Visko‐elastisches Verhalten von Holz‐Klebstoff‐Verbindungen unter zyklischer Zug‐Druck‐Belastung

Visco‐elastic behavior of bonded wood under cyclic tensile and compression loading In the present contribution, a test method is used to simulate the static, cyclic loading of adhesive joints due to swelling and shrinking of the wood and to demonstrate the plastic deformation in the low load range. For tensile shear specimens prepared from beech wood and bonded with three different adhesives (MUF, PRF, PUR), the elastic behavior under cyclic tensile and compression loading was investigated and the loss and storage of energy was determined. All tested adhesives showed viscose parts even at a very low load level of 3 MPa. At a load level of 7 MPa, the PRF joints revealed a more elastic behavior than the other. The increased loss energy determined for the PUR bonding indicates a softening of the adhesive joint.     

Ascot Racecourse – Stahl‐Kunststoff‐Sandwich für die Tribüne des neuen Grandstand

Die erste kommerzielle Tribünenanwendung mit dem Sandwich‐Plate‐System (SPS) – einem Stahl‐Kunststoff‐Sandwich‐System – konnte durchgeführt werden, weil besondere Umstände an dem gerade fertiggestellten Tribünengebäude der Pferderennbahn in Ascot [1] eine zusätzliche dringliche Anpassung erforderten. Ascot Racecourse – Steel‐plastic sandwich for the terrace of the new Grandstand. The first commercial application of the sandwich‐plate‐system (SPS) – a steel‐plastic sandwich system – could be carried out because special circumstances at the just completed building of the Grandstand at Ascot Racecourse [1] required additional urgent adjustments.     

Ein elasto‐plastisches Materialmodell zur realistischen Berechnung der Verformungen mechanisch überhöhter Stahl‐ und Verbundträger

Zur anteiligen Kompensation der Verformungen von Stahl‐ und Verbundträgern werden diese häufig überhöht eingebaut. Wird die Überhöhung durch eine mechanische Beanspruchung (Boxen) aufgebracht, so besitzt der Stahlträger bereits im Einbauzustand plastische Dehnungen. Wie von Grages in [3] gezeigt, führt dies bei der Rückbelastung zu einer Abnahme der Steifigkeit. Dieses Phänomen ist auch als Bauschinger‐Effekt [2] bekannt und kann bei praktischen Verformungsberechnungen zu einer Unterschätzung der tatsächlich auftretenden Deformationen führen. In dem aktuellen Aufsatz wird ein ratenunabhängiges elasto‐plastisches Materialmodell mit linearer und nichtlinearer isotroper sowie nichtlinearer kinematischer Verfestigung vorgestellt, welches eine realistische Abschätzung der Verformungen erlaubt. Das gezeigte Materialmodell ist durch die geringe Anzahl an Eingangsparametern sehr einfach anwendbar. Eine gezielte Anpassung der einzelnen Parameter wird in anschaulicher Form dargestellt. Die Qualität des Modells wird anhand der Nachrechnung von in [3] durchgeführten Versuchen überprüft. An elasto‐plastic material model for realistic calculation of the deformations of mechanical cambered steel‐ and composite girders. Steel and composite girders are often cambered before they are embedded into structures to compensate deflections. In case of a mechanical pre‐deformation via a hydraulic press the steel girder contains plastic strains before assembly. As shown by Grages [3] this leads to a reduction of stiffness and strength during reverse loading. This phenomenon is also known as the Bauschinger‐Effect [2] and may lead to an underestimation of the real deformation during a structural analysis. In this paper a rate‐independent elastoplastic material model with linear and nonlinear isotropic as well as nonlinear kinematic hardening is introduced which allows a realistic estimation of deformations. The material model presented can be easily applied due to its low number of parameters. A straightforward algorithm to adjust the single parameters is described. The quality of the model is verified by a recalculation of experimental data from [3].     

Entwicklung hybrider Stahl‐Glas‐Träger

Immer höhere Anforderungen an die Transparenz von Bauwerken führen zu einer stetigen Weiterentwicklung von tragenden Glaselementen, wobei dem Glas neben seiner ausfachenden Funktion mehr und mehr konstruktive Aufgaben zugedacht werden. Aktuelle Entwicklungen stellen die hybriden Stahl‐Glas‐Träger dar, bei denen schlanke Stahlflansche und Glasstege mittels geklebter Schubfugen zu einem “I”‐Träger zusammengesetzt werden [1]. Der Vorteil der linienförmigen Verklebung liegt hierbei in der gleichmäßigen Lasteinleitung in das Glasbauteil. Die Wahl des eingesetzten Klebstoffsystems trägt in Abhängigkeit seiner mechanischen Eigenschaften maßgeblich zum Tragverhalten des Verbundquerschnittes bei. Ebenso ist der Fugengeometrie eine große Bedeutung hinsichtlich der Tragfähigkeit und Herstellbarkeit zuzuordnen. Allerdings bestimmen Langzeiteffekte, wie Alterung des Klebstoffs und zeitabhängiges Tragverhalten, die Bemessung. Der Beitrag gibt einen Überblick über das systematische Vorgehen zur Untersuchung des Tragverhaltens und nennt wichtige Kriterien und Anforderungen beim Entwurf hybrider Stahl‐Glas‐Träger in Hin blick auf eine optimierte Ausnutzung der einzelnen Komponenten. Development of hybrid steel‐glass‐beams. Increasing higher requirements for transparent and filigree structures result in continuous development and improvement of load‐bearing glass elements, not only in a classic way being a space enclosing element but also increasingly offering load carrying functions. To realize architectural attractive transparent and lightweight constructions bonded hybrid steel‐glass beams have been developed, composed of slim steel flanges and glass webs which were assembled to ”I”‐beams using adhesives. The adhesive system significantly account for the structural behavior of the bonded composite section. Furthermore the joining geometry is of great importance concerning load carrying capacity and producibility but the design is governed by long‐term effects like ageing, creeping and time dependent load‐carrying behavior. This contribution shows the general systematic approach for the analysis of the load bearing behavior of hybrid steel‐glass beams and introduces important criteria and requirements for the design and its mechanical properties focused on an optimized exploitation of the components.     

Behaviour of uni‐axially loaded concrete‐filled‐steel‐tube columns confined by external rings

Concrete‐filled‐steel‐tube (CFST) columns have been widely adopted for column construction of tall buildings due to its superior strength and ductility performance contributed by the composite action. However, this beneficial composite action cannot be fully developed at early elastic stage as steel dilates more than concrete and thereby causing imperfect interface bonding. Hence, it reduces the elastic strength and stiffness of the CFST columns. To resolve the problem, external confinement in the form of steel rings is proposed in this study to restrict the lateral dilation of concrete and steel at initial elastic stage. In this paper, CFST columns of various dimensions cast with normal‐strength or high‐strength concrete and installed with external steel rings were tested under uni‐axial compression. From the results, it was evident that (a) the external steel rings could restrict the lateral dilation of CFST columns and improve the interface bonding condition and (b) externally confined CFST columns had uni‐axial strength and stiffness larger than those of unconfined CFST columns. With the experimental results, an analytical model taking into account the confining effects of steel tube and rings has been developed to predict the uni‐axial strength of ring‐confined CFST columns. Copyright © 2012 John Wiley & Sons, Ltd.     

Semi‐volatile organic compounds in heating,ventilation, and air‐conditioning filter dust in retail stores

Retail stores contain a wide range of products that can emit a variety of indoor pollutants. Among these chemicals, phthalate esters and polybrominated diphenyl ethers (PBDEs) are two important categories of semi‐volatile organic compounds (SVOCs). Filters in heating, ventilation, and air‐conditioning (HVAC) system collect particles from large volumes of air and thus potentially provide spatially and temporally integrated SVOC concentrations. This study measured six phthalate and 14 PBDE compounds in HVAC filter dust in 14 retail stores in Texas and Pennsylvania, United States. Phthalates and PBDEs were widely found in the HVAC filter dust in retail environment, indicating that they are ubiquitous indoor pollutants. The potential co‐occurrence of phthalates and PBDEs was not strong, suggesting that their indoor sources are diverse. The levels of phthalates and PBDEs measured in HVAC filter dust are comparable to concentrations found in previous investigations of settled dust in residential buildings. Significant correlations between indoor air and filter dust concentrations were found for diethyl phthalate, di‐n‐butyl phthalate, and benzyl butyl phthalate. Reasonable agreement between measurements and an equilibrium model to describe SVOC partitioning between dust and gas‐phase is achieved.     

Berechnung von Holz‐Beton‐Verbunddecken

Evaluation of Timber Concrete Composite Slabs If timber and concrete acts together in a cross section, the carrying and deformation behaviour of this new composite section is affected by many influences e.g. geometry, material properties, connecting device, long‐term behaviour, etc. The advantages of this building method and an optimization of the cross sections only can be achieved, if these influences are known and can be taken into account in the structural design. For the proof of the short term behaviour different procedures, e. g. [1], are available. Regarding the long‐term behaviour of the timber concrete composite floors it becomes obvious that the rheological behaviour of timber concrete composite structures is not considered sufficiently by the design method in [1].     

Zur Berechnung von Holz‐Beton‐Verbundträgern mit Methoden der mathematischen Optimierung

Holz‐Beton‐Verbundkonstruktionen weisen als hybride Tragwerke gegenüber reinen Holz‐ bzw. Stahlbetonkonstruktionen zahlreiche Vorteile auf. Wesentlich für die Effizienz der Hybridbauweise ist die Ausbildung der Verbundfuge. Der vorliegende Beitrag stellt ein neues Verbundelement für Straßenbrücken in Holz‐Beton‐Verbundbauweise vor, welches im Rahmen eines Forschungsprojekts an der Bauhaus‐Universität Weimar entwickelt wurde. Die rechnerische Analyse der Verbundkonstruktion erfolgt — abweichend von den im Hybridbau bisher üblichen Berechnungsmethoden — unter Anwendung von Energiemethoden und Nutzung von Algorithmen der mathematischen Optimierung. Calculation of timber‐concrete composite structures using mathematical optimization methods. Timber‐concrete composite structures have many advantages over conventional timber and re inforced concrete structures. The efficiency of such hybrid structures significantly depends on the properties of the com posite joint. This article presents a novel structural element for the joint of timber‐concrete composite bridges, which was developed as part of a research project at the Bauhaus‐Universität Weimar. In contrast to standard calculation methods, an energy method and the mathematical optimization is applied for the numerical analysis of the hybrid structure.     

Diagramme zur Auswertung der Johansen‐Formeln für einschnittige Holz‐ bzw. Holzwerkstoff‐Verbindungen

Diagrams to evaluate the Johansen‐formulas for single shear connections of timber or wood‐based panels. Metal dowel‐type fasteners are nowadays designed by using the Europe‐wide applied, but rather complex Johansen Theory, which is effectively used by means of computer programs. This however can yield a diminished overview. In this contribution the evaluation is done by use of diagrams, which should facilitate the application of the new design rules in addition or as an alternative to other methods.