## 5 Testing and Development of Hybrid Connections (PRESSS Research Program 1989)  
[PERSON] [6] reported an overview of the Precast Seismic Structural Systems (PRESSS) Research program. This research program was started in 1989 as part of the United States-Japan protocol on large-scale testing for the seismic response of precast concrete buildings [6]. The purpose of this program was to develop recommendations for the seismic design of precast buildings. The hybrid precast connection was developed during the four phases of this program (I, II, III, IV (A and B)). During Phase I, [PERSON] and [PERSON] (1991) [30] experimented with precast concrete beam column assembly using high-strength post-tensioning bars fully grouted as shown in Figure 10. The specimens failed because of their inability to carry higher loads due to the yielding of post-tensioning bars at the column-beam interface. A plastic hinge formed in the interface region. The gap opening at the interface was roughly 13 mm. The yielding occurred at this interface, which resulted in loss of post-tensioning force. The precast column beam assembly was found to be as strong as the monolithic sub-assembly. The post-tensioning force contributed to strength. Precast assembly was found to be ductile in a similar manner to monolithic sub-assembly. The yielding and gap opening at the column-beam interface provided ductility. The energy dissipated per cycle by the post-tensioned specimen was 30% compared to its monolithic counterpart. Total energy dissipation to failure was 80%. Overall, the assembly performed well, but energy dissipation, which is very important factor for seismic resistance, was very low on per cycle basis. In Phase II, [PERSON] and [PERSON] (1993) [31] experimented with a precast assembly using two beam specimens. For specimen 1 (Figure 11), strands were used. For specimen 2 (Figure 12), PT bars were used and moved closer to the center. Both specimens were fully grouted. The failure was similar for both specimens. They failed due to the yielding of post-tensioning steel, the crushing of concrete, and a gap opening at the column and beam interface. The width of the opening increased as the post-tensioning steel moved closer to the centroid. The type of post-tensioning steel did not affect the width of the gap opening. The precast specimen strength exceeded that of their monolithic counterpart. Moving the post-tensioning steel closer to the centroid did not have any adverse effect on connection strength. It reduced strain in the post-tensioning steel. The precast assembly was found to be more ductile than its monolithic counterpart. The energy dissipated by the strand post-tensioned specimen was greater than that of post-tension bar specimen. The energy dissipated per cycle was less than that of the monolithic specimen. Total energy dissipation for the precast post-tensioned specimen was higher than that of the monolithic specimen because of the higher displacement ductility achieved by the precast specimen. Overall, the precast connection performed well but the energy dissipation per cycle was still low. In Phase III, [PERSON] and [PERSON] (1993) [32] performed a numerical analysis of a column and beam assembly with partially unbonded post-tensioned tendons (Figure 13). Tendons were unbonded along the column length and extended 48 in (1.22 m) into the beam. The tendons were placed closer to the beam centroid, as shown in Figure 13. [PERSON] and [PERSON] [32] performed a dynamic nonlinear analysis to study the concept of partially unbonded frames. They predicted that the debonded system would result in the maintenance of prestress compression after seismic displacement. Residual displacement would be negligible. They suggested special spiral reinforcement at the beam ends to ensure satisfactory performance in the plastic hinge regions. Ductility demands of partially debonded and fully bonded tendons would be the same; however, for short period structures high displacement ductility demands can be expected.