Enhancement of collapse-resistant capacity of non-seismically designed RC frames using various CFRP strengthening schemes

Abstract

The existing studies have demonstrated relatively weak robustness of non-seismically designed reinforced concrete (RC) frames against collapse than seismically designed RC frames, causing the demand of efficient strengthening schemes to enhance their collapse-resistant capacity. Therefore, this paper presents an experimental program aiming at strengthening the collapse-resistant capacity of non-seismically designed RC frames using carbon fiber reinforced polymer (CFRP). A total of seven sub-frames were tested, of which the penultimate column or edge column was notionally removed to replicate the initial damage caused by accidental loads. Two sub-frames without strengthening were tested first as reference tests. Similar to existing research outcomes, the referential sub-frames experienced premature rebar fracture at the beam ends near the removed column, resulting in a severe softening in load resistance. Whilst the load resistance could reascend, the fracture of the rebar at the beam ends near the side columns only allowed an insufficient mobilization of catenary action (CA). In addition, the side joint of the referential sub-frame representing a penultimate column removal scenario suffered significant damage. Subsequently, five strengthening schemes were applied to the referential sub-frame, they are designed to increase the compressive arch action (CAA) capacity or CA capacity, or both the CAA and CA capacities through CFRP strengthening. Test results demonstrated that the proposed strengthening schemes in this paper can efficiently increase the load resistance at the CAA and CA stages but failed to mitigate the severe load resistance softening. The strengthening scheme planned to increase the CAA capacity unexpectedly decreased the deformation capacity of the strengthened sub-frame due to premature fracture of beam rebar near the side columns. The strengthening schemes planned to increase the CA capacity or both the CAA and CA capacities could increase not only the CAA capacity but also the CA capacity. The enhanced load resistance at the CA stage was mainly attributed to the continuous CFRP strips attached to the soffits. Unfortunately, the energy method demonstrated that the dynamic load resistance of the tested sub-frames to prevent collapse was achieved at the CAA stage rather than the CA stage because of the severe load resistance softening. Thus, the efforts devoted to increasing the CA capacity by this paper were valid in a real collapse of building scenario. In addition, 113 available test results were collected to compare with the acceptance criteria in existing codes for collapse-resistant design. Based on the test results and comparison, design suggestions were given for the collapse-resistant design.