Dünnwandige,mikrobewehrte Betonbauteile unter Querkraftbeanspruchung

Shear strength of thin‐walled, micro‐reinforced concrete members Within the second funding period of the DFG Priority Programme 1542 ”Concrete Light“ investigations on the load‐bearing behavior of ”Ultra‐lightweight, Thin‐walled Tubular Concrete Members“ are carried out at the IBMB, division of concrete construction of the TU Braunschweig. Due to the currently insufficient amount of investigations dealing with the shear strength of micro‐reinforced concrete (ferrocement) members, fundamental studies had to be performed. These include experimental and theoretical investigations on the shear strength of profiled, thin‐walled concrete beams with micro‐reinforced webs. The performed tests showed the improved cracking behavior and failure notice of micro‐reinforced specimens, when compared to conventionally reinforced specimens. Furthermore, the conservative and thus secure estimation of the shear strength of micro‐reinforced beams using the design approaches provided by EC2+NA or ACI 318 is proven.     

Deformation capacity of ultra‐high strength concrete flexural elements subjected to inelastic load reversals

Ductility and plastic rotational capacity of flexural elements constructed with concrete strengths up to 175 MPa were investigated experimentally. Ten exterior beam–column sub‐assemblages were tested under cyclic displacements that represented a severe seismic event. The test variables included the concrete strength, the bottom/top reinforcement ratio, the transverse reinforcement ratio, and the shear‐span‐to‐depth ratio. The increase of concrete brittleness with the higher strength concretes did not inhibit the reinforced concrete flexural elements from attaining comparable ductility and deformation capacity with respect to counterpart elements constructed with normal strength concrete. However, the maximum concrete strength that could be used in achieving highly ductile elements depended on the other test variables of the beam element including the bottom/top reinforcement ratio, the transverse reinforcement ratio in the hinge region, and the shear‐span‐to‐depth ratio. Copyright © 2010 John Wiley & Sons, Ltd.     

Zur Mindestquerkraftbewehrung von Stahlbetonbalken

On minimum shear reinforcement amounts for reinforced concrete beams A ductile shear bearing behaviour of reinforced concrete beams is recently ensured by providing standardized amounts of minimum shear reinforcement in compliance with established detailing provisions. However, lower than minimum shear reinforcement amounts do not necessarily go along with a brittle mode of failure in all cases. Dependent on the individual layout and amount of stirrups, the contributions of other bearing capacities and the crack formation itself, the initial cracking force might be overtaken even along with further load gains and reasonable limited shear crack widths. Especially the latter one holds potential to rate the failure mode – brittle or ductile – on site. Experiments on lightly shear reinforced single span concrete beams are evaluated and rated with respect to minimum shear reinforcement amounts required. Thereby, special interest is set on the load‐deformation response obeying the critical shear crack and the structural design. Finally, the overall shear resistance as well as the stirrups' efficiency in the effective shear domain are analysed to derive suggestions for future practical application on‐site.     

集中荷载作用下钢筋混凝土无腹筋梁的受剪承载力分析

根据对国内外343根钢筋混凝土无腹筋梁在集中荷载作用下的试验数据,分别用混凝土轴心抗压强度fc、混凝土抗拉强度ft和fc建立了无腹筋梁受剪承载力平均值的计算公式;并对现行规范无腹筋梁受剪承载力的可靠度进行了分析;最后提出了用fc表达的钢筋混凝土无腹筋梁受剪承载力计算公式。     

Segmentbrücke aus textilbewehrtem Beton: Konstruktion,Fertigung, numerische Berechnung

Segmental Textile Reinrorced Concrete Bridge Design, Manufacturing and Numerical Simulation Concrete provides a brought variety of construction and design possibilities. The low tensile capacity of concrete is taken by reinforcement of steel, short fibres or textile fabrics. Textile reinforced concrete (TRC) is a high performance composite in which technical textiles made of high performance fibers are embedded in a fine‐grained concrete matrix. Because of the corrosion resistance of the textile materials, thick concrete covers as known in ordinary reinforced concrete are no longer needed. Slender new concrete elements extend concrete application to completely new fields and gives architects and engineers more design possibilities. Design, reinforcement concept, production, approval tests, and numerical simulations considering uncertain data are demonstrated for the first bridge made of textile reinforced concrete. It is shown that application of this new, sophisticated composite material is already possible, although more research is needed.     

Tragverhalten von Verbundbauteilen aus bewehrtem UHFB und Stahlbeton

Ultra‐Hochleistungs‐Faserbetone (UHFB) eignen sich aufgrund ihrer hohen Festigkeiten, des hohen Verformungsvermögens und der geringen Permeabilität zur Verbesserung und Instandsetzung bestehender Betonbauten. Mit dünnen Schichten von bewehrtem UHFB, die auf bestehende Stahlbetonbauteile aufgetragen werden, können der Tragwiderstand und die Gebrauchstauglichkeit deutlich gesteigert werden. In einer umfangreichen Versuchsreihe wurden die Eigenschaften von mit zusätzlich zu den Fasern auch mit Stabstahl bewehrtem UHFB untersucht. Die Bewehrung des UHFB mit Stabstählen ist vorteilhaft, um den Verfestigungsbereich des UHFB zu erweitern, seinen Tragwiderstand zu erhöhen und die Streuung seiner mechanischen Eigenschaften zu reduzieren. Zur Bewehrung können hoch‐ oder niederfeste Stähle mit unterschiedlichen Oberflächenstrukturen zum Einsatz kommen. Abschließend werden zwei Anwendungen vorgestellt. Structural Behaviour of Composite Elements Combining Reinforced Ultra‐High Performance Fibre‐Reinforced Concrete (UHPFRC) and Reinforced Concrete Due to their high strengths, high deformability and low permeability Ultra‐High Performance Fibre‐Reinforced Concretes (UHPFRC) are suitable for the improvement and rehabilitation of existing concrete structures. Thin layers of reinforced UHPFRC that are applied to existing concrete members, increase both the load bearing capacity and the serviceability. By comprehensive experimental studies the behaviour of UHPFRC with additional bar reinforcement was investigated. The reinforcement of UHPFRC is advantageous in order to increase the strain hardening capacity of UHPFRC, its load bearing capacity and to reduce the scatter of its mechanical properties. Low or high strength steel grades with various surface characteristics can be used as reinforcement of UHPFRC. Finally two on site applications are presented.     

薄壁钢筋混凝土剪力墙结构的性能

延性等级较低的钢筋混凝土剪力墙会突然发生失效,并导致灾难性的破坏,因此建立一个精确的钢筋混凝土剪力墙模型是必要的。故用改进的可有效进行计算的纤维梁构件模拟薄壁钢筋混凝土剪力墙的性能。混凝土和钢筋作为两种独立的材料,组合在一起构成组合截面,用来描述钢筋混凝土的性能。将混凝土模型作为一种正交性材料,假设总应力的主方向与总应变主方向一致,因此在荷载过程中不断地改变方向。本构模型效仿软化膜模型(SSM),根据横向应变来减小混凝土压缩强度。该模型之后被用于一系列的数值研究中,从而评估几种参数对剪力墙非线性性能的影响。这些参数包括长宽比、横向钢筋配筋率、轴向力和混凝土抗压强度。最后得出了几个关于这种墙体系统整体性能和局部性能的重要结论。     

Model for post-yield tension stiffening and rebar rupture in concrete members

A tension stiffening model is presented which enables the calculation of average tensile stresses in concrete, after yielding of reinforcement, in reinforced concrete elements subjected to uniaxial tension, shear or flexure. To determine the average tensile stress-strain relationship for concrete, a crack analysis approach is employed taking into account the bond mechanism between concrete and deformed reinforcing bars, and numerical analyses are conducted to determine the tensile behavior of reinforced concrete members including post-yield response. Analytical parametric studies are conducted to determine the influence of various parameters including concrete compressive strength and reinforcement yield strength, ultimate strength, hardening stress, and hardening strain. Analysis results obtained from the proposed model, when compared to experimental results for uniaxial members, indicate good agreement for structural behavior after yielding of reinforcement. The proposed model makes it possible to accurately calculate reinforcement stresses at crack locations and, thus, average strain conditions which result in rupture of reinforcement. This leads to more realistic predictions of the uniaxial, flexural, and shear ductility of reinforced concrete members.     

Biegetragfähigkeit von textilbetonverstärkten Stahlbetonplatten

Textilbeton (TRC) ist eine sehr effektive Methode zur Verstärkung von Stahlbetonkonstruktionen. An der TU Dresden wurden im Rahmen des SFB 528 umfangreiche Forschungen zum Einsatz von Textilbeton zur nachträglichen Biegeverstärkung bestehender Beton‐ und Stahlbetonbauteile durchgeführt. Die experimentellen Untersuchungen erfolgten im Regelfall an textilbetonverstärkten kleinformatigen Stahlbetonplatten mit Spannweiten von 1,60 m und Plattendicken von 0,10 m. Parallel zu diesen Versuchen erfolgte die Entwicklung von Berechnungsmodellen, mit denen unter anderem die maximale Biegetragfähigkeit der verstärkten Bauteile vorhergesagt werden kann. Der vorliegende Aufsatz beschreibt experimentelle und theoretische Untersuchungen zur Überprüfung der Übertragbarkeit der bisher gewonnenen Ergebnisse auf großformatige Stahlbetonplatten mit Spannweiten von 6,75 m und Plattendicken von 0,23 m. Durch die Verwendung textiler Hochleistungsbewehrungen aus Carbon auf Basis von so genannten Heavy‐Tow‐Garnen wurden sehr hohe Verstärkungsgrade realisiert. Die Ergebnisse zeigen signifikante Steigerungen der Tragfähigkeiten im Vergleich zu unverstärkten Referenzplatten. Dadurch konnte die sichere Anwendung von Biegeverstärkungen aus Textilbeton auch für Bauteile mit großen Spannweiten und großen Verstärkungsgraden gezeigt werden. Gleichzeitig wurde bei vergleichbarem Lastniveau mit zunehmendem Verstärkungsgrad eine deutliche Verringerung der Durchbiegungen nachgewiesen. Die experimentell ermittelten Tragfähigkeiten sind mit dem vorgestellten Berehnungsansatz zur überschläglichen Biegebemessung textilbetonverstärkter Stahlbetonplatten gut nachvollziehbar. Bending Capacity of Reinforced Concrete Slabs Strengthened with Textile Reinforced Concrete Textile Reinforced Concrete (TRC) is a very effective method to strengthen reinforced concrete constructions. The SFB 528 of the TU Dresden has been carrying out vast research concerning the use of TRC for a subsequent bending strengthening of existing concrete and reinforced concrete components. As a rule the experiments TRC strengthened small format reinforced concrete slabs with span widths of 1.60 m and thicknesses of 0.10 m were used. Parallel to these tests calculation models were developed enabling a prediction of the maximum bending load carrying capacity of the strengthened units among others. The paper describes the experimental and theoretical research for checking the transferability of the results gained until now onto large‐size reinforced concrete slabs with span widths of 6.75 m and thicknesses of 0.23 m. Through the use of high performance textile reinforcements based on carbon Heavy‐Tow‐Yarns very high reinforcement degrees were realized. The results show significant increases of the load carrying capacity compared to the unstrengthened reference slabs. Thus the safe use of bending strengthening out of TRC for components with large span widths and high reinforcement degrees could be proven. At the same time we were able to demonstrate considerably lower deflection with growing reinforcement degrees. The experimentally determined load bearing capacity can be well comprehended with the introduced calculation models of the bending measurement of TRC strengthened reinforced concrete slabs.     

Querkraftwiderstand von Stahlbetonbauteilen ohne Querkraftbewehrung

Shear Strength of Reinforced Concrete Members without Transverse Reinforcement A new theoretical model concerning the shear strength of reinforced concrete members without transverse reinforcement is presented, considering free‐bodies unilaterally bounded by a fictitious crack, inclined at π/4 to the tension chord and extending from the tension chord to the compression chord. It is assumed that the shear stress that can be transferred across a crack decreases linearly with the crack width and that the crack width at the level of the tension chord is proportional to the product of the tension chord strain at the crack and the chord distance. This results in a linear‐hyperbolic relationship between the shear strength and the (elastic) tension chord force at the crack, allowing to determine the governing crack location and other relevant quantities via equilibrium considerations. Due to potential strut or arch action for the transfer of loads applied in the vicinity of supports the crack location is somewhat restricted; it is assumed that the corresponding length is equal to twice the chord distance.     

Mit Schrauben Bewehren – Neue Ergebnisse

Self‐tapping screws as reinforcement – new results. Self‐tapping continuously threaded screws can be used as elements for reinforcement or joints in timber construction because of their high axial strength and excellent bonding properties. The layout of the reinforcing or joining screws can be derived from strut‐and‐tiemodels which are common in concrete design and follow the internal flow of forces. Several series of tests in 2007 already showed the high potential of this construction method on various joint details and reinforced timber elements. Based on this success in 2008 further tests were taken out on joints and timber beams with altered and more sophisticated screw configurations. Hereby the load‐bearing capacity of timber beams reinforced with steel lamellas and rigid frame corners connected with self‐tapping screws was extraordinary and demonstrated once more the high potential of this design method.     

Behavior of shear-dominant thin-walled RC structures

Reinforced concrete (RC) shear-dominant walls can fail suddenly at lower ductility levels, which can lead to catastrophic damage. Accurate modeling of shear-dominant RC walls is therefore essential. In this paper, fiber beam elements, which are proven to be computationally very efficient, were developed to model the behavior of thin-walled RC shear walls. Concrete and steel were considered as separate materials, and are combined at the section level to describe the behavior of the reinforced concrete member. Concrete was modeled as an orthotropic material in which the principal directions of total stresses were assumed to coincide with the principal directions of total strains, thus changing the directions continuously during the loading. The constitutive model follows the Softened Membrane Model (SMM) in which the compressive strength of concrete is reduced as a function of the lateral strain. The model was subsequently used to conduct a series of numerical studies to evaluate the effect of several parameters affecting the nonlinear behavior of the shear dominated wall. These parameters include the aspect ratio, the transverse reinforcement ratio, the axial force, and the concrete compressive strength. These studies resulted in several important conclusions regarding the global and local behavior of wall systems.     

钢纤维高强混凝土梁斜截面受力性能试验研究

根据 1 7根钢纤维混凝土强度等级为CF65～CF90的钢纤维高强混凝土无腹筋梁的受力性能试验结果 ,分析了钢纤维体积率变化对钢纤维高强混凝土梁裂缝分布形态及破坏特征的影响规律 ,验证了钢纤维对高强混凝土梁裂缝发生与发展的有效约束作用及对斜截面破坏形态的改善作用。经过统计分析 ,提出了钢纤维高强混凝土梁斜截面抗裂和斜截面承载力计算方法 ,供修订《纤维混凝土结构技术规程》参考。     

钢纤维混凝土无腹筋梁抗剪强度的回归分析

通过对大量试验结果的回归分析 ,总结了剪跨比、混凝土强度、钢纤维含量和纵筋配筋率对钢纤维混凝土无腹筋梁抗剪强度的影响 ,并指出这些因素在影响程度上的不同 .分析结果表明 ,剪跨比对钢纤维混凝土无腹筋梁抗剪强度的影响最大 .提出了预测钢纤维混凝土无腹筋梁抗剪强度的回归计算公式 .     

Versuche zur Tragfähigkeit von Ankerplatten mit einbetonierten Kopfbolzendübeln in schmalen Stahlbetonstützen

Müssen große Lasten von Stahl‐ in Betonbauteile eingebracht werden, sind Ankerplattendetails mit aufgeschweißten Kopfbolzendübeln oft praktikable Lösungen. Dies gilt auch für die Befestigung von Stahlträgern an Stützen aus Stahlbeton. Hier können die Ankerplatten z. B. bauseits an der Schalung befestigt und mit dem Ortbeton einbetoniert werden. Die Dimensionierung dieser Einbauteile kann, wie auch die Berechnung von nachträglich installierten Befestigungsmitteln, derzeit nach Europäischen Technischen Zulassungen (ETA) erfolgen. Im Unterschied zu nachträglich installierten Befestigungen ist für die Ankerplatte mit einbetonierten Kopfbolzendübeln eine rechnerische Nutzung der Tragfähigkeit der Bewehrung möglich. Allerdings führen die diesbezüglichen Regelungen – im Vergleich zu den in den Versuchen ermittelten Tragfähigkeiten – rechnerisch zu stark eingeschränkten Tragfähigkeiten. Dies beruht einerseits auf der Reduktion der Tragfähigkeit wegen geringer Randabstände und andererseits auf der stark eingeschränkten Nutzbarkeit der Bewehrung. Im folgenden Beitrag werden Versuchsergebnisse vorgestellt, welche den Einfluss der Betondruckfestigkeit, der Verankerungslänge, der Lastexzentrizität sowie der Bewehrungsmenge und ‐position auf das Versagensgeschehen und die Tragfähigkeit zeigen. Results of Experimental Investigations on the Load‐Bearing Capacity of Steel Anchor Plates with in Concrete Encased Headed Studs in Reinforced Narrow Concrete Columns Anchor plates with welded shear studs are often used to transfer high loads from steel to reinforced concrete elements. This is for example the case for the fixation of steel beams to reinforced concrete columns. The anchor plates are fixed on site on the formwork and cast in‐place in the concrete. The design is calculated according to European Technical Approvals like the design of post‐installed anchors. In contrast to post‐installed fastenings, the utilisation of the reinforcement of in concrete encased headed studs is theoretically possible. But the regulations of the ETAs lead to great reductions in the analysed load‐bearing capacities compared to the capacities, obtained in tests. These tests have been conducted to investigate the load bearing behaviour and failure modes of anchor plates, especially in narrow reinforced concrete columns. In the following article the results of the experimental investigations are presented, which show the influence of concrete compression strength, shear stud length, load eccentricity as well as amount and position of reinforcement.     

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.     

Finite‐Elemente‐Berechnungen zum Trag‐ und Verformungsverhalten von Kopfbolzendübeln

Finite Element Calculation of load bearing and deformation behavior of headed studs. In the scope of numerical simulations the composite joint between concrete slab and steel girder is represented by springs or interface elements considering bi‐linear as well as tri‐linear shear‐slip behavior. This method is established to determine the global load bearing behavior for any degree of shear connection. In order to illustrate the concentrated load introduction a three‐dimensional modeling of the shear connector is required. This paper describes a three‐dimensional nonlinear finite element model, which offers the possibility to investigate the local load bearing and deformation behavior of headed studs under shear load. It has been calibrated on shear tests performed by the Institute for Structural Concrete, of University of Technology, Aachen. On the basis of parametrical studies the influence of the concrete strength, the length of the stud as well as the formation of the welded collar have been determined. Furthermore, the limits of the simulation of shear tests are detected.     

Stahlbetonträger mit zweiachsiger Querkraftbeanspruchung

Reinforced Concrete Beams subject to biaxial Shear Forces: strut‐and‐tie models, experiment and design approach Spatial strut‐and‐tie models are developed for the design of reinforced concrete beams subject to biaxial shear forces. They are derived from a resolution of the vector of the resultant shear force and valid for beams with rectangular cross sections. The models consist of tensile struts of stirrups and longitudinal bars, compressive concrete struts directed to each separate area of the longitudinal reinforcement in the cracked part of the cross section as well as compressive struts stiffening the stirrups. They are qualitatively verified by experiments and further elaborated to derive design charts for the design of the stirrups. An example shows the practical application of the charts.     

Schubversuche an simulierten Durchlaufbalken ohne Querkraftbewehrung

Shear test on simulated continuous beams without shear reinforcement This paper presents an experimental investigation on the shear behavior of continuous beams without shear reinforcement. A total of five rectangular 2‐span reinforced concrete beams under a uniform load were tested. In each beam the shear failure was arranged to take place in the main span, while the short span was highly reinforced by stirrups. The reaction at the end support of the short span was adjusted by different schemas during the test in order to obtain different M/V‐ratios at the main span. By this means significant differences in the failure location and thus the maximum shear force at failure between continuous beams have been observed. The experimental observations showed that the shear resistance is not a pure sectional property but it represents rather a structural character, which cannot be described by the model included in EC 2. The observations were interpreted using the proposed approach and generally could be well explained. Based on the knowledges obtained from this investigation and some other experimental programs, this paper suggests some remarks on the planning of test for continuous beams that conforms to the behavior of concrete structures in the practice.