Discrepancies in Shear Strength of Prestressed Beams with Different Specifications Catherine French, Principal Investigator Department of Civil Engineering University of Minnesota January 2010 Research Project Final Report #2010-03 Technical Report Documentation Page 1. Report No. 2. 3. Recipients Accession No. MN/RC 2010-03 4. Title and Subtitle 5. Report Date January 2010 Discrepancies in Shear Strength of Prestressed Beams with 6. Different Specifications 7. Author(s) 8. Performing Organization Report No. Ozer Dereli, Carol Shield, Catherine French 9. Performing Organization Name and Address 10. Project/Task/Work Unit No. Department of Civil Engineering University of Minnesota 11. Contract (C) or Grant (G) No. 500 Pillsbury Drive SE (c) 89261 (wo) 15 Minneapolis, MN 55455 12. Sponsoring Organization Name and Address 13. Type of Report and Period Covered Minnesota Department of Transportation Final Report Research Services Section 14. Sponsoring Agency Code 395 John Ireland Boulevard, MS 330 St. Paul, MN 55118 15. Supplementary Notes http://www.lrrb.org/pdf/201003.pdf 16. Abstract (Limit: 250 words) Although Mn/DOT inspection reports indicate that prestressed concrete bridge girders in service do not show signs of shear distress, girders rated with the Virtis-BRASS rating tool and Load Factor Rating (LFR) have indicated that a number of the girders have capacities lower than design level capacities. One of the reasons for the discrepancy was suspected to be conservatism of the rating methods (i.e., LFR). Other suspected reasons included potential flaws in the rating tools used by Mn/DOT (i.e., Virtis-BRASS software) including neglecting possible additional shear capacity parameters (e.g., end blocks). As a consequence, the rating methods have made it difficult to discern the cases for which shear capacity may be a real concern. In order to identify the reasons for the discrepancies and inconsistency in rating results relative to observed performance of the prestressed bridge girders, an analytical research program was conducted. The report provides a brief description of the models that provide the basis for the AASHTO shear design provisions and descriptions of the provisions through the 2002 AASHTO Standard specifications. This is followed by a description of the Virtis-BRASS rating tool, which was verified with example bridges provided by Mn/DOT. To investigate prestressed bridge girders within the inventory that might be most at risk for being undercapacity for shear, 54 girders were selected from the inventory for further evaluation. Some of the 54 girders were found to have larger stirrup spacings than required at the time of design. These girders were subsequently rated and evaluated per the 2002 AASHTO Standard Specifications to determine the adequacy of the designs based on the LFR inventory and operating rating methods. Potential sources for increased shear capacity were identified and reviewed. 17. Document Analysis/Descriptors 18. Availability Statement Prestressed concrete, Load Factor Rating, Shear, Load rating, No restrictions. Document available from: Bridge girders National Technical Information Services, Springfield, Virginia 22161 19. Security Class (this report) 20. Security Class (this page) 21. No. of Pages 22. Price Unclassified Unclassified 242 Discrepancies in Shear Strength of Prestressed Beams with Different Specifications Final Report Prepared by: Ozer Dereli Carol Shield Catherine French Department of Civil Engineering University of Minnesota January 2010 Published by: Minnesota Department of Transportation Research Services Section 395 John Ireland Boulevard Mail Stop 330 St. Paul, MN 55118 This report represents the results of research conducted by the authors and does not necessarily represent the views or policies of the Minnesota Department of Transportation or the University of Minnesota. This report does not contain a standard or specified technique. The authors, the Minnesota Department of Transportation, and the University of Minnesota do not endorse products or manufacturers. Trade or manufacturers’ names appear herein solely because they are considered essential to this report. Acknowledgments Funding for this research was provided by the Minnesota Department of Transportation (Mn/DOT), and it is gratefully acknowledged. Also, this research would not have been possible without the assistance and technical expertise of Lowell Johnson and Edward Lutgen. Their cooperation and assistance are appreciated. Table of Contents Chapter 1. Introduction ................................................................................................................... 1  1.1 Status of Bridges ....................................................................................................................1  1.2 Load Rating ............................................................................................................................1  1.2.1 Load Factor Rating (LFR) ............................................................................................. 2  1.2.2 Rating Aids for LFR ...................................................................................................... 3  1.3 Problem Statement .................................................................................................................3  1.4 Organization of the Document ...............................................................................................4  Chapter 2. Models for Shear Capacity and AASHTO Shear Provisions ........................................ 6  2.1 Introduction ............................................................................................................................6  2.2 Shear Transfer Mechanisms in Prestressed Concrete Beams ................................................6  2.3 AASHTO Standard Shear Provisions ....................................................................................7  2.3.1 AASHTO 1979 Interim Specifications .......................................................................... 8  2.3.2 2002 AASHTO Standard Specification ....................................................................... 10  2.3.3 Concrete Contribution – Flexure-Shear Case .............................................................. 10  2.3.4 Concrete Contribution - Web-Shear Case .................................................................... 11  2.3.5 Web Reinforcement Contribution ................................................................................ 12  2.3.6 Horizontal Shear .......................................................................................................... 12  2.3.7 Critical Section ............................................................................................................. 13  2.4 Summary of Differences in the Shear Provisions of the AASHTO Standard Specifications .............................................................................................................................................13  2.5 Strut-and-Tie Method ..........................................................................................................14  2.5.1 Strength of Ties ............................................................................................................ 15  2.5.2 Strength of Struts ......................................................................................................... 15  2.5.3 Strength of Nodes ........................................................................................................ 16  2.6 Evaluation of AASHTO Shear Provisions, NCHRP Report 549 ........................................16  2.7 Mn/DOT Report 2007-47 (University of Minnesota Tests) ................................................18  2.8 Summary of the Findings .....................................................................................................18  Chapter 3. Description and Verification of Virtis – BRASS Software ......................................... 20  3.1 Introduction ..........................................................................................................................20  3.2 Software Description ...........................................................................................................20  3.3 Verification Procedure with Load Factor Rating (LFR) ......................................................20  3.3.1 PCI Bridge Example .................................................................................................... 21  3.3.2 Mn/DOT Bridge Examples .......................................................................................... 22  3.4 Summary of the Errors Found in Virtis-BRASS .................................................................25  Chapter 4. Selection of Bridges to Investigate for Shear .............................................................. 26  4.1 Introduction ..........................................................................................................................26  4.2 Properties of Selected Bridge Examples from Mn/DOT Inventory ....................................26  4.3 Redesign of the Selected Girders for Shear .........................................................................26  4.4 Observations on Design of Girders for Shear ......................................................................27  Chapter 5. Shear Capacity Evaluation and Operating and Inventory Rating of Bridges In Accordance with the 2002 AASHTO Standard Specifications.................................... 30  5.1 Introduction ..........................................................................................................................30  5.2 Shear Design per 2002 AASHTO Standard Specifications .................................................30  5.2.1 Effect of Horizontal Shear on Design of Girders per 2002 Standard Specifications ... 30 5.3 Observations on Shear Design of Girders per 2002 Standard .............................................31  5.4 Shear Capacities and Ratings per 2002 AASHTO Standard Specifications .......................33  5.5 Observations on Shear Capacity and Rating of Girders per 2002 AASHTO Standard Specifications ......................................................................................................................33  5.6 Mn/DOT Report 2007-47 ....................................................................................................35  5.6.1 Application of Mn/DOT Report 2007-47 .................................................................... 36  Chapter 6. Investigation of Additional Shear Capacity ................................................................ 38  6.1 Introduction ..........................................................................................................................38  6.2 Investigated Parameters for Additional Shear Strength .......................................................38  6.2.1 Contribution from End Blocks at the Beam Ends ........................................................ 38  6.2.2 Effect of Concrete Strength on Shear Strength ............................................................ 39  6.2.3 Arching Action ............................................................................................................. 45  6.3 Investigated Parameters for Reduced Shear Demand ..........................................................49  6.3.1 Live Load Distribution Factors for Shear .................................................................... 49  6.3.2 Effect of End Diaphragms on Shear LLDF ................................................................. 58  Chapter 7. Summary, Conclusions and Recommendations .......................................................... 60  7.1 Summary and Conclusions ..................................................................................................60  7.2 Recommendations ................................................................................................................62  References ..................................................................................................................................... 64  TABLES ...................................................................................................................................... 68  FIGURES .................................................................................................................................... 115  APPENDIX A Sample Shear Calculations APPENDIX B Bridge Inspection Report APPENDIX C Core Test Data from Literature APPENDIX D: Procedure for Finding the Truck Loading Configuration to Check the Applicability of Arching Action APPENDIX E Sample Calculations of Shear Live Load Distribution Factors List of Tables Table 1.1 Load Factors for Rating Levels .................................................................................... 68 Table 2.1 Comparison of AASHTO Shear Provisions ................................................................. 69 Table 2.2 Comparison of AASHTO Horizontal Shear Provisions ............................................... 71 Table 2.3 Comparison of V /V for AASHTO Shear Provisions ........................................... 72 test pred Table 3.1 Dead Load Effects ........................................................................................................ 72 Table 3.2 Live Load Effects with WAD¹=100 (default) ............................................................... 72 Table 3.3 Comparison of Shear Capacities for PCI Bridge Example ........................................... 73 Table 3.4 Comparison of Shear Rating Factors with WAD = 100 ............................................... 73 Table 3.5 Live Load Effects with WAD = 1000 ........................................................................... 73 Table 3.6 Discrepancies in Calculation of Strand Locations ........................................................ 74 Table 3.7 Girder Properties of the Bridge No. 27068_2 and Bridge No. 83022_1-3 ................... 74 Table 3.8 Comparison of Shear Capacities for Bridges 27068_2 and 83022_1-3 ........................ 75 Table 3.9 Comparison of Shear Rating Factors for Bridges 27068_2 and 83022_1-3 ................. 76 Table 4.1 Properties of the Girder Sections of Selected Mn/DOT Bridges .................................. 76 Table 4.2 Girder Properties of the Selected Mn/DOT Bridges Grouped by Year ........................ 77 Table 4.3 Provided and Required Stirrup Spacings for Group 1 Bridges (Designed by 1961 AASHTO Standards) ................................................................................................... 78 Table 4.4 Provided and Required Stirrup Spacings for Group 2 Bridges (Designed by 1965-1969 AASHTO Standards) ................................................................................................... 78 Table 4.5 Provided and Required Stirrup Spacings for Group 3 Bridges (Designed by 1973-1977 AASHTO Standards) ................................................................................................... 79 Table 4.6 Provided and Required Stirrup Spacings for Group 4 Bridges (Designed by 1983 AASHTO Standards) ................................................................................................... 80 Table 4.7 Provided and Required Stirrup Spacings for Group 4* Bridges (Designed by 1977- 1979 Interim AASHTO Standards) .............................................................................. 80 Table 4.8 Differences Between Provided and Required Stirrup Spacings for Groups 1 & 2 ....... 81 Table 4.9 Differences Between Provided and Required Stirrup Spacings for Group 3 ............... 82 Table 4.10 Differences Between Provided and Required Stirrup Spacings for Group 4 ............. 83 Table 4.11 Differences Between Provided and Required Stirrup Spacings for Group 4* ........... 83 Table 4.12 Distribution of Differences Between Provided and Required Stirrup Spacings for Vertical Shear ............................................................................................................... 84 Table 4.13 Distribution of Differences Between Provided and Required Stirrup Spacings for Vertical Shear as a Percentage of Beam Depth ............................................................ 84 Table 4.14 Distribution of Differences Between Provided and Required Stirrup Spacings, Horizontal Shear Requirements Included .................................................................... 85 Table 5.1 Provided and Calculated Stirrup Spacings per 2002 Standard (1961-65-69) ............... 86 Table 5.2 Provided and Calculated Stirrup Spacings per 2002 Standard (1973-77) .................... 87 Table 5.3 Provided and Calculated Stirrup Spacings per 2002 Standard (1983 or 1977) ............ 88 φV Table 5.4 n,STD2002 at Point of Interests (1961-1969) ............................................................... 89 V u Table 5.5 Inventory and Operating Ratings at Point of Interests (1961-1969) ............................. 90 Table 5.6 φV /V at Point of Interests (1973-1977) ............................................................ 91 n,STD2002 u Table 5.7 Shear Inventory and Operating Ratings at Point of Interests (1973-1977) ................... 92 φV Table 5.8 n,STD2002 at Point of Interests (1983) ........................................................................ 93 V u Table 5.9 Shear Inventory and Operating Ratings at Point of Interests (1983) ............................ 94 Table 5.10 Controlling V for 2002 Standard Equation at Point of Interests ............................... 95 c Table 5.11 Parameters for All Girders at Critical Section ............................................................ 97 Table 5.11(Continued) Parameters for All Girders at Critical Section ......................................... 98 φV Table 6.1 n,STD2002 and Shear Inventory Rating Factors (RFs) for Girders without Considering V u End Blocks ................................................................................................................... 99 Table 6.2 Statistical Parameters for 28-day Concrete Compressive Strength for Elk River Cylinders .................................................................................................................... 100 Table 6.3 Statistical Parameters for 28-day Concrete Compressive Strength from Nowak and Szerszen (2003) .......................................................................................................... 100 Table 6.4 Variation in f ' Normalized with Age to f’ for Moist-Cured Specimens ......... 100 c c,28_day Table 6.5 Variation in f ' Normalized with Age to f’ for Specimens Stored Outdoors ... 101 c c,28_day Table 6.6 Core Test Data from Literature ................................................................................... 102 Table 6.7 Comparison of Data from Wood (1991) and Core Test Data ..................................... 103 φV Table 6.8 Calculated n,STD2002 and Shear Rating Factors at Critical Section Based on 20% V u Increase in f ' ............................................................................................................. 104 c φV Table 6.9 Calculated n,STD2002 and Shear Rating Factors at 0.1L Based on 20% Increase in f ' V c u .................................................................................................................................... 105 φV Table 6.10 Calculated n,STD2002 and Shear Rating Factors at 0.3L and 0.4L Based on 20% V u Increase in f ' ............................................................................................................. 106 c Table 6.11 Shear Test Results of Deep Pretensioned I-Girders Compared to the 2002 AASHTO Standard Specifications .............................................................................................. 107 Table 6.12 Comparison of Maximum Distance to the Rear Tandem from the Support for φV /V < 1 with 2.5h to Check the Applicability of Arching Action ............. 108 c n,STD2002 u Table 6.13 Shear Live Load Distribution Factors for Exterior Girders (Huo et al. 2003) .......... 108 Table 6.14 Shear Live Load Distribution Factors for Interior Girders (based on Huo et al. 2003) .................................................................................................................................... 109 Table 6.15 Ratio of Shear Live Load Distribution Factors from Simplified Methods to those from grillage analysis for Exterior Girders (Puckett et al. 2007) ....................................... 110 Table 6.16 Ratio of Shear Live Load Distribution Factors from Simplified Methods to those from grillage analysis for Interior Girders (Puckett et al. 2007) ........................................ 110 Table 6.17 Live Load Shear Calibration Factors (Puckett et al. 2007) ....................................... 111 Table 6.18 Analysis Factors, γa, for Shear Based on One-Half STD¹ (Puckett et al. 2007) ..... 111 Table 6.19 Parameters of Selected Bridges for Shear LLDF Comparison ................................. 112 Table 6.20 Diaphragm Effect on Finite Element Analysis Results for Precast Concrete Beams113 Table 6.21 Diaphragm Effect on Grillage Analysis Results at Obtuse Corner, Beam 1, Bridge No. 24 ................................................................................................................................ 113 Table 6.22 Diaphragm Effect on Grillage Analysis Results at Obtuse Corner, Beam 8, Bridge No. 24 (Puckett et al., 2007) ............................................................................................. 114 List of Figures Figure 1.1 AASHTO HS-20 Loading ......................................................................................... 115 Figure 2.1 Shear Transfer/Actions Contributing to Shear Resistance ........................................ 115 Figure 2.2 Stirrup Contribution ................................................................................................... 116 Figure 2.3 Types of Shear Cracks in Prestressed Concrete Beams ............................................ 116 Figure 2.4 D-Regions in a Frame ................................................................................................ 116 Figure 2.5 Strut-and-Tie Model for a Simple Deep Beam Figure .............................................. 117 Figure 2.6 Range in Parameters for Prestressed Concrete Members .......................................... 117 V Figure 2.7 1979 Interim test vs. f ’ for 85 Prestressed Members (Hawkins et al., 2005) ......... 118 c V pred V Figure 2.8 1979 Interim test vs. Stirrup Reinforcement Ratio for 85 Prestressed Members V pred (Hawkins et al., 2005) ................................................................................................ 118 Figure 3.1 PCI Bridge Example – Bridge Cross Section ............................................................ 119 Figure 3.2 Bridge No. 83022_1-3 Cross Section ........................................................................ 119 Figure 3.3 Interior Girder Composite Cross Section for Bridge No. 83022_1-3 ....................... 119 Figure 3.4 Idealized Bilinear Relationship Between Steel Stress and Distance from the Free End of Strand ..................................................................................................................... 120 Figure 4.1 Number of Bridges Built with Respect to Time ........................................................ 120 Figure 4.2 Distribution of Inadequate Stirrup Spacing for Vertical Shear (Provided – Required) .................................................................................................................................... 121 Figure 4.3 Distribution of Inadequate Stirrup Spacing (Provided – Required) (Horizontal Shear Included) .................................................................................................................... 121 Figure 5.1 Distribution of Bridges in Design Groups ................................................................. 122 Figure 5.2 Distribution of Bridges in Design Groups with Stirrup Spacing Differences ........... 122 f Figure 5.3 Required Spacing per Eqn. (5.3) vs. y with and without 24 in. Spacing Limit ...... 123 b v Figure 5.4 Shear Inventory RF at the Critical Section vs. Beam Depth ..................................... 123 Figure 5.5 Shear Operating RF at the Critical Section vs. Beam Depth ..................................... 124 φV L Figure 5.6 n,STD2002 at the Critical Section vs. for All Girders ........................................ 124 V S u g L Figure 5.7 Inventory Rating Factor at the Critical Section vs. for All Girders ................... 125 S g Figure 6.1 Geometry and Dimensions of End Blocks for 54, 63 and 72 in. Deep Girders ........ 126 Figure 6.2 PC Type III - Outdoor Exposure from Wood (1991) and Core Test Data from the Literature .................................................................................................................... 127 Figure 6.3 Arching Action in a Beam ......................................................................................... 127 V Figure 6.4 test vs. a/d for Different Types of Failure ..................................................... 128 V n,STD2002 Figure 6.5 Lever Rule LLDFs vs. Grillage Analysis LLDFs for Interior Girders, for Two-Lanes Loaded, at the End of Span ........................................................................................ 128