Comprehensive and Automatic Monitoring of Footbridge to Mitigate Potential Structural Risks
The main campus of The Hong Kong Polytechnic University (PolyU) is located in Hung Hom, while the subsequently built Block Z is situated along Chatham Road. Separated by the multi-lane West Kowloon Corridor, the two campuses are linked by a footbridge, which is the main passage for students and faculty to commute to and fro.
Principles of Bridge Design and Multiple Aspects of Safety Consideration
Excellent bridge design primarily focuses on two key indicators: safety and functionality. Safety refers to the bridge's ability to withstand internal and external forces, thus avoiding the risk of failure or collapse. Functionality, on the other hand, pertains to its performance in fulfilling its intended purpose. For example, when pedestrians walk across the bridge, it should not induce severe vibrations that can cause dizziness.
Professor Xia believes that in addition to design, materials, and construction details, monitoring the surrounding environment and the bridge itself during its usage also plays a crucial role in ensuring the safety and functionality of the footbridge.
Apart from the suitability of materials and structural needs, an excellent bridge design should also take into account other external factors, including whether wind speed can cause turbulence, whether the number of vehicles and pedestrians exceeds the bridge's capacity, and whether the air and environment of coastal areas can cause corrosion of the structure.
Therefore, in general, engineers cope with extreme situations and reduce the probability of accidents by increasing the load carrying capacity of the bridge.
However, bridge safety is not only important during the stages of design and construction, but is also essential upon the completion of the construction with various indices being monitored so that extreme conditions can be dealt with promptly. As a result, Professor Yong Xia and his team from the Department of Civil and Environmental Engineering at PolyU have developed an interdisciplinary real-time monitoring system that continuously and automatically measures changes in data on the pedestrian bridge throughout the year.
Automated Collection of Real-time Data with Comprehensive Interdisciplinary Monitoring
By installing different types of sensors at various positions on the bridge, this system enables real-time data collection and analysis for various indicators. "Because there are too many external factors that affect the bridge structure, different sensors are needed to obtain the required data on a comprehensive scale," explained Professor Xia.
A total of 11 sensors are utilised in the system, which can be broadly categorised as follows:
Accelerometers: When pedestrians walk on and vehicles pass under the bridge causing vibrations, the level of comfort for pedestrians is compromised and the bridge's structural integrity can even be jeopardised. By installing eight accelerometers on the east and west steel beams of the bridge, accelerations in different directions can be measured.
Strain gauges: Certain external forces, including the bridge's own weight, pedestrians, temperature, wind, and earthquakes, generate internal forces within the bridge. If the internal forces exceed the allowable values, the bridge's safety can be compromised. By installing strain gauges at critical parts of the bridge, changes in internal forces can be detected.
Solar radiometers: Daily and seasonal temperature variations can cause structural changes in bridge components. However, due to the varying distribution of heat, it is difficult to install temperature sensors on every component. Therefore, Professor Xia installed a solar radiometer on Block X to measure the solar radiation intensity in the vicinity of the bridge, thereby estimating the temperatures of various bridge components.
Anemometers: Hong Kong is frequently attacked by typhoons, and excessive wind force and speed can cause the bridge to oscillate, lowering the level of comfort for pedestrians. By installing an anemometer on Block X, wind force on the bridge can be detected.
Weather station: Installed at the end of the bridge, the weather station monitors temperature, humidity, atmospheric pressure, rainfall, and other factors.
Fibre optic sensors: The bridge is equipped with two sets of fibre optic sensors. The first set consists of 20 interconnected fibre optic gratings that measure strain in the concrete bridge deck, while the second set includes a special fibre optic sensor for distributed Brillouin scattering analysis, which measures the temperature distribution of the bridge deck. Both sets of sensors are laid in parallel on the top surface of the bridge deck, forming a circular structure. The data is transmitted through the main optical cable to the fibre optic grating demodulator and Brillouin scattering analyzer in Block Z for analysis.
Global Navigation Satellite System (GNSS) receivers: By installing four GNSS receivers on the bridge deck sides and glass canopy, the three-dimensional movement of the bridge can be monitored in a continuous, real-time, automatic, and accurate manner to assess its functionality.
Displacement metres: The bridge undergoes deformation under forces. By installing a displacement metre at expansion joints on the bridge, displacements in the longitudinal direction of the bridge can be measured.
The weather station installed at Block X of the pedestrian bridge's end can monitor changes in temperature, humidity, atmospheric pressure, and rainfall.
Since the sensors detect different parameters, the accuracy of collected data is also influenced by the sensor positions. "Each discipline requires its own expertise and theory to maximise the effectiveness of each sensor," Professor Xia continued, "so it is indeed an interdisciplinary collaboration."
The collected data will be analysed using big data and artificial intelligence, and relevant campus departments will be notified in real-time in case of extreme situations. Additionally, the real-time data can be accessed on the screen next to the bridge and through the system's website, allowing pedestrians to see the changes in data as they cross the bridge.
Similar systems have already been implemented on the Hong Kong-Zhuhai-Macao Bridge, and the team was involved in the system operation, providing professional advice on sensor optimisation, data collection and analysis, as well as structural calculations. "By doing so, we can monitor the entire lifecycle of the bridge, from construction to operation, and have a more comprehensive understanding of its functionality and safety," Professor Xia said.
The screen next to the footbridge can display real-time data, including the vibrations generated when pedestrians cross the bridge. Moreover, the interactive platform stimulate the students’ and the general public’s interest in applied science.
