## 5 Testing and Development of Hybrid Connections (PRESSS Research Program 1989)
They suggested experimental research to confirm the numerical results. Figure 11: PT strand fully grouted. Figure 10: PT bar (fully grouted). [PERSON] and [PERSON] (1994) [33] experimented using two specimens (Figures 14 and 15), similar to in phase II. The only change was that the prestressing steel was unbonded at the joint region. Both specimens failed in same way. Failure was due to post-tensioning steel yielding, crushing of concrete, and a gap opening at the column and beam interface. The width of the gap opening was not affected by partially debonding prestressing steel. The specimen strength exceeded that of its monolithic counterpart. Moving post-tensioning steel closer to the centroid did not have any adverse effect on connection strength. The precast assembly was found to be more ductile than its monolithic counterpart. The energy dissipation was improved compared to fully bonded post-tensioning steel. Total energy dissipation for the precast post-tensioned specimen was greater than for the monolithic specimen because of the higher displacement ductility achieved by the precast specimen. Overall, the precast connection performed well, but the energy dissipation was still around 60% compared to its monolithic counterpart. [PERSON] and [PERSON] (1994a,1994b) [12; 34] experimented using a 2/3 scale unbonded post-tensioned precast concrete column beam assembly, with details as shown in Figure 16. The behavior of specimen was characterized by the gap opening and closing at the column-beam interface. They reported that the provision of suitable calculated unbonded length was sufficient to prevent yielding of the tendons. However, it should be ensured that the anchorage details of the post-tensioned tendons and compression zones in the column beam contact area are detailed satisfactorily. The strength of the connection was found to be satisfactory. No significant stiffness degradation was noticed even at a 4% story drift. The  
Figure 12: PT bar placed closer to the center and fully grouted. Figure 13: PT strand placed closer to the center and partially unbonded. sub-assembly behaved well and attained large lateral drifts without significant damage. The energy dissipation was very little. The residual displacement after unloading was found to be negligible, indicating self-centering capability. In Phase IV-A, [PERSON] and [PERSON] (1993) [13] experimented with precast assembly (Figure 17) using five beam specimens, as shown in Figure 18. The variables were the type of post-tensioning steel and the type and amount of low-strength steel and partially and fully bonded PT steel. The fully bonded strand and mild steel specimen failed prematurely due to bond failure of the mild steel at 1.7% story drift. The unbonded PT bar and fully bonded mild steel specimen failed due to the fracture of the mild steel bar at 3% and 3.6% story drift, respectively. Specimens A and B were not tested to failure. The purpose of this test was to determine which type of PT steel behaved best (bar or strand). The strand type lost 30% of the initial force, while the bar type lost 80% of the initial force. Loss of force in the strand type was due to reseating of the chunks, while that lost in PT bars was due to yielding and crushing of beams. Failure of specimen C was due to the shear cracks that  
Figure 14: PT strand (partially unbonded). Figure 15: PT bar (partially unbonded). formed at the interface of the T section and dog bone region. All specimens performed well in terms of connection strength. The low drift for the bonded strand and the mild steel specimen was due to premature bond failure. The location of PT steel in the center improved the cyclic energy dissipation. Energy dissipation dropped after the fracture of mild steel. In Phase IV-B, [PERSON] and [PERSON] (1994) [13] experimented with precast beam column assembly (Figure 19) using four beam specimens, as shown in Figure 13.0 Unbonded post-tensioning steel and a bonded mild steel bar were used. The variables were the amount and type of low-strength steel.