A DETAILED NUMERICAL ANALYSIS FOR THE PROGRESSIVE COLLAPSE OF CONCRETE FRAME SUBSTRUCTURES STRENGTHENED WITH FRP STRIPS

Externally bonded FRP strips can effectively improve the progressive collapse-resisting performance of existing structures, but the seismic performance of structures and the ease of construction were not taken into consideration in existing FRP strengthening schemes. Numerical simulation was performed to study the influences of strengthening schemes using FRP strips on the seismic and progressive collapse-resisting behavior of cast-in-site and precast concrete frame substructures, by which the strengthening schemes were consequently optimized. The detailed numerical models of concrete frame substructures strengthened with FRP strips were established using the general finite element (FE) software LS-DYNA, in which the concrete, steel reinforcement and FRP strips were simulated by solid, beam and shell elements, respectively. The bond-slip of steel bars and FRP strips, the bond failure between precast and cast-in-site concrete and the loss of cross-sectional areas of bars at the mechanical sleeves were considered in the numerical models. The numerical models were validated by experimental results, which showed that the failure modes and the strengths of the substructures in the experiment were well captured by the numerical models. The results of the different strengthening schemes suggested that the bonding of the longitudinal FRP strips at the beam bottoms and the neutral axes of beam sides and U-shaped transverse FRP strips in the plastic hinge regions of the beams hardly improved the structural collapse resistances under small deformations for cast-in-site concrete substructures (the maximum percentage increase was only 2.6%). Such a strengthening scheme had almost no effect on the seismic performance of the substructures, while the progressive collapse resistance under large deformations was increased by at most 49.5%. For precast concrete substructures, applying longitudinal FRP strips at the beam tops, beam bottoms and bottoms of the beam sides and U-shaped transverse FRP strips in the plastic hinge regions would improve their maximum resistance under small and large deformations by at most 24.2% and 48.1%, respectively. Under small deformations, their collapse resistance was increased to the same level as cast-in-place ones, which was advantageous for improving the seismic performance of precast concrete substructures. A further analysis of the aforementioned optimal schemes shows that keeping the amount of FRP unchanged and at the same time increasing the covering length and the number of U-shaped transverse FRP strips applied in the plastic hinge regions of the beams could improve the structural collapse resistance under large deformation, while the effect of FRP strengthening on the collapse resistances under small deformation remained unchanged.