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Papers by Thomas Ummenhofer

Eurocode 3 Bemessung und Konstruktion von Stahlbauten, Band 2: Anschlüsse, 2015

FE simulation of welds from austenitic steel was carried out in the current paper. Two different ... more FE simulation of welds from austenitic steel was carried out in the current paper. Two different multi-pass welds were modelled. Measurements of welding residual stresses, which were found in literature, were applied for the validation of the results. The validated models were then used as basis for sensitivity analysis. The influence of differentiating the welding speed, the heat input of the weld heat source, intermediate cooling between consecutive weld-passes and welding sequence on the welding residual stresses was investigated.

Welding in the world, Mar 11, 2024

This paper analyses the applicability of a modified strain approach to predict the fatigue life o... more This paper analyses the applicability of a modified strain approach to predict the fatigue life of HFMI-treated transverse stiffeners under variable amplitude loading (VAL) with random load sequences of a p(1/3) and linear shaped spectrum. Local stresses are determined using linear-elastic finite element analyses. The measured weld geometry and component imperfections are considered. From the hardness of the HFMI-treated zone and the base material, the elastic-plastic material behaviour and Coffin-Manson parameters to describe the damage parameter Woehler curve are estimated. Based on a hysteresis counting method (HCM), the damage for each closed hysteresis is calculated. The applied notch strain approach includes the impact of residual stresses and the influence of surface roughness. Thus far, the application of similar approaches has only been validated for welded components with comparatively low residual stresses and HFMI-treated welds subjected to constant amplitude loading. To validate the accuracy of the approach for HFMI-treated welds under variable amplitude loading, the approximated fatigue life is compared to the number of cycles derived from experimental investigations. In this study, it is shown in conjunction with experimental results that it is essential to consider the strength of the base material near the weld when assessing the service life. This area can be more critical than the HFMI-treated weld toe. Keywords Notch strain approach • Elastic-plastic material behaviour • High-frequency mechanical impact (HFMI) • Constant and variable amplitude loading Recommended for publication by Commission XIII-Fatigue of Welded Components and Structures

This research report will be put into the web since it's stadium nascendi and will be updated eve... more This research report will be put into the web since it's stadium nascendi and will be updated every weekend latest. Colleagues of the shell buckling community are cordially invited to contribute material, ideas, hints and their opinion.

This paper reflects the application of high frequency peening in order to increase the fatigue li... more This paper reflects the application of high frequency peening in order to increase the fatigue life of welded steel structures. Recent studies demonstrate that high frequency peening can increase the fatigue life of welded joints by 5 to 15 times. Furtheron high frequency peening methods which are accredited by a German institute (Karlsruhe Institute of Technology , KIT) feature very small scatter in the results. Therefore, the quality control can be achieved by a procedure similar to the quality control system for welding, which is completed by a visual inspection. A developed design concept allows calculating the increased residual fatigue life of the structures by accounting for the locally increased fatigue strength. The present paper reports on an example of a fatigue loaded welded steel bridge which has been repaired by applying high frequency hammer peening in order to extend the remaining service life.

ce/papers, May 1, 2018

Im konstruktiven Glasbau wird die Transparenz alleine durch die Verbindungselemente behindert. Pu... more Im konstruktiven Glasbau wird die Transparenz alleine durch die Verbindungselemente behindert. Punkthalter stören das Erscheinungsbild dabei am wenigsten, allerdings muss das Glas in der Regel durchbohrt werden. Dies kann zu Spannungsspitzen im Glas und Dichtigkeitsproblemen bei Isolierglas führen. Um diesen Nachteilen entgegenzuwirken, wurde ein transparenter Klebstoff entwickelt, mit dem kreisrunde Punkthalter auflaminiert werden können: das Transparent Structural Silicone Adhesive (TSSA). Dieser Beitrag beschreibt Grundlagenuntersuchungen zu rechteckigen TSSA-Verbindungen, wobei die Klebefläche der experimentellen Untersuchungen stark von der ursprünglichen runden Geometrie des Punkthalters abweicht. Die Übertragbarkeit der Festigkeitskennwerte und Versagensmodelle für die Dimensionierung von Klebungen in anderen Anwendungsbereichen wird untersucht. New applications for transparent structural bonding. In structural glazing applications only connection points do disturb the transparency. Point fixing solutions are the less obstructive, but in general require drilling of the glass. This leads to stress concentration in the borehole area and jeopardises the tightness of insulated glazing units. For this purpose the Transparent Structural Silicone Adhesive (TSSA) was developed, a lamination foil which allows to structurally bond the point fixings to the glass. Baseline investigations on longitudinal rectangular connections with TSSA have been carried out and are presented hereafter. The bonding surface strongly differs from the original round point fixing geometry. The usability of the design values and failure criterias for different applications are investigated.

ce/papers, Sep 1, 2017

ABSTRACTA careful consideration of imperfections is critical to the design of shell structures to... more ABSTRACTA careful consideration of imperfections is critical to the design of shell structures to ensure that they are both adequately safe and economically built. Following the development of the Eurocode for shell structures EN 1993‐1‐6, the situation has considerably improved: the extensive literature on the imperfection sensitivity of shells has been introduced into practical designs; fabrication tolerance classes have been defined, which allow the designer to adjust his design to the professional efforts of the fabricator; and numerical and experimental results have been matched by defining an axisymmetrical weld depression as a substitute imperfection in the recent amendment. These advances seem to have established a very good position, and it might be thought that little more needs to be done. However, a closer look at some of the above items reveals the need to reconsider the management of imperfections, especially from the designers point of view. In this paper the authors attempt to make a first step to resolve the role of imperfections from different points of view. In addition, some unpublished results from researches at KIT are presented.

Key Engineering Materials, Sep 1, 2016

Within the code-family of the Eurocodes, provisions for aluminium shells are given in EN 1999-1-5... more Within the code-family of the Eurocodes, provisions for aluminium shells are given in EN 1999-1-5 (EC9) [1]. EC9-1-5 is listed in the Bavarian List of Technical Building Regulations. Thus, in Bavaria as well as in other Federal States of Germany it is mandatory to use EC9-1-5 for the verification of silos. A typical aluminium silo for industrial products might have a diameter of 3 m, a bin height of 10 m and wall thicknesses of 4 mm / 5 mm. The aluminium alloy EN AW-5754 [Al Mg3] O/H111 (EN 485-2 [2]) would be typical as well. Relevant for determining the required wall thickness is the buckling resistance under axial compression in the skirt and axial compression with coexisting internal pressure in the silo bin. When some obvious shortcomings in the formulae for coexisting internal pressure were investigated, it was found that there is a big discrepancy between scientific research, which has been done on the imperfection sensitivity of aluminium shells and the design equations in EC9-1-5. In the present paper an effort was made, in order to tackle these discrepancies and make clear, in which points the code needs amendment.

Stahlbau, Sep 22, 2022

Fachwerkkonstruktionen aus direkt verschweißten Hohlprofilen unter vorwiegend ruhender Beanspruch... more Fachwerkkonstruktionen aus direkt verschweißten Hohlprofilen unter vorwiegend ruhender Beanspruchung werden in vielen Bereichen des Stahlhochbaus und der Infrastruktur, wie bei Fußgängerbrücken, sowie im Anlagen‐ oder Mobilkranbau eingesetzt. Die Verwendung hochfester Stähle ermöglicht bei hohen Tragfähigkeitsanforderungen und großen Spannweiten eine materialsparende Bauweise. Zur Ausnutzung dieses Potenzials sind Bemessungsregeln erforderlich, die die spezifischen Eigenschaften dieser Stähle zutreffend erfassen. Die derzeit gültigen Bemessungsregeln für Hohlprofilanschlüsse wurden anhand von Versuchen mit normalfesten Stählen auf semiempirische Weise hergeleitet. Im Rahmen dieser Versuche zeigte sich, dass durch Plastizieren lokale Spannungsspitzen abgebaut und sekundäre Biegemomente umgelagert werden. Im Unterschied zu normalfesten Stählen weisen hochfeste Stähle ein geringeres plastisches Umlagerungsvermögen auf. Dem wird in den Bemessungsregeln derzeit durch pauschale, abgeschätzte Abminderungsfaktoren Rechnung getragen, die für hochfeste Hohlprofile bislang nicht ausreichend experimentell validiert wurden. Durch die Projekte zu den „Hochfesten Hohlprofilknoten“ im FOSTA Forschungsverbund HOCHFEST sollen differenzierte und abgesicherte Bemessungsregeln hergeleitet werden, die einen wirtschaftlichen und ressourcenschonenden Einsatz hochfester Stähle in Hohlprofilkonstruktionen ermöglichen.

Stahlbau, Sep 1, 2016

Im Rahmen des Forschungsprojekts "FIT" wurden Ermüdungsfestigkeitsuntersuchungen an geschweißten ... more Im Rahmen des Forschungsprojekts "FIT" wurden Ermüdungsfestigkeitsuntersuchungen an geschweißten Konstruktionsdetails, die häufig in Gründungsstrukturen von Offshore-Windenergieanlagen (OWEA), aber auch im Stahlbrückenbau eingesetzt werden, durchgeführt. Der Schwerpunkt der Untersuchungen wurde auf einseitig geschweißte Kreishohlprofile (KHP) gelegt. Für einseitig stumpfgeschweißte Kreishohlprofile wurden die derzeitigen auf unzureichender Versuchsbasis erstellten Kerbfalleinstufungen geltender Regelwerke überprüft. Hierzu wurden umfangreiche experimentelle Untersuchungen zur Ermüdungsfestigkeit dieses Details durchgeführt. Die den Ermüdungswiderstand maßgeblich beeinflussenden Parameter, wie vorhandene geometrische Imperfektionen und Schweißnahtausbildung, wurden identifiziert, bewertet und deren Einfluss im Rahmen numerischer Berechnungen untersucht. Basierend auf diesen Ergebnissen wurde eine Kerbfallempfehlung erarbeitet, die eine zutreffende Lebensdauerabschätzung ermöglicht. Diese Kerbfallempfehlung soll als Grundlage für die zukünftige Aufnahme in Normen und Regelwerke dienen. Fatigue performance of girth welded butt weld of circular hollow sections. Within the research project "FIT", the fatigue behavior of welded joints frequently used in supporting structures for offshore wind energy plants as also bridges was investigated. The research work was focused on the fatigue performance of girth welded circular hollow sections (CHS). For girth welded CHS being welded from one side only, the FAT-classification according to different standards and recommendations was compared, and extensive experimental investigations were performed. Based on the results the fatigue strength of girth welded CHS was determined and a recommendation for FAT-classification was given. Additional studies contribute to identify and evaluate effects which are relevant to fatigue such as the influence of misalignment. These influences have been taken into account within the numerical studies and a recommendation for FAT classification is given. The developed recommendation can serve as a basis for future consideration in standards and guidelines.

For the energetic use of sunlit roofs, photovoltaic and solar thermal elements are mounted using ... more For the energetic use of sunlit roofs, photovoltaic and solar thermal elements are mounted using different fastening systems on existing roof surfaces. For the installation on trapezoidal profiles or sandwich panels, hanger bolts and solar fasteners belong to the most prevalent fastening systems. For the structural analysis of these systems, however, only insufficient design approaches are available until up to date. Especially for the application on roofs covered with sandwich panels no design criteria is available at the present. The introduction of roof parallel loads from roof shear, wind and snow into the substructure can be specified as pressing problem. Within the scope of a research project sponsored by the DIBt (German Institute for Building Technology) design criteria for hanger bolts and solar fasteners in sandwich panels with a plastic foam core were developed. When roof parallel forces are applied to the fastener the fastener is supported by the sandwich panelsŽ surface...

Construction and Building Materials, 2021

Abstract The average condition of German road bridges has deteriorated significantly in recent ye... more Abstract The average condition of German road bridges has deteriorated significantly in recent years. Steel bridges often show fatigue crack at points of increased stress concentrations. For increasing the remaining service life, effective reinforcement methods for reducing the crack growth are of increasing importance. Adhesively bonded reinforcements show many advantages compared to screwed or welded reinforcements, as the components are not damaged by heat input or drill holes. In this paper, a common EP adhesive and a toughened EP adhesive are characterised and compared by means of standardized experimental investigations. Due to the dynamic loading and the service temperatures occurring in steel bridges, the toughened EP adhesive is especially suitable for the strengthening application and is therefore prioritized for further experimental investigations. Due to the pre-stressing of the CFRP (Carbon Fiber Reinforced Plastics)-lamellas, the creep resistance of the bonded joint is of central importance. Creep tests show that the expected creep deformations are in irrelevant orders of magnitude. Fatigue tests on centre-notched steel specimens using the toughened EP adhesive in combination with CFRP lamellas show great potential for decreasing the crack tip stress and therefore increasing the remaining service life. With a reinforcement on both sides with pre-stressed plates, up to 7.9 times the remaining service life can be determined in comparison to unreinforced specimens.

Im Rahmen des abgeschlossenen AiF Projektes „Systematische Untersuchungen zur Verstarkung von Sta... more Im Rahmen des abgeschlossenen AiF Projektes „Systematische Untersuchungen zur Verstarkung von Stahlkonstruktionen mit kohlefaserverstarkten Kunststoffen (CFK) – STAKOK“ wurde die geklebte Verstarkungsmasnahme mit unidirektionalen CFK-Lamellen als Alternative zu den ublichen Verstarkungsmethoden, wie Anschrauben oder Anschweisen von Stahllaschen, untersucht. Aufgrund von sich andernden Anforderungen an Stahlkonstruktionen ist oft keine ausreichende Tragfahigkeit oder Gebrauchstauglichkeit mehr gewahrleistet. Deshalb ist die Verstarkung solcher Konstruktionen unumganglich. Im Vergleich zur Verstarkungsmasnahme mit Stahllaschen weist die Methode mit geklebten faserverstarkten Kunststoffen Vorteile bezuglich der Fertigung, des geringeren Gewichtes, der verringerten Kerbwirkung und einer guten Korrosionsbestandigkeit auf.