The loss of life and property in catastrophic events such as earthquakes, hurricanes, fires, or bomb blasts is mainly the result of damages to or the collapse of structures. Consequently, engineers worldwide are continually trying to validate structural models and iterate structural designs to reduce tragedies caused by these types of events.
In 2004, the China Earthquake Administration (CEA), the governmental body managing the country’s earthquake preparedness and disaster mitigation, selected seven newly constructed megastructures as the test bed for structural health monitoring (SHM) technology. These landmark structures include the 2008 Summer Olympic venues in Beijing (including both the Beijing National Stadium and the Beijing National Aquatics Center), the 104-story World Trade Center in Shanghai, the 66-story Park Hyatt Hotel complex in Beijing, the 240 m concrete arch dam in Ertan, the 8266 m cable-stayed bridge in Shantou, and the base-isolated CEA data center in Beijing.
The main objective of this major civil engineering project is to develop a state-of-the-art solution to monitor structural health characteristics, including stability, reliability, and livability, in real time by using contemporary computing, sensor, and communication technology.
CGM Engineering Inc., a California-based company, won the international bid to develop a solution for this project with a remote demonstration. During this demonstration, officials in China observed the real-time client application detecting the vibrations generated by a paper clip dropped onto a table. Subsequently, we designed nine 64-channel and two 36-channel embedded monitoring systems using CompactRIO embedded controllers that use remote network monitoring and configuration for deployment by the CEA.
Performing Continuous, Real-Time Structural Health Monitoring
Based on LabVIEW and CompactRIO, our systems are designed to capture the vibration signatures of a structure and detect any sudden shifts of structural characteristics. Detected vibrations can be caused by a variety of stimuli, ranging from natural geotechnical waves to event spectators. Much like cardiologists diagnose human heart disease by measuring pulse and blood pressure, structural engineers determine structural performance by continually monitoring the natural frequency, damping ratio, and hysteresis diagram derived from the acceleration time history that accelerometers measure. For example, if a high-rise office building incurred permanent deformation from an earthquake in its key structural members, such as beams or columns, there will likely be a decrease of its natural frequency (function of rigidity over mass).
Two key requirements for the system were continuous and real-time structural monitoring. Because most disasters strike in an abrupt and unpredictable manner, emergency management and effective reactions to sudden disasters must be based on real-time knowledge of how a structure performs during and immediately after adverse events. Additionally, because the health of structures gradually degrades over time, by performing continuous monitoring and capturing early symptoms of health decay, engineers can compare key health indicators against previously recorded levels.
Developing Structural Health Monitoring Systems with LabVIEW and CompactRIO
Using the NI platform, we developed two different customized systems to meet the CEA’s SHM requirements. Nine 64-channel and two 36-channel systems in a client-server architecture encapsulated in a rugged NEMA enclosure – which permits the systems to operate in high-humidity environments and in temperatures ranging from -40 to +70 °C –are deployed at critical points in the six selected sites throughout China.
The nine 64-channel units each consist of three CompactRIO systems, while the two 36-channel devices each contain two. Each device also incorporates multiple accelerometers for vibration measurements as well as a GPS receiver for real-time synchronization. We use the LabVIEW FPGA Module and the GPS disciplined clocks to achieve real-time, intrachassis synchronization within ±10 µs. In areas where GPS signals are not available, engineers can synchronize the systems using a computer clock. Additionally, we use the LabVIEW Real-Time Module for user-configurable filtering to improve the accuracy of the low-frequency measurements the system is taking and to prevent unwanted noise.
The acquired data are stored on embedded single-board computers (SBCs) within each system. By using the LabVIEW shared variable engine in the system software architecture, multiple users can remotely access and analyze recorded data concurrently, in real time, from the embedded SBC via the Internet. We can also configure the systems, using a single or multivariate architecture, to notify offline users via e-mail when an event has occurred.
Advantages of a System Based on NI Software and Hardware
There were several reasons the CEA chose our solution based on LabVIEW over our competitors’ solutions. Two key factors were the highly accurate, real-time GPS synchronization capabilities and the ability to remotely access data from any location in the world. Our system also offers the highest channel count for the lowest cost. By designing our system based on CompactRIO and modular NI C Series I/O hardware, we can achieve any channel count, up to 128, at an average per-channel cost of about $500 USD for a 16-bit system and $800 USD for a 24-bit system (excluding sensors), and use GPS synchronization to extend to higher channel counts. Additionally, our system offers a simple out-of-the-box setup with a variety of off-the-shelf I/O options that can be quickly and easily reconfigured to meet changing system requirements.
Using National Instruments hardware and software, we designed, prototyped, and deployed a high-channel-count, SHM system with GPS synchronization in less than one year. We deployed an embedded monitoring system with unmatched competitive accuracy, price, and flexibility by using LabVIEW and CompactRIO as the computing platform. With this combination, we provided the CEA a system that is 10 times more accurate than initially thought possible, at the lowest cost per system.
The Future of Structural Health Monitoring In China
According to The World Bank, by 2015, half of the world’s new building construction will take place in China. Because most large structures located in countries such as the United States were built before the development of advanced monitoring systems, the opportunity is now available to monitor these buildings and perform research that will ultimately help improve the safety of future buildings and reduce the number of lives lost from catastrophic events.
CGM Engineering, Inc.
882 N. Fairoaks Ave
Pasadena, CA 91103