Fatigue of prestressed beams with web reinforcement under cyclic shear Many existing bridge structures built in the 1960s and 1970s typically feature a high degree of prestressing and low shear reinforcement ratios. Therefore, their shear bearing capacity, originally determined based on the principal tensile strength criterion, can often not be verified with the strut and tie models of DIN-Fachbericht 102. To investigate the shear strength of prestressed concrete beams with web reinforcement under cyclic loading, fatigue tests on 14 beams with a low amount of shear reinforcement (ρ w = 0,15 %, ρ w = 0,22 % and ρ w = 0,33 %) were performed. The present paper describes the fatigue tests and the occurring gradually stirrup fractures with increasing number of load cycles. Based on comparative calculations and the experimentally determined strut inclinations, a modified approach to calculate the stirrup stress amplitude under cyclic loading has been developed. It is based on a truss model with variable strut inclination.Bild 1 Nachweiskonzept zur Bewertung der Querkrafttragfähigkeit von vorgespannten Bestandsbrücken Basic structure of shear assessment of existing prestressed concrete bridges
Bild 3 Querschnitt (oben) und Längsschnitt (unten) der T-Träger Cross section (top) and half-elevation (bottom) of the T-beamsBild 2 Querschnitt (oben) und Längsschnitt (unten) der I-Träger Cross section (top) and half-elevation (bottom) of the I-beams
Bisher durchgeführte Nachrechnungen bestehender Spannbetonbrücken auf Basis der vom Bundesverkehrsministerium veröffentlichten Nachrechnungsrichtlinie haben in vielen Fällen Defizite der Querkraft‐ und Torsionstragfähigkeit sowie des Ermüdungswiderstands ergeben. In einem Forschungsvorhaben im Auftrag der Bundesanstalt für Straßenwesen wurden daher Bemessungsverfahren formuliert, die in die erste Ergänzung zur bestehenden Nachrechnungsrichtlinie implementiert wurden. Der vorliegende Beitrag beschreibt die entsprechenden Verfahren zur Ermittlung der Querkrafttragfähigkeit unter statischer und zyklischer Belastung ohne Berücksichtigung einer Querkraftbewehrung. Die Verfahren berücksichtigen neben neueren Forschungsergebnissen die technischen Regelwerke für Betonbrücken und basieren auf einem Nachweis der Hauptzugspannungen. Anhand einer typischen Hohlkastenbrücke des Bundesautobahnnetzes werden die Anwendung der Verfahren exemplarisch erläutert und die wesentlichen Ergebnisse dargestellt.
Principal tensile stress criterion for the shear assessment of existing concrete bridges
The structural assessment of existing road bridges that has been recently performed on the basis of the equally named provisions, issued in 2011 by the Federal Ministry of Transport and Urban Affairs, have revealed significant deficits in shear and torsion as well as fatigue resistance especially in prestressed concrete bridges. In a research project, funded by the Federal Highway Research Institute of Germany, design methods have been developed that were incorporated into the first addendum of the Structural Assessment Provisions. This paper describes the respective design methods for shear under static and cyclic loading without shear reinforcement. The procedures take into account current research results as well as the design codes for concrete bridges and are based on a verification of the principal tensile stresses. The use of the procedure and the main results are shown exemplarily by a design example of a typical box girder bridge of the German road network.
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