Zentrische Druckversuche an schlanken UHPC‐Stützen

Axial compression tests on slender columns – unreinforced slender columns: a taboo? – Debate on potential and risk Within the project of the Priority Programme (SPP) 1542 ”Concrete light“, which is funded by the German Research Foundation (DFG), unreinforced UHPC columns with square and rectangular cross‐section were tested under axial compression in the subproject ”Cross sectional adaption for rod‐shaped elements in compression“. The results were compared with test series of other researchers. The focus of the study was on the buckling behavior of columns with different conditions of end support. A hinged and a fixed support have been applied for these experiments. In addition to an analysis of the failure process, the results were compared with the Euler buckling load and the normal stress in consideration of second‐order effects. Concluding, advantages and disadvantages of both approaches are presented, compared, and discussed.     

Durchstanzen von Flachdecken bei hohen Stützenlasten

Sollen Hochbauten eine flexible Raumnutzung erlauben, stellen Systeme aus Stahlbeton‐Flachdecken und Stützen eine wirtschaftliche und effiziente Lösung dar. Die Stützen dieser Bauten werden häufig aus hochfestem Beton vorfabriziert und stark bewehrt, um hohe Normalkräfte mit minimalen Querschnitten aufnehmen zu können. Der oft aus normalfestem Beton bestehende Deckenteil, der zwischen den übereinanderstehenden Stützen liegt, wird dabei ganz beträchtlichen Druckbeanspruchungen ausgesetzt. Zudem erfährt die Deckenplatte um die Stütze eine starke Querkraftbeanspruchung, so dass der Durchstanzwiderstand maßgebend werden kann. Der vorliegende Aufsatz untersucht die Interaktion zwischen den vertikalen Druckbeanspruchungen in der Deckenplatte und dem Durchstanzwiderstand mit Hilfe einer Versuchsreihe an sechs Plattenausschnitten (3,00 x 3,00 x 0,25 m) unter verschiedenen Lastkombinationen. Aufgrund der Versuchsresultate wird ein einfaches Bemessungsmodell vorgestellt. Dieses Modell basiert auf dem Ansatz der DIN 1045‐1 für den Durchstanzwiderstand von Decken ohne Querbewehrung, die einer horizontalen Druckspannung ausgesetzt sind (z. B. Vorspannung). Die mit dem Modell errechneten Resultate stimmen gut mit jenen der Versuche überein – sowohl beim Widerstand wie bei der beobachteten Bruchart. Interaction between column loading and punching shear strength in flat slabs without transverse reinforcement Flat slabs supported by columns are cost efficient solutions for high‐rise buildings. To reduce the cross section of the columns, the columns are typically precast with high strength concrete and include large amounts of longitudinal reinforcement. On the contrary, the slab between columns is typically cast‐in‐place with normal strength concrete. Thus, the slab is subjected in the support region of the columns to large transverse compressive stresses, which may exceed its uniaxial compressive strength. In addition, the flat slab around the column region has to carry large shear stresses, which may potentially lead to punching shear failures. In this paper, the interaction between column loads and shear forces in the support region of a flat slab is investigated by means of an experimental programme consisting of 6 slabs without transverse reinforcement (3.0 x 3.0 x 0.25 m). The flexural reinforcement of the slab was varied as well as the ratio between the column loads and the applied shear forces. On the basis of these results, a theoretical model based on the punching shear approach of DIN 1045‐1 is presented. This model accounts for the compressive stresses in the support region due to column loads and for the potential increase of punching shear strength due to it. A very good agreement between the test results and the theoretical model is obtained both in terms of strength and observed failure modes.     

Rissbildung von biegebeanspruchten Bauteilen aus Ultrahochfestem Faserbeton

Das Konstruieren und Bemessen von Bauwerken aus faserbewehrtem UHPC erfordert sehr detailliertes Wissen über das Zugtragverhalten von Faserbeton. Es ist beeinflusst von der Fasergeometrie, der Fasermenge, der Schubverbundfestigkeit zwischen Faser und Matrix, der Matrixfestigkeit, dem Schwinden und der Faserorientierung. Leutbecher beschreibt ein Modell für reine Zugbeanspruchungen, indem all diese Parameter Eingang finden. Im hier vorliegenden Artikel wird dieses Modell für Biegebeanspruchungen erweitert und anhand von Biegeversuchen an Platten aus UHPC verifiziert. Dabei werden sowohl Faserbewehrung als auch konventionelle Stabbewehrung berücksichtigt. Crack Formation of Flexural Members made of UHPFRC For the design and planning of buildings made of fibre reinforced ultra high performance concrete one needs very detailed knowhow about the tensile carrying behaviour of fibrated concrete. It is affected by various parameters such as the fibre geometry, the content of fibres, the bond strength between fibre and binder matrix, the strength of the matrix, the shrinkage of the concrete and the orientation of the fibres. All these parameters are considered in a theoretical model for UHPFRC under tension (Leutbecher). In the paper presented here the range of validity of this model is extended to bending stress. Experimental investigations on UHPC‐plates under bending complete and verify the theoretical results. The tests cover fibre reinforcement as well as conventional passive reinforcement.     

Zum Tragverhalten von Stützen semi‐integraler Brücken

Load‐bearing behaviour of columns of integral bridges – realistic calculation of columns of integral bridges for the ultimate limit state Due to the complex and interactive load‐bearing behaviour, calculation of integral bridges is more expensive than the calculation of bridges with its superstructure uncoupled from the substructure. Apparently conservatively chosen reinforcement ratios lead to increased stiffness of the columns and thus inherently increased the constraining forces. Therefore, a greater reinforcement ratio is not imperatively connected with a better load‐bearing behaviour of the columns. To better understand the load‐bearing behavior of integral bridges, it is necessary to consider the physical non‐linearity in the calculation. In this contribution, the non‐linear behaviour of integral columns is examined by varying the cross section dimensions and the reinforcement ratios. The results are analyzed with regard to the columns ductility and the constraining forces. Finally, this contribution offers conceptual proposals, which can be used for the dimensioning of integral bridges.     

Bemessung von Stützmauern aus Stahlbeton nach Eurocode

Design of Concrete Retaining Structures basing on Eurocode Standards Design of retaining structures bases on current concrete and geotechnical design standards. At this time the basis of the Austrian state of the art is changing from a national system to the System of European standards (EUROCODES). This contribution shows the actual situation of the Austrian standardisation and the use of the Eurocode 2 (EN 1992‐x) and the Eurocode 7 (EN 1997‐1) for designing of concrete retaining structures. For this the essential design approaches and there standardised regulations are shown.     

Bauzeit und Baukosten für Stahlbetonarbeiten – Berechnungsmethoden und Anwendung

Construction Time and Cost of Reinforced Concrete Works – Calculation Methods and Application Construction time is of crucial importance when it comes to utilizing the production factors in an effective and efficient way. Construction periods that are too short usually result in higher cost, poorer quality and a larger number of disputes. This paper sets out to demonstrate the calculation of construction time whilst considering key construction management parameters. Beyond a simple, deterministic method, other options for calculation are shown that rely on probability calculus. The approaches described to determine construction time and cost are illustrated by a building project example. The deterministic method results in one value per each calculation process (calculation mode 1). In calculation mode 2, probability calculus is applied in a simple fashion. Both range and probability of occurrence can be considered for the relevant input variables. For the third calculation mode, the Monte‐Carlo method is applied using the @RISK software. For each of the parameters to be determined, this method shows a probability distribution.