Rissbreiten in Stahlbetonbauteilen vergrößern sich unter Langzeitbeanspruchung durch das zeitabhängige Materialverhalten des Betons. Während viele Rissbreitenansätze das Betonschwinden bei der Rissbreitenberechnung vernachlässigen, zeigen Untersuchungen in der Literatur, dass Schwinddehnungen bei der Rissbreitenberechnung berücksichtigt werden sollten. Um die einzelnen Effekte aus Schwinden, Verbund- und Betonkriechen auf das Rissbild einzuordnen, wurden die verschiedenen Anteile unter einer Lastbeanspruchung analysiert. Hierzu wurde im abgeschlossenen Rissbild eine Parameteranalyse mit einem an experimentellen Versuchen validierten rheologischen Modell durchgeführt und der Anteil des Betonschwindens an der Rissbreitenvergrößerung festgestellt. Ein Vergleich mit dem Rissbreitenansatz des Model Code 2010 zeigt, dass der Ansatz des Model Code 2010 potenziell gut geeignet ist, zeitabhängige Effekte wirklichkeitsnah abzubilden, diese allerdings überschätzt. Aus diesem Grund wurde der Ansatz modifiziert, sodass in einem weiteren Vergleich sehr gute Ergebnisse erzielt wurden, ohne die in der Praxis etablierte Rissbreitenformel grundlegend zu verändern.

Influence of concrete shrinkage on crack widths in reinforced concrete members under direct loading
Under long-term loading, crack widths in reinforced concrete members increase due to the time-dependent material behaviour of concrete. While many crack width approaches neglect shrinkage in crack width calculations, investigations in the literature indicate that shrinkage strains should be taken into account for crack width calculation. In order to classify the individual effects of shrinkage, bond creep and concrete creep on the crack pattern, they were analysed separately under direct loading. For this purpose, a parameter analysis was performed in the stabilized cracking stage with a rheological model validated by experimental tests to determine the proportion of concrete shrinkage on crack width increase. A comparison with the crack width approach of Model Code 2010 shows that the approach of Model Code 2010 is potentially well suited to describe time-dependent effects realistically, but overestimates them. For this reason, the approach was modified without fundamentally changing the established crack width formula. With that modification, very good results were achieved in a further comparison.

Beton ist eines der wichtigsten Baumaterialien in der modernen Ingenieurspraxis und findet sein Anwendungsgebiet im Brückenbau, Hochbau und Grundbau. Beton ist ein Material, welches sein Verhalten im Laufe der Zeit in Abhängigkeit von den Umweltbedingungen verändert. Besonders Verformungen, die durch Kriech- und Schwindprozesse hervorgerufen werden, spielen eine wichtige Rolle in der Bemessung von Betonstrukturen - vor allem im Hinblick auf die Gebrauchstauglichkeit. Bereits in den ersten Bemessungsnormen für Betonstrukturen gab es Regelungen hinsichtlich der Berücksichtigung dieser Einflüsse. Zu Beginn wurden die Verformungen mit einem äquivalenten Temperaturgradienten berücksichtigt. Erst mit der Herausgabe der Spannbetonnormen wurden entsprechende Kriech- und Schwindmodelle vorgeschlagen und bis hin zur heute gängigen Praxis weiterentwickelt. Im ersten Teil dieser Arbeit wird ein Überblick über die historische Entwicklung dieser Modelle bis zu den heute geltenden Eurocode 2 und fib Model Code-Richtlinien gegeben. Im zweiten Teil wird eine Sensitivitätsanalyse der Eingangsparameter der aktuellen Modelle präsentiert, gefolgt von einem Anwendungsbeispiel, in dem die Unterschiede der verschiedenen Modelle auf Basis einer Durchbiegungsberechnung veranschaulicht werden.

Evolution of creep and shrinkage models for concrete structures in Austria and Germany - Evaluation of the current models regarding the sensitivity of the input parameter
Concrete is one of the most important materials in civil engineering structures and finds its application in the building of bridges, ground engineering and building construction. As widely known, concrete is a material which changes its behavior with time in dependency of the environmental conditions. Especially the long-term deformations caused by creep and shrinkage processes are of great interest regarding the durability and sustainability of infrastructures. Early versions of design codes gave some guidelines on how to treat deformations caused by these sources. In the beginning, these deformations were taken into account by an additional temperature drop. The first sophisticated approaches on how to deal with these long-term processes were published in the standards for prestressed concrete structures. These early models were updated and improved to today's prevailing documents such as the Eurocode 2 and the fib Model Code. The first part of this paper gives an overview of the historical development of creep and shrinkage models in Austria and Germany until today's guidelines. The second part presents a sensitivity study of the current models regarding the input parameters for some given scenarios. Lastly, the differences between the models are highlighted based on an application example taken from literature.

The analysis of the mechanical behaviour of a reinforced concrete tie subjected to a monotonic loading in the stabilized cracking stage is performed here by way of a theoretical general model that considers the effect of the so-called Goto cracks (secondary cracks). It is shown, in particular, that the average bond stress along the transmission length depends not only on the concrete strength as assumed by the fib Model Code for Concrete Structures 2010, but also on reinforcement ratio and bar diameter. In this regard, tabulated theoretical values of the average bond stress are proposed as a function of the aforementioned parameters. Moreover, the secondary cracks reduce the effect of tension stiffening on the relative mean strain. On the basis of the main results obtained with the general model, some improvements are suggested for the calculation methods proposed by fib Model Code 2010 and Eurocode 2 concerning the average value of the bond stress and taking into account the influence of the secondary cracks on the mean deformation. An improved calculation method is therefore performed. Finally, the theoretical results of crack spacing and crack width obtained with the general and improved methods are compared with experimental data obtained from extensive research on RC ties.

This paper presents a procedure for designing precast tunnel segments for mechanically excavated tunnel linings in fibre-reinforced concrete, without any traditional steel reinforcement. Both ultimate and serviceability limit states are considered as well as structural checks at different construction stages of the segment, including demoulding, positioning on floor, storage, transportation, handling and the final stage concerning the loads due to the ground pressure.
The structural checks are performed by means of bending moment-axial force interaction envelopes for both the considered limit states, once the constitutive relationship of the material is defined for each stage. Traditional interaction envelopes are drawn for the ultimate limit state check, whereas for the serviceability limit state check, envelopes obtained by limiting the maximum crack opening and maximum concrete compressive stress are proposed. The shear action is also accounted for by reducing the bending moment-axial force envelope. The possibility of having the assistance of a test procedure for particular loading situations is also proposed. Finally, a case study related to a precast steel fibre-reinforced concrete segment is analysed in order to clarify the procedure and show, practically, how to define the actions and evaluate the interaction envelopes.

In the fib Model Code for Concrete Structures 2010, fibre-reinforced concrete (FRC) is recognized as a new material for structures. This introduction will favour forthcoming structural applications because the need of adopting new design concepts and the lack of international building codes have significantly limited its use up to now. In the code, considerable effort has been devoted to introducing a material classification to standardize performance-based production and stimulate an open market for every kind of fibre, favouring the rise of a new technological player: the composite producer.
Starting from standard classification, the simple constitutive models introduced allow the designer to identify effective constitutive laws for design, trying to take into account the major contribution in terms of performance and providing good orientation for structural uses. Basic new concepts such as structural characteristic length and new factors related to fibre distribution and structural redistribution benefits are taken into account. A few examples of structural design starting from the constitutive laws identified are briefly shown.
FRC can be regarded as a special concrete characterized by a certain toughness after cracking. For this reason, the most important constitutive law introduced is the stress-crack opening response in uniaxial tension. A wide discussion of the constitutive models introduced to describe this behaviour, which controls all the main contributions of fibres for a prevailing mode I crack propagation, is proposed. The validity of the models is discussed with reference to ordinary cross-sections as well as thin-walled elements by adopting plane section or finite element models.

Structures made of a material with a very high standard deviation, such as fibre-reinforced concrete, exhibit an exceptionally safe prediction of the maximum bearing capacity when this is derived from characteristic values identified by means of small specimens. This is emphasized when the structures are characterized by high redundancy. In this regard, two reference tests representing two extreme situations are considered: a) simply supported unnotched full-scale beams characterized by a statically determinate loading scheme and b) full-scale slabs on the ground characterized by a statically indeterminate loading scheme. The Italian standard and, more recently, the fib Model Code for Concrete Structures 2010 have introduced a coefficient (structural redistribution factor) that is able to take into account the reduced variability of mechanical bearing capacity when associated with a large volume involved in the failure process and/or when the structure is able to redistribute stresses significantly, thus favouring the average rather than the minimum strength. A numerical procedure taking into account the expected heterogeneity of the mechanical characteristics in the structure is introduced for the first time to evaluate the redistribution factor.

This paper focuses on the reliability of the design approach proposed in the fib Model Code for Concrete Structures 2010 for estimating the ultimate capacity of fibre-reinforced concrete (FRC) elevated slabs on the basis of different tests for material characterization. The fracture properties of the material are determined through three-point bending tests on notched beams and through double edge wedge splitting (DEWS) tests carried out on cylinders cored in the full-size test structure. As a case study, an FRC elevated flat slab 0.2 m thick is considered which consists of nine bays (panels) measuring 6 × 6 m (overall size 18.3 × 18.3 m) and is supported by 16 circular concrete columns. The ultimate bearing capacity of the slab determined experimentally is compared with the design value predicted by means of a procedure based on limit analysis following fib Model Code 2010. The results show that the method proposed in fib Model Code 2010 using the characteristic values and the classification is reliable. Even if the tests are affected by a significant standard deviation and the two experimental campaigns with three-point bending tests give a significant difference between class “5c” and class “3e”, the structural test results in a loadbearing capacity that is always larger than the predicted one, which considers a safety coefficient for the material &ggr;F = 1.5.

Mit dem Ziel, einen Überblick über aktuelle normative Modelle zur Berechnung der Rissbreite im Stahlbetonbau zu erhalten, wurden diese am iBMB, Fachgebiet Massivbau der TU Braunschweig zusammengestellt und verglichen. Um einen unabhängigen Vergleich der Modelle sicherzustellen, wurde eine Datenbasis mit Versuchswerten zu Rissbreiten zusammengestellt. Der Modellvergleich wurde auf zentrische Zugversuche, bei denen charakteristische Werte der gemessenen Rissbreiten (95 %-Quantilwerte) angegeben waren bzw. ermittelt werden konnten, beschränkt. Durch die Vergleichsberechnungen konnte gezeigt werden, dass zwischen den unterschiedlichen Modellen signifikante Abweichungen auftreten. Während der Eurocode 2 die im Versuch gemessenen Rissbreiten tendenziell überschätzt, werden die Rissbreiten mit den Nationalen Anhängen für Deutschland bzw. Österreich überwiegend unterschätzt. Mit dem Modell nach Model Code 2010 werden die Versuchsergebnisse im Mittel ebenfalls überschätzt, die Ergebnisse liegen jedoch zwischen denen nach Eurocode 2 sowie den Nationalen Anhängen für Deutschland bzw. Österreich. Des Weiteren kann mit den Modellen der Nationalen Anhänge für Deutschland und Österreich sowie des Model Code 2010 eine, gegenüber dem Eurocode 2, erhöhte Vorhersagegenauigkeit erreicht werden.

Comparison of code provisions for the calculation of crack widths
In order to give an overview of current code provisions for the calculation of crack widths, these provisions were compiled and compared at the iBMB, Department of Concrete Construction of TU Braunschweig. In order to ensure an independent comparison, a database with test values for crack widths was compiled. The comparative calculations were limited to tensile tests in which characteristic values of the measured crack widths (95 %-quantile values) were or could be determined. As a result, it can be stated that significant deviations occurred for the different code provisions. While the provisions by Eurocode 2 tend to overestimate the test values, the National Annexes for Germany and Austria mainly underestimate the measured crack widths. Model Code 2010 also overestimates the test results on average, but the results are between those obtained with Eurocode 2 and the national annexes for Germany and Austria. Furthermore, the models in the national annexes for Germany and Austria as well as the Model Code 2010 achieve a higher prediction accuracy than Eurocode 2.

The scope of fib Model Code for Concrete Structures 2010 includes the fully fledged performance- and displacement-based seismic design of new structures and assessment of existing ones. This part of fib Model Code 2010 covers buildings, bridges or similar concrete structures and aims to provide well-defined performance levels for specific seismic hazard levels. Detailing of members for ductility is not based on opaque prescriptions, as in current codes, but on transparent, explicit verification of inelastic deformation demands against capacity limits. The reference analysis method is non-linear dynamic, but under certain conditions inelastic deformation demands may be estimated from linear analysis and the 5%-damped elastic response spectrum; in that case force demands on force-controlled, brittle failure modes are estimated from the plastic mechanism through equilibrium. In order to predict the seismic deformation demands with some confidence, the analysis should use realistic values for the member secant stiffness up to the yield point. The paper explains the background to the expressions given for this property in fib Model Code 2010 as well as of those for the deformation limits used when verifying seismic deformation demands. The modifications to the shear resistance approach of fib Model Code 2010, which takes cyclic loading into account, are also explained and justified.

Während die Druckfestigkeit des Betons durch gleichzeitig wirkenden Querdruck gegenüber der einaxialen Druckfestigkeit erheblich gesteigert werden kann, führen Querzugbeanspruchung und Rissbildung zu einer Abminderung der Tragfähigkeit. Dies gilt für unbewehrten Beton und Stahlbeton gleichermaßen. In den einschlägigen Regelwerken finden sich hierzu international sehr unterschiedliche Bemessungsansätze, wobei die vorgesehenen Abminderungsbeiwerte für denselben Anwendungsfall um das bis zu Zweifache differieren. Die Frage der Druck-Zug-Festigkeit von Stahlbeton wurde in den vergangenen 40 Jahren von zahlreichen Wissenschaftlern untersucht. Ihre Ergebnisse sind allerdings zum Teil ebenso widersprüchlich wie die aktuelle Normensituation. Basierend auf eigenen experimentellen Untersuchungen sowie einer kritischen Auswertung und Einordnung als richtungweisend angesehener, früherer Versuchsreihen wird ein Vorschlag zur Abminderung der Druckfestigkeit des gerissenen Stahlbetons entwickelt. Erstmals wird dabei auch der Einfluss einer Faserzugabe in Kombination mit Stabstahlbewehrung berücksichtigt. Ein Vergleich mit den in DIN 1045-1, CEB-FIP Model Code 1990, Eurocode 2 und ACI Standard 318-05 angegebenen Bemessungsregeln zeigt, dass allein DIN 1045-1 die in den Versuchen beobachtete maximale Abminderung der Druckfestigkeit durch Querzug und Rissbildung zum Teil erheblich unterschätzt, so dass eine konservative Auslegung der Tragwerke nicht immer sichergestellt ist.

Biaxial Compression-Tension-Strength of Reinforced Concrete and Reinforced Steel Fibre Concrete
The compressive strength of concrete can be substantially increased in relation to uni-axial compressive strength by transverse compression acting at the same time. In contrast, transverse tension and cracking lead to a reduction of the load-carrying capacity. This holds true for plain concrete as well as for reinforced concrete. In international standards very different calculation rules can be found on this subject, whereby the provided reductions differ up to a factor of two for the same application. The question of biaxial compression-tension-strength of reinforced concrete was examined in the past 40 years by numerous scientists. Their results are, however, partially contradictory in the same way as the current standard situation. Based on own experimental investigations as well as on a critical review and classification of former test series regarded as trend-setting, a proposal for the reduction of the compressive strength of cracked reinforced concrete is developed. For the first time, also the influence of fibres in addition to bar reinforcement is considered thereby. A comparison with the calculation rules in DIN 1045-1, CEB-FIP Model Code 1990, Eurocode 2, and ACI Standard 318-05 shows, that exclusively DIN 1045-1 underestimates sometimes substantially the maximum reduction of the compressive strength by transverse tension and cracking observed in the tests, so that a conservative design of structures cannot always be ensured.

A concrete interface is a material discontinuity that requires special care with respect to structural design and assessment. Therefore, the definition of design expressions based on experimental testing data must ensure the necessary reliability depending on the type of structure and its use. The present work describes a new proposal for the design of concrete interfaces subjected to shear loading for different roughness profile types. The proposal is characterized by three linear branches (for monotonic loading) and an S-N curve (for cyclic loading) and is the result of a parametric analysis of existing experimental data (obtained by the authors and also from an extensive literature search) based on statistical and probabilistic methods. Design expressions were defined in order to minimize the dispersion and variability of the safety factor values for each experimental test considered and also to assure that those values are within a target range (defined according to reliability considerations). These improvements became clearer when the new proposal was compared with the most common design code recommendations.

Mit der Einführung des fib Model Code for Service Life Design werden dem bemessenden Ingenieur anerkannte und bewährte Vorhersagemodelle zur Verfügung gestellt, mit denen die Bestimmung der zeitabhängigen Carbonatisierungstiefe bzw. Chlorideindringtiefe bis zur Depassivierung des Betonstahls möglich ist. Unter Einbezug statistischer Kennwerte (z. B. Verteilungstyp, Mittelwert, Standardabweichung) lässt sich eine vollprobabilistische Dauerhaftigkeitsberechnung durchführen, mit der im Rahmen des leistungsbezogenen Entwurfsverfahren nach DIN EN 206-1, Anhang J, die zeitabhängige Zuverlässigkeit gegenüber Carbonatisierung bzw. Chlorideindringen bestimmbar ist. Rechnerische Untersuchungen zeigen jedoch, dass die vom Bindemittel abhängige, rechnerische Bandbreite der Zuverlässigkeit für jede Expositionsklasse variiert. Der DAfStb empfiehlt nun in einem in diesem Heft ebenfalls abgedruckten Positionspapier expositionsabhängige Mindestzuverlässigkeiten, die bei einer Anwendung von leistungsbezogenen Entwurfsverfahren nach DIN EN 206-1, Anhang J, nachzuweisen sind. In diesem Beitrag werden die wichtigsten Hintergrundinformationen zu verwendeten Modellen, Daten und Nachweiskonzepten geliefert.

This paper presents an experimental campaign aiming to assess the cracking behaviour of flexural members made with self-compacting concrete (SCC) and reinforced with both rebars and steel fibres recycled from end-of-life tyres (ELT). The characteristics, constructability and performance of this new type of fibre are first discussed. The results of the tests carried out are then presented and discussed. The parameters that have been investigated are: &phgr;/&rgr;s,ef, concrete cover and fibre content. The results obtained show improvement in cracking behaviour, especially for low reinforcement ratios and large covers. Results are compared with the predictions of the recently published fib Model Code for Concrete Structures 2010. The main objective of this investigation is to evaluate the efficiency of a new type of fibre technology for crack width control of RC elements, with advantages in sustainability from the point of view of recycling and durability.

This article investigates the transferability of the Simplified Modified Compression Field Theory (SMCFT) [2], which is known in reinforced concrete design and included in the fib Model Code for Concrete Structures 2010 (Volume 3) [1], to reinforced or prestressed masonry beams (RM beams) with or without an additional layer of concrete. The investigation for this work is the obsolete shear design concept that has been used until now for reinforced masonry under shear loading, which does not adequately reflect the actual load-bearing behaviour of significant areas of masonry. The fundamentals of the SMCFT are explained and the transferability of the theory to RM beams is examined, taking into account in particular the different material properties of masonry compared to reinforced concrete. A first approach for future application is represented by the equations presented here for the determination of the shear force capacity of RM beams. The verification is performed through a comparison of the shear resistances determined experimentally (exp.) and by calculation (calc.).

EN 1992 [1] is currently under revision. In that context it is justified to pass a critical eye over the standard, particularly if the models and resistance functions for the ultimate limit state comply with results from tests. The present contribution is limited to S&T models. The results of the analysis will primarily have relevance to corbels; however, some conclusions are general and should be considered when choosing the mathematical model for any S&T model. Ref. [1] is not sufficiently detailed to provide a basis for the complete design of, for example, corbels. The European Concrete Platform [2], which contains worked examples, has been prepared in accordance with and to support [1].
The two topics dealt with are 1) selection of the main model, and 2) details and use of the S&T model in [1] and [2], and particularly its ability to predict results from tests. The tests for the analysis are taken from a thorough study described in [3]. To evaluate results from using [1] and [2], a concept for the design of corbels suggested earlier has been used to calculate the same tests [4].
It will be claimed that the models in [1] and [2] and their ability to predict the outcome of tests have some flaws compared with calculations according to [4]. The detailed calculations are given in annexes, i.e. Part 1 for design according to [1] and [2] and Part 2 for design according to [4]. The annexes as well as [4] can be obtained from the publisher or the author.

Der Einsatz von Makrofasern aus Stahl gewinnt seit der bauaufsichtlichen Einführung der Richtlinie “Stahlfaserbeton” des Deutschen Ausschusses für Stahlbeton (DAfStb) auch in tragenden Betonkonstruktionen zunehmend an Bedeutung. Insbesondere beim Nachweis ausreichender Querkrafttragfähigkeit erweist sich die Fasertragwirkung als günstig, da eine Querkraftbewehrung auch bei Balken rechnerisch vollständig durch Fasern gebildet sein kann - Bügel also entfallen. Haupteinflussparameter ist die Nachrisszugfestigkeit, die aufgrund vielfältiger interagierender Einflussparameter hohen Streuungen unterliegt und nach Ansätzen der DAfStb-Richtlinie bzw. den Modellen des Model Codes 2010 deutlich unterschiedlich einfließt. Der Beitrag vergleicht die Prognosegenauigkeiten rechnerischer Querkrafttragfähigkeiten, indem rechnerische Bruchlasten experimentellen Daten von Schubversuchen aus der Literatur gegenübergestellt werden. Die aufgebaute Schubdatenbank umfasst dabei über 250 Versuche mit praxistypischen Konfigurationen, d. h., dass Querschnittsgeometrie, Längs- und Bügelbewehrungsgehalt, Nachrisszugfestigkeit des Stahlfaserbetons sowie Betondruckfestigkeit variable Parameter sind.

Database with shear tests on steel fibre reinforced concrete girders
The application of macro fibres made of steel is recently growing in concrete engineering since valid standards like the DAfStb-Guideline “Steel fibre reinforced concrete” of the German Committee for Structural Concrete (DAfStb) are available. In particular, it is meaningful to take into account fibre's effect in shear design to considerably reduce the amounts of stirrups. The most significant parameter is the steel fibre's post-cracking tensile strength that exhibits pronounced scattering due to several interacting parameters. It is differently considered in the design approaches of DAfStb-Guideline and fib Model Code 2010, respectively. The paper investigates accuracies of predicted shear resistances by comparing calculative and experimental ultimate loadings. For this purpose, a shear database is build up comprising more than 250 test results from the literature that differ in cross-section's geometry, ratios of longitudinal rebars and stirrups, fibre amounts and concrete compressive classes, respectively.

Die Regelungen zur Durchstanzbemessung im Eurocode 2 basieren im Wesentlichen auf den Modellvorstellungen in Model Code 90, der die Durchstanztragfähigkeit auf die Querkrafttragfähigkeit zurückführt. Nach Versuchsauswertungen ist diese Analogie für punktgestützte Platten jedoch nicht in jedem Fall zutreffend. Insbesondere für bügelbewehrte Platten ist bei der Bemessung nach Eurocode 2 ein Sicherheitsdefizit möglich, und es erscheint eine Anpassung der Bemessungsgleichungen zwingend erforderlich. Dieser Beitrag erläutert die Sicherheitsdefizite und schlägt verbesserte Bemessungsgleichungen vor.

Punching of Flat Slabs according to Eurocode 2
The punching shear provisions according to Eurocode 2 are based on the design recommendations in Model Code 90 where no differentiation between shear and punching shear design is made. This approach is not fully confirmed by test results for flat slabs. In particular for flat slabs with stirrups as shear reinforcement some design provisions according to Eurocode 2 do not achieve the required safety level. Therefore adjustments to the design equations seem to be necessary. In this paper the Eurocode 2 provisions are critically reviewed and improved design regulations are proposed.

CEB/FIP Model Code 1990 (MC-1990) did represent the technology and focus some 20 years ago. However, it soon became evident that the document had some notable lacunas. In 1995 the general assemblies of the two organizations endorsed CEB/FIP bulletin No. 228, extensions to MC 1990 for high-strength concrete, and in 2000 a similar extension to MC 1990 for lightweight aggregate concrete as bulletin No. 4.
The fib approved bulletin No. 34 Model Code for Service Life Design (MC SLD) in 2006. All these three additions have since matured and are now incorporated in the new fib Model Code for Concrete Structures 2010 (MC-2010).
The main purpose of an fib Model Code is to act as a model for operational standards. The obvious counterpart for a body such as fib operating worldwide is ISO. The initiative taken by MC SLD has therefore further matured in ISO TC-71/SC-3/WG-4 and it was accepted as ISO 16204 “Durability - Service Life Design of Concrete Structures” during the summer of 2012.
According to the obligations given in the WTO Agreement on Technical Barriers to Trade, it is hoped that these principles will be further implemented in national and regional standards.
This article describes the need for a transparent methodology when dealing with service life design, and the process - originating from a group of enthusiasts one decade ago - through fib and finally reaching international consensus in ISO.

No short description available.

Das ständig steigende Verkehrsaufkommen sowie die im Laufe der Jahre überarbeiteten Bemessungsnormen erfordern eine neue Beurteilung der angestrebten Nutzungsdauer bestehender Brücken. Die rechnerische Schubtragfähigkeit erweist sich hierbei in vielen Fällen als besonders problematisch. Im Zuge der Überarbeitung der österreichischen Nachrechnungsrichtlinie ONR 24008 “Bewertung der Tragfähigkeit bestehender Eisenbahn- und Straßenbrücken” wurden Vergleichsberechnungen bezüglich der rechnerischen Querkrafttragfähigkeit von Bestandsbrücken nach Eurocode 2 und fib Model Code 2010 durchgeführt. Der vorliegende Beitrag fasst die wesentlichen Ergebnisse aus dem Vergleich zwischen fib Model Code 2010 und dem Eurocode 2 zusammen und erläutert die Besonderheiten der Berechnung der rechnerischen Querkrafttragfähigkeit gemäß fib Model Code 2010.

Comparison of the calculative shear resistance of existing bridges according to Eurocode 2 and fib Model Code 2010
Due to the constantly increasing traffic loads and the revised design approaches it becomes necessary to reassess the expected service life of existing bridges. In many cases the calculative shear resistance turns out to be particularly problematic. In the course of the revision of the Austrian Assessment Technical Regulation ONR 24008 “Evaluation of load capacity of existing railway and highway bridges” comparative calculations according to the shear carrying capacity of existing bridges conforming to Eurocode 2 and fib Model Code 2010 were carried out. The present paper summarizes the main results of the comparison between the fib Model Code 2010 and Eurocode 2 and explains characteristics of the calculations of the shear resistance according to fib Model Code 2010.

The fatigue behaviour of concrete is gaining new relevance against the backdrop of continuous developments in concrete construction. Modern types of concrete are achieving ever higher strengths; hence, concrete structures are becoming increasingly attractive for new fields of application such as onshore and offshore wind turbines. The fatigue of concrete has a special relevance for these cyclically loaded structures and knowledge of the number of cycles to failure is no longer sufficient for their design. There are further questions concerning strain and stiffness development and the combination of fatigue loading and maritime environmental conditions which have been investigated with new testing methods at Leibniz Universität Hannover within the scope of the “ProBeton” research project. The first results of this project, which is supported by the Federal Ministry for Economic Affairs and Energy, are presented here.

Part I of this paper introduces an experimental programme carried out on RC members with thick-walled hollow circular cross-sections to study their behaviour under combined bending and shear. The study looked at ultimate resistance and propagation of characteristic crack pattern as well as the shape and behaviour of the failure sections as a function of wall thickness, amount of longitudinal and transverse reinforcement, shear span and axial force. Test results were used to verify a newly developed calculation model describing the behaviour of the members investigated at failure under combined bending and shear. This model will be presented in Part II.

Mit der bauaufsichtlichen Einführung des Eurocode 2 in Deutschland wurde die Durchstanzbemessung von Fundamenten und Bodenplatten neu geregelt. Für die Bemessung der Durchstanzbewehrung wird im Eurocode 2 unabhängig vom Bauteil ein 33°-Fachwerk mit konstantem Betontraganteil vorgeschlagen. Aufgrund von Sicherheitsbedenken wurde im Deutschen Anhang zum Eurocode 2 (EC2+NA(D)) davon abweichend ein Aufhängefachwerk ohne Betontraganteil für die Bemessung der Durchstanzbewehrung in Fundamenten und Bodenplatten eingeführt. Während zur Bewertung der Bemessungsgleichungen für die Ermittlung der Durchstanztragfähigkeit von Fundamenten ohne Durchstanzbewehrung bzw. der maximalen Durchstanztragfähigkeit verschiedene Versuchsergebnisse vorliegen, fehlen bislang systematische Versuchsreihen an Fundamenten mit einem Durchstanzversagen innerhalb des durchstanzbewehrten Bereichs (Bemessung der Durchstanzbewehrung).
Zur systematischen Untersuchung des Durchstanztragverhaltens von Fundamenten mit Bügeln als Durchstanzbewehrung wurden insgesamt drei Versuchsserien (elf Versuche) mit variierendem Durchstanzbewehrungsgrad durchgeführt. Dabei erfolgte die Variation der Durchstanzbewehrungsmenge über den Bügeldurchmesser. Weitere untersuchte Einflussparameter waren die Schubschlankheit und die statische Nutzhöhe. Im vorliegenden Beitrag werden die durchgeführten Versuche vorgestellt und die Versuchsergebnisse diskutiert. Durch den Vergleich der experimentell ermittelten Bruchlasten mit den rechnerischen Durchstanztragfähigkeiten nach Eurocode 2 und EC2+NA(D) können die bestehenden normativen Regeln bewertet werden. Darüber hinaus werden die Bemessungsregeln nach Model Code 2010 in die Bewertung mit einbezogen.

Punching strength of footings with varying shear reinforcement ratios
With the introduction of Eurocode 2 in Germany, the punching shear design provisions of footings and ground slabs were revised. According to Eurocode 2, the design of the shear reinforcement is performed by means of a design model considering a strut inclination of 33° and a constant contribution of concrete. Due to safety related concerns, in German Annex to Eurocode 2 (EC2+NA(D)), a different concept was introduced. In this context, the design of the shear reinforcement in footings and ground slabs is conducted without consideration of a concrete contribution. Various test series on reinforced concrete footings without shear reinforcement and with high amounts of shear reinforcement are available and can be used for the evaluation of the current provisions. Nevertheless, the evaluation of the code equations for the design of the shear reinforcement (failure inside the shear-reinforced zone) is still not possible since systematic test series on footings with a varying amount of shear reinforcement have not yet been conducted.
To investigate the punching shear behavior of reinforced concrete footings with stirrups as shear reinforcement, three systematic test series (eleven specimens) with varying shear reinforcement ratios were performed. In the tests, the shear reinforcement ratio was varied by changing the stirrup diameter only. Further investigated influences were the shear span-depth ratio and the effective depth. In this paper, the results of the tests are discussed and compared to the predictions according to Eurocode 2 and EC2+NA(D). Moreover, the punching design provisions according to Model Code 2010 are evaluated.

Um wirtschaftliche hochfeste Konstruktionsleichtbetone mit möglichst geringer Trockenrohdichte und guter Verarbeitbarkeit zu entwickeln und bestehende Forschungslücken zum Tragverhalten zu schließen, wurde ein mehrjähriges Verbund-Foschungsvorhaben aufgelegt, dessen Ergebnisse im Rahmen eines Pilotprojektes erstmals erfolgreich eingeflossen sind. Dieser Aufsatz widmet sich den durchgeführten Untersuchungen zum Tragverhalten. In mehreren Versuchsserien an Probebalken wurde für Konstruktionsleichtbetone unterschiedlicher Festigkeiten der Einfluß seiner Normaldruckkraft auf die Schub- und Biegetragwirkung untersucht mit dem Ziel, die Bemessungsansätze nach DIN 1045-1, Model Code MC 90 und ENV 1992-1-1 (EC 2) zu überprüfen bzw. ggf. zu modifizieren sowie konstruktive Details für Bauteile aus vorgespanntem Leichtbeton bereitzustellen. Die Untersuchungen stellen die Voraussetzungen für den sicheren Einsatz von hochfestem Konstruktionsleichtbeton im Spannbetonbau dar.

Seit der Veröffentlichung normativer Vorschriften zur Bemessung von Stahlfaserbeton in Deutschland und Europa gewinnt dessen Einsatz für tragende Konstruktionen des Hoch- und Ingenieurbaus stetig an Bedeutung. Nach Eintreten der Rissbildung im Beton ermöglichen Makrofasern aus Stahl eine effektive Übertragung von Zugspannungen über den Riss hinweg, was sich in Bezug auf das Durchstanztragverhalten als besonders vorteilhaft erweist. Während für die Durchstanztragfähigkeit stahlfaserbewehrter Flachdecken mit Plattendicken bis 150 mm zahlreiche Versuchsergebnisse vorliegen, fehlen Untersuchungen an praxisrelevanten Plattenstärken von 200 mm bis 300 mm nahezu gänzlich. Zudem versprechen Fortschritte in der Leistungsfähigkeit moderner Stahldrahtfasern ein verbessertes Trag- und Verformungsverhalten des Stahlfaserbetons.
Zur Untersuchung des Durchstanztragverhaltens stahlfaserbewehrter Flachdecken mit praxisrelevanten Plattendicken wurden acht Durchstanzversuche an Flachdeckenausschnitten mit unterschiedlichen Plattenstärken durchgeführt. Als weitere Einflussparameter standen der Stahlfasertyp und insbesondere der Stahlfasergehalt im Vordergrund. Der vorliegende Beitrag liefert einen Überblick über die durchgeführten Versuche und diskutiert die gewonnenen Ergebnisse. Anhand eines Vergleichs der experimentell gewonnenen Versagenslasten mit den rechnerischen Durchstanztragfähigkeiten nach der Richtlinie “Stahlfaserbeton” des Deutschen Ausschusses für Stahlbeton (DAfStb) sowie nach fib Model Code 2010 erfolgt eine Bewertung der Bemessungsansätze.

Increase of the punching shear capacity and structural ductility achieved through utilisation of modern high-performance steel fibres
Since the introduction of normative regulations for the design of steel fibre reinforced concrete in Germany and Europe, the application is recently growing in concrete engineering. After crack formation has occurred in the concrete, macro fibres made of steel allow an effective transfer of tensile stresses across the crack, which proves to be particularly advantageous regarding punching shear failure. While there are numerous test results available for the punching shear capacity of steel fibre reinforced concrete slabs with slab thicknesses of up to 150 mm, there is an almost complete lack of tests on practically relevant slab thicknesses. In addition, advances in the performance of modern steel fibres promise improved load-bearing and deformation behaviour of the steel fibre concrete.
Eight punching shear tests were carried out on flat slab specimens with different slab thicknesses to investigate the punching shear behaviour of steel fibre reinforced flat slabs with practically relevant slab thicknesses. Further investigated influencing parameters were the steel fibre type and in particular the steel fibre content. This article provides an overview of the tests carried out and discusses the results obtained. Based on a comparison of the experimentally obtained failure loads with the calculated punching shear capacities according to the DAfStb-Guideline “Steel fibre reinforced concrete” of the German Committee for Structural Concrete (DAfStb) as well as according to fib Model Code 2010, an evaluation of the design approaches will be provided.