Damage detection of truss bridge elements using an enhanced pseudo-local flexibility method

Vibration-based structural health monitoring aims to not only detect the occurrence of the damage but also identify the location of damage. The pseudo-local flexibility method (PLFM) is a vibration-based approach that only requires the identified modal parameters of the structure to perform damage detection. Thus, the cost of constructing a finite-element model of the structure and the modeling error of the finite-element model can be circumvented. In addition, the PLFM is based on the flexibility matrix of the structure, which is practical, because only the first few modes are required to estimate the necessary flexibility matrix and only the first few modes can be identified accurately in real applications. However, the potential damage region that is identified using the PLFM is indicated by the location of the center of the applied virtual forces, but not the potential damage elements. Hence, in this study, an enhanced PLFM (EPLFM) is proposed to improve the resolution of damage localization in the conventional PLFM. The regional rigidity ratios obtained by the PLFM are distributed to each element based on the virtual strain energy corresponding to virtual forces. The damage locations indicated by the EPLFM are marked at each element, and hence, more specific damage locations can be identified by the elements with smaller elemental rigidity ratios. Herein, the present EPLFM was numerically and experimentally validated with a simply supported steel-truss bridge. In the numerical validation, a simplified two-dimensional finite-element model for the truss structure was constructed using SAP2000 software package, and seven damage scenarios and two setups of measurement degrees of freedom (DOF) were investigated. It is found that accurate damage localization of the 2D simply supported truss structure was achieved when both the vertical and horizontal DOFs of all nodes were measured and the virtual force configurations acting on both ends of each element were used. In the in-field experimental validation where the mode-truncation errors and measurement noises were unavoidably introduced, it was observed that the EPLFM could identify the full-cut vertical member at the mid-span or 5/8^th span, even though the cut member was identified in a group with some adjacent un-damaged members.