Brussels, Jun 2003
New hardware and software tools are being developed by the IMAC project to allow rapid assessment of the state of Europe's many bridges. They will help to keep the Trans-European Networks open to traffic.
Our main networks of transnational, primary and secondary roads in Europe depend on no less than three quarters of a million bridges. A high proportion of them were built over 30 years ago and are close to a critical stage when they will need more inspection and maintenance to guarantee their continued service. A huge peak of repair and retrofit investment is expected to be needed from 2005, which could cost up to three times the present value of such work.
The Commission-funded IMAC project was designed to help bridge owners cope with this increased burden. It is developing tools and related software to monitor bridge cables and assess their condition. It is also devising a new methodology to assess the condition of the large number of pre-stressed structures. The results will permit repair at the most effective time, avoiding unnecessary strengthening. This will help to keep Europe's traffic moving and reduce the congestion and disruption caused by the often lengthy diversions that are due to unserviceable bridges.
Many kinds of cable
The stay cables that support bridges may be clearly visible as in cable-stayed bridges, suspension bridges, or hidden within concrete box girders. Because of the problems of inspecting them, some countries have banned the use of pre-stressed internal cables.
"Modern bridge cables are composite," explains Dr Helmut Wenzel of VCE Holding in Austria, the prime contractor for the IMAC consortium. "They have a core of twisted wire rope made of high strength steel, covered with a material like PVC or polyurethane. They resist corrosion but visual inspection of the steel is impossible. Each is individually specified according to its end use and new assessment tools are needed for them."
There are two strands to the IMAC project:
Improving the hardware, software and methodology for assessing the huge number of existing pre-stressed structures; and
Creating methods and tools for monitoring and assessing new external and stay cables, to improve construction and reduce future maintenance problems.
Non-destructive testing tools
Work began with a review of available non-destructive techniques for detecting faults in bridge cables, followed by extensive field-testing of the best ones on various types of bridge. This led to the selection of acceleration sensors using low frequency vibrations of the order of > 0 Hz.
The partners have classified vibration intensities into four classes from non-critical to critical. The goal is a highly portable, compact and accurate instrument that gives a result in a single step. Progress is well advanced and ahead of schedule and the consortium is confident that it will succeed in producing a viable instrument. The target for accuracy is +3% in all weathers and conditions. New software is needed for on-the-spot interpretation of sensor readings making use of modern portable computer power.
A major data collection exercise has been to gather as much information as possible on existing bridge cables. All known composite bridge cables have been recorded in detail and will form a useful databank for future bridge building. In addition, a far larger number than expected of cable-stayed bridges has been found, and many of them will be used for testing prototype systems. The European Commission's Joint Research Centre (JRC) is performing the final round of tests under laboratory conditions in its European Laboratory for Structural Assessment (ELSA) in Ispra, Italy.
"One success of the project so far was that we found unsuspected defects in two bridges during the testing phase," comments Dr Wenzel. "This illustrates very graphically the advantage that will accrue from the IMAC tool. It not only shows up strains but also gives a better quality of information. Furthermore, inspection costs are only a fraction of what they have been with present inspection methods.
"Safety is also one of the main keywords. The European transport network will be safer and more reliable when IMAC is in use. Its development has only been possible by the joint efforts of consortium partners with their range of expertise. The French partner TUS has also been part of the consortium since the project began. Its non-linear acoustic damage protection approach, based on work in the nuclear field, has promise for inspecting steel beams embedded in concrete."
Benefits to the environment too
The failure of a single bridge can entail a huge cost to business and society in terms of extra distance travelled and time taken. The resulting traffic jams increase noise, vibration and vehicle emissions, with local health and environmental problems. Road safety is reduced on detours because people are driving on poorer, unfamiliar roads, generally in a bad temper. Prolonged repair works can affect local employment and cause business failures, because mobility is so important. An entire regional economy can suffer, as has been shown by analysis of bridge failures in Austria.
Anticipation of increasing bridge failures as the older bridges reach an age where they are more susceptible to strain was one of the main reasons for setting up this project. The IMAC strain assessment tool will increase European safety and prosperity and reduce environmental pollution.