Experimental Research And Prediction Method Of Mechanical Properties Of Tibetan Populus Cathayana In Ancient Buildings

Mr.Liang Ssu-ch’eng once said that “The Chinese wooden structure system can survive in such a vast area and for a period of four thousand years,and still being used today without changing its basic characteristics.Only the continuity of Chinese civilization can be compared with this phenomenon.Because Chinese architecture is an inseparable part of this civilization.” The Chinese nation has a long history.In the long history,the wooden structure is the crystallization of the wisdom and sweat of the working people in ancient times.As a biological building material,wood has many advantages such as light weight,high strength,easy processing,convenient recycling,pollution-free,etc.However,due to the erosion of natural environmental factors and the destruction of war,there are very few ancient wooden structures that have survived to this day.Protecting the wooden structure of ancient buildings not only means protecting the history it carries,but also protecting China’s excellent traditional culture.The wooden structure of ancient buildings can stand for hundreds of years without falling down.In addition to the scientific nature of the structural system itself,it is also largely due to the excellent mechanical properties of wood.For the study of wooden structures of ancient buildings,whether it is static calculations such as structural bearing capacity and deformation,dynamic response under earthquake or environmental vibration,or finite element analysis based on simulation models,etc.All of these require accurate mechanical parameters of building materials as a basis.However,ancient building wood are different from new wood.Under long-term loading history and continuous erosion of environmental factors,not only different types of ancient building components,but also the wood from different part of the same component would have different mechanical properties,which brings great difficulties to the protection and study of ancient buildings.Address this issue,this paper takes the Tibetan ancient building wooden structure as the research object,and uses a combination of on-site testing,laboratory testing,numerical simulation and theoretical analysis to discuss the present situation,the degradation law and the prediction method of the mechanical properties of ancient building wood,as well as the prediction method of the remaining life of ancient building wooden components.The main research contents of the thesis are as follows:(1)On the basis of considering the difference of loading conditions,a replaced Tibetan Populus cathayana beam component from a typical Tibetan ancient wooden structure was meticulously sectioned and sawed,the modulus of elasticity,Poisson’s ratio,shear modulus,compressive strength,shear strength,along grain tensile strength and bending strength of the small clear specimens were tested.In the test,considering the rarity of the aged wood material,smaller compression,tensile,and shear specimens were designed.The tension and compression inequality of elastic parameters,the effect of loading rate on wood tensile properties,and the influence of the opening form of the Iosipescu shear specimen on the shear stress distribution were studied,which provides a reference for the study of mechanical properties of the ancient building wood.(2)Through the significant analysis of the difference in the mechanical properties of Tibetan Populus cathayana at different sawing positions,the necessity of distinguishing the loading history when evaluating the mechanical properties of ancient building wood materials was verified.Combined with the mechanical properties variation model between heartwood and sapwood in existing studies,and the creep numerical simulation results of Tibetan ancient wooden beams during 300 years of service,the reasons for the significant difference in the compressive strength were quantitatively analyzed.(3)Nondestructive testing(NDT)was carried out in laboratory and on site for small clear specimens and in-service components of aged Tibetan Populus cathayana.Based on the linear correlation analysis,the functional relationship between mechanical properties and NDT indexes was established,and the influence of the type of NDT index,as well as the number and location of measuring points on correlation coefficient was discussed.Based on the study of small clear specimens,the mechanical properties of inservice components were predicted.(4)Based on the real microscopic composition of wood,an improved continuous micromechanical model of wood was proposed.The effectiveness of the model in predicting some elastic parameters of poplar was verified by comparing the calculated values of the model with the experimental values of Tibetan Populus cathayana,and the influence of the volume ratio of various microscopic compositions on the elastic parameters of poplar was discussed.(5)Aiming at the Gerhards model which is commonly used in wood life prediction,the scope of application of different model parameters was analyzed.The SGR and SGC models for predicting wood life under linear loading and constant loading conditions were derived.Based on the SGC model,the strength degradation model was proposed for the constant loading condition of wood components,and the relationship between wood residual strength and residual life was analyzed.91 diagrams(267 sub-diagrams),90 charts,266 references...