One year monitoring of the Z24-bridge : Environmental influences versus damage events

When using the analysis of vibration measurements as a tool for health monitoring of bridges, the problem arises of separating abnormal changes from normal changes in the dynamic behaviour. Normal changes are caused by varying environmental conditions such as humidity, wind and most important, temperature. The temperature may have an impact on the boundary conditions (frozen soil) and the Young's modulus of the material of which the structure consists. Abnormal changes on the other hand are caused by a loss of stiffness somewhere along the bridge. It is clear that the normal changes should not raise an alarm in the monitoring system (i.e. a false positive), whereas the abnormal changes may be critical for the structure's safety. This paper tries to give an answer to the question whether it is possible to separate the environmental influences from damage events. In the frame of the European SIMCES-project, the Z24-bridge in Switzerland was monitored during almost one year before it was artificially damaged; what makes it an excellent object to study methods that try to filter out the environmental influences. The paper presents the results of the measurements on the post-tensioned concrete bridge and shows that it is indeed possible to distinguish between abnormal and normal changes of its dynamic characteristics.