gt strudl users group 22 nd annual meeting & training seminar june 24, 2010

GT STRUDL GT STRUDL Users Group22nd Annual Meeting & Training

SeminarJune 24, 2010

Practical Modeling Technique for Transfer Length

Chris Carroll, Ph.D.

Assistant Professor

Department of Civil Engineering

University of Louisiana at Lafayette

Overview

Introduction

Top-strand Effect

Test Speciemens

GT STRUDL Model

Background

Practical Modeling Techniquefor Transfer Length

BackgroundDevelopment length (standard reinforcing steel)

• The length required to anchor the reinforcing to fully develop the stress in the reinforcing at the nominal moment capacity of the member (AASHTO)• The length of embedment required to prevent slip between reinforcing and the surrounding concrete when that reinforcing is placed in tension (or

compression)

Practical Modeling Technique

for Transfer Length

Background

'

3

40 b tr

b

y t e sd bc K

dc

fL d

f

Required stress in steel

Concrete Strength

Location of the bar Coating of the bar

Size of the bar

Diameter of the bar

Cover and confinement

Effect of lightweight concrete

Required stress in steel

Diameter of the bar

Development length (standard reinforcing steel)


Background

Development Length• The length required to

anchor the strand to fully develop the stress in the strand at the nominal moment capacity of a member

3000 1000ps sese

d b b

f ffL d d

Lt Lfb

fse

fps

Ld

2

3d ps se bL f f d

ACI

AASHTO


BackgroundTransfer Length• The bonded length of

strand required to transfer the prestress force in the strand to the surrounding concrete

3000 1000ps sese

d b b

f ffL d d

Lt = 50db

Lt = 60db

3000se

t b

fL d

Lt

fseACI

AASHTO


Unconservative

BackgroundTransfer Length (Code Provisions)

UnconservativeUnconservative



Background

Top-strand Effect

> 12”

Deformed Bar

• Provisions exist for development length of deformed bars

• Ld multiplied by 1.3 (ACI) and 1.4 (AASHTO) with > 12 inches of fresh concrete below the bar

• Provisions do not exist for the development or transfer length of prestressing strands


BackgroundTop-strand Effect

a

b

a

b


Background

Top-strand Effect– Is top-strand effect a factor of the amount of

concrete beneath the strand?– New hypothesis: Top-strand effect may be a factor

of the amount of concrete above the strand rather than the amount below or a combination thereof

12 ft 12 ft

Block A Block B


Test SpeciemensT-beams

30 in.

4 in.

5 in.

24 in.

2 in.

8 in.

Large

3 in.

19 in.

2 in.

4 in.

24 in.

Medium

8 in.

24 in.

2 in.

17 in.

2 in.

4 in.

Small

8 in.

½” regular ½” special 0.6”


Test Specimens

300 ksi

270 ksi

Normal InvertedA

A

B

B

Normal

Inverted

Normal

Inverted


Test SpecimensT-beams


Test SpecimensTop-strand blocks

12 ft 12 ft

Block A Block B

24”

4”

2”

5”

5”

5”

5”

2”

2”

5”

5”

2”

14”

4”

A

B

CC

D

E

F

G

H



Single Strand Blocks

Five Strand Blocks

Three Strand Blocks


Test SpecimensTransfer Length

50 mmspacing

100 mmspacing


Test SpecimensTransfer Length

50 mmspacing

100 mmspacing

≈ 30,000 measurements


Test SpecimenTransfer Length


Test SpecimensBond/Shear Failure


Top-strand EffectTransfer Length• Influence of Release Method• Influence of Strand Strength• Influence of Strand Diameter/Area• Influence of Effective Prestress• Influence of Concrete Strength• Influence of Time• Influence of Casting Orientation• Proposed Transfer Length Equation


Top-strand EffectTransfer Length (Influence of Casting Orientation)



Amount of Concrete Below

Amount of Concrete Above



Same Amount of Concrete Below

Same Amount of Concrete Above


Amount of Concrete Above

Amount of Concrete Below



Top-strand EffectTransfer Length (Proposed Transfer Length Eq.)

'

sib

ci

fX d

f

12 in.

24castd 24castd 0castd

1

Z

Tra

nsfe

r Le

ngth

A B

C

Amount of Concrete Above the Strand

'

sib

ci

fX d

f

12 in.

24castd 24castd 0castd

1

Z

Tra

nsfe

r Le

ngth

A B

C

Amount of Concrete Above the Strand


Top-strand EffectTransfer Length (Proposed Transfer Length Eq.)

2'

2435

40castsi

t b

c

dfL d

f

= 1 = 2

R2 = 0.176R2 = 0.206


Top-strand EffectTransfer Length (End-slip)


Top-strand Effect

Conclusions

• Top-strand effect was more dependent on the amount of concrete cast above the strand

• On average Lt increased ½ in. for every 1 in. reduction in the amount of concrete cast above the strand

2'

2435

40castsi

t b

ci

dfL d

f


GT STRUDL Model


GT STRUDL Model

$$===================================================$$ CONCRETE ELEMENT DATA$$===================================================

TYPE PLANE STRESSGENERATE 6 ELEMENTS ID 'AB-1', 1 FROM 'A1',1 TO 'A2',1 TO 'B2',1 TO 'B1',1GENERATE 6 ELEMENTS ID 'BC-1', 1 FROM 'B1',1 TO 'B2',1 TO 'C2',1 TO 'C1',1

ELEMENT PROPERTIES TYPE 'IPLQ' THICK 4'AB-1' TO 'AB-6''BC-1' TO 'BC-6‘

CONSTANTS E 3949 -'AB-1' TO 'AB-6‘ –'BC-1' TO 'BC-6‘

G 1688 -'AB-1' TO 'AB-6' -'BC-1' TO 'BC-6‘

POI 0.17 -'AB-1' TO 'AB-6' -'BC-1' TO 'BC-6'

A1 A2 A3 A4 A5 A6 A7

C1 C2 C3 C4 C5 C6 C7

B1 B2 B3 B4 B5 B6 B7

AB-1 AB-2 AB-3 AB-4 AB-5 AB-6

BC-1 BC-2 BC-3 BC-4 BC-5 BC-6


GT STRUDL Model

$$==================================================================$$ SPECIFY JOINT COORDINATES$$==================================================================

GENERATE 5 JOINTS ID 'C1',1 X 0. -DIFF -1 2 AT 1 2 AT 2. Y 2. Z 0.

C1 C2 C3 C4 C5

(-1,2) (0,2) (1,2) (3,2) (5,2)


GT STRUDL Model

$$==================================================================$$ SPECIFY STRAND PROPERTIES $$==================================================================

TYPE PLANE TRUSSGENERATE 4 MEMBERS ID 'STRND-0',1 FROM 'Cd0', 1 TO 'Cd1'

MEMBER PROPERTIES PRISMATIC AX 0.153'STRND-0' TO 'STRND-3'

Cd0 Cd1 Cd2 Cd3 Cd4

STRND-0 STRND-1 STRND-2 STRND-3


GT STRUDL Model

$$===================================================$$ SPECIFY BOND ELEMENT PROPERTIES$$===================================================

ELEMENT INC'BOND-1' 'Cd1' 'C1''BOND-2' 'Cd2' 'C2''BOND-3' 'Cd3' 'C3''BOND-4' 'Cd4' 'C4'

NONLINEAR SPRING PROPERTIESCURVE 'BOND' FORCE VS DISPL0.0 0.0 -50.0 -1.0END

ELEMENT PROPS'BOND-1' TO 'BOND-4' TYPE 'NLS'

NONLINEAR SPRING ELEMENT DATASTIFFNESS'BOND-1' TO 'BOND-4' X CURVE 'BOND'END

50 kip/in.

100 kip/in.

150 kip/in.

200 kip/in.

250 kip/in.


GT STRUDL Model

61100 6.5 10 28500 0.153 31.2

P A EA

T

P T EA

P x

$$==================================================================$$ SPECIFY TEMPERATURE LOADINGS$$==================================================================

LOADING 'TRANSFER' '-1100 TEMPERATURE CHANGE'MEMBER TEMPERATURE LOADS'STRND-0' TO 'STRND-3' AXIAL -1100

Cd0 Cd1 Cd2 Cd3 Cd4

STRND-0 STRND-1 STRND-2 STRND-3


GT STRUDL Model• 4x4 in. 12 ft concrete prism (k = 50 kip/in.)

• 4x4 in. 12 ft concrete prism (k = 50 kip/in.)

• 4x4 in. 12 ft concrete prism (k = 250 kip/in.)

• Excel Spreadsheet


GT STRUDL Model

99% max force


GT STRUDL Model


GT STRUDL Model

• 4x24 in. 12 ft concrete block (k = variable)

• 17 in. deep T-beam with eccentric strands

• 17 in. deep T-beam with eccentric strands

• 8 ft deep 96 ft long I-beam (End-zone)

• 8 ft deep 96 ft long I-beam (End-zone)

???Questions

gt strudl users group 22 nd annual meeting & training seminar june 24, 2010

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