Integral Mechanics Property Analysis And Safety Evaluation Of Ancient Timber Structures

Chinese ancient timber structures are a unique building system in the world for their abundant history, culture, science and art, as well as economic and social value. Ancient timber structures have survived for hundreds of years, so damages are hard to avoid. Ancient timber structures must be reinforced properly to guarantee both original appearance and safety, so the integral mechanics property analysis and safety evaluation are necessary before reinforcement.A timber structure has thousands of components which are connected by mortise-tenon joints. Mortise-tenon joints are semi-rigid joint and the working mechanism of them is very complex. Their flexural behavior plays an important role in the lateral stiffness, bearing capacity, integrity and stability for structures. But in general their bearing capacity is lower than beams or columns because the cross section is weakened. For the accurate mechanics calculation of timber structures, the mechanical property analysis of joints is therefore necessary, including relationship between moment and rotation, ultimate bearing capacity, rotation and failure modes.Scholars all over the world have done much specific research on mortise-tenon joints using the method of modern mechanics. However, there are still many problems need to be solved. In this paper, the insufficiency of current research was concluded and summarized. Then the mechanical property of joints, frames and integral structures were studied further by combining experiment study, numerical simulation and theoretical analysis.(1) Experiment study, numerical simulation and theoretical analysis of mortise-tenon jointsSix kinds of mortise-tenon joints were designed and manufactured. Relationships of moment and rotation, relationships of tenon pull-out and rotation as well as the failure modes were obtained from experiments under monotonic loading. Results showed gaps had a great influence on their flexural behavior. The main failure mode of straight joints, dovetail joints, half joints and straight joints at the column bottom was pull-out of tenons. The failure mode undertaking downward load of penetrate joints and shizigutou joints was tearing along the grain in the variable cross section,while the failure mode undertaking upward load of them was bending damage under the tenon. To half joints and penetrate joints, the flexural behaviour undertaking upward load was better than that undertaking downward load.To make up the lack of experiment, finite element analyses were conducted using ABAQUS to examine the deformation state and the stress distribution of tenons. The simplified mechanics models of relationship between moment and rotation for above joints were proposed based on experiment study and numerical simulation. These models contain the basic parameters, including material properties, the size of joints and the thickness of gaps, so these models are universal.(2) Experiment study and numerical simulation of two typical framesTwo typical timber frames, named Tailiang and Chuandou respectively, were designed. Relationships of load and displacement, peak bearing capacity and the failure modes were obtained from experiments under monotonic loading. Results showed the penetrate joints breaking off and the half joints pulling out gradually with the increasement of loading displacement. Finally, two frames collapsed under vertical load.Based on the simplified mechanics models of joints, beam element and connect element were used to build models for frames. The distribution of force and influence of vertical load were studied. The accuracy of simplified mechanics models and the way of modelling can be checked out by comparing experiment and simulation restlts. Finally, the way to model and analyze ancitent timber structures was obtained.(3) Integral mechanics property analysis and safety evaluation of ancient timber structuresThe space models for Tailiang and Chuandou structures were built based on beam element, connect element and friction isolator element. The calculation method of two structures under vertical load, wind and earthquake was developed. Results showed the largest deflection and moment located at midspan of ridge purline in the central room. The largest shear force located at the end of ridge purline in the edge room. The axial force of middle columns in central room was the biggest. The lateral displacement of Tailiang under wind was much smaller than Chuandou, which means the lateral resistance behavior of Tailiang was better. The dynamic coefficient of both structures was less than one and the foot of columns slipped a litter, which making the aseismic performance well. Rarthquake energy was mainly absorbed by structural damping (abput 70%), hysteresis of mortise-tenon joints (abput 20%) and kinetic energy. Finally, the technical route of safety evalutation was developed according to related standards.