Flexural Performance Of Concrete-filled FRP Ellipse Tube With Longitudinal Steel

Coupling beam are horizontal members connecting different walls in shear wall structures,Its shear strength and ductility play an important role in improving the seismic performance of coupled shear wall structures.The span-depth ratio of coupling beams is usually very small.it is easy to occur that the shear failure is earlier than the bending failure,So that the interaction between adjacent walls will be lost and the seismic performance of the structure will be reduced.The core of coupling beam design is to improve its shear strength and ductility.Professor J.G.Teng from the Hong Kong Polytechnic University presented an idea which using FRP tube replace stirrups in reinforced concrete,and the section shape is changed to ellipse to form Concrete-Filled FRP ellipse Tubes with Longitudinal Steel.In order to study the performance of concrere-filled FRP ellipse tube with longitudinal steel,it is necessary to systematically study their mechanical properties.In order to study the flexural behavior of beams,a total of 13 beams were made for four-point bending tests.In this test,three different ratio of long axis to short axis,two different FRP thickness and three different steel-reinforcement ratio are taken as parameters.A large number of resistance strain gauges are pasted on the surface of steel and FRP to measure their strain response.In addition,displacement transducer is used to measure the deflection of beams at midspan,fulcrum and loading point.Through experimental study and drawing load-displacement curve can get the following conclusions:(1)The beams have good flexural capacity and ductility;load-deflection curve of specimens has been rising;mid-span deflection can even reach nine percent of the span;(2)Under the same conditions,the flexural capacity of the beams increases with the increase of the long axis to the short axis ratio,while the ultimate deformation of the beams decreases with the increase of the long axis to the short axis ratio.(3)Under the same conditions,the bearing capacity of specimens obviously increases with the increase of steel-reinforcement ratio.(4)Under the same conditions,the bearing capacity of specimens slightly increases with the increase of FRP thickness.(5)When the composite pipe is thick enough,the failure mode of the specimens is that the longitudinal steel bar breaks and loses its bearing capacity.Compared with the crushing failure of ordinary reinforced concrete beams,the brittle failure of concrete is avoided and the ductility of the specimens is greatly improved.In addition,the constitutive relationship of FRP-confined elliptical column concrete is obtained by Teng et al's 2016 design model,The moment-curvature curve of the specimen is obtained by using the plane section hypothesis principle.Then the load-displacement curve of the specimen is obtained by integration.The theoretical load-displacement curve is compared with the test to verify the applicability of the theoretical calculation.