Publications

Force identification, as a specific type of inverse problem, i nvolves reconstructing the force distribution on a structure from measured data. Having an accurate knowledge of the exact loading on a structure or mechanical system is important in any situation where the loading forms a crucial component in the design of the structure, i.e. when the unknown load can be regarded as critical and dictates the design, but measuring these loads is not always possible resulting in the necessity to obtain them by means of such reconstruction. Since the wind load may constitute a critical loading for high towers or mass structures and since, furthermore, it is in general dif ficult to measure a time-varying force on a structure, especially if the force acts as a continuous load ing, the identification of time-varying wind loads on a structure presents itself as an interesting and ch allenging force identification application. Obtaining these forces requires an indirect computation using measured response data in conjunction with a mathematical model of the structure. In its broadest sense force reconstruction can be viewed as being comprised of two main steps, the first of which involves the creation of a coefficient matrix wh ich relates the measured data to the unknown forces. The content and structure of this matrix depends on the specifics of the formulation used and are dictated by, for instance, the domain in which the solution is obtained (frequency/time), the available information used to establish the input-output l ink (impulse response functions, the output from a strategically chosen forward problem, etc.) as well as other less fundamental issues pertaining to the flexibility of the formulation in regard to prescribed initial conditions, the input quantities used (displacements/accelerations), and so forth. The second step, referred to as regularization, comprises t he manipulation of this, mostly illconditioned, coefficient matrix in an attempt to make it bett er suited for meaningful inversion. These manipulations are a function of the specific type of regulari zation used which, in turn, is a problemspecific issue. In the context of a wind load identification application the i nverse problem will be stated in the time domain and the coefficient matrix can be created either by usi ng a state-space model of the structure, with impulse response functions serving to establish the input-output link, or by using an external FE

No abstract is provided for this article.

One of the goals of seismic imaging is to determine the reflection contrast function of an interface at a particular target depth, independent of geological structure and the geometry of the acquisition setup (seismic aperture). Analysis of variations of seismic amplitude with source-receiver offset or immergence angle at the target interface may reveal information about the lithology and/or fluid content of the underlying target zone.

The Bears' Cage footbridge is a slender steel structure with a single span. Its dynamic behaviour is predicted based on a refined finite-element (FE) model and the vibration serviceability is assessed according to the current codes of practice. The assessment indicates a high susceptibility to human-induced vibrations with disturbing vibration levels even for sparse pedestrian densities. To validate the predicted behaviour of the structure, an extensive experimental study is performed including static deflection and dynamic vibration tests. The analysis shows that statically, the longitudinal movement of the supports on one side of the span can be considered unconstrained, indicating a behaviour of the sliding pot bearings as designed. Due to the footbridge's arch-like shape, the longitudinal stiffness of the supports highly influences the natural frequency of the fundamental bending mode. The analysis shows that the longitudinal stiffness of sliding pot bearings and the structural inherent damping ratio of the fundamental mode significantly reduces once an initial friction level in the sliding pot bearings is overcome as the result of a significant movement at the supports. The vibration serviceability is reassessed based on the calibrated FE model and shows that even for high pedestrian densities, maximum vibration comfort is ensured.