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The key objective of this research is to study fracture with a meshfree method, local maximum entropy approximations, and model fracture in thin shell structures with complex geometry and topology. This topic is of high relevance for real-world applications, for example in the automotive industry and in aerospace engineering. The shell structure can be described efficiently by meshless methods which are capable of describing complex shapes as a collection of points instead of a structured mesh. In order to find the appropriate numerical method to achieve this goal, the first part of the work was development of a method based on local maximum entropy (LME)
shape functions together with enrichment functions used in partition of unity methods to discretize problems in linear elastic fracture mechanics. We obtain improved accuracy relative to the standard extended finite element method (XFEM) at a comparable computational cost. In addition, we keep the advantages of the LME shape functions,such as smoothness and non-negativity. We show numerically that optimal convergence (same as in FEM) for energy norm and stress intensity factors can be obtained through the use of geometric (fixed area) enrichment with no special treatment of the nodes
near the crack such as blending or shifting.
As extension of this method to three dimensional problems and complex thin shell structures with arbitrary crack growth is cumbersome, we developed a phase field model for fracture using LME. Phase field models provide a powerful tool to tackle moving interface problems, and have been extensively used in physics and materials science. Phase methods are gaining popularity in a wide set of applications in applied science and engineering, recently a second order phase field approximation for brittle fracture has gathered significant interest in computational fracture such that sharp cracks discontinuities are modeled by a diffusive crack. By minimizing the system energy with respect to the mechanical displacements and the phase-field, subject to an irreversibility condition to avoid crack healing, this model can describe crack nucleation, propagation, branching and merging. One of the main advantages of the phase field modeling of fractures is the unified treatment of the interfacial tracking and mechanics, which potentially leads to simple, robust, scalable computer codes applicable to complex systems. In other words, this approximation reduces considerably the implementation complexity because the numerical tracking of the fracture is not needed, at the expense of a high computational cost. We present a fourth-order phase field model for fracture based on local maximum entropy (LME) approximations. The higher order continuity of the meshfree LME approximation allows to directly solve the fourth-order phase field equations without splitting the fourth-order differential equation into two second order differential equations. Notably, in contrast to previous discretizations that use at least a quadratic basis, only linear completeness is needed in the LME approximation. We show that the crack surface can be captured more accurately in the fourth-order model than the second-order model. Furthermore, less nodes are needed for the fourth-order model to resolve the crack path. Finally, we demonstrate the performance of the proposed meshfree fourth order phase-field formulation for 5 representative numerical examples. Computational results will be compared to analytical solutions within linear elastic fracture mechanics and experimental data for three-dimensional crack propagation.
In the last part of this research, we present a phase-field model for fracture in Kirchoff-Love thin shells using the local maximum-entropy (LME) meshfree method. Since the crack is a natural outcome of the analysis it does not require an explicit representation and tracking, which is advantageous over techniques as the extended finite element method that requires tracking of the crack paths. The geometric description of the shell is based on statistical learning techniques that allow dealing with general point set surfaces avoiding a global parametrization, which can be applied to tackle surfaces of complex geometry and topology. We show the flexibility and robustness of the present methodology for two examples: plate in tension and a set of open connected
pipes.
A safe and economic structural design based on the semi-probabilistic concept requires statistically representative safety elements, such as characteristic values, design values, and partial safety factors. Regarding climate loads, the safety levels of current design codes strongly reflect experiences based on former measurements and investigations assuming stationary conditions, i.e. involving constant frequencies and intensities. However, due to climate change, occurrence of corresponding extreme weather events is expected to alter in the future influencing the reliability and safety of structures and their components. Based on established approaches, a systematically refined data-driven methodology for the determination of design parameters considering nonstationarity as well as standardized targets of structural reliability or safety, respectively, is therefore proposed. The presented procedure picks up fundamentals of European standardization and extends them with respect to nonstationarity by applying a shifting time window method. Taking projected snow loads into account, the application of the method is exemplarily demonstrated and various influencing parameters are discussed.
This term paper presents a literature review and discusses concepts of the following point: 1- Factors affecting small-strain stiffness in soil; 2- Methods to determine small-strain shear stiffness in laboratory and in-situ; 3- Brief introduction into wave propagation and 4- Bender elements technique to determine shear wave velocity in soil.
The initial shear modulus, Gmax, of soil is an important parameter for a variety of geotechnical design applications. This modulus is typically associated with shear strain levels about 5*10^-3% and below. The critical role of soil stiffness at small-strains in the design and analysis of geotechnical infrastructure is now widely accepted.
Gmax is a key parameter in small-strain dynamic analyses such as those to predict soil behavior or soil-structure interaction during earthquake, explosions, machine or traffic vibration where it is necessary to know how the shear modulus degrades from its small-strain value as the level of shear strain increases. Gmax can be equally important for small-strain cyclic situations such as those caused by wind or wave loading and for small-strain static situations as well. Gmax may also be used as an indirect indication of various soil parameters, as it, in many cases, correlates well to other soil properties such as density and sample disturbance. In recent years, a technique using bender elements was developed to investigate the small-strain shear modulus Gmax.
The objective of this thesis is to study the initial shear stiffness for various sands with different void ratios, densities, grain size distribution under dry and saturated conditions, then to compare empirical equations to predict Gmax and results from other testing devices with results of bender elements from this study.
Die Bauhausstraße 11 war in der NS-Zeit Sitz von zahlreichen Institutionen der Gesundheitspolitik. Jetzt ist das Gebäude zum Gegenstand eines Forschungsprojektes geworden, in Zukunft wird auch vor Ort an seine Einbindung in nationalsozialistische Verbrechen erinnert. Dieses Buch dokumentiert und reflektiert die Erinnerungsarbeit auf dem Campus der Bauhaus-Universität Weimar und darüber hinaus. Anhand der interdisziplinären Beiträge wird das Gebäude in der heutigen Bauhausstraße 11 räumlich in Weimar und Thüringen, erinnerungspolitisch aber in einer seit Jahrzehnten erkämpften Landschaft des Gedenkens an nationalsozialistische Verbrechen verortet.
Identifying cable force with vibration-based methods has become widely used in engineering practice due to simplicity of application. The string taut theory provides a simple definition of the relationship between natural frequencies and the tension force of a cable. However, this theory assumes a perfectly flexible non-sagging cable pinned at its ends. These assumptions do not reflect all cases, especially when the cable is short, under low tension forces or the supports are partially flexible. Extradosed bridges, which are distinguished from cable-stayed bridges by their low pylon height, have shorter cables. Therefore the application of the conventional string taut theory to identify cable forces on extradosed bridge cables might be inadequate to identify cable forces.
In this work, numerical modelling of an extradosed bridge cable saddled on a circular deviator at pylon is conducted. The model is validated with the catenary analytical solution and its static and dynamic behaviours are studied. The effect of a saddle support is found to positively affect the cable stiffness by geometric means; longer saddle radius increases the cable stiffness by suppressing the deformations near the saddle. Further, accounting the effects of bending stiffness in the numerical model by using beam elements show considerable deviation from models with truss elements (i.e. zero bending stiffness). This deviation is manifested when comparing the static and dynamic properties. This motivates a more thorough study of bending stiffness effects on short cables.
Bending stiffness effects are studied using two rods connected with several springs along their length. Under bending moments, the springs resist the rods' relative axial displacement by the springs' transverse component. This concept is used to identify bending stiffness values by utilizing the parallel axis theorem to quantify ratios of the second moment of area. These ratios are calculated based on the setup of the springs (e.g. number of springs per unit length, transverse stiffness, etc...). The numerical model based on this concept agrees well with the theoretical values computed using upper and lower bounds of the parallel axis theorem.
The proposed concept of quantifying ratios of the second moment of area using springs as connection between cable rods is applied on an actual extradosed bridge geometry. The model is examined by comparison to the previously validated global numerical model. The two models showed good correlation under various changing parameters. This allowed further study of the effects of stick/slip behaviour between cable rods on an actual bridge geometry.
Mitigating Risks of Corruption in Construction: A theoretical rationale for BIM adoption in Ethiopia
(2021)
This PhD thesis sets out to investigate the potentials of Building Information Modeling (BIM) to mitigate risks of corruption in the Ethiopian public construction sector. The wide-ranging capabilities and promises of BIM have led to the strong perception among researchers and practitioners that it is an indispensable technology. Consequently, it has become the frequent subject of science and research. Meanwhile, many countries, especially the developed ones, have committed themselves to applying the technology extensively. Increasing productivity is the most common and frequently cited reason for that.
However, both technology developers and adopters are oblivious to the potentials of BIM in addressing critical challenges in the construction sector, such as corruption. This particularly would be significant in developing countries like Ethiopia, where its problems and effects are acute. Studies reveal that bribery and corruption have long pervaded the construction industry worldwide. The complex and fragmented nature of the sector provides an environment for corruption. The Ethiopian construction sector is not immune from this epidemic reality. In fact, it is regarded as one of the most vulnerable sectors owing to varying socio-economic and political factors. Since 2015, Ethiopia has started adopting BIM, yet without clear goals and strategies. As a result, the potential of BIM for combating concrete problems of the sector remains untapped. To this end, this dissertation does pioneering work by showing how collaboration and coordination features of the technology contribute to minimizing the opportunities for corruption. Tracing loopholes, otherwise, would remain complex and ineffective in the traditional documentation processes.
Proceeding from this anticipation, this thesis brings up two primary questions: what are areas and risks of corruption in case of the Ethiopian public construction projects; and how could BIM be leveraged to mitigate these risks? To tackle these and other secondary questions, the research employs a mixed-method approach. The selected main research strategies are Survey, Grounded Theory (GT) and Archival Study. First, the author disseminates an online questionnaire among Ethiopian construction engineering professionals to pinpoint areas of vulnerability to corruption. 155 responses are compiled and scrutinized quantitatively. Then, a semi-structured in-depth interview is conducted with 20 senior professionals, primarily to comprehend opportunities for and risks of corruption in those identified highly vulnerable project stages and decision points. At the same time, open interviews (consultations) are held with 14 informants to be aware of state of the construction documentation, BIM and loopholes for corruption in the country. Consequently, these qualitative data are analyzed utilizing the principles of GT, heat/risk mapping and Social Network Analysis (SNA). The risk mapping assists the researcher in the course of prioritizing corruption risks; whilst through SNA, methodically, it is feasible to identify key actors/stakeholders in the corruption venture. Based on the generated research data, the author constructs a [substantive] grounded theory around the elements of corruption in the Ethiopian public construction sector. This theory, later, guides the subsequent strategic proposition of BIM. Finally, 85 public construction related cases are also analyzed systematically to substantiate and confirm previous findings.
By ways of these multiple research endeavors that is based, first and foremost, on the triangulation of qualitative and quantitative data analysis, the author conveys a number of key findings. First, estimations, tender document preparation and evaluation, construction material as well as quality control and additional work orders are found to be the most vulnerable stages in the design, tendering and construction phases respectively. Second, middle management personnel of contractors and clients, aided by brokers, play most critical roles in corrupt transactions within the prevalent corruption network. Third, grand corruption persists in the sector, attributed to the fact that top management and higher officials entertain their overriding power, supported by the lack of project audits and accountability. Contrarily, individuals at operation level utilize intentional and unintentional 'errors’ as an opportunity for corruption.
In light of these findings, two conceptual BIM-based risk mitigation strategies are prescribed: active and passive automation of project audits; and the monitoring of project information throughout projects’ value chain. These propositions are made in reliance on BIM’s present dimensional capabilities and the promises of Integrated Project Delivery (IPD). Moreover, BIM’s synchronous potentials with other technologies such as Information and Communication Technology (ICT), and Radio Frequency technologies are topics which received a treatment. All these arguments form the basis for the main thesis of this dissertation, that BIM is able to mitigate corruption risks in the Ethiopian public construction sector. The discourse on the skepticisms about BIM that would stem from the complex nature of corruption and strategic as well as technological limitations of BIM is also illuminated and complemented by this work. Thus, the thesis uncovers possible research gaps and lays the foundation for further studies.
Zwischen den Jahren 1920 und 1930 kam es an der kalifornischen Küste zu Bauschäden an Brücken und Fahrbahnen, die sich vor allem in einer deutlichen Rissbildung äußerten. Seither werden immer wieder Bauschäden beschrieben, deren Ursache in der Reaktion von Zuschlägen, die „reaktive“ Kieselsäure enthalten, mit der Porenlösung des Betons zu sehen ist. Diese Reaktion wird als Alkali-Kieselsäure Reaktion (AKR) bezeichnet. Seit der ersten Veröffentlichung von Stanton über die „alkali-aggregate reaction“ an opalhaltigen Zuschlägen sind hunderte von Forschungsarbeiten zu diesem Thema durchgeführt und deren Ergebnisse veröffentlicht worden. Trotz eingehender Forschung seit mehr als 8o Jahren ist weder der Mechanismus der AKR vollständig geklärt noch eine eindeutige Voraussage über die Gefährdung von Bauwerken oder Bauteilen mit potentiell AKR-empfindlichen Zuschlägen möglich. Das liegt vor allen Dingen daran, das es sich bei der AKR um eine Reaktion handelt, die aus einer komplexen Abfolge chemischer und physikalischer Prozesse besteht, die in ihrer Gesamtheit zu einer Schädigung von Beton bzw. Betonbauteilen und Bauwerken führen können. Eine geschlossene Beschreibung und Behandlung dieser Reaktion ist nicht möglich, solange keine befriedigende Kenntnis über den Ablauf der einzelnen Schritte vorliegt. Dazu bedarf es grundsätzlicher Untersuchungen der einzelnen chemischen und physikalischen Reaktionsschritte sowie einer möglichst quantitativen Bewertung der verschiedenen Einflussfaktoren. Grundsätzlich gibt es weltweit eine ganze Reihe von Richtlinien und Normen , die dazu verhelfen sollen, Schädigungen an Bauwerken durch AKR zu verhindern. In Deutschland ist das momentan gültige Regelwerk die sogenannte Alkali-Richtlinie des deutschen Ausschusses für Stahlbeton (DAfStb). Sie dient zur Beurteilung von Zuschlag nach DIN 4226 [6, 7, 8] mit alkaliempfindlichen Bestandteilen. Dabei bezieht sich der Teil 2 der Richtlinie auf Zuschläge mit Opalsandstein, Kieselkreide und Flint aus bestimmten Gewinnungsgebieten. Hier wird eine reine Zuschlagprüfung gefordert. Teil 3 der Richtlinie bezieht sich auf präkambrische Grauwacken und andere alkaliempfindliche Gesteine. Gefordert werden hier Prüfungen der Zuschläge selbst sowie Prüfung an Betonbalken und 30er Würfeln in der Nebelkammer. Für die meisten in der Richtlinie genannten Zuschläge bilden die Prüfungen und Vorschriften eine ausreichende Sicherheit, um eine AKR zu vermeiden. Dennoch treten immer wieder Schäden mit Zuschlägen auf, die nach der Alkali-Richtlinie als unempfindlich eingestuft werden müssten. Dabei handelt es sich in der Regel um Schadensfälle, die erst nach mehreren Jahren mit spät reagierenden AKR-empfindlichen Zuschlägen auftreten. Zu diesen Zuschlägen, die gegebenenfalls speziell im Nebelkammertest innerhalb von neun Monaten keine signifikante Dehnung (<0,6mm/m) aufweisen, gehören Stressquarze, Kieselkalk, Granit, Porphyr, Kieselschiefer und Grauwacke. Die vorliegende Arbeit dient speziell der Beurteilung und Einordnung von unterschiedlichen kristallinen Quarzmodifikationen sowie der Ermittlung geeigneter Untersuchungsmethoden für die Beurteilung der AKR-Empfindlichkeit von Quarz.
In the last decades, Finite Element Method has become the main method in statics and dynamics analysis in engineering practice. For current problems, this method provides a faster, more flexible solution than the analytic approach. Prognoses of complex engineer problems that used to be almost impossible to solve are now feasible.
Although the finite element method is a robust tool, it leads to new questions about engineering solutions. Among these new problems, it is possible to divide into two major groups: the first group is regarding computer performance; the second one is related to understanding the digital solution.
Simultaneously with the development of the finite element method for numerical solutions, a theory between beam theory and shell theory was developed: Generalized Beam Theory, GBT. This theory has not only a systematic and analytical clear presentation of complicated structural problems, but also a compact and elegant calculation approach that can improve computer performance.
Regrettably, GBT was not internationally known since the most publications of this theory were written in German, especially in the first years. Only in recent years, GBT has gradually become a fertile research topic, with developments from linear to non-linear analysis.
Another reason for the misuse of GBT is the isolated application of the theory. Although recently researches apply finite element method to solve the GBT's problems numerically, the coupling between finite elements of GBT and other theories (shell, solid, etc) is not the subject of previous research. Thus, the main goal of this dissertation is the coupling between GBT and shell/membrane elements. Consequently, one achieves the benefits of both sides: the versatility of shell elements with the high performance of GBT elements.
Based on the assumptions of GBT, this dissertation presents how the separation of variables leads to two calculation's domains of a beam structure: a cross-section modal analysis and the longitudinal amplification axis. Therefore, there is the possibility of applying the finite element method not only in the cross-section analysis, but also the development for an exact GBT's finite element in the longitudinal direction.
For the cross-section analysis, this dissertation presents the solution of the quadratic eigenvalue problem with an original separation between plate and membrane mechanism. Subsequently, one obtains a clearer representation of the deformation mode, as well as a reduced quadratic eigenvalue problem.
Concerning the longitudinal direction, this dissertation develops the novel exact elements, based on hyperbolic and trigonometric shape functions. Although these functions do not have trivial expressions, they provide a recursive procedure that allows periodic derivatives to systematise the development of stiffness matrices. Also, these shape functions enable a single-element discretisation of the beam structure and ensure a smooth stress field.
From these developments, this dissertation achieves the formulation of its primary objective: the connection of GBT and shell elements in a mixed model. Based on the displacement field, it is possible to define the coupling equations applied in the master-slave method. Therefore, one can model the structural connections and joints with finite shell elements and the structural beams and columns with GBT finite element.
As a side effect, the coupling equations limit the displacement field of the shell elements under the assumptions of GBT, in particular in the neighbourhood of the coupling cross-section.
Although these side effects are almost unnoticeable in linear analysis, they lead to cumulative errors in non-linear analysis. Therefore, this thesis finishes with the evaluation of the mixed GBT-shell models in non-linear analysis.
Heutige Methoden zur Soll-Spezifikation von Bauleistungen (Kostenermittlung und zeitliche Ablaufplanung) gehen von einer abstrahierten und vereinfachten Betrachtung der Zusammenhänge bei Bauprojekten aus. Leistungsverzeichnisse, Kostenermittlungen und Bauzeitpläne orientieren sich nur indirekt an der Geometrie des Bauwerks und der Baustelle. Die dabei verwendeten Medien wie Papier, 2D-Dateien, digitale Leistungsbeschreibungen oder 3D-Darstellungen lassen die Suche nach Informationen auf der Baustelle zu einem zeitaufwändigen und in Anbetracht existierender Medientechnologien ineffizienten Prozess werden. Interaktive virtuelle Umgebungen erlauben die Auflösung starrer Zusammenhänge durch interaktive Eingriffe des Anwenders und visualisieren komplexe bauproduktionstechnische Vorgänge. Das Konzept der visuellen interaktiven Simulation der Bauproduktion sieht vor, die Soll-Spezifikation anhand eines interaktiven 3D-Modells zu entwickeln, um räumliche Veränderungen und parallele Prozesse auf der virtuellen Baustelle im Rahmen der Entscheidungsfindung zum Bauablauf besser berücksichtigen zu können. Verlangt man einen hohen Grad an Interaktivität mit dem 3D-Modell, dann bieten sich Computerspieltechnologien sehr gut zu Verifikationszwecken an. Die visuelle interaktive Simulation der Bauproduktion ist damit als eine 3D-modellbasierte Methode der Prozessmodellierung zu verstehen, die Entscheidungen als Input benötigt und die Kostenermittlung sowie die zeitliche Ablaufplanung als Output liefert.