IMPACT OF AASHTO LRFD SPECIFICATIONS ON THE
IMPACT OF AASHTO LRFD SPECIFICATIONS ON THE DESIGNOF PRECAST, PRETENSIONED U-BEAM BRIDGESA ThesisbyMOHSIN ADNANSubmitted to the Office of Graduate Studies ofTexas A&M Universityin partial fulfillment of the requirements for the degree ofMASTER OF SCIENCEDecember 2005Major Subject: Civil Engineering
IMPACT OF AASHTO LRFD SPECIFICATIONS ON THE DESIGNOF PRECAST, PRETENSIONED U-BEAM BRIDGESA ThesisbyMOHSIN ADNANSubmitted to the Office of Graduate Studies ofTexas A&M Universityin partial fulfillment of the requirements for the degree ofMASTER OF SCIENCEApproved by:Co-Chairs of Committee,Committee Member,Head of Department,Mary Beth D. HuestePeter B. KeatingTerry KohutekDavid V. RosowskyDecember 2005Major Subject: Civil Engineering
iiiABSTRACTImpact of AASHTO LRFD Specifications on the Design of Precast, Pretensioned UBeam Bridges. (December 2005)Mohsin Adnan, B.S., NWFP University of Engineering and TechnologyCo-Chairs of Advisory Committee: Dr. Mary Beth D. HuesteDr. Peter B. KeatingTexas Department of Transportation (TxDOT) is currently designing its highwaybridge structures using the AASHTO Standard Specifications for Highway Bridges, andit is expected that TxDOT will make transition to the use of the AASHTO LRFD BridgeDesign Specifications before 2007. The objectives of this portion of the study are toevaluate the current LRFD Specifications to assess the calibration of the code withrespect to typical Texas U54 bridge girders, to perform a critical review of the majorchanges when transitioning to LRFD design, and to recommend guidelines to assistTxDOT in implementing the LRFD Specifications. This study focused only on theservice and ultimate limit states and additional limit states were not evaluated.The available literature was reviewed to document the background researchrelevant to the development of the LRFD Specifications, such that it can aid in meetingthe research objectives. Two detailed design examples, for Texas U54 beams using theLRFD and Standard Specifications, were developed as a reference for TxDOT bridgedesign engineers. A parametric study was conducted for Texas U54 beams to perform anin-depth analysis of the differences between designs using both specifications. Majorparameters considered in the parametric study included span length, girder spacing,strand diameter and skew angle. Based on the parametric study supplemented by theliterature review, several conclusions were drawn and recommendations were made. Themost crucial design issues were significantly restrictive debonding percentages and thelimitations of approximate method of load distribution.
ivThe current LRFD provisions of debonding percentage of 25 percent per sectionand 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 strengthconcretes. Based on previous research and successful past practice by TxDOT, it wasrecommended that up to 75% of the strands may be debonded, if certain conditions aremet.The provisions given in the LRFD Specifications for the approximate loaddistribution are subject to certain limitations of span length, edge distance parameter (de)and number of beams. If these limitations are violated, the actual load distribution shouldbe determined by refined analysis methods. During the parametric study, several of theselimitations were found to be restrictive for typical Texas U54 beam bridges. Two caseswith span lengths of 140 ft. and 150 ft., and a 60 degree skew were investigated bygrillage analysis method.
vDEDICATIONI dedicate this thesis to my grandfather Saeed Ahmed Khan, my parents NuzhatMufti and Ahmed Zia Babar and my wife Zubia Naji.
viACKNOWLEDGMENTSMy utmost gratitude is to Almighty God, who has been very kind to me duringall these years.It is a pleasure to thank many who made this thesis possible. Firstly, I would liketo 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 formy M.S., and without her common-sense, knowledge and perceptiveness I would neverhave finished. I appreciate the contribution of Dr. Peter Keating and Dr. Terry Kohutekfor their helpful review of this document. I am grateful to my father-in-law Dr. AhmedRiaz 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) whofunded this research through the Texas Transportation Institute. I also wish toacknowledge the financial support provided by the Department of Civil Engineering atTexas A&M University. I greatly benefited from very many technical discussions withMr. Mohammad Safiudin Adil, a graduate student at Texas A&M University.Finally, and most importantly, I am forever indebted to my parents Nuzhat Muftiand 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.
viiTABLE OF CONTENTSPageABSTRACT. iiiDEDICATION.vACKNOWLEDGMENTS. viTABLE OF CONTENTS . viiLIST OF FIGURES. ixLIST OF TABLES . xii1.INTRODUCTION.11.1 Background and Problem Statement .11.2 Objectives and Scope .31.3 Research Methodolgy.31.4 Organization of Thesis .72.LITERATURE REVIEW.92.12.22.32.42.52.62.73.Introduction .9AASHTO Standard and LRFD Specifications.9Code Calibration and Application of Reliability Theory .21Development of Vehicular Live Load Model .28Vehicular Live Load Distribution Factors .30Debonding of Prestressing Strands .46Refined Analysis .57PARAMETRIC STUDY OUTLINE AND ANALYSIS PROCEDURES .673.13.23.33.43.53.63.7General .67Bridge Geometry and Girder Section.68Design Parameters.70Detailed Design Examples .72Verification of Design Approach .73Design Loads and Distribution.75Analysis and Design Procedure.85
viiiPage4.PARAMETRIC STUDY RESULTS .1234.1 Introduction .1234.2 Live Load Moments and Shears.1254.3 Service Load Design .1414.4 Ultimate Limit State Design.1665.GRILLAGE ANALYSIS .1835.15.25.35.45.55.65.75.85.96.Introduction .183Problem Statement .184Verification of Finite Element Analysis.184Calibration of Grillage Model .189Grillage Model Development.194Application of HL-93 Design Truck Live Load.200Grillage Analysis and Postprocessing of Results.202LRFD Load Distribution Factors .204Summary of Results and Conclusion .204SUMMARY, CONCLUSIONS AND RECOMMENDATIONS .2076.16.26.36.4Summary .207Design Issues and Recommendations .209Conclusions .215Recommendations for Future Research .221REFERENCES.222APPENDIX APARAMETRIC STUDY RESULTS .227APPENDIX BDETAILED DESIGN EXAMPLES FOR INTERIOR TEXASU54 PRESTRESSED CONCRETE BRIDGE GIRDER DESIGNUSING AASHTO STANDARD AND LRFDSPECIFICATIONS .275APPENDIX CILLUSTRATIONS OF DERHERSVILLE BRIDGE USED FORTHE VERIFICATION OF FINITE ELEMENT ANALYSISMODEL IN SECTION 5.406VITA.410
ixLIST OF FIGURESPageFigure 2.1Reliability Indices for LRFD Code, Simple Span Moments inPrestressed Concrete Girders (Nowak 1999).27Figure 2.2Reliability Indices for AASHTO Standard (1992), Simple SpanMoments in Prestressed Concrete Girders (Nowak 1999).27Figure 2.3Grillage Bending Moment Diagram for Longitudinal Member(Hambly and Pennels 1975).61Figure 2.4Principle Modes of Deformation (a)Total, (b) Longitudinal Bending,(c) Transverse Bending, (d) Torsion, (e) Distortion (Hambly 1991).62Figure 3.1Typical Girder Bridge Cross Section.68Figure 3.2Typical Section Geometry and Strand Pattern of Texas U54 Beam(Adapted from TxDOT 2001).69Figure 3.3Beam End Detail for Texas U54 Beams (TxDOT 2001).72Figure 3.4HL93 Design Truck (AASHTO 2004) .77Figure 3.5HS20-44 Design Lane Load (AASHTO 2002).77Figure 3.6HS20-44 Design Truck Load (AASHTO 2002) .78Figure 3.7Placement of Design Live Loads for a Simply Supported Beam. .81Figure 3.8Definition of de (for This Study).83Figure 3.9Various Choices for Web and Flange Lengths, and Thicknessesfor Texas U54 Beam to Calculate the Reduction Factor, φω .92Figure 3.10Neutral Axis Location.103Figure 4.1Comparison of Live Load Distribution Factor for Moment. .129Figure 4.2Comparison of Live Load Distribution Factor for Shear.130Figure 4.3Comparison of Undistributed Live Load Moment. .131Figure 4.4Comparison of Undistributed Live Load Shear Force at CriticalSection. .134Figure 4.5Comparison of Distributed Live Load Moment. .135Figure 4.6Comparison of Distributed Live Load Shear Force at CriticalSection. .138Figure 4.7Comparison of Undistributed Dynamic Load Moment at Midspan. .139
xPageFigure 4.8Comparison of Undistributed Dynamic Load Shear Force atCritical Section. .140Figure 4.9Maximum Span Length versus Girder Spacing for U54 Beam. .143Figure 4.10Comparison of Initial Concrete Strength (Strand Diameter 0.5 in.).152Figure 4.11Comparison of Final Concrete Strength (Strand Diameter 0.5 in.). .156Figure 4.12Comparison of Initial Prestress Loss (Strand Diameter 0.5 in.). .157Figure 4.13Comparison of Final Prestress Loss (Strand Diameter 0.5 in.). .162Figure 4.14Comparison of Factored Design Moment.169Figure 4.15Comparison of Factored Design Shear at Respective CriticalSection Location (Strand Diameter 0.5 in.). .171Figure 4.16Comparison of Nominal Moment Resistance (Strand Diameter0.5 in.). .174Figure 4.17Comparison of Nominal Moment Resistance (Strand Diameter0.6 in.). .175Figure 4.18Comparison of Camber (Strand Diameter 0.5 in.).176Figure 4.19Comparison of Transverse Shear Reinforcement Area (StrandDiameter 0.5 in.). .181Figure 4.20Comparison of Interface Shear Reinforcement Area (StrandDiameter 0.5 in.). .182Figure 5.1Illustration of the Finite Element Model Used for Verification .186Figure 5.2Comparison of Experimental Results vs. FEA Results .189Figure 5.3Grillage Model No. 1 .190Figure 5.4Grillage Model No. 2 .191Figure 5.5Location of Longitudinal Member for Grillage Model No. 1.191Figure 5.6Grillage Model (for 60 Degree Skew). .195Figure 5.7Calculation of St. Venant’s Torsional Stiffness Constant forComposite U54 Girder.197Figure 5.8T501 Type Traffic Barrier and Equivalent Rectangular Section.198
xiPageFigure 5.9Cross-Sections of End and Intermediate Diaphragms. .199Figure 5.10Application of Design Truck Live Load for Maximum Momenton Grillage Model.201Figure 5.11Application of Design Truck Live Load for Maximum Shear onGrillage Model.201Figure 5.12Design Truck Load Placement on a Simply Supported Beam forMaximum Response. .203409
xiiLIST OF TABLESPageTable 2.1Comparison of Serviceability and Strength Limit States . 12Table 2.2LRFD Live Load Distribution Factors for Concrete Deck onConcrete Spread Box Beams. 15Table 2.3Comparison of Relaxation Loss Equations in the LRFD and StandardSpecifications . 19Table 2.4Statistical Parameters of Dead Load (Nowak and Szerszen 1996) . 23Table 2.5Statistical Parameters for Resistance of Prestressed Concrete Bridges(Nowak et al. 1994) . 25Table 3.1Section Properties of Texas U54 Beams (Adapted from TxDOT2001). 69Table 3.2Proposed Parameters for Parametric Study . 70Table 3.3Additional Design Parameters. 71Table 3.4Comparison of Detailed Example Design for LRFD Specifications(PSTRS14 vs. MatLAB) . 74Table 3.5Comparison of Detailed Example Design for Standard Specifications(PSTRS14 vs. MatLAB) . 74Table 3.6Formulas for Different Live Load Placement Schemes in Figure 3.7(Adapted from PCI Bridge Design Manual). . 80Table 3.7LRFD Live Load DFs for Concrete Deck on Concrete Spread BoxBeams (Adapted from AASHTO 2004). 82Table 3.8Spacings – Reasons of Invalidation . 84Table 3.9Allowable Stress Limits for the LRFD and Standard Specifications. 91Table 4.1Summary of Design Parameters . 123Table 4.2Comparison of Moment Distribution Factors for U54 Interior Beams . 127Table 4.3Comparison of Live Load Distribution Factors . 128Table 4.4Comparison of Distributed Live Load Moments . 136Table 4.5Range of Difference in Distributed Live Load Shear for LRFDRelative to Standard Specifications. 137
xiiiPageTable 4.6Range of Difference in Undistributed Dynamic Load Moment andShear for LRFD Relative to Standard Specifications. 139Table 4.7Maximum Differences in Maximum Span Lengths of LRFD DesignsRelative to Standard Designs . 142Table 4.8Comparison of Maximum Span Lengths (Strand Diameter 0.5 in.) . 144Table 4.9Comparison of Maximum Span Lengths (Strand Diameter 0.6 in.) . 145Table 4.10Comparison of Number of Strands (Strand Diameter 0.5 in.,Girder Spacing 8.5 ft.). 147Table 4.11Comparison of Number of Strands (Strand Diameter 0.5 in.,Girder Spacing 10 ft.). 148Table 4.12Comparison of Number of Strands (Strand Diameter 0.5 in.,Girder Spacing 11.5 ft.). 149Table 4.13Comparison of Number of Strands (Strand Diameter 0.5 in.,Girder Spacing 14 ft.). 150Table 4.14Comparison of Number of Strands (Strand Diameter 0.5 in.,Girder Spacing 16.67 ft.). 150Table 4.15Comparison of Initial Concrete Strength (Strand Diameter 0.5 in.) . 151Table 4.16Comparison of Final Concrete Strengths Required for LRFDRelative to Standard Specifications (Strand Diameter 0.5 in.) . 154Table 4.17Comparison of Initial Prestress Loss for LRFD Relative to StandardSpecifications (Strand Diameter 0.5 in.). 155Table 4.18Comparison of Elastic Shortening Loss for LRFD Relative toStandard Specifications (Strand Diameter 0.5 in.). 159Table 4.19Comparison of Initial Relaxation Loss for LRFD Relative toStandard Specifications (Strand Diameter 0.5 in.). 160Table 4.20Comparison of Final Prestress Loss for LRFD Relative to StandardSpecifications (Strand Diameter 0.5 in.). 163Table 4.21Comparison of Steel Relaxation Loss for LRFD Relative to StandardSpecifications (Strand Diameter 0.5 in.). 164Table 4.22Comparison of Creep Loss for LRFD Relative to StandardSpecifications (Strand Diameter 0.5 in.). 165Table 4.23Comparison of Factored Design Moment . 168Table 4.24Comparison of Factored Design Shear at Respective Critical SectionLocation (Strand Diameter 0.5 in.). 172
xivPageTable 4.25Comparison of Nominal Moment Capacity (Strand Diameter 0.5 in.) . 173Table 4.26Comparison of Camber (Strand Diameter 0.5 in.) . 177Table 4.27Comparison of Transverse Shear Reinforcement Area (StrandDiameter 0.5 in.). 178Table 4.28Comparison of Interface Shear Reinforcement Area (StrandDiameter 0.5 in.). 180Table 5.1Parameters for Refined Analysis. 184Table 5.2Comparison of Experimental Results with Respect to Finite ElementAnalysis Results (Lanes 1 and 4 Loaded) . 187Table 5.3Comparison of Experimental Results with Respect to Finite ElementAnalysis Results (Lane 4 Loaded) . 187Table 5.4Comparison of FE Analysis Results with Respect to Grillage ModelNo. 1 . 192Table 5.5Comparison of FE Analysis Results with Respect to Grillage ModelNo. 2 . 192Table 5.6Various Cases Defined for Further Calibration on Grillage ModelNo. 1 . 192Table 5.7Comparison of Results for FEA with Respect to the Grillage ModelNo. 1 (Case No. 1). 193Table 5.8Comparison of Results for FEA with Respect to the Grillage ModelNo. 1 (Case No. 2). 193Table 5.9Comparison of Results for FEA with Respect to the Grillage ModelNo. 1 (Case No. 3). 193Table 5.10Comparison of Results for FEA with Respect to the Grillage ModelNo. (Case No. 4). 193Table 5.11Composite Section Properties for U54 Girder . 197Table 5.12LRFD Multiple Presence Factors. 202Table 5.13Maximum Moment and Shear Response on a Simply SupportedBeam. 204Table 5.14LRFD Live Load Moment and Shear Distribution Factors. 204Table 5.15Comparison of Moment DFs. 205Table 5.16Comparison of Shear DFs . 205Table 6.1Summary of Design Parameters for Parametric Study. 209
xvPageTable 6.2Parameters for Refined Analysis. 212Table 6.3Maximum Differences in Maximum Span Lengths of LRFD DesignsRelative to Standard Designs . 217404
11.1.1INTRODUCTIONBACKGROUND AND PROBLEM STATEMENTUntil the mid-1990s, the design of bridges in the United States was governed bythe AASHTO Standard Specifications for Highway Bridges (AASHTO 1992). To ensurea more consistent level of reliability among bridge designs, research was directedtowards developing an alternate design philosophy. As a result, the AASHTO Load andResistance Factor Design (LRFD) Bridge Design Specifications were introduced in 1994(AASHTO 1994). The LRFD Specifications are based on reliability theory and includesignificant changes for the design of highway bridges. The latest edition of the StandardSpecifications (AASHTO 2002) will not be updated again, and the Federal HighwayAdministration (FHWA) has established a mandatory goal of designing all new bridgestructures according to the LRFD Specifications no later than 2007.Until 1970, the AASHTO Standard Specifications were based on the workingstress design (WSD) philosophy, alternatively named allowable stress design (ASD). InASD, the allowable stresses are considered to be a fraction of a given structuralmember’s load carrying capacity and the calculated design stresses are restricted to beless than or equal to those allowable stresses. The possibility of several loads actingsimultaneously on the structure is specified through different load combinations, butvariation in likelihood of those load combinations and loads themselves is notrecognized in ASD. In the early 1970s, a new design philosophy, load factor design(LFD), was introduced to take into account the variability of loads by using differentmultipliers for dead, live, wind and other loads to a limited extent (i.e., statisticalvariability of design parameters was not taken into account). As a result, the ASD andLFD requirements, as specified in the AASHTO Standard Specifications (AASHTOThis thesis follows the style of ASCE Journal of Structural Engineering.
21992, AASHTO 2002), do not provide for a consistent and uniform safety level forvarious groups of bridges (Nowak 1995).AASHTO’s National Cooperative Highway Research Program (NCHRP) Project12-33 was initiated in July of 1988 to develop the new AASHTO LRFD Specificationsand Commentary (AASHTO 1998). The project included the development of loadmodels, resistance models and a reliability analysis procedure for a wide variety oftypical bridges in the United States. To calibrate this code, a reliability index related tothe probability of exceeding a particular limit state was used as a measure of structuralsafety. About 200 representative bridges were chosen from various geographical regionsof the United States based on current and future trends in bridge designs, rather thanchoosing from existing bridges only. Reliability indices were calculated using aniterative procedure for these bridges, which were designed according to the StandardSpecifications (AASHTO 1992). In order to ensure an adequate level of reliability forcalibration of the LRFD Specifications, the performance of all the representative bridgeswas evaluated and a corresponding target reliability index was chosen to provide aminimum, consistent and uniform safety margin for all structures. The load andresistance factors were then calculated so that the structural reliability is close to thetarget reliability index (Nowak 1995).This study is part of the Texas Department of Transportation (TxDOT) project 04751 “Impact of AASHTO LRFD Specifications on Design of Texas Bridges.” TxDOTis currently designing its highway bridge structures using the Standard Specifications,and it is expected that TxDOT will make transition to the use of the LRFDSpecifications before 2007. It is crucial to assess the impact of the LRFD Specificationson the TxDOT bridge design practice because of the significant differences in the designphilosophies of the Standard and LRFD Specifications.
31.2OBJECTIVES AND SCOPEThe major objectives of the study described in this thesis are (1) to evaluate theimpact of the current LRFD on the design of typical Texas precast, pretensioned U54bridge girders, (2) to perform a critical review of the major changes when transitioningfrom current TxDOT practices to LRFD based design, and (3) to recommend guidelinesto assist TxDOT in implementing the LRFD Specifications.The scope of this study is limited to precast, prestressed Texas U54 beams.Detailed design examples were developed and a parametric study was carried out onlyfor interior beams. The provisions in TxDOT Bridge Design Manual are based onprevious research and experience, and these provisions address the needs that are typicalfor Texas bridges. So, in general TxDOT’s past practices, as outlined in their BridgeDesign Manual (TxDOT 2001, are considered in this study when possible. For example,although the modular ratio is usually less than unity in bridge design practice because ofbeam elastic modulus being greater than the deck slab elastic modulus, it is considered tobe unity for the service limit state design, based on TxDOT practice. The actual va
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 Specifica
AASHTO LRFD Bridge Design Specifications, 4th Edition, with 2009 interims AASHTO LRFD Bridge Design Specifications, 5th Edition AASHTO LRFD Bridge Design Specifications, 5th Edition, with 2010 interims AASHTO LRFD Bridge Design Specifications, 6th Edition AASHTO LRFD Bridge Design
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Recently, we were made aware of some technical revisions that need to be applied to the AASHTO LRFD Bridge Design Specifications, 6th Edition. Please replace the existing text with the corrected text to ensure that your edition is both accurate and current. AASHTO staff sincerely apologizes for any inconvenience.File Size: 2MBPage Count: 104Explore furtherAASHTO LRFD 2012 Bridge Design Specifications 6th Ed ( US .archive.orgAASHTO Issues Updated LRFD Bridge Design Guideaashtojournal.orgAASHTO Publishes New Manual for Bridge Element Inspection .aashtojournal.orgAASHTO LRFD Bridge Design Specifications. Eighth Edition .trid.trb.orgSteel Bridge Design Handbook American Institute of Steel .www.aisc.orgRecommended to you b
AASHTO LRFD Bridge Design Specifications, 7th Edition, 2014 (AASHTO LRFD) v. AASHTO LRFD Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals, First Edition, 2015 (AASHTO Signs) vi. Washington State Department of Transportation Bridge Design Manual (LRFD), 2016 (BDM) vii. American Institute of Steel .
AASHTO LRFD Bridge Design Specifications, 8th Edition (LRFD-8) May 2018 . Dear Customer: Recently, we were made aware of some technical revisions that need to be applied to the AASHTO LRFD Bridge Design Specifications, 8th Edition. Please scroll down to see the full erratum.File Size: 2MB
Sep 05, 2021 · Development of the LRFD Specifications. LRFD Specifications were adopted by AASHTO in 1996 Period of transition – Two specifications were in use – AASHTO did not eliminate the Standard Specifications – Most States continued to use the Standard Specifications in early 2000s Maintain
AASHTO T 217 AASHTO T 85 AASHTO T 2 AASHTO T 255 AASHTO T 217 AASHTO T 85 Report on Form BMT-122 and Plant Diary. Test Cylinders One set per PTU*. A set of cylinders consists of two 6” x 12” cylinders, to be tested at 28 days of age. During placement of concrete. AASHTO T 141 AASHTO T 141 ALDOT-210 AASHTO T
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