Ancient Timber Structures Research Articles

Investigation on the Seismic Damage and Identification of Lateral Stiffness for Ancient Timber Structures

ABSTRACTThis paper investigates the seismic damage and identification of lateral stiffness for ancient timber structures. Based on the shaking table test of a palace‐style timber structure, the lateral displacement responses, interstory equivalent lateral stiffness (IELS), and sensitivity of lateral displacement to degradation of IELS were analyzed to investigate the degradation and identification of IELS. The state and observation equations of a simplified mechanical model of the timber frame considering the friction slipping of the column base were predicted. Considering the noise disturbance in the test, the IELS of the model was identified by the partial least squares–singular value decomposition (PLS‐SVD) and extended Kalman filter (EKF) method. Results shown that the ratio of the IELS of the column frame layer to Ru‐Fu layer decreased as the seismic damage increased, and the peak lateral displacement was more sensitive to the damage of the column frame layer than the Ru‐Fu layer. Under nondamage conditions, the identification error of the IELS was about 10%, while the error ranged from 15% to 20% under damage conditions. Through the identification of the equivalent lateral stiffness of the Xi'an bell tower, it was validated that the hybrid method is effective in monitoring the IELS of ancient timber structures.

Seismic behavior of looseness-induced inclined mortise and tenon joints in ancient timber structures: Experimental tests, multi-scale finite element model, and behavior degradation

The inclination damage of mortise and tenon (M-T) joints, caused by the looseness between the mortise and tenon, has an detrimental impact on the overall safety of structures. To investigate the effects of varying inclination rotation on the seismic behavior of ancient M-T joints, four 1/3.2-scaled straight M-T joints with different inclination rotation, one intact joint for comparison and three looseness-induced inclined joints, were cyclically tested. Typical failure modes of the damaged joints were identified. The influence of the tilt deterioration on the joint's hysteretic and rotational behavior was evaluated. The test results showed that with an increase in the inclination rotation, the size of the hysteretic loops of the inclined joints was increasingly smaller, the negative initial slippage and pinching effects were more apparent, the ultimate moment-resisting, initial elastic stiffness, and ductility of the specimens degraded significantly. Furthermore, the multi-scale finite element models of the looseness-induced inclined M-T joints were established using the ABAQUS. Based on the validated numerical model, the parameter analysis was performed. The influence of the frictional coefficient of wood, inclined rotation, and horizontal gap between the tenon and mortise on the joint's hysteretic behavior and energy dissipation mechanism was quantified. It is found that with a friction coefficient of 0.6, the positive and negative ultimate moments-resisting of the joint improved 65.99 % and 35.89 %, respectively; the joint's slip moment and energy dissipation increased by 12.18 and 3.78 times, respectively. With a 0.043 rad inclined rotation, the positive and negative ultimate moments of the joint decreased 23.32 % and 30.46 %, respectively; the joint's positive and negative initial elastic stiffnesses and cumulative energy dissipation reduced 49.12 %, 28.96 %, and 56.33 %, respectively. When the inclined rotation remained at 0.022 rad, the positive and negative ultimate moments of the joint including a 6 mm horizontal gap reduced 26.63 % and 34.20 %, respectively; the joint's positive and negative initial elastic stiffnesses, and accumulative energy dissipation decreased 42.42 %, 59.43 %, and 79.50 %, respectively.

Seismic vulnerability analysis of palace-style timber structures base on lumped mass model considering column rocking effect

To investigate the influence of joint reinforcement on the vulnerability of palace-style ancient timber structures, a spring-rigid rod analysis model was established, considering the mechanical behavior of tenon-mortise joints, column base joints, and bracket set joints. A lumped mass model was proposed using the centroid method and stiffness equivalent method and the accuracy of the simplified model is verified through shake table tests. Additionally, the influence of reinforcement on tenon-mortise joints and bracket set joint on the seismic displacement response was investigated, the optimal reinforcement range for joints was determined. Finally, incremental dynamic analysis was conducted on the model considering joint reinforcement, and the influence of optimal joint reinforcement on the seismic vulnerability of timber structures was researched. The study found that the optimal stiffness enhancement of column frame mortise-tenon joints range from 4 to 6 times and the stiffness of bracket set joints by 2.9 times. The probability of structural collapse of reinforced both mortise-tenon joints and bracket set joints was reduced by 15.56 %. However, individually reinforcing either tenon-mortise joints or bracket set joints only slightly decreases the probability of collapse.

Effects of external loads-induced inclination damage on the seismic performance of traditional straight M-T joints

Under the earthquakes, wind, and other external loads, ancient timber structures have experienced various degrees of damages, in which the inclination damage of mortise and tenon (M-T) joints, induced by the external loads, has an adverse effect on the safety of the whole structure. To study the effect of various degrees of inclination damage on the seismic behavior of straight M-T joints, we fabricated four 1/3.2-scaled ancient straight M-T joint specimens, one without inclination and three with different degrees of inclination, and subjected them to the cyclic loading tests. The inclination of the joint was simulated artificially, i.e., pushing the intact joint to the predetermined inclination rotation and holding the loads, then unloading when the part of tenon entered a nonrecoverable plastic state. The failure modes, initial elastic stiffness, moment-resisting capacity, strength and stiffness degradation laws as well as energy dissipation and deformation capacities of the joints were investigated. The results showed that the plastic deformation of the tenon and mortise edge, tenon pull-out, and longitudinal splitting of the tenon were exhibited in the inclined joint specimens. Compared to the intact joint, the plastic deformation of the tenon and tenon pull-out for the inclined M-T joints were more apparent during the positive loading. The hysteretic curve of each joint exhibited an anti- "Z" shaped, and the greater the inclination degree, the more apparent the negative initial slipping and pinching effect. With an increase in the damage degree, the initial stiffness, ultimate moment-resisting capacity, and hysteretic energy dissipation behavior of the inclined M-T joint gradually degraded. When the damage degree was 3.89%, the positive yielding moment, peak moment, and initial stiffness of the inclined M-T joint degraded to 52.41%, 86.09%, and 67.19% of that of the intact joint. Based on the hysteretic characteristics and test results of the inclined M-T joints, the degradation formulas of the key parameters of seismic behavior for the inclined M-T joints were established, and an improved degraded hysteretic model, which can reflect the hysteretic behaviors of inclined M-T joint, was proposed. Good agreements between the model predictions and the test results were observed.

Structural detection and reinforcement methods for existing buildings

With the popularity of building construction worldwide, building design, construction technology and quality are faced with inherent deficiencies and may be hit by disasters. Therefore, the diagnosis and reinforcement of building structure has become a major problem to be solved urgently in the current building construction. This paper introduces simple detection methods of different building structures and reinforcement techniques under different conditions. In the part of detection methods, various basic detection methods of normal building structure are studied. Then, a special damage appearance is discussed. Besides the traditional electronic imaging method to detect cracks, a special method is proposed to solve the crack problem, namely Min-Max Gray Level Discrimination (M2GLD). This method can improve the detection accuracy and reduce the influence of human factors. At the same time, several different reinforcement techniques are proposed, including traditional reinforcement techniques and advanced reinforcement techniques. This paper introduces the traditional technology of ancient timber structures and the technology of spraying and external bonding FRP to enhance flexural, shear and compressive strength. This paper can be used as reference for structural inspection and reinforcement of existing buildings.

Investigation on seismic performance of inclined Dou-Gong bracket between columns of ancient timber structures

To investigate the seismic behavior of the inclined Dou-Gong (DG) brackets, three full-scaled Dou-Gong bracket between columns with different inclinations along the width direction were tested under the pseudo-static test. The failure mode, hysteretic characteristic, stiffness degradation, deformation and energy dissipation capacity as well as the deformation modes of DG were discussed. Besides, numerical analysis of DG’s stress distribution and seismic behavior of DG brackets inclined in the overhanging direction was also studied. Parameter analysis was conducted to study the effect of frictional coefficient, vertical load and wood properties on the bearing capacity of DG. The results demonstrated that the DG bracket rotates as a whole and the relative slip of Da-Dou and Pingban-Fang leads to the shear damage of Mantou-tenon when DG failed. As the inclination in width and overhanging direction increases, the positive bearing capacity and lateral stiffness decrease, while the negative direction increases. The DG’s negative deformation ability is better than the positive direction and the larger the inclination, the stronger the deformation capacity. However, inclination reduces the energy dissipation capacity of DG. Rotation deformation is the main deformation mode of the DG bracket. Increasing the frictional coefficient, vertical load and compressive strength along the grain improves the bearing capacity of the DG, but the bearing capacity is slightly affected by the elastic modules along the grain. The research results can provide a scientific basis for the performance evaluation and repair protection of inclined DG brackets.

Experimental study on fracture damage and seismic performance of loose through-tenon joints in ancient timber structures

A looseness is typical damage of mortise-tenon joints in ancient timber structures. Brittle fractures of loose joints are likely to occur, leading to joint failure during earthquakes. This study conducts pseudo-static tests on three groups of full-scaled through-tenon joints to examine the fracture damage and seismic behavior of the loose mortise-tenon joints and determines fracture damage modes of through-tenon joints with different looseness. It uses the acoustic emission method to measure the ring count on the tenon surface, along with each joint's critical load and critical energy release rate and then analyzes the fracture damage performance of different tenon joints. In addition, this research investigates the hysteretic and skeleton curve, stiffness degradation curve, and tenon pulling quantity of the joint under various fracture damage states. The results showed that the fracture failure mode of the through-tenon joint is a fracture in the variable cross-section and along the local tenon neck. During the loading process, the critical energy release rate initially increases and then decreases as the looseness grows. In addition, with the increase of crack mouth opening displacement (CMOD) and crack length, the bending moment of the joint firstly increases and then decreases. When the variable cross-section breaks and the tenon neck partially cracks, the joints' bending moment and stiffness reduce with increasing looseness. With the increase in local embedment compression in the lower side of the beam end and the mortise, the amount of tenon pull-out of loose joints decreases under positive and negative loading.

Application of SPO2IDA in Seismic Performance Evaluation of Ancient Timber Structures

ABSTRACT Ancient timber structures have a high cost for conducting dynamic time-history response analysis due to the presence of many nonlinear components, such as mortise-tenon joints and Dou-Gon. To enhance the efficiency of evaluating the seismic performance of ancient timber structures through dynamic analysis, a simplified dynamic response analysis method based on SPO2IDA was adopted to evaluate the seismic performance of a timber frame palace with nine-purlin in Qing Dynasty of China and the results were compared with those obtained using the traditional pushover analysis method. The results showed that there were only slight differences in the seismic response of the ancient timber structure determined by the two methods when the structure was in a nonlinear stage. During a Ⅸ degree rare earthquake, the displacement response differed by 0.004 m, while the median seismic intensity during the third level of damage limit state differed by 0.079 g. The traditional pushover analysis method may underestimate the seismic resistance of the ancient timber structure, but the SPO2IDA algorithm can better capture the uncertainty of the structural vulnerability model at different limit stages caused by ground motion input, achieving the goal of quickly evaluating the seismic performance of the ancient timber structure with acceptable accuracy.

Experimental study on seismic performance and deformation damage of loose dovetail-tenon joints in ancient timber structures

A looseness is typical damage for mortise-tenon joints in ancient timber structures. The tenon of the loose dovetail-tenon joints is prone to be pulled out, leading to joint failure in the earthquake. Hence, it is necessary to investigate the seismic performance and deformation damage of loose dovetail-tenon joints. This study conducts pseudo-static and acoustic emission (AE) tests on three full-scaled dovetail-tenon joints to determine the four stress stages of each joint, including slight damage, medium damage, severe damage, and nearly failure. It examines the hysteretic and skeleton curve, stiffness and strength degradation, energy dissipation performance, and the variation raw of the amount of tenon pulled out at each stage. The deformation damage mode, damage degree, and damage evolution equation of the joint at each stage are obtained based on the AE characteristic parameters measured at each stage. The results indicate that as the rotation angle increases, each joint's stiffness and energy dissipation capacity decrease, while the joint bending moment rises and then reduces, and the amount of tenon pulled out increases. The greater the degree of looseness, the lower the bearing capacity of each joint, and the more pronounced the degradation of stiffness and strength. At each stage, the maximum decrease of initial stiffness is 85.75%, and the maximum amount of tenon pulled out is 93 mm. The failure modes of joints are mainly tenon pulled out failure, and extrusion deformation greatly influences the tenon pulled out failure. As the degree of looseness grows, joints are more prone to severe deformation damage under large rotation angles. At the slight and medium stages, the joints primarily suffer from tensile damage of material, but at the severe damage and nearly failure stage, they mainly suffer from shear damage of material. The damage value of each joint increases with the relative stress ratio, and looseness significantly impacts the damage degree of the joint.

Experimental research on progressive collapse behavior of Chinese ancient timber buildings with different joint strengthening methods

Many Chinese ancient timber buildings present various damages due to material performance degradation, natural environmental influence, and human improper action during the long history. These existing damages make them vulnerable to different progressive collapse scenarios. Investigation of scientific strengthening approaches to prevent them from progressive collapse is urgent. This study compared the effect of different joint reinforcing approaches that are commonly employed for Chinese ancient timber buildings through experimental studies. The structural performance of frames reinforced by U nails, steel plate, or Carbon Fiber Reinforced Polymers (CFRP) sheet under Single-Column Removal (SCR) scenario was compared. Then, the deformation and failure modes of these reinforced joints were investigated. Finally, the moment and axial force in the beam before and after the reinforcement were concerned to understand the strengthening mechanism of the above reinforcement approaches. The results show that: the reinforcement can improve both the load bearing capacity and the ductility of the structure in the collapse. The improvement of the ultimate load is because the reinforcement can increase the ultimate moment at the joint. A better ductility of the collapse should be ascribed to a large axial force in the beams due to the reinforcement. Compared with the no-reinforced frame, improvement of the load bearing capacity due to the aforementioned three types of strengthening is ordered as follows: steel-plate reinforcement (359–639%) > CFRP reinforcement (297–430%) > U-nail reinforcement (90–286%). The failure of the steel-plate reinforcement is comparatively brittle while the failure of the other two presents great ductility. Besides, according to the experimental results, several suggestions for anti-collapse strengthening in engineering practice with respect to each aforementioned reinforcement are proposed. This suggestion is also applicable to the engineering strengthening practice of ancient timber structures in Asia.

Experimental Study on the Bonding Performance of FFRP Reinforced Timber Interface

ABSTRACT Although there are lots of researches about the application of FRP composites on the external bonding and reinforcement of timber structures, they are mostly aimed at Carbon FRP, which has a large difference in mechanical properties and inconsistent deformation with timber. Meanwhile, research about the interface between timber and Flax Fiber Reinforced Polymer (FFRP) with similar properties with timber is limited; hence, relevant models have not been established yet. Similar to linear elastic FRP confined timber structure, the failure of FFRP-timber structure interface is a process in which the interfacial stress is gradually transferred from the loaded end to the free end, and the interfacial stress transfer area, that is an effective bond length of the FFRP-timber interface. In order to study the interfacial stress transfer characteristics between FFRP and timber structure, based on the existing CFRP-timber structure interface bonding theory and mechanism, a single lap shear test is used to conduct a systematic study on bonding length and width are studied to optimized the application of FFRP. The bond-slip behavior is studied and fitted to provide theory support for the application of FFRP in the restoration of ancient timber structures.

The medieval carpentry of the Basilica of St. Anthony in Padua

Attics are often the most interesting rooms where to investigate historical construction techniques. Above the eight domes of the medieval Basilica of St. Anthony in Padua, lightweight timber structures attest to the use of rather archaic frameworks. The ongoing research on the domed roofs confirmed the preservation of their 13th-century configuration and the perpetuation of this model during later interventions until the 18th century. Based on on-site measurements, the study of archival material, and dendro-dating, this paper aims at shedding light on the constructing techniques and dating of the timber domes of St. Anthony. Results from the dendro-sampling campaigns provide evidence of 13th-century elements still in place. Moreover, cross-references between on-site findings and archival materials enable the tracing of the dendro-provenance of replacements from the 16th century. Finally, a short comparison with the timber domes of St. Mark in Venice and St. Justine in Padua enhances the importance of the ancient timber structures of St. Anthony.

Numerical analysis of traditional Chinese timber structure: Simplified finite element model and composite elements of joints

Mortise-tenon (M-T) joint, column foot joint, and Dou-Gong joint play a key role in the seismic behavior and energy dissipation of traditional timber structures. To simplify the modeling process and predict accurately the hysteretic behaviors of traditional timber joints as well as the global seismic responses of traditional timber structures, three types of composite elements, which were composed of varying non-linear one-dimensional spring elements available in the ANSYS finite element (FE) software, were proposed to characterize the hysteretic performance of the M-T joint, the column foot joint, and the Dou-Gong joint, respectively. The beam element was used for simulating the timber components with large size and not easily damaged. The parameters of the composite elements were identified from the simplified hysteretic model or test hysteretic curve of each type of joint. The simplified FE models of the M-T joint, M-T jointed timber frame, column foot joint, and Dou-Gong joint were developed, and verified using the cyclic loading tests of joints, respectively. Based on the validated composite elements and simplified FE models of varying joints, the three-dimensional simplified bar system FE model of a 1/3.52-scaled palace-style ancient timber structures was established using the ANSYS, and also validated using the shaking table tests, in terms of its dynamic characteristics and responses. Good agreements between the model predictions and test results were observed. Additionally, parametric analyses were performed, while revealing and quantifying the effects of performance degradation for the M-T joint, column foot joint, and Dou-Gong joint on the dynamic responses to the structure. It is found that the damage to the Dou-Gong joint had little effect on the maximum story drift of column frame layer, however, the damage to the M-T joint and column foot joint would significantly increase the maximum story drift of column frame layer. When the damage degree of the M-T joint, Dou-Gong joint, and column foot joint in the structure was up to 60%, the maximum story drift ratios of column frame layer were close to 1/48, 1/50, and 1/39, respectively, the maximum story drift ratios of Dou-Gong layer were approximately 1/164, 1/98, and 1/125, respectively.

Seismic Performance Evaluation of Damaged Ancient Timber Structures with Looseness Mortise-Tenon Joints

ABSTRACT In order to investigate the seismic performance of damaged ancient timber structures with looseness mortise-tenon joints, the shaking table test of an intact ancient timber structure was carried out. The experimental model’s failure modes, main modal parameters, and dynamic response curves were investigated. In addition, the finite element models (FEMs) with looseness mortise-tennon joints were built by OpenSees, and the natural periods, acceleration, and displacement time history curves of intact FEM and experimental model are compared. Moreover, incremental dynamic analysis (IDA) of damaged models was performed, where the ground peak acceleration (PGA) and maximum inter-story drift ratio θ max are taken as intensity measure (IM) and damage measure (DM) of the motions of the ground, respectively. The seismic fragility of damaged models was studied by probabilistic seismic demand analysis to evaluate the seismic performance of the damaged structures from the perspective of probabilistic.

An Experimental Study of the Hysteresis Model of the Kanchuang Frame Used in Chinese Traditional Timber Buildings of the Qing Dynasty

Kanchuang frames are important parts of traditional timber architecture in China. This paper used experimental and numerical methods to study the restoring force model of Kanchuang frames, which were used frequently in Chinese ancient timber structures, particularly in North China. The prototyped test model is a type of Chinese traditional timber architecture named Qilinyingshan. It was widely used in ancient timber buildings preserved from the Ming and Qing dynasties. This study analyzed the loading process and failure modes of the test model, and the skeleton curve and hysteretic curve data were collected. Moreover, a dimensionless skeleton curve model was developed based upon the findings. The hysteresis loops of the test model were also analyzed, and it was found that each hysteresis loop can be divided into several feature segments according to their stiffness at different loading stages. Regression analysis was also used to obtain the stiffness degradation curvilinear equations of the feature segments. Finally, a hysteresis force model of a Kanchuang frame was established. This study also found that the loading process can be divided into three stages: the elastic stage, in which all of the components are in good condition; the elastic–plastic stage, in which cracks gradually develop on the wall; and the new elastic–plastic stage, after which the wall collapses. It was found there was consistency between the restoring force model and the test results, indicating that the model is valid and reliable. The skeleton curve model and hysteretic model provide reference for the nonlinear seismic response of ancient timber architecture.

Experimental study on mechanical properties of column foot of ancient timber structures – Take the drum-shaped plinth as an example

The column foot is a typical structural feature of ancient buildings in China. The sliding rotation of the joint is related to the overall structural safety of the ancient wooden structure. In order to study the mechanical properties of column foot under horizontal force, based on the investigation of column foot forms and referring to the engineering practice of Qing Dynasty, the 1:3.2 scale models of three typical column foundation forms, i.e. Floating, Guan-jiao, and Tao-ding, were made, and the static and quasi-static loading tests were carried out. The horizontal static loading is carried out under the vertical force, and the parameters such as the angle change of the axis, the horizontal displacement load curve, the lateral stiffness and the failure mode are obtained. The results show that: during the loading process, there is no slippage of the wooden column, and the failure mode of the column foot is the overturning failure of the wooden column; The angle formed by the axis of the timber column accounts for more than 25% of the overall lateral displacement, which should not be ignored in the mechanical analysis. The hysteretic curve, skeleton curve, stiffness degradation rate and energy dissipation capacity of different column foot is obtained in the quasi-static test of horizontal low cycle repeated loading. The results show that the shape of the hysteretic loops of the column foot presents an inverse Z shape, and the area of the hysteretic loops increases with the increase of the horizontal displacement; The results show that the energy dissipation capacity of the flat Floating is better than that of the other two forms, and that of Guan-jiao is the second; The results show that Tao-ding has the strongest lateral stiffness, while Floating is slightly weaker.

Shaking table test on a low-damage controlled multiple-rocking-column steel frame

Ancient timber structures represented by the Shaka Pagoda can remain intact after strong seismic actions. Inspired by the rocking and energy dissipation mechanisms of ancient timber structures, a multiple-rocking-column steel frame (MRCSF) system was proposed in this study. The proposed system consists of continuous beams, segmented columns and self-centering friction connections. Friction joints are employed at the beam-column and column base connections to dissipate energy, and the self-centering capacity is mainly provided by the gravity load and additional springs at the connections. A 1/3-scale model of a three-story MRCSF was designed, fabricated and tested on a shaking table at Tongji University. Test results showed that the columns started to rock when subjected to excitations beyond the frequent earthquake level, as expected. The tested frame successfully underwent a series of strong excitations and exhibited excellent low-damage characteristics. Negligible residual drifts and structural damage were measured after the tests. Finally, a feasible design process for the newly proposed MRCSF system was proposed.

Effects of dowels on the mechanical properties of wooden composite beams in ancient timber structures

In order to provide more accurate suggestions for the restoration of ancient timber buildings, five types of specimens were designed for static loading tests. The tree species used for the specimens was larch. The wooden composite beams were composed of purlins, tie plates, and fangs. The study analyzed the effects of the number and position of dowels on the mechanical behaviors of wooden composite beams in ancient timber buildings. The bending moment, slippage, strain of the wooden composite beams under the deflection of the beam allowed according to code, and the ultimate bearing capacity of the wooden column composite beams under failure conditions were examined. The test results showed that the dowels could improve the bending capacity of the wooden composite beams. The even distribution of the dowels was beneficial in reducing the sliding effect of the wooden composite beams. Under the amount of deflection allowed by the code, the mid-span section strain along the height of the wooden composite beam approximately conformed to the plane section assumption. The wooden composite beam still had bending capacity after each member failed. The results of this study illustrated that dowels improved the overall mechanical properties of the wooden composite beams.

RESEARCH ON ASEISMIC BEHAVIOR OF STRAIGHT TENON JOINTS REINFORCED BY SMA STRINGS IN ANCIENT TIMBER BUILDINGS

The semi-rigid mortise-tenon joints in the wooden structures of ancient buildings are often destroyed before beams and columns under major earthquakes, which are the important positions for reinforcement and protection. A new type of shape memory alloy (SMA) preventive reinforcement device is designed, and low-cyclic loading tests are carried out on five reinforced joints and one unreinforced joint. The failure modes, moment-rotation relationship, strength degradation, stiffness degradation, energy dissipation, self–centering ability and deformation capacity of the joints reinforced by SMA strings with different numbers and different pretension strains are studied. A calculation method for bending capacity of straight tenon joints reinforced by SMA strings is established. The results show that the bending capacity of the joints after SMA reinforcement is improved. The maximum bending capacity of the reinforced joint is 1.49 times that of the unreinforced joint. Compared with the unreinforced joint, the strength degradation of the reinforced joints is not serious, which indicates that the SMA strings can still continue to provide the bending capacity for the joints when the rotation is large. The maximum rotational stiffness of the reinforced joint is 2.24 times that of the unreinforced joint. With the increase of the rotation of joints, the energy dissipation of unreinforced joint gradually decreases, however the energy dissipation of reinforced joints increases first and then decreases. At the same rotation, the relative residual deformation decreases with the increase of numbers and of pretension strains of SMA strings, and the self-centering ability is enhanced. The results of research can provide a reference for the preventive conservation of the mortise-tenon joints in ancient timber structures.

Application of Nondestructive Testing Technologies in Preserving Historic Trees and Ancient Timber Structures in China

In China, preserving historic trees and ancient timber structures has attracted more attention from researchers and institutions. Nondestructive testing (NDT) technology has gained momentum in the application and research in recent years. This paper presents some examples of the application of NDT techniques in the protection of historic trees and ancient timber structures in China. A total of nine NDT techniques were applied, including five techniques of visual inspection, sounding, Resistograph, stress wave transmission velocity, stress wave tomography for both historic trees and ancient timber structures, two techniques of the pulling-test and ground-penetrating radar (GPR) for historic trees alone, and two techniques of acoustic emission (AE) and digital imaging for ancient timber structures. Based on field testing, there is a very good detecting effect for visual inspection and Resistograph, but sounding works unsatisfactorily. The stress wave transmission velocity is found to be excellent in screening for the internal defects of historic tree trunks, but performs badly in screening for the internal defects within the wood members of ancient timber structures. AE is less successful but digital imaging performs well in long-term monitoring and damage warning of wood structures. The stress wave tomography successfully obtains images of internal defects inside tree trunks. The anchorage of some historic trees cannot be quantitatively evaluated with the pulling test if the tree is too large. GPR’s resolution and accuracy are very low, which limit its application. Theoretical research on NDT and promotion of NDT technology are still an essential part of the work in the future.