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AbstractToday, short- and long-term structural health monitoring (SHM) of bridge infrastructures and their safe, reliable and cost-effective maintenance has received considerable attention. From a surveying or civil engineer’s point of view, vibration-based SHM can be conducted by inspecting the changes in the global dynamic behaviour of a structure, such as natural frequencies (i. e. eigenfrequencies), mode shapes (i. e. eigenforms) and modal damping, which are known as modal parameters. This research work aims to propose a robust and automatic vibration analysis procedure that is so-called robust time domain modal parameter identification (RT-MPI) technique. It is novel in the sense of automatic and reliable identification of initial eigenfrequencies even closely spaced ones as well as robustly and accurately estimating the modal parameters of a bridge structure using low numbers of cost-effective micro-electro-mechanical systems (MEMS) accelerometers. To estimate amplitude, frequency, phase shift and damping ratio coefficients, an observation model consisting of: (1) a damped harmonic oscillation model, (2) an autoregressive model of coloured measurement noise and (3) a stochastic model in the form of the heavy-tailed family of scaled t-distributions is employed and jointly adjusted by means of a generalised expectation maximisation algorithm. Multiple MEMS as part of a geo-sensor network were mounted at different positions of a bridge structure which is precalculated by means of a finite element model (FEM) analysis. At the end, the estimated eigenfrequencies and eigenforms are compared and validated by the estimated parameters obtained from acceleration measurements of high-end accelerometers of type PCB ICP quartz, velocity measurements from a geophone and the FEM analysis. Additionally, the estimated eigenfrequencies and modal damping are compared with a well-known covariance driven stochastic subspace identification approach, which reveals the superiority of our proposed approach. We performed an experiment in two case studies with simulated data and real applications of a footbridge structure and a synthetic bridge. The results show that MEMS accelerometers are suitable for detecting all occurring eigenfrequencies depending on a sampling frequency specified. Moreover, the vibration analysis procedure demonstrates that amplitudes can be estimated in submillimetre range accuracy, frequencies with an accuracy better than 0.1 Hz and damping ratio coefficients with an accuracy better than 0.1 and 0.2 % for modal and system damping, respectively.

Test strategy for evaluation of strain gauge long term stability In the field of structural health monitoring the measuring of strain is prevalent beside temperature and displacement. The strain is used to determine the utilization rate and to estimate the fatigue strength of a structure. A very common measurement method for strains at and in components is the usage of strain gauges. Further, a climate cycle for analysis of long term stability of strain gauges is introduced, that simulates an accelerated temperature and humidity stress. This cycle consists of four phases, which lasts 240 h and covers different combinations of temperature and humidity conditions up to freezing and thawing. The results of a 85 d lasting long term test are presented and used to describe characteristic values for long term stability of strain gauges. The measurement results show significant differences for the strain gauge types in the zero point drift and underline the role of correct choice and protection to achieve reliable strain measurements.

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Der Ablauf der tatsächlichen Schädigungsentwicklung unter Berücksichtigung von Spannungsumlagerungen an ermüdungsbeanspruchten großformatigen Bauteilen wird unter Anwendung von neuen experimentellen Bauteilprüfverfahren und Messmethoden untersucht und durch numerische Simulationen validiert. Mit zunehmender Lastwechselzahl kommt es zu Veränderungen der gemessenen Dehnungen im Querschnitt. Im höchstbelasteten Bereich der Betondruckzone konnte die Degradation durch Anwendung von Ultraschalllaufzeitmessungen beobachtet werden. In der numerischen Simulation wurden Spannungsumlagerungen nachgewiesen. Durch das Resonanzprüfverfahren können Ermüdungsuntersuchungen an großen Bauteilen in hohen Lastwechselzahlbereichen sehr effektiv und effizient durchgeführt und dabei präzise Prüflasten aufgebracht werden, wodurch sich ein umfangreiches neues Untersuchungsfeld erschließen lässt. Die Ultraschalllaufzeitmessung zeigt sich als geeignete Messmethode zur Untersuchung der Betondegradation auch bei starken Erschütterungen während des Versuchs und könnte daher auch bei Versuchen an realen Bauwerken Anwendung finden.