Dynamische Verformungsmessungen an Eisenbahnbrücken mittels Mikrowelleninterferometrie

Dynamic measurements of railway bridge displacements through microwave interferometry – Part 1: measurement method The microwave interferometry is a rather new measuring technique, yet little‐known in civil engineering applications. It allows the non‐contact acquisition of structural displacements with accuracy in the sub‐millimetre range at a sampling rate of up to 4 kHz. The high sampling frequency allows also the caption of dynamic structural responses, which can be used for a straightforward determination of the main modal parameters of the structures (natural frequencies, damping ratios). Furthermore, the synchronous acquisition of the overall motion of the targeted object is possible due to a high range resolution, which facilitates a direct identification of modal shapes. This paper gives a short introduction of the measurement method and outlines its boundary conditions and limitations with respect to applications in railway bridge dynamics. The knowledge has been gained on the basis of comprehensive systematic experimental investigations performed within the frame of a cooperation project with the German Railways (Deutsche Bahn AG). As a result an evaluation matrix was created, which clearly illustrates the applicability of the microwave interferometry for different railway‐specific tasks. The second part will present selected results of microwave interferometry measurements of railway bridges in comparison to parallel conventional measurements and the corresponding numerical investigations, which were used for the validation of the measurement technique.     

Empfehlung Nr. 5 “Punktlastversuche an Gesteinsproben“ des Arbeitskreises 3.3 “Versuchstechnik Fels“ der Deutschen Gesellschaft für Geotechnik

Der Arbeitskreis AK 3.3 “Versuchstechnik Fels“ der Deutschen Gesellschaft für Geotechnik e. V. erarbeitet Empfehlungen für felsmechanische Labor‐ und Feldversuche sowie Messungen im Gebirge und an Bauwerken. Die vorliegende Neufassung der Empfehlung Nr. 5 behandelt den Punktlastversuch an Gesteinsprobekörpern und berücksichtigt die Weiterentwicklung der Versuchstechnik und ‐auswertung seit der gleichnamigen Empfehlung Nr. 5 von 1982. Es werden die Anforderungen an die Prüfeinrichtung und die Probekörper sowie die Vorgehensweisen für die Durchführung und Auswertung von Punktlastversuchen festgelegt. Die wesentliche Neuerung besteht darin, dass nicht mehr der Lastpunktabstand allein, sondern die Probekörperfläche zur Berechnung des Punktlastindex verwendet wird. Beim Punktlastversuch wird ein Indexwert für die Festigkeit eines Gesteins bestimmt, indem ein zylinderförmiger, quaderförmiger oder unregelmäßig geformter Probekörper zwischen zwei Lasteinleitungsspitzen bis zum Bruch belastet wird. In dieser Empfehlung werden der Zweck, die Begriffe, die Prüfeinrichtung, die Anforderungen an den Probekörper und die Versuchsdurchführung erläutert. Drei mögliche Optionen der Versuchsauswertung werden aufgezeigt, die Darstellung der Ergebnisse beschrieben und mit Hilfe von Beispielen illustriert. Abschließend wird erläutert, wie die einaxiale Druckfestigkeit aus der Punktlastfestigkeit abgeleitet werden kann. Recommendation No. 5 (revised) of the Commission on Rock Testing of the Deutsche Gesellschaft für Geotechnik e.V. — “point load tests on rock samples”. The Commission on Rock Testing of the Deutsche Gesellschaft für Geotechnik e.V. (German Geotechnical Society) is compiling instructions for rock tests conducted in the laboratory and in‐situ, and for performing monitoring of rock masses and civil engineering structures. The revised version of recommendation No. 5 deals with the point load test on rock samples and incorporates recent developments in testing machines and testing procedures since the first version from 1982. The requirements for the testing machines and the specimens are given, as well as the procedures to perform the tests and evaluate the data. The essential modification is the use of the sample area instead of the platen tip distance alone. The point load test is used to derive an index value for rock strength. Therefore rock specimens in the form of core, cut blocks or irregular lumps are loaded until failure between a pair of load tips. In this recommendation, scope, terms, apparatus, specimen requirements and procedure of the test are explained. Three possible options of test evaluation are given, the reporting of results described and illustrated by examples. Finally it is shown, how the uniaxial compressive strength can be derived by the point load strength.     

Methoden zur Analyse des Bruchverhaltens hochfester Stahlfaserbetone

Methods for analyzing the fracture behavior of high‐strength steel fiber‐reinforced concretes High‐strength and ultra‐high strength fiber‐reinforced concretes are most suitable for applications with extreme mechanical loads. These extreme conditions require a ductile behavior under tensile loading, which is obtained solely by the addition of steel fibers and their working mechanism. Profound know ledge on the working mechanism of the steel fibers is necessary for optimizing this material. Usually, this knowledge is obtained by means of classical measuring techniques of destructive tests. Adopting measuring techniques from non‐destructive material testing helps to analyze and to identify the different stages of the fracture mechanism of high‐strength and ultra‐high strength fiber‐reinforced concretes in detail. The application of different non‐destructive measuring techniques is shown exemplary on tensile tests conducted on an ultra‐high strength fiber‐reinforced concrete and its applicability for analyzing the fracture behavior is discussed. The main focus is on the characterization of the relevant failure modes under tensile loading by the different measuring techniques and the comparison with classical measuring techniques (e. g. extensometer). The tensile tests have been analyzed by optical deformation measurements using digital image correlation (DIC), acoustic emission analysis (AE), and 3D computed tomography (CT).