1.

INTRODUCTION

China has a long history, left many ancient buildings with different styles. They were produced in different ages and they were the crystallization of the wisdom of people in different dynasties, and they showed the architectural styles of different dynasties, and reflected the development and progress of different dynasties. The existence of these ancient buildings not only shows the treasure of the great culture of the Chinese nation, but also provides a lot of valuable resources for the development of the domestic construction industry.

However, with the increase of the number of urban vehicles and the gradual formation of rail transit network, the vibration environment of ancient buildings in the central area of the city is gradually deteriorating. The impact of traffic vibration on ancient buildings is mainly reflected in the long-term lasting effect of vibration, which may not only lead to fatigue damage of fragile building components, but also lead to uneven settlement of foundation soil, leading to cracking and even collapse of ancient buildings. At the same time, due to the combined action of environmental erosion, natural disasters and human factors, the wood structure of ancient buildings will also suffer from roof leakage, tenon pulling of wood components and wall looseness, which will further lead to the decline of safety performance.

Therefore, how to protect the existing ancient buildings and repair the damaged ancient buildings scientifically and effectively is an important responsibility in the development of modern construction industry. The maintenance of ancient buildings has important historical and economic value.

2.

RESEARCH STATUS

For the protection and maintenance measures of ancient buildings, Zhai Xiaoyun¹ studied several improvement routes from the aspect of the protection and maintenance system of ancient buildings, such as establishing a diversified investment mechanism for the protection and maintenance of ancient buildings, establishing a long-term mechanism with “government investment as the main, supplemented by private investment”, updating the development concept, following the principle of “repairing the old as the old” put forward by Mr. Liang Sicheng, and exploring the protection system of ancient buildings in line with China’s national conditions. Rong Qingwen² based on the experience model of preventive protection of European architectural heritage and China’s actual national conditions, provided a feasible way for the regional overall preventive protection of Chinese architectural heritage, solved the actual and urgent needs, and also provided Chinese experience for the international practice network of preventive protection of international architectural heritage.

In view of the impact of vibration on ancient buildings, Zhang Yijing³ measured the vibration sources of rail transit lines, obtained the vibration acceleration responses of rails and sleepers of different sections in the tunnel when the train passed through, and studied the impact of line type and speed on the vibration of track structure. Xia Qian et al.⁴ systematically summarized the research methods of the impact of traffic vibration on ancient buildings, put forward the existing problems on the basis of summarizing and analyzing the current situation, gave relevant suggestions, and creatively put forward the idea of calculating the fatigue life of ancient masonry buildings. Ma Meng et al.⁵ analyzed and expounded the position of micro vibration control in the protection of cultural relics, proposed that the micro vibration control of ancient buildings should focus on the three values that do not affect cultural relics, and divide the micro vibration control level based on the vibration bearing capacity of ancient buildings and their components, and suggested that the formulation of vibration standards should be combined with the daily repair of cultural relics. Zhu Liming et al.⁶ took the ancient building drum tower in Nanjing as the research object, combined with the tunnel construction scheme of Nanjing Metro Line 4, established an analysis model by using ANSYS general finite element software, simulated and calculated the drum tower vibration caused by tunnel blasting excavation, and verified the feasibility and reliability of the numerical analysis method by experiments. Gao Yuhui⁷ took the wooden structure of the North palace gate of the summer palace in Beijing as an example to explore the vibration impact of the adjacent subway lines on the wooden structure of ancient buildings. Using the idea of control variable method, by changing the corresponding parameters in the model, he studied the impact of different factors on the vibration response of the subway operation of the wooden structure of ancient buildings. Zhang Kai and others⁸ took Xi’an Metro Line 2 crossing or bypassing national key cultural relics protection units as an example, obtained the vibration response level of the tunnel and ancient buildings since the metro operation through on-site monitoring, and evaluated the vibration reduction effect of the steel spring floating slab track bed.

In view of the application of modern technical means in the protection of ancient buildings, Xie Hui⁹ put forward the advantages of Bim in assisting the maintenance of ancient buildings and the difficulties in the application of BIM Technology in the protection and repair of ancient buildings. In combination with the architectural characteristics of Guanyin Pavilion in dule temple, he studied the selection of BIM software and model creation, the implementation process of Guanyin Pavilion maintenance information modeling, the role and precautions of Bim in assisting the maintenance of ancient buildings. Taking xiannongtan as an example, Sun Xue¹⁰ digitized the Taisui Hall of xiannongtan and established a three-dimensional display platform by using three-dimensional laser and skyline technology. Wang Fei et al.¹¹ used the extension set theory to take the health status of wood structures, evaluation indicators and their eigenvalues as matter elements. After normalizing the evaluation criteria and measured data, they obtained the classical domain, section domain, weight coefficient and correlation degree of the model, and then established the matter element model for health diagnosis of wood structures, which was reasonably applied to the safety evaluation of the Yellow Emperor hall in Qingcheng Mountain.

In summary, it is found that the current protection and maintenance scheme for ancient buildings is biased towards theoretical research or research on independent components such as vibration and frame structure, and lacks a system scheme for real-time state monitoring. With reference to the idea of active operation and maintenance of power system¹², research and implement the ancient building health monitoring system based on real-time situation awareness. By selecting the key monitoring indicators of the ancient buildings, using the sensor equipment to collect the monitoring data in real time, and designing the health evaluation model of the ancient buildings, the health status of the ancient buildings is dynamically calculated based on the real-time collected data, and the specific repair and maintenance work is targeted based on the calculation results.

3.

SYSTEM SOLUTION

The main function of the health monitoring system for ancient buildings based on real-time situational awareness is to dynamically evaluate the health of buildings through real-time monitoring of key parameters of ancient buildings. In addition, it can also generate corresponding evaluation reports and maintenance plans after disasters to guide the repair and maintenance of ancient buildings.

The architecture of the ancient building health monitoring system is shown in Figure 1, which is a typical three-tier architecture. From bottom to top, there are data acquisition layer, data processing layer and business application layer. The data acquisition layer realizes the data acquisition tasks of the sensors deployed at the monitoring points of the ancient buildings, including environmental monitoring (such as temperature and humidity, wind speed, wind direction, etc.), tilt monitoring, stress-strain, displacement monitoring, crack monitoring and vibration monitoring; After receiving the data collected by each sensor, the data processing layer performs unified analysis and processing, including data analysis, data storage, model calculation, health calculation, status monitoring and threshold alarm; The business application layer realizes the basic parameter configuration and data display of the system, including parameter configuration, monitoring information display, health display, statistics and reports, analysis and early warning, and maintenance plan.

Figure 1.

System architecture diagram.

4.

MODEL DESIGN

The hierarchical analysis model is adopted for the health monitoring and evaluation of ancient buildings. Specifically, AHP is used. For the current system, the apex of the health tree is designed as “ancient building health monitoring system”. The model is shown in Figure 2 below.

Figure 2.

Health assessment model.

As the top node of the ancient building health monitoring system, the system layer is divided into four modules: foundation, frame, environment and vibration. The lowest layer is the index layer. The foundation is mainly the bearing factor and settlement component, the frame is the displacement, inclination and crack component, the environment is the temperature, humidity and wind speed component, and the vibration is the vibration level component. The health value calculation process is from bottom to top. The data of the indicator layer comes from the real-time data collection and data analysis of the sensor. The quantitative indicators are divided into two types: benefit type (the larger the better) and cost type (the smaller the better). The health value of the subsystem is calculated from the indicator layer, and then the health value of the system layer is summarized and calculated from the subsystem layer.

The overall calculation process of the model adopts the layer-by-layer evaluation method of the health state of the system. The steps are as follows.

(a) Select evaluation indicator I

Based on the measurable indicators of the system, K key evaluation indicators are selected, which are respectively expressed as I1, I2 …IK. Meanwhile, the measured value range of each indicator is normalized to 0-1. For the benefit type (the bigger the better) indicator, 1 represents the optimal critical value, and for the cost type (the smaller the better) indicator, 0 represents the optimal critical value.

(b) Calculate the index evaluation matrix D

To simplify the calculation process, the health status of the system is divided into five levels (from excellent to poor, they are health, sub-health, mild illness, moderate illness, and severe illness), and the evaluation matrix of each indicator is determined by the equal division method. The result of D_i is [λ_i(1) λ_i(2) λ_i(3) λ_i(4) λ_i(5)], the value range of i is 1-k.

For benefit type indicators, c(1)=0, c(2)=0.25, c(3)=0.5, c(4)=0.75, c(5)=1,z is the measured value of the indicator, and the calculation steps of λ_i(s) are as follows:

For cost type indicators, c(1)=0, c(2)=0.25, c(3)=0.5, c(4)=0.75, c(5)=1,z is the measured value of the indicator, and the calculation steps of λ_i(s) are as follows:

(c) Calculate the health evaluation value HE

The process of calculating the health evaluation value is as follows:

First, calculate the immune sequence KX, which is [K₁ K₂ … K_k], where K_i is , the value range of i is 1-k.

Next, calculating the influence factor RE of the indicator layer on the element layer, which is KX x BX^T.

Next, the health evaluation value HE is calculated, which is 1–RE.

Table 1.

T.L.Saaty 1-9 Scaling method value description.

5.

EXAMPLE ANALYSIS

The calculation process of the model is illustrated by an example.

(a) Select evaluation indicator I

This example is illustrated by selecting four monitoring indicators in the system, which are respectively I1, I2, I3 and I4. Among them, I1, I3 and I4 are cost type indicators and I2 is benefit type indicator. The measured values of each indicator are normalized to the range of 0-1, and the measured data are 0.45, 0.85, 0.4 and 0.68 respectively.

(b) Calculate the index evaluation matrix D

After calculation, the evaluation matrix D of the four indicators is

(c) Calculate the health evaluation value HE

After calculation, the immune sequence KX of the indicator is [0 0 0 1], the influence factor RE of the indicator layer on the element layer is 0.11, and the health evaluation value HE is 0.89. The system health status is determined as level 3 (slight pathological). Meanwhile, the immune sequence KX shows that the indicator I4 is a weak link. Therefore, the next step can focus on the optimization of indicator I4 to carry out maintenance work.

In conclusion, through the case analysis, it can be found that the health status of the current ancient buildings can be effectively judged by selecting appropriate key monitoring indicators. Meanwhile, the immune sequence of the model can also be used to quickly locate the indicators affecting the health degree, and the maintenance program can be targeted in the future.

6.

CONCLUDING REMARKS

For the preventive maintenance of ancient buildings, by using the latest research results and technical means at home and abroad for reference, a real-time situation awareness-based health monitoring system for ancient buildings is researched and realized. By using the health evaluation model to monitor the health status of ancient buildings from multiple dimensions, the first is the change of surrounding geological environment and the change of foundation caused by urban excavation and construction, and the second is the impact of long-term environmental factors on components. The specific implementation is to dynamically generate the health monitoring report of ancient buildings through real-time monitoring of environmental categories such as temperature and humidity, wind speed, wind direction and wind pressure, and structural response categories such as local strain, crack development and beam column inclination. At the same time, after the occurrence of earthquake and flood, it can also quickly generate the evaluation report to guide the maintenance of ancient buildings. Through the application exploration, it is found that applying the health evaluation model to the maintenance practice of ancient buildings is conducive to promoting the maintenance of ancient buildings in China. Further, as the selection of key indicators of the model and the construction of indicator correlation matrix depend on certain experience, there is still a lot of room for improvement in how to design parameters more scientifically.