Traditional bridge monitoring systems often require wired connections between sensors, a data acquisition system, and data center. The use of extension wires, conduits, and other costly accessories can dramatically increase the total cost of bridge monitoring. With the development of wireless technologies and the notable cost benefits, many researchers have been integrating wireless networks into bridge monitoring system. In this study, a wireless bridge monitoring system has been developed based on the Sub-1 GHz network. The main functional components of this system include sensors, wireless nodes, gateway and data center. Wireless nodes can convert analog signals obtained from the sensors to digital signals, then transmit the collected data to the gateway using the Sub-1 GHz network. The gateway receives and sorts data from different wireless nodes and then forwards these data to the data center wirelessly. All collected data are processed in the data center using the data processing software developed in this study. In order to validate the performance of the wireless system developed in this study, a steel girder bridge was monitored in the field during the concrete deck construction. The field results were also compared with the theoretical values obtained from finite element models to ensure the accuracy and reliability of the wireless system. The results indicate that the wireless bridge monitoring system developed in this study is effective and affordable. The Sub-1 GHz network can be a better solution for bridges with complicated site conditions because of the extended data transmission distance. Although the power consumption can be controlled by using low-power consumption components, including the power control in software design can also dramatically reduce the system’s power consumption.
Long-term or short-term bridge monitoring often requires many sensors and data acquisition systems (DAS). The traditional bridge monitoring system uses extension wires to connect sensors with the monitoring system, conduits to protect the wires, and other costly accessories.
One of the major concerns for developing a wireless bridge monitoring system is the data transmission approach. With the rapid development of wireless technologies in recent years, many wireless transmission methods, such as Bluetooth, WIFI, Zigbee, cellular network, and Sub-1 GHz, can be implemented when integrating wireless sensor networks into a bridge monitoring system. The data transmission rate, range, power consumption, and overall reliability are different for each method. A comparison among different commonly used wireless technologies is shown in
Bluetooth low energy | WIFI | Zigbee | Cellular (2G/3G) | Sub-1 GHz | |
---|---|---|---|---|---|
Frequency | 2.4 GHz | 2.4 GHz |
868/915 MHz |
850/900 MHz |
433/868/915 MHz |
Nominal range | 50 m (164 ft) | 10–100 m (33 to 328 ft) | 10–100 m (33 to 328 ft) | Cellular network | 2.5 km (8202 ft) |
Maximum signal rate | 1 Mbps | 54 Mbps | 250 Kbps | 10 Mbps | 4 Mbps |
Power consumption | Low | Medium | Low | High | Low |
In this paper, a low-cost bridge monitoring system using the Sub-1 GHz network was developed for both long-term and short-term bridge monitoring. The system mainly includes four functional components: sensors, wireless nodes, data gateway, and data processing center. Multiple sensors can be attached to a single wireless node, and the data can be transferred to the gateway through the Sub-1 GHz radio network. The computer located in the data center processes the raw data coming from these wireless nodes, turning them into the desired format. Furthermore, this system was implemented during the deck construction of a bridge, and the results were validated by comparing them with the computer simulation using finite element (FE) analysis. The comparison indicates that the system developed in this study can provide reliable and accurate results during bridge monitoring.
The design of the wireless bridge monitoring system considers data transmission distance and rate, power consumption, cost, and reliability. In this study, the wireless bridge monitoring system mainly consists of sensors, wireless nodes, gateway, and data processing station, as shown in
The wireless nodes collect analog signals directly from the connected sensors and convert these signals into digital ones in a binary format. After each measurement, the binary data are transmitted from the wireless nodes to the gateway that is generally connected to the data station. Sub-1 GHz radio network was selected for the data transmission between the wireless nodes and gateway because the range can be up to 2.5 km (1.55 miles). The gateway can also be used to extend the wireless transmission distance through the Sub-1 GHz or cellular networks.
The gateway in this system accepts and sorts the binary data based on the address of each node. The data processing software installed in the data station is designed to convert binary data to decimal format and store all the decimal data into the data station. Another essential function of the data processing software is to control the wireless nodes and gateway to reduce power consumption.
The wireless node development includes two parts: hardware design and operating system (OS) design. The hardware design of the wireless node is shown in
The material used in developing the wireless node is shown in
Component | Name/Model | Manufacturer | Quantity | Cost (USD/Unit) |
---|---|---|---|---|
Power supply unit | 18650 battery | EBL (East Hartford, CT, USA) | 2 | 3.00 |
Voltage regulator | Seeed studio (Shenzhen, China) | 2 | 4.50 | |
Power control unit | MOSFET power control kit | SparkFun (Boulder, CO, USA) | 2 | 4.00 |
ADC | ADS-1256 PCB | Texas instruments (Dallas, TX, USA) | 2 | 22.00 |
MCU & radio transmitter | CC1310 launchpad | Texas instruments (Dallas, TX, USA) | 1 | 29.00 |
The development of the operating system for a wireless node needs to consider the power consumption and the communication between the node and the gateway. The general workflow is shown in
In order to receive data from the wireless node, the CC1310 Launchpad is also used as gateway. The gateway can also send commands to wireless nodes and communicate with the data processing software. The workflow of the operating system for the gateway is shown in
The data processing software includes three different functional modules: “Check Status”, “Data Collection”, and “Stop”. The workflow for all three modules is shown in
In order to validate the wireless bridge monitoring system as well as evaluate the exterior girder rotation during concrete deck placement, a bridge located in the state of Illinois was selected to be instrumented with different types of sensors during construction. The bridge is a three-span continuous bridge with span lengths of (23.5 + 29.3 + 23.5) m (77 + 96 + 77 ft). Six American W40 steel girders with a spacing of 2.21 m (7.25 ft) between each girder were utilized. In order to reduce the rotation in the exterior girder, pipe-tie systems [
Parameter | Value |
---|---|
Beam type | W40 |
Beam depth | 40 in (102 cm) |
Skew angle | 0 degrees |
No. of girders | 6 |
No. of span | 3 |
Span length | 23.5 m + 29.3 m + 23.5 m (77 ft + 96 ft + 77 ft) |
Overhang width | 1.05 m (3.46 ft) |
Girder spacing | 2.21 m (7.25 ft) |
Diaphragm type | C15 × 40 |
Temporary rotation prevention system | Pipe-tie system |
Tilt sensors and strain gauges were installed on the bridge to evaluate the rotation of bridge girders during deck construction. The locations of the sensors were determined based on the preliminary FE analysis results, which indicated that the largest transverse girder rotation occurs in-between the diaphragms that have the maximum longitudinal spacing; hence the girder transverse unbraced length [
The tilt sensors, CXTLA02 manufactured by MEMSIC Inc. (Andover, MA, USA), were used to evaluate the rotation of exterior girder during deck construction. These sensors were protected with open aluminum boxes, shown in
Foil strain gauges (CEA-06-125UN-350) manufactured by Micro-Measurements (Raleigh, NC, USA) were installed at the bottom flanges of the exterior girder and first interior girder, as shown in
All sensors were connected to the wireless nodes developed in this study.
The wireless node measures the voltage changes in the tilt sensor; therefore,
Because voltage changes for strain gauges are generally very small, the Wheatstone quarter bridge circuit was used to detect the small voltage changes in strain gauges. Resistive bridge adapters, 4WFBS350 manufactured by Campbell Scientific (Logan, UT, USA), were connected between the wireless node and strain gauges. Therefore, the strain values can be computed using
In the equation, G is the gauge factor, and
The field monitored rotation data using the wireless bridge monitoring system are shown in
The strain values were measured in the diagonal pipes to evaluate the performance of the pipe-tie rotation prevention system as well as the wireless bridge monitoring system. The field results of the strain in the diagonal pipe are shown in
The strain in the bottom of the exterior and first interior girders was also monitored during deck construction. The results are shown in
Both rotation and strain data show very low noise during monitoring, mainly due to the use of the low-noise ADC. The ADS-1256 ADC used in this study offers outstanding noise performance that can be optimized by adjusting the data rate or programmable gain amplifier (PGA) setting. When the data rate is programmed using 100 Hz and the PGA is set as 64, the system can convert an analog signal to a high-resolution digital signal with an effective number of bits (ENOB) at 20 [
In order to validate the performance of the wireless bridge monitoring system, the field monitoring results were compared with results from a FE analysis. The FE model was established using SAP2000 software and has been verified in previous research studies [
Structural component | Element type | Number of element |
---|---|---|
Steel girder | Shell | 52,513 |
Diaphragm | Shell | 25,204 |
Overhang bracket | 3-D frame | 750 |
Screed machine rail | 3-D frame | 3,120 |
Tension only 3-D truss | Transverse ties | 620 |
Compression only 3-D truss | Diagonal pipes | 1240 |
Since it is difficult to confirm the exact locations of the construction equipment from the field data, the comparison between the FE and the field results is conducted by comparing only the maximum rotation and strain/stress values to assess the accuracy and reliability of the wireless monitoring system.
The comparison between FE and field strain/stress results is shown in
In this study, a Sub-1 GHz network-based wireless bridge monitoring system was developed. The main functional components in this system include sensors, wireless node, gateway, and data center. The wireless node collects the analog signals directly from sensors, and transmits all the data through the Sub-1 GHz network to either the data center directly or the gateway. The gateway can receive and sort the data from all the wireless nodes, and then forward data to the data center using the wireless network such as cellular or Sub-1 GHz network, depending on the communication distance. To evaluate the performance of the system, a bridge was instrumented with different types of sensors and the wireless monitoring system developed in this study during the deck construction. The following conclusion can be drawn based on the findings of this study. The wireless monitoring system developed in this study is demonstrated to be an effective and cheap solution for bridge monitoring. The condition of the bridge site can be very complex and sometimes requires long data transmission distances. Sub-1 GHz network can be a better solution for bridge monitoring since the data transmission distance can be several kilometers and does not require a cellular network. The communication distance between the wireless nodes and the data center can be extended by using the gateway. The gateway forwards all the received data to the data center using the Sub-1 GHz network or cellular network, depending on the distance between the gateway and the data center. High accuracy ADC should be used in the wireless bridge monitoring system in order to obtain high accuracy and reliable field results. When connecting strain gauges, the Wheatstone bridge circuit should be used in order to detect the minor changes of the voltage/current in the strain gauges. Power consumption can be controlled by using low-power consumption components. Besides, the design of the software, including the OS for the wireless node, gateway, and data processing software, can dramatically affect the power consumption of the system.