Behavior of Two-Span Continuous Pier Caps with Varying Levels of Prestress

There are currently two different design philosophies present in American codes and standards. For conventionally reinforced members, design is based on the ultimate limit state with service limit state checks while members with prestressing steel are designed based on the service limit state with ultimate limit state checks. As well, there is presently no provision for using anything but full prestressing based on the service limit state if any prestressing is used. These different philosophies often lead to confusion in design and in many cases uneconomical designs.

The goal of this study is to develop a clearer design method that may be applied to structures using varying amounts of non-prestressed and prestressed reinforcement. In particular, two-span continuous stocky beams are examined in conjunction with a unified design philosophy. The philosophy is one that bases flexural and shear designs on the ultimate limit state, carrying out adequate serviceability checks.

Four two-span continuous model beams were constructed and loaded to failure. The model designs included one conventionally reinforced and one fully prestressed design in accountance with the 1992 AASHTO Standard Specifications for Highway Bridges. Also designed were a model with 100% prestressed reinforcement to carry ultimate loads and a model with 71% prestressed reinforcement and 29% non-prestressed reinforcement to carry ultimate loads.

Behavior of these models during loading was recorded in terms of cracking behavior, load-deflection and moment-deflection response, moment redistribution, and ultimate load carrying capacity. Predicted fatigue capacity as well as constructibility and economic considerations were examined. Many conclusions and design recommendations are made.