In late 2008, a fiber optic structural health monitoring (SHM) system was installed on the Chiapas Bridge in Mexico. This installation represents the first SHM instrumented bridge in Mexico using optical fiber sensors and SHM techniques. The project was conceived and managed by the Structures Department within the Institute of Engineering at the National Autonomous University (UNAM) in Mexico, under contract from the Mexican Transportation Undersecretary. The system was designed by MCH Engineering LLC, equipment and sensors provided by Micron Optics Inc. and the system installed by Chandler Monitoring Systems Inc. (www.chandlermonitoring.net).

Figure 1: Aerial view of the Chiapas Bridge in Southwest Mexico

The Chiapas bridge was opened to traffic in 2003 and is the most important structure of an entire highway going through the state of Chiapas, in Southwest Mexico, and part of the Transamerican highway system. The superstructure is a continuous grade 50 steel orthotropic box of constant height comprising 8 spans, with a total length of 1208m. Piers heights range from 27 to 89 m measured from the bottom of the dam. Steel jackets of the offshore type, were used as the main elements of the piers.

Figure 2: Aspect and topology of the installed sensor network inside the bridge

During construction of the superstructure, a basic monitoring system was initially implemented to monitor and assess the structural behavior of the bridge. However, advances in SHM techniques and the importance of the Chiapas Bridge in the region, prompted the need for a new and advanced instrumentation system. The new solution is based on a multi-point, multi-sensor monitoring system based on optical fiber Bragg grating sensors and opto-electronic interrogators. The system is self contained, works as a stand-alone equipment and allows for the in-situ, real-time monitoring of the bridge as well as its long-term condition. A total of 82 fiber optic strain and temperature sensors were installed in key bridge locations. Local electric power is supplied by a solar panel system.

The monitoring system instrumentation is composed of a single optical interrogator (Micron Optics model sm130-500) with 4 optical channels, scanning at 1khz; a 4×16 channel sensor multiplexer (model sm041-416) and a sp130 controller and data acquisition module (Figure 3). The system can be configured to record data at any specific interval and to any threshold level.

Figure 3: Micron Optics Interrogation System on Chiapas Bridge

Admin edited to add images 4/6/09

Applications of FOS technology are growing for industrial process monitoring. Engineers in plants producing glass, chemicals, metals, paper and plastics are using FOS to measure where other sensing technologies perform poorly or not at all. Fiber optic technology provides solutions for high EMI environments, areas where explosion risks persist, and where sensor density requirements make installation and cabling too expensive and cumbersome.

One recent example is an installation by Hatch Ltd. Hatch is a designer and supplier of custom designed furnaces and furnace components for the production of metals. They are using arrays of FBG sensors inside the furnaces used in smelting operations to provide a dense map of temperatures in the critical zones — information that can detect hot spots. FBG sensors provide superior temperature measurements and measurement density compared to thermocouples, which are traditionally used. This information guards against costly and potentially dangerous breaches of the furnace insulation systems and will allow maintenance to be driven by the actual furnace condition rather than a simple schedule for preventive maintenance. The use of FBG technology is part of an on-going effort by Hatch to develop a Diagnostic System (patent pending) for furnaces and other metallurgical reactors that will estimate the health or remaining life of the equipment.

Phil Shadlyn, a key engineer on this project, told me: “It really couldn’t have gone better. [Hatch's] sensor design worked as planned, and installation was smooth. [Our customer] was pleased with how easy it was to use the Micron Optics instruments and software. We will continue to monitor the installation and if everything progresses as planned, we’ll deploy several more installations throughout the next two years.”

One of Micron Optics’ largest market segments for optical sensing systems is for monitoring of bridges. Hundreds of bridges today are monitored using FBG based sensors, but >98% of these are in Asia and Europe. But, only a few structures in North America use this technology. Why?

I think one answer is resistance to change. Owners are more comfortable using their 35 year old visual inspection protocols than adopting better technology. And, there has been no FHWA mandate in the US that all bridges of certain classes or deteriorated condition must use objective measurements to support the visual bridge inspection process, unlike China.

An Atlanta company, LifeSpan Technologies, on behalf of our industry, is promoting the use of advanced technologies that are more precise and objective to help owners and funding agencies make better decisions. In brief, they conclude that deployment of advanced condition assessment technologies will lower both the risk and life cycle costs for bridge owners and taxpayers. I agree.

LifeSpan makes a solid case for their argument in their October 2008 white paper, A Better Way to Fix our National Bridge Problem. Find it here: www.lifespantechnologies.com.

This proposed solution is independent of the assessment technology choice. And since there are number of competent technology suppliers in this business, finding what you need isn’t that difficult. Most technologies have unique features, so it pays to do your homework and compare both technical approaches and costs.

I believe that it’s only a matter of time before FHWA and the state DOTs will begin to use these advanced technologies routinely. Proposed solutions like this one from LifeSpan are sure to help accelerate the process.

Tom Graver
Director, Optical Sensing Group

Most commercialized fiber optic sensors operate at temperatures between -40 and 150 C. Higher temperatures can degrade polyimide fiber coatings (above 250 C) and erase FBGs from the fiber core (above 400 C).

Some industrial applications (e.g., oil refining, steel production and chemical formulation) require much higher temperatures, and often conventional thermocouples do not last long enough or do not operate reliably due to high EMI or cumbersome cabling.

Several research organizations and commercial companies are working to develop high temperature optical sensors that are both accurate and reliable in high temperature environments. One such company is Chiral Photonics, Inc. in Pine Brook, New Jersey. They recently announced a novel optical sensor that uses twisted fibers to create a thermally sensitive spectral response. The sensor appears to be stable, accurate and repeatable at temperatures up to 1000 C.

Find details at: www.chiralphotonics.com/

Now that the optical core is nicely characterized, Chiral is working on ruggedized packaging for large-scale field deployments. Sensor readings are made simple with self-contained laser instrumentation and software from Micron Optics — so the user does not need optical systems expertise to use these sensors. They are as easy to use as conventional sensors.