1 Introduction

Confucian temples are temples built to commemorate the social philosopher, Confucius, and aimed at the veneration of Confucius and the sages and philosophers of Confucianism in Chinese folk religion and other East Asian religions. They were formerly the site of the administration of the imperial examination in China and Vietnam, and often housed schools and other educational facilities. It is an ancient architectural complex with oriental architectural features at a grand scale as well as unique artistic and historical cultural features. As a reflection of the profound Confucianism, which deeply impresses the Chinese thought for more than 2000 years, the Confucian temple has high cultural and architectural research values (Wei 2003; Qin 2003).

The two main structures comprising the Shuanghe Confucian temple, which is the subject of this paper, are shown from different views in (Figs. 1–3). The temple sits in Yibin city, Sichuan province, and was first built at about 1241. For the following 600 years after construction, the temple has been exposed to various gradual damage and deterioration, but was consistently refurbished. The last major refurbishment, or mainly retrofit from a structural point of view, was finished on April 11, 1851. The area of the main palace is nearly 350 m2. The length is 22 m and the width is 15.8 m. Although the Sichuan province is an earthquake prone area (Yuzhu 2003; Zhang 2003), the Shuanghe Confucian temple damage have been mostly caused by wars and invasions instead of earthquakes, until recently as revealed in this paper (County and annals compilation committee 1993).

Fig. 1
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Aerial images of the Shuanghe Confucian temple

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Fig. 2
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The main palace of the Shuanghe Confucian temple

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Fig. 3
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The back (complementary) palace of the Shuanghe Confucian temple

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As a typical traditional Chinese timber structure, the Shuanghe Confucian temple comprises most of the Chinese timber structural characteristics (Fang et al. 2001a, b). (1) The structural skeleton is constructed majorly with wooden columns and beams. (2) These components are connected by mortise and tenon joints, with the tenon as one component and mortise as another. The mortise and tenon joints have various types and shapes and dimensions according to different functions. (3) The roof is the heaviest part of the structure and it accounts for most of the weight of the temple. (4) The columns are directly supported on plinths. (5) No nails are used in the wood frames.

In order to protect such type of structures through better understanding of their dynamic behavior, Dina D'Ayala and Hui Wang (2006a, b) report the most important rule of Conservation Practice of Chinese Timber Structures are ‘No Originality to be Changed’ or ‘Conserve as Found’. Tsai Pin‐Hui et al. (2011) analyzed the performance of Chines Temples exposed to earthquake action and offered the possible non-invasive restoration and strengthening approaches. Andre R Barbosa et al. (2021) presents the behavior of floor diaphragms of a shake-table experiment of a full-scale 2-story mass-timber building structure. Chengqing Liu et al. (2021,2013) present some timber structural damage during the 2008 Wenchuan earthquake and the 2015 Nepal earthquake. Fang et al. (2001a, b) conducted full-scale on-site tests and finite element analyses to offer fundamental information like frequencies and damping ratios. In Europe, Cestari et al. (2018) conducted diagnosis, assessment and intervention on conservation of historic timber roof structures of Italian architectural heritage. These studies found that the bearing capacity of wood has continually been underestimated. It was also reported that many structural renovations or retrofits, that have been carried out in recent times, have betrayed the idea of conservation where sometimes, unjustified demolition of centuries-old roofs was reported. Furthermore, the timber mechanical properties, especially in bending strength and stiffness remain unchanged over the time, or decrease in a negligible way. De Liam et al. (2018) used prestress to strengthen the timber beams, while Roensmaens et al. (2018) used Cross-laminated timber (CLT) panels to refurbish existing timber floors. Xie et al. (2018) proposed methods to enhance the seismic performance of historic timber buildings in Asia by applying super-elastic alloys to a Chinese complex bracket system. Gao et al. (2017) reported the use of viscoelastic dampers to protect Chuan-Dou timber building. In order to keep the original material during the renovation, Kunecký et al. (2018) and Fajman and Maca (2018) offered a possible technology using all wooden lap scarf joints.

On 22:55(UTC + 8), June 17th, 2019, an Ms 6.0 earthquake with a depth of 16 km stroke Changning County (2019). For the next 20 days following that “main shock”, another 4 strong “aftershocks” with magnitude larger than Ms 5.0 occurred. The Shuanghe Confucian temple was very close to the epicenter and suffered damage for the first time as a result of earthquakes. After the main shock, the following general main damages were observed: (1) one corner of the roof overhangs dropped; (2) one column of the back palace ruptured; and (3) some surrounding fences and walls cracked and damaged. However, more components were damaged as a result of the aftershocks. More surrounding fences and walls collapsed, and one specific surrounding wall collapsed towards the temple and collided with the adjacent columns. The collapsed wall caused permanent deformation at column bottom because of the resulting induced impact loads.

The overall goal of this paper is to enrich literature on reconnaissance and heritage preservation by comprehensively documenting and assessing the damage of the Shuanghe Confucian temple after the 2019 Changning earthquake Series. The paper first present in Sect. 2 sufficient information on the Changning earthquake series including acceleration and velocity histories as recorded at Gongxian middle school station. A discussion of the structural system and components of the Confucian temple is provided next in Sect. 3. This section also discusses the evolution and mechanical functions of these components. Brief preliminary results from free vibration tests and damage details of the temple are presented and discussed in Sect. 4 and 5, representatively. The provided information in this paper helps better understand the seismic performance of traditional timber structures and identify potential weaknesses which might have adverse influences on timber structures. Thus, lessons learnt from the Shuanghe Confucian temple can be used in future to better protect other historical timber structures from earthquakes.

2 The changning earthquake series

This section provides sufficient data on the Changning earthquake series as recorded in close-by station to the temple for completeness. Such complementary data can be utilized in future studies that would consider relating the earthquake characteristics to the observed damage and conduct analytical or computational investigations. Figure 4 provides the earthquake intensity map of the Ms 6.0 Changning earthquake in Sichuan Province released by the Sichuan Earthquake Administration on June 20, (2019). As for the earthquake series, Table 1 shows basic information of all five earthquakes with magnitude larger than Ms 5.0 that happened between June 17 and July 4, 2017, which are referred to herein as the Changning earthquake series. The strong motion recording station (28°26'N, 104°42'E) installed at Gongxian middle school is the nearest station to the epicenter and the temple. Ground motions recorded at this station during the Ms 6.0, Ms5.4 and Ms 5.6 earthquake are presented in this section. Figure 5 gives the recorded raw acceleration ground motions without any corrections. For the selected major three earthquakes, Table 2 lists the Peak ground acceleration (PGA) values for all three components of the earthquakes. Additionally, Fig. 6 provides the recorded velocity histories, and Fig. 7 shows the acceleration response spectra of the selected ground motions from Gongxian station.

Fig. 4
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Earthquake intensity map of Ms 6.0 Changning earthquake in Sichuan

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Table 1 Earthquakes with magnitude larger than Ms 5.0 occurred as part of the Changning earthquake series
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Fig. 5
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Acceleration histories recorded at the Gongxian middle school station

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Table 2 Recorded PGA at Gongxian middle school station during the Changning earthquake series
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Fig. 6
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Velocity histories recorded at the Gongxian middle school station

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Fig. 7
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Response spectra of acceleration histories recorded at the Gongxian middle school station

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It can be seen from the tables and figures above that the maximum PGA recorded was 0.60 g. The bracket duration (Housner 1965; Bolt 1973; Bommer and Martinez-Pereira 2000) of the three recorded ground motions with a threshold amplitude of 0.05 g, i.e. June 17, June 22, and July 4, 2019 earthquakes, were 8.2 s, 6.8 s and 5.1 s, respectively. From Fig. 7, it can also be seen that the acceleration spectra amplitude decreases very fast as period increases compared with the typical design response spectra such as the Chinese Code of Seismic Design (2010) or American ASCE 7–16 code (2016).

3 Basic structural information of the Shuanghe Confucian temple

The Confucian temple is of traditional oriental characteristic and unique art and historical values. It is the reflection of Confucianism, which has thoroughly affected the Chinese culture for more than 2000 years. With the rapid development of economy and urbanization, several temples were inappropriately demolished, and many of the remaining ones have not been properly protected. As for the Shuanghe Confucian temple, only the two main palaces (see Figs. 1–3 above) survived until now, while all the other affiliations have been damaged. The major components of the temple, i.e. the two standing palaces, like the roof, beams, and columns are introduced in this section.

3.1 Roofs

Roofs are usually the most eye-catching component of ancient architecture, which also contribute to the important function of the facade and include the most distinct ritual feature. The major roof forms of ancient Chinese timber structures are shown in Fig. 8 (Li et al. 2017).

Fig. 8
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Typical ancient Chinese timber structural roof types

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A hip roof consists of five ridges and four slopes. It is the supreme grade and can only be used by the royal palaces and some Confucian temples in federal society. Among hip roofs, the double eave hip roof is more important than the single eave hip roof. The main hall of the Shuanghe Confucian temple uses a double eave hip roof as shown before in Figs. 1 and 2. A gable-hip roof has nine ridges, two full slopes, and two half slopes. This style of roof appears similar to a flush gable roof or overhanging gable roof superimposed onto a hip roof and is ranked second only to the hip roof because of its owner’s official position. The back hall of the Shuanghe Confucian temple uses gable-hip roof as shown in Figs. 1 and 3 above.

Other roof types like flush gable and overhanging gable, as shown in Fig. 8(d) and (e), are majorly used for residential houses. Pyramidal pavilion roof is usually used for Chinese sightseeing pavilions. Another characteristic of the Chinese roofs is that their shapes are markedly flexible and are curved or concaved. Part of the reason is for architectural beauty, another reason is to protect the wood against moisture migration, i.e. rain or melting snow seeping back into the roof (Goldberg 1983).

3.2 Beams and columns

Post and lintel wooden construction, as shown in Fig. 9(a), was initially developed between BC 770 and BC 476 and has been widely applied after that as a very typical construction type in Chinese architectural history. One beam is supported by two columns. Another two secondary columns are placed on the lifted beam and the span decreases. This type of construction is easy to fabricate, erect, and install. However, the size of the beams and columns timber cross-sections need to be huge, which is an issue if the structural depth is limited.

Fig. 9
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Beam and column construction types

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Column and tie wooden construction, as shown in Fig. 9(b), is another widely used construction type, which remedies the shortcomings of post and lintel wooden construction. It can fulfil relatively larger structural depth with smaller timber size. A series of columns with different heights are arranged and are used to support the roof directly. Beams are still used but to mainly connect the columns to increase the integrity. However, the column space for column and tie wooden construction is limited and indoor space is also limited. The main hall of the Shuanghe Confucian temple is the post and lintel wooden construction type structure as illustrated in the details shown in Fig. 10.

Fig. 10
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The post and lintel wooden construction type of the main hall of the Shuanghe Confucian temple

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3.3 Columns

In the very early ancient China, the wooden column bottoms were buried in the soil. The burial depth decreased gradually as time goes. Two reasons account for this trend. On the one hand, wooden columns are perishable at humid environment. Shallow burial columns can avoid excessive rain soak. On the other hand, compared with deep buried columns, shallow buried columns suffer less shear forces and moments facing wind and earthquake loads. This is because such columns are less constrained and the footings work more similar to hinge joints. This is important for timber structures because timber, especially the timber joints, usually does not have very high strength.

Starting from the West Han dynasty (between BC 202–AD 8), structural columns became simply floated on plinths as illustrated in Fig. 11. This is a big breakthrough for traditional construction methods and is the most outstanding characteristic for the ancient Chinese architecture. The maximum potential structural lateral shear force is limited to less than the total friction forces. This also reveals that the upper timber components had already reached good integrity and stiffness at that time. Another trend shows that the upper surface of the plinths become smoother and smoother, which indicates the friction coefficient is decreasing. Meanwhile, the surface radius of the plinths is larger than that of the column sections. The larger size of the plinth could be arguably an original design consideration to accommodate potential displacements in events like earthquakes.

Fig. 11
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Column simply supported on Cornerstone

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3.4 Dougong and inclined braces

Dougong is a bucket arch system consists of brackets unique to traditional Chinese architecture (D’Ayala and Tsai 2008; Yeo 2018). It is a structural member found between the top of a column and a cross beam. Each dougong is formed of a double bow-shaped arm (Gong), which supports a block of wood (Dou) on each side. Fixed layer upon layer, the arrangements bear the load of the roof. It helps increase the span of the structures as well as provides an important decoration element.

In the Shuanghe Confucian temple, Dougongs are replaced by the inclined braces as shown in Fig. 12. This is a very important local feature in the south western areas. These inclined braces can also help increase the roof span. Although its construction is not as complicated as Dougong, all the braces are all dedicatedly sculptured, which is very time consuming and required high specifications on the wood species. It is worth noting that outdoor wood sculptures do not last long in most parts of the world. Changes of humidity and temperature may cause it to split, and it is subject to attack by insects and fungus. Thus, the surviving inclined braces in the Shuanghe Confucian temple are very rare and special, which deserves to be carefully protected and properly preserved.

Fig. 12
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Views of sample inclined braces

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3.5 Sparrow brace (Chinese pronunciation, Queti)

Sparrow brace, as illustrated in Fig. 13, like a pair of wings, is a component installed at the corner of the beams and columns. It has two mechanical functions (Tang and Zhong 2015). The first is to improve the conjunction rigidity of beam and column as well as the integrity of the whole framework. The second is to support the beams and reduce the clear span of beams, and thus enhance the bearing capacity. Because the sparrow brace surface is relatively large and flat and it is right in front of the viewer’ eyes, sparrow braces have generally changed from pure mechanical components into a unity of mechanics and aesthetics. Sparrow braces have experienced various changes in shape, crafts, decoration, color, etc. during its hundreds of years’ history. Craftsmen often carve sparrow braces into dragon, phoenix, magpie, flowers and golden hoptoad, which all have nice metaphors in Asian culture. Round, relief and piercing sculpture methods are majorly used to carve sparrow braces. The development and the evolution of the sparrow brace had obeyed the rules that from large to small, intuitive to complex, gorgeous to graceful, and structural to decorative. The main hall of the Shuanghe Confucian temple is decorated with sparrow braces at all the outermost columns.

Fig. 13
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A view of sample sparrow brace

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3.6 Hump support

Hump supports, as shown in Fig. 14, are usually installed on top of beams to help support the purlins and roof. It is called hump support because its shape looks like camel hump. Hump supports are usually made of flat planks. This is another place to do the decoration sculptures. The hump supports of the Shuanghe Confucian temple are mainly sculptured with flowers and painted with golden lacquer as shown in Fig. 14.

Fig. 14
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Hump supports and decorations from the Shuanghe Confucian temple

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4 Fundamental frequencies of the temple palaces

During the post-earthquake reconnaissance efforts, the natural frequencies of both palaces at the Shuanghe Confucian temple were obtained. For each palace, three dimensional acceleration sensors were placed on the ground and on top of the main beam and vibration of the palace was induced. Collecting and processing the acceleration data, the fundamental frequencies of both the main palace and the back palace at the two horizontal perpendicular directions were obtained. Table 3 provides the summary of the obtained frequencies for first two modes of vibration. A sample of the corresponding Fourier spectra, processed using Fast fourier transform (FFT) of the monitored vibration time histories, in both X and Y directions is plotted in Fig. 15. It is noted that the X and Y directions are designated in Fig. 1 above. The sample FFT frequency-domain diagrams provided in Fig. 15 show at least dominant frequencies of first two modes for both the main and back palaces. It should be mentioned that the frequencies are tested after the earthquakes and some damage has happened. The frequencies might be not the same as the original ones. Considering the damage levels, the deviation might be small.

Table 3 Natural frequencies of the main and back palace of the Shuanghe Confucian temple
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Fig. 15
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Natural frequencies of the main palace and back palace of the Shuanghe Confucian temple

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From Table 3 and Fig. 15, it can be clearly seen that the two dominant frequencies of the main palace are about 1.5 and 4.3 Hz (0.67 s and 0.23 s) in one direction, and 1.5 and 6.2 Hz (0.67 s and 0.16 s) in the other direction. These modes are mostly related to the roof type. The main hall uses double eave hip roof, which is like two layer roofs and thus, the whole structure vibrates like a two Degree of freedom (DOF) system. Meanwhile, the back palace seem to have mostly one obvious dominant frequency controlled by the first mode at each direction, which is also consistent with the single layer, i.e. single DOF, gable-hip roof type. As for vertical direction, responses of both palaces show little amplification compared with ground responses. The dominant frequency is about 20 Hz for both palaces in the vertical direction.

The dominant frequencies for the main and back palaces are 1.5 Hz and 1.6 Hz (0.67 s and 0.63 s), respectively. Compared with other structure types, like masonry and concrete, at similar size, the frequencies of the timber structures are relatively small, i.e. longer vibration periods. In other words, the timber structures are usually less rigid and more flexible, so the smaller stiffness result in a smaller frequency. Furthermore, it should be mentioned that the structural frequencies obtained here are based on small amplitude ambient vibrations. The frequencies might change when larger excitations such as earthquakes occur, because the columns are directly floating on the ground and they might slide during such events. The period might be extended to a larger number when the excitation increases. Besides, the connection method of beams and columns made the structure dynamic characteristics vary facing small and big excitations when rocking and friction works. This also affect the structural period at big excitations. The elongated vibration periods, i.e. reduced frequencies, in the face of large excitations help the timber structures perform well during earthquakes by roughly considering the spectral characteristics. Usually the dominant earthquake frequencies, which might lead to resonance, are in the range of 1–10 Hz, are likely avoided by the reduced frequencies following large excitation effects.

5 Earthquake damage

During the Shuanghe earthquake, the Confucian temple suffered some hard-to-neglect damage. All the damage details are carefully documented and described in this section in full details. An overview of the damage includes the following. As a result of the mainshock: (1) one corner of the roof overhangs of the main palace dropped; (2) one column of the back palace ruptured; and (3) some surrounding fences and walls, which were newly built to isolate the temple from the surroundings, cracked and collapsed. More components that survived the mainshock were damaged during the aftershocks where more surrounding fences and walls collapsed. Specifically, one surrounding wall collapsed towards the main hall and collided with adjacent columns. The collision led to some permanent deformation at the column bottom. The details of such outlined damage are illustrated using photos collected by the research team as provided and discussed next.

5.1 Roof damage

As mentioned before, the main hall is the double eave hip roof. It has eight corners in total. As can be seen in Fig. 16, the corners of the upper eave are curvier than those of the lower eave. In addition, the length and size of the overhung upper eave corners are also larger than those of the lower eaves. As a result, the moment induced by the overhung upper eave corner is much larger than that of the lower eave corner during earthquakes. It can be seen that all the inertia forces of the corners are supported by the log beneath the corner. It works as a cantilevered system and is a statically determinate system, which means there is no extra redundancy. Once the log breaks, the corner drops. A whiplash effect might also aggravate the dynamic forces beard by the log beneath the corner, and this also account for the break of the log and the failure of the corner. Luckily, the eave corner is more a decoration component, instead of a structural component. Its failure did not cause any casualties or affect the structural overall safety directly.

Fig. 16
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Roof damage of the main palace

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From Fig. 16c, it can be seen from the log section that the log remains in relatively good condition. Insects and fungus does not seem to have very obvious influence during its hundred years of history. Some slight but fresh fungus grew at the surface layer after the break of the log. The picture was taken on July 11, 2019 which was 24 days after the corner failure following the mainshock earthquake. The environment in July at Shuanghe is humid and hot, which made the log easy to mildew. This then should alert people that repair work should be undertaken as soon as possible following such extreme events. Otherwise, the condition of the rupture surface of the remaining log on the roof will only get worse. Caution should be exercised to deal with the rupture surfaces during the repair work if the part of the broken log, which remain on the roof, will be retained. Fungus might be a big problem at the rupture surface and this has already been demonstrated from the rupture surface, in only 24 days, as shown in Fig. 16d.

Although it is very important to preserve the historical architecture in its original format while repairing, tiles made of composite materials might be considered as an alternative to replace the broken ones at the broken corner as they are lighter. Thus, such alternative solutions of composite materials will result in less inertia forces being transferred to the log during earthquakes.

5.2 Column damage

One outermost column of the back palace was fully ruptured during the earthquake as shown in Fig. 17. Firstly, it can be seen that the rupture is close to the corner brace connection joint and the cracks occurred along the grain direction. Secondly, the wood near the rupture section has not been properly protected and has been corroded. Compared with other columns, the outermost columns have less shadow and have experienced more rain. Another factor need to mentioned is that there is a newly built surrounding wall right next to the broken column.

Fig. 17
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Column damage of the back palace

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Many researchers investigated the variation of timber mechanical properties over time, e.g. (Cavalli et al. 2016). It is difficult to give accurate and consistent conclusions because it is hard to determine the original mechanical properties of the old wood and the effect of different factors like the state of conservation, the load history, the damages occurred during the service life, and the mounting/dismantling operations. Most researches on variation of timber mechanical properties over time studied the variation of bending strength and stiffness. The majority of research works agreed on the fact that the bending strength and bending stiffness remain unchanged over time or just slightly decrease in an insignificant way.

The bending stiffness and strength reductions that have been reported for structural timber are mostly a result of the in-service conditions, e.g. duration of load, state of conservation, and dismantling damages as opposed to a direct consequence of aging. In most cases, the damage observed in historical timber structures deals with biodeterioration and mechanical deterioration both induced by excessive moisture content. If no damaging action occurred, the impact of loading history and time on the strength and stiffness of structural timber elements seems to be limited. The time-in-service effect on old timber strength and stiffness parallel to grain is now evident, possibly due to the low stress level during service. (Feio and PB Louren O et al.. 2007).

As for the column that was fully ruptured in the Shuanghe Confucian temple, the following is noted. (1) Moisture induced biodeterioration seem to be the major reason of the seismic induced rupture. It can be clearly seen from the pictures that the anticorrosive paintings have been peeled and the log has been seriously decayed. (2) The rupture took place near the inclined brace connection joint. In order to mount the corner brace, a mortise need to be excavated onto the column. The cut of timber might decrease the timber integrity and weaken the log bearing capacity. This also accounts for the log rupture. (3) Given that there is a surrounding wall right next to the column, the rain drops on top of the wall will directly splash on the column. The narrow space between the surrounding brick wall and the timber structural wall is at the shady side and is not well ventilated. This might have aggravated the biodeterioration. On top of the status of the column before the earthquake, (4) the collision between the column and the wall might be then the direct reason of the log rupture. The wall and the column vibrate individually and independently during the earthquakes. However, the wall is much more rigid compared with the overall palace where the columns vibrate as part of the whole structure. Due to frequency differences, the wall and the column likely collided as they moved towards each other. The collision took place at the wall top, which was also close to the rupture section. The bending moment in the column at the collision increases suddenly and tremendously, which might lead to short-column failure. With the additional partition wall with a less height than the column, the free part of the timber column became shorter (with same moment demands) and flexible deformations are prevented. While the shear demands during the earthquake increases, with the additional effects of biodeterioration, the shear strength of the column was not enough. As a result, a brittle rupture due to the short-column mechanism occurred.

5.3 Column malposition

Column(s) dislocation and malposition was observed at the Shuanghe Confucian temple following the Changning earthquake series, which was caused by collision of adjacent wall collapse. As shown in Fig. 18, the surrounding wall collapsed towards the temple and collided with the columns and plinths. The traces of impact, such as deep scratches, can be clearly seen in the figure. About 5 to 10 cm permanent displacement of the plinths took place. As mentioned in Sect. 3, the plinths are floating and directly supported on the ground, i.e. not buried in the soil. There is no connection between the different plinths. Although the columns suffered this large malposition, no other visually detected damage occurred except some painting peeled at the beam column connection joint as shown in Fig. 18.

Fig. 18
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Malposition of the columns of the main palace

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As for the collapsed wall, the impact load was limited but still led to relatively large column malposition to occur. The first reason of this malposition is attributed to the base connection type. The connection surfaces have been designed to be flexible to reduce seismic adverse influences and the friction coefficient is thus small. Once the lateral force is larger than the static friction force, column base displacement happens. The second reason is because the timber structural weight is generally light. Although the collapsed surrounding wall was not very heavy, it still caused a relatively big impact when compared with the light structural weight of timber components.

Compared with other structure types, like masonry structures or concrete structures, the timber structure column connection type is very unique where sliding is allowed. Excessive moments are generally avoided and the moment at the column top and bottom can be reduced at a designed level, which is majorly related to weight, friction coefficient, column height and diameter. The design strategy is consistent with today’s base isolation concept. The smooth plinth works just as the sliding isolation bearings. When facing strong ground motions, the column bottoms slide and reduce excessive moments to protect the timber structural components. However, unlike the Friction pendulum system (FPS), the column connection type system lacks self-centering capacity, the residual deformations might be large and permeant. Meanwhile, the surface radius of the plinths is larger than that of the column sections. The larger size of the plinth could be arguably an original design consideration to accommodate potential displacements in events like earthquakes.

5.4 Surrounding wall damages

Due to the lack of enough emphasis, the original walls that surrounded the temple had disappeared for many years. Some new surrounding walls were built in recent years to isolate the Confucian temple from nearby residential area as part of the efforts to protect it. However, the unexpected lesson learned from the Changing earthquake and highlighted here in this paper, is the very poor seismic performance of those relatively new surrounding walls, which not only collapsed but adversely aggravated the Confucian temple damage.

Figure 19 shows a collection of pictures for the damage of the surrounding walls. Figure 19a was taken right after the main earthquake by the Beijing Youth daily (https://baijiahao.baidu.com/s?id=1636837120918612340&wfr=spider&for=pc). It can be seen that although the front surrounding wall cracked, it remained sound and standing. The other photos were taken by the authors after the aftershocks. The front surrounding wall collapsed after the aftershocks as shown in Fig. 19b. The surrounding walls all have heavy heads, which significantly increase/shift the height of the wall gravity center. From Fig. 19c, d, it can be seen that the walls were just directly built on top of the platform. There was no inserted base connections. From Fig. 19e, f, it can be seen that cracks occurred. No reliable connection between longitudinal and lateral walls was constructed. From Fig. 19g, h, the surrounding wall was long but no concrete constructional columns were considered. Thus, the out-of-plane stiffness of the wall is hard to be guaranteed in such setting, which violates the design code (GB 2011). Moreover, and as mentioned in Sect. 5.2, the surrounding walls were built too close to the structure at the back side. It is unfortunate that the walls were initially built to protect the temple, yet they ended up causing more damage to the temple. This is then a cautionary to practice highest levels of carefulness and place more emphasis on regular repair or preservation works to avoid basic causes of damage resulting from overlooked mechanical and structural measures.

Fig. 19
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Damage of the surrounding wall

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5.5 Bookshelf overturn

The Shuanghe Confucian temple has been used as a library for the Shuanghe middle school before the Changning earthquake. Only one bookshelf was toppled and overturned during the earthquake series, but all the other bookshelves remained standing. Various photographs of the bookshelves inside the temple following the earthquake, including the toppled one, are provided in Fig. 20. The overturned bookshelf was right in front of the inclined wooden partition. There is a masonry wall right beside that wooden partition. Because the stiffness of the masonry wall is much higher than that of the wooden partition, and the masonry wall is very close to the wooden partition, the deformation of the wooden partition during the earthquakes was restricted to the opposite direction towards the masonry wall. This is the direction towards the bookshelf. Accordingly, the overturn of the bookshelf was more likely induced by the collision of the adjacent partition and not a direct result of internal inertial forces. Thus, the improperly built surrounding walls are again to be blamed for this damage and not a structural shortcoming.

Fig. 20
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Bookshelf overturn

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6 Summary and concluding remarks

Timber structures form an important structural system and structures type in Asian areas and the world. The Confucian temples, built to commemorate Confucius, are good examples to study the traditional timber structures in China. The Shuanghe Confucian temple, which sits in the southwestern area of China, have experienced some damage in the 2019 Changning earthquake series. This temple has the supreme ancient structural grade and dedicated local characteristics, and thus, was selected to be the subject of this paper. The paper aims at relating the structural system of this temple along with new additions such as surrounding walls to the observed seismic damage, with an attempt to motivate the need to rethink architectural and heritage preservation. Basic information of the Changning earthquake were provided and the architectural characteristics of the Shuanghe Confucian temple were presented. As part of the reconnaissance efforts, the authors conducted some free vibration tests to obtain the fundamental frequencies of the temple structures, which were reported in this paper for completeness. The earthquake damage was then carefully reported and discussed. The work presented in this paper helps better understand the timber structure overall seismic performance and the potential weaknesses which might have adverse influences on timber structures.

Overall, the Shuanghe Confucian temple (both main and back palaces) showed very good earthquake resistance capacity and performed well during the Changning earthquake series. Some components suffered damage while most other structural components remained intact. However, there are important lessons that can be learnt from the Shuanghe Confucian temple. The following remarks can provide more insight to better protect other historical timber structures from earthquakes and inform future preservation works.

  • The eave corners of the roofs are fragile components. The corners are cantilevered and form statically determinate system, which lacks extra redundancy, especially the upper eave corners which are curvier and have larger overhung length and size. The curvy shapes of the eaves are not only designed for architectural aesthetics but also for roof drainage. It is not practical to change the roof shapes nor materials for seismic protection of ancient timber structures. Thus, extra emphasis should be put on the mechanical properties and quality checks of the supporting log beneath the eave corners during structural maintenance with paying attention to bio-deterioration.

  • The floating placement of the columns is a unique characteristic of Chinese timber structures, which radically changes the structural internal force distribution. The bending moment at the columns bottom is decreased to almost zero and the maximum potential lateral shear forces is restricted to a certain value, which is determined by the structure gravity and friction coefficients. Nevertheless, the floating placement weakens the integrity between the structure and the base, which made this type of timber structures very susceptible and vulnerable to impact loads. During the Changning earthquake series, one column of the Shuanghe Confucian temple experienced serious malposition because of the collision of an adjacent collapsed surrounding wall. The column dislocation did not have serious implications and did not lead to collapse, yet it uncovers an alarming need to pay attention to such connections in preservation works.

  • For seismic protection of historical architectures, every single measure or step should be carefully studied and interrogated in light of potential effects and consequences on the preserved structures. This study highlights one example associated with the relatively recent surrounding walls built around the temple. Such walls were intended to protect the Shuanghe Confucian temple, yet they ended up hurting the structures and causing adverse consequences to the temple during the Changning earthquake series. The improper design of the wall led to its collapse during the earthquake and eventually permanent malposition of one of the temple columns. On the other hand, the surrounding wall was built too close to the shadow side of the back hall, which restricted the free movement of the columns during earthquake, and formed a narrow moist space aggravating the timber columns biodeterioration.