A Study of Member Displacement According to Seasonal Climate of the Sungnyemun Gate, a Korean Wooden Architectural Heritage Site

Abstract

This study analyzes the results of a displacement measurement of the Sungnyemun Gate’s structural members, such as column, girder, and hip rafter, carried out by the National Institute of Cultural Heritage for about 10 years from December 2013 to October 2022. Through this, we attempt to examine the behavior of wooden architectural heritage sites according to seasonal changes and infer the factors influencing structural deformation. As a result of the analysis, it was confirmed that the structural members of the Sungnyemun Gate, including the columns, girders, and hip rafters, continued to move and that the displacement of members was accumulated, and the structure was deformed. It was also confirmed that member displacements accumulated in a specific direction. In the case of the Sungnyemun Gate, the column leaning south, the hip rafters’ endpoint sagging, and the girders’ center deflecting were continuously observed. Furthermore, the behavior of wooden architectural heritage sites, where displacement accumulates as it undergoes repeated deformation and recovery according to seasonal changes, was also revealed in detail. The deformation of the Sungnyemun Gate’s members shows a pattern that reflects the mechanical properties of wood, which repeatedly increases and decreases displacement depending on the season. However, seasonal deformation did not appear the same in all the members. Even the same member has an uneven drying speed due to differences in the amount of sunlight it receives depending on its location, which leads to uneven distribution of deformation. The significance of this study is that it examined the behavior of a wooden architectural heritage site in detail based on the quantitative results of long-term measurements and prepared primary data for the future management of wooden architectural heritage sites.

1. Introduction

Korea has four distinct seasons and a climate with significant differences in temperature and humidity between summer and winter. A seasonal climate like this is one factor that significantly influences the management of Korea’s architectural heritage sites. In wooden construction, the movement of the structure is greatly sensed depending on the season because the wood shrinks or relaxes depending on the climate. Although it is a slight movement, it is essential to examine structure changes according to the climate in the management of wooden architecture and cultural heritage, as these movements affect the entire building if they move repeatedly for a long time.

Structural Health Monitoring (SHM) is a technology designed to monitor structural deformation and diagnose damage or deterioration in structures [1]. Accordingly, SHM has been introduced to manage architectural heritage and is currently the focus of active research, development, and field application [2,3]. In particular, as destructive testing to assess structural performance is not easy to conduct for wooden architectural heritage sites, SHM is emerging as a prominent technology in the field of safety management for such heritage sites [4,5,6].

However, in Korea, the attachment of SHM sensors to architectural heritage sites is permitted only in limited cases. There are only three instances of wooden architectural heritage sites in Korea where sensors have been installed for SHM and used to monitor for more than three years [7]. This is attributed to the strict regulations of Korea’s architectural heritage conservation system concerning alterations to existing conditions. Among conservation experts, there is a perception that adhesives and equipment used for sensor attachment may damage architectural heritage sites. Consequently, implementing SHM using sensors in Korea requires approval from the Cultural Heritage Committee [8].

Therefore, NIRCH has proposed methodologies to minimize damage while monitoring structural behavior [9,10]. The measurement method using a total station, as suggested by NIRCH, involves attaching only small reflective sheets measuring 1 cm² without requiring additional equipment installation on the cultural heritage site.

The subject of this study, the Sungnyemun Gate, has been periodically monitored for structural displacement using a total station since December 2013 [7]. The Sungnyemun Gate, designated as Korea’s first National Treasure, was destroyed by fire in 2008 and restored in 2013 [11]. The restoration of the Sungnyemun Gate was carried out through the complete dismantling of the structure and the subsequent replacement of the damaged components. Approximately 90% of the upper wooden components and 10% of the lower components were damaged, with most structural elements of the upper section being replaced. The Sungnyemun Gate is primarily constructed using Korean red pine (Pinus densiflora), which was also utilized as the primary material in its restoration (Figure 1) [11].

Korean red pine is a structural wood mainly used for the columns and beams of Korea’s wooden architectural heritage sites. Since structural wooden members are subject to a large dead load, the mechanical properties of wood are an important factor to consider in the drying process or structure planning. Although the mechanical properties of Korean red pine have been suggested through experiments during the drying process [12], specific properties such as creep deformation used as structural members for architectural heritage still need to be verified. Therefore, it is difficult to closely calculate the deformation of architectural heritage structural materials established based on the carpenter’s empirical knowledge, and its stability can only be inferred from the fact that there have been no structural defects from the time it was first built to the present. However, in the case of the Sungnyemun Gate, it was recently revealed that there is room for structural capacity through analysis of dynamic behavior using a structural analysis program (Midas Gen) [13].

Korea’s wooden architectural heritage is a structure in which hundreds of thousands of wooden members are assembled to create a framework, and a heavy roof is placed on top of it. In the case of the Sungnyemun Gate, the roof loads on the first and second floors are estimated to be 7.2 kN/m² and 8.3 kN/m², respectively [13]. Because hundreds of thousands of wooden members are assembled despite a large vertical load, various distances occur at the member’s joints depending on the location and conditions (Figure 2).

The distance of the members is basically due to the mechanical properties of the wood. However, the behavior of the entire structure is a complex intertwining of the behavior of each member, so it is not easy to separate and analyze the mechanical properties from the movement of the structure. However, by identifying behavior trends, abnormal behaviors can be detected, and structural instability can be prevented in advance. The National Institute of Cultural Heritage installed reflective sheets at 50 locations. The displacement of the entire structure is monitored through the tendency of these 50 points to move. This study examined the trend characteristics of the displacement of 16 points on the northeastern elevation.

2. Materials and Methods

The process of examining the displacement of the Sungnyemun Gate involved the installation of 50 points inside and outside the building. These points were then measured with a total station, ensuring precise data collection. The measurement data analyzed in this study are the displacement data of 16 points installed outside the northeast side of the Sungnyemun Gate. The northeast side was chosen for this study due to its relatively stable daily temperature change compared to the southwest side (Figure 3).

Figure 2 illustrates the locations of the 16 points, which were measured using Leica’s Total Station TS60. The target was a reflective sheet and was measured at a distance of less than 50 m. The TS60 can measure distances of up to 1000 m without a prism, achieving an accuracy of 2 mm + 2 ppm for distances up to 500 m (Figure 4).

The points were installed at the upper part of the columns on the first and second floors, the center of the central girder, and the hip rafter’s end on the first floor. The points installed on the columns were observed around horizontal displacement (x-axis, y-axis), and the points installed on the girder and hip rafter were observed with level displacement (z-axis) (Figure 5).

Periodically, the displacement of each point using a total station was measured. Thirty-one measurements were made until 2022, and the first measurement occurred on 18 December 2013. One month later, the second measurement occurred on 16 January 2014. After these tow test measurements, the points were measured four times a year until 2019. However, in 2018, measurements were made only three times due to field conditions. Since 2020, the structure behavior has been confirmed as stable and measured twice yearly. The specific measurement date and the average temperature and humidity of the measuring day in the area where the Sungnyemun Gate is located are as follows (

Table 1. Temperature and relative humidity at survey date.

No.	Date	Temperature (°C)	Relative Humidity (%)	Location
1	18 December 2013	−0.2	84	Seoul, Korea 37° 33′ 35″ N, 126° 58′ 31″ E
2	16 January 2014	0.1	70
3	27 January 2014	1.9	61
4	22 May 2014	20.2	72
5	13 August 2014	22.9	74
6	18 November 2014	3.7	59
7	5 February 2015	−1	55
8	22 April 2015	12.1	63
9	14 July 2015	24.3	73
10	14 October 2015	16.6	71
11	3 February 2016	−1.6	61
12	18 April 2016	8.8	82
13	19 July 2016	25.8	70
14	11 October 2016	16.8	79
15	6 March 2017	−1	43
16	22 May 2017	19	50
17	17 August 2017	24.3	80
18	02 November 2017	15	79
19	14 March 2018	16.4	63
20	1 October 2018	12.9	72
21	26 November 2018	4.9	80
22	5 March 2019	7.5	66
23	21 May 2019	14.5	73
24	23 September 2019	16.9	68
25	12 November 2019	7.6	72
26	12 October 2020	11.8	66
27	11 November 2020	7.7	61
28	14 April 2021	7.2	84
29	18 October 2021	18.9	89
30	11 April 2022	17.8	83
31	12 October 2022	12.5	78

) [14].

3. Results

Table 2. Measurement results (In November 2014, the 1G point could not be measured due to field conditions).

	1F															2F
	1C1		1C2		1C3		1C4		1C5		1C6		1G	HR1	HR2	2C1		2C2		2C3		2C4		2C5		2C6		2G
	x	y	x	y	x	y	x	y	x	y	x	y	z	z	z	x	y	x	y	x	y	x	y	x	y	x	y	z
Dec. 2013	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Jan. 2014	3	−4	3	−6	7	−5	9	1	11	−10	12	2	−2	−4	−6	1	−6	4	−4	7	−7	11	−4	10	1	13	−3	−2
Jan. 2014	7	−1	7	−3	7	−7	7	3	6	−3	5	4	−2	−1	0	6	−2	8	1	8	−4	8	−3	8	−1	7	−4	−1
May. 2014	3	0	3	−4	5	−1	6	4	5	0	8	0	−3	−8	−8	1	−4	2	−4	3	−6	6	−2	5	1	6	−3	−4
Aug. 2014	0	0	2	−1	2	−3	4	5	4	−1	6	1	−2	−7	−4	−2	−4	0	−1	2	−2	4	0	5	−2	5	−1	−2
Nov. 2014	6	0	5	−4	6	−2	7	2	7	−3	7	3		−14	−10	3	−5	5	−2	5	−9	7	−7	6	−1	7	−1	−6
Feb. 2015	3	1	2	−2	4	−2	4	1	5	0	4	2	−3	−14	−12	1	−1	3	2	3	−5	4	−3	5	−1	5	−6	−7
Apr. 2015	2	0	2	−1	2	−2	4	1	3	−4	5	1	−4	−15	−11	0	−3	2	−4	2	−9	3	1	4	−2	4	−2	−8
Jul. 2015	1	0	2	0	2	−2	4	2	5	−4	4	1	−4	−17	−13	−3	−4	0	−4	2	−9	4	−7	5	−4	5	−4	−5
Oct. 2015	−2	1	−2	1	−2	0	0	6	−2	2	−1	1	−6	−22	−18	−3	−3	−2	−2	−1	−8	0	−4	0	−4	−1	−2	−11
Feb. 2016	6	1	4	−6	6	−3	7	3	7	−2	9	−2	−4	−21	−15	5	−4	6	−2	5	−9	9	−8	8	−4	7	−3	−9
Apr. 2016	4	0	5	1	4	−2	7	0	6	2	6	2	−6	−23	−15	2	−2	5	−2	5	−6	6	−2	6	−1	6	1	−9
Jul. 2016	2	2	3	−1	2	−2	6	2	6	−7	6	−1	−3	−20	−12	0	−3	1	−2	2	−6	4	−7	6	−4	5	−1	−5
Oct. 2016	2	1	1	−3	2	−4	4	5	4	−4	6	−1	−4	−24	−17	0	−3	2	−3	2	−10	4	−7	6	−5	4	−2	−7
Mar. 2017	−1	0	0	−3	1	−3	2	2	2	−1	3	0	−6	−25	−19	−1	−4	1	−3	1	−9	1	−4	2	−2	2	−2	−11
May. 2017	2	−1	2	−3	2	−5	3	3	3	−3	3	2	−5	−27	−22	−1	−5	1	−5	1	−10	2	−5	3	−3	2	−3	−12
Aug. 2017	0	0	1	−2	2	−2	4	1	4	−3	4	0	−6	−23	−17	−2	−3	0	−4	1	−9	3	−6	4	−3	4	−3	−9
Nov. 2017	0	0	1	−2	2	−2	3	2	2	−2	4	−1	−7	−27	−21	−2	−3	1	−3	0	−9	1	−4	2	−1	2	−1	−11
Mar. 2018	0	−1	0	−3	2	−2	4	4	3	−2	4	2	−7	−28	−21	−1	−3	1	−6	2	−9	3	−6	3	−1	3	−1	−11
Oct. 2018	1	−1	2	−3	2	−4	4	2	3	−3	4	−1	−6	−30	−23	−1	−4	2	−4	1	−10	2	−7	2	−3	2	−3	−12
Nov. 2018	2	2	1	−2	2	−2	3	4	3	−3	3	1	−7	−30	−24	−2	−4	1	−4	1	−10	2	−6	1	−1	2	−3	−12
Mar. 2019	2	2	1	−2	2	−3	3	3	2	−3	4	−2	−8	−33	−25	−1	−3	1	−7	2	−10	1	−7	2	−3	2	−3	−13
May. 2019	2	0	1	−4	2	−5	3	1	3	−4	4	−3	−7	−34	−26	−1	−4	1	−7	1	−12	1	−8	2	−5	1	−4	−14
Sep. 2019	0	−1	1	−2	2	−6	3	2	3	−3	4	−2	−7	−31	−23	−2	−4	1	−5	1	−11	1	−7	2	−5	2	−3	−12
Nov. 2019	1	2	1	−1	2	−4	3	3	2	−2	3	1	−7	−33	−26	−2	−2	1	−4	1	−10	1	−7	2	−3	2	−3	−13
Oct. 2020	0	0	1	−1	2	−3	3	3	3	0	5	0	−7	−34	−25	−2	−4	−1	−6	1	−11	2	−7	3	−3	2	−2	−14
Nov. 2020	1	1	0	−3	1	−4	3	3	2	0	4	−1	−7	−35	−27	−2	−4	1	−5	0	−11	0	−7	2	−3	0	−1	−14
Apr. 2021	3	2	2	−2	4	−1	5	3	4	−1	5	0	−8	−38	−29	0	−3	3	−3	2	−12	3	−8	4	−4	3	−3	−15
Oct. 2021	0	−4	0	−5	1	−6	2	4	2	−3	3	0	−7	−37	−26	−3	−7	1	−7	0	−14	2	−8	3	−3	2	−3	−13
Apr. 2022	0	0	1	−2	2	−2	4	1	3	0	5	1	−8	−39	−28	−1	−3	1	−3	1	−12	2	−7	4	−3	3	−1	−15
Oct. 2022	−1	2	−1	−1	0	−2	2	1	2	−2	3	−1	−8	−37	−28	−3	−3	−1	−4	−1	−11	0	−5	2	−6	1	−6	−13

shows the column displacement measurement results. The initial data were set to 0, and the amount of displacement compared to that initially observed was recorded in millimeters. The x value of the columns means the amount of change in the southeast and the northwest; the southeast was recorded as plus and the northwest as minus. In addition, the y value recorded the amount of change in the northeast and southwest, and the northeast was recorded as plus and the southwest as minus. The measurement points (1G, 2G, HR1, HR2) installed in the central girder and the hip rafter recorded the level of change sagging due to the load.

The point installed on the columns has a more significant displacement of the second floor than the first floor, estimated because the second-floor columns receive a relatively more minor load than the first-floor columns. In addition, the initial measured displacement value is larger than the later-measured displacement value, which reflects how the structure stabilizes under the roof’s load. According to the result values, the points on the upper part of the column are generally stabilizing as they move in the southeast direction. As for points installed on girders, the second-floor point (2G) has a more significant displacement than the first-floor point (1G). 2G moved up to 15 mm lower compared to the first time. As for the point installed at the end of the hip rafter on the first floor, point HR1, installed in the southeast, has a more significant displacement than HR2. HR1 moved downward up to 39 mm (Figure 6).

The restoration of the Sungnyemun Gate was completed in June 2013. Moreover, in December of that year (18 December 2013), installation and the first measurement of the points were conducted. The measurement of the Sungnyemun Gate continues to this day. The Sungnyemun Gate is the only case in Korea where individual wooden architectural heritage measurements have been conducted periodically over a long time.

While it has been understood that the structure of wooden architectural heritage sites moves seasonally in response to temperature and humidity, specific trends such as the amount or direction of displacement have yet to be identified. The data presented in Table 2 are significant, as they provide quantitative information on the moves in the Sungnyemun Gate over the past 10 years, offering valuable insights into the seasonal behavior of wooden structures. These data also serve as the basis for the displacement graph presented in the next section.

4. Discussion

4.1. Structural Stabilization Process

Wooden structures undergo a relatively large deformation immediately after construction and during stabilization. Looking at the observed movement of the hip rafter, we can see that it is steadily sagging downward (Figure 6). In particular, rapid and significant deformation is observed immediately after completion. Looking at the amount of downward movement of the HR1 point over a two-year cycle, a deflection of 22 mm was measured from 2014 to 2015, 5 mm from 2016 to 2017, and 6 mm from 2018 to 2019. The amount of deflection in the first two years is approximately four times the amount in each of the subsequent two years. It can be seen that relatively fast changes and large displacements appeared in the initial behavior. This sagging is a process in which the roof load gradually stabilizes the building’s structure. Traditional Korean buildings are built by assembling wooden members, so after the building is completed, the members are closely attached, resulting in acceptable deformation. In addition, deformation occurs as the wood used in the building dries and shrinks over a long period.

4.2. The Effects of the Seasonal Climate

The girder’s center and the hip rafter’s endpoint continue to sag and sometimes show recovery movements. Deflection is mainly observed in winter measurements, and movement to recover deflection is observed chiefly in summer measurements. Korea’s climate is primarily divided into a hot and humid summer and a cold and dry winter. The average temperature in Seoul in January, which entered full-scale winter in 2017, was −1.8 °C (the lowest daily average temperature recorded in January was −8.9 °C). The average relative humidity in January was 55%, and in March, the driest month, the average relative humidity fell to 48%. On the other hand, the average temperature in July, which entered summer, was 26.9 °C (the highest recorded daily average temperature in July was 29.9 °C), the average relative humidity was 77%, and July, during the rainy season, was the most humid month in 2017 (Figure 7) [13].

Wood is a material that swells in a wet environment and shrinks in a dry environment. The movement of the hip rafter’s endpoint intuitively shows the behavior in summer and winter, reflecting the characteristics of wood. In Figure 7, the hip rafter’s endpoint (HR1, HR2) clearly shows that the deflection measured through the winter until May 2017 showed a significant recovery in August, when summer arrives, and then sagged again in the winter. The girder’s center also appears to have recovered from deflection in the summer. Although there are differences in the recovery amount and start time at the 1G and 2G measurement points, there was a relative deflection recovery in the summer measurements (Figure 7).

The movement of the columns also shows that the displacement increased due to the expansion of the member during a hot and humid summer. The graph on the bottom left of Figure 7 shows the behavior of 2C5 around 2017 displayed in a coordinate system in the order of measurement. Point ④ measured in the summer of 2017 (August) shows movement away from the original point (0,0) of the measurement point, and point ⑤ measured in winter shows a behavior of returning to the original point (0,0), it seems. This behavior shows that the column’s lean is also affected by the difference in climate between summer and winter.

Meanwhile, in this phenomenon, displacements occurring in the member may appear to recover with seasonal changes. However, as can be seen from the measurement results, this recovery does not recover all of the displacement. The member repeats large deformation and slight recovery, eventually accumulating displacement in a specific direction.

4.3. The Influence of Viscoelastic Behavior of Wood

The seasonal member behavior discussed above can be assumed to be one of the causes of the mechanical properties of wood that respond to long-term loads. Wood has the characteristic of continuing to deform when a long-term load is applied, even if the magnitude of the load does not change [15]. Thus, various studies have been attempted to reveal the viscoelastic behavior of wood precisely [16], and as one of the results, research has been actively conducted to specify the mechanisms that determine creep deformation such as viscoelastic creep, mechanosorptive creep, and irrecoverable mechanosorptive creep and to present mathematical models for them [17,18,19,20].

Recently, a study was conducted to examine mathematical models reflecting this mechanism and to investigate the long-term behavior of wood in real climates with changing moisture content. This study presents notable conclusions regarding wet and dry periods of wood deformation. Studies have shown that the irrecoverable part of mechanosorption increases only during wetting periods. Therefore, mechanosorption and irrecoverable mechanosorption add up during wetting periods. However, during drying periods, mechanosorption develops in the opposite direction, up to a value close to the irrecoverable mechanosorption, thus significantly reducing the total mechanosorptive strains [21].

Meanwhile, when a constant load is applied to wood long-term, creep deformation can be divided into three phases. First, primary creep is characterized by a significant increase in creep after a load is applied and then a gradual decrease in the deformation rate. Secondary creep is a stage in which strain increases slowly over a long period. The final phase, tertiary creep, increases strain rapidly before failure occurs [22].

When looking at the trendline by calculating the displacement of each member in the behavior of the Sungnyemun Gate members, a relatively large amount of displacement is shown in the beginning. However, the displacement gradually decreases and converges to around 1 to 2 mm (Figure 8). This change trend in displacement is similar to that of secondary creep deformation in that the deformation rate decreases, and the strain rate decreases slowly. On the other hand, the direction of displacement occurs in opposite directions during the dry and humid seasons, reducing the total displacement (Figure 9). However, once a large displacement occurs, it is not entirely recovered, resulting in a continuous increase in cumulative displacement over time (Figure 6). This tendency of displacement occurrence can be attributed to the accumulation of mechanosorptive creep caused by repeated humidity changes, leading to long-term structural deformation. This tendency can be further explained in detail through recent research findings related to the mechanosorptive deformation of wood [21].

Of course, because adjacent members influence a member’s behavior within the entire structure, the change in displacement investigated through the movement of a point set on a member cannot be defined solely by the mechanical properties of the wood. However, since the trend of change in displacement reflects the characteristics of the viscoelastic behavior of wood that have been revealed to date, the mechanical properties of wood can be seen as one of the influencing factors on the overall behavior of the Sungnyemun Gate structure.

4.4. The Influence of Geographical Characteristics

The repetition of swell and shrink of wood caused by seasonal climate change leads to long-term structural deformation. Figure 9 shows a coordinate representation of the displacement of the x- and y-values of the points of the columns. Since the lower part of the column is fixed, the movement of the points set in the upper part indicates the column’s lean. The measuring points that significantly moved while showing a large amount of displacement before the structure was stabilized gradually converged to a certain point while reducing the amount of displacement (Figure 9 and Figure 10).

The analyzed columns generally tend to lean toward the interior of the building (southwest) and, at the same time, toward the southeast. This tendency is more evident in the second-floor columns than on the first floor. X marked in red in Figure 9 is the result of the last measurement. Comparing the first and second floors, the range of behavior on the first floor is smaller than that on the second floor, and the result of the last measurement is also closer to the origin on the first floor than on the second floor. This is because the first floor is close to the foundation, so horizontal deformation is relatively small. Furthermore, this is because the second floor is more affected by horizontal loads than the first floor, and the bending moment of the building accumulates more on the second floor.

The column displacement measurement results ultimately mean that the upper part of the structure moves generally to the south. The columns’ tendency to lean south can be interpreted as a phenomenon caused by Korea’s geographical characteristics. In Korea, the south is the direction with the most sunlight, which means that the drying rate in the south is relatively fast. Therefore, even for members of the same type, differences occur in the amount and direction of displacement depending on the location, and these differences combine to create directionality in the overall behavior of the building.

The Sungnyemun Gate is a building with its front facing southwest, so the east and south sides receive more sunlight than the west and north sides. Figure 11 is a diagram analyzing the amount of sunlight each facade of the Sungnyemun Gate receives during the summer and winter solstices. The upper part of the diagram analyzes the amount of sunlight on the south and east sides, and the bottom section analyzes the amount of sunlight on the north and west sides. The analysis results show that while the north and west sides receive almost no sunlight during the day, the south and east sides receive ample sunlight from around 11:00 AM to 5:00 PM.

The above results obtained from investigating the behavior of the columns for 10 years provide important facts for managing the deformation of the Sungnyemun Gate structure. Columns exhibit slightly different behavior depending on the load conditions or environment at each location, affecting the overall behavior trend of the structure. Thus, the behavioral characteristics of each column need to be identified and managed. According to the investigation results, the displacement of the upper part of each column continues to decrease, and the range of behavior becomes smaller than initially. Through this, the direction in which each column leans can be specified, and the direction of this deformation can be defined as a unique characteristic of each column’s behavior.

4.5. The Follow-Up Steps

The tendency of member displacement revealed in this study can be used to manage the structural stability of the Sungnyemun Gate. Displacement accumulation resulting from seasonal climate changes affects structural stability. As displacement accumulates, the column leans, causing height changes and vertical alignment problems, and the bending strength of horizontal members such as girders decreases due to increased sagging. Therefore, it is necessary to detect abnormal signs in advance through periodic monitoring and take measures to strengthen the structure. Behavioral characteristics, such as the tendency of member displacement, can be used to predict abnormal signs in the structural health monitoring of the Sungnyemun Gate.

The behavior characteristics obtained through 10 years of follow-up observation are important information for managing not only the Sungnyemun Gate but also Korea’s architectural heritage. In Korea, it is rare for long-term measurements like those at the Sungnyemun Gate to be made and the results to be revealed and analyzed quantitatively. Currently, in Korea, architectural heritage for priority management is being selected to monitor the transformation of wooden architectural heritage sites over a long period. The specifically revealed trends of structural behavior in the Sungnyemun Gate will be analyzed with other wooden cultural heritage monitoring results and used to manage the structural risks in Korean wooden architectural heritage sites.

5. Conclusions

This study observed the displacement of the structural members of the Sungnyemun Gate from 2013 to 2022 and proposed the behavioral characteristics and influencing factors. The findings are as follows:

• After the restoration was completed, a stabilization process accompanied by significant member displacement was observed for about 2–3 years.
• The behavior of the Sungnyemun Gate’s members showed a repeated increase and decrease in displacement depending on the season. Generally, the recovery amount due to the decrease was smaller than the increased displacement, leading to a steady accumulation over 10 years.
• Specific types of deformation and directional tendencies in the displacement of each member were observed. Hip rafters and girders consistently showed sagging, while columns displayed leaning. The direction and amount of leaning varied depending on their location, but columns generally tended to lean southward.
• The cumulative displacement of the Sungnyemun Gate’s members, which repeated cycles of increase and decrease, is closely related to creep deformation caused by seasonal humidity changes. Creep deformation accelerates under mechanosorption when humidity changes are added. When humidity changes repeatedly, mechanosorption and viscoelastic behavior act simultaneously, causing the deformation of the wood to increase nonlinearly.
• The general southward lean of the columns is thought to be influenced by Korea’s geographic characteristics in the Northern Hemisphere. In Korea, the southern side receives more sunlight than the northern side, leading to faster drying of the southern face. Differences in displacement patterns among columns based on location can also be attributed to variations in sunlight exposure.

These results will be used as primary data to manage the structural risks of Sungnyemun Gate in the future. Architectural heritage sites exposed to the outdoors are transformed and damaged over time. Therefore, exceptional management is required to prevent irreversible damage and pass the sites on to future generations. It is hoped that this study will be helpful for the management of architectural heritage in Korea and other countries that share the wooden construction tradition.