Research On Evaluation Method Of Reliability-based Residual Life Of Historic Timber Structure

Ancient timber structures in China,unique in design and construction,have great historic,cultural and artistic value.However because of hundreds years of material degradation and natural biological attack,many timber structures have been damaged, some even vanished.Therefore it is very important to prolong the service life of these historic timber structures by studying the rule of resistance degradation,i.e.,the time-dependent models of the timber decay,termite attack,duration of load(DOL) and dry shrinkage crack.Based on the factors which affect the structure's service-life, the mechanics,quantitative model,time-dependent multi-factor resistance degradation model of the timber and the residual service-life prediction of the structures are studied in this paper.Considering the hygrothermal effects on load-duration behavior,the cumulative damage model for DOL in short time is improved to that for predicting long-time strength.Then the model is integrated and modified to the long-time strength degradation model to be consistent with the resistance theory.The random model for termite attack,composed of four stages,proposed by Leicester R.H etc.in Australia,is modified to two-stage model according to the resistance theory.And the time-dependent function of the damage variable of erosion area,defined to describe the erosion degree,is suggested based on the in-site tests and survey of termite damage.Finally the time-dependent resistance model of the timber member attacked by termite is deducedBecause of different location and different stress state,the strength decrease varies.The method to determine the strength reduction coefficient according to the decay class is proposed and the loss in geometrical property is calculated by the outside decay depth.Then the time-dependent resistance models of the timber beam and timber column are calculated in which the empirical formula in China is used for above- ground,exposed timber beam and column and the decay depth model for under-ground timber in Australia is adepted the timber embedding wall.Based on the test data about the moisture content and dry shrinkage crack development of air-dry timber,the curve between crack depth and moisture content, and that between crack depth and time are fit.Then the relationship between the crack depth and timber member resistance is analyzed by finite element method. Furthermore the different effect on the member between single crack and double cracks is also compared.Consequently the model of crack's effect on the resistance of timber member is obtained.The multi-factor resistance degradation model is synthesized by assuming the factors,DOL,termite attack,decay and dry-shrinkage crack are mutually independant random variables.In order to consider that the in-site testing data about historic timber structure has the effect on its future resistance,the Bayesian method is employed to improve the multi-factor resistance degradation model by testing sample.The program to predict the residual life of the historic timber structure is developed,in which stress level is calculated by the FEM and resistance degradation model is realized by MATLAB.Finally a historic timber truss and a beam-column frame are analyzed to predict their residual life,and the effectiveness of the proposed model is verified.In word,breaking through the past empirical method,the research provides a more scientific prediction method for historic timber structure's maintenance and reinforcement,which can prolong the historic structure's life because the maintenance period can be determined scientifically and economically.