Matthias Hillebrand

More and more often, semi‐precast reinforced concrete slabs with lattice girders are employed for industrial buildings or bridges, where they are exposed to high cycle fatigue loading. Despite research in recent years that has led to the formulation of an S–N curve for lattice girders, the effects of cyclic loading on semi‐precast slabs have not yet been sufficiently clarified. Moreover, research has so far been limited to single‐span slabs. The effects of continuous slab systems, which are mainly realized in buildings and bridges, cannot be considered in the calculation of fatigue resistance yet. To improve the fatigue design concept for shear and interface shear, theoretical and experimental investigations were conducted at the Institute of Structural Concrete, RWTH Aachen University. In addition to the fatigue behavior of 16 tests under cyclic loading, particular attention is paid to the increase in shear and interface shear resistance at the inner support of continuous slabs. Furthermore, the influences of the support detailing were investigated. The findings illustrate further potential for optimization of the design under cyclic loading.

Semi‐precast slabs, where lattice girders serve both as reinforcement in the construction and in the final state, are characterized by a high flexibility and cost efficiency. Due to their technical and economic advantages, they are widely employed in constructions all over the world. In Germany, for example, they are used in about 70% of all floor slabs. To address this economic potential and to improve the design concept for shear and interface shear, theoretical and experimental investigations were conducted at the Institute of Structural Concrete (IMB), RWTH Aachen University. In addition to presenting the general structural behavior of the 16 tests, particular attention is paid to the increase in shear and interface shear resistance at the inner support of continuous slabs and the positive influences of the investigated support detail. The findings illustrate further potential for optimization under monotonic loading, which will also have a positive effect on the fatigue behavior under cyclic loading.

Due to their high cost efficiency and flexibility, semi-precast concrete slabs with lattice girders are widely used in constructions all over the world. Prefabricated concrete slabs, combined with in situ concrete topping, exhibit a quasi-monolithic structural behavior in which lattice girders serve as vertical shear reinforcement and ensure the transfer of longitudinal shear within the interface, acting in combination with concrete-to-concrete bonding mechanisms. To be applicable in industrial and bridge construction, semi-precast slabs need to have sufficient resistance against fatigue failure. To improve and expand the limits of application, theoretical and experimental investigations are conducted at the Institute of Structural Concrete (IMB), RWTH Aachen University. To investigate the fatigue behavior of lattice girders, small size tests with lattice girder diagonals were carried out. These test results have been used to derive an S–N curve (S: stress, N: number of load cycles) for lattice girders for a more refined fatigue design. Subsequently, the fatigue behavior of semi-precast slabs with lattice girders was investigated by fatigue tests on single-span slab segments. The fatigue design regulations of lattice girders according to technical approvals can generally be confirmed by this test program; however, they tend to be conservative. The use of the derived S–N curve leads to significantly improved agreement of fatigue behavior observed in tests and design expressions.

In the last decades, increasing traffic loads have led to higher requirements on the bearing capacity of existing and also new bridges. In particular, many bridge structures built in the 1960s and 1970s often contain less web reinforcement than the nowadays required minimum web reinforcement. In this context, the shear resistance under cyclic loading is of special interest. For this reason, experimental tests were conducted on prestressed concrete beams with and without shear reinforcement at the Institute of Structural Concrete of RWTH Aachen University within the last years to investigate the shear fatigue strength. The specimens were able to resist more load cycles than predicted by the approaches implemented in the Eurocodes for bridges. Based on the test results, design models for shear under cyclic loading should be reviewed and improved, especially with regard to the assessment of existing structures. This paper describes the recent tests on 20 I‐shaped prestressed beams with web reinforcement.

In the recent years, bridges, as an important part of the national and international infrastructure, had to comply with stricter requirements due to increased heavy load traffic. Many of these bridge structures built in the 1960s and 1970s often contain less web reinforcement than the modern required minimum web reinforcement. In this context, the shear resistance under cyclic loading is of special interest. For this reason, experimental tests were conducted on prestressed concrete beams with and without shear reinforcement at the Institute of Structural Concrete of RWTH Aachen University to investigate the shear fatigue strength. This paper describes the recent tests on ten Tshaped prestressed beams with web reinforcement. The specimens were able to resist more load cycles than predicted by the approaches implemented in the Eurocodes for bridges. Based on the test results, design models for shear under cyclic loading should be reviewed and improved, especially regarding the assessment of existing structures.

Die Anforderungen an Brückenbauwerke erhöhen sich seit Jahren durch steigende Verkehrszahlen, die insbesondere bei Brücken im Bestand zu einer hohen Ausnutzung der Querkraftbewehrung unter zyklischer Belastung führen. Zur Beschreibung des Ermüdungstragverhaltens von querkraftbewehrten, profilierten Spannbetonbalken unter zyklischer Querkraftbeanspruchung wurden am Institut für Massivbau der RWTH Aachen Versuche durchgeführt, um insbesondere die Einflüsse aus Querkraftbewehrungsgrad, Oberlast und Amplitude zu untersuchen. Die Versuche mit einem Querkraftbewehrungsgrad von ca. 0,67 % ergänzen bereits in der Vergangenheit getestete Balken mit geringerer bzw. ohne Querkraftbewehrung. Die zyklische Belastung führt zu einem sukzessiven Versagen der Querkraftbewehrung und ausgeprägten Umlagerungen der Kräfte, die sich über mehrere Hunderttausend Lastwechsel erstrecken. In den Versuchen wurden deutlich größere Lastspielzahlen erreicht, als sich nach aktuellen Nachweisen für Querkraft unter Ermüdung ergeben. Im Rahmen des Beitrags werden die Ergebnisse von sechs neuen Versuchen mit I‐Profil vorgestellt.