Behavior and Design of Precast Bridge Cap Beams with Pocket Connections

Link to  FINAL REPORT

BACKGROUND
In conventional reinforced concrete bridge construction, cap beams and their connection to columns are designed to be capacity protected under strong earthquakes. This is because cap beams and connections are difficult to repair. The same design philosophy is mandatory for precast cap beams that are used in accelerated bridge construction (ABC), particularly in moderate and high seismic zones. ABC relies heavily on prefabricated reinforced concrete members. The NCHRP report 698 provided a synthesis of different promising ABC connections. Pocket connections were identified as practical means of joining prefabricated columns and pier caps. The AASHTO Scan 11-02 revealed more recent studies about seismic performance of pocket connections.

OBJECTIVE
The main objective of this study is to compile and interpret data on seismic performance of cap beams with pocket connections and identify behavior, design, detailing, and construction considerations for successful implementation of this category of connections. The results of the study will be transformed into design guidelines for possible adoption by AASHTO.

SCOPE
Six tasks will be completed:

  1. conducting literature review,
  2. determining seismic performance and behavior of pocket connections and cap beams,
  3. evaluating constructability of pocket connections,
  4. evaluating different cap beam and pocket detailing methods to ensure capacity protected behavior,
  5. developing design and detailing guidelines for cap beams with pocket, and
  6. summarizing the investigation and the results in a draft final report.

The pocket is formed in the precast cap beam normally using corrugated steel pipes (Fig. 1a). The cap beam is placed on top of precast columns subsequently and the joint is completed. There are two types of pocket connections: cast-in-place (Fig. 1b) and precast (Fig. 1c). In the former type, the column is prefabricated only up to bottom of the cap beam with dowels extending into the pocket. The connection is then filled with concrete placed from a hole at the top of the cap beam. In the second connection type, the column is fully precast and is inserted into the cap beam pocket. Subsequently, the gap between the column and pocket is filled with grout.

Regardless of the construction method, there is discontinuity between the cap beam and column reinforcement, thus there is a concern about the effectiveness of the connection and design of the cap beam particularly adjacent to the joint. Design methods will be presented to ensure linear-elastic behavior of precast cap beams in high seismic regions.

RESEARCH TEAM
Principal Investigators: M. Saiid Saiidi, PhD, PE. and Ahmad Itani, PhD, PE.
Research Associate: Mostafa Tazarv, PhD; Research Assistant: Mehrdad Mehraein, PhD Candidate.

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