Besonderheiten bei der Beurteilung des Ankündigungsverhaltens von Spannbetonbrücken nach dem Riss‐vor‐Bruch‐Kriterium

Particularities in Interpretation of the Preannouncement Behaviour of Prestressed Concrete Bridges by Applying the “Crack‐before‐Collapse‐Criterion” Part I: Temperature Loads, Relocation of Internal Forces Bridges without sufficient preannouncement behaviour with regard to the “crack‐before‐collapse‐criterion” bear risks of failure. The risks are especially high for prestressed concrete bridges with tendons endangered to stress‐corrosion‐cracking. Effects out of the structure and imponderableness during construction work affect the preannouncement behaviour additionally. To evaluate the existing risk for the owner different influences of the structure will be investigated by numerical calculations: Influences of temperature loads as well as influences of failure of cross‐section with relocation of internal forces will be investigated more closely.     

Zur Beurteilung des Ankündigungsverhaltens von Spannbetonbrücken nach dem Riss‐vor‐Bruch‐Kriterium. Vorschläge zur Vorgehensweise bei gefährdeten Brücken

Interpretation of the Preannouncement Behaviour of Prestressed Concrete Bridges by applying the “Crack‐before‐Collapse‐Criterion” Proposals for further Proceeding with Critical Bridges Due to extending lifetimes of prestressed concrete bridges the importance of the durability increases. With numerical calculations and investigations of structures according to DIN 1076 it is intended to recognize and prevent possible failure of the structures in time and to minimize the risk for the owner during the remaining lifetime. Especially older prestressed concrete bridges with tension wires endangered to stress‐corrosion‐cracking need to be numerically investigated and judged concerning their preannouncement behaviour according to the “crack‐before‐collapse‐criterion”. According to the numerical results of the “crack‐before‐collapse‐criterion” and supplemental calculations it is possible to evaluate the existing risk for the owner concerning failure of the structures. The design of the concerned structures will be discussed. Based on results of performed investigations proposals for further proceeding with older critical bridges will be given along with recommendations how to limit the remaining risks. For newer bridges the analysis according to the “crack‐before‐collapse‐criterion” is a measure of a sufficient sturdiness.     

Erfahrungen bei der Beurteilung der Dauerhaftigkeit vorgespannter Bewehrung von älteren Spannbetonbrücken

Experiences due to the evaluation of the durability of prestressed reinforcements in old prestressed concrete bridges. Old prestressed concrete bridge superstructures exhibiting cracks across the prestressed reinforcement need detailed inspections for estimating maintenance requirements. Only this type of evaluation makes it possible to determine the required extent and nature of maintenance. In this paper essential steps of the “Directive of the durability of prestressed reinforcements in old prestressed concrete bridge superstructures” of the Federal Highway Research Institute are explained, also against the effects of growing heavy traffic with expected higher gross weights and axle loads.     

Beurteilung des Tragverhaltens von Spannbetonbrücken mit spannungsrisskorrosionsgefährdetem Spannstahl

Die Beurteilung des Tragverhaltens von Spannbetonbrücken, die in den 1960er und 1970er Jahren in Deutschland und Österreich gebaut wurden und für deren Herstellung vergütete Spannstähle Verwendung fanden, ist seit vielen Jahren Gegenstand der Forschung mit starkem Praxisbezug. Im vorliegenden Beitrag werden nach einer theoretischen Erläuterung des Tragverhaltens die praxisrelevanten Fragestellungen wie das Vorgehen bei der Beurteilung sowie weiterführende Untersuchungen erörtert. Zudem werden aktuelle Entwicklungen in der Forschung zur Beurteilung von gefährdeten Spannbetonbrücken aufgezeigt und diskutiert. Assessment of Load Bearing Behaviour of Post‐Tensioned Concrete Bridges Equipped with Prestressing Steel Susceptible to Stress Corrosion Cracking Basics, Practical Application and Current Developments Due to extending lifetime of infrastructure in general and posttensioned concrete bridges in particular the importance of durability aspects increases. Especially older prestressed bridges built in the 1960s and 1970s in Germany and Austria are subject of these investigations. Some of these post‐tensioned concrete bridges consist of quenched and tempered prestressing wires with high susceptibility to stress corrosion cracking as we know nowadays. This article deals with recalculation, assessment and evaluating methods for post‐tensioned concrete bridges including further analytic for endangered bridges. The procedure is shown by an example of an existing multi girder bridge and furthermore, future prospects are given.     

Instandsetzung von Spannbetonbrücken bei laufender Schadensanalyse

Bei der Instandsetzung von zwei etwa 30 Jahre alten Spannbetonbrücken traten eine Reihe von Problemen auf, die einer raschen Klärung bedurften, um unter den komplizierten Randbedingungen das Vorhaben fortführen zu können. Dies betraf unter anderem sofortige Materialanalysen und Erarbeitung von Lösungen zu den Themen Abdichtung/Aus kris talli sa tionen mit etwaigen Treibreaktionen bzw. gefährdeter Betonzusammensetzung und Rissen sowie ganz besonders zu den Lagern, Betonschäden an Endquerträgern und Auflagerbänken. Ein weiterer Schwer punkt war die Instandsetzung von Spann gliedern, da Mängel bei der Endverankerung festgestellt wurden. Die nötigen Unter suchungen nebst Lösungen der Pro bleme werden dargestellt. Repair of prestressed concrete bridges with continuous failure analysis. During the reinstatement of two 30 year old pre‐stressed concrete bridges a great range of problems occurred. These problems required a rapid clarification to be able to continue the project under the complicated boundary conditions. This concerned among others immediate material analyses and development of solutions to the topics sealing/crystallizations with possible expansive reactions or endangered concrete composition and cracks as well as in particular bearings, concrete damages to end transversal girders and bridge seats. A further main focus was the repair of tendons because there were detected defects at the end anchorage. The necessary investigations together with the solutions of the problems are illustrated.     

Rechnerische Beurteilung der Schubtragfähigkeit einer Spannbetonbrücke mit geringem Querkraft bewehrungsgrad

Assessment of the shear strength of an existing post‐tensioned concrete bridge with a low amount of shear reinforcement – First application of the ”Flexural Shear Crack“ model in practice In recent years it was reported several times, that within a static assessment according to current standards the shear strength of post‐tensioned bridges, which were built in Austria before 1989, cannot be fulfilled any more. However, test results prove, that especially prestressed structures have additional load bearing capacities, which cannot be reproduced with current calculation models. In the framework of a pilot project a new developed shear model (Flexural Shear Crack Model) was used. In this paper the approach as well as the main results of a statical reevaluation of an existing post‐tensioned road bridge at the Tauern Autobahn A10 will be presented.     

Berücksichtigung von Viskoelastizität bei der Beurteilung von Zwangbeanspruchungen erhärtender Massenbetonbauteile

Der nachfolgende Aufsatz behandelt die Berechnung und Messung von hydratationsbedingten Zwangspannungen in massigen Betonbauteilen. Dabei wird auf das zeitveränderliche und viskoelastische Materialverhalten des Betons und dessen Berücksichtigung innerhalb eines numerischen Berechnungsverfahrens besonders eingegangen. Die vorgestellten Zusammenhänge werden anhand eines Dehnstabmodells mathematisch beschrieben und innerhalb eines FE‐Modells numerisch implementiert. Im Zuge des Betonierens einer Sohlplatte eines Schleusenbauwerks werden die Berechnungsergebnisse mit Messdaten für die Spannungsentwicklung verglichen und bewertet. Consideration of Viscoelasticity in Computation of Hydration induced Stresses in Massive Concrete Members The present document reports about the computation and measurement of hydration induced stresses in massive concrete structures. It focuses on the time dependent and viscoelastic material properties of concrete and their consideration within a numerical computation procedure. The presented relationships will be described mathematically by using a strain‐rod‐model and be implemented in a FEM‐Model. The computed results will be compared and evaluated by using measured stress data obtained during the casting of a lock's base slab.     

FEM‐Untersuchung zur Beurteilung des Einflusses von geometrischen Imperfektionen auf die Lebensdauer von Schraubenverbindungen bei Windkraftanlagen

FEM‐Investigation to evaluate the influence of geometric imperfections on fatigue of preloaded bolt connections of Wind Energy Converters. There is a strong influence on fatigue of preloaded bolted ring flange connections with geometric imperfections, as they are used for tubular steel towers of wind turbines. This can result in fatigue failure of the connections because the bolts could not take the high dynamic loads sufficiently. This paper is dealing with the results of [1] in which the influence on fatigue of variing geometric imperfections at L‐flanges was investigated numerically. The results of this thesis paper lead to a gener criteria, which can be very useful in order to rate geometric imperfections, as they appear in production line. It should be used to sensitize manufactorers and quality responsible staff in reason of allowable tolerances in the assembly process of windturbine towers.     

Beurteilung des Ermüdungsverhaltens von Krankonstruktionen bei Einsatz hoch‐ und ultrahochfester Stähle

Analysis of fatigue details typically used in crane structures. For joints and attachments in crane structures usually the verification of the fatigue strength governs the design. When using the nominal stress approach some details often used in crane structures, especially welded details, cannot be classified according to the tables of fatigue classes given in the standards. Together with representatives from the crane industry such details have been identified. In the frame of the FOSTA research project P 512 fatigue tests have been carried out, so that proposals for the classification according to Eurocode 3 could be made. In addition, fatigue tests on structural details already covered by the codes have been executed to check the influence of material strength. A general increase of the fatigue strength when using high strength steel could not be observed but details with low notch effects show a better behaviour compared with normal strength steel. This is attributed to higher fabrication qualities. The research project P 512 was funded by the Stiftung Stahlanwendungsforschung, Essen, and has been carried out in behalf of the Forschungsvereinigung Stahlanwendung e.V.