A midwestern research laboratory has installed what is believed to be the first global remote monitoring system on a highway bridge. The system-installed on a rolling bascule bridge over Sturgeon Bay in Door County, Wis.-allows engineers located at the laboratory in Evanston, Ill., to closely monitor crack propagation and other conditions on the bridge, which is near the end of its design life.
The system, installed by Northwestern University's Basic Industrial Research Laboratory (BIRL), is intended to serve as the prototype for a new method to monitor the condition of aging bridges in order to ensure public safety and allocate scarce rehabilitation and replacement funds in an optimal manner.
A Federal Highway Administration (FHWA) study has shown that 42% of the 542,000 bridges in the U.S. are either structurally or functionally deficient. Similarly, 40% of the total square footage of U.S. bridges are 15 to 35 years old-prime candidates for major rehabilitation. Nowhere near the level of funding required to repair these bridges is expected to be available anytime soon. It is imperative, therefore, that a method be found to evaluate the condition of these bridges and, where a problem is discovered, continuously monitor their condition to detect further degradation long before failure occurs.
One of the strongest non-destructive evaluation (NDE) teams in the country has been formed at Northwestern to address this problem. Under the Bridge Condition Monitoring Program, this team is engaged in:
- Improving NDE equipment and techniques for steel and concrete bridges,
- Guiding users in proper utilization of these methods, and
- Commercializing equipment and services for NDE of bridges.
This method was recently applied for the first time on the Sturgeon Bay lift bridge built in 1930. Each span, including counterweights, weighs over 1,500,000 lb (680 tons). The mechanism that drives the bridge is nearly worn out. The bridge is scheduled for replacement in several years. A visual inspection found a considerable amount of damage, particularly in the lift mechanism.
The basic approach to global monitoring pioneered by BIRL requires the use of one or more remotely mounted data acquisition systems linked by RF transceivers to a host computer connected to a phone line. Concern about pier movement made it necessary to monitor sites on both sides of the shipping channel. These sites could not be hard wired to a single data acquisition system so the RF transceivers were used on each side to individually communicate with the host.
The host sits in the operator's shack on the bridge. The host computer in the bridge operator's shack is connected to a dedicated phone line that makes it possible to monitor each lift-and-lower cycle from the university 240 miles away or at the Wisconsin Department of Transportation (WISDOT) headquarters in Madison.
A major obstacle to the development of such a system in the past was the unavailability of a suitable data acquisition system. A key requirement is being able to withstand the extremes in temperature, moisture and vibration associated with a lifting bridge. The system needs to be small and self-contained so that it can be hidden away from prying hands. Finally, the system has to be capable of reducing the vast amounts of data generated in unattended bridge monitoring.
The SoMat Model 2100 field computer system from SoMat Corp. of Champaign, Ill., met these requirements. The Model 2100 is a very compact and self-contained data acquisition and analysis system consisting of compact, stackable modules with a common bus architecture. Complete signal conditioning for many analog transducers, including strain gauges and tilt meters, are available in individual modules.
A lithium battery-backed memory ensures the safety of the data even if the host computer or RF transceiver should go down. SoMat's Test Control Software (TCS) includes a networking feature that allows multiple Model 2100 field computers to communicate with a single host.
The field computers are located in shielded boxes attached to opposite sides of the bridge structure. Locating the field computers on the bridge structure avoids the need for stringing wires around the bridge that will inevitably be damaged. Stringing wire also is an expensive and potentially dangerous activity when instrumenting bridges. BIRL engineers mounted a combination of strain gauges and tilt meters to the track castings on the bridge. Each field computer system is comprised of a total of six channels, four strain gauges and two tilt meters. The field computers communicate with the host computer via a pair of SoMat Wireless Data Links (WDL), a rugged spread-spectrum RF transceiver. With the proper antennae, a WDL pair is capable of communication and control of a Model 2100 at distances up to one mile from the host computer.
Data reduction is accomplished by configuring the field computers to acquire burst histories on both opening and closing cycles. This is done using the 2100's test control software running on the host computer.
The remote monitoring system has recorded each of the 20 to 25 lift cycles performed each day since July 1995. Once a day, the data is analyzed and engineers look for any changes that would indicate that the bridge's condition has worsened. The field computers and TCS also provide a real-time strip chart DataMode that makes it possible to watch the bridge lift cycle in real time. With a notebook computer and a cellular phone, engineers can view the real-time bridge data from any location.
Early results showed both sides of the lift mechanism working satisfactorily. In the first week of August 1995, changes occurred, at first sporadically, in one of the strain gauge readings. The use of multiple redundant sensors meant that the failure was detected on several sensors, thus eliminating the possibility of a sensor failure.
The damage worsened steadily, so BIRL engineers visited the site and took boroscope readings through holes in the structure that had been put in to stop other cracks. They saw cracks growing in the bottom of the casting and moving towards the strain gauge area. These cracks would have been virtually impossible to detect by visual inspection. This is believed to be the first time a crack was detected on a bridge as soon as it formed.
Since then, engineers have closely monitored the progress of the crack. The castings have keys that mate with openings in the girders to raise the bridge. If allowed to progress unimpeded, the crack would have eventually destroyed a tooth, jamming the bridge in the semi-open position. This would have made it impossible for cars to use the bridge and also blocked shipping in the channel. The damage would not have been serious enough to raise the possibility of catastrophic failure. At the same time, WISDOT has accelerated the replacement of the casting with a machined steel plate. The design of the plate will be optimized based on the data from the monitoring system.
Currently plans are to maintain the monitoring system for the life of the bridge, which is projected to be about six years. When the new plate is installed, strain gauges will be attached to it to provide baseline readings, which were unavailable in the past because the exact physical properties of the old casting were unknown.
