Comparison of natural frequencies and strain-based influence lines as damage indicators for structural health monitoring of railway bridges

Structural health monitoring (SHM) of civil engineering structures relies on the recording and evaluation of the structural response, either on a permanent or a periodic basis. The assessment of the structural health mostly involves the derivation of so-called damage indicators from the raw response data. In vibration-based SHM, those typically consist of the natural frequencies and displacement or strain mode shapes. Recent work has also considered the use of strain-based unit influence lines as alternative damage indicators in the particular case of railway bridges. While natural frequencies are determined by the global stiffness distribution of the structure, the influence lines of quantities of interest such as strains or internal forces are more sensitive to local stiffness. In this way, a change in influence lines also reveals a change in internal force distribution, which is of particular interest when it comes to the assessment of the structural health. This paper compares the use of natural frequencies and strain-based influence lines for a case study on a Vierendeel railway bridge in Mechelen, Belgium. The natural frequencies are derived from acceleration measurements on the bridge deck, using operational modal analysis (OMA). The influence lines are extracted from the dynamic strain response of the bridge during train passages, using an estimate of the axle loads and locations, derived from simultaneous strain measurements on the rails. In this study, both indicators are compared (1) in terms of their variation under environmental and operational variability and (2) in terms of their sensitivity to local damage, which is modeled using a detailed finite element model of the railway bridge.