Engineered for Earthquakes: Preparing for the Next Big One
"The research project was 100 percent successful. There’s no question—we wouldn’t be doing these kinds of tests without National Instruments technology. All data was collected accurately and then processed and stored as required. Due to the size and cost of this one experiment, everything has to work perfectly every time you push the ‘start’ button, and NI hardware and LabVIEW helped us do exactly that."
- Dr. Patrick Laplace, Ph.D.,
University of Nevada, Reno
At the Earth’s surface, earthquakes manifest by shaking and sometimes displacing the ground. It is estimated around 500,000 earthquakes occur each year that are detectable with current instrumentation, and about 100,000 of those can be felt. Additionally, this natural phenomenon is inherently unpredictable. Therefore, to ensure humans remain safe during earthquakes, it is important to determine how man-made structures, such as bridges, will behave under stresses caused by earthquakes.
To simulate live loads on bridges under seismic conditions, University of Nevada, Reno (UNR), researchers constructed a lab with a to-scale curved bridge on hydraulic shakers with six pick-up trucks filled with 2 tons of sand on top of the bridges.
Dr. Patrick Laplace, Ph.D. - University of Nevada, Reno
Minor earthquakes occur nearly constantly around the world. With the San Andreas Fault, a major fault line located along the coast of California near one of the United States’ most populous areas, earthquakes are California’s costliest disaster, producing more than $60 billion USD in building and bridge damage and business interruption losses since 1971. Because of this, UNR researchers are focused on helping minimize the destruction earthquakes cause by testing bridges in earthquake-like scenarios in their large-scale bridge research laboratory outfitted with two 50-ton payload capacity shake tables.
UNR is now one of the leading research facilities for earthquake engineering, which involves the analysis and design of structures that support or resist loads, including service loads, such as traffic loads on a bridge, or extreme loads, like winds, floods, and earthquakes. Engineers usually design bridges using individual loading conditions such as live vehicle loads or earthquake loads, but they do not typically design for both of these types of conditions at the same time. Thus, UNR researchers and graduate students developed a series of live-load bridge tests that help practicing engineers better understand the effects vehicles have on a bridge during an earthquake.
Figure 1. Attaching Instruments and Cables
Earthquake and structural engineering involves the analysis and design of structures that support or resist loads. These loads may be “service” loads, such as traffic loads on a bridge or furniture loads in a building, or they may be “extreme” loads such as winds, floods, and earthquakes. Although you can model complete bridge systems in a computer, how accurate are your modeling assumptions without the physical tests to back them up? What happens if an earthquake occurs during rush hour, when the bridge is loaded with bumper-to-bumper traffic?
The series of tests performed allow UNR researchers and graduate students to calibrate their computer models and simulations, ebaling practicing engineers to understand the effects of vehicles on bridge response during an earthquake and may ultimately lead to changes in the design codes.
Figure 2: Conceptual Model
Figure 3: One of Three Bridge Decks "In Flight"
Researchers wanted the ability to create an earthquake at any time to measure its effects on both a bridge and the vehicles on top of the bridge. To achieve this goal, the engineers built a model of a curved bridge, spanning the entire length of the laboratory, on top of four MTS Systems shake tables. To fit everything in the laboratory, a 40 percent-scale model was used to represent a three-section bridge measuring 145 ft long and 16 ft high, with an 80 ft radius. To simulate live loads, researchers placed a series of six commercial trucks filled with sand on top of the bridge. The researchers wanted to see if these loads dampened or amplified the response of the bridge.
The data acquisition system needed to measure hundreds of conditioned channels in real time to ensure all the data was captured reliably. The shake tables, data acquisition system, controllers, and data analysis machines needed to work together flawlessly every time, and all of the systems needed to communicate and operate in lock-step with each other.
The NI LabVIEW Real-Time Module made creating a deterministic system a quick process since it can be easily programmed. A key element of LabVIEW Real Time is its unique ability to operate on multiple cores, where every core is utilized on every machine distributing the processing load to optimize the task as much as possible. To easily facilitate the inter-machine communication, researchers chose Curtiss-Wright/Systran SCRAMNet reflective memory, which allowed for shared memory between physically separate computers. Data written to one memory location is transmitted almost instantaneously to all of the other nodes through fiber optic cabling. This transmission is handled in the ScramNET hardware, and LabVIEW allows reading, writing, and IRQ monitoring, thus providing the required communication and clocking.
Figure 4: Example of Reflective Memory Operation
We used NI hardware, LabVIEW software, and the LabVIEW Real-Time Module to seamlessly connect the following systems, which are distributed over a large area:
- Four physically independent DAQ systems each containing
- 400+ instruments, all converted in house to TEDS plug-and-play sensors
- Two shake-table controllers with RM nodes
- Two Windows PC hosts running LabVIEW
- One single-core controller running LabVIEW Real-Time with two RM nodes acting as a bridge between two memory loops
- One quad-core controller running LabVIEW Real-Time with an RM node orchestrating shake-table commands
- One quad-core controller LabVIEW Real-Time with an RM node collecting all RM data for storage
- One dual-core controller running LabVIEW Real-Time with an RM node transmitting real-time data to the web
- Four NI Compact FieldPoint systems monitoring each shake table
- One NI CompactRIO system monitoring the shake-table hydrostatic bearings
- One single-core controller running LabVIEW Real-Time acquiring SMPTE and GPS timestamps
- One quad-core controller running LabVIEW Real-Time with an RM node creating timing signals for an external vision system
- One dual-core controller running LabVIEW providing instrument calibration checks
- Three Windows PCs running LabVIEW for analyzing post-test data
- Custom data analysis software created in LabVIEW for researchers and students
Figure 5: Example LabVIEW Analysis Software Built for Researchers and Students
Benefits and Outcome
NI hardware, along with the IEEE 1451.4 plug-and-play standard, greatly reduced setup time to minutes instead of days, and eliminated human error related to instrument calibration information. The LabVIEW Real-Time Module ensured all the systems ran synchronously and deterministically, a major requirement for the servo-hydraulic shake tables and data acquisition systems. The command generators programmed in LabVIEW Real-Time ran on schedule, successfully keeping the shake tables in sync. LabVIEW helped researchers analyze data instantaneously during testing and to instantly replicate critical data to be backed up on multiple FTP machines throughout the campus and NEES repository.
According to the researchers, this project was 100 percent successful and made possible by the National Instruments platform. All data was collected accurately, then processed and stored as required. The shake table system accurately produced the earthquake records required by the investigators, and the NI CompactRIO system successfully monitored the hydraulic system, ensuring any potential problems would be immediately recognized. Due to the size and cost of this one experiment, everything has to work perfectly every time you push the "start" button, and National Instruments and LabVIEW did exactly that.
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