Study On Deterioration Mechanism And Mechanical Performance Of Structural Cables And Cable Supported Structures

Cable-supported structures have high structural efficiency and are gradually becoming one of the most frequently studied,the fastest growing and the widely used structural systems.This structural type has been defined as one of the most frequently used structural systems in Code for Seismic Design of Buildings.Since the first application of cable-supported structures in China,some of the applications have been served for almost 20 years.Because of material degradation,environmental erosion,human induced accidents and natural disasters,structure members will inevitably be damaged and thus lead to force variations in structure members.As the key component of the structure,the prestressed cables may get break in some extreme conditions and then affect the safety of the whole structure.Then in this paper,the pretension loss factors that may happen during the whole period from the erection of the structure throughout the in-service life were firstly studied.Then systematic experimental tests and numerical explorations were performed on the stress relaxation problems of structural cables,and on the cable break reactions in a suspendome model.Then a time-varying degradation model of pretension loss of cable in cable supported structures was proposed and corresponding safety protection mechanisms were suggested based on obtained test data and degradation properties.Several research findings and conclusions with great importance in scientific research as well as engineering application have been derived.Firstly,different factors that may induce pretension loss during the whole life cycle of cable supported structures were summarized,and the calculation methods and compensation measures of different pretension loss factors were also studied.Then a comprehensive pretension loss model was built.Prestress loss during structure erection period includes anchorage loss,batched tension loss and friction loss,which all belong to short-term pretension loss and need to be considered during the construction period.The in-service tension loss factors include stress relaxation loss,creep loss,cable corrosion loss and temperature induced loss,which are all long-term tension loss and need to be considered in the structural design period.Then stress relaxation tests were performed on five typical structural cables,which are high strength steel wires,spiral strands,semi-parallel wire strands,steel tie rods and Galfan spiral strands.Before the test,special onsite anchoring methods and anchorage devices for semi-parallel wire strands and Galfan spiral strands were put forwarded to ensure reliable anchorage.Relaxation rate of tested cables under different initial stress levels and temperatures were derived,and the varying trends of relaxation performances were also obtained.Long-term relaxation rates were calculated based on the test data,which can be used into the design of cable supported structures for stress degradation evaluation.A simplified finite element modeling method for stress relaxation simulation of high strength steel wire,semi-parallel wire strands and spiral strands were proposed.Simplified relationship between stress relaxation and creep of steel wire was derived and a simplified modeling method on spiral wire composition was proposed.The whole numerical approach incorporated the steel wire creep model and the simplified cable compositions to realize the simulation of stress relaxation behavior of semi-parallel wire strands and spiral strands.Based on the new model,a series of numerical simulations were performed on a wide range of semi-parallel wire strands and spiral strands that from 19-wire to 127-wire.The stress relaxation performance,relaxation varying trends and the induced inner wire force degradations were all obtained and studied.This study provides a numerical method on stress relaxation behavior of large diameter structural cables.Another important part was the cable break study in suspendome structure.A new cable rupture device was firstly invented to realize the sudden break condition of tensioned cable.Two impact cases were considered,namely,single hoop cable breaking under normal service conditions and rupture responses with only one tensioned hoop cable under self-weight.The strain histories,dynamic deformations,and membermoving procedures during the cable break were recorded by high-speed video camera and dynamic strain amplifiers.The dynamic reactions and internal force redistribution under local break,together with the shock effect under different tension levels and cable-strut node clamping conditions were all carefully studied.At the same time,corresponding numerical cable break simulations were also performed using an explicit,dynamic,and finite-element analysis program called ANSYS/LS-DYNA.Two indexes,the dynamic amplification factor and dynamic coefficient were proposed to evaluate the dynamic effect from the cable rupture.The suggested values for the two indexes were also derived based on the test data,which could easily be used in the structural design process.Based on the results and data of stress relaxation and corrosion tests,the corresponding pretension loss model caused by relaxation and corrosion can be calculated and the comprehensive time varying model was derived.With the degradation model,the tension loss at selected time during the whole in-service period can be predicted.Together with the adverse effect caused by cable break,a set of safety protection mechanisms for structural cables were put forwarded,which possess guiding significance for the design and safety evaluation of cable-supported structures.