Study On Mechanical Properties Of Reinforced Mortise-tenon Joints In Ancient Timber Structures

Large numbers of exiting ancient timber structures have been in services for lots of years and most of them have been greatly damaged,therefor,it is necessary to find out a reasonable strengthened method for Chinese Ancient Timber Structures.The mortise-tenon is an important part of the timber structures.However,there have been a number of studies on mortise-tenon joints,but theoretical researches of strengthened joints still need to be advanced.This paper studies the mechanical properties of strengthened mortise-tenon joints after summarize and analyze the existing achievements.In this paper,3 primitive joints,3 joints strengthened by CFRP and 3 joints strengthened by steel hoop were tested by monotonic loading.The results of joints tests showed that:the typical damage mode of straight tenon was tenons pulled out for mortises,while the beams and columns were intact,the straight mortisejoints can undertake a certain moment but tenon and mortise extruded deformation obviously,such as the semi-rigid model.And the relationship between the tenon-pulling-out length and rotation was linear.On the basis of the tests results,The curve performed as three sections of broken lines,which is divided into"rising,horizontal,descent" three section.With the increase of load,the stiffness decline gradually.The bearing capacity of joints increases greater than the stiffness after reinforced with steel hoop,while the stiffness of joints increases greater than the bearing capacity after reinforced with FRP.The trend of every group's M-?is consistent in the range of allowable error.Besides,the curve of FRP reinforcement has a slightly different,at the rising period,the bearing capacity fells sharply because of the joints' angle which made the glue fall off.In this paper,solid elements in ABAQUS software was used to simulate the joints' deformation state and stress distribution,use " Static-Kinetic friction formulation " and "hard model" to simulate the effect between the joints' normal and tangential directions,use "cohesive zone " to simulate the contact effect between FRP and timber.the test results were compared to verify the accuracy of the model.The stress and strain distribution of the key parts was analyzed at the same time,the results shows that:Tenons' compressive stress approximate triangular distribution along the tenons height before reinforced,steel hoops' stress distribution along the width is roughly into trapezoidal after reinforced,both the horizontal direction and vertical direction of FRP's strain approximate triangular distribution.Based on the models,this paper used the width as the independent variable to consider its influence for the mechanical properties of the straight tenon.And the analysis results show that wider steel hoops the greater initial stiffness,and the hoops' width has a small promotion effect on joints' stiffness.The capacity increases with the increase of the steel hoops' width.In conclusion,steel width is better to use 1/3 tenons' height.On the basis of the analysis of the mechanics mechanism of the mortise-tenon joints after the reinforcement,establish calculation model by simplifying assumptions,and deduce the mortise-tenon joints'bearing capacity calculation formula after reinforcement,which has a good precision and could provide a theoretical basis for ancient building reinforcement design by compared with the experimental results and the finite element analysis results.