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Wooden construction of the movable roof of the garden pavilion in Český Krumlov castle
Bláha, Jiří; Kloiber, Michal
2022 - anglický
A remarkable technical monument is situated on one of the garden terraces belonging to the state castle and chateau in Český Krumlov. The umbrella-shaped roof of the lookout pavilion can be lifted about a meter above the masoned parapet allowing the visitor a beautiful view of the World Heritage town panorama when the weather is fine. In windy conditions or in case of heavy rain the roof could be lowered and the pavilion thus closed. The lifting is operated by means of a wooden lever mechanism with a counterweight box hidden in the basement level of the building. The unique construction was built in 1823 and needed to be partially altered in 1830-1835. However, since then it has been preserved in a surprisingly authentic state even allowing practical demonstrations of its original function. Prior to recent repairs in 2016, a comprehensive non-destructive survey was conducted. The extent of the damage discovered was determined by measuring the speed of elastic wave propagation using stress waves (Fakkop 2D) and measuring the mechanical resistance when drilling with a thin bit using resistance micro-drilling (Resistograph). Another method based on loading timber by means of a\nminiature loading jack inserted into a drilled hole was selected to determine the current mechanical properties.
Klíčová slova:
lookout pavilion; gazebo; non-destructive testing; roof; timber; wood
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Wooden construction of the movable roof of the garden pavilion in Český Krumlov castle
A remarkable technical monument is situated on one of the garden terraces belonging to the state castle and chateau in Český Krumlov. The umbrella-shaped roof of the lookout pavilion can be lifted ...
Aerodynamic interference forces acting on two square prisms in a model atmospheric boundary layer
Macháček, Michael; Lo, Y. L.; Máca, J.
2022 - anglický
The presented experimental analysis focuses on the definition of interference factors for aerodynamic forces acting on two square prisms depending on their relative position. Both prisms used had the same dimensions and a height to width ratio of 6 - this ratio is relatively common in high-rise buildings. The experiment was performed in a wind tunnel with a model atmospheric boundary layer, which represented a dense urban area. The forces acting at the base of one of the buildings were measured, and by moving the other building it was possible to measure 204 mutual configurations, i.e., 204 interference positions. In all positions, the windward facades of the small-area model were placed perpendicular to the direction of the incoming flow. This created a detailed map of the interference effect on a wind-loaded pair of buildings. The results show a significant effect of the relative position of buildings on wind loads. A significant increase in\nload occurred due to the proximity of buildings. Due to the interference effect, wind load fluctuations can increase by up to 28% and average values by up to 36%.
Klíčová slova:
interference effect; two square prisms; wind tunnel experiments; turbulent boundary layer
Plné texty jsou dostupné v digitálním repozitáři Akademie Věd.
Aerodynamic interference forces acting on two square prisms in a model atmospheric boundary layer
The presented experimental analysis focuses on the definition of interference factors for aerodynamic forces acting on two square prisms depending on their relative position. Both prisms used had the ...
Tomographic investigation of the sandstone fracture toughness
Vavřík, Daniel; Beneš, Pavel; Fíla, Tomáš; Koudelka_ml., Petr; Kumpová, Ivana; Vavro, Martin
2022 - anglický
It is well known that the measured values of the fracture toughness of quasi brittle materials are influenced by material heterogeneity, dimensions, boundary conditions, and unequal tension and compression properties. Standard testing methods supposing isotropic material, in contrary the quasi-brittle materials differ from this theoretical expectation, therefore this approach may fail. The authors present Local Fracture Toughness Testing (LFTT) method to overcome this obstacle. LFFT based on a complex methodology using a series of tomographic reconstructions recorded during specimen loading is calculated independently of the outside boundary conditions.
Klíčová slova:
fracture process zone; crack path; quasi-brittle material; X-Ray computed tomography; four-point bending test
Plné texty jsou dostupné v digitálním repozitáři Akademie Věd.
Tomographic investigation of the sandstone fracture toughness
It is well known that the measured values of the fracture toughness of quasi brittle materials are influenced by material heterogeneity, dimensions, boundary conditions, and unequal tension and ...
Stone surface topography of Prague historic monuments over the centuries
Kovářová, K.; Cihla, Michal; Malát, R.; Semerád, M.; Tryml, M.
2022 - anglický
Stonemasons tool traces on stone surface are undoubtably a significant part of monument historic value. As any other profession, stonemasons craft develops over time and has its own specifics in every period. Each work has its own unique pattern and bears traces of individual stonemason workshops, tradition, etc. The study of building stone cutting is based on evolving methods of mechanoscopy and analytical traceology. Stone traceology deals with traces in material and the subsequent reconstruction of tools and the processes of stoneworking. The interpretation of data in terms of determining the actual trace is called a mechanoscopy. When studying a stone surface, the latest 3D modelling technology is used with subsequent analyses by means of Global Mapper software. The use of these methods enables reconstruction of the craftsman’s tools. The presented article is an extract of a study that systematically maps the stone cutting work in the territory of Prague from the oldest buildings to the present day. For example, stonework in 9th century Prague was gradually evolving from simple stone block modelling to sophisticated cutting of blocks in the 12th century. This information can be very useful in the process of monument care.
Klíčová slova:
stone topography; Prague; craftmanship; stonemason tool; tool traces
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Stone surface topography of Prague historic monuments over the centuries
Stonemasons tool traces on stone surface are undoubtably a significant part of monument historic value. As any other profession, stonemasons craft develops over time and has its own specifics in every ...
Creep of oak dowel: various loading and environmental conditions
Hasníková, Hana; Kunecký, Jiří; Hataj, M.
2022 - anglický
For research of timber joints, the crucial role in force distribution inside the joint is played by distribution and variability of stiffness of the dowels. Not only the instant stiffness, but the one that we can encounter after some longer period. Also, this value and its statistical distribution is influenced by many other factors, however, humidity and temperature are in timber (oak) dowels of utmost importance. In the work results of a creep experiment made using a special testing rig is presented. It has been found, that biggest changes in creep behavior are in case if very humid conditions are present, and, also, that cycling the temperature in high humidity conditions can produce about 104% of the original instantaneous displacement. The result is not surprising, however, new insights are made thanks to relatively high number of samples and ability to produce some statistics. Another outcome is relation of dowel stiffness in time to the level of applied stress, which is quantified in the article.
Klíčová slova:
oak dowel; creep; humidity
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Creep of oak dowel: various loading and environmental conditions
For research of timber joints, the crucial role in force distribution inside the joint is played by distribution and variability of stiffness of the dowels. Not only the instant stiffness, but the one ...
Comparison of mechanical properties of wood determined by local gently destructive tests and fully destructive tests on building elements
Drdácký, Miloš; Kloiber, Michal; Valluzzi, M. R.; Casarin, F.
2022 - anglický
Demolition or repair of historic buildings is a valuable source of test materials for testing nondestructive and gently destructive methods for estimating the mechanical properties of built-in wood. The article uses the results of one such opportunity, when it was possible to study the mechanical properties of wood obtained by local measurements on a larger sample of historic and new wooden beams and compare them with the properties calculated from destructive tests of structural elements. Thirty-one wood elements (nineteen recovered from disassembled buildings and twelve new) were subjected to considerately destructive local tests using a loading mini-jack method. In this method, a small loading jack is inserted into a precisely drilled hole and a load test of the wood in compression along the fibers is performed on a part of the hole wall while measuring the achieved deformation under load. Red and white firs, and pine species constituted the recovered elements - red fir and larch the new ones. Destructive tests (in bending and compression) were also carried out on ten recovered and six new elements, and results were correlated with those of considerately destructive tests. Comparison of the assessed mechanical properties shows reasonably good correlation and promising conclusions for practical applications.
Klíčová slova:
considerately destructive tests; mini-jack method; destructive tests; assessment of mechanical characteristics of wood; historic timber
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Comparison of mechanical properties of wood determined by local gently destructive tests and fully destructive tests on building elements
Demolition or repair of historic buildings is a valuable source of test materials for testing nondestructive and gently destructive methods for estimating the mechanical properties of built-in wood. ...
Modeling the moving cogwheel load - analysis using ANSYS & Matlab
Bayer, Jan
2022 - anglický
A moving impulse load generated by a heavy cogwheel (CW) can be used as a testing excitation for bridges. This previously proposed type of dynamic testing offers theoretically short testing times. Laboratory experiments confirmed already its capability of damage indication by repeated testing. This contribution suggests an approach how to solve the cogwheel movement using a Finite Element (FE) solver and Matlab. As the solution is theoretically nonlinear, the necessary limiting conditions for its application are formulated. Then the analysis is applied to simulate the passage of the CW over a simple laboratory model in order to compare it to laboratory experiments.
Klíčová slova:
moving impulse load; modal analysis; finite element models
Plné texty jsou dostupné v digitálním repozitáři Akademie Věd.
Modeling the moving cogwheel load - analysis using ANSYS & Matlab
A moving impulse load generated by a heavy cogwheel (CW) can be used as a testing excitation for bridges. This previously proposed type of dynamic testing offers theoretically short testing times. ...
Aeroelastic instability of differently porous U-profiles in crosswind direction
Hračov, Stanislav; Macháček, Michael
2022 - anglický
Flow-induced vibrations of the flexibly mounted slender U-shaped beams allowed to oscillate in the crosswind direction only are studied experimentally in the wind tunnel. All beams are characterized by a cross section having a side ratio of along-wind to across-wind dimension equal to two. The effects of two depths of U profiles and two porosities of their flanges ( 0 % and 75 %) onto a loss of aeroelastic stability are investigated under the smooth flow conditions and for low Scruton numbers. The results indicate almost similar proneness of the non-porous beams to galloping-type oscillations to a rectangular prism with the same side ratio regardless their depth. The onset of across-wind galloping occurred in these cases at wind velocity very close to von-Kármán-vortex-resonance flow speed, even though the critical velocity predicted by the quasisteady theory is much lower. For porous and shallower U profile this asynchronous quenching also takes\nplace. However, the higher flange porosity reduces significantly not only the vortex-shedding effect, but also causes an increase in the onset galloping velocity above the critical speed determined for non-porous profiles. In the case of deeper U-shaped beam, the effect of higher porosity even suppresses the proneness to galloping
Klíčová slova:
aeroelastic instability; galloping; vortex shedding; asynchronous quenching; U profile
Plné texty jsou dostupné v digitálním repozitáři Akademie Věd.
Aeroelastic instability of differently porous U-profiles in crosswind direction
Flow-induced vibrations of the flexibly mounted slender U-shaped beams allowed to oscillate in the crosswind direction only are studied experimentally in the wind tunnel. All beams are characterized ...
Solving the moving mass problem on large finite element models with modal analysis – estimation of the discrete movement error
Bayer, Jan
2021 - anglický
Solving transient dynamic problems on large finite element (FE) models using a direct integration requires a high sampling rate and therefore also considerable computing times because of the large system matrices. Movement of a mass on a FE model has to follow the discrete pattern of the FEs, and is therefore of a discrete character. This introduces an unknown\nerror into the analytical results. Applying modal analysis (MA) reduces the number of equations used in the subsequent numerical integration by orders of magnitude. Resampling of the mode shapes to the required sampling rate makes it possible to solve the moving mass problem much more quickly and quasi-continuously. The performed comparative analytical study using ANSYS and MATLAB showed that using a discrete movement on the FE mesh when solving a moving mass problem can cause a considerable error.
Klíčová slova:
moving mass; FE analysis; modal analysis; discret-movement error; ANSYS & MATLAB
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Solving the moving mass problem on large finite element models with modal analysis – estimation of the discrete movement error
Solving transient dynamic problems on large finite element (FE) models using a direct integration requires a high sampling rate and therefore also considerable computing times because of the large ...
Description of the quasi-periodic response caused by combined harmonic and random excitation
Náprstek, Jiří; Fischer, Cyril
2021 - anglický
The generalized van der Pol equation exposed to combined harmonic and random excitation can exhibit a quasi-periodic response. The existence of this particular type of response depends on the detuning between the driving and resonance frequencies. The response is stationary for a ”small” or ”large” value of detuning. The contribution specifies in detail the detuning interval in which the quasi-periodic response occurs.
Klíčová slova:
van der Pol equation; resonance; vibrations
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Description of the quasi-periodic response caused by combined harmonic and random excitation
The generalized van der Pol equation exposed to combined harmonic and random excitation can exhibit a quasi-periodic response. The existence of this particular type of response depends on the detuning ...
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