Keywords: structural monitoring system, weigh in motion, distortion fatigue, vibration fatigue, railway bridge, equipped sleeper.

The fatigue assessment of railway bridges is one of the main issues in current practice arising from the concurrent events including: the rapid development of the European railway network, the increase of train traffic, the introduction of new materials, and the influence of particular effects such as distortion and local vibration not covered by the present design codes.

An important aspect in designing new lines or in the evaluation of upgrading operations is the convenience of not restricting the use of the railway lines to a limited train typology in order to enable the interoperability of railway infrastructures, as suggested by the European directive. Furthermore, the possible presence of new train types, characterised by several layouts of weights and axels distributions, introduces new uncertainties with respect to the expected traffic loads and the consequent loading spectra to be adopted in fatigue design. Therefore the new operating conditions of the European Railway Networks require the assessment of the adequacy and effectiveness of actual loading spectra provided by design codes and regulations. Following the previous considerations, the evaluation of the effective capability of existing bridges to guarantee the necessary safety level under present traffic conditions requires the improvement and enhancement of the available design procedures and increasing levels of knowledge and information concerning the behaviour of structural components and details.

Most cracks found in bridges were caused by the distortion of member cross sections and out-of-plane deformation of webs that induced localised bending stresses. Besides distortion effects, local vibration modes are excited during train passage producing high local stresses and hence a reduction of the expected fatigue life (vibration induced fatigue).

The main objective of the European research project FADLESS "Fatigue damage control and assessment for railways bridges", funded by the European Research Fund for Coal and Steel (RFCS) and currently ongoing, is to investigate the aforementioned uncertainties on the structural behaviour and integrity of steel and composite steel-concrete bridges with particular attention to the fatigue phenomena. With this aim, six bridges were selected as case studies in order to apply experimental and numerical methodologies. On the basis of performed analyses, the definition of innovative technical guidelines for the assessment and control of existing and new bridges, regarding fatigue phenomena induced by vibrations and distortions produced by train passage, has been possible.

In this paper, preliminary results obtained by the long-term monitoring system developed for one of the Italian case studies, the Panaro bridge, are reported. Such a monitoring system is composed of: four designed instrumented sleepers, for the train-axle weigh in motion and for the measurement of dynamic train-bridge interaction forces; strain gauges, for the measurement of strain patterns acting in the bridge; accelerometers, for the measurement of acceleration fields for critical details; and two video cameras for a visual estimation of train typologies.

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