Structural Performance And Evaluation Method Of The Connecting Joints In Heritage Timber Buildings Under Damage State

The heritage timber buildings are the precious treasure with high historical,cultural,artistic and scientific values.However,these heritage buildings have shown different types and extent of damage under the long-term effects of various environmental and external loads after hundreds of years of service.These damages seriously reduce the structural performance and even lead to the local failure or overall collapse of the structures.Therefore,it is very important to evaluate the safety of the heritage buildings with considering the current damage condition of the structures.At present,many scholars have conducted lots of research work on the mechanical performance of traditional timber structures and the connecting joints.But most of these studies are focused on the undamaged structural model such that the corresponding research findings cannot be directly applied to condition assessment of the real historical structures.Moreover,due to the fuzzification in the service history process of the heritage timber buildings,there are large uncertainties in the influencing factors such as material properties,component size and external loads,which leads to a great difficulty and challenge in safety evaluation and state assessment of the heritage structures in service.For the above problems,the thesis conducted a series work on structural performance and reliability assessment of the heritage timber buildings in damage condition with research techniques including field investigation,model test,numerical simulation and theoretical calculation.The main work and important results are as follows:(1)Based on the results of field investigation,the typical damage types of the ancient timber buildings are systematically classified and described.The effect of structural damage is analyzed and the reason of damage is discussed.This part of work provides an important basis for in-depth study on the structural performance of heritage timber buildings in damage conditions.(2)With consideration of the effects of long-term load,decay and shrinkage crack,a multivariate cumulative damage model of timber is proposed.For the difference between the material test results and the existing strength prediction model,a correction factor is introduced to develop the time-dependent strength degradation model applicable to the timber components of ancient buildings.Two failure criteria based on the ultimate strength capacity and allowable deformation are proposed to predict the service life of timber component.To verify the feasibility of modified model and prediction method proposed,reliability-based analysis is conducted for the residual strength and service life prediction of timber component with consideration of the statistical uncertainty of different influencing factors.(3)According to the damage investigation and dynamic test on a heritage timber structure,it is found that the column foot damage has an influence on the structural dynamic characteristics.Based on the refined finite element modeling analysis,the moment-rotation characteristics of a column footing joint is obtained and the effects of vertical load and effective diameter are studied.A simplified model of column footing joint for structural dynamic analysis is proposed.By considering the degradation of rotational stiffness of the column footing joints,the seismic response of the timber structures under undamaged and damaged conditions is compared and analyzed,and the influence of the column foot damage is illustrated.According to the relationship between the rotational stiffness of the column footing joint and the structural frequency,the sensitive domain influencing the dynamic characteristics of the structure is determined.(4)Queti inclination is a typical damage type of beam-column joints in Tibetan heritage buildings.Tests on full-scale models including one normal joint and four joints with different Queti inclination angles under static vertical load are conducted.The deformation feature and failure modes of components,and the load-transferring mechanism of the joint are studied.Furthermore,a mathematic model of the load-displacement relationship is obtained from nonlinear regression of the experimental data,and the structural performance of the joint in terms of the stiffness,yield load,ultimate load-carrying capacity,and their degradation with respect to the Queti-inclination angle is investigated.(5)Based on some boundary condition assumptions,a simplified calculation model of Tibetan beam-column joint considering Queti inclination damage is proposed.Three failure modes of the joint under eccentric-compression,rotation and shear force are determined by the conceptual analysis.The calculation forulas of the yield load and ultimate load of the joint under different failure modes are derived.The test results are used to verify the accuracy of the calculation method.The change law of load-carrying capacity with angle of Queti-inclination is analyzed.The variation of material property,Queti width and height,dowel height as well as column inclination on load effect of the joint have also been studied.(6)Considering the uncertainty of the influencing factors,a method for reliability analysis and evaluation of the heritage timber building is proposed.Based on the theoretical model of the joint with Queti inclination,the sensitivity analysis method is used to calculate the influence weight of each parameter on the ultimate load-carrying capacity of the inclined joint.The Monte Carlo Simulation is then used to establish the statistical model of the structural resistance of each joint with only considering the uncertainty of the key parameters.According to the site investigation and relevant specifications,the statistical characteristics of the operation loads is determined.Finally,the reliability analysis and evaluation on the service state of the joints and whole structure are completed.