Vibration based structural health monitoring of prebended steel - VHPC beams

This paper deals with a vibration based damage detection method applied to the monitoring of two large scale prebended beams in laboratory conditions. Prebended steel - VHPC beam consists of a type of composite structure in which very high performance concrete (VHPC) is cast around the lower flange of a prebended steel girder. When the two local prebending loads are released, concrete round the bottom flange is submitted to compression. After completion, the beams were submitted to quasi-permanent and fatigue loading test programs. Finally the beams were loaded quasi-statically up to serviceability limit state (SLS), ultimate limit state (ULS) and failure. In addition to an extensive survey of the deformations using classical displacement and strain measurement techniques, transient acceleration signals were periodically recorded for subsequent modal analysis. Output-only data were processed, using a subspace covariance driven algorithm implemented in the free software COSMAD, in order to extract the modal characteristics. The variations of the modal properties are discussed in relation with the visual inspections and static load-deformation relationships. The loss of stiffness induced by loading the beam over the SLS was detected from deflection measurements. However, no significant shift of the resonant frequencies occurred because the compressive stresses were high enough to close the cracks.