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Blog
What can be done to monitor bridge structural condition?
August 27, 2007
The short answer is: A lot!
The problem, of course, is that bridges are great big things with important bits that are difficult (spelled “d a n g e r o u s”) to inspect visually and often impossible to access directly. What you really want to do is monitor strain building up in structural components, applicable to factory automation and process control applications, as well.
Strain, for those who’ve forgotten their Intro to strength of materials class, is the percentage deformation of a sample. The sample, of course, doesn’t have to be sitting out on a lab bench, ready to go into a Universal Testing Machine. The important samples are any portions of a bridge’s structure carrying loads—specially, those carrying loads in tension. Given half a chance, structural members fail under tension.
Anyway, as bridge structures deteriorate, they develop microscopic cracks in those parts under tension. Those cracks allow the material to yield (strain increases), which acts to relieve some of the tensile stress.
Initially, strain accumulates rapidly as the weaker areas within the structure yield, transferring some of the stress onto stronger parts. Unless the design, construction, or material was badly flawed, however, the structure quickly settles into a stable configuration where stress is carried by those parts most able to bear the load. This configuration should last for decades or even centuries. Some Roman structures have been in this state for over two millennia.
Overall strain accumulation rate changes over time by many orders of magnitude.
Stress does, however, continue to accumulate imperceptibly. Eventually, tides and time have their way, and the structure begins to weaken. Strain starts to accumulate at a faster—and accelerating—rate. Finally, the strain accumulation rate becomes rapid, leading inexorably to failure.
What we want to do is catch that transition from stability to deterioration before it becomes too far advanced. Therefore, the best way to catch structural deterioration is to slap strain gauges on the bits carrying the most tensile stress, and chart strain versus time over a long time.
Here, we’d best consult a current article by
“On-line monitoring and sensor fusion can cover failure mechanisms and unpredictable deterioration. Physical Acoustics Corp. (PAC), a member of Mistras Group, has developed digital wireless sensor technology that assists bridge operators in preventing bridge failures. PAC applies sound technology called acoustic emission (AE) for bridge monitoring. Cracks can be heard from remote distances by using AE sensors similar to using geophones for earthquake detection.
‘It is only because of state DOTs and government funding that has prevented wide scale deployment of on-line monitoring using sensor fusion and wireless technologies on bridges,’ states Dr. Sotirios J. Vahaviolos, founder, chairman, and CEO of PAC and Mistras Group. ‘We have the technology for bridge health monitoring now and are ready to team up with bridge operators in order to prevent catastrophic failures from naturally occurring structural flaws that are the result of an aging infrastructure,’ concludes Dr. Vahaviolos.”
For related news, see this
Structural testing: Sensors, computer modeling, wireless technologies can help
Posted by Charlie Masi on August 27, 2007 | Comments (2)
In response to: What can be done to monitor bridge structural condition?
Remote Monitoring and data logging commented:
You cannot control what you have not measured. In order to understand the material performance of bridge or other structural materials, you need to monitor their performance under real-world conditions. This requires instrumenting the structure and recording the relevant data. A variety of remote monitoring stand-alone dataloggers exist on the market today. There is a large amount of University research in the US that is funded by the DOT to do just that. But with the aging of our infrastructure, more monitoring is required to ensure the safety of our bridges. www.datataker.com
In response to: What can be done to monitor bridge structural condition?
Install Strain Gauge Sensors commented:
I was inolved with installing a strain gauge monitoring system to use for measuring the build up of ash on boiler tubes. The process involved strain sensor epoxied to the sides of the hanger bolts and sent to a SCADA package to monitor the strain at each area. Strain gauges could get imbedded into the rebar to measure stresses on the bridge at critical areas. This could be sent to a website or a system monitor to when there is a problem.