Modal Analysis
The numerical modal analysis constitutes a methodology for the examination of the dynamic behaviour of structures, systems and buildings. The mechanical characteristics of structural systems are described by the parameters’ frequencies, damping values and
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Modal Analysis Werner Ru¨cker
The numerical modal analysis constitutes a methodology for the examination of the dynamic behaviour of structures, systems and buildings. The mechanical characteristics of structural systems are described by the parameters’ frequencies, damping values and vibration modes. The knowledge of these modal parameters facilitates to interpret the vibration of a system on the basis of measurements. Furthermore, these parameters can be used to formulate requirements for retrofitting and modifying the structural system. Modal data can be used for the following tasks: • Design verification, modification and optimisation • Prototype development • Improvement of numerical dynamic models • Measures for vibration reduction • Verification of the refurbishment and retrofitting success of structures • Condition monitoring and damage identification. This chapter describes the principles and methods of modal analysis and presents examples showing the application variety of numerical and analytical case studies.
6.1
Dynamically loaded structures have to be designed so that the excitation frequencies do not overlap with their natural frequencies. Therefore, it is of outstanding importance to understand and to visualise the modal parameters of different structural types in the design process which also helps to identify weak structural areas. The knowledge of the modal damping is important as it often determines the service life of a structure. On the basis of the results of numerical modal analysis it can be decided whether a numerical modal analysis model complies with the existing structure. A damage assessment of dynamically loaded structures may be achieved by the determination of the natural frequencies, mode shapes and modal damping. A possible necessary reduction of non-tolerable dynamic loading caused by, e.g. changing operation conditions of production facilities is often only technically practicable after the knowledge of the modal parameters.
6.2
W. Rücker (&) Bundesanstalt für Materialforschung und–prüfung, Unter den Eichen 87, 12205 Berlin, Germany e-mail: [email protected]
Scope of Modal Analysis
Excitation of Structures and Systems for Modal Analysis
The appropriate methodology for the excitation of structures and systems has to be chosen in such a way that the required modal parameters can be determined uniquely, precisely and
H. Czichos (ed.), Handbook of Technical Diagnostics, DOI: 10.1007/978-3-642-25850-3_6, Springer-Verlag Berlin Heidelberg 2013
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Fig. 6.1 Comparison of result of dynamic analysis with impact and ambient excitation of a reinforced steel bridge for pedestrians
comprehensively. This leads to requirements for the scale and for the frequency range as well as for the spatial location and distribution of the excitation on the structure. In general, ambient excitations such as seismic activities, wind and wave loading, traffic, operation excitations and others or excitations such as an impact hammer, drop weights and oscillator systems can
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