Browsing by Author "Akis, Tolga"

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Citation Count: 0
Amendments on "application of Mises Yield Criterion To Rotating Solid Disk Problem" by N. Aleksandrova [int. J. Eng. Sci. 51 (2012) 333-337]
(Pergamon-elsevier Science Ltd, 2012) Eraslan, Ahmet N.; Akış, Tolga; Akis, Tolga; Civil Engineering
[No Abstract Available]
Citation Count: 10
Automated Selection of Optimal Material for Pressurized Multi-Layer Composite Tubes Based on an Evolutionary Approach
(Springer London Ltd, 2018) Azad, Saeid Kazemzadeh; Akış, Tolga; Akis, Tolga; Azad, Saeıd Kazemzadeh; Civil Engineering; Department of Civil Engineering
Decision making on the configuration of material layers as well as thickness of each layer in composite assemblies has long been recognized as an optimization problem. Today, on the one hand, abundance of industrial alloys with different material properties and costs facilitates fabrication of more economical or light weight assemblies. On the other hand, in the design stage, availability of different alternative materials apparently increases the complexity of the design optimization problem and arises the need for efficient optimization techniques. In the present study, the well-known big bang-big crunch optimization algorithm is reformulated for optimum design of internally pressurized tightly fitted multi-layer composite tubes with axially constrained ends. An automated material selection and thickness optimization approach is employed for both weight and cost minimization of one-, two-, and three-layer tubes, and the obtained results are compared. The numerical results indicate the efficiency of the proposed approach in practical optimum design of multi-layer composite tubes under internal pressure and quantify the optimality of different composite assemblies compared to one-layer tubes.
Citation Count: 6
Damage Observation of Reinforced Concrete Buildings After 2020 Sivrice (elazig) Earthquake, Turkey
(Asce-amer Soc Civil Engineers, 2021) Cenan Mertol, Halit; Tunç, Gökhan; Tunc, Gokhan; Akış, Tolga; Akis, Tolga; Civil Engineering
An earthquake with a magnitude of M-w = 6.8 and a depth of approximately 8.06 km occurred on January 24, 2020, with the epicenter near the town of Sivrice in Elazig province, Turkey. The earthquake resulted in numerous collapses and severe structural damage to the buildings located in the downtowns and villages of Elazig and Malatya provinces. A technical team investigated the earthquake-affected areas, and this study presents their observations and findings with respect to reinforced concrete buildings. It was concluded that almost all of the collapsed or severely-damaged reinforced concrete buildings were constructed between 1975 and 1998. The design and construction of these buildings did not comply with the specifications of the Turkish earthquake codes valid at that time. The failures and severe damage of the buildings are associated with poor material quality, inadequate reinforcement configuration, and framing problems related to their lateral load-carrying systems. Therefore, it is recommended that buildings constructed between 1975 and 1998 in the region be structurally re-evaluated to avoid additional loss of life and property in future earthquakes. (C) 2021 American Society of Civil Engineers.
Citation Count: 0
Discussion of the Article "approximate Solution of Limit Angular Speed for Externally Loaded Rotating Solid Disk" by S. Bhowmick, D. Misra and Nk Saha [int J Mech Sci 2008;50:163-74]
(Pergamon-elsevier Science Ltd, 2008) Eraslan, Ahmet N.; Akış, Tolga; Akis, Tolga; Civil Engineering
[No Abstract Available]
Citation Count: 42
Elastoplastic Analysis of Functionally Graded Spherical Pressure Vessels
(Elsevier Science Bv, 2009) Akis, Tolga; Akış, Tolga; Civil Engineering
Purely elastic, partially plastic and fully plastic stress states of internally pressurized functionally graded spherical pressure vessels are investigated analytically in the framework of small deformation theory. The modulus of elasticity and the uniaxial yield limit of the spherical pressure vessel material are assumed to vary nonlinearly in the radial direction. The plastic model is based on Tresca's yield criterion and ideal plastic material behavior. It is shown that, unlike in the case of a homogeneous spherical pressure vessel, different modes of plasticization may take place due to the radial variation of the functionally grading parameters. (C) 2009 Elsevier B.V. All rights reserved.
Citation Count: 7
Evaluation of Masonry Buildings and Mosques After Sivrice Earthquake
(Croatian Soc Civil Engineers-hsgi, 2021) Mertol, Halit Cenan; Mertol, Halit Cenan; Tunc, Gokhan; Tunç, Gökhan; Akis, Tolga; Akış, Tolga; Civil Engineering
The evaluation of masonry and mosque type structures after the Sivrice Earthquake is presented in this study. Stone masonry buildings exhibited damage such as vertical cracks and splitting at corners, wedge shaped corner failures, diagonal cracking on walls, out-of-plane splitting of walls, and separation of walls from flooring/roofing systems. On the other hand, the separation of flags and caps of minarets was a common example of damage in mosques. Future earthquake damage can be prevented by following design codes and providing adequate supervision for new structures, while strengthening measures are recommended for the existing buildings.
Citation Count: 29
Exact Solution of Rotating Fgm Shaft Problem in the Elastoplastic State of Stress
(Springer, 2007) Akis, Tolga; Akış, Tolga; Eraslan, Ahmet N.; Civil Engineering
Plane strain analytical solutions to estimate purely elastic, partially plastic and fully plastic deformation behavior of rotating functionally graded (FGM) hollow shafts are presented. The modulus of elasticity of the shaft material is assumed to vary nonlinearly in the radial direction. Tresca's yield criterion and its associated flow rule are used to formulate three different plastic regions for an ideal plastic material. By considerina different material compositions as well as a wide range of bore radii, it is demonstrated in this article that both the elastic and the elastoplastic responses of a rotating FGM hollow shaft are affected significantly by the material nonhomogeneity.
Citation Count: 3
Influence of Cement Replacement by Calcinated Kaolinitic and Montmorillonite Clays on the Properties of Mortars
(Springer Heidelberg, 2023) Akış, Tolga; Akis, Tolga; Civil Engineering
This study aims to investigate the decomposition and pozzolanic reactivity of two different clays (kaolinitic and montmorillonite) from different origins and to determine their effects after calcination on the properties of cement mortars when used to replace Portland cement partially. Mineralogical and chemical compositions of the clay samples were determined using XRD (X-ray Diffractometer) and XRF (X-ray Fluorescence) tests, respectively. TG-DTA (Thermogravimetry-Differential Thermal Analyses) was used to determine the temperature profiles and the burning temperatures of the clays. The density and fineness of the burnt clays were also determined. In order to investigate the optimum material properties, different burning temperatures and replacement levels were considered. It was found that for all temperatures, the two burnt clays possess good pozzolanic activity. The highest compressive strength and lowest water absorption capacity were achieved when the clay determined as kaolinitic was burned at 700 & DEG;C and with 10% replacement level. While for the clay determined as montmorillonite, the optimum properties were obtained at 700 & DEG;C with a 20% substitution level. Kaolinite had better pozzolanic reactivity than montmorillonite, achieving higher strength performance with lower water absorption when partially replaced with cement. Moreover, it had compressive strength values even higher than plain cement.
Citation Count: 11
Investigation of Rc Buildings After 6 February 2023, Kahramanmaras, Turkiye Earthquakes
(Mdpi, 2023) Mertol, Halit Cenan; Mertol, Halit Cenan; Tunc, Gokhan; Tunç, Gökhan; Akis, Tolga; Akış, Tolga; Kantekin, Yunus; Aydin, Ishak Can; Kantekin, Yunus; Aydın, İshak Can; Civil Engineering; Department of Civil Engineering
Two major earthquakes struck Pazarcik and Elbistan, towns in Kahramanmaras, Turkiye, on 6 February 2023, approximately 9 h apart. The first earthquake, recorded at 04:17 local time, had a M-w = 7.7, with a focal depth of 8.6 km. At 13:24 local time, a second earthquake occurred with M-w = 7.6 at a focal depth of 7 km, approximately 90 km north of the first one. A total of 11 provinces were severely affected by these earthquakes. As of 15 April 2023, they caused close to 51,000 deaths and almost 215,000 completely destroyed/severely damaged buildings. At some locations, the largest horizontal peak ground acceleration (PGA) values of the first and second earthquakes exceeded the code-generated PGAs by almost 3 and 1.75 times, respectively. A technical team visited these areas within 15 h of the first earthquake. The purpose of this article is to present their observations, findings, and the characteristics of the two earthquakes, with comprehensive site survey results supported by photographs. This study concludes that most of the collapsed and severely/moderately damaged buildings in the region were built between 1975 and 2000, when site inspections were rare or non-existent. In addition to the high PGAs recorded in these earthquakes, it was verified that the design and construction of these buildings did not fully comply with the earthquake codes valid at the time. The collapsed buildings and their damage patterns confirm inadequate development length, violation of bending stirrup ends at 135 & DEG;, deficiencies in construction materials and reinforcement configuration, noncompliance with confinement zones, violation of the strong beam-stronger column analogy, and issues related to building inspection. Based on the extent of the damage, it is strongly recommended that the structural performance inspection of all other buildings located near major fault lines, specifically those constructed between 1975 and 2000, should be completed. Since these earthquakes generated much higher PGAs, which is believed to be one of the main reasons for the extensive damage, a re-evaluation of all other PGAs along major fault lines is also recommended.
Citation Count: 0
Lessons Learned From Four Recent Turkish Earthquakes: Sivrice-Elazığ, Aegean Sea, and Dual Kahramanmaraş
(Springer, 2024) Tunc, Goekhan; Mertol, Halit Cenan; Mertol, Halit Cenan; Akış, Tolga; Akis, Tolga; Civil Engineering
T & uuml;rkiye is located in an earthquake-prone region where almost all of its population resides in risky areas. In the past 100 years, there has been a strong earthquake every two years and a major one every 3 years. This study investigates the impact of four recent earthquakes, that occurred between 2020 and 2023, on reinforced concrete (RC) buildings. The first, Sivrice-Elaz & imath;& gbreve;, struck the eastern part of T & uuml;rkiye on January 24, 2020, with a moment magnitude of Mw = 6.8. The second, the Aegean Sea, hit the western part of the country on October 30, 2020, with an Mw of 6.6. The third and fourth are the February 6, 2023 dual Kahramanmara & scedil; earthquakes with Mws of 7.7 and 7.6, which struck the eastern part of T & uuml;rkiye approximately 9 h apart. Immediately following these earthquakes, a technical team investigated each of the damaged areas. This study summarizes their findings on RC buildings. It was discovered that the majority of the collapsed or severely damaged RC buildings were constructed before 2000. The main reasons for this included technological limitations, specifically on producing high-quality concrete, as well as a lack of public policies and enforced laws in the construction sector to maintain an acceptable international standard. Furthermore, the damage patterns of buildings from these four earthquakes indicated poor workmanship, low material quality, improper structural framing, a common appearance of soft and weak stories, the inadequate use of shear walls, and defective reinforcement configuration. The significance of soil studies and the enforcement of building inspections are also discussed, along with the earthquake codes. The study concludes that the maximum peak ground accelerations from the dual Kahramanmara & scedil; earthquakes were almost triple the code-prescribed values. Therefore, it is recommended that the current mapped spectral acceleration values be revised and that buildings constructed before 2000 should be prioritized while determining their structural performances.
Citation Count: 1
Metaheuristic Optimization of Rotating Multilayer Composite Tubes Under Internal Heating and Pressure
(Springer, 2022) Akış, Tolga; Akis, Tolga; Azad, Saeıd Kazemzadeh; Civil Engineering; Department of Civil Engineering
Although analysis/design of multilayer assemblies has been always an active field of research, works on the optimal design of rotating multilayer composite tubes are very limited. This paper addresses the design optimization of rotating multilayer composite tubes under internal heating and pressure. For determining the structural responses, analytical solutions are provided based on different boundary conditions. The automated selection of optimal material as well as thickness optimization of pressurized multilayer assemblies is carried out under different angular speed and internal heating conditions using a metaheuristic algorithm. The corresponding optimum design for each angular speed as well as internal heating condition is sought, and the numerical results are discussed. The study provides general guidelines for conceptual design of rotating multilayer composite tubes subjected to internal heating and pressure.
Citation Count: 2
Modeling of Asymmetric Shear Wall-Frame Building Structures
(Architectural inst Japan, 2009) Akış, Tolga; Tokdemir, Turgut; Yilmaz, Cetin; Civil Engineering
Based on the conventional wide column analogy, two different three-dimensional shear wall models for open and closed sections are proposed. These approximate models are verified in comparison to not only the results available in the literature but also the ones obtained by Using models containing shell elements. With the help of these new models five different groups of shear wall-frame structures with different floor plans and different heights are analyzed. The first three natural vibration periods are determined and time history analyses are performed. The results Of these computations are observed to be in good agreement with those obtained by detailed models containing shell elements.
Citation Count: 2
On the Yielding of Two-Layer Composite Spherical Pressure Vessels
(Gazi Univ, 2017) Akış, Tolga; Civil Engineering
The yielding of two-layer composite spherical pressure vessels under either internal or external pressure is investigated analytically in the framework of small deformations and von Mises yield criterion. It is shown for both pressure cases that depending on the material properties and sphere dimensions, different modes of plasticization may take place. Unlike the deformation behavior of a single layer spherical pressure vessel, yielding may commence at the inner layer or at the outer layer or simultaneously at both layers of the assembly.
Citation Count: 48
Plane Strain Analytical Solutions for a Functionally Graded Elastic-Plastic Pressurized Tube
(Elsevier Sci Ltd, 2006) Eraslan, Ahmet N.; Akış, Tolga; Akis, Tolga; Civil Engineering
Plane strain analytical solutions to functionally graded elastic and elastic-plastic pressurized tube problems are obtained in the framework of small deformation theory. The modulus of elasticity and the uniaxial yield limit of the tube material are assumed to vary radially according to two parametric parabolic forms. The analytical plastic model is based on Tresca's yield criterion, its associated flow rule and ideally plastic material behaviour. Elastic, partially plastic and fully plastic stress states are investigated. It is shown that the elastoplastic response of the functionally graded pressurized tube is affected significantly by the material nonhomogeneity. Different modes of plasticization may take place unlike the homogeneous case. It is also shown mathematically that the nonhomogeneous elastoplastic solution presented here reduces to that of a homogeneous one by appropriate choice of the material parameters. (c) 2006 Elsevier Ltd. All rights reserved.
Citation Count: 50
Pull-Out Behavior of Prestressing Strands in Steel Fiber Reinforced Concrete
(Elsevier Sci Ltd, 2012) Baran, Eray; Akış, Tolga; Akis, Tolga; Yesilmen, Seda; Baran, Eray; Yeşilmen, Seda; Civil Engineering; Department of Civil Engineering
This paper presents the results of an experimental study investigating the effects of steel fibers on the mechanical properties of concrete and the enhancement of bond strength of prestressing strands in steel fiber reinforced concrete (SFRC). The first part of the experimental program consisted of compression, tension and flexural tests on SFRC. Two types of steel fibers with 30 mm and 60 mm fiber lengths were used with five different fiber contents. The second part of the study consisted of simple pull-out tests on 12.7 mm and 15.2 mm diameter seven-wire untensioned prestressing strands embedded in concrete blocks. The pull-out tests were conducted with two different fiber lengths and five different fiber contents for each strand diameter. The steel fibers were observed to improve the pull-out resistance of strands by controlling the crack growth inside concrete blocks. (C) 2011 Elsevier Ltd. All rights reserved.
Citation Count: 2
A Site Survey of Damaged Rc Buildings in Izmir After the Aegean Sea Earthquake on October 30, 2020
(Croatian Soc Civil Engineers-hsgi, 2023) Mertol, Halit Cenan; Mertol, Halit Cenan; Tunc, Gokhan; Tunç, Gökhan; Akis, Tolga; Akış, Tolga; Civil Engineering
An earthquake with a magnitude of Mw = 6.6 and a depth of approximately 16.5 km occurred on 30 October 2020 off the cost of Samos, a Greek island 35 km southwest of Seferihisar, a town in Izmir. The earthquake caused several collapses and severe structural damage in approximately 6,000 buildings, specifically in the Bayrakli District in Izmir Bay. This paper presents the observations and findings of a technical team that visited the earthquake -affected areas immediately after the earthquake. Eleven partially or fully collapsed and several severely damaged reinforced concrete buildings were investigated. Based on the site investigations, we observed that almost all of the collapsed or severely damaged reinforced concrete buildings in the region were built between 1975 and 2000. Site observations also confirmed that the construction of these collapsed or damaged buildings did not conform to the requirements outlined in the Turkish Earthquake Codes used at the time. The failures and severe damage to buildings in earthquake-affected areas are primarily related to inadequate reinforcement configuration, poor material quality, the absence of geotechnical studies, and framing problems related to their lateral load-carrying systems. Therefore, it is recommended that all the buildings located in and around Izmir Bay, particularly those built between 1975 and 2000, be structurally evaluated to prevent any further loss of life and property during future earthquakes.
Citation Count: 16
The Stress Response of Partially Plastic Rotating Fgm Hollow Shafts: Analytical Treatment for Axially Constrained Ends
(Taylor & Francis inc, 2006) Eraslan, Ahmet N.; Akış, Tolga; Akis, Tolga; Civil Engineering
analytical solutions to estimating the elastoplastic response of rotating functionally graded (FGM) hollow shafts with fixed ends are presented. The modulus of elasticity, as well as the uniaxial yield limit of the shaft material, are assumed to vary nonlinearly in the radial direction. The plastic model is based on Tresca's yield criterion, its associated flow rule, and ideal plastic material behaviour. Elastic, partially plastic, fully plastic, and residual stress states are investigated. It is shown that the elastoplastic stress response of a rotating FGM hollow shaft is affected significantly by the nonhomogeneity of the material. Unlike the case of a homogeneous hollow shaft, plastic deformation may commence at the inner surface, at the outer surface, or simultaneously at both surfaces. Accordingly, each case requires different mathematical treatment to arrive at its partially plastic solution. It is also shown that, by taking a numerical limit, the complete FGM solution presented herein converge to the solution of a homogeneous rotating shaft.
Citation Count: 2
Structural Design Optimization of Multi-Layer Spherical Pressure Vessels: a Metaheuristic Approach
(Springer, 2019) Akis, Tolga; Akış, Tolga; Azad, Saeid Kazemzadeh; Azad, Saeıd Kazemzadeh; Civil Engineering; Department of Civil Engineering
This study addresses the optimum design problem of multi-layer spherical pressure vessels based on von Mises yield criterion. In order to compute the structural responses under internal pressure, analytical solutions for one-, two-, and three-layer spherical pressure vessels are provided. A population-based metaheuristic algorithm is reformulated for optimum material selection as well as thickness optimization of multi-layer spherical pressure vessels. Furthermore, in order to enhance the computational efficiency of the optimization algorithm, upper bound strategy is also integrated with the algorithm for reducing the total number of structural response evaluations during the optimization iterations. The performance of the algorithm is investigated through weight and cost minimization of one-, two- and three-layer spherical pressure vessels and the results are presented in detail. The obtained numerical results, based on different internal pressures as well as vessel sizes, indicate the usefulness and efficiency of the employed methodology in optimum design of multi-layer spherical pressure vessels.
Citation Count: 3
A Study of Shrink-Fitting for Optimal Design of Multi-Layer Composite Tubes Subjected To Internal and External Pressure
(Springer, 2019) Azad, Saeid Kazemzadeh; Akış, Tolga; Akis, Tolga; Azad, Saeıd Kazemzadeh; Civil Engineering; Department of Civil Engineering
This paper addresses the effect of shrink-fitting on the optimal design of pressurized multi-layer composite tubes. Analytical solutions for structural response calculations are provided for axially constrained two- and three-layer shrink-fitted tubes under both internal and external pressure. A recently developed numerical evolutionary optimization algorithm is employed for weight and cost minimization of these assemblies. In order to investigate the effect of shrink-fitting, first, optimal material selection and thickness optimization of tightly fitted tubes, under either internal or both internal and external pressure, are accomplished without shrink-fitting. Next, under the same loading and boundary conditions the assemblies are optimized where shrink-fitting parameters are taken into account for weight and cost minimization. The numerical results obtained for multi-layer composite tubes with and without shrink-fitting indicate that more economical or lightweight assemblies can be obtained if shrink-fitting parameters are treated as additional design variables of the optimization problem. Furthermore, it is observed that considering the shrink-fitting parameters for optimal design becomes more advantageous in the test cases with a higher ratio of internal pressure to external pressure.
Citation Count: 2
Yielding of Radially Pressurized Functionally Graded Long Tubes Based on Von Mises Criterion
(Gazi Univ, 2015) Akış, Tolga; Eren, Omur; Civil Engineering
The elastic behaviour of functionally graded long tubes with axially constrained ends subject to either internal or external pressure is investigated analytically. The modulus of elasticity and the yield limit of the tube material are assumed to vary radially in nonlinear forms. Making use of von Mises yield criterion, the effect of grading parameters on the yielding behavior of the tube is investigated and it is observed that for both pressure cases, the plastic flow may commence at the inner surface, at the outer surface or simultaneously at both surfaces depending on the functionally grading parameters.