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A Numerical Analysis of the Shear Key Cracking Problem in Adjacent Box Beam Bridges
Abstract
This objective of this thesis is to evaluate the potential of selected shear key modifications in reducing tensile stress in the shear key region of adjacent box beam using numerical analysis. In order to meet the objective, a comprehensive literature review of the current design and construction practices was conducted. Shear key configuration, grouting material, posttensioning, and bearing pad details were identified as four possible sources for cracking from the design perspective. A grillage analysis was conducted to determine the maximum moment and shear experienced by shear key. The information was then used in the finite element analysis to evaluate the potential of each shear key modification in reducing the tensile stress in the shear keys of adjacent box beam bridge. The most effective shear key modifications in reducing tensile stress, which was the main mechanism for cracking, were concluded at the end of the thesis.
TABLE OF CONTENTS
List of Figures…………………………………………………………………………………………………………………… vi
List of Tables…………………………………………………………………………………………………………………… viii
Acknowledgement ……………………………………………………………………………………………………………………….ix
Chapter 1 – Introduction ………………………………………………………………………………………………………………. 1
1.1. Introduction ……………………………………………………………………………………………………………………. 1
1.2. Objective ………………………………………………………………………………………………………………………… 2
1.3. Tasks ………………………………………………………………………………………………………………………………. 2
1.4. Outline of Thesis ……………………………………………………………………………………………………………… 2
Chapter 2 – Literature Review ……………………………………………………………………………………………………….. 4
2.1. Literature Review …………………………………………………………………………………………………………….. 4
2.1.1. Shear Key Geometry …………………………………………………………………………………………….. 4
2.1.2. Grouting Material ………………………………………………………………………………………………… 6
2.1.3. Transverse Reinforcement Details ………………………………………………………………………….. 7
2.1.4. Bearing Pad Details ………………………………………………………………………………………………. 9
2.1.5. Numerical Analysis ……………………………………………………………………………………………… 10
2.2. Summary ………………………………………………………………………………………………………………………. 13
Chapter 3 – Grillage Analysis ……………………………………………………………………………………………………….. 14
3.1. Introduction ………………………………………………………………………………………………………………….. 14
3.2. Loads ……………………………………………………………………………………………………………………………. 14
3.3. Modeling Description ……………………………………………………………………………………………………… 15
3.3.1. Grillage ……………………………………………………………………………………………………………… 15
3.3.2. Material Properties and Member Section Properties ……………………………………………… 16
3.3.3. Load Combinations …………………………………………………………………………………………….. 20
3.4. Results and Discussion ……………………………………………………………………………………………………. 21
3.5. Distribution Factor …………………………………………………………………………………………………………. 25
Chapter 4 – Finite Element Analysis ……………………………………………………………………………………………… 28
4.1. Modeling of the Shear Test ……………………………………………………………………………………………… 28
4.1.1. Introduction ………………………………………………………………………………………………………. 28
4.1.2. Geometry and Boundary Conditions …………………………………………………………………….. 29
4.1.3. Material Models …………………………………………………………………………………………………. 30
4.1.4. Elements and Mesh ……………………………………………………………………………………………. 33
4.1.5. Results and Discussion ………………………………………………………………………………………… 35
4.1.6. Modeling Remarks ……………………………………………………………………………………………… 39
4.2. Parametric Study of Shear Key Modifications ……………………………………………………………………. 39
4.2.1. Modeling Approach ……………………………………………………………………………………………. 39
4.2.2. Analysis of Typical PennDOT Shear Keys ……………………………………………………………….. 42
4.2.3. Grouting Material ………………………………………………………………………………………………. 44
4.2.4. Shear Key Configuration ……………………………………………………………………………………… 46
4.2.5. Post-tensioning Effect …………………………………………………………………………………………. 48
4.2.6. Bearing Pad ……………………………………………………………………………………………………….. 55
4.2.7. Table Summary ………………………………………………………………………………………………….. 58
Chapter 5 – Conclusion and Recommendations ……………………………………………………………………………… 59
5.1. Conclusion …………………………………………………………………………………………………………………….. 59
5.2. Recommendation for future research ………………………………………………………………………………. 59
Appendix A – Grillage Analysis Section Property Calculations ………………………………………………………….. 60
Appendix B – Grillage Analysis Results ………………………………………………………………………………………….. 64
Appendix C – Distribution Factor Calculations ……………………………………………………………………………….. 72
Appendix D – Finite Element Analysis Summary …………………………………………………………………………….. 75
Reference …………………………………………………………………………………………………………………………………. 77
Chapter 1 – Introduction
1.1. Background Information
This thesis is originated from the Pennsylvania Department of Transportation research project work order 14 – determining more effective approaches for grouting shear keys of adjacent box beams. The project ended on May 30th, 2010. This thesis is an update on the existing PennDOT WO14 final report to incorporate the analysis information that was not included in that report.
The use of bridges built with adjacent precast, prestressed concrete box beams is favored by several State Departments of Transportation, due to the efficiency of their construction. Box beams are typically connected by shear keys with some type of transverse reinforcement (mild or prestressed) and are grouted. Although the structural performance of such bridges has been successful, a common problem that has been observed is shear key grout failure at early stages – cracking, see Figure 1.1. In many cases this cracking has resulted in water leakage, which could lead to corrosion problems both on the mild and prestressed reinforcement. Severe cracking can eventually affect the load transfer ability of the shear key unit. (Macioce et al. 2007)
Figure 1.1 Shear key cracking during construction (William Koller 2008)
A typical adjacent box beam bridge cross section is shown in Figure 1.2. A series of prestressed box beams are placed together next to each other. The box beams are joined together by a space of ¼” to ¾” filled with grouting material. The joint is called shear key as shown in the figure. Shear key is a unique and important structural component in adjacent box beam bridges. It acts as a connector between two adjacent box beams to transfer the loads across the entire superstructure. One advantage pertaining to adjacent box beam bridge is the small distribution factor, which means the vehicle load is spreading uniformly across the superstructure.
Figure 1.2 A typical adjacent box beam cross section
1.2. Objective
This thesis aims to evaluate the potential of shear key modifications in reducing tensile stress and thus cracking in the shear key region of adjacent box beam bridge. This evaluation will be conducted numerically based on experimental material characterization.
In order to meet this objective, the following tasks were conducted:
- A literature review was conducted to identify possible shear key modifications that have the potential in reducing the likelihood of cracking. The review also looked into existing grillage analysis methods and finite element analysis used to model concrete bridge structures.
- A grillage analysis was conducted to identify the maximum moment and shear experienced by the shear key under full live load. In addition, the distribution of loads in the bridge structure was examined.
- Finite element analysis was conducted to evaluate the potential of each shear key modification in reducing stress level in the shear key under full live load.
1.3. Scope of research
In this research, a state of practice literature review was conducted to identify components that related to shear key cracking. Material characterization was conducted to provide the basic material properties needed for the numerical analysis. A typical PennDOT box beam bridge (80 feet span) was analyzed for the critical load cases that produced the maximum load effects in the shear key. Based on the results and the shear key modifications identified in literature review, numerical evaluation using finite element analysis was conducted to achieve the objective of the research.
1.4. Outline of thesis
Chapter 1 provides the introduction. Chapter 2 of the thesis presents the findings from a comprehensive literature review of the current practices and researches regarding shear key cracking. In Chapter 3, a grillage analysis is conducted to identify the maximum forces effect experienced by the shear key. The results from the grillage analysis are then used in finite element model to assess the effect of different modifications to shear key. The parametric study of the shear key modifications is discussed in Chapter 4. The final Chapter, Chapter 5, presents conclusions from the analysis and recommendations for future research.
