Dynamic Analysis Of Ancient Timber-frame Buildings Under Seismic Excitations

Dynamic analysis of ancient timber buildings under seismic excitations is criticalfor its rehabilitation, and also a difficult problem need to be solved. In this paper, thestructural characteristics and seismic conceptual design ideas of the ancient timberbuildings are qualitatively analyzed. Then a single-storey and intermediate-bay palacehouse was fabricated according to Yin Zao Fa Shi, and shaking table tests wereconducted under unidirectional earthquake excitations. The dynamic characteristics andseismic responses of this palace house are quantitatively studied. Based on hystereticbehaviors of structural members, a3-D beam-spring FEM model was established. And asimplified calculation model was put forward, taking into account of the basic structuralfeatures of ancient timber buildings such as sliding isolation of column root, semi-rigidcharacteristics of mortise-tenon joints, and energy dissipation mechanism of Dou-gonlayers. On the basis of dynamic nonlinear time-history analysis, the collapseperformance was studied using IDA method. The main research work this paperaccomplished is as follows:(1) By analyzing the structural configuration, layout of structural system andconstructional measures, the seismic conceptual design ideas of ancient buildings arediscussed. The results show that: The ancient buildings have a simple and regular planelayout, a symmetry column grid so that the planar and vertical stiffness distributeuniformly. The aspect ratio is strictly limited. The shape of the structure iscorresponding to existing principles on seismic concept design. Columns simple placingon top of plinth, mortise-tenon connections between structural members and the use of Dou-gon and heavy roof make the ancient timber buildings a multi-layered system ofthe vibration isolation and shock absorption. The classical modulus design methods‘Cai-fen’ and “Dou-kou” not only ensure its structural safety, but also take into accountof the adverse effects of wood creep and relaxation on the structural performance. Thespecial constructional measures such as “Ce-jiao” and “Sheng-qi”, the use of diagonalcolumn and “Que-ti” improve its seismic performance greatly. The use of “raised-beam”roof achieves the purpose of saving material.(2) A single-bay palace house with a scale ratio1:3.52was fabricated and shakingtable tests were conducted. Displacement and acceleration responses at column root,column head and roof top were studied under different input seismic excitations. Thedynamic characteristics of the structure, failure mode, dynamic lateral stiffness andhysteretic energy features were analyzed. Those following conclusions are made:The range of natural period and damping ratio of the test model is0.49~0.67s and2.9%~4.6%respectively. The sliding of column foot and deformation of mortise-tenonjoints play a role in vibration isolation. But the isolation role of Dou-gon layers dependsprimarily on relative value between the spectral characteristics of the seismic wave andthe structure’s natural frequency. When the nominal PGA reached to300gal,400gal,600gal and900gal, maximum deformation angle of wooden frame reached to1/70,1/50,1/35and1/21respectively, showing a greater deformation capacity than modernstructures. With the increase in seismic intensity, the overall stiffness of the structuregradually decreased, with a range of0.346~0.028kN/mm. The lateral deformation ofwooden frame dissipates most of the seismic energy. By contrast, the sliding of columnroot and deformation of Dou-gon dissipates less energy.(3) A3-D beam-spring FEM model was established based on structural features ofthe ancient timber structures and restoring force characteristics of structural members.Element damping matrix proportional to its initial stiffness was put forward to simulatethe damping mechanism of mortise-tenon joints and Dou-gon sets, instead of usingclassic Rayleigh damping. Dynamic nonlinear time-history analysis was conducted tocompare with experimental results, which verified the reliability of the calculationmodel and selection of parameters. On this basis, parametric analysis was conducted ondynamic performance of palace buildings. The results show that: whether the column roots are rigid or hinged, acceleration amplification factor of the structure significantlysurpasses the original structure, which weakens the seismic capacity of ancient timberbuildings. The settings of Dou-gon can significantly prolong the natural period of thestructure and present a function on shock absorption. The larger the stiffness ofDou-gon is, the weaker the ability of shock absorption becomes. The increase ofstructural damping ratio ζ can effectively reduce the displacement and accelerationresponses of the structure. The increase of roof mass prolongs the natural period of thebuilding, and magnifies the relative displacement. Roof mass is also helpful inimproving the ability of shock absorption of Dou-gon.(4) A simplified calculation model was put forward to take into account of sliding ofcolumn root, semi-rigid connection of mortise-tenon joints, shock isolation of Dou-gonlayers. An aggregate mass model was established and a unified motion equation was putforward to represent different status of sliding and stick phase. And the critical conditionwas give between the two phases. Nonlinear time-history analysis according toWilson-θ method was conducted using MATLAB. The comparison between calculationvalues and experimental results shows its feasibility.(5) The collapse criterions of both collapse due to sliding of column root andinterlayer collapse due to lateral deformation of wooden frame are put forward. Thesetwo collapse mechanisms are studied using IDA method, and results show: the slidingcollapse is less prone to happen under common seismic excitations. The most likelycollapse mechanism under long period seismic waves is the interlayer collapse. Thelong period seismic waves are more hazardous than common ones.