Developing Extended Strands in Girder-Diaphragm Connections for Positive Moment Resistance

Thesis (Master's)--University of Washington, 2017In bridges with precast prestressed concrete girders, the resistance to seismic effects is achieved by the interaction between the columns, the cap beam and the girders. Said components must be connected such that flexural resistance is provided. Under the impact of longitudinal seismic motion, the bottom flanges of the two girders, meeting end-to-end at the cap beam, will be under tension and compression, respectively. The tension connection is presently made by extending some of the bottom strands into the cast-in-place diaphragm. At this location, the space available is too small for development by bond in the straight strands alone. Since concrete in the diaphragm is highly confined, it can probably carry high bearing stress and a small bearing area may be possible. Thus, the goal of this project is to create a reliable, effective, as well as practically applicable way of anchoring strands extended from the girder into the cap beam. The first stage in the development of the girder-diaphragm seismic connection consists of establishing the adequacy of the smallest possible strand anchor capacity such that a strand ductile failure due to yielding and fracture is achieved before the strand anchor fails by crushing the concrete at the bearing surface. As a second stage, the impact of the possible failure mechanisms of the strands embedded in the diaphragm on the development of the girder-cap beam positive moment connection was investigated. Finally, the distribution of girder bending moments across the bridge deck was evaluated, while investigating the influence on that distribution of the most important bridge parame- ters, such as cracking of bridge components, as well as varying cross sectional dimensions