Study On Suitable Strengthing For Ancient Timber Buildings

The ancient settlement wood structure is an important part of ancient architecture in China.It contains rich historical and cultural information,high artistic and technological accomplishments,and needs long-term protection through reinforcement and maintenance.However,the traditional reinforcement and repair methods are based on the experience of mentoring and their scope of use is strictly limited.There is lack of understanding and evaluation of recovery level of structural performance after reinforcement.Those methods are unable to adapt to the current engineering structure reliability evaluation system and difficult for modern engineer and technical personnel to master it.On the other side,it is easy to damage the integrity,authenticity and cultural tradition of ancient buildings that simply transplanting the reinforcement method of the modern structure to the ancient building structure,as well as overstrengthening the characteristics of the original structure.It is,therefore,necessary to study on suitable strengthening.Based on the principle of cultural relic protection,this paper proposes suitable reinforcement methods for column root decay,split crack and decay of beam heads,and loosening and dislocation of mortise-tenon joints in ancient timber buildings.Intensive research on the mechanical properties of the reinforced members and joints are conducted in terms of experimental,numerical and theoretical analyses.The main methods and conclusions are summarized as follows:A splice joint reinforced by a steel jacket was proposed to address the limitation in traditional splice techniques due to a short splice height and influence on appearance.Axial compression tests were performed on the columns reinforced by steel jacket with different length of splice,steel jacket with different extended length of the steel jacket beyond the splice joint(L_e),steel jacket with adhesive,ear hoops and carbon fiber polymer,respectively.Performances among these enhancement methods had been analyzed with respect to the failure modes,ultimate bearing capacity,bending stiffness,compression stiffness and load-strain curves.The degree of recovery of load-bearing capacity and bending stiffness using different reinforcements has been compared.The digital image correlation(DIC)method was used to analyze the buckling directions,displacement and strain of intact reference columns.The results showed that,similar to the reference columns,the spliced columnsfailed mainly because of their instabilities.The load-bearing capacity of the spliced columns reinforced with steel jacket was close to reference columns.The load-bearing capacity of the spliced columns reinforced with steel jacket and further enhanced by epoxy adhesive could recover about the full capacity of the reference columns.The retrofitted columns reinforced by ear hoops performced well on axial compressive capacity and bending stiffness.Due to their low lateral stiffness,colums wrapped with carbon fiber polymers exhibited relatively poor loading capacity.The DIC method can capture the buckling path of axis-symmetrical circular columns.The displacement and strain distribution of column surface were also presented to improve the results of test analysis.To overcome the influence of material property variation and processing error on stress mechanism of the specimens,the numerical models of spliced columns were modelled using finite element(FE)software ABAQUS.The buckling modes of splice columns,splice parameters and stress distribution of splice joints were thoroughly investigated.The simulation results showed that the first mode corresponds to the III pattern of lateral deflection of splice columns in test.It was the dominant mode of buckling directions.The reasonable range of L_e and the length of the splice were 0.5 to 1.5 times and 1 to 1.5 times of the column diameter,respectively.L_e was the main parameter of the bearing capacity and the lateral stiffness of the splice columns.The distribution rules of positive pressure and frictional force between the steel jacket beyond the splice joint and columns changed with L_e.The theoretical model of axial compressive strength for the spliced columns retrofitted with steel jacket was proposed accordingly based on the rules of contact stresses at splice joint and the second kind of stability theory.The theoretical model was verified by comparing with the test data and simulation results in terms of parameters,such as tree species,column length and diameter.Several parameters,such as L_e,slenderness ratio and initial eccentricity of columns were considered in this model.This modle had certain universality and can provide support for the retrofitted design of ancient timber buildings.Based on the principle of suitable reinforcement,using the methods of reinforcement and connection of modern wooden structures,a method of reinforcing the spliced beam joints with self-tapping screws was proposed without affecting the appearance of the spliced beam,avoiding cracking,and with high operating efficiency.An experimental program based on four-point bending test configuration was conducted to study the strength,stiffness,ductility and failure modes of the spliced beams.Based on the experimental results,an analytical model that addressed the specific head pull-through failure mode was proposed to predict the flexural capacity of spliced beams.The analytical model was calibrated by the data of the head pull-through resistance test results.Comparisons between the analytical and the test results revealed that the model can adequately predict the moment bearing capacity of the spliced beams with the corresponding specific failure mode.Besides,the location of joint could be controlled due to the small relative variability of mechanical behavior,which is valuable for further engineering application.The detachable flat steel reinforcement technology without damage strengthening was proposed to address the limitation in traditional steel reinforcement due to the damage of timber elements'the section.The mechanical behaviors of the joints with the new method were experimentally studied by comparing with those of the joints without strengthening or retrofitted with FRP.The failure modes,ultimate bearing capacity,relationship between moment and rotation(M-?)curves of the retrofitted joints were thoroughly analyzed.The results showed that the failure mode of the retrofitted joints with undamaged flat steel still possessed the semi-rigid property similar to the unreinforced mortise-tenon joints.Such new method can efficiency prevent the tenons from pulling out.However,the joints reinforced with FRP presented rigid failure mode.Then,FE models for the joints without strengthening and retrofitted with undamaged flat steel were constructed and their mechanical behaviors were numerically studied.On the basis of the contact stress between the wood braces,and the relationships of tenon pull-out and rotation,the theoretical model of M-?curve was proposed.This model showed good consistency with the experimental results.Since the dimension and properties of the timbers were considered in this model,it has certain universality.