STRAND GAUGE Optical fiber is glued on prior to erection. (Photo courtesy of Mouchel, PLC)

Europe's first public highway bridge to be built with structural plastics opened Oct. 29 near Oxford, England. It not only pioneers the use of composite materials on an active highway river crossing, but also is a test bed for a novel fiber optic structural monitoring system.

The monitors on the 10-m-long West Mill Bridge use optical strands just 0.0125 cm thick that have been laser-etched with grating patterns in 8-cm-long segments at various points on their lengths. The fibers are bonded to the surfaces of structural elements.

When light is passed through the strands, it reflects at the etched areas in wavelengths that shift as those areas change length. The shifts can be interpreted to accurately represent structural strain at those locations on the fiber, says Beverley Meggit, chief executive officer of the system's London-based developer, EM Technology Ltd.

Strands can be etched in several places and can monitor multiple locations. The number depends on the bandwidth of light used to light up the fiber, says Meggit. The West Mill fibers, for example, monitor up to eight locations each.

The fibers, within protective sleeves, are bundled in groups of eight into 0.3-cm-dia cables that converge from various parts of the bridge at a small junction box on one side. A single cable runs from there to a hut on the river bank and a computer that processes the signals. Software converts the torrent of responses into structural monitoring data that can be accessed over the Internet.

In trials last year, EM Technology attached monitors to the steelwork of a Norwegian bridge. West Mill expands the potential by having the fibers built into structural elements made of polymer reinforced with glass and carbon fibers. EM Technology does not have a Website, but can be reached by telephone at 44 20 8766 6292.

Having validated the optical system with conventional gauges, Sam Luke, a director of the bridge's designer, Mouchel Group, West Byfleet, is happy with the results. "We think it will be a fantastic way to monitor structures," he says. "We would consider it for some of our large infrastructure clients."