2009 Caltrans-PEER Seismic Seminar Series Seismic Response and Seismic Response and Capacity Evaluation of Exterior Capacity Evaluation of Exterior Sacrificial Shear Keys of Bridge Sacrificial Shear Keys of Bridge Abutments Abutments Scott A. Ashford, PE, PhD Professor and School Head Oregon State University Corvallis, Oregon
History of UCSD Work on Abutments • Post-tensioned Abutments (2002) – Megally, Silva, and Seible • Sacrificial Shear Keys (2007) – Bozorgzadeh, Bauer, Restrepo, and Ashford • Abutment Backfill Models (2007) – Bozorgzadeh, Ashford, and Restrepo – Initially considered diaphragm-type abutments – Worked with EMI on backfill soil types – Completed testing on seat-type abutment
Acknowledgments Acknowledgments • The California Department of Transportation (Caltrans) is gratefully acknowledged for the financially support of the experimental research on sacrificial exterior shear keys. • This research was carried out at UC San Diego. The participation of the UCSD faculty, staff, and students is also acknowledged. 3
Schematic of Exterior Shear Key in Schematic of Exterior Shear Key in Bridge Abutments Bridge Abutments Exterior Wing-wall shear key Superstructure Back-wall Stem wall Footing Vertical pile Battered pile 4
1994 Northridge Earthquake 1994 Northridge Earthquake Over half of damaged bridges Over half of damaged bridges suffered abutment damage suffered abutment damage 5
Experimental Work Experimental Work • Five test series, each including two test units were built at a 40% scale. • Main variables: – Construction joint. – Amount and configuration of vertical reinforcement. – Amount and configuration of horizontal reinforcement. 6
Elevation View of Reinforcement Elevation View of Reinforcement Layout of the Proposed Model Layout of the Proposed Model 5B 5A Vertical Shear Key 24” 48” 24” Vertical Shear Key Reinforcement,4#4 16.75” Rough Reinforcement,4#4 Construction Joint Foam (0.5” thick) 24” Headed Bars, 14#4 Smooth 30 1/2” Construction Joint 7
Shear Key Test Specimens Shear Key Test Specimens Under Construction Under Construction 8
9 Guiding Frame Test Setup Test Setup Loading Arm Hydraulic Actuator
Failure of Exterior Shear Keys of Failure of Exterior Shear Keys of Two Test Series Two Test Series � Failure occurred in a diagonal Current Model strut in the stem wall. � No sliding at the interface of shear key-abutment stem wall occurred . � Performed as structural fuses Proposed Model with sliding shear failure. � Diagonal cracks formed in the stem wall but the maximum width of cracks were 0.012". 10
Load- -Displacement Curves Displacement Curves Load Current Monolithic 300 Model Construction V Load (kips) Joint 200 100 0 Displacement (in) 0 1 2 3 4 Proposed 180 using foam Model V Load (kips) with bond 120 breaker 60 0 Displacement (in) 0 0.5 1 1.5 2 11
Conclusions Conclusions • A model for evaluation of capacity of shear keys under lateral force was developed based on Strut-and-Tie models. • The proposed model showed better agreement with test results than the current shear friction model. • The current shear friction model underestimates shear key capacity which may lead to damage of abutment wall or supporting piles. 12
Conclusions Conclusions • Based on results of experimental work, several recommendations were proposed for construction details: – Using smooth construction joint. – Shear key vertical reinforcement bars should be the only reinforcement connecting the shear key to the abutment stem wall. – Headed bars or hanger bars can be used in the stem wall to carry the force transmitted by the shear key. 13
Research results Research results implemented along I- - implemented along I 905 in San Diego in 905 in San Diego in September 2007 September 2007 Most testing carried out Most testing carried out in 2003/4, final report in 2003/4, final report dated October 2007 dated October 2007 Caltrans implements Caltrans implements good results quickly, in good results quickly, in this case before final this case before final report issued. report issued.
Shear Key Construction Shear Key Construction
Completed Keys Completed Keys
Results Available at UCSD Results Available at UCSD • SSRP-2001/22, May 2002: “Seismic Response of Sacrificial Shear Keys in Bridge Abutments,” S.H. Megally, P.F. Silva, F. Seible, 215p. • SSRP-04/14, October 2007: "Seismic Response and Capacity Evaluation of Exterior Sacrificial Shear Keys in Bridge Abutments,” A. Bozorgzadeh, H.L. Bauer, J.I. Restrepo, S.A. Ashford, 40p. • SSRP-07/12, May 2007: “Experimental and Analytical Investigation on the Stiffness and Ultimate Capacity of Bridge Abutments,” A. Bozorgzadeh, S. Ashford, and J. Restrepo, 196p.
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