Research On Damage Identification Method Of Ancient Wooden Structure Under Random Excitation

Ancient architecture is the witness of history,which has a positive meaning in reflecting the cultural heritage and characteristics of the city.It is also an important medium for attracting tourists of the world to learn about Chinese history and traditional culture.Chinese ancient buildings are mainly wooden structures,which have been built for hundreds or even thousands of years.Because the wood material is easy to decay and aging,in addition to impacts of the long-term overload and the surrounding environment,many ancient buildings are being in danger.Because damage detection on the destructive test of ancient buildings can not be actualized,it is necessary to use the environment such as vehicle load to identify the damage of ancient buildings.Therefore,the research on damage identification of ancient timber structures under random excitation is not only of great value to the protection of cultural relics,but also can be the promotion to the sustainable development of cultural tourism.In this paper,different methods are selected to establish damage identification indexes for damage identification and safety assessment of ancient wooden structures under random excitation.The main research contents and results are as follows:(1)A damage identification method for ancient wooden structures based on the cross-correlation function of the dynamic response of random vibration structures is proposed.The results show that the damage index based on the dynamic response cross-correlation function can effectively identify the damage of timber beams,and accurately locate the single damage and double damage at different positions.However,due to the damage of mortise and tenon joints,the index loses locating ability.The degree of relative change of indexes before and after the structural damage increases roughly in proportion to the degree of damage,which can be used to judge the damage degree of the structure.(2)Based on the cross-correlation function,a damage identification method based on the auto-correlation function of structural dynamic response is proposed,which studies the damage identification and localization capability of the damage indexes.The results show that the damage index based on the auto-correlation function of dynamic response,which is not limited by the reference point,has outstanding effects on single-damage and double-damage localization of beam or mortise-and-tenon joint of wooden structure.In addition,the damage degree of the structure can be determined according to the relative change degree of the damage index before and after the damage of the structure.(3)Based on the correlation theory of cross-correlation function and auto-correlation function,a damage identification method based on quadratic correlation function of structural dynamic response is further proposed,and the peak vectors of the quadratic correlation function of the dynamic response of the damaged structure and the intact structure are constructed respectively.The damage of the structure is identified according to the cross-correlation coefficient.According to the relative degree of the peak vectors before and after the damage,not only the single and double damage positions of the beam and mortise-and-tenon joint can be accurately located,but also the damage degree of the structure can be determined.(4)After a 24-hour real-time vibration monitoring of Xi’an Bell Tower under the excitation of ground traffic load was carried out,the vibration characteristics of the measured response signals of each monitoring point at different time periods were analyzed,and the variation law of the peak value of the correlation function of the measured vibration signals at two measuring points on the wooden structure was studied.The results show that the correlation function of measured vibration response signals can be applied to sensitively characterize the influence of traffic load-induced micro-amplitude vibration on the timber structure of the Bell Tower and the variation of the timber structure vibration characteristics.It provides a basis for damage identification and safety assessment of ancient timber structures.(5)According to the architectural and structural characteristics of Xi’an Bell Tower,the overall finite element model of " upper wooden structure-high platform-base foundation " was established with the measured vibration signal as a random excitation.The three damage indexes were applied to the wooden structure of Xi’an Bell Tower.The applicability of different indexes for damage identification of complete and complex wood structures was studied.The results show that the damage index based on the cross-correlation function of the dynamic response is affected by the reference point.The damage location may be misjudged.The damage index based on the auto-correlation function and the quadratic correlation function is not limited by reference point,and the single damage and double damage location of beam or mortise and tenon joint of the complete ancient wooden structure can be accurately located.The relative change degrees of the three indexes before and after the structural damage increased in roughly the same proportion as the damage degree.The deviations of the three indexes are all within 10%.Therefor,the judgment of the damage degree of the complete wooden structure is realized.(6)Through comparing the damage localization ability and anti-noise interference ability of three damage indexes applied to the wooden structure of the bell tower,the results show that in terms of damage localization of wooden structures and anti-noise interference,the damage index established based on the quadratic correlation function of structural dynamic response is superior to the damage index based on the auto-correlation function and the damage index based on the cross-correlation function.The damage index based on quadratic correlation function and auto-correlation function has good value in engineering application and popularization.