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End Region Detailing of Pretensioned Concrete Bridge Girders

End Region Detailing of Pretensioned Concrete Bridge Girders

Trey Hamilton, PE, PhD, Univ. of FloridaBrandon E. Ross, PE, PhD, Clemson Univ.

Technical Coordinator: Sam Fallaha, PEFDOT Contract BDK75 977-05

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Presentation Outline

Prestressing basics

Background on end region reinforcement

FIB54 Test Program – Service and Ultimate

FIB63 Test Program – Service

Findings

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End Region

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Effect of Prestressing

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Prestressing-Hoyer Effect

Flame-cut

Unstressed strand expands back to original diameter

Girder

strand

Girder

strand

Concrete restrains expansion of strandResult: tension stress

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Prestressing-Transfer Length

Transfer Length (60ds)

TendonForce

Effective prestress force in tendon

Tendon FBD

Girder FBD

Bond Stress

Bond Stress

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Prestressing-Transfer Length

Transfer Length (60ds)

TendonForce

Effective prestress force in tendon

Tendon FBD

Girder FBD

Bond Stress

Bond Stress

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Prestressing Effect-Splitting

ez

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Splitting-Web

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Splitting-Flange

Z

X

Tadros et al. (2010)

exX

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Prestressing Effect-Partial Debonding

Excerpt:

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Ultimate Strength-Strut and Tie

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Ultimate strength-Strut and Tie

ConsiderationsFactored Load

Flange GeometryBearing

GeometryStrand Locations

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Lateral-Splitting Failure

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LRFD “Confinement”

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FDOT Standard Details

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Research Approach

FIB54: Flange Cracks & Ultimate CapacityFIB63: Web Cracks

Finite Element Analysis

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FIB54 10 Specimens (5 beams)

Fabrication Strain data Crack data

Load tests Behavior Capacity

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FIB54 Reinforcement and Prestressing Scheme

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FIB54 Reinforcement and Prestressing Scheme

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FDOT Confinement

Modified Confinement No Confinement (2)

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No Bearing Plate

Bearing Plate

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FIB54 Debonding

45% debonded

13% debonded

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FIB54-ResultsFlange Splitting Cracks

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44.0

99.0

219.5

134.0

104.0

41.5

107.0

0

50

100

150

200

250

Tota

l len

gth

(in.)

Length of flange-splitting cracks

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Analytical Program

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FEA + Experimental-Transfer Length

Stage 1 – outer strands released Stage 2 – all strands released

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FEA + Experimental-Web Stresses

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FEA + Experimental-Transverse Tension

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FIB54-Topping

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Shear-span = 10 ft.a/d ~ 2

FIB54 w/ 8 in. deck50 ft. long44 ft. span

62 in.

0.6in. Dia. strands

FIB54-Test Setup

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FIB54-Test Procedures

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FIB54-Test Procedures

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FIB54-Test Procedures

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FIB54-Test Procedures

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FIB54-ResultsUltimate Capacity

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Lateral-Splitting(4) specimens

Failure Modes

Web-Shear(4) specimens

Bond-Shear(2) specimens

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Displacement / Slip (in.)

Supe

rimpo

sed

Shea

r (ki

p)

0 0.25 0.5 0.75 1 1.250

100

200

300

400

500

600

700

FNFB WN

WB

Vn = 454kACI

Vn = 528kAASHTO

Bond-shear

Lateral-splitting

Ultimate Capacity

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766

666 698635

508

612

375409

753703

0

100

200

300

400

500

600

700

800

900

HC HU VC VU WN WB FN FB DC DM

Max

imu

m S

hea

r (k

ip)

Lateral-splitting failure

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Ultimate Capacity-Transverse Strain

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14.0

4.4

10.0

29.8

13.1

35.8

12.8

0

5

10

15

20

25

30

35

40

WN WB FN FB

Ave

rag

e st

ress

(ks

i)

V = 375 kip

Ultimate

Includes stress from loading & prestressing

Ultimate Capacity-Transverse StrainSt

ress

in

conf

inem

ent

rein

forc

emen

t

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Ultimate Capacity-Transverse Strain

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75.978.8

74.9

44.2

72.178.5

70.773.070.1

63.9

37.0

57.9 59.7 60.3

0

20

40

60

80

100

HC VC WB FB DC DM Avg.

% of tran

sverse (x‐x) force 

carried by bearing plate 

V=375

Ultimate

Ultimate Capacity-Transverse Strain

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FIB63 4 specimens (2 beams)

Monitor cracks after release

Load test

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FIB63 Reinforcement and Prestressing Scheme

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FIB63 Test Specimens

SL

CN

LB

PT

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2 ksi

0 ksi

-2 ksi

-4 ksi

-6 ksi

Release Stresses

CalculatedAllowable

0ft. 5ft. 10ft. 15ft. 20ft. 25ft. 30ft. 35ft. 40ft. 45ft. 50ft.

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SL

CNLB

PT

FIB63 Test Specimens

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Web Splitting Cracks

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Web Splitting Cracks

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Conclusions-Ultimate Strength

Horizontal reinforcement – Negligible

Bearing plate - 9% to 21% improvement

FDOT confinement reinforcement-13% increase over no reinforcement

Bonded Strand - 43% increase for strands placed under web

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Conclusions–Flange Splitting

Tension stress Outer strands Hoyer effect

Strategies Bonded strand arrangement Confinement reinforcement Bearing plate Partial debonding

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Conclusions–Web Splitting

Post-tensioning (PT) Promising Additional research

Large diameter bars (LB) Reduce crack width No effect on crack length

45% strand shielding (SL) Shorter, smaller cracks Affects strength

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Report BDK75 977-05

End Region Detailing of Pretensioned Concrete Bridge Girders

Available on theFDOT Research Website

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Thank You Experimental work conducted at

M. H. Ansley Structures Research Center Many thanks to Sam Fallaha and the Center’s

staff! David Allen Adam Brennan Frank Cobb Steve Eudy Tony Hobbs Will Potter Paul Tighe David Wagner Chris Weigly Steven Nolan, P.E., FDOT Structures Design Office

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Wish List

Bottom Flange

Flange Strands

Flange Strands

Web Strands