transfer and development length of strands in post-tensioned members after anchor head failure

101 Pages·2010·3.99 MB·English

by  Elie El Zghayar

TRANSFER AND DEVELOPMENT LENGTH
OF STRANDS IN POST-TENSIONED MEMBERS
AFTER ANCHOR HEAD FAILURE

by

ELIE ATALLAH EL ZGHAYAR
B.S. University of Central Florida, 2007

A thesis submitted in partial fulfillment of the requirements
for the degree of Master of Science of Civil Engineering
in the Department of Civil, Environmental, & Construction Engineering
in the College of Engineering and Computer Science
at the University of Central Florida
in Orlando, Florida

Summer Term
2010
©2010 Elie El Zghayar

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ABSTRACT

Post-tensioning tendons in segmental bridge construction are often only anchored within
the deviator and pier segments. The effectiveness of the post-tensioning (PT) system is therefore
dependent on proper functioning of the anchorages. On August 28, 2000 a routine inspection of
the Mid-Bay Bridge (Okaloosa County, Florida) revealed corrosion in numerous PT tendons.
Moreover, one of the 19-strand tendons was completely slacked, with later inspection revealing a
corrosion-induced failure at the pier anchor location. Anchorage failure caused all PT force to
transfer to the steel duct located within the pier segment that in turn slipped and caused the
tendon to go completely slack. After the application of PT force, the anchorage assembly and
steel pipes that house the tendon are filled with grout. These short grouted regions could, in the
event of anchorage failure, provide a secondary anchorage mechanism preventing the scenario
mentioned above from occurring. This paper presents the results of a full-scale experimental
investigation on anchorage tendon pull-out and a finite element model to support the
experimental results and interpretation. The study focuses on the length required to develop the
in-service PT force within the pier segment grouted steel tube assembly. Seven, twelve, and
nineteen 0.6” diameter strand tendons with various development lengths were considered.
Recommendations for pier section pipe detailing and design will be discussed.

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ACKNOWLEDGMENTS

I would like to thank Dr. Kevin Mackie for his continuous support and guidance in every
step of this project and my time at UCF. I would also extend my special thanks for Zachary
Haber and his invaluable input into this project and his great help in writing of the FDOT report
as well as the ASCE conference paper related to this project. I also thank Dr. Necati Catbas and
Dr. Manoj Chopra for serving on my committee and providing great feedback for improving this
manuscript.
I would like to express gratitude to the Florida Department of Transportation for funding
this project (Project BD550-11), the staff at the FDOT Structures Research Center in
Tallahassee, Florida, Dr. Lei Zhao for his contributions to this work, and Robert Slade for his
assistance with the project. Robert’s drafting skills were a great value to this project.
I thank all my friends and fellow graduate students at UCF for providing their support
and friendship throughout my years at UCF.
Finally to the best parents any one can wish for, I thank you for your continuous support,
encouragement and high standards and example you have set for me.

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TABLE OF CONTENTS

LIST OF FIGURES ..................................................................................................................... viii
LIST OF TABLES ....................................................................................................................... xiii
LIST OF ACRONYMS/ABBREVIATIONS .............................................................................. xiv
CHAPTER 1: INTRODUCTION ................................................................................................... 1
Research Objectives .................................................................................................................... 1
Background and Motivation ........................................................................................................ 2
CHAPTER 2: LITERATURE REVIEW ........................................................................................ 5
Introduction ................................................................................................................................. 5
Definitions ................................................................................................................................... 5
Transfer Length or Transmission Length ................................................................................ 5
Development Length ............................................................................................................... 5
Flexural Bond Length .............................................................................................................. 6
Embedment Length .................................................................................................................. 6
Hoyer Effect ............................................................................................................................ 7
Stress Transfer Mechanism in Post Tensioned Members ........................................................... 8
Stress Transfer in Prestressed Members ...................................................................................... 8
Bond Mechanisms ................................................................................................................... 8
Transfer and Bond Stresses ..................................................................................................... 9
Bond Failure .......................................................................................................................... 11
Factors Affecting Transfer Length ............................................................................................ 11
Strand Diameter ..................................................................................................................... 12
Strand Surface Condition....................................................................................................... 12
Method of Release ................................................................................................................. 13
Other Related Research ............................................................................................................. 13
Diephuis, Xia, VSL and FDOT on Pipe Slipage ................................................................... 13
Construction Technology Laboratories Bonded and Unbonded Post-Tensioned Systems ... 14
Theoretical Approach for Transfer Lengths .......................................................................... 15
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Transfer and Development Length Equations ........................................................................... 16
CHAPTER 3: EXPERIMENTAL PROGRAM DESIGN AND SET-UP .................................... 18
Loading Procedures ................................................................................................................... 22
Tendon Stressing Procedure .................................................................................................. 22
De-stressing Procedure .......................................................................................................... 23
Active Loading Procedure ..................................................................................................... 23
Testing Reaction Fixtures.......................................................................................................... 24
Specimen Set-up ........................................................................................................................ 25
Instrumentation.......................................................................................................................... 26
Test Fixtures .......................................................................................................................... 26
Specimen Block ..................................................................................................................... 29
CHAPTER 4: TEST OBSERVATIONS, RESULTS, AND DISCUSSION ................................ 30
Seven-strand Tendon Specimen ................................................................................................ 31
Test Details ............................................................................................................................ 31
Numerical Test Results .......................................................................................................... 32
Post-test Observations ........................................................................................................... 41
Twelve-strand Tendon Specimen .............................................................................................. 42
Test Details ............................................................................................................................ 42
Test Results............................................................................................................................ 43
Post-Test Observations .......................................................................................................... 51
Nineteen-strand Tendon Specimen ........................................................................................... 52
Test Details ............................................................................................................................ 52
Test Results............................................................................................................................ 54
Post-Test Observations .......................................................................................................... 63
CHAPTER 5: ANALYTICAL MODEL ...................................................................................... 66
Modeling ................................................................................................................................... 66
Model Geometry and Materials ............................................................................................. 66
Elements and Constitutive Model.......................................................................................... 67
Mesh ...................................................................................................................................... 67
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Boundary Conditions and Loads ........................................................................................... 69
Load Pattern and Analysis ..................................................................................................... 69
FEM Results .............................................................................................................................. 70
CHAPTER 6: NASP ..................................................................................................................... 77
NASP Background .................................................................................................................... 77
Application in This Project ....................................................................................................... 77
Instrumentation and Testing ...................................................................................................... 78
CHAPTER 7: DISCUSSION AND CONCLUSIONS ................................................................. 83
Future Research ......................................................................................................................... 84
LIST OF REFERENCES .............................................................................................................. 86

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LIST OF FIGURES

Figure 1. Mid-Bay Slipped Pipe (FDOT) ....................................................................................... 4
Figure 2. Mid-Bay Anchorage (FDOT) .......................................................................................... 4
Figure 3. Stress Distribution in a Strand (PCI, 1978) ..................................................................... 6
Figure 4. Hoyer Effect (Sengupta & Menon) ................................................................................ 7
Figure 5. Bond Stress Distribution in a Prestressed Beam without Anchorage (Rajagopalan,
2002) ............................................................................................................................................. 10
Figure 6. Specimen details ............................................................................................................ 19
Figure 7. Specimen Pipe Details ................................................................................................... 20
Figure 8. Specimen Blocks Prior to Casting ................................................................................. 21
Figure 9. Enerpac Jacks ................................................................................................................ 22
Figure 10. Enerpac Flow Coupler ................................................................................................. 22
Figure 11. Reaction Fixtures ......................................................................................................... 24
Figure 12. Experimental set-up ..................................................................................................... 25
Figure 13. Test set-up photos ........................................................................................................ 25
Figure 14. Load Cell Locations .................................................................................................... 27
Figure 15. Sliding Fixture Tie-downs and Instrumentation .......................................................... 28
Figure 16. Specimen Instrumentation Details ............................................................................... 29
Figure 17. Hydrostone Joint .......................................................................................................... 31
Figure 18. Seven-strand tendon load history. ............................................................................... 34
Figure 19. Seven-strand strain history: gages R1 and R6. ............................................................ 34
Figure 20. Seven-strand strain history: gages R5 and R10. .......................................................... 35
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Figure 21. Corner Displacements of the Specimen ...................................................................... 35
Figure 22. Axial top strain profile during 7-strand de-stressing. .................................................. 36
Figure 23. Axial side strain profile during 7-strand de-stressing.................................................. 37
Figure 24. Circumferential top strain profile during 7-strand destressing .................................... 37
Figure 25. Circumferential side strain profile during 7-strand destressing .................................. 38
Figure 26. Axial top strain profile during 7-strand active loading. .............................................. 39
Figure 27. Axial side strain profile during 7-strand active loading .............................................. 40
Figure 29. Circumferential side strain profile during 7-strand active loading .............................. 41
Figure 30. Grout Blocker at Free End ........................................................................................... 42
Figure 31. Crack Pattern within Bearing Plate ............................................................................. 42
Figure 32. Twelve-strand tendon load history. ............................................................................. 44
Figure 33. Twelve-strand strain history: gages R1 and R6. .......................................................... 44
Figure 34. Twelve-strand strain history: gages R5 and R10. ........................................................ 45
Figure 35. Specimen Displacement .............................................................................................. 47
Figure 36. Axial top strain profile during 12-strand destressing .................................................. 47
Figure 37. Axial side strain profile during 12-strand destressing ................................................. 48
Figure 38. Circumferential top strain profile during 12-strand destressing .................................. 48
Figure 39. Circumferential side strain profile during 12-strand destressing ................................ 49
Figure 40. Axial top strain profile during 12-strand during active loading .................................. 49
Figure 41. Axial side strain profile during 12-strand active loading ............................................ 50
Figure 42. Circumferential top strain profile during 12-strand active loading ............................. 50
Figure 43. Circumferential side strain profile during 12-strand active loading ............................ 51
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Figure 45. Free-end Grout Block Wedge Plate ............................................................................. 53
Figure 46. New Grout Block Device ............................................................................................ 53
Figure 47. 19-strand During Active Loading ................................................................................ 54
Figure 48. De-stressing Load History ........................................................................................... 55
Figure 49. De-stressing Pipe Displacement History ..................................................................... 56
Figure 50. De-stressing Strain History: Gages 1 and 6 ................................................................. 56
Figure 51. De-stressing Strain History: Gages 5 and 10 ............................................................... 57
Figure 52. 19-strand Axial Strain Distribution: 00 Gage ............................................................... 58
Figure 53 19-strand Axial Strain Distribution: 1200 Gage ............................................................ 58
Figure 54. 19-strand Circumferential Strain Distribution: 00 Gage .............................................. 58
Figure 55. Circumferential Strain Distribution: 1200 Gage .......................................................... 58
Figure 56. Loading History During Active Loading .................................................................... 59
Figure 57. Pipe Displace History during Active Loading ............................................................. 59
Figure 58. Active Loading Strain History for Gages 1 and 6 ....................................................... 60
Figure 59. Active Loading Strain History for Gages 2 and 7 ....................................................... 60
Figure 60. Active Loading Strain History for Gages 5 and 10 ..................................................... 61
Figure 61. 19-strand Axial Strain Distribution During Destressing: 00 Gage .............................. 62
Figure 62. 19-Strand Axial Strain Distribution During Destressing: 1200 Gage .......................... 62
Figure 63. 19-Strand Circumferential Strain Distribution During Destressing: 00 Gage ............. 62
Figure 64. 19-Strand Circumferential Strain Distribution During Destressing: 1200 Gage ......... 62
Figure 65. Photo of 19-Strand Specimen Block Cracking ............................................................ 64
Figure 66. Photo of Radial Cracking of the specimen around the tube ........................................ 64
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