Study On Anchorage System And Its Long-Term Performance For Large Capacity FRP Cable

CFRP(Carbon Fiber Reinforced Polymer)has excellent properties such as high tensile strength,light weight,fatigue resistance and corrosion resistance,etc.As a cable material,disadvantage of obvious sag effect and low bearing capacity caused by the increase of bridge span and the coupling effect of fatigue-corrosion on the service life of the cable can be alleviated.Due to the anisotropy of CFRP material,the transverse mechanical properties are much lower than that of the longitudinal direction,so the anchoring method of steel cables is not ideal for CFRP cables.Up to now,numerous scholars have conducted extensive research on CFRP cable anchorage system,but there are still few anchorage system that could satisfy the short and long-term requirements,and most of them cannot be successfully applied to practical engineering.Therefore,it is necessary to further develop and study the CFRP cable anchorage system,especially for multiple CFRP tendons with large capacity.This paper focuses on the anchorage problem of CFRP cable with large capacity.The main research contents and results are as follows:(1)A bond-extrusion anchorage system with segmented variable stiffness was developed.A 3D refined simulation model was established to simulate the effects of variable stiffness,geometric and interface parameters on the mechanical properties of the cable in the anchorage zone.Based on the numerical analysis and experimental data of anchorage systems with 37 CFRP parallel tendons,an evaluation method with the minimum compressive strength in load transfer mediums as the optimal control objective was established,and the design parameters of the anchorage system with the bearing capacity of 500-2000 tons were predicted.The results show that gradient changes of the stiffness is helpful to reduce the radial and shear stress concentration of the cable in the anchorage zone,and make the stress distribution more gentle.The reduction of the roughness and internal taper of the steel sleeve is beneficial to the overall wedging action of the load transfer mediums,thus increasing the anchoring force of the cable.Increasing the length of steel sleeve and the radius of load transfer medium at loading end is helpful to reduce the radial and shear stress peaks of the cable.Increasing wall thickness of steel sleeve has little effect on the stress of the cable,but has obvious effect on reducing the circumferential stress of the steel sleeve.The recommended design parameters of 500,1000,1500,2000 ton anchorage system are obtained.The preliminary design for anchorage system with the capacity of 500-2000 ton can be carried out by fitting equation(2)The static performance of CFRP cable anchorage system(37,04mm)was experimentally studied.The construction technology of the cable anchorage system was introduced,the axial compressive strength of load transfer mediums was tested and the relationship between transversal compressive strength and transversal displacement of the tendon was also studied.The failure mode of the cable anchorage system was analyzed and its anchoring efficiency was evaluated.The load-strain relationship of the cable in free portion,the stress and displacement distribution of the cable in the anchorage zone,the relationship between wedging displacement of load transfer medium and the radial stress of the cable were studied.The accuracy of the results of finite element simulation was also verified.The results show that layered casting technology is helpful to enhance the density of load transfer medium.The compressive strength of load transfer medium is the controlling factor for the anchorage design,not the transverse compressive strength of CFRP cables.The transversal compressive strength of CFRP cables is positive correlation with the transversal displacement.The failure mode of static test is fiber bursting and the average anchoring efficiency is 97%.The stress and displacement of the cable in the anchorage zone are smooth,and the radial stress of the cable increases gradually with the wedge displacement of load transfer medium.The experimental results are in good agreement with the simulation results of the finite element model(3)The long-term performance of cable anchorage system was studied.Firstly,the anti-fatigue behavior of cable anchorage system was verified by taking the load conditions of the maximum 45%fu,the stress amplitude of 200MPa and the frequency of 4Hz.Then,based on the maximum stress of 45%fu and the frequency of 2 Hz,cable anchorage systems with high stress range(500,700,900MPa)were investigated experimentally and the fatigue life prediction was carried out.The damage evolution mode of cable anchorage system was analyzed,and the effects of cyclic load on stiffness and strength of the cable,the cable stress in the anchorage zone,the wedging action of load transfer medium,the axial and circumferential stress of steel sleeve were also studied.The results show that cable anchorage system can withstand 2 million cycles of cyclic loads,and the cable and anchorage zone do not produce any macroscopic cracks Compared with the initial performance,the retention rates of stiffness and strength of the cables are 95%and 96%,which has a high safety reserve,it can meet the service requirements of the bridge under static and fatigue loads.The relative displacement of the anchorage components under the cyclic load has little change,and the cyclic load has little effect on the shear stress of the cable in the anchorage zone.The experimental fitting equation shows that the fatigue life of the anchorage system can reach 126 million cycles under stress range of 200 MPa,and in order to realize 2 million cycles,the designed stress range needs less than 473 MPa(4)Based on the designed long-span cable-stayed bridges,the fatigue life of CFRP.,BFRP and steel cables considering the sag effect with different safety factors was evaluated Firstly,the fatigue cycles of the cables during design and service period was studied by statistical analysis of vehicle characteristics.Through finite element analysis,the most disadvantageous cable and its stress amplitude were determined.Then a combined algorithm of equivalent transformation was established to evaluate the fatigue life of the cables based on design stress and service life.Finally,the designed safety factors for CFRP,BFRP and steel cable were recommended to realize the design life of 100,200 and 300 years.The results show that the cable can withstand about 1.14×109 load cycles in 100 years under vehicle loads.C130 is the most disadvantageous cable in mid-span position.The equivalent stress amplitude of the cable is almost 12 times of the actual stress amplitude.The relationship between safety factor and fatigue life can be characterized by parabolic equation.The increasing trend of fatigue life of CFRP cables is much higher than that of BFRP and steel cables with the increase of safety factor.When the safety factor adopts 2.55,the fatigue life of CFRP and BFRP cables is almost 3 and 1.4 times that of steel cables.For 100-year designed fatigue life,the safety factors of CFRP,BFRP and steel cables are recommended to be taken 2.55,3.27 and 3.64 respectively.When the safety factor of 3.54 for CFRP cable,it can realize the fatigue life of 300 years,and has obvious advantages in long-span cable-stayed Bridges(5)A long-span cable-stayed bridge with a main span of 2056m was preliminarily designed and a simulation model was established.The lane load was arranged after forming the reasonable bridge state,and the internal forces and displacements of the cables,the girder and the pylon were analyzed.The installation parameters of all CFRP cables on cable-stayed bridges were calculated.The cables were simulated by the combination method of catenary theory and nonlinear finite element analysis.Taking the longest cable as an example,the installation parameters of CFRP and steel cable were compared and analyzed.The main parameters were the angle correction of the anchorage on the girder and the pylon,the difference of the cable forces at two ends,the length and the sag of the cable.The results show that the difference of cable forces between the anchoring position at the girder and the pylon,CFRP cable is 1/6 of the steel cable.The correction angle of the anchorage is positively correlated with the length of the cable,and the correction angle for CFRP cable is only 1/4 of the steel cable,which is more conducive to the cable installation.CFRP cable is superior to long-span cable-stayed bridges in construction and installation.