Seismic Behavior Of Reinforced Concrete Columns With Different Slenderness Ratios Confined By Bidirectional Fiber Reinforced Polymer

Bidirectional fiber reinforced polymer can reinforce concrete column combined flexure and shear because of its bidirectional stress characteristics and tear resistance.The flexural and shear bearing capacity of the column can be improved simultaneously by anchoring the longitudinal fiber with fiber reinforced polymer anchors.In this paper,the seismic behavior of 7 bidirectional basalt fiber reinforced polymer(BFRP)confined reinforced concrete columns under low cyclic lateral load is investigated.In this paper,two groups of reinforced concrete rectangular columns with slenderness ratio of 4,6 and 8 were designed and manufactured respectively.Parameter settings included bidirectional BFRP reinforcement,FRP anchor and slenderness ratio.Based on the failure mode of anchor,a design method of FRP anchor in bidirectional BFRP reinforced concrete column was proposed.The results showed that:The annular and longitudinal fibers of bidirectional BFRP had good integrity,which could limit the development of cracks in compression zone and tension and compression side,and effectively improved the seismic performance of columns.The reinforced column changed from flexural and shear failure to flexural failure mode.The hysteretic curve of the reinforced column was full and the bearing capacity increased.Besides,the deformation performance and energy dissipation capacity improved.The improvement of bearing capacity and ductility of flexural failure columns decreased after reinforcement.In addition,slenderness ratio had significant effect on the seismic performance of bidirectional FRP constrained columns.With the increasing of slenderness ratio,the hysteretic curve of the column reduced,the ultimate displacement of the column increased,and the bearing capacity of the reinforced column decreased more than that of the unreinforced column.The ductility of columns with large slenderness ratio strengthened by bidirectional FRP was weaker,but the energy dissipation capacity was higher.The ductility of columns with slenderness ratio strengthened by bidirectional FRP increased by 70.8%,82.5% and 66.5%,and the corresponding ξe increased by 25%,23.3% and 66.3%.With the increasing of slenderness ratio,the rotational capacity and deformation performance of column improved,the initial stiffness and stiffness degradation rate decreased,and the annular fiber strain of the reinforced column enhanced.With the increasing of the displacement,the fiber of anchor gradually broke until the anchor was pulled off.Under the same slenderness ratio,the longitudinal fiber strain increased and the utilization ratio of longitudinal fiber increased obviously after the anchor was anchored.The flexural capacity,stiffness and deformation performance of the column improved,and the bearing capacity and ductility coefficient increased by 6.7% and 12.8% respectively.The plastic hinge length of the test column was determined by using "longitudinal reinforcement yield zone method",and the plastic hinge length parameters were analyzed.It was found that the plastic hinge length increased obviously with the increasing of slenderness ratio,and the plastic hinge length also increased after using bidirectional BFRP reinforcement and FRP anchor.Finally,based on the experimental data and theoretical analysis,the calculation formula of equivalent plastic hinge length and bearing capacity of bi-directional BFRP confined concrete columns was proposed.