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Stress ribbon curved structure
Kocourek, Petr ; Stráský, Jiří (advisor)
The theme of this doctoral thesis is research of footbridges curved in plan that are formed by stress ribbon. Recently, several noteworthy curved pedestrian bridges, which decks are suspended on their inner edges on suspension or stay cables, have been constructed. However, curved stress ribbon bridges have not been built so far. The goal of this study was to answer the question whether the use of curved stress ribbon structures is even possible. For this reason, in the first stage feasibility study of these structures has been prepared. Knowledge of both curved pedestrian bridges, which decks are suspended on their inner edge and straight stress ribbon has been used. For the mathematical modeling FEM software ANSYS was used. The obtained findings were further used to design a new type of a modern, aesthetic curved bridge for pedestrians. The structure is formed by slender reinforced deck, which is through the steel brackets on the inner side stiffened by steel section. Torsion of the deck, caused by curved structure and asymmetrical cross-section, is reduced by cable situated in the handrail. Footbridge span is 45 m, arc camber in plan is 10 m, free bridge width is 3 m. Described is a general structure effect, detail static and dynamic analysis was carried out. Designed structure including construction stages were experimentally verified on a fully physical functional 1:6 scale model. The thesis describes the model analogy used for the design of the model, its structural design and its implementation. The model was subjected to a series of load tests including the final ultimate strength test. Performed tests confirmed the good match of calculation with the reality, correctness of the design and high resistance of designed construction. Acquired results and experiences from design and realization of model form the basis for practical realization of studied structures.
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Analysis of a plan curved cable stayed and suspension footbridges
Koláček, Jan ; Stráský, Jiří (advisor)
The aim of the doctoral thesis focuses on the static and dynamic analysis of a plan curved cable stayed and suspension pedestrian bridges suspended on a single-side of their deck. The first part of this thesis deals with an analysis of the equilibrium in the transversal direction of a deck cross section suspended on a single-side. The section and its arrangement should be designed so that the torsion caused by the deck self-weight and dead load would be minimal. This theory was verified on a simple study of a single-sided suspended section with and without prestressing. Second part of the thesis deals with the design of a study of a plan curved cable stayed pedestrian bridge. The study describes in detail a finding of an initial state of the structure and the static and dynamic analysis performed by software ANSYS. The static analysis describes the response of the structure on the most frequent variable loads only, but not design of dimensioning according to the valid codes. The dynamic analysis verifies a predisposition of the structure to the vibrations and others harmful oscillation effects. The next objective of the thesis was to design a study of a plan curved suspension pedestrian bridge suspended on a single-side. A process of an analysis of these structures has not found in any available references. Especially, the finding of an initial state of a suspension cable has not been documented anywhere and by anybody. The study was analyzed with the same geometry as the cable stayed variation in order to compare both structures. The static and dynamic analysis was performed, too. The last part of the thesis describes the verification of structural solution on a fully functional model in a 1:10 scale, proposed process of the initial state finding, response of the structures on the loading and ultimate load test. The important step is the comparison of the results of the completed physical model and the calculation model.
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Pedestrian bridges formed by a flat arch
Jurík, Michal ; Stráský, Jiří (advisor)
This doctoral thesis focuses on the research of the pedestrian bridges formed by the flat arch. To understand the basic static behaviour of the flat arch it was necessary to make a study of the development of the direct flat arch as footbridge with large span and the impact of stiffness on its camber. For the mathematical modeling FEM software ANSYS were used. The calculation has shown that a design of purely concrete flat arch would demand enormous bending stiffness, which can be achieved only through a massive cross-section. The findings gained in this chapter were further used to design a unique pedestrian bridge formed by the curved in plan flat arch, where to transfer of the large bending moments a steel pipe was designed. Several variants with different span and rise of the arch in plan were tested. From the tested variants was then selected footbridge with a span of 45 m and with the rise of the arch 10 m, which seemed to be the best solution according to the calculations and it was further analyzed in detail. The studied structure is formed by curved concrete slab that is stiffened through the steel brackets on the inner side of a steel tube with a graded thickness. The external cables that are situated in the handrail pipe balance the dead load torsional moment. Designed structure and the static analysis procedure were verified on a fully functional 1:6 scale model. The thesis describes the model analogy used for the design of the model, its structural design and its implementation. Load tests on the model confirmed correctness of the design of the proposed curved in plan pedestrian bridge, its high carrying capacity and the accuracy of the developed procedure of static analysis. Results and experiences acquired from the design and the realization of model are the basis for a practical realization of studied structures. The last part of the thesis deals with the possibility of replacement of the steel components with concrete in pedestrian bridges formed
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Pedestrian Bridge
Hibš, Daniel ; Kocourek,, Petr (referee) ; Stráský, Jiří (advisor)
This diploma thesis deals with the design of a new bridge construction over a multi-lane communication. The converted road is a pedestrian and biking trail connecting significant agglomerations. Three studies were worked out in this thesis and the C variant was selected for further detailed processing. It is an unsymmetrical suspended bridge with a cast-in-place deck. In the section above the communication, the bridge is made up of prefabricated segments. Suspension of the structure is made of two sloping pylons forming a V-shape. The analysis of the construction was carried out in the MIDAS Civil program, where a complete model of construction was entered, including a subsequent estacade. The model describes the construction process and includes creep and shrinkage of the concrete. The traffic, temperature and wind loads were used. The design was assessed for the ultimate limit states and serviceability limit states. The assessment was carried out in the IDEA StatiCa program, supplemented by manual calculations. The static calculations also include assessment of the harmonic response. Subsequently, overview drawings, detailed drawings and visualizations were developed.
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Form finding of shell structures
Musil, Jiří ; Bažant, Zdeněk (referee) ; Vítek,, Jan (referee) ; Stráský, Jiří (advisor)
The theme of this doctoral thesis is the design of concrete shell structures with the focus on finding their optimal shape. The optimal shape of a concrete shell is the shape in which for a given load (usually the dead weight of the structure) no significant bending moments are generated in the shell and the structure is in the so-called membrane state. The inspiration for this thesis is the work of Swiss engineer Heinz Isler, who developed the shapes of shell structures using model tests of appropriately loaded flexible membranes. He developed the shell structure for large spans by inverting the resultant shape, which carried its weight almost entirely via membrane forces. The numerical solution of the above experiments using Midas Civil is presented herein. The basic principles of the method are demonstrated on the example of sagged cable. The numerically found shapes are compared with the analytical solution. A shell is designed based on the numerically found shapes and its stress response to dead load is described, particularly in relation to the membrane action. In the next part, the acquired knowledge and methods were used to design three relatively complicated shell structures. Each structure was statically analysed and its static behaviour was described. Structures with perfectly rigid or flexible supports, which simulate real behaviour of the supports, were studied. In the final phase, the results of static analysis of the selected shell were experimentally verified on a physical model in a scale of 1: 55.56. The model has been built using 3D printing. The thesis describes the use of a modelling similarity, the model design, the production process, and the experiment. The load test confirmed the optimal design of the shell structure and the validity of the numerical method for finding their shapes.
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Microstructure and mechanical properties of ultra-fine grained titanium alloys
Václavová, Kristína ; Stráský, Josef (advisor) ; Šíma, Vladimír (referee)
Title: Microstructure and mechanical properties of ultra-fine grained titanium alloys Author: Bc. Kristína Václavová Department / Institute: Department of Physics of Materials Supervisor of the master thesis: PhDr. RNDr. Josef Stráský, Ph.D. Abstract: Metastable β-Ti alloys Ti-15Mo and Ti-6.8Mo-4.5Fe-1.5Al (TIMETAL LCB) were subjected to severe plastic deformation by high pressure torsion. Microhardness of Ti-15Mo and TIMETAL LCB alloys increases with increasing inserted deformation, i.e. with increasing number of HPT rotations and also with increasing distance from the centre of the sample. The highest microhardness after HPT exceeds significantly the microhardness of two- phase α + β heat-treated material. Increasingly deformed microstructure was also demonstrated by scanning electron microscopy and by electron back-scatter diffraction. Significant twinning was observed in both studied alloys. Mechanism of multiple twinning contributes notably to the fragmentation of grains and thus to the refinement of the microstructure. Defect structure in Ti-15Mo alloy was studied by positron annihilation spectroscopy. It was proved that dislocations are the only detectable defects in the material by positron annihilation spectroscopy and that dislocation density increases with the number of HPT revolution and with...
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Microstructure and mechanical properties of Ti15Mo alloy prepared by ECAP
Terynková, Anna ; Stráský, Josef (advisor) ; Šíma, Vladimír (referee)
In the bachelor thesis, mechanical properties and microstructure of Ti-15Mo alloy prepared by equal channel angular pressing were studied along with their dependence on the number of passes. Microhardness was studied by Vickers method, microstructure was studied by scanning electron microscopy and by electron back-scattered diffraction (EBSD). Finally, the elasticity modulus was measured by resonant ultra-sound spectroscopy. Mircohardness significantly increased after the first ECAP pass. After the second and the third pass it was almost constant and it again distinctly rose after the fourth pass. Grains with the size of hundreds of micrometres were observed in all samples. The volume fraction of twinned material increased with the number of passes. Deformation bands were also observed, namely in samples after two and three passes. Elastic modulus significantly increased with increasing deformation by ECAP and the evolution of elastic modulus is qualitatively similar to the evolution of microhardness. Considering that the material was processed by ECAP at 250řC, the omega phase may have formed during the processing. The increase of microhardness and elastic modulus can be explained by the increasing volume fraction of the omega phase.
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Microstructure and Texture of Titanium Prepared by Powder Metallurgy
Kozlík, Jiří ; Stráský, Josef (advisor) ; Chráska, Tomáš (referee)
Bulk commercially pure titanium was prepared by powder metallurgy, namely by cryogenic milling and spark plasma sintering, with aim to produce ultra-fine grained material with enhanced strength. The microstructure of milled powders was investigated in detail by a novel method called transmission EBSD, which allowed the first direct observation of texture within the powder particles. This texture is similar to rolling texture, because of the similar nature of the defor- mation during milling. Microstructure observations revealed grains with the size under 100 nm. The influence of sintering parameters on material properties were studied by scan- ning electron microscopy including EBSD, X-ray diffraction and by microhardness measurements. The trade-off relationship between porosity and grain size was identified, fully dense material with ultra-fine grained microstructure could not be produced. Increased oxygen content was identified as a main strengthening factor, while porosity has significant deteriorating effect on mechanical properties. The texture of powder was retained in the bulk material. The possibility of stabilizing the microstructure by mechanical alloying of Ti with yttrium oxide nanoparticles was investigated with mixed results. The stabiliza- tion was successful, but several issues...
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Oxygen-strengthened biomedical beta titanium alloys
Preisler, Dalibor ; Stráský, Josef (advisor) ; Kužel, Radomír (referee)
Oxygen strengthened biomedical beta titanium alloys Beta titanium alloy Ti-35Nb-6Ta-7Zr-0.7O (wt. %) exhibits a low Young's modulus and high strength thanks to interstitial strengthening effect of oxygen. These mechanical properties are promising for implant production. In this thesis, the aforementioned alloy is thoroughly studied in conditions prepared by various thermomechanical treatments, namely: material after casting, hot compression testing, die-forging, hot rolling and finally, cold-swaged and annealed condition. Mechanical properties and microstructure of prepared conditions are studied with special interest of enhancing the fatigue performance that is poor after casting due to porosity and large grains. The hot rolling was shown to improve the fatigue performance significantly, mainly through grain refinement. It is shown that by combining the effects of cold-swaging and recrystallization annealing, extensive refinement of grain structure can be achieved. Industrial aspects of implant production by the studied methods were discussed. In the second part of the thesis, the possibility of lowering the Young's modulus by reducing the beta phase stability (by reducing of Nb and Ta content), while retaining high strength caused by high interstitial oxyen content is explored. Several alloys were...
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Morphology and hardness of particles of titanium powder prepared by cryogenic milling
Ibragimov, Ilya ; Stráský, Josef (advisor) ; Janeček, Miloš (referee)
The effect of cryogenic milling on morphology and microhardness of titanium powder was investigated. Initial powder of commercially pure titanium was subjected to cryogenic milling in liquid argon (LAr) using two different milling speeds. Prepared powders were subsequently cleaned by ethanol. The cleaning procedure was done in two ways: in the air and in the inert atmosphere in the glovebox. Particle size and morphology were investigated my scanning electron microscopy and subsequent automated image analysis. Microhardness of powder particles was determined by Vickers hardness measurement using small loads. The milling did not cause significant powder refiniment, while the shape of powder particles changes substantially. Milling speed affected particle shape and powder contamination. Application of stearic acid as the processing control agent prevented cold-welding of powder particles, but contaminated the powder by oxygen and hydrogen. Stearic acid could be successfully removed by cleaning in ethanol. The using of glovebox for cleaning did not have significant effect on the resultiing contamination.
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