Optimization Design And Performance Of Variable-stiffness Wedge Anchorage Based For FRP Tendon

In the severe environment,the traditional prestressed reinforced concrete structure will appear the phenomenon of steel corrosion,using good corrosion resistance fiber reinforced composite(FRP)to replace the prestressed steel bar/strand is an effective way to solve the above problems.However,FRP materials have anisotropic properties and their transverse shear strength is only one tenth of the ultimate tensile strength.Because of its advantages such as convenient construction,good interchangeability and clear anchoring mechanism,clip anchorage has gradually become the main anchoring form of prestressed tendons(stranded wires)in prestressed structures.When the traditional clamping anchors are used to anchor the FRP bars,the clamping will produce a large transverse compression on the FRP bars,and the radial compressive stress concentration(notch effect)will appear at the loading end of the clamping bars,and Cause the phenomenon of low anchorage efficiency.Based on the FRP bar composite variable stiffness laminated anchor developed by the research group,this paper further optimized it from three aspects of three-dimensional finite element modeling data analysis,short-term mechanical properties and long-term mechanical properties of FRP bar variable stiffness laminated anchor.The production process,static tensile performance,long-term fatigue performance and relaxation performance of the new type of variable stiffness clamping anchor system with FRP bars are illustrated,and a comparative analysis is made between the variable stiffness clamping anchor system with FRP bars and the steel anchor system with FRP bars.The main content of this paper is as follows:(1)Parametric analysis of finite element modeling of FRP bar anchorage system with variable stiffness.The anchoring performance of 7mm and 10mm smooth circular BFRP bars and variable stiffness clip anchoring system of CFRP bars after optimized design was tested respectively.Based on ANSYS three-dimensional finite element modeling of variable stiffness FRP bars,the influences of anchoring length,anchoring form,stiffness distribution length of clips,stiffness ratio of clips and number of segments on the radial stress of FRP bars were analyzed,and the finite element parameterization optimization was carried out for BFRP and CFRP bars with different diameters.The main dimension parameters,material selection and mechanical properties of the variable stiffness clamp anchors with good anchoring properties,which can significantly reduce the radial stress on the surface of FRP bars,are determined.The variable stiffness clamping anchor size of 10mm FRP bars is optimized as anchor cup length 120mm(7mm BFRP bars 80mm),clamping length 130mm(7mm BFRP bars 90mm),clamping Angle is 5°,stiffness distribution length is 1:1:3,Stiffness size is 3GPa-8GPa-20GPa(CFRP bar is 3GPa-8GPa-30GPa).The design highlight of the variable stiffness clip designed in this paper is that the outside of the clip is pasted with a steel sheet,which reduces the friction from the anchor cup when the clip is followed up.Compared with the previous generation of variable stiffness clip,the anchor size is greatly shortened,which improves the portability and practicability of the anchor.(2)Experimental study on static performance of variable stiffness clamped FRP bar anchorage system.The anchoring performance of 7mm and 10mm smooth circular BFRP bars and variable stiffness clip anchoring system of CFRP bars after optimized design was tested respectively.For BFRP bars with different diamets,the overall anchoring efficiency of FRP bars with variable stiffness is 88.8-91.0%,and the ultimate stress of the anchoring system is above 1400MPa,which is 79.4%higher than that of the traditional steel clip anchors.The overall anchorage efficiency is increased by about 10%,and the anchoring effect is significantly improved.The overall anchoring efficiency of CFRP bars with variable stiffness is 95.7%,and the overall anchoring efficiency of steel clips anchors is 104.3%.The anchoring effect meets the requirements of the code.The comprehensive analysis of the economy,applicability and practicability of the two anchors shows that compared with steel anchors,the variable stiffness FRP bar anchors have advantages of low cost,FRP bars suitable for a variety of materials and good anchoring effect,so they have a good application prospect in engineering applications.(3)Experimental study on fatigue and relaxation performance of variable stiffness clamped FRP bar anchorage system.Firstly,the fatigue performance of 10mm smooth circular BFRP bar variable stiffness clip anchorage system was tested.The fatigue life of the component with the fatigue stress amplitude of 0.04f_uis more than 2 million times.With the increase of the fatigue stress amplitude,the fatigue life becomes shorter rapidly.The fatigue life of variable stiffness clamping anchor member of BFRP bar with 0.08f_ufatigue stress amplitude is 94590 times.When the fatigue stress amplitude of the component increases to0.12f_u,the fatigue life of the BFRP bar variable stiffness clip anchorage system is only 14272times.Therefore,the fatigue stress amplitude is an important factor affecting the fatigue life of BFRP bars.The S-N fatigue life prediction model based on the fatigue performance data of BFRP bars with variable stiffness was established to provide data support and theoretical guidance for the setting of fatigue load.The relaxation performance of FRP bar with variable stiffness clamp anchorage system is also studied.Three groups of FRP bars with different initial stress levels of 0.4f_u,0.5f_uand 0.6f_uwere set for structural relaxation tests.The results show that the relaxation rate increases linearly with the increase of the initial stress of the specimens,and the relaxation rate of the components in 1000 hours is 4.0%,5.4%and 6.9%respectively.Based on the test data,the load retention model of multi-million-hour BFRP bar variable stiffness sandwich anchor is established,and the relaxation performance of BFRP bar variable stiffness sandwich anchor under the initial stress level of 0.5f_ucan meet the requirements of engineering practice and prestress retention.