moment demand (at the interface of the column), only the results for the positive bending moment are discussed next. Referring to Figure 10 it is clear that some level of inelastic deformations will develop in the bent cap. However, these will not be significant and only minor spalling or yielding of the longitudinal reinforcement is likely to occur. This will be addressed during retrofit, and CFRP sheets will be placed on the bent cap longitudinally to increase the flexural capacity of the bent cap.

0 10 20 30 40 50

Curvature Ductility (mF=Fm/Fy)

0

2000

4000

6000

8000

Mom

ent (

kips

-in)

5% f'c Ag-Axial LoadPositive Bent Cap CapacitySteel YieldingConcrete Spalling

0

200

400

600

800

Mom

ent (

kN-m

)

Figure 10 Bent Cap Flexural Assessment

Column/Bent Cap Shear Capacity Evaluation

Two different models were implemented to determine the theoretical shear capacity of the column and bent cap. One of the models used to determine the shear capacity of both the column and the bent cap was the ACI 426 shear model was (ACI, 2002). The second model used to determine the shear capacity of both the column and the bent cap was the shear model developed at the University of California San Diego (UCSD) (Priestley et al., 1995). Unlike the ACI model, in the UCSD model the shear strength of the concrete, Vc, changes as the section is subjected to varying amounts of ductility. This accounts for a loss of shear strength in the area of the plastic hinge as cracks form and the section begins to open. In order to quantify the loss of strength due to increased ductility demand, an additional factor, “k”, is used to reduce the concrete shear strength. In addition, in the UCSD model the shear strength enhancement due to the axial load strut is also considered in the evaluation of the shear capacity evaluation. Referring to Figure 11, it is clear that according to the UCSD shear model, shear failure of the column is likely to occur at a curvature ductility level of 10, which correspond to a displacement ductility

10

level of 2. This indicates that shear failure of the column was expected at the onset of spalling of the cover concrete. However, according to the ACI shear model, shear failure of the column is not likely to occur. Test results clearly show that shear failure of the column was the controlling failure of the column during Phase I. On the other hand, shear failure of the bent cap is not likely to occur as depicted by the evaluation conducted by either of the models employed.

0 10 20 30 40 50Curvature Ductility (mF=Fm/Fy)

0

50

100

150

200

250

Shea

r (ki

ps)

0

400

800

Shea

r (kN

)

Figure 11 Shear Capacity Analysis

Joint Shear Capacity Evaluation

Extensive experimental investigation of joints in the literature (Silva et al., 1999; Sri et al., 1997; Sri, 1998) indicate that joint shear cracking is not expected when the principal tensile stresses are below:

]psi[f5.3 'ct ≤ρ Eq. 2

Above the limits given below joint shear failure is highly likely. ]MPa[f0.5 '

ct ≥ρ Eq. 3 Evaluation of the test units principal tensile stresses indicate that stresses above these

limits are likely and as a result joint shear failure will most likely develop for Unit 1 during Phase II of testing. As a result, Units 1 and 2 were retrofitted for joint shear.

DESIGN OF THE UN-STRENGTHEN TEST UNITS

Unit 1 was only strengthened at certain prescribed performance levels, while Unit 2 was strengthened according to the same retrofit scheme as in Unit 1, but strengthening was fully completed prior to testing. The strengthening scheme is described in more detail in this section.

5% f'c Ag-Axial LoadColumn ACI ShearColumn UCSD ShearPositive CapacityBent Cap ACI ShearBent Cap UCSD ShearSteel YieldingConcrete Spalling

11

Column Strengthening Scheme

Column strengthening was accomplished according to two design protocols. One design protocol was performed in regions where significant plastic rotations will develop, and the second protocol for elastic regions governed by shear.

Confinement Enhancement

The first protocol was to strengthen the column section within inelastic regions with the main objective of enhancing its ductility. The thickness of the required jacket was given by: (Priestley, et al., 1995)

( ) 'cc

ujuj

cuj Df

f004.010.0

tεε −

=

Eq. 4

In order to implement this equation in design it is necessary to estimate an ultimate

displacement ductility demand. It was stipulated that the column must be able to achieve a displacement ductility level of 12.0, which corresponds to an ultimate limit state stipulated at fracture of the longitudinal reinforcement due to low cycle fatigue. Converting the displacement ductility to curvature ductility for this structure yields a curvature ductility of approximately 20.0. Given the material properties of the carbon fiber presented in the material properties section, and using a moment curvature analysis in order to estimate the design concrete strain, εcu, at the design curvature ductility of 20.0, a total of 9 plies were required within: (Priestley, et al., 1995)

yblyblCP fd30.0fd15.0L08.0L ≥+= Eq. 5 Strengthening in the plastic hinge region is shown in Figure 12.

2'6 Plies

2" Clear Gap

1'-10"9 Plies

No Plies

In LoadStud

1'-7 1/4"3 Plies

2'6 Plies

2" Clear Gap

1'-10"9 Plies

No Plies

In LoadStud

1'-7 1/4"3 Plies

Figure 12 Column Strengthening Details

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Shear Enhancement

In regions that perform either in the elastic or inelastic range it was also necessary to investigate design of the strengthening scheme for shear considerations according to the following: (Priestley, et al., 1995)

( )

θπφ

cotDf50.0VVV/V

tj

PSCsO

j++−−

= Eq. 6

Where fj is the design jacket strength, and is given by: (Priestley, et al., 1995) jj E004.0f = Eq. 7

Once again, given the material properties of the carbon fiber presented in the material properties section, and using the moment curvature analysis in order to estimate the moment capacity of the column section at the design curvature ductility of 20.0, a total of 2 plies were required on all other sections outside of the plastic hinge. However, because shear failure is defined by an extremely brittle behavior, and because carbon fiber has no yield plateau, it was decided, as an additional factor of safety, that one additional ply of FRP should be used to strengthen the column for shear. In addition the number of plies was tapered from 9 to 3 according to the illustration shown in Figure 12.

Cap Beam/ Joint Strengthening Scheme

Strengthening of the bent cap/joint was accomplished with the main objective to ensure that the bent cap and joint regions would remain elastic under the ultimate lateral loads, which were governed by the column ultimate moment and shear capacity. A strut and tie model within the joint region was derived according to the scheme depicted in Figure 13.

TCCC

FH

FV

TBL

CBL TBR

CBR Figure 13 Interior Mechanism Strut-and-tie Model

Referring to Figure 13, analysis of the strut-and tie model was carried out by ensuring

that the tension forces TC and TBR were clamped, respectively, by the horizontal and vertical component of the diagonal CFRP sheet crossing the joint. Conservatively it can be shown that:

⎟⎟⎠

⎞⎜⎜⎝

⎛ −=

b

UCH h5.0

c5.0D7.0TF Eq. 8

13

And

⎟⎟⎠

⎞⎜⎜⎝

⎛ −=

D5.0c5.0d

TF YBLV Eq. 9

Once again assuming conservatively that these forces are perpendicular to each other it can be shown that the CFRP sheet tensile diagonal force is given by:

( ) ( )2

V2

HCFRP FFF += Eq. 10 As before, given the material properties of the carbon fiber presented in the material

properties section, and using the moment curvature analysis for the strengthen sections, in order to estimate the column tensile force, TC, the beam tensile force, TBL, and the neutral axis at ultimate, cU, and yielding, cY, a total of 3 plies were required in the CFRP sheet placed in the diagonal direction of the joint. In order to enhance the clamping resistance for the diagonal sheets and to enhance the flexural capacity of the cap beam layers of 6 additional sheets were placed in the other directions as shown in Figure 14.

Through the testing of the first specimen it became evident that certain modifications

needed to be made to the design of the FRP retrofit to improve its effectiveness. As testing proceeded on the first specimen, it was noted that the FRP retrofit was not acting to its full potential as the sheets began to delaminate in areas of high tensile concentrations as well as in locations where the FRP sheets were subjected to compressive stresses. Additionally, it was noted that the FRP retrofit did very little to prevent up lift of a conical concrete core that developed in the region of the bent cap/column joint during testing. Therefore, FRP anchors were used in Unit 2 to: (1) prevent the delamination of the ends of the FRP sheets in areas of tensile stress concentrations; (2) prevent the buckling of the FRP sheets in areas of compressive stresses, and (3) anchor the conical concrete core of the bent cap/column joint.

GFRP Anchors Layout CFRP Sheets LayoutGFRP Anchors Layout CFRP Sheets Layout Figure 14 Bent Cap Strengthening Details

Strengthening of the Test Units

As previously discussed the first test specimen was tested in three stages (to column shear failure, to joint shear failure, and to ultimate). Following each of the first two stages, testing was stopped to strengthening the structure for the demonstrated performance level. Figure 15 depicts strengthening of the column for shear and confinement enhancement, and Figure 16 depicts strengthening of the bent cap for joint shear and flexural capacity enhancement. During the last few years, extensive techniques have been developed using FRP composites for repair and strengthening of RC members walls. Details on the application of CFRP sheets have been

14

extensively published in the literature (ACI 440, 2001; Nanni and Dolan, 1993; Nanni, 1993; Nanni et al., 2001) and for brevity they are not presented herein.

Figure 15 Retrofit of Column Figure 16 Retrofit of the Bent Cap

LOADING PROTOCOL

Loading of the test unit was performed according to the loading sequence presented in Figure 17. The first step in the testing procedure consisted of applying the vertical loads for gravity load simulation. Following the application of the initial axial loads the test unit was first subjected to single cycles under force control at 25%, 50%, 75% and 100% of the theoretical first yield lateral force. First yield of the test specimen was obtained from a moment curvature analysis of the column, and corresponded to the tension yielding of the column. The structure was then loaded under displacement control with three cycles applied at each specified displacement ductility level.

0 5 10 15 20 25 30Number of Cycles (#)

-14

-12

-10

-8

-6

-4

-2

0

2

4

6

8

10

12

14

Disp

lace

men

tDuc

tility

( mD)

Loading LevelsLocations where dynamic testing was performedLocations where retrofit was added to the structure

(Specimen #1 Only)

V=1

9.43

k

V=3

8.85

k

V=5

8.28

k

Single Cyclesto First Yield

(LoadControl)

3 Cycles at EachDisplacement Level

Above Yield(Displacement Control)

m1.0

Dy=0.78"

m1.5

D=1.17"

m2.0

D=1.56"

m3.0

D=2.34"

m4.0

D=3.12"

m5.0

D=3.90"

m6.0

D=4.68"

m8.0

D=6.24"

m12.0

D=9.36"

0 5 10 15 20 25 30Number of Cycles (#)

-14

-12

-10

-8

-6

-4

-2

0

2

4

6

8

10

12

14

Disp

lace

men

tDuc

tility

( mD)

0 5 10 15 20 25 30Number of Cycles (#)

-14

-12

-10

-8

-6

-4

-2

0

2

4

6

8

10

12

14

Disp

lace

men

tDuc

tility

( mD)

Loading LevelsLocations where dynamic testing was performedLocations where retrofit was added to the structure

(Specimen #1 Only)

V=1

9.43

k

V=3

8.85

k

V=5

8.28

k

Si

Loading LevelsLocations where dynamic testing was performedLocations where retrofit was added to the structure

(Specimen #1 Only)

V=1

9.43

k

V=3

8.85

k

V=5

8.28

k

Single Cyclesto First Yield

(LoadControl)

3 Cycles at EachDisplacement Level

Above Yield(Displacement Control)

m1.0

Dy=0.78"

m1.5

D=1.17"

m2.0

D=1.56"

m3.0

D=2.34"

m4.0

D=3.12"

m5.0

D=3.90"

m6.0

D=4.68"

m8.0

D=6.24"

m12.0ngle Cyclesto First Yield

(LoadControl)

3 Cycles at EachDisplacement Level

Above Yield(Displacement Control)

m1.0

Dy=0.78"

m1.5

D=1.17"

m2.0

D=1.56"

m3.0

D=2.34"

m4.0

D=3.12"

m5.0

D=3.90"

m6.0

D=4.68"

m8.0

D=6.24"

m12.0

D=9.36"

Figure 17 Fully Reversed Cyclic Loading Protocol

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EXPERIMENTAL RESULTS - TEST UNIT 1

General Observations

Observations were recorded at each cycle peak reversal. The first stage of testing consisted of applying the axial load to the specimen in order to simulate the dead load on the structure. The axial load was increased at a slow rate from 0.0 kip to 160.0 kip.

Testing During Phase I: Seismic response of the test unit was simulated using a lateral

load sequence with fully reversed cycles shown in Figure 17. Onset of flexural cracking occurred at the interface of the column to the bent cap in the first cycles under force control. In the last cycles in force control onset of diagonal shear cracking in the joint regions was observed as depicted in Figure 18. At this stage only hairline cracks were observed indicating that column or joint shear failure had not yet occurred. During the first cycles in displacement control and at the displacement ductility of 2 onset of spalling in the columns was observed and during the second cycle to displacement ductility of 4 shear failure of the column was observed (see Figure 19). This failure mode was characterized by wide-open shear cracks and loss in the lateral load. This marked the end of Phase I of testing, which was followed by retrofit of the column according to the retrofit scheme depicted in Figure 12 and Figure 15.

Figure 18 Test Unit 1 At First Yield

Figure 19 Test Unit 1 At Onset of Column Shear Failure

Testing During Phase II: After retrofit of the column was accomplished, testing proceeded in displacement control and according to the loading protocol. Testing proceeded until the next failure mode was observed. The next failure mode occurred in the joint region, characterized by wide open cracks in the joint region and significant dilation of the joint in the transverse direction to the bent cap, as depicted in Figure 20. After removal of the loose concrete wide open cracks extended through the entire width of the joint exposing the column longitudinal reinforcement, as depicted in Figure 21. This marked the end of Phase II of testing, which was followed by retrofit of the bent cap according to the retrofit scheme depicted in Figure 14 and Figure 16. However, for Unit 1 no GFRP anchors were provided.

16

Testing During Phase III: After joint shear retrofit of the bent cap was accomplished, testing proceeded in displacement control and according to the stipulated loading protocol. Testing was carried out until ultimate failure of the unit was observed, see Figure 22. The test unit ultimate failure mode was characterized by pull-out of column longitudinal reinforcement from the joint region.

Figure 20 Test Unit 1 At Onset of Joint Shear Failure

Figure 21 Test Unit 1 At Onset of Joint Shear Failure

Figure 22 Test Unit 1 - At Ultimate State

Load-Deformation Response

Figure 23 shows the load deformation response of the test unit along with the predicted yield and ideal capacities. Response of the test unit shows that the predicted ultimate flexural strength of the column was achieved; however, significant strength degradation was observed beyond the displacement ductility of 4. This marked the end of Phase II, which corresponded to the onset of joint shear failure. Although, during Phase III first cycle the test unit was able to regain some of its capacity before joint shear failure occurred, beyond the second cycle it can be

17

observed a drastic decrease in the unit capacity. This indicates that joint shear retrofit of the bent cap was not adequate in preventing pullout failure of the column longitudinal reinforcement from the joint. The test unit hysteretic loops displayed considerable energy absorption capacity but with significant pinching, which is characteristic of joint shear failure.

-10 -8 -6 -4 -2 0 2 4 6 8 10Displacement (in)

-200

-150

-100

-50

0

50

100

150

200La

tera

l Loa

d (k

ip)

-800

-600

-400

-200

0

200

400

600

800

Late

ral L

oad

(kN

)

-250 -200 -150 -100 -50 0 50 100 150 200 250Displacement (mm)

mD=1 2 3 4 6 8 10

V'y

Vy

12

mD=12 123468

V'y

Vy

10

Figure 23 Unit 1; Load-Deformation Response

EXPERIMENTAL RESULTS - TEST UNIT 2

General Observations

As in Unit 1, in Unit 2 observations were recorded at each cycle peak reversal. As before the first stage of testing consisted of applying the axial load to the specimen in order to simulate the dead load on the structure. The next stage of testing consisted of applying the pseudo-dynamic lateral load to the top of the test column in load control up to the first yielding of the column longitudinal reinforcement, and in displacement control thereafter.

Testing During Phase IV: As previously described, Unit 2 was strengthened according to the same retrofit scheme as in Unit 1, but strengthening was fully completed prior to testing. Unit 2 testing proceeded according to the same loading protocol as in Unit 1 for comparison reasons. Testing was carried out until ultimate failure of the unit was observed. The test unit ultimate failure mode was characterized by fracture of column longitudinal reinforcement. Unlike Unit 1, this observation indicates that joint shear retrofit of the bent cap was adequate in preventing pullout failure of the column longitudinal reinforcement from the joint. The test unit remained basically crack free (see Figure 24) and with significant plastic rotations at the base of the column (see Figure 25)

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Figure 24 Test Unit 2 - At Ultimate State Figure 25 Test Unit 2 - At Ultimate State

Load-Deformation Response

Figure 26 shows the load deformation response of the test unit along with the predicted yield and ideal capacities. Response of the test unit shows that the predicted ultimate flexural strength of the column was achieved, with significantly lower levels of strength degradation as compared to Unit 1. During testing to the second cycle at the displacement ductility of 9 fracture of the column longitudinal reinforcement was observed. This indicates that joint shear retrofit of the bent cap was, in this case prior to testing, adequate in preventing pullout failure of the column longitudinal reinforcement from the joint. The test unit hysteretic loops displayed considerable energy absorption capacity. Unlike Unit 1, pinching of the hysteresis loops is significantly lower, indicating once again that joint shear failure was not the prevalent failure mode.

CONCLUSIONS

Unit 1 test results indicate that: 1) shear and confinement capacity of the column were enhanced by strengthening of the column with CFRP sheets in the hoop direction; and 2) strengthening of the joint region was not adequate in preventing joint shear failure leading to anchorage failure of the column longitudinal reinforcement. In real applications when there has been significant damage to the joint region it is unfeasible to strengthen the joint region by simply applying CFRP sheets. Post tensioning may be required to completely close the cracks through the joint region before applying CFRP sheets. Unit 2 test results indicate that: 1) as in Unit 1, shear and confinement capacity of the column were enhanced by strengthening of the column with CFRP sheets in the hoop direction; and 2) unlike Unit 1, strengthening of the joint region with CFRP sheets prior to testing was adequate in preventing joint shear failure. Failure of Unit 2 was characterized by fracture of the column longitudinal reinforcement with minimum strength degradation at ultimate. These results indicate that retrofitting with CFRP sheets was adequate in improving the seismic response of bridges.

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-10 -8 -6 -4 -2 0 2 4 6 8 10Displacement (in)

-160

-120

-80

-40

0

40

80

120

160

Load

(kip

)

-600

-400

-200

0

200

400

600

Load

(kN

)

-250 -200 -150 -100 -50 0 50 100 150 200 250Displacement (mm)

mD=1 2 3 4 6 8 9

mD=9 123468

V'y

V'y

Vy

Vy

Figure 26 Unit 2; Load-Deformation Response

REFERENCES

ACI 440 (2001), “Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures,” American Concrete Institute Committee 440, Detroit, Michigan, pp103.

American Concrete Institute (ACI), (2002), “Building Code Requirements for Structural Concrete” ACI318-2002,

Farmington Hills, MI, January 2002. Anderson N, Baker H, Chen G, Hertell T, Hoffman D, Luna R, Munaf Y, Prakash S, Santi P, and Stephenson R.

(2001), “Earthquake Hazard Assessment Along Designated Emergency Vehicle Priority Access Routes,” Final Report RDT REDT01-009, Missouri Department of Transportation.

Nanni, A. and Dolan, C.W., Editors (1993), "FRP Reinforcement for Concrete Structures," Proceedings of the ACI

SP-138, American Concrete Institute, Detroit, MI, pp. 977. Nanni, A. (1993), “Fiber-Reinforced-Plastic (FRP) Reinforcement for Concrete Structures: Properties and

Applications,” Developments in Civil Engineering, Vol. 42, Elsevier, Amsterdam, The Netherlands, pp. 450. Nanni, A., P.C. Huang and G. Tumialan (2001), “Strengthening of Impact-Damaged Bridge Girder Using FRP

Laminates,” Proceedings of the Ninth International Conference, Structural Faults and Repair, London, UK, July 4th –6th, 7 pp.

Penzien J. (2001), “Earthquake Engineering for Transportation Structures – Past, Present, and Future.” Earthquake

Spectra, Vol. 17, No. 1, pp. 1-34.

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Priestley, M.J.N., Seible, F., and Uang, C.M. (1994), “The Northridge Earthquake of January 17, 1994: Damage Analysis of Selected Freeway Bridges,” Department of Structural Engineering, Report No. SSRP-94/06, University of California San Diego, La Jolla, California, Aug 1994, 260 pp.

Priestley, M. J. N., Seible, F., and Calvi, M. (1995), Seismic Design and Retrofit of Bridges, John

Wiley & Sons, Inc., New York, NY, Sep 1995, 672 pp. Silva, Pedro F., Sritharan, S., Seible, F., Priestley, M.J.N. (1999), “Full-Scale Test of the Alaska Cast-In-Place Steel

Shell Three Column Bridge Bent”, Division of Structural Engineering, University of San Diego, La Jolla, California, February 1999, Report # SSRP-98/13

Sritharan, S., Priestley, M. J. N., Seible, F. (1997), “Seismic Design and Performance of Concrete Multi-Column

Bents for Bridges”, Department of AMES - Division of Structural Engineering , University of California San Diego, La Jolla, California, June 1997, Report # SSRP-97/03.

Sritharan, S. (1998), “Analysis of Concrete Bridge Joints Subjected to Seismic Action”, Doctoral Dissertation,

Department of AMES - Division of Structural Engineering, University of California San Diego, La Jolla, California.

ACKNOWLEDGEMENTS

The research project described in this report was funded by the Federal Highway Administration, the states of Missouri and Alaska Department of Transportation and Public Facilities, and the University Transportation Center located at UMR.

LIST OF NOTATIONS

cU Neutral Axis at Ultimate cY Neutral Axis at First Yielding D Column Diameter

bld Main longitudinal bar diameter

jE CFRP Jacket Modulus of Elasticity FH Horizontal Clamping Force FV Vertical Clamping Force

'ccf Compression Strength of Confined Concrete

ujf CFRP Jacket Ultimate Stress

yf Steel Yield Stress hb Beam Depth LC Column Height LB Beam Length

pL Length of the plastic hinge CCM Column Moment Capacity DBM Bent Cap Moment Demand

NA Neutral Axis at Yielding P Column Axial Load TC Column Tension Force tj FRP Jacket Thickness V ° Shear Demand on Column at Column Over-

strength Flexural Capacity VC Concrete Component Shear Resisting Mechanism VS Transverse Reinforcement Shear Resisting

Mechanism VP Axial Load Shear Resisting Mechanism εcu Ultimate Concrete Strain εuj CFRP Jacket Ultimate Strain µφ Curvature ductility µ∆ Displacement ductility

sφ Shear Strength Reduction Factor

θ Greater of 35° or the column corner to corner angle