Flex Piers

8/11/2019 Flex Piers

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Alkhrdaji, T and A. Nanni, "Flexural Strengthening of Bridge Piers Using

FRP Composites," ASCE Structures Congress 2000, Philadelphia, PA,M.Elgaaly, Ed., May 8-10, CD version, #40492-046-008, 8 pp.


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FLEXURAL STRENGTHENING OF BRIDGE PIERS USING FRP COMPOSITES

Tarek Alkhrdaji, Graduate Research Assistant/Ph.D. Candidate

Antonio Nanni, Ph.D., P.E., V&M Jones Professor of Civil Engineering, FASCE

Center for Infrastructure Engineering Studies (CIES)University of Missouri-Rolla

Abstract:

The effectiveness of FRP jackets for increasing the shear capacity and the

flexural ductility of reinforced concrete (RC) columns was demonstrated in manystudies. However, for smaller axial loads, the contribution of FRP jackets to flexural

strength is minimal. Using FRP sheets in the direction of a column with end

anchorage to improve its flexural capacity at the base is not easily achieved. Thispaper reports on a research project aimed at upgrading the flexural capacity of RC

piers using near-surface mounted (NSM) FRP rods. Flexural strengthening and

testing to failure of the piers were carried out on a bridge that was scheduled for

demolition during the Spring of 1999. Three of the four piers of the bridge werestrengthened with different configurations using FRP rods and jackets. The flexural

strengthening was achieved using NSM carbon FRP rods that were anchored into the

footings. The piers were tested under static push/pull load cycles. An analyticalmodel was developed to determine the net forces acting on a bridge pier at a given

load level based on the measured response. Strengthening techniques, test results,

modes of failure, and sample analytical results of tested bridge piers are described

and the effectiveness of this technology is demonstrated.

Keywords:

Bridge piers, Carbon fibers, Fiber Reinforced Polymer (FRP), Flexural strengthening,Near-surface mounted (NSM) reinforcement, Structural modeling.


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INTRODUCTION

Many reinforced concrete (RC) bridge piers, constructed in the first half ofthis century, were designed as gravity piers with minimal flexural capacity. The

potential risk of failure of these piers under a moderate earthquake is becoming a

growing concern to states DOTsRC piers can be seismically deficient in shear and flexural strength, and

flexural ductility. Due to lack of seismic detailing requirement, it is common to find

minimal amount of transverse reinforcement in gravity piers constructed prior to1970. However, they can be adequate to resist the earthquake induced shear forces

due to their large cross sections. Inadequate flexural strength, on the other hand, may

arise from the low seismic lateral forces that were typically considered in earlier

designs. Inadequate flexural strength may also arises from the premature terminationof the main reinforcement or its inadequate splicing. One method for retrofitting

piers with flexural strength deficiency consists of the addition of a RC jacket. This

method is also effective in improving the shear strength and the ductility of a pier.

However, it may not be very practical due to undesirable section enlargement orconstruction constraint.

Previous work on strengthening of columns with FRP composites hasdemonstrated the effectiveness of jacketing with FRP in the hoop direction in

improving the shear capacity and the flexural ductility of RC rectangular columns

(Seible et al, 1995). Since some gravity piers are designed to carry axial loads that

are only a small fraction of their axial load capacity, the influence of jacketing onenhancing the flexural capacity is minimal. This is because jacketing can only

improve the flexural capacity through concrete confinement if failure was governed

by concrete crushing (compression-controlled failure). Strengthening of columns forflexure using FRP sheets with the fibers aligned in the column direction is not

practical due to anchorage requirement at the base of column. New techniques for

the flexural retrofit of RC piers, especially gravity piers, are therefore required.In an attempt to improve the flexural capacity of columns jacketed with FRP

sheets, researchers have used steel plates with bolt connections accompanied by

section enlargement at the base of the column (Hakamada, 1997). This method

resulted in a slight improvement of the flexural capacity. However, such mechanicalanchors, although effective in the laboratory, are not very practical for field

application due to drawbacks such as stress concentration, which can cause the

premature rupture of FRP. In addition, where carbon FRP is used, the likelihood ofgalvanic corrosion due to steel-carbon fiber contact is an additional concern.

Strengthening of RC members with near-surface mounted (referred to as

NSM) FRP rods is another technique that consists of embedding FRP rods in groovesmade on the surface of the concrete and bonded in place with epoxy. This technique

was successfully used to upgrade Pier 12 at the Naval Station in San Diego, CA to

meet demand of operational changes accompanied by higher vertical loads (NavalFacilities, 1998). The use of NSM rods is more practical than externally bonded FRP

laminates when the end anchorage of the FRP reinforcement is an essential design

requirement or when the installation of laminates involves extensive surface

preparation work.


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A research program at the University of Missouri-Rolla was tailored to

investigate the applicability and effectiveness of NSM rods in improving the flexural

capacity of RC piers. Bridge J857, located in Phelps County-Missouri, wasscheduled for demolition during the Spring of 1999. The bridge was, therefore,

considered for the strengthening and testing to failure of its RC piers. A structural

model was developed to reflect the observed behavior of the bridge piers. The modelwas analyzed using the matrix displacement method of structural analysis to

determine the internal forces (moments) and external force acting on the pier at any

applied load level by using the measured deformations as an input.

DESCRIPTION OF THE BRIDGE PIERS

Bridge J857, was build during the early 1930s and represented typicalconditions of existing bridges in mid-America. It consisted of three simply supported

solid RC decks with an original roadway width of 7.6 m (25 ft). Each simply

supported deck spanned 7.9 m (26 ft). The bridge bents (see Figure 1) consisted of

two piers connected at the top by a RC cap beam. The piers had a 0.6

0.6 m (2

2ft) square cross-section and were reinforced with four 19 mm (#6) deformed steel

rods. The transverse reinforcement consisted of 6 mm (#2) steel ties spaced at 457

mm (18 in). Each pier was supported by 1.2 1.2 0.75 m (4 4 2.5 ft) squarefooting. The actual length of the piers varied from 1.8 to 3.4 m (6 to 11 ft). No

corrosion of reinforcement or concrete spalling was observed on the bridge piers.

STRENGTHENING SCHEMES

Seismic performance category (SPC) B was selected for the analysis of the

bridge piers since it is relevant to Missouri (AASHTO, 1996). Under SPC B

condition, the seismically induced lateral load at the top of the piers was determinedto be 160 KN (36 kips). The computed shear capacity of the piers was 338 KN (76

kips). For flexure, the capacity for lateral load applied at the top of the piers in thelongitudinal direction varied from 98 KN (22 kips) to 53 KN (12 kips) for the shortest

and tallest piers, respectively. The piers were therefore adequate in shear and

deficient in flexure. Three piers were strengthened and the fourth pier was used as abenchmark.

Two piers were strengthened for flexure using near-surface mounted carbon

FRP rods. One pier was strengthened with 14 NSM rods, mounted on two opposite

faces, seven on each face. A second pier was strengthened with six NSM rods, threeon each face. The NSM rods considered for this application were 11 mm (7/16 inch)

diameter smooth carbon rods with surface roughened by sandblasting. The rods werefully anchored (minimum 380 mm (15 in.)) into the footing of each pier. Finally, thetwo piers were wrapped with 4-ply of carbon FRP jacket. The third pier was

externally jacketed with six plies of glass FRP sheets. As will be discussed later, the

test setup was designed such that the lateral movement of the piers was allowed at thetop while restrained at the base. Therefore, it was expected that the maximum

moment would occur at the base of the pier. Consequently, the NSM rods were only

anchored to the footing. In addition, anchoring the NSM rods to the top flare can not


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be easily achieved due to its shape (see Figure 1). The mechanical properties of the

FRP sheets and rods are given in Table 1. Figure 2 summarizes the strengthening

schemes of the bridge piers.

Table 1: Mechanical properties of FRP reinforcement

FRP Type

Dimension

mm

[in]

Design

Strength

MPa [ksi]

Design Strain

mm/mm

or in/in

Tensile

Modulus GPa

[ksi]

Glass

sheets*

0.353

[0.0139]

1520

[220]0.0210

72

[10,500]

Carbon

sheets*

0.165

[0.0065]

3800

[550]0.0170

228

[33,000]

Carbon

rods**

11

[7/16]

1240

[180]0.0105

119

[17,200]Sheet thickness or bar diameter * Fiber properties ** Rod properties

STRENGTHENING PROCEDURE

The NSM FRP rods were embedded in grooves that were 19 mm ( in.)

deep, and 14 mm(9/16in.) wide cut along the length of the piers. The grooves were

made using conventional hand-held tools. The grooves were cleaned using sand

blasting to remove all loose particles and dust. Surface preparation is important since

the tensile stresses are transmitted from the concrete to the FRP rod through thebinding paste by means of tangential stresses. To anchor the rods, 400 mm (16-in)

deep holes were drilled into the footings. The holes were aligned with the grooves on

the pier sides. The grooves and the drilled holes were then filled halfway with a

viscous epoxy grout and the carbon FRP rods were installed. Another layer of epoxygrout was then applied and the surface was leveled.

All FRP jackets were installed by the wet lay-up process. The carbon andglass FRP sheets covered the entire height of the piers with the fiber direction

perpendicular to the pier axis. The corners of the rectangular piers were rounded to 13

mm (0.5 in.) radius to prevent stress concentrations in the FRP sheets.

TEST SETUP

The loading system was designed such than it could apply a maximum loadmuch larger than theoretically predicted. This was done to account for the possibility

of higher actual material strengths than initially presumed as well as for thestrengthening effect. The desired level of loading could only be applied by means ofhydraulic jacks. The test setup was designed such that it could induce reversing

loading cycles in which the piers were allowed to displace laterally at the top. To

achieve this, a 250-mm (10-inch) strip of the deck was saw cut and removed. The

central portion of the cap beams were also saw cut and removed and a hydraulic jackwas inserted in the gap. A schematic of the test setup is shown in Figure 3. The

function of the internal jack was to apply the outward push force to the piers cap


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beam. Saw cutting the bridge deck and the cap beam allowed for the relative

movement of the cap beams and the topping deck. To pull the piers together, a

reaction frame was constructed such that it confined the cap beams. A set of twohydraulic jacks was then attached to the reaction frame. The internal and external

jacks were used alternately to create a static lateral loading cycles.

INSTRUMENTATION

The bridge piers were instrumented with electric strain gages installed on themounted rods as well as on existing steel reinforcement. Strain gages were also

installed on the FRP sheets. An 890 KN (200 kips) capacity load cell was used to

measure the applied force. The lateral displacement of the each pier was measured at

mid-height and at the top of the pier using linear variable displacement transducers(LVDTs). The LVDTs were mounted on steel towers that were fixed to the footing

using conventional drop anchors. The rotation of the cap beam, the pier, and the

footing was measured by means of inclinometers at three locations (see Figure 4).

TESTING PROCEDURE

Once the setup was erected, instrumentation was connected to the dataacquisition system and zero reading were taken. For safety reasons, the first loading

cycle was always a pull-in load condition. When the desired lateral force was

achieved, the system was unloaded and the hydraulic hoses were disconnected fromthe external jacks and connected to the internal jack. A push-out force was then

applied. These two loading cycles were repeated until the weaker pier failed. To test

the second pier of the same bent, a diagonal bracing was installed against the failed

pier, as shown in Figure 4. Prior to the testing of the piers of the second bent, thedeck slabs resting on the bent were jacked up using hydraulic jacks lubricated steel

plates were inserted between the cap beam and the deck slabs. This action was

intended to eliminate some of the frictional forces at the top of the piers.

TEST RESULTS AND OBSERVATIONS

The failure loads of the bridge piers exceeded in magnitude the predictedloads. In addition, all the piers underwent a double curvature type of behavior. The

rotation restraint of the superstructure on the cap beam was larger than expected even

for the bent with reduced friction. For the piers with reduced friction, larger rotationswere measured on the cap beam. In these piers, as the cap beam rotated it pushed the

topping decks upward. As a result, the point of application of the vertical force due

to the deck weight shifted to the edge of the cap beam. This behavior resulted in anadditional moment that acted at the top of the pier.

For the unstrengthened pier, the applied lateral load at failure was 351 KN (79

kips) and the measured maximum lateral displacement at the top of the pier wasaround 15.5 mm (0.61 in.). Figure 5(a) illustrates the measured rotations at the last

loading cycle (push-out) of the unstrengthend pier. The continuous rotation under

constant force was related to the yielding of the reinforcement as well as soil failure,

which is represented by the continuous rotation of the footing at failure. One major


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crack was observed on the pier, which occurred at the upper third of the pier height,

close to the termination point of the flare reinforcement, as shown in Figure 5(b). For

the pier strengthened with 7 CFRP bars on opposite sides and CFRP jacketing, thefailure was initiated by a crack occurred at the pier-flare intersection where the

mounted rods were terminated. After the crack occurred, the pier went through a

continuous rotation with no increase in load carrying capacity. The applied lateralload at failure was 360 KN (81 kips) and the measured lateral displacement at the top

of the pier varied from 1.5 mm (0.058 in.) just before cracking to 2.9 mm (0.116 in)

at loading termination. Figure 6 illustrate the measured rotation at the last loadingcycle (push-out) of this pier. This figure indicates that at maximum load, the whole

pier experience a rigid body rotation for a while, then a crack occurred at the top of

the pier causing the cap beam rotation to reduce significantly due to the yielding of

the reinforcement and the formation of a plastic hinge. After the formation of theplastic hinge and the redistribution of moments, the pier and the footing continued to

rotate at a faster rate indicating soil failure.

For the pier strengthened with three CFRP bars on two opposite sides and

CFRP jacketing, cracks occurred at the top and the base of the pier, as shown inFigure 7. The footing of this pier was originally cast in a bedrock therefore no

rotation was measured at this footing, as shown in Figure 8. The figure illustrates themeasured rotations at the last loading cycle (push-out) of this pier. As a result of the

large rotational stiffness of the footing, larger moment were developed at the base of

the pier. Failure was initiated by the rupture of the FRP rods at the base of the pier at

a load level of 382 KN (86 kips) with a maximum lateral displacement measured atthe top of 21.8 mm (0.86 in.). This indicates that the full capacity of the NSM rods

can be achieved, giving that the rods are adequately anchored. As for the pier with

GFRP jacket only, the pier started to rotate as a rigid body at 222 KN (50 kips). Thetest was terminated when the lateral displacement exceeded 38.1 mm (1.5 in.). The

failure mode of this pier was, therefore, a soil failure. It should be mentioned that the

above given displacements at maximum loads are the absolute displacement withoutaccounting to the rotation of the footing. The variation in failure modes and lateral

load capacity may be related to the influence of superstructure/substructure

interaction, variation in the boundary conditions of each pier (e.g., footing rotation

stiffness and friction forces), and the skew effect of the bridge bents.

ANALYTICAL MODELING

The basic objective of modeling is to provide the simplest mathematical

formulation of the true behavior of the pier, which satisfies a particular set of known

values (in this case, the measured response) for quantitative determination of theinternal forces. The developed structural model simulating the observed behavior of

a bridge pier is shown in Figure 9(a). The pier is simulated by a column, which is

free to displace laterally at the top and is restrained laterally at the bottom. Theflexibility of the footing is represented by a rotational spring with unknown constant

k1 at the base of the column, which models the effect of footing rotation due to soil

deformation. Another spring with unknown stiffness k3 is used at the top of the

column to model the effect of cap beam rotation due to the applied loading. The


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frictional force between the deck and the cap beam is represented by a linear spring

connected to the top joint. The spring constants, k5, may vary at each load level due

to softening of the boundary conditions under repeated loading cycles. The model isanalyzed using the matrix displacement method. The overall structural stiffness

matrix, internal forces, and external loading due to a given structural response can,

therefore, be determined by simple matrix operations. For simplicity, the unknowninternal forces are determined at the locations of the nodes were displacements and

rotations were measured experimentally. Accordingly, each pier is represented by a

two-element, three-node column, as shown in Figure 9(b). The unknown rotations ofthe joints are denoted as X1, X2, and X3 and the unknown displacements of the joints

are denoted as X4 and X5. The nodal displacements are used as the degrees of

freedom (DOFs) of the column. Thus, the column has five degrees of freedom. This

model is only applicable prior to pier cracking, after which a non-linear analysis isrequired. For the current investigation, the capacities of the piers were slightly larger

than the cracking capacity and for some cases failure was governed by the rigid body

rotation of the pier, therefore the elastic analysis approach was valid at higher load

levels. An example of the analytical results is given in Figure 10 for the pier

strengthened with 7 NSM-rods on two opposite sides. Figure 10(a) shows themeasured deformations at the three nodes of the model due to a lateral load of 178

KN (40 kips). The effect of footing rotation was included in determining the lateral

displacements (X4 and X5) of the joints. Figure 10(b) shows the calculated external

loads and reaction of the column while Figure 10(c) shows a plot of the momentdiagram along the length of the column. The results indicated that the frictional force

is in the order of 57.6 KN (13 kips). However, this value was found to be reduced at

higher load levels. The maximum moment occurred at the top of the pier due to thelarger rotational stiffness exerted by the superstructure on the cap beam than the

rotational stiffness of the soil. The analytical behavior correlates well with the

experimental results where the first crack on this pier occurred at the pier-flareintersection, the location of the maximum moment. Due to limited space, a

comprehensive documentation of the structural modeling, structural analysis and

analytical results will be reported in a future publication.

CONCLUSION

The objective of this research program was to demonstrate the use near-surface mounted FRP rods to improve the flexural capacity of rectangular RC piers.

Prior, to demolition, full-scale bridge piers were strengthened with FRP rods and

sheets and tested to failure. Test results indicate that this strengthening technique iseffective in increasing the flexural capacity of the piers. Test results also indicate that

the capacity and failure modes of the bridge piers are closely related to the

superstructure/substructure interaction and the pier boundary conditions. Flexuralstrengthening of piers may cause the structural deficiency problem to shift another

location within the structure. Therefore, flexural strengthening may require the

retrofitting of beam-pier joints and foundations to account for the upgraded flexural

capacity of the pier. In general, the determination of the elastic structural response


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under any load value is quite achievable with a reliable model in terms of well-

defined boundary conditions and reasonably accurate material properties and

stiffness.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the funding provided by the Missouri

Department of Transportation (MoDOT), Mid-America Transportation Center

(MATC), and the University of Missouri-Rolla/University Transportation Center(UMR-UTC). Master Builders Technologies, Cleveland, OH, and Structural

Preservation Systems, Baltimore, MD, provided and installed the FRP systems,

respectively.

REFERENCES

Standard specifications for Highway Bridges. (1996). American Association of

State Highway and Transportation Officials (AASHTO), Washington, D.C.Hakamada, F. (1997), Experimental Study on Retrofit of RC Columns Using CFRP

Sheets. Proc., Third Int. Sym. on Non-Metallic (FRP) Reinforcement for

Concrete Structures (FRPRCS-3), Japan Concrete Institute, Tokyo, Japan, 1,

419-426.

Macrae, G. A., Nosho, K., Stanton, J., and Myojo, T. (1997), Carbon Fiber Retrofit

of Rectangular RC Gravity Columns in Seismic Regions. Proc., Third Int. Sym.on Non-Metallic (FRP) Reinforcement for Concrete Structures (FRPRCS-3),

Japan Concrete Institute, Tokyo, Japan, 1, 371-386.

Naval Facilities Engineering Service Center. (1998), Navy Advanced Compositetechnology in Waterfront Infrastructure. Special Publication Sp-2046-SHR.

Seible, F., Hegemier, G., Priestly, M. J. N., and Innamorato, D. (1995), Rectangular

Carbon Fiber Jacket Retrofit Test of a Shear Column with 2.5% Reinforcement.Report No. ACTT-95/05, University of California, San Diego.


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Figure 1. The Two Bents of the Bridge

Figure 2. Strengthening Schemes of the Piers

Bent 2, pier 1

(6 CFRP rods, 4 CFRP hoop plies)

Bent 2, pier 2

(5 GFRP hoop plies)

Bent 1, pier 1( 14 CFRP rods, 4 CFRP hoop plies)

Bent 1, pier 2(No Strengthening)


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26-3

2W14 90

HydraulicJack

Hydraulic

Jack

Cut throughbridge deck

Dywidag

Rod

25 Saw Cut 3-6

Pier

Bent 1 Bent 2

Above bridge deck Below bridge deck

Figure 3. Schematic of the Test Setup (1 in. = 25 mm)

Figure 4. Diagonal Bracing of the Failed Pier.

54o

LVDT

Inclinometer

F

steel pipe

RC footing

Failed


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Figure 5. Behavior of the Unstrengthened Pier (1 KN = 0.225 kip)

Figure 6. Behavior of the Pier with 14 NSM rods (1 KN = 0.225 kip)

(a) Measured Rotations (b) Final crack

(a) Measured Rotations (b) Final crack at flare


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Figure 7. Cracks at Failure of the Pier with 6 NSM Rods

Figure 8. Measured Rotation of the Pier with 6 NSM rods (1 KN = 0.225 kip)

(a) Top crack at pier-flare intersection (b) Pier base crack showing FRP rupture


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Figure 9. Analytical model of a bridge pier

Figure 10. Measured deformation and analytical results for pier with 14 NSM rods

P

k1

k3k5

L

(a) structural model

X1

X2

X4

X3

X5

2

1

L/2

L/2

(b) two-elements analytical model

156.9 KN-M

194.7 KN-M

120.4 KN

(b) calculated external forces(a) measured response

P = 178 KN

2.97 m

X3 = 0.022 deg.

X5=0.515 mm

X2 = 0.046 deg.

X4=0.304 mm

X1 = 0.0306 deg.

194.7 KN-M

156.9 KN-M

(c) moment diagram

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