Browsing by Author "Altintas, G"

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    Adventure of Democracy Seen Through Animal Metaphors Created By 6th Grade Students
    Selcuk, G; Altintas, G; Ciritci, HM
    A group of 41 students in a 6th grade class of 75. Yil Secondary School in the province of Demirci in Manisa, Turkey, are asked to create animal metaphors about perception, constitutionalism, secularism, monarchy, oligarchy, freedom and executive for the unit on democracy adventure, a subject discussed in Social Science class. The animals that the students have chosen for the representation are from different habitats but through the different techniques of analysis, it was seen that this choice is not arbitrary. The animal metaphors represent their mental images of the related concepts. At the same time, the animals used by the students are in connection with their own environment and also their knowledge about these animals and their characteristics. (C) 2015 The Authors. Published by Elsevier Ltd.
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    Mechanical properties of Acropora cervicornis aragonite skeleton by using multiscale models based on micro-CT data
    Altintas, G; Cankal, D; Kilic, G; Ergun, S
    Corals, crucial for ocean ecosystems, face threats such as ocean acidification from global warming and pollution, which weaken their skeletons. This study focuses on Acropora cervicornis, known for its hard but fragile structure, requiring strength and flexibility to withstand the forces from climate-driven atmospheric events. An experiment using uniaxial mechanical loading from the initial stage to complete failure at a very low strain rate (1.2821 x 10-5 s-1) was conducted to ascertain the mechanical properties of corals. The geometric properties and Young's modulus were analysed based on various levels of micro-architectural details from micro-CT data, with resolution values influencing the measurements. The highest resolution model showed a Young's modulus approaching 22.265 GPa and porosity at 40.448%. Calibration of finite element models incorporating micro-architectural details enabled a precise comparison of parameter effects and more accurate results, highlighting the significance of resolution in modelling coral mechanical properties.
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    The effects of orthotropic materials on the vibration characteristics of structural system
    Altintas, G; Goktepe, AB
    The rapid growth in anisotropic material (such as composite materials) usage marks a new era in material science. Obviously, in order to understand the physical behavior of structural systems constituted by composite materials, several parameters, such as natural frequencies, mode shapes, and transmissiblities, must be incorporated into anisotropic elastic analyses. Orthotropy, as a special case of anisotropy, is common in almost all fields of civil and mechanical engineering. The objective of this effort is to investigate the effect of orthotropy on the behavior of vibrating plate systems near degenerate modes. A degenerate plate may be lack of geometric stability since the plate may exhibit a qualitatively different behavior under an arbitrarily small change in special parameters. The special parameters considered in this study are orthotropy and the location of additional mass, which have the effect of removing the system symmetry. The method, which depends on a variational procedure in conjunction with a finite difference method, is employed to examine free vibration characteristics as well as to characterize steady state response to a sinusoidally varying force applied to orthotrophic elastic rectangular plate. The problem is reduced to the solution of a system consisting of algebraic equations by using the variational difference method (VDM). In order to sort the effect of orthotropy on the vibration behavior from other factors, locations of external force, and mass are treated as additional parameters in the analyses. Furthermore, supports are constituted as elastic so as to be able to obtain force transmissibility curves. Finally, plate systems having two different boundary conditions, namely elastically supported and elastically point supported, are utilized to observe the generalization of the outcomes. Results reveal that in case of material orthotropy existence in a plate system, there are several keynote issues that must be taken into account in vibration analyses, which are not important under isotropic conditions. Obviously, these effects are of theoretical importance and should be considered in practical applications.
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    Effect of strategic intelligent games on gathering attention
    Yesiltepe, M; Altintas, G
    Nowadays, while raising the humans of information age, to which extent we can provide guidance under the roof of the school that will ensure them to tackle the problems that they will encounter in the future? Currently, to what extent do we assume our children whom we prepare for currently unknown professions of future ages the name, function and the requirements for equipment of which are not known by us now, for their future, country and the world will have what degree of strategic thinking skills and whether they will be attentive in case of confusion regarding the target. If we assume that the individuals who will keep pace with rapidly changing world and direct it should have very good level of attention, activities to improve it should be arranged. We should consider making foundations available for them to manage daily problems or possible obstacles by using different thinking methods and by managing them with the plans a, b even c prepared by them, and finding alternative responses in the competition environment and creating added value for them, their country and the world. For this purpose, increase in attention gathering level at the beginning of mental activities would positively affect this process. What would be the effect of plays in developing attention giving and gathering features that are not felt more until coming to the school age? Our study started with this question and the voluntarily participated students were observed during the study. Study covers 3 students playing at very low, middle and high frequency who are selected among universe of 10 students, 2 of which were girls who are the members of strategic mind games activities which is a working group indicating maximum diversity. Subject who are taken under the scope of the study are selected from upper socio-cultural level. Students were expected to participate in thinking skills developing paper-and-pencil activities and strategic mind games. Strategy developing games that are used in the study are used for development of gathering attention.
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    Collapse mechanism estimation of a historical slender minaret
    Nohutcu, H; Hokelekli, E; Ercan, E; Demir, A; Altintas, G
    The aim of this study is to accurately estimate seismic damage and the collapse mechanism of the historical stone masonry minaret Hafsa Sultan, which was built in 1522. Surveying measurements and material tests were conducted to obtain a 3D solid model and the mechanical properties of the components of the minaret. The initial Finite Element (FE) model is analyzed and numerical dynamic characteristics of the minaret are obtained. The Operational Modal Analysis (OMA) method is conducted to obtain the experimental dynamic characteristics of the minaret and the initial FE model is calibrated by using the experimental results. Then, linear time history (LTH) and nonlinear time history (NLTH) analyses are carried out on the calibrated FE model by using two different ground motions. Iron clamps which used as connection element between the stones of the minaret considerably increase the tensile strength of the masonry system. The Concrete Damage Plasticity (CDP) model is selected in the nonlinear analyses in ABAQUS. The analyses conducted indicate that the results of the linear analyses are not as realistic as the nonlinear analysis results when compared with existing damage.
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    Effect of voxel size in flow direction on permeability and Forchheimer coefficients determined by using micro-tomography images of a porous medium
    Ucar, E; Mobedi, M; Altintas, G; Glatt, E
    The permeability and Forchheimer coefficients of a porous medium, volcanic rock, are determined using micro-tomography images. A cubic volume in the middle of the images is extracted as REV (representative volume). The voids in the REV are discretised into anisotropic voxels using the commercial program of GeoDict. Seven computational domains with different voxel size in flow direction are generated. The velocity and pressure fields in the voids are obtained for Reynolds numbers ranging from 0.01 to 10. The obtained fields are used to determine the permeability and the Forchheimer coefficients. The performed calculations show that the nominal pore size changes with the voxel size in flow direction, however permeability and the Forchheimer coefficient approaches to the constant values.
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    Effect of model calibration on seismic behaviour of a historical mosque
    Demir, A; Nohutcu, H; Ercan, E; Hokelekli, E; Altintas, G
    The objective of the study is to investigate the effects of model calibration on seismic behaviour of a historical mosque which is one of the most significant Ottomon structures. Seismic analyses of calibrated and noncalibrated numeric models were carried out by using acceleration records of Kocaeli earthquake in 1999. In numerical analysis, existing crack zones on real structure was investigated in detail. As a result of analyses, maximum stresses and displacements of calibrated and noncalibrated numerical models were compared each other. Consequently, seismic behaviour and damage state of historical masonry Hafsa Sultan mosque was determined as more realistic in the event of a severe earthquake.
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    Proposal for a novel technological damper system (TDS) for the retrofit of reinforced concrete frame structures
    Suk, R; Demir, A; Altintas, G; Altiok, TY
    Considering the severe earthquakes that have occurred worldwide, it has been observed that even buildings which are constructed in compliance with the seismic regulations, have suffered heavy damage or collapsed. Similarly, in earthquake on February 6, 2023, in Kahramanmaras,, Turkey, many old and new structures were damaged or even collapsed. This situation has compelled researchers to focus on the most current and technological applications for earthquake protection. In the latest research topics, there has been a growing emphasis on the impact of new technological systems targeted at improving the seismic performance of structures constructed with reinforced concrete systems, given the notable occurrence of collapsed buildings with such structural systems in this earthquake. In this study, the developed Technological Damper System (TDS) was employed to enhance the earthquake performance of reinforced concrete structures. TDS is a versatile and adjustable frictiontype damper. For this reason, two identical 1/2 scaled reinforced concrete frames were fabricated for the study, one representing a traditional structure as a reference frame (REF), and the other a frame with the TDS device. After applying quasi -static cyclic tests to the frames, the force -displacement, and force -rotation relationships, as well as energy dissipation capacities were determined for both frames. In addition, the damage conditions of the columns during the tests were assessed based on the plastic rotation limit conditions outlined in Turkish Building Earthquake Code (TBEC 2018). As a result, the frame retrofitted with the TDS device exhibited a significant increase in horizontal load -carrying capacity, ranging from 42.04% to 75%, when compared to the REF. Additionally, it was observed that energy consumption increased between 85% and 220%. The REF reached significant damage levels at 1% and 2% story drift ratios and an advanced damage state at about 3%, in accordance with TBEC 2018. Finally, the behavior of the frame retrofitted with TDS considerably improved according to REF and it reached a significant damage state at 3% story drift ratio.
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    Advancing earthquake resistance: Hybrid retrofitting of RC frames with FRP and TDS
    Suk, R; Demir, A; Altiok, TY; Altintas, G
    Traditional and technological retrofitting methods have been proposed over time to enhance earthquake resistance in structures. Friction-type dampers have attracted considerable interest from both researchers and the construction industry because of their versatility in retrofitting, quick installation, and non-destructive characteristics. Moreover, the integration of damping systems with various retrofitting elements and the resulting impact of these hybrid systems on building performance have consistently been subjects of interest. This study involved the construction of three identical half-scale reinforced concrete (R.C) frames. One frame served as the reference (REF), the second was wrapped with Fiber Reinforced Polymers (FRP) material (REF-FRP), and the third was retrofitted using both FRP wrapping and the developed Technological Damper System (TDS-FRP). Quasi-static cyclic experiments were performed on the three structural frames, providing force-displacement and force-rotation relationships. The acquired data were then used to assess the damage states of the frames according to the Turkish Building Earthquake Code (TBEC 2018), and energy consumption rates were determined. Moreover, Finite Element Method (FEM) analyses were performed on REF, REF-FRP, and TDS FRP frames to derive force-displacement relationships, which were subsequently compared with experimental findings. The experiment results indicate that the horizontal load-carrying capacity of the TDS-FRP frame increased by 76 % to 122 % compared to the REF frame, while the REF-FRP frame showed a maximum increase of 14 %. Additionally, the cumulative energy consumption capacity of the REF-FRP frame increased by a maximum of 42 % compared to the REF frame, and the TDS-FRP increased between 51 % and 156 %. At a 1 % drift ratio, shear cracks at the beam ends and column-beam intersections of the REF frame were observed to be significantly reduced in the REF-FRP frame and eliminated in the TDS-FRP frame. Additionally, upon reaching a 3 % drift ratio, it was observed that the TDS-FRP frame remained within acceptable limits as per TBEC 2018, whereas the REF and REF-FRP frames exceeded the advanced damage limit. Additionally, it has been observed that the results obtained from FEM analyses coincide with the experimental results. In this context, the TDS-FRP hybrid application can be considered as an effective and alternative solution for the R.C buildings.
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    Effect of 3D isotropic resolutions of sequenced images on natural vibration properties of trabecular bone
    Altintas, G; Ergut, A; Goldepe, AB
    The voxel based finite element (FE) method used to obtain primary data for non-invasive imaging techniques has emerged as a major focus of interest in several disciplines such as medicine, mechanics and material engineering for solving micro and nano-scale problems. Owing to the fact that, voxel based FE models are directly affected by parameters of imaging techniques such as Computed Tomography (CT) and Magnetic Resonance Imaging (MRI), the consequences of these effects on the natural vibration analysis of structures having complex geometry in micro scale, are investigated in this study. In this context, voxel based FE models are obtained using Micro-CT imaging data that has three different resolutions of vertebral trabecular bone tissue. Furthermore, resolutions of image data sets are artificially increased and equalized for evaluating voxel based FE models that are free from FE size effects. Natural vibration characteristics of voxel based FE models are investigated not only numerically but also including associated mode shapes. Unpredictable vibrational behavior for various voxel sizes, is, thus, revealed. Element size effects of voxel based FE models are considerably different from the effects on structural components with regular prismatic shapes. Obtained results show that, investigated parameters have a crucial influence on the natural vibration behavior of trabecular bone tissue which is selected as an example of complex geometries. Modal behaviors that are effective in micro local regions, but less in the whole body, where there are possibilities for working with approximate geometry without considering the micro structure have been observed. Moreover, the results are new from a theoretical point of view, and they also represent the importance of quality in imaging data, which, in practical applications must be taken into consideration, (C) 2013 Sharif University of Technology. Production and hosting by Elsevier B.V. All rights reserved.
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    EFFECT OF MATERIAL PROPERTIES ON VIBRATIONS OF NONSYMMETRICAL AXIALLY LOADED THIN-WALLED EULER-BERNOULLI BEAMS
    Altintas, G
    This paper focuses on effect of material properties on free vibration characteristics of nonsymmetrical axially loaded thin-walled Euler-Bernoulli beams. Many scientists studied the vibration of coupled systems but adequate consideration has not been given to detailed and parametric studies on the effect of material properties on vibration properties. The current findings of this study show that there have been quite different effects of material properties on the systems without symmetry axis than on the systems with symmetry axis. Material properties are properties affecting the free vibration behavior. Moreover, effect of axial load on natural frequencies was also investigated in this study. Convergence work revealed that the current mathematical model simulated the problem with excellent accuracy.
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    Natural vibration behaviors of heterogeneous porous materials in micro scale
    Altintas, G
    Natural porous materials have a large area of usage, from micromechanical designs to medical applications, due to their geometrical forms and chemical properties. Mechanical properties of porous materials are usually performed after several homogenized applications because of their complex geometric forms. It is known that usage of homogenized details of microarchitecture in macro structural analyses provides savings in terms time and computational effort. However, realistic analyses are performed by including microarchitectural details in models, due to advancing technology and reverse engineering techniques. In this study, modal properties of a porous structure are investigated by taking microstructural details into consideration according to the voxel-based finite element model, and significant modal behavior properties, which cannot be observed in routine modal analyses, are determined. In addition, analyses using the homogenized model of porous structure are performed, and it is seen that the obtained results are not only different numerically but also have different modal behavior from the porous model. Special modes, defined as local modes, which are not seen in homogenized models, are observed in models by using the voxel-based finite element method and analyses are comprehensively performed.
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    Behavior of multidirectional friction dampers
    Suk, R; Altintas, G
    Earthquakes are catastrophic events causing loss of lives, injuries, and extensive losses in properties. Majority of the life and property losses of earthquakes are dependent on the incapabilities of the building stock to resist earthquakes. Although unsuitable design, analyses, and production techniques play a major role as the main reasons for the poor performance of buildings against earthquakes, buildings constructed in accordance with building codes also suffer from the devastating impact of earthquakes. In this context, the lack of proper management and adequate damping of the energy caused by earthquakes is a major cause of structural damage in earthquakes. The efficiency of conventional basic elements in structures with energy damping is very limited and may not be sufficient for the damping of a large amount of earthquake-induced energy. Thanks to the rapid advances in technology and associated engineering techniques, numerous new products, and production and calculation techniques are underway to mitigate the devastating effects of earthquakes on buildings. In this study, it was aimed to theoretically and experimentally investigate the performance of a versatile friction-type seismic damper that eliminates earthquake energy. The damper is designed using a spherical surface friction joint to respond to all loads regardless of the loading direction. The damper can be easily adjusted to the desired capacity by means of bolt tensioning elements. Experiments have been carried out for various shear loads and damping parameters. Furthermore, numerical analysis of the model was carried out by use of the finite element method. The results of this study revealed that the shear load capacity of the device did not change at different frequencies. Analyzing the effect of the equipment on a structure, it was understood that it reduces roof displacement and periods of the structure. The analysis revealed that the damper significantly improved the earthquake performance of the structure.
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    Artificial neural network analysis of infilled planar frames
    Bagci, M; Altintas, G
    In this study, infilled planar frames have been analysed using an artificial neural network. The data used were provided by a finite element model (FEM) in which non-linearity of materials and the structural interface were taken into account under increasing lateral load. For the skeleton frame, the panel was modelled by a two-noded frame element and a four-noded isoparametric element respectively. The Von Mises failure criterion was used for the infill wall. Values obtained from FEM were used in training the network established. An artificial neural network (ANN) architecture was chosen in which a multilayer, feed-forward and back-propagation algorithm was used. The controls conducted in the test phase showed that training was satisfactory. The study has proved that the ANN could be successfully used for analysis of infilled planar frames.
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    Effect of micro-ct slice intensity on natural vibration behavior of cancellous bone models based on reverse engineering techniques
    Altintas, G; Erdem, RT
    Identification of natural vibration properties of bone structure in macro and micro levels is important because of increasingly vibration inclusion of current methods in recognition and treatment of bone diseases. In this scope, results which are obtained from non-destructive reverse engineering approaches are crucial especially in living tissue studies. The results may differ from each other because of the multi-staged methods and differences in constituted models due to the redundancy of options. In this paper, effect of differences due to the slice intensity on natural vibration analysis results by using constant resolution Micro-Computational tomography (Micro-CT) data of L3 vertebrae in voxel based finite element models are investigated. In addition to this investigation, another approach which is different from classic approach where non-cubic voxel based finite elements can be generated and only about the effect of slice thickness and cubic voxels is improved. The results obtained from the study show that solutions of proposed model are more conservative than the results of voxel based finite element (V-FEM) and these results are purified from the effects of element aspect ratio
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    Effect of Mass Based Imperfections on Behavior of Linear Vibrating Plates Near Degenerate Modes
    Altintas, G
    The principal objective in this study is to investigate the special behaviors of linear vibrating plates with special parameters near degenerate modes. Near degenerate modes, plates may lack geometric stability because they exhibit a qualitatively different behavior under an arbitrarily small change in special parameters. The special parameters considered in this study are the location and quantity of an additional mass, which have an effect on removing the system symmetry. To this end, analyses are carried out for three plate systems with each having different boundary conditions. The effect of the location and the quantity of additional mass on degenerate modes are significantly different from those of other mode types. Clearly, these effects are of theoretical importance and should be taken into consideration in practical applications. With the view of the deterministic approach, it can be stated that the study determines some unspecified parts of forced vibration behaviors of mass loaded plates.
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    Determination of the steady-state response of viscoelastically supported rectangular orthotropic mass loaded plates by an energy-based finite difference method
    Altintas, G; Bagci, M
    A method based on a variational procedure in conjunction with a finite difference method is used to examine the free vibration characteristics and steady-state response to a sinusoidally varying force applied orthotropic elastic rectangular plate carrying masses, Using the energy-based finite difference method, the problem reduced to the solution of a system of algebraic equations. Due to the significance of the fundamental natural frequency of the plate, its variation is investigated with respect to the mechanical properties of the plate material, the translational spring coefficient of the supports, the mass distribution, the mass locations and the quantity of mass. The steady-state response of the viscoelastically supported plates was also investigated numerically for the damping coefficient of the supports and the force distribution in addition to the characteristics of the plate system. Many new results are presented and the validity of the present approach is demonstrated by comparing the results with other solutions based on the Kirchhoff-Love plate theory.
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    Effect of application area distributions of mass, force, and support on free and forced vibration behavior of viscoelastically supported plates
    Ergut, A; Altintas, G
    In real world systems, implementation areas of mass, force and supports have considerable quantity, unlike homogenized areas in numerical solutions. In this study, the effect of area distributions of non-structural additional mass, external force and supports on linear vibrating plate systems are analyzed. There are two key questions addressed in this study. First, the majority of the previous works cover the range of a fundamental mode, and do not answer the question of how results of the fundamental analyses can be generalized for the higher modes based on investigated parameters, particularly for the case of forced vibration. Second, how structural safety is threatened by focused idealizations has not been adequately studied before. Numerical results of this study are obtained by using the variational difference method (VDM), which is based on a variational procedure in conjunction with traditional finite difference method. VDM is applied for determining free vibration characteristics and steady-state responses to a sinusoidally varying force applied to a viscoelastically supported plate. Because the results of the investigated systems showed that the vibration characteristics had a high sensitivity to analyzed parameters (the amount of area distributions of non-structural additional mass, external force and supports), the idealization process of areas must be attentively employed. The study provides insight not only for a fundamental mode but also for higher modes. Obviously the results of this deterministic study have showed that theoretical importance shall be considered in practical applications.
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    Shear strength estimation of plastic clays with statistical and neural approaches
    Goktepe, AB; Altun, S; Altintas, G; Tan, O
    Because shear strength parameters highly influence the bearing capacity of soils, several researchers have carried out large number of experimental and theoretical studies aimed at understanding soil strength behaviors. Within this context, the determination of correlations between soil index properties and shear strength parameters for specific soil types is possible. The aim of this study is to observe the performance of statistical and artificial neural network (ANN)-based methods on establishing correlations between index properties and shear strength parameters of normally consolidated plastic clays. To collect modeling data, consolidated-undrained triaxial tests were performed oil normally consolidated plastic clays obtained from the same region. Additionally, detailed statistical analyses were conducted on the test data. Results indicate that the ANN-based model is superior in determining the relationships between index properties and shear strength parameters. However, in order to get appropriate outcomes, specific care must be dedicated when applying ANN-based correlation models. (C) 2007 Elsevier Ltd. All rights reserved.
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    Investigation of a historic masonry structure by numerical and operational modal analyses
    Nohutcu, H; Demir, A; Ercan, E; Hokelekli, E; Altintas, G
    This paper presents the results of model calibration conducted on a historical mosque called Hafsa Sultan in Manisa, Turkey. The finite element model of the mosque was calibrated by the use of the results obtained from ambient vibration tests of the structure. In order to develop a solid model of the structure, the dimensions of the structure, defects such as cracks and material degradations in the structure, and the materials used in different parts were identified. For the evaluation of the material properties of the structure, nondestructive and destructive testing methods were used. The numerical and experimental modal parameters of the structure were obtained by finite element method (FEM) and Operational Modal Analysis (OMA), respectively. The natural frequencies and corresponding mode shapes were obtained from both FEM and OMA and compared with each other. While a good compatibility was achieved between mode shapes, some differences between natural frequencies occurred. It was thought that the differences resulted from variations in the Young's modulus of masonry, cracks in elements or boundary conditions. Therefore, the finite element model was calibrated by changing material parameters. Finally, a more realistic numerical model of the mosque was put forward and the results were discussed in detail. Copyright (c) 2015 John Wiley & Sons, Ltd.