impact of aashto lrfd specifications on the design of precast, pretensioned u-beam bridges

425 Pages·2005·2.39 MB·English

by  Mohsin Adnan

IMPACT OF AASHTO LRFD SPECIFICATIONS ON THE DESIGN
OF PRECAST, PRETENSIONED U-BEAM BRIDGES

A Thesis
by
MOHSIN ADNAN

Submitted to the Office of Graduate Studies of
Texas A&M University
in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE

December 2005

Major Subject: Civil Engineering
IMPACT OF AASHTO LRFD SPECIFICATIONS ON THE DESIGN
OF PRECAST, PRETENSIONED U-BEAM BRIDGES

A Thesis
by
MOHSIN ADNAN

Submitted to the Office of Graduate Studies of
Texas A&M University
in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE

Approved by:

Co-Chairs of Committee, Mary Beth D. Hueste
Peter B. Keating
Committee Member, Terry Kohutek
Head of Department, David V. Rosowsky

December 2005

Major Subject: Civil Engineering
iii
ABSTRACT

Impact of AASHTO LRFD Specifications on the Design of Precast, Pretensioned U-
Beam Bridges. (December 2005)
Mohsin Adnan, B.S., NWFP University of Engineering and Technology
Co-Chairs of Advisory Committee: Dr. Mary Beth D. Hueste
Dr. Peter B. Keating

Texas Department of Transportation (TxDOT) is currently designing its highway
bridge structures using the AASHTO Standard Specifications for Highway Bridges, and
it is expected that TxDOT will make transition to the use of the AASHTO LRFD Bridge
Design Specifications before 2007. The objectives of this portion of the study are to
evaluate the current LRFD Specifications to assess the calibration of the code with
respect to typical Texas U54 bridge girders, to perform a critical review of the major
changes when transitioning to LRFD design, and to recommend guidelines to assist
TxDOT in implementing the LRFD Specifications. This study focused only on the
service and ultimate limit states and additional limit states were not evaluated.
The available literature was reviewed to document the background research
relevant to the development of the LRFD Specifications, such that it can aid in meeting
the research objectives. Two detailed design examples, for Texas U54 beams using the
LRFD and Standard Specifications, were developed as a reference for TxDOT bridge
design engineers. A parametric study was conducted for Texas U54 beams to perform an
in-depth analysis of the differences between designs using both specifications. Major
parameters considered in the parametric study included span length, girder spacing,
strand diameter and skew angle. Based on the parametric study supplemented by the
literature review, several conclusions were drawn and recommendations were made. The
most crucial design issues were significantly restrictive debonding percentages and the
limitations of approximate method of load distribution.
iv
The current LRFD provisions of debonding percentage of 25 percent per section
and 40 percent per row will pose serious restrictions on the design of Texas U54 bridges.
This will limit the span capability for the designs incorporating normal strength
concretes. Based on previous research and successful past practice by TxDOT, it was
recommended that up to 75% of the strands may be debonded, if certain conditions are
met.
The provisions given in the LRFD Specifications for the approximate load
distribution are subject to certain limitations of span length, edge distance parameter (d )
e
and number of beams. If these limitations are violated, the actual load distribution should
be determined by refined analysis methods. During the parametric study, several of these
limitations were found to be restrictive for typical Texas U54 beam bridges. Two cases
with span lengths of 140 ft. and 150 ft., and a 60 degree skew were investigated by
grillage analysis method.
v
DEDICATION

I dedicate this thesis to my grandfather Saeed Ahmed Khan, my parents Nuzhat
Mufti and Ahmed Zia Babar and my wife Zubia Naji.
vi
ACKNOWLEDGMENTS

My utmost gratitude is to Almighty God, who has been very kind to me during
all these years.
It is a pleasure to thank many who made this thesis possible. Firstly, I would like
to gratefully acknowledge the guidance and support that I received from my advisor, Dr.
Mary Beth D. Hueste. I could not have imagined having a better advisor and mentor for
my M.S., and without her common-sense, knowledge and perceptiveness I would never
have finished. I appreciate the contribution of Dr. Peter Keating and Dr. Terry Kohutek
for their helpful review of this document. I am grateful to my father-in-law Dr. Ahmed
Riaz Naji as he has been a continual source of guidance and support during my M.S.
I wish to acknowledge the Texas Department of Transportation (TxDOT) who
funded this research through the Texas Transportation Institute. I also wish to
acknowledge the financial support provided by the Department of Civil Engineering at
Texas A&M University. I greatly benefited from very many technical discussions with
Mr. Mohammad Safiudin Adil, a graduate student at Texas A&M University.
Finally, and most importantly, I am forever indebted to my parents Nuzhat Mufti
and Ahmed Zia Babar, and my wife Zubia Naji for their understanding, endless patience,
encouragement and love when it was most required. I am also grateful to Tashfeen,
Farhan and Hassan for their love and support.
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TABLE OF CONTENTS

Page
ABSTRACT ............................................................................................................iii
DEDICATION ..............................................................................................................v
ACKNOWLEDGMENTS................................................................................................vi
TABLE OF CONTENTS................................................................................................vii
LIST OF FIGURES..........................................................................................................ix
LIST OF TABLES..........................................................................................................xii
1. INTRODUCTION..................................................................................................1
1.1 Background and Problem Statement.............................................................1
1.2 Objectives and Scope....................................................................................3
1.3 Research Methodolgy....................................................................................3
1.4 Organization of Thesis..................................................................................7

2. LITERATURE REVIEW.......................................................................................9
2.1 Introduction...................................................................................................9
2.2 AASHTO Standard and LRFD Specifications..............................................9
2.3 Code Calibration and Application of Reliability Theory............................21
2.4 Development of Vehicular Live Load Model.............................................28
2.5 Vehicular Live Load Distribution Factors..................................................30
2.6 Debonding of Prestressing Strands.............................................................46
2.7 Refined Analysis.........................................................................................57

3. PARAMETRIC STUDY OUTLINE AND ANALYSIS PROCEDURES..........67
3.1 General........................................................................................................67
3.2 Bridge Geometry and Girder Section..........................................................68
3.3 Design Parameters.......................................................................................70
3.4 Detailed Design Examples..........................................................................72
3.5 Verification of Design Approach................................................................73
3.6 Design Loads and Distribution....................................................................75
3.7 Analysis and Design Procedure...................................................................85
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Page
4. PARAMETRIC STUDY RESULTS.................................................................123
4.1 Introduction...............................................................................................123
4.2 Live Load Moments and Shears................................................................125
4.3 Service Load Design.................................................................................141
4.4 Ultimate Limit State Design......................................................................166

5. GRILLAGE ANALYSIS...................................................................................183
5.1 Introduction...............................................................................................183
5.2 Problem Statement....................................................................................184
5.3 Verification of Finite Element Analysis....................................................184
5.4 Calibration of Grillage Model...................................................................189
5.5 Grillage Model Development....................................................................194
5.6 Application of HL-93 Design Truck Live Load........................................200
5.7 Grillage Analysis and Postprocessing of Results......................................202
5.8 LRFD Load Distribution Factors..............................................................204
5.9 Summary of Results and Conclusion........................................................204

6. SUMMARY, CONCLUSIONS AND RECOMMENDATIONS......................207
6.1 Summary...................................................................................................207
6.2 Design Issues and Recommendations.......................................................209
6.3 Conclusions...............................................................................................215
6.4 Recommendations for Future Research....................................................221

REFERENCES ..........................................................................................................222
APPENDIX A PARAMETRIC STUDY RESULTS...............................................227
APPENDIX B DETAILED DESIGN EXAMPLES FOR INTERIOR TEXAS
U54 PRESTRESSED CONCRETE BRIDGE GIRDER DESIGN
USING AASHTO STANDARD AND LRFD
SPECIFICATIONS..........................................................................275
APPENDIX C ILLUSTRATIONS OF DERHERSVILLE BRIDGE USED FOR
THE VERIFICATION OF FINITE ELEMENT ANALYSIS
MODEL IN SECTION 5..................................................................406
VITA ..........................................................................................................410
ix
LIST OF FIGURES

Page

Figure 2.1 Reliability Indices for LRFD Code, Simple Span Moments in
Prestressed Concrete Girders (Nowak 1999)..........................................27
Figure 2.2 Reliability Indices for AASHTO Standard (1992), Simple Span
Moments in Prestressed Concrete Girders (Nowak 1999)......................27
Figure 2.3 Grillage Bending Moment Diagram for Longitudinal Member
(Hambly and Pennels 1975)....................................................................61
Figure 2.4 Principle Modes of Deformation (a)Total, (b) Longitudinal Bending,
(c) Transverse Bending, (d) Torsion, (e) Distortion (Hambly 1991)......62
Figure 3.1 Typical Girder Bridge Cross Section......................................................68
Figure 3.2 Typical Section Geometry and Strand Pattern of Texas U54 Beam
(Adapted from TxDOT 2001).................................................................69
Figure 3.3 Beam End Detail for Texas U54 Beams (TxDOT 2001)........................72
Figure 3.4 HL93 Design Truck (AASHTO 2004)...................................................77
Figure 3.5 HS20-44 Design Lane Load (AASHTO 2002).......................................77
Figure 3.6 HS20-44 Design Truck Load (AASHTO 2002).....................................78
Figure 3.7 Placement of Design Live Loads for a Simply Supported Beam...........81
Figure 3.8 Definition of d (for This Study).............................................................83
e
Figure 3.9 Various Choices for Web and Flange Lengths, and Thicknesses
for Texas U54 Beam to Calculate the Reduction Factor, φ ..................92
ω
Figure 3.10 Neutral Axis Location...........................................................................103
Figure 4.1 Comparison of Live Load Distribution Factor for Moment.................129
Figure 4.2 Comparison of Live Load Distribution Factor for Shear......................130
Figure 4.3 Comparison of Undistributed Live Load Moment...............................131
Figure 4.4 Comparison of Undistributed Live Load Shear Force at Critical
Section..................................................................................................134
Figure 4.5 Comparison of Distributed Live Load Moment...................................135
Figure 4.6 Comparison of Distributed Live Load Shear Force at Critical
Section..................................................................................................138
Figure 4.7 Comparison of Undistributed Dynamic Load Moment at Midspan.....139
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Page

Figure 4.8 Comparison of Undistributed Dynamic Load Shear Force at
Critical Section.....................................................................................140
Figure 4.9 Maximum Span Length versus Girder Spacing for U54 Beam............143
Figure 4.10 Comparison of Initial Concrete Strength (Strand Diameter 0.5 in.).....152
Figure 4.11 Comparison of Final Concrete Strength (Strand Diameter 0.5 in.)......156
Figure 4.12 Comparison of Initial Prestress Loss (Strand Diameter 0.5 in.)...........157
Figure 4.13 Comparison of Final Prestress Loss (Strand Diameter 0.5 in.)............162
Figure 4.14 Comparison of Factored Design Moment.............................................169
Figure 4.15 Comparison of Factored Design Shear at Respective Critical
Section Location (Strand Diameter 0.5 in.)..........................................171
Figure 4.16 Comparison of Nominal Moment Resistance (Strand Diameter
0.5 in.)...................................................................................................174
Figure 4.17 Comparison of Nominal Moment Resistance (Strand Diameter
0.6 in.)...................................................................................................175
Figure 4.18 Comparison of Camber (Strand Diameter 0.5 in.)................................176
Figure 4.19 Comparison of Transverse Shear Reinforcement Area (Strand
Diameter 0.5 in.)...................................................................................181
Figure 4.20 Comparison of Interface Shear Reinforcement Area (Strand
Diameter 0.5 in.)...................................................................................182
Figure 5.1 Illustration of the Finite Element Model Used for Verification...........186
Figure 5.2 Comparison of Experimental Results vs. FEA Results........................189
Figure 5.3 Grillage Model No. 1............................................................................190
Figure 5.4 Grillage Model No. 2............................................................................191
Figure 5.5 Location of Longitudinal Member for Grillage Model No. 1...............191
Figure 5.6 Grillage Model (for 60 Degree Skew).................................................195
Figure 5.7 Calculation of St. Venant’s Torsional Stiffness Constant for
Composite U54 Girder..........................................................................197
Figure 5.8 T501 Type Traffic Barrier and Equivalent Rectangular Section..........198