In China,the working state evaluation of piers is mainly based on the Code for Rating Existing Railway Bridges.However,the vibration amplitudes of the bridge piers under train excitations vary with train types,train s...In China,the working state evaluation of piers is mainly based on the Code for Rating Existing Railway Bridges.However,the vibration amplitudes of the bridge piers under train excitations vary with train types,train speeds,axle loads and track irregularities,so the damages of piers are difficult to quantitatively evaluate by the vibration amplitude.In this paper,the impact vibration test method(IVTM) was systemically studied through theoretical analyses,numerical simulation calculation,laboratory tests and field experiments.The soundness evaluation method based on the eigenfrequencies of bridge piers was proposed.The main work done in this dissertation is as follows.(1) The fundamental of IVTM was deduced from theory in detail and the computer software for processing the impact vibration test signal was worked out.(2) The principle of determining the eigenfrequencies of structures was confirmed by laboratory tests and some numerical examples including a cantilever pier,a simply-supported beam,a two-span bridge with a single pier,a multi-span bridge with piers and a spatial truss beam model.The fundamentals of IVTM are explained as: for velocity response signal,the dominant frequency,at which the phase is 180° or 360° and the amplitude spectrum shows a dominant peak,is the natural frequency of a structure.For displacement and acceleration response signal,the dominant frequency,at which the phase is 90° or 270° and the amplitude spectrum shows a dominant peak,is the natural frequency of a structure.On the basis of field test data,the analysis results by IVTM,ambient vibration method and modal test method were compared.The frequency identification results of the three methods agree well,which proves the correctness of the fundamental of IVTM from the aspect of engineering application.(3) When bridge piers are impacted laterally by a weight hammer,their response spectra may emerge dominant frequencies at the frequencies of the adjacent piers or beams.At this time,the frequencies can be identified according to the phase condition: in the velocity response spectrum of the pier,the dominant frequency satisfying the 180° phase is the eigenfrequency of the test pier,while those dissatisfying the 180° phase may be the eigenfrequencies of the adjacent piers or beams.(4) The residual free-vibration responses of piers under train excitations are complex.The frequency identification results by IVTM showed that those frequencies identified by RFVM were not the eigenfrequencies of piers,but those of the adjacent beams.Therefore,RFVM is not suitable for the frequency identification of the bridge pier.(5) Four healthy bridges with spread foundation piers were tested,and the eigenfrequencies of piers were analyzed by IVTM.On the basis of the test data,the standard value formula for spread foundation piers of single-track bridges was established by statistical regression method.(6) According to the Chinese Code for Bridge Foundation Design,the critical value of soundness index was determined as 0.7 on the basis of numerical examples and real engineering cases,based on which the soundness evaluation criteria and the evaluation method for piers were proposed.(7) A field experiment was carried out on a bridge which had been judged by a common dynamic test as damaged.The engenfrequencies of the piers and the beams were measured by IVTM,and a conclusion was drawn that No.5 pier was greatly damaged,while No.6 pier was healthy according to the e soundness evaluation criteria for bridge piers.The subsoil around the foundations of the two piers was excavated,by which the damage of No.5 pier was proved and the influence laws of subsoil excavation on the dynamic properties of piers were studied.The in-situ excavation results are identical with the identified results,which confirm that the proposed IVTM and soundness evaluation method for piers in this paper are precise and effective.(8) Since the first-order eigenferquency and the first-order mode shape of a pier can be easily and precisely obtained by IVTM,the ratio between frequency shifts and other indexes based on the first-order modal parameters such as mode shape ratio,MAC coefficient,COMAC coefficient,eigenparameter and curvature difference mode shape were used to detect the damage of a reinforced concrete cantilever.The sensitivities of these indexes to damages were compared and it was pointed out that the first-order mode shape ratio,the first-order eigenparameter and the first-order curvature difference mode shape were indexes suitable for damage identification of the bridge pier.展开更多
文摘In China,the working state evaluation of piers is mainly based on the Code for Rating Existing Railway Bridges.However,the vibration amplitudes of the bridge piers under train excitations vary with train types,train speeds,axle loads and track irregularities,so the damages of piers are difficult to quantitatively evaluate by the vibration amplitude.In this paper,the impact vibration test method(IVTM) was systemically studied through theoretical analyses,numerical simulation calculation,laboratory tests and field experiments.The soundness evaluation method based on the eigenfrequencies of bridge piers was proposed.The main work done in this dissertation is as follows.(1) The fundamental of IVTM was deduced from theory in detail and the computer software for processing the impact vibration test signal was worked out.(2) The principle of determining the eigenfrequencies of structures was confirmed by laboratory tests and some numerical examples including a cantilever pier,a simply-supported beam,a two-span bridge with a single pier,a multi-span bridge with piers and a spatial truss beam model.The fundamentals of IVTM are explained as: for velocity response signal,the dominant frequency,at which the phase is 180° or 360° and the amplitude spectrum shows a dominant peak,is the natural frequency of a structure.For displacement and acceleration response signal,the dominant frequency,at which the phase is 90° or 270° and the amplitude spectrum shows a dominant peak,is the natural frequency of a structure.On the basis of field test data,the analysis results by IVTM,ambient vibration method and modal test method were compared.The frequency identification results of the three methods agree well,which proves the correctness of the fundamental of IVTM from the aspect of engineering application.(3) When bridge piers are impacted laterally by a weight hammer,their response spectra may emerge dominant frequencies at the frequencies of the adjacent piers or beams.At this time,the frequencies can be identified according to the phase condition: in the velocity response spectrum of the pier,the dominant frequency satisfying the 180° phase is the eigenfrequency of the test pier,while those dissatisfying the 180° phase may be the eigenfrequencies of the adjacent piers or beams.(4) The residual free-vibration responses of piers under train excitations are complex.The frequency identification results by IVTM showed that those frequencies identified by RFVM were not the eigenfrequencies of piers,but those of the adjacent beams.Therefore,RFVM is not suitable for the frequency identification of the bridge pier.(5) Four healthy bridges with spread foundation piers were tested,and the eigenfrequencies of piers were analyzed by IVTM.On the basis of the test data,the standard value formula for spread foundation piers of single-track bridges was established by statistical regression method.(6) According to the Chinese Code for Bridge Foundation Design,the critical value of soundness index was determined as 0.7 on the basis of numerical examples and real engineering cases,based on which the soundness evaluation criteria and the evaluation method for piers were proposed.(7) A field experiment was carried out on a bridge which had been judged by a common dynamic test as damaged.The engenfrequencies of the piers and the beams were measured by IVTM,and a conclusion was drawn that No.5 pier was greatly damaged,while No.6 pier was healthy according to the e soundness evaluation criteria for bridge piers.The subsoil around the foundations of the two piers was excavated,by which the damage of No.5 pier was proved and the influence laws of subsoil excavation on the dynamic properties of piers were studied.The in-situ excavation results are identical with the identified results,which confirm that the proposed IVTM and soundness evaluation method for piers in this paper are precise and effective.(8) Since the first-order eigenferquency and the first-order mode shape of a pier can be easily and precisely obtained by IVTM,the ratio between frequency shifts and other indexes based on the first-order modal parameters such as mode shape ratio,MAC coefficient,COMAC coefficient,eigenparameter and curvature difference mode shape were used to detect the damage of a reinforced concrete cantilever.The sensitivities of these indexes to damages were compared and it was pointed out that the first-order mode shape ratio,the first-order eigenparameter and the first-order curvature difference mode shape were indexes suitable for damage identification of the bridge pier.
