Unstrengthened Specimens
Displacement controlled monotonic loading tests were conducted on the five specimens by gradually increasing the displacement at the center column until the displacement capacity of the hydraulic actuator was reached. The force–displacement relationship of the gravity load-resisting specimen not strengthened by strands or side plates is presented in Fig. 8. The arrow mark ↙ represents the fracture of re-bars. The specimen started to yield at the displacement around 40 mm, and full plastic hinge formed at the displacement of about 90 mm. When the displacement reached about 180 mm the strength increased again due to the activation of catenary force of the re-bars. The curve for the specimen dropped slightly at displacement of 293 mm when one of the tension re-bars fractured. The strength further increased and more re-bar fractured as more displacement was imposed on the specimen, and at displacement of 426 mm one of the interior ends of the beam was completely separated from the column and the specimen failed. The failure criterion for beams recommended in the GSA guidelines is 0.015 radian, which corresponds to vertical displacement of 214 mm in the specimen with clear span of 2035 mm. Therefore the displacement at failure of the specimen far exceeded the limit state specified in the guidelines. It also can be noticed that the GSA-specified failure point corresponds to the displacement where the catenary force started to be activated. Figure 9 shows the photograph of the specimen at failure. Major cracks were observed only at both ends of the beams, and relatively few cracks formed in the external columns.
The force–displacement relationship of the lateral load-resisting specimen is presented in Fig. 10. The specimen started to yield at the displacement around 100 mm, which is significantly larger than the first yield point of the gravity load-resisting system. The yield strength of the specimen is also more than twice as high as that of the gravity load-resisting system. After the strength reached 41 kN at the displacement of 144 mm, the flexural strength of the specimen started to decrease as a result of yield of re-bars and formation of microcracks. The strength dropped to 54 % of the peak value due to fracture of tension re-bars at the displacements of 198 and 209 mm. The strength increased again up to 27 kN before another re-bar fractured and the strength suddenly dropped. The strength increased again to 44 kN and dropped again at the displacement of about 400 mm due to fracture of another re-bars. Then the strength re-increased due to catenary force of remaining re-bars. The maximum displacement capacity of the actuator was reached and the test stopped right after the strength re-increased to another peak point. Figure 11 shows the overall view of the damaged specimen after the test was over. It was observed that the plastic hinges formed away from the column faces where the closer stirrup spacing was no longer required. Due to the enhanced shear reinforcement at the ends of the beams and the seismic detailing of re-bars at the beam-column joints including anchoring of bottom re-bars using standard hooks, the effective length of the beam was reduced and consequently the maximum bending strength was increased. The number of fractured re-bars was also reduced compared with the case of gravity load-resisting system. The number of cracks formed in the exterior column was slightly larger than that of the specimen designed only for gravity load. Figure 12 shows the damaged region of the specimen, where it was noticed that the cross-sectional areas of the fractured re-bars were slightly reduced representing typical tension failure.
The variation of rebar strain in the gravity-load resisting specimen is shown in Fig. 13. It can be observed in Fig. 13a that the strain of the top rebars of the left end of the right-hand-side beam (location RL in Fig. 2a), which was initially subjected to compression, started to resist tension after the displacement exceeded 40 mm. The tension increased rapidly at the displacement of 330 mm due to the initiation of catenary action. Similar phenomenon was observed in the re-bars located in the right end of the beam (location RR in Fig. 2a) as shown in Fig. 13b. It can be observed that the strain of the bottom bar rapidly increased starting from the displacement of 330 mm due to catenary action. Figure 14 shows the rebar strain in the lateral-load resisting specimen, where it can be noticed that the activation of catenary action is apparent compared with the case of the gravity load resisting specimen. It can be observed that the bottom rebar in the right-end of the right-hand-side beam (location RR in Fig. 2b), which was initially under compression, started to be subjected to tension at the vertical displacement of 400 mm. Therefore if the test had not been terminated early due to the limitation of the displacement capacity of the actuator, more catenary force might have been observed in this specimen. The activation of catenary action in beams with different boundary conditions can be found in Kim and An (2009).
Strengthened Specimens
The specimen of gravity load system, which showed inferior performance to the specimen designed for seismic load, was reinforced by either high strength strand or side plates to enhance its progressive collapse resisting capacity. The force–displacement relationships of the specimens strengthened by bonded and unbonded strands are shown in Fig. 15. The force–displacement curve for the unstrengthened specimen was also plotted in each figure for comparison. The specimen reinforced with bonded strand showed similar force–displacement relationship to that of the unstrengthened specimen, except that the maximum strength increased by 56 % and the specimen did not fail completely when the test was over. The number of re-bars fractured was seven, which is the same with that observed in the test of the unstrengthened specimen. In the specimen with unbonded strand, the maximum strength turned out to be 145 % higher than the maximum strength of the specimen without the strand. It was also observed that two re-bars fractured at the displacement of 350 mm, and the force was reduced for about 50 kN. As displacement further increased the force increased again until the strength reached the maximum value of 84 kN at the displacement of 436 mm. The number of fractured re-bars was reduced to three. It can be observed from the figures that the specimen with an unbonded strand showed superior catenary action to that of the specimen with bonded strand.
Figure 16 depicts the strain history of the high strength strand located at the right-end of the left-hand-side beam (LR) and in the right-end of the right-hand-side beam (RR). It can be observed that, except the strain at RR of the bonded specimen, the strains increased almost linearly as the displacement increased. The strain of the strand located at LR of the specimen reinforced with bonded strand is larger than that of the strand at RR, which is due to the larger cracks formed near LR than those formed near RR. However the variations of the strains observed in the specimen with unbonded strand are similar to each other, which implies that damages occurred nearly symmetrically in the two beams.
Figures 17 and 18 depict the variation of the rebar strain in the specimens strengthened with bonded and unbonded strand, respectively. It can be observed that at the vertical displacements around 350 mm the rebars which were initially under compression started to resist large tensile force due to catenary action.
Figure 19 shows the photographs of the damaged specimens reinforced with tendons. It was observed in the force–displacement relationship that the specimen with an unbonded strand showed catenary action superior to that of the specimen with bonded strand. This is due to the fact that in the specimen with a bonded strand the catenary force of the strand was transmitted to the beams evenly along the length, which resulted in separation of the beam from the column face as can be observed in the photograph of the specimen at failure shown in Fig. 19a. However in the specimen strengthened with unbonded strand, where all catenary force in the strand acts on the far face of the exterior column, the beam end was not separated from the column face even at the maximum displacement as can be observed in Fig. 19b. It was observed that, compared with the crack formation of the specimen without strand (shown in Fig. 9), smaller cracks formed relatively uniformly along the beam length reinforced with high strength strands. The number of cracks formed in the exterior beam-column joint of the specimen with unbonded strand turned out to be smaller than that in the specimen with bonded strand due mainly to the larger confinement effect of the unbonded strand and its anchorage. The cracks formed in the side columns of the specimens, however, may not be found in real buildings because the axial load imposed on the columns and the bending moment of the adjacent beam will compensate for the resultant tensile stress in the column joints.
Figure 20 shows the force–displacement relationship of the specimen reinforced with side plates at both sides of the beam-column joints. Plastic hinges formed at the displacement of about 90 mm, and the specimen showed ductile behavior until the force increased again at the displacement of around 200 mm due to activation of catenary force. As no re-bar was fractured until maximum displacement was reached, the force kept increasing without sudden drop as observed in the other specimens. Compared with the performance of the specimen strengthened with the high strength strands, the specimen reinforced with side plates showed slightly smaller strength but more stable behavior. Moreover, considering the higher expanse involved in the anchoring of strands, the side plate strengthening scheme seems to be more practical means of enhancing progressive collapse resisting capacity of RC moment frames.
The variation of re-bar strain in the specimen reinforced with side plates is shown in Fig. 21. It can be observed in Fig. 21a that the strain of the bottom bars of the left end of the right-hand-side beam, which was initially subjected to tension, increased until the displacement reached about 180 mm and decreased due to formation of cracks. However the strain of the top bars at the same location, which was initially under compression, started to resist tension after the displacement exceeded 247 mm due to the initiation of catenary action. Similar phenomenon was observed in the re-bars located in the right end of the right-hand-side beam as shown in Fig. 21b. In this case the bottom bars were subjected to tension starting from the displacement of 290 mm due to catenary action.
Figure 22 shows the damaged ends of the specimen reinforced with side plates after the test is over. It can be seen that major cracks formed at the end of the side plates, which is 160 mm away from the column face. No major crack was observed within the region covered by the side plates, which is probably due to the confining effect of the plates with stud bolts. It was also observed that due to the catenary force many tension cracks formed along the beam length.