EXPERIMENTAL IDENTIFICATION OF AN ELASTO-MECHANICAL MULTI-DEGREE-OF-FREEDOM-SYSTEM USING STOCHASTIC SIGNALS

The determination of dynamic parameters are the central points of the system identification of civil engineering structures under dynamic loading. This paper first gives a brief summary of the recent developments of the system identification methods in civil engineering and describes mathematical models, which enable the identification of the necessary parameters using only stochastic input signals. Relevant methods for this identification use Frequency Domain Decomposition (FDD), Autoregressive Moving Average Models (ARMA) and the Autoregressive Models with eXogenous input (ARX). In a first step an elasto-mechanical mdof-system is numerically modeled using FEM and afterwards tested numerically by above mentioned identification methods using stochastic signals. During the second campaign, dynamic measurements are conducted experimentally on a real 7-story RC-building with ambient signal input using sensors. The results are successfully for the relevant system identification methods

Whole body vibration on offshore structures: an evaluation of existing guidelines for assessing low-frequency motions

An extensive literature research has been conducted to create an insight into the existing norms and standards regulating the assessment of human exposure to motions in offshore environments. A summary of current threshold values and their specific fields of application is included. The presented literature is analysed with respect to their applicability for assessing low frequency oscillatory motions of floating offshore wind turbines to which technicians are exposed during maintenance tasks. The review identifies the need for a consistent assessment method in combination with threshold values for floating structures

An Endogenously Derived AK-model of Economic Growth

Assuming a production process with returns to scale that vary with the intensity it is operated at, an AK-model of endogenous growth with constant returns to scale in production is shown to arise due to replication driven by profit-maximization. If replication occurs at the efficiency-maximizing scale, the result applies also when the number of production processes must be discrete, thus overcoming the so-called integer problem. When competition is imperfect, there is only convergence toward the AK-model for large enough input use, so an economy is more prone to stalling in a steady-state without growth, the smaller and less competitive it is. Inefficient scaling also raises the risk of stalling

Flüssigkeitsdämpfer zur Reduktion periodischer und stochastischer Schwingungen turmartiger Bauwerke

The tuned liquid column damper (TLCD), patented by Frahm already in 1910, consists of a U-shaped tube system filled with a Newtonian liquid. The tuned parameters of the liquid damper enable the liquid mass to oscillate with a 90° phase shift with respect to the motion of the structure, leading to interaction forces on the primary structure such as viscous damping loads. The oscillation energy of the liquid dissipates in the tube system because of turbulence effects and local pressure loss caused by friction. The mathematical expression of the liquid motion is carried out using the instationary Bernoulli equation. The natural frequency of the liquid damper depends on the geometry of the tube system. The dissipation forces of the absorber result from the impulse, which is caused by the liquid column motion. In order to determine the parameters of the liquid damper the tuning criteria of Den Hartog, Warburton and Lyapunov, which are valid for mechanical dampers, can be applied through the already known analogy approaches. The results of these criteria are the optimal natural frequency and the optimal damping ratio of the liquid damper. The geometry factors of the liquid damper affect the effective liquid mass ratio, which is critical for the reduction of the oscillation. The geometry factors are defined by the angle of inclination of the vertical stream line of fluid flow, the sectional area and the length of the liquid column. In order to solve the optimization problem in the research works so far developed, the geometry factors have always been assumed as known values. As the absorber efficiency depends not only on the tuning of the natural frequency and the damping ratio but also on the effective liquid mass ratio, the geometry factors must be included in the optimization procedure of the liquid damper. To achieve this purpose in this research work the tuning criteria of Den Hartog, Warburton and as well as Lyapunov are set up with a new expanded optimization approach. The practical application of the new optimization approach is demonstrated with the aid of two examples. The first example deals with a wind turbine, which is periodically excited by a non-uniform wind flow. The second example shows the application on a seismically excited benchmark building. The efficiency of the liquid damper and the influence of the absorber parameter are calculated through sensitivity analysis and compared with the conventional mechanical tuned mass dampers. This research work includes the conceptual design of a new semi-active liquid damper. The nonlinear equations of motion as well as the interaction forces are mathematically derived and the damper efficiency is numerically validated