A new collapse mechanism of RC frame structures under earthquakes: Shaking table tests and numerical analysis

For the reinforced concrete (RC) frame structures under mega earthquakes, it is well acknowledged that damage primarily concentrates at beam ends and column ends. Successively, a certain number of plastic hinges develop which can trigger the structural collapse. However, the present study found that the plastic hinges could form at the mid-span of the first story beams in an RC frame structure configured with a slab opening on one side of the first floor. To investigate the failure behavior and collapse mechanism, first, shaking table tests were conducted on three 1/10-scale three-story RC frame models M1, M2 and M3, each configured with a slab opening on one side of the first floor. The models M1 and M2 were identical and conducted for repetition, while the model M3 served as a reference and was strengthened at the mid-span of the first story beams to prevent from forming a plastic hinge at this location. Then, the time history analysis and pushover analysis were performed using the finite element method. Finally, the prototype RC frame structures were numerically simulated till collapse under five bidirectional earthquakes. Test and numerical results found that for the models M1 and M2, the plastic hinges consistently formed at the mid-span of the first story beams and contributed to the new collapse mode towards the side with the slab opening. However, for the model M3, the plastic hinges concentrated at the beam ends and columns ends, behaving in the commonly recognized collapse towards the side without the slab opening. The pushover analysis found that the slab on one side of the first floor significantly influenced the distribution of cross-sectional bending moments along the first story beams, leading to the plastic hinges developing at the mid-span of the first story beams.