1. Introduction
Sungnyemun as shown in Fig. 1(a)~(c) is currently No.1 national treasure of South Korea. It was built in 1398 - the early establishment stage of Chosun Dynasty - in the capital city of Chosun Dynasty, in the process of building new capital city. At that time, it took about three years to build Sungnyemun. Sungnyemun served as a main gate of Seoul and survived numerous wars and natural disasters for about 600 years. But, unfortunately, Sungnyemun was burned out by arson 10th Feb. 2008 as shown in Fig.1(d)~(e). Most parts of the 2nd floor were demolished, but fortunately most parts of the 1st floor survived the fire. After about five years of hard restoration work, Sungnyemun took back its original appearance as shown in Fig 1(f). In the restoration processes, traditional materials and construction methods were taken.
Researches on the Korean traditional timber structures have been carried out consistently by domestic researchers. Generally, researches were carried out on the (1) traditional timber residential house, which is called Hanok, (2) Buddhist temple building, and (3) palace buildings. Min et al.(2011) performed dynamic analysis for Heunginjimum, which has similar appearance to the Sungnyemun, and evaluated its structural capacity, and Han et al.(2005) and Seo et al.(1999) studied on the structural performance of tenon joints under lateral load. Hwang et al.(2009) performed shaking table test on the timber Buddhist temple building and evaluated its dynamic characteristics. Lee et al.(2006) and Lee et al.(2007) performed both experiments and structural analysis and evaluated lateral load capacity of Korean traditional timber structures.
Figure 1
The history of Sungnyemun-before fire and after restoration.
(Reference : (a)~(c) : Report on detailed measurement survey of Sungnyemun 2006, (d)~(e) :Yonhap news 2008)
As researches on the new-styled Hanok, recently built by the modernized construction method, Kim (2014a) performed both eigenvalue analysis and dynamic experiments for a two-storied new-styled Hanok and extracted its dynamic characteristics. Large sized timber, which is used as main structural members of Hanok, has characteristics of cracking and splitting as it dries. The most influential factor to the splitting of timber is moisture contents. Kim(2014b) monitored moisture contents for both traditional and new-styled Hanok and analyzed the effecting factors to the moisture contents.
In abroad, on researches to the timber structure, Kang et al.(2011) evaluated stiffness of mortise and tenon joint in Chinese traditional column and tie timber structure. Lindt et al.(2010) performed shaking table test on a light frame wooden building to evaluate seismic response. Fang et al.(2001a, b) performed full-scale shaking table test on an ancient Chinese timber structure to analyze load-displacement relationship.
In this research, structural analysis and safety evaluation for Sungnyemun was performed. Roof loads were calculated in detail, and analysis model was constructed using structural analysis software, Midas Gen ver.820. Static structural analysis under vertical loads was performed and safety of main structural members and serviceability of main horizontal members were investigated. To evaluate dynamic characteristics of Sungnyemun, eigenvalue analysis was performed using structural analysis software, and these results were compared with the field measurements of impact hammer test done by Prof. Hwang, from Cheonnam University.
2. Constructing the Analysis Model
Sungnyemun is two-story Korean traditional timber structure used as official building to guard the main gate to the capital city, Seoul. The size of Sungnyemun is 28m in width, 12.6m in depth and 11.4m in height as shown in Fig. 2. The main vertical loads acting on Sungnyemun is its roof loads. The roof load of 2nd floor is about 8.3kN/m2, and that of 1st floor is about 7.2kN/m2. The roof loads act as shown in Fig. 3. The timbers used in Sungnyemun are 1st grade Pine, and this was adopted in analysis model.
Due to the heavy roof loads, long-spanned main horizontal framing members of Sungnyemun undergo severe deflection which can be seen by naked eyes as shown in Fig. 4(a)~(b). And large sized timbers are used as the structural members of Sungnyemun, most members undergo severe splitting as shown in Fig. 4(c)~(d).
Fig. 5 shows column types of both on 1st and 2nd floors according to their locations and compositions method. Fig. 6 shows structural analysis model of Sungnyemun constructed by structural analysis software, Midas Gen ver. 820. In constructing structural analysis model, the construction methods and connection types were considered in detail. Fig. 6(a) shows the whole structural analysis model, Fig. 6(b) shows analysis model hiding roof envelope, and Fig. 6(c)~(d) show structural frames in both Y and X directions.
3. Results of Structural Analysis
3.1 Static Analysis
The static structural analysis of Sungnyemun under long-term vertical loads was performed by structural analysis software, Midas Gen. The overall deflection is shown in Fig. 7(a). In Fig. 7(b)~(d), deflection of main frames both in Y and X direction are shown.
Fig. 8 shows detailed deflection of peripheral main horizontal members. In Table 1, long-term deflections are compared both by field measurement and structural analysis. The field measurement was done by Prof. Hong, Seoul national University. Although structural analysis is very hard to simulate the actual situations, the results show that the structural analysis was done in a very accurate level. But, the deflection of ChangBang in south of 1st floor showed much difference between measured and analyzed deflection. It is thought that, the material properties of this member is somewhat lower than the ordinary members.
Table 1
Structural evaluation of long-term deflections
| Member | Measured Deflection(mm) | Analyzed Deflection(mm) | Allowable Deflection(mm) |
|---|---|---|---|
| South of 2nd floor | 33.0 | 27.4 | 29.2 |
| North of 2nd floor | 33.0 | 27.4 | 29.2 |
| South of 1st floor | 45.0 | 12.3 | 29.2 |
| North of 1st floor | 20.0 | 12.3 | 29.2 |
Fig. 9 shows axial forces of columns on both 2nd and 1st floors. In Table 2 and 3, axial stresses of columns on both 2nd and 1st floors are evaluated. The maximum axial stress ratio of columns on 2nd floor is 0.21 and that of the 1st floor is 0.28.
Table 2
Axial stress evaluation of 2nd floor columns
| Member | Diameter(mm) | Maximum Axial Force(kN) | Maximum Axial Stress(MPa) | Allowable Axial Stress(MPa) | Axial Stress Ratio |
|---|---|---|---|---|---|
| Type A | 524 | 265 | 1.23 | 6.75 | 0.18 |
| Type B | 524 | 306 | 1.42 | 6.75 | 0.21 |
| Type C | 554 | 307 | 1.27 | 6.75 | 0.19 |
Table 3
Axial stress evaluation of 1st floor columns
Table 4
Flexural stress evaluation of peripheral main horizontal members
Table 5
Shear stress evaluation of peripheral main horizontal members
It shows that columns have much room in their loading capacity. In Table 4 and 5, both flexural and shear stresses of peripheral main structural members are evaluated. The results show that the main horizontal members satisfy stress requirements.
3.2 Dynamic Analysis
Two methods were taken to evaluate dynamic characteristics of Sungnyemun. One method is field measurement using impact hammer test done by Prof. Hwang, Cheonnam University, and the results are shown in Table 6. The other method is eigenvalue analysis using structural analysis software. The mass for the eigenvalue analysis was constructed by converting roof loads and framing weight into masses, and it is shown in Table 7.
To perform dynamic analysis, the rotational stiffness of the member joints should be defined in advance. In this research, trial and error method was taken to define these relative rotational stiffness of the main structural joints between column and beam as shown in Fig. 11. By comparing results of eigenvalue analysis to those of the field measurements, 5% of relative rotational stiffness was chosen as an adequate value. The dynamic characteristics from eigenvalue analysis is shown in Table 8. Vibration mode shapes for the first three modes are shown in Fig. 12. Sungnyemun had its first vibration mode in X direction, second vibration mode in rotational direction and third vibration mode in Y direction.
Table 6
Dynamic characteristics from field measurement
| Mode | Natural Frequency(Hz) | Natural Period(sec) | Modal Participation Factors(%) | ||
|---|---|---|---|---|---|
| X | Y | RZ | |||
| 1st | 1.350 | 0.753 | 100 | 0 | 0 |
| 2nd | 1.603 | 0.622 | 0 | 0 | 100 |
| 3rd | 1.780 | 0.553 | 0 | 100 | 0 |
Table 7
Mass of Sungnyemun for eigenvalue analysis
| Floor | Mass(kN sec2/m) | ||
|---|---|---|---|
| Roof Loads | Frame Loads | Total | |
| 2nd floor | 306.2 | 66.1 | 372.3 |
| 1st floor | 194.8 | 64.7 | 259.5 |
| Summation | 501.1 | 130.8 | 631.8 |
4. Conclusions
In this research, structural analysis and safety evaluation for Sungnyemun was performed both by analysis software and field measurements. Analysis model was constructed considering in detail the actual construction method and materials. Structural analysis was performed by Midas Gen ver.820. As a result of static analysis, peripheral main horizontal members are somewhat dissatisfy serviceability criteria, but they satisfy stress requirements. Column members show much room in their stress capacity. As a result of dynamic analysis, Sungnyemun showed its first vibration mode in X direction, second vibration mode in rotational direction and third vibration mode in Y direction. The relative rotational stiffness of the main structural joints such as between column and beam was extracted by comparing dynamic characteristics of the field measurement to those of the eigenvalue analysis, and the relative stiffness of the joint was turned out to be 5%.
