Rissbildung und Zugtragverhalten von mit Fasern verstärktem Stahlbeton am Beispiel ultrahochfesten Betons

Wegen des Problems möglicher Inhomogenitäten der Faserverteilung/‐ orientierung ist der Einsatz von Fasern als alleinige Bewehrung im konstruktiven Ingenieurbau auf wenige Anwendungsgebiete beschränkt (z. B. Aufnahme von Zwangbeanspruchungen). Unter Zugbeanspruchung lässt sich zudem mit den in der Praxis für normalfeste Betone gebräuchlichen Fasergehalten nach der Erstrissbildung kaum ein verfestigendes Verhalten erzielen. Kombiniert man jedoch Stabstahlbewehrung und Faserbewehrung zu einem stahlfaserverstärkten Stahlbeton, so addieren sich die Vorteile beider Verbundwerkstoffe gleichermaßen. Besonders bei erhöhten Anforderungen an die Rissbreite (Größenordnung: unter 0, 1 mm) kann durch gemischte Bewehrung aus Stabstahl und Fasern eine wesentliche Verbesserung gegenüber Stahlbeton erzielt werden. Im Teil 1 dieses Beitrags wurden die für das Verständnis der unterschiedlichen Wirkungsweisen der beiden Bewehrungselemente “Stabstahl” und “Fasern” erforderlichen mechanischen Zusammenhänge dargestellt. Im Teil 2 erfolgt eine Überprüfung der abgeleiteten Beziehungen anhand experimenteller Untersuchungen an gemischt bewehrten Zugelementen aus ultrahochfestem Beton (UHPC). Für UHPC erreicht die Thematik besondere Aktualität, da aus Gründen der Duktilität der Einsatz von Fasen bei diesen Betonen die Regel ist. Der Nachweis der Begrenzung der Rissbreite bei kombinierter Bewehrung wird zudem an zwei Rechenbeispielen veranschaulicht. Crack Formation and Tensile Behaviour of Concrete Members Reinforced with Rebars and Fibres exemplified by Ultra‐High‐Performance Concrete. Part 2: Experimental Investigations and Examples of Application Due to the problem of possible inhomogeneities of fibre distribution/orientation the use of fibres as sole reinforcement is limited in engineering practice to few applications (e. g. coverage of stresses due to constraints). In addition, it is hardly possible to obtain strain hardening after first crack formation with fibre contents commonly used for normal strength concretes. However, if steel fibre reinforcement is used in combination with bar reinforcement, the advantages of both components are additive in the composite material. Especially for enhanced requirements concerning the crack width (order of magnitude: 0.1 mm), with combined reinforcement of rebars and fibres an essential improvement compared to reinforced concrete can be achieved. In part 1 of this contribution the mechanical relationships required for the understanding of the different behaviours of the two reinforcing elements “rebars” and “fibres” have been presented. In part 2 the derived relationships are validated on the basis of experimental investigations on tensile members with combined reinforcement made of ultra‐high‐performance concrete (UHPC). For UHPC the topic is of special interest, because fibres are added to these concretes generally to improve ductility. The crack width control with combined reinforcement is furthermore illustrated by means of two examples.     

Rissbildung und Zugtragverhalten von mit Fasern verstärktem Stahlbeton am Beispiel ultrahochfesten Betons

Crack Formation and Tensile Behaviour of Concrete Members Reinforced with Rebars and Fibres exemplified by Ultra‐High‐Performance Concrete Part 1: Crack Mechanical Relationships When combining conventional non‐pre‐stressed or pre‐stressed reinforcement with fibres, differences in the load‐carrying and deformation behaviour arise in comparison to the well‐known reinforced and pre‐stressed concrete. This fact holds true comparably for all concrete classes. However, it is of special interest for ultra‐high‐performance concretes (UHPC), because fibres are added to these concretes generally to improve ductility. With regard to durability, the positive influence of the fibres on the crack formation process and the crack widths in the serviceability range is significant. Especially for enhanced requirements concerning the crack width (order of magnitude: 0.1 mm) with combined reinforcement of rebars and fibres an essential improve compared to reinforced concrete can be achieved. The analysis of the crack formation process presumes the understanding of the different behaviours of the two reinforcing elements ”rebars” and ”fibres”. In part 1 of this contribution, the therefore required mechanical relationships are presented and linked with each other considering equilibrium and compatibility. In part 2 the derived relationships are validated on the basis of experimental investigations on tensile members with combined reinforcement made of UHPC. The application is furthermore illustrated by means of two examples. Because of its universal formulation, the presented proposal is generally applicable to all types of concrete reinforced with rebars and fibres, i.e. it is not limited to ultra‐high‐performance concrete.     

Aktualisierte Diagramme zur Bemessung von Stahlkonstruktionen für den Brandfall nach Eurocode 3

Updated diagrams for fire design of steel structures based on Eurocode 3. The forthcoming design methods for steel structures subjected to fire in Eurocode 3 [1] differ considerably from the methods in the current German standard DIN 4102. The required fire protection material cannot be specified by simplified tables any longer. The structure will now be designed for the case of fire like it is done for ambient temperature. With this method the cost‐intensive structural fire protection can be optimized or even be avoided. However, the more realistic design comes along with a higher effort of calculation. To minimize this effort, in [3] a design tool was developed and published in [4]. In this tool the design methods of the Eurocode have been diagrammed for simple use. However, it is based on the prestandard and cannot be used any longer due to some changes in the current Eurocode. Therefore updated diagrams are presented and further diagrams are added to reduce the calculation effort for the design of steel structures in fire as much as possible.     

Gemeinsame Grundlagen von Methode 1 (wirksame Breiten) und Methode 2 (Beulspannungsbegrenzung) beim Plattenbeulnachweis nach Eurocode 3 – Teil 1‐5

Demonstration of the common basis of method1 (effective width approach) and method2 (stress limit approach) for the plate‐buckling assessment of built‐up steel components according to Eurocode 3 – Part 1‐5. Eurocode 3 – Part 1‐5 gives two methods for the plate buckling verification of built up members: method 1 with an effective width approach and method 2 with a stress limit approach. These methods have a common basis in the form of a bilinear stress‐strain diagram with a “plastic plateau”, the level of which is determined by the buckling strength of the plate element considered. The integration of the plate buckling strengths of all its plated elements give the resistance of the full cross‐section in analogy to plastic design of cross‐sections. The mobilization of the strength reserves of plated elements stronger than others by this integration effects a modification of the stress fields initially determined by elastic design in the weaker plate elements. This modification leads to an increase of shear stresses and a decrease of longitudinal stresses following the interaction curve for plate buckling strengths. This modification is also the cause for the shape of the interaction formula for the cross‐sectional resistances for plate buckling. Method 2 therefore provides two levels of resistance, one without the mobilisation of strength reserves suitable for serviceability limit checks, the other with mobilisation of strength reserves applicable to ultimate limit state verifications. Method 2 with mobilizing strength reserves can be also expanded to cover the plate‐buckling of stiffened plates. Though so far no design code with an explicite rule for this case exists, the reassessment of various plated bridge structures using different criteria for the size of admissible yield zones demonstrates that such an expansion of the method would be consistent with the results of the traditional plate buckling design according to DASt‐Ri 012.     

Das Druckgurtmodell für Stahlbetonbauteile

In den vergangenen Jahren wurden verschiedene auf die Bemessung von Stahl‐ und Spannbetonbauteilen ausgerichtete theoretische Modelle entwickelt. Heute stehen das Zuggurtmodell, das Modell der gerissenen (Steg‐)Scheibe und das Druckgurtmodell zur Verfügung, auf deren Grundlage Verformungsverhalten und Tragwiderstände umfassend beurteilt werden können. Auf das Druckgurtmodell wird im vorliegenden Beitrag detailliert eingegangen. Es berücksichtigt die Festigkeits‐ und Duktilitätssteigerung durch eine Umschnürungsbewehrung sowie die bruchmechanisch begründete Entfestigung des Betons und die damit einhergehende Verformungslokalisierung. Auf der Grundlage neuerer Versuchsergebnisse können für die komplexen Interdependenzen zwischen diesen Effekten plausible Beziehungen angegeben werden. Die mithilfe des Druckgurtmodells gewonnenen Erkenntnisse sind für die Baupraxis von Bedeutung: Das Verhalten von auf Druck beanspruchten Bauteilen (z. B. Druckplatten von Brückenquerschnitten, Stützen) lässt sich zuverlässig erfassen; darüber hinaus ergeben sich wichtige Hinweise für die konstruktive Durchbildung. Compression Chord Model for Structural Concrete In recent years, several theoretical models have been developed with the scope on the design of reinforced and prestressed concrete structures. Today, the tension chord model, the cracked membrane model and the compression chord model are available, with which the deformation behaviour as well as ultimate loads can be determined. In the present contribution the compression chord model is discussed in detail. The model takes the increase of strength and ductility due to a confining reinforcement into account and considers the softening as well as the localisation of deformations accompanying the fracture of concrete. On the basis of new test results plausible relations for the complex interdependencies between these effects are found. The results of the compression chord model are relevant for practical applications: The behaviour of compressed members (e.g. in bridge girders or columns) can be assessed reliably; moreover, hints for the detailing of reinforcement can be deduced.     

Normen für den Konstruktiven Ingenieurbau

Codes in Structural Engineering In structural engineering codes are of essential importance. Codes have to represent accepted rules as well as the state of the art in science and technology and they provide a basis for the communication between all involved professionals. In codes of the future all procedural steps of planning have to be included – design, execution, service, conservation and dismantlement – and an integral approach to safety and quality has to be adopted. In comparison to present codes, the technical breadth and the level of detail will increase. Nonetheless, the overall aim of code writers shall be to come up with regulations that are easy to understand and to apply. Codes of the future have to (re‐)gain their significance and acceptance.     

Ein einheitliches dreiaxiales Bruchkriterium für alle Betone

Die Auswertung zahlreicher mehraxialer Versuche hat ergeben, dass die auftretenden Versagensmechanismen unter mehraxialer Beanspruchung für alle Betonarten prinzipiell gleich sind. Dies gilt sowohl für Normal‐ bis ultrahochfesten Beton, für Leichtbeton als auch für Faserbeton. Das vorgestellte Bruchkriterium orientiert sich an diesen Versagensarten und wird über eine entsprechende Kalibrierung an das Verhalten des jeweiligen Betons angepasst. Im Zuge der Aktualisierung des CEB‐FIP Model Codes 90 wird es Einzug in die Bemessungsvorschriften finden. A Unified Multiaxial Fracture Criterion for all Concretes The evaluation of numerous multiaxial tests has shown that the occurring failure mechanisms under a multiaxial load are basically the same for all types of concrete. This is true for normal to ultra high performance concrete, lightweight concrete, as well as for fibre concrete. The presented fracture criterion is based on these failure types and is adjusted by a corresponding calibration to the behaviour of each concrete. In the course of the upgrade of the CEB‐FIP Model Code 90, it will find entry into the dimensioning specification.