A view of the tower at Cut River. The Mackinac Bridge and Cut River Bridge are part of a $400,000 ongoing data collection project that uses fiber optic gauges and sensors and wireless communication technology to monitor strains and stresses placed on the structures through normal use. It helps engineers track the lifespan of various bridge components. Powered by solar energy, it also tracks traffic and weather patterns.

The unique project uses a wireless transmission network to send data from the Cut River Bridge to the Mackinac Bridge and eventually to Lansing, is funded through combined state and federal money set aside for state planning and research and specifically for "intelligent transportation system" projects.

"This is one-of-a-kind, to my knowledge," said Steve Cook, an operations engineer working on the project with the Michigan Department of Transportation. "I know there have been bridges instrumented, but not combined with multi-use cases with data sent 25 miles via canopy wireless, then off to the Internet to a server for analysis. I don't know where that's being done to this degree anywhere [else] in the world."

The finished Bridge Monitoring System will send data collected at the Cut River Bridge 25 miles to a dish atop the Mackinac Bridge's south tower, which will transmit that and data collected on the Mackinac Bridge to the Bridge Authority office. From there, the data will be sent via an Internet connection to state highway department computer servers in Lansing and Kansas City. The $400,000 project also logs all data collected at each site and collects weight data from vehicles driving along US-2 one mile east of the Cut River Bridge. (Michigan Department of Transportation image) The project began with a study of the Mackinac Bridge walk for four days in 2007, an exercise that showed engineers just how the structure absorbs the movement of tens of thousands of walkers.

This graph shows 22,000 data points plotted every eight seconds along the X-axis and stress in two directions plotted along the Yaxis. This large downward spike that appears about one-third of the way into the 48-hour monitoring period shows stress on Span 23 of the Mackinac Bridge during the 2007 Labor Day Bridge Walk. Stress on bridge components during the annual event was found to exceed normal operating stresses, although was well within the 10,300 poundsper square-inch maximum stress allowed by bridge engineers for Span 23. (Michigan Department of Transportation image) A wireless network and four vibrating wire gauge sensors were first installed on the Mackinac Bridge August 27, 2007, and a test data collection period ran from September 1 to September 4, 2007, in which sensors collected stress data during the annual Labor Day Bridge Walk.

Data collection during the Bridge Walk confirmed the suspicions of bridge staff that the huge group of walkers cause a great deal of stress to the bridge during the annual event.

"The beams were actually lifting up in the air," said Bob Sweeney, executive secretary of the Mackinac Bridge Authority. "You wouldn't even notice it by looking at it, since it was such a minor change, but they did move."

As walkers approached Pier 23 along the north approach span, Span 23 between Pier 23 and the north anchor pier showed a slight lifting with a force of negative 2,100 pounds per square inch (psi). As the walkers continued south over Pier 23 toward the north anchor pier, the stress on bridge components changed from negative 2,100 psi to a force of positive 1,100 psi. Once walkers were off Span 23, the stress levels returned to their ordinary values as compared to the other times during the data collection period.

Engineers who built the bridge rated Span 23 for a maximum allowable stress of 10,300 psi, meaning the stress caused by the walk was only 20% of the allowable stress force. This amount of stress, Mr. Sweeney said, is considered acceptable, although it will eventually wear down the bridge components.

"It was significantly below what our threshold was," he said. "One good thing about the program is we found out we were well below our allowable strain on our beams."

Each beam is designed to withstand a certain amount of pressure a certain number of times, Mr. Sweeney said. This data, he said, will help the bridge authority better track the lifespan of its bridge components.

"Every loading that you have creates fatigue on a beam," he said of stresses like the Bridge Walk and normal daily traffic. "It sort of takes a little bit of the life out, but that's what they are designed to do."

Mr. Sweeney said they charted torsion, or twisting, forces during the walk, as well, part of the reason why bridge staff attempts to maintain a full load of cars in the western lanes during the walk. Keeping cars stopped or moving slowly on the other side, he said, helps balance out the weight put on the east side of the span by tens of thousands of walkers.

Before sensors were installed on either bridge, each structure was modeled using a finite analysis program, which determines which bridge components are main loadcarrying members under normal service conditions. These crucial pieces are then outfitted with the sensors, which monitor strain on the main load-carrying bridge components, which is converted to stress and compared to pre-existing stresses and loads created by bridge design.

Significant changes in these stress amounts can be set to trigger alert notifications, which would be sent to bridge staff and the state's department of transportation, engineers with the department said. Consultants specializing in structural analysis and design are on contract with Michigan Department of Transportation to analyze the data collected through this monitoring system and help engineers there interpret what it means for current and future management of the two structures.

When the Cut River Bridge on US-2 west of the Straits was repaired this fall, a similar system was put in place there.

A stress-monitoring system similar to the Mackinac Bridge's was installed on the Cut River Bridge during the project completed October 17, 2009. This system includes a wireless communication system and 16 fiber optic strain gauges installed to monitor stresses and strains on major bridge components; two environmental sensors to detect bridge deck moisture content, deck temperature, chloride content, and icing conditions; and two traffic sensors to determine traffic speed, volume, and occupancy.

Monitoring chloride content, triggered by the presence of calcium chloride, or road salt, is important for bridge decks, Mr. Sweeney said, because it is a factor in the degradation of the deck itself. The salt can penetrate the concrete road deck and corrode the steel reinforcement in it, which can eventually force repairs and replacement of the road deck.

The two towers installed at the Cut River Bridge also hold five solar panels to power the system, a Roadway Weather Information System (RWIS) to collect weather data, closed-circuit television cameras to monitor traffic and visibility, and a wireless weigh-in-motion station one mile east of the bridge on US-2 to weigh vehicles approaching the bridge.

The two towers near the Cut River, 30 feet and 70 feet tall, will collect all this information and transmit it over the tree line to the Mackinac Bridge, where it will be relayed to the Bridge Authority office and streamed through the Internet onto the Michigan Department of Transportation network based in Lansing. This connection to MDOT's Internet server is still being developed, but, when complete, the department plans to archive the data and use it with other collected data for a variety of purposes.

Mr. Cook said this can be used to increase the efficiency of weather data collection and reporting, help them better understand traffic congestion, keep road maintenance up-to-date, and assist the department with planning and asset management for maintenance, repair, and replacement projects.

"We want to use this data to support and enhance our current manual data collection systems," he said. Using machinery to collect the data, he said, makes it more efficient.

The state highway department plans to expand the wireless monitoring system on the Mackinac Bridge, as well, installing eight wireless strain gauges at critical locations near the south tower to monitor loads placed on the bridge 24 hours a day, seven days a week. This second phase of the project is expected to be installed next year, Mr. Sweeney said, although he acknowledged state funding concerns may slow its progress.

The new sensors will not be installed on the Mackinac Bridge until spring 2010, owing to winter weather conditions, and data collection on the suspension bridge will begin shortly after the installation is complete.