Studies On Dyanamic Detection Technique Of Pretension Deviation And Load-adaptive Retrofits With SMA On Tensile Structures

This work focuses on the safety issues induced by pretension deviation on tensile structures,and investigates solutions in two aspects:(1)Identification of pretension deviation in key members based on ambient testing techniques.(2)Adaptive retrofitting of tensile structures based on shape memory alloy(SMA)to accommodate extreme loading condition and pretension deviation,utilizing the softening mechanism and super-elasticity of the material.Listed below are the major contents of this thesis:1)By means of stiffness decomposition,tensile structures can be classified into three categories according to their dependency on geometric stiffness to sustain stability.The mathematical models of most unfavorable structural pretension deviation(MUSPD)are extended by five different criteria.The influence of MUSPD on different categories of tensile structures is also investigated,along with the sensitivity analyses on pretension,natural modes and structural stability.The results prove that the most unfavorable pretension deviation may exacerbate the stability issue of geometrically sensitive tensile structures when subjected to loads,and may infuse overstress to both "robust" structures and some of the geometrically sensitive structures?2)In view of the difficulties in separating closely spaced modes and exciting a large span structure,operational modal analysis(OMA)techniques based on ambient response are proposed to help identifying the geometric stiffness of tensile structures.An optimal sensor placement approach considering the contribution of geometric stiffness is developed for the purpose of cable force measurement.A localized technique for cable force detection based on a substructure strategy is also proposed to overcome the limitation on field measurements.Summarizing from a health monitoring project on as-built tensile structures,this paper also discusses the materialization methods and existing problems of ambient testing on large span structures3)A series of modal tests were carried out on a 1:10 scaled model of a herringbone ribbed cable dome structure subjected to random excitation.By multiple sets of testing,most part of its modal information has been obtained despite of its complexity.In comparison with the experimental modal analysis results,the OMA methods have been verified regarding to the identification performance on tensile structures.A discussion is raised on the difference between experimental and theoretical modes and how it affects the identified modal information.A modal strain energy based method for structural damage identification has been modified,so as to detect the stiffness deviation caused by pretension loss and allocate the loosen cable.4)A novel cable force detection method based on recursive model updating technique driven by data has been proposed,assuming the cable pretensions as undetermined parameters.The multiplicity of solutions has been avoided by considering the continuity of cable forces in sequential cables.This approach can circumvent the difficulties in identifying closely spaced modes and matching of the leaping modes.It exhibits great potentials in online monitoring of cable forces.5)The microscope mechanism and macro-mechanical behaviors of Nitinol shape memory alloy have been discussed in detail.A simplified multi-linear hysteretic model that can meet the engineering requirements has been established for SMA.The model is then calibrated using the tensile test data of SMA bundle specimens.It is capable of simulating fatigue effect,rate effect and mechanical behaviors of uniaxial hybrid SMA members such as tension-compression dampers6)Adopting the Vector Form Intrinsic Finite Element method(VFIFE)in the full-process simulation of defective tensile structures subjected to strong typhoon or heavy snow,the mechanical behaviors of these structures with or without SMA retrofitting have been compared and studied.The differences in structural responses have been investigated in a comparative study between different cable length errors,lengths of SMA retrofits,and whether the fatigue effect or rate effect of the SMA material is considered.Accordingly,a design method is proposed for the SMA retrofits.The analytical results show that,the stress within a defective tensile structure can be maintained within a safe level even under extreme loads by self-accommodation as long as the SMA retrofits are properly designed.Meanwhile,the global stiffness and stability of the structure show little sign of compromise,and therefore the retrofit is consistent with the design philosophy of self-adaptation and load-relieving.