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Behavior and Design of Large Structural Concrete Bridge Pier Overhangs

Scott D. Armstrong, Reuben M. Salas, Brad Wood, John E. Breen and Michael E. Kreger

1997

In designing large cantilever bent caps for use on recent projects under current AASHTO design specifications, designers were faced with considerable uncertanties. Questions arose when designers attempted to satisfy both serviceability and strength reqiurements for a series of cap design that mixed both non-prestressed and prestressed concrete solutions. The problems were complicated because of uncertainty whether AASHTO corber or deep beam provisions would apply. The resulting designs were highy congested, had poor constructibility and high costs.

This report outlines development of a new design approach involving use of strut-and-tie models (STM), as well as a mix of prestressed and non-prestressed main flexural reinforcement. A series of sixteen bent cap overhangs were designed, built as a reduced scale, and loaded failure. Deflections, crack patterns and widths, reinforcement strains, and overall behavior were observed. Economic and constructibility issues were examined. The use of T-headed anchorages were explored.

A comprehensive design approach that considers both serviceability issues (deflections, cracking, crack widths, fatigue stress ranges, and side face crack control) and strength issues (ductility, adequacy of STM design and analysis of flexural capacity) was developed. Recommendations for design and detailing to improve behavior, reduce congestion and improve constructibility are provided.

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