# Validity of Anchorage Zone Design for Post-Tensioned Concrete Members with High-Strength Strands
## 4 Experimental Results and Analysis
In Group B, regardless of the number of anchorages, the strains from the bearing force of Specimens CH-3 and CH-4, which were reinforced by hairpins, were 0.000948 and 0.001195, respectively, and were the smallest. Compared with the specimens that were reinforced with U-shaped hairpins, the strains of specimens that were reinforced by Type 1 and Type 2 details increased by 9.88% and 3.39%, respectively, when two anchorages were applied, and by 15.56% and 2.86% when three anchorages were applied. Therefore, continuous reinforcement with hairpin rebars in the direction of member length was the most effective approach for resisting the bearing force in the anchorage zone. Reinforcement with Type 1 details, which used equally spaced closed stirrups in the anchorage zone, was the worst for resisting the bearing force; resistance decreased as the number of applied tendons increased. Reinforcement with Type 2 details, which used a distance of approximately 0.5 h from the end, showed similar resistance to reinforcement with hairpins as the number of applied tendons increased. The design standards require that the allowable bearing force exceeds the pre-stressing force. The applicability of the methods outlined in the design codes for the design of anchorage zones to resist bearing force when using high-strength strands was evaluated. The allowable bearing force and pre-stressing force according to anchorage shape are shown in Figure 10. For strands with tensile strength of 2400 MPa, the pre-stressing force was equal to 319.6 kN. The allowable bearing forces calculated by the ACI and AASHTO design codes were 224.4 kN and 457.07 kN, respectively, for rectangular anchorages, and 258.5 kN and 529.3 kN for circular anchorages. The allowable bearing force calculated by the ACI design code was smaller than the pre-stressing force regardless of anchorage shape; the compressive strength of the concrete or the bearing plate area should increase when tensile force is applied. However, there were no cracks or failures of concrete from the bearing force, which was smaller than the maximum compressive strain of concrete. The allowable bearing force calculated by the AASHTO design code exceeded approximately 35% of the pre-stressing force. Thus, the post-tensioned concrete members had adequate compressive  
Figure 9: Comparison of bursting stresses from design code and test results. concrete strength and bearing plate size to resist the pre-stressing force. However, the difference between allowable bearing force and pre-stressing force was quite large. It was inferred that the AASHTO design code is intended for civil structures, and so does not work well for architectural structures, which have smaller members. The ACI design standard considered the bearing plate area when calculating allowable bearing force, while the AASHTO design code assumed that the area where the bearing force is transmitted in the member was larger than the bearing plate area. Therefore, the AASHTO codes calculated a larger allowable bearing force than the ACI code. In this study, stress distributions were analyzed in the anchorage zone when the ultimate flexural load was applied on a post-tensioned concrete member. The strain was estimated using SL steel gauges, which were attached at the leg of the rebar in the anchorage zone. The concrete gauges CT1 and CT2 are shown in Table 4, where \(P_{u}\) is the maximum load on the specimens. The crack patterns and failure aspects after the experiment are shown in Figure 11. In Group A, the concrete strain by bursting force when applying circular anchorages rather than rectangular anchorages decreased by 6.5% when two tendons were applied, and by 74% when four tendons were applied. The concrete strain estimated by bearing force with circular anchorages decreased by 24%, 71%, and 31%, respectively, as the number of  
Figure 10: Comparisons of bearing force to design code.