Structural Health Monitoring
Structural Health Monitoring (SHM) is the practice of identifying and tracking performance of a structure by analytical simulation and measured data. This project applied SHM to the Tacony-Palmyra Bridge (near Philadelphia), which was originally constructed in 1929. In general, structures that reach the end of their design life are replaced. However, structures of exceedingly high importance to society must be sustained. Therefore, for these structures the objective is to indefinitely preserve them by leveraging modern technology. The solution is a comprehensive SHM plan that addresses operational, maintenance, and engineering needs of the bridge. Our approach was to develop a computer based system to continuously monitor record and report important bridge parameters such as load, stress, weather conditions, etc. Identified issues and potential problems are then handled through maintenance and upgrades.
Drexel University, Princeton University, and Process Automation Corporation partnered together to develop the Tacony-Palmyra Bridge SHM system, as part of the Learning Bridge Project which is funded by the National Science Foundation. Drexel University was awarded the project with the objective to transform the Tacony-Palmyra Bridge into a living laboratory. Princeton University and Process Automation were added to the project as technical experts in SHM and LabVIEW based Supervisory Control and Data Acquisition (SCADA) software development, respectively. There are two primary objectives of the project: 1) help indefinitely preserve the bridge, and 2) provide engineering students ‘hands on’ bridge evaluation and testing experience.
Using two NI CompactRio (cRIO) systems and a server computer running NI LabVIEW software, we built and programmed a customized SHM that incorporates TrendSafe-History (PAC’s LabVIEW based SCADA software) to monitor and record multi-modal sensor data along with time-synchronized video images. During each significant bridge event, the images from four cameras are combined with sensor data, and stored in files. The stored files contain data taken before, during and after detected events. The files may be replayed and analyzed offline using a separate LabVIEW based Playback program.
We used the following hardware:
- National Instruments: Two cRIO industrial computers
- Axis Communications: Four IP Video Cameras
- Campbell Scientific: AVW200, Vibrating Wire Spectrum Analyzer and; AM16-32, VW Multiplexer
- Columbia Weather Systems: Orion Weather Station with MicroServer Internet interface.
The SHM system currently focuses on the two-leaf movable bascule system (260 ft span). The instrumentation installed on the bridge includes electrical resistance strain gauges (30 samples/second) and vibrating wire strain gages (3 seconds/sample), tilt sensors, temperature sensors, and a weather station. Resistance strain sensors are wired directly to each cRIO via NI9236 modules. The Campbell Scientific Vibrating Wire AVW200 Analyzer communicates via the cRIO’s serial port. All hardware except the sensors is mounted inside weatherproof enclosures attached to each bridge leaf. In addition, Axis IP cameras were placed at four strategic locations on and around the structure. The cRIO’s and cameras are then connected via Ethernet to a remote server computer that executes the LabVIEW based programs. The server supports an Internet connection for processing and broadcasting the data.
Tacony-Palmyra Bridge Server, Historian and Playback Software
The LabVIEW based SCADA software that was used as a platform on which to build the SHM is comprised of the following four main components.
- A Real Time Database can acquire and store instantaneous values for process variable data acquired from process sensors/actuators.
- A Data Historian that can compress, store, retrieve and display the current and past states of stored variable data, and display it via trend graphs and printed reports.
- A web portal program accessible from the internet to show the current status of the bridge.
- An Event Playback program (separate from the server) that can process the high speed data event files to create event displays consisting of camera images synchronized with trended stress data.
When a bridge opening event is triggered, the server copies all of the buffered data (i.e. 3 minutes long) to a new directory that is named for the triggered event. During an opening event, and for 3 minutes after, data is copied into the new opening event’s directory.
When a bridge overload event is triggered the server saves the currently active 15-second data file and the following 15-second data file. Event data may be downloaded manually from the server disk for playback and analysis.
Web Portal and Playback Program
Effective data visualization is a challenge in developing effective SHM systems. Data acquisition methods have become commonplace for structural monitoring systems. However, visualization of the data has proven crucial for maximizing the ultimate benefit of the system. It requires synchronization of the data from the many different sources and the creation of effective displays. For this project the integration of data and video was accomplished through the development of live web portals and a customized playback program using LabVIEW software. The live web portals allow for real-time remote viewing of the data and video over the internet. The structural monitoring software provides the ability to record important events such as bascule openings and overloaded vehicle passage. Both of these types of events can be viewed in the playback program using event file data. This program allows the data to be viewed both spatially and temporally so as to maximize data interpretation for the benefit of the end user. The system will be equipped with trigger and alert functionality to send e-mail or text messages for specified events. In addition, an automatic reporting feature will be incorporated that will generate time period based (e.g. hourly or monthly) reports, which will include short and long term reliability and health indices. The final goal is to better understand and maintain this invaluable structure.