With this habilitation thesis I am applying for the venia docendi in structural dynamics. To undergraduate students of civil engineering, the importance of this field in engineering is sometimes demonstrated by means of structures under resonance excitation. In the resonance scenario, harmonic excitation close to one of the system’s natural frequencies leads to large vibration amplitudes and high cyclic structural stresses, limited only by the damping behavior of the structure and by the duration of its exposure to the load. Students can impressively observe how only small inputs result in large dynamic responses, but resonance excitation of structures is not only relevant for academic examples or teaching purposes. Modern railway bridges encounter this condition at constant travel speeds of passing trains. Here, regularly repetitive axle loads form the periodic pattern of load with an excitation frequency that is eventually close enough to a resonance frequency of the bridge-train interaction system. Since in the standardized design process of railway bridges static load patterns must be applied, which often lead to higher internal forces than the dynamic resonance case, the investigation of structural failure is not the primary objective in this context. For railway bridges subjected to high-speed traffic, the acceleration response and serviceability limits for deflections are in most cases the critical design parameters. Both must be limited to prevent derailments and destabilization of the ballast superstructure.

Scientific questions regarding this complex interaction system of moving train and bridge present great relevance. Especially nowadays, when the reality of climate change is accepted and no longer denied by most of us, the development of the rail network as a clean public transport system must be of global interest. In order to enhance the attraction of this transport network for users, lines are being expanded and increased in efficiency, which goes hand in hand with an augmentation of travel speed and an increase in operating frequencies, ultimately leading to higher demands on the engineering design as well as on the structural components of the railway tracks and bridge structures along the line. In the field of engineering sciences, we should not forget about the engineering background and the practical implications and applications of our scientific work. In my own research efforts at the Unit of Applied Mechanics, it has always been my goal to keep this applicability in mind. In the long run, scientific methods in structural dynamics can lead to practical design concepts and computational methods that contribute to the state of the 1 art and beyond. The attempt to do so and to develop new approaches and methods for a straightforward estimation of dynamic effects on railway bridges, as well as to contribute to the updating of established design approaches and methods, motivates the topics presented here in the area of railway bridge dynamics. This process involves many steps and contributions of different types, such as experimental, theoretical, and numerical findings. Collectively, this work consists of seven peer-reviewed papers (six published and one reviewed and accepted for publication) focusing on numerical simulation and theoretical investigations addressing three topics of structural dynamics problems that are of current and significant interest to the scientific community and to practical engineers working in the field of railway bridge dynamics. However, the methods and ideas presented in the following papers of this theses are not restricted to the field of railway bridge dynamics but may be also applied to other engineering fields within structural dynamics.

The first topic is concerned with the response amplification effects due to irregular rail track surfaces and the need for an efficient design approach to address them. Since the irregular surface of the rail track is generally random in nature, the most appropriate approach to these problems is a probabilistic design method that takes randomness into account. The second topic is the investigation of soil-structure interaction effects on railway bridge vibrations. A realistic quantification and subsequent exploitation of radiation damping effects of the subsoil would yield a huge gain for a resource efficient bridge design. The third topic addresses the development of simplified methods for practical engineers, to yield tools for quick dynamic checks in the design or first analysis phase.

With this habilitation thesis, the author applies for a Venia Docendi in Mechanics. Since this discipline has its theoretical foundation in Isaac Newton’s Philosophiae Naturalis Principia Mathematica published in 1687, one might ask: ”Is there really an urge to pursue a line of research that has been studied for more than 300 years ?”
Unsurprinsingly, the author’s response to this rhetorical question is ”Yes”. Although Newton’s laws and theories derived therefrom in the last centuries have proven to be inappropriate in some subfields of physics, a predominant number of physical phenomena can be described sufficiently accurate by them. At the same time, with technical progress constantly increasing in the Industrial Age, challenges (not only, but also related to Mechanics) arising from this have increased equally. It is the author’s strong belief that as long as mankind lives within an environment dominated by technology, these challenges will continue to appear. Therefore, the urge for ”engineering solutions” to ”engineering problems”, in this case related to mechanics, will not cease to exist. 

Historically, theory and experiment are the two foundations of natural sciences. With the advent of computer power in the 20th century, simulation has emerged from theory, representing a third foundation in the present day. Based on these approaches, the title of this thesis includes
”analytical, experimental and computational”, since all three of them have their legitimate fields of application. However, in the opinion of the author they should not be understood as separate sub-disciplines. In many cases, an appropriate combination of analytical, experimental and computational methods is the preferred approach. For instance, a new theory or model
derived analytically is the basis for a numerical implementation oftentimes. Without proper application of the former, the latter therefore is irrelevant. On the other hand, both analytical and computational investigations are useless without experimental validation. Again, a proper application of the latter is required for this validation to be correct. 

This thesis consists of seven peer-reviewed and published papers and one paper submitted for publication where all three of the approaches mentioned are employed. The topics addressed cover a relatively wide range, demonstrating the broad field of active research in Mechanics,
focusing mainly on applications in structural dynamics.

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