Browsing by Author "Aktaş, Engin"

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Application of artificial neural networks to structural reliability problems
(01. Izmir Institute of Technology, 2023-06) Köroğlu, Fahri Baran; Aktaş, Engin; Maguire, Marc
The contemporary approach in structural engineering indirectly addresses uncertainties arising from load and resistance parameters by using safety factors. To consider these uncertainties in structural engineering, it is necessary to incorporate their statistical properties into the analysis and design process. However, this approach requires the calculation of challenging multi-fold probability integrals. Approximate methods known as FORM and SORM have been developed as an alternative to calculating those integrals. Unfortunately, these methods might have accuracy and convergence problems depending on the problem at hand. Simulation-based structural reliability methods have been developed to overcome the problems associated with approximate methods. The main problem with these methods is that they are often computationally expensive when along with finite element analysis, or it is hard to implement them when a more specific method is chosen to reduce computational costs. In this study, artificial neural networks have been applied to structural reliability problems to obtain accurate probability estimates with low computational cost. A special type of learning algorithm called Bayesian Regularization was used in the training of artificial neural networks. Additionally, details of the application of artificial neural networks to structural reliability problems are provided. At the end of the study, the advantages and disadvantages of applying artificial neural networks to structural reliability problems are presented and compared with other known structural reliability methods. Additionally, a new convergence criterion and an adaptive algorithm have been developed. It was observed that applying artificial neural networks to structural reliability problems provides both efficient and accurate probability estimates.
A benefit/cost analysis for the seismic rehabilitation on existing reinforced concrete buildings in Izmir
(Izmir Institute of Technology, 2005) Boylu, Mert; Aktaş, Engin
The Aegean is one of the most seismically active regions of Turkey. The Eqarthquake disasters which occurred in Turkey recently, obligate the seismic rehabilitation of existing buildings and the new construction projects produced to require well-detailed seismic strengthing. On the other hand, before starting an expensive project such as seismic rehabilitation, a benefit/cost analysis is needed to determine whether the proposed project is economically feasible. In this study, a benefit/cost analysis model for the seismic rehabilitation of existing reinforced concrete buildings in Izmir is presented. To express the seismicity of Izmir City, between 1900-2003, the magnitude of Ms 4.9 earthquakes which occurred in Izmir and its vicinity are used as data. The Poisson Model is used to calculate the probability of occurrence. For the seismic risk of Izmir which includes damage to buildings and loss, various damage estimation methods are described. As a generalization of all of the existing reinforced concrete buildings in Izmir Region, two existing reinforced concrete buildings with total building area of 735 square meters and 716 square meters, respectively, which have different social functions, are chosen to estimate whether the seismic rehabilitations of the investigated buildings are economically justified. As a result of the analysis, benefit/cost ratios of both structures are found greater than one which means both rehabilitation projects are economically feasible.
Evaluation of behaviour factors of steel moment resisting frames
(Izmir Institute of Technology, 2019-07) Baran, İbrahim; Aktaş, Engin; Kanyılmaz, Alper
Builders of structures throughout the history of civilizations have been challenged with various threats. Earthquakes, which may be considered as the most critical challenge of the built environment have been one of the main concerns of builders at all times especially in regions with high earthquake risk. In this study, Response Modification Factor, (also known as ‘R’ or ‘q’ factor) which is used by seismic codes to determine force demand considering the ductile behaviour of the structure during the earthquake, has been analyzed. Steel frames (36 frames) with a different number of stories (3,6,9) and bays (3 to 8) were designed with normal ductility (18 frames) and high ductility (18 frames) moment resisting steel frames. The Turkish Building Earthquake Code-2018 (TBEC 2018) was followed for the seismic design process. All frames were assumed to be constructed in a region with the highest seismic demand defined in the web-based Hazard Map in TBEC 2018. The performance evaluation of the designed frames is completed using non-linear static analyses following FEMA, ATC-40 and NEHRP guidelines. The non-linear static analyses were implemented with SAP2000 v20.2 commercial software. Static pushover analysis as well as the capacity curve graph, known as the N2 method, has been obtained, and the position of the frame capacity in the spectral acceleration graph has been determined and presented visually. The results were summarized and the R factors obtained from these analyses were compared with the values given in the Turkish Building Earthquake Code.
Finite element modeling of historical masonry structures; case study: Urla Kamanli Mosque
(Izmir Institute of Technology, 2004) Teomete, Egemen; Aktaş, Engin
Historical structures are our cultural values that left behind by the thousands of year.s cultural accumulation. They pass our identity and civilization to the next generations. Conservation of historical structures and carrying them to the future are our inevitable duty.Before restoration and strengthening, historical structures must be analyzed thoroughly. The problems of the structure should be well identified; taking into account the special conditions of the structure, assessmental applications should be planned and applied. The assessment procedure generally consists of history investigation, measurement survey, material and structural tests, foundation survey, long term observations and structural analyses. Today, finite element analyses is the most common method of structural analyses of historical masonries. In this thesis finite element (FE) modeling of historical masonry structures was investigated. The study has been conducted on a case study, Urla Kamanli Mosque. History investigation, measurement survey, long term observations, material tests have been conducted. The seismicity of the region and local soil conditions were investigated. By using the data collected, FE analyses of the structure was performed by use of LUSAS, a commercial FE software. The structure is safe under its self weight while high tensional stresses occur at the drum-dome connections and at the second level windows and drum windows. The seismic analyses were conducted by use of response spectrum. The empty lintel places and the drum-dome connections are vulnerable under seismic loads. Linear elastic settlement analyses with self weight enlightens the reason of the existing cracks of the structure Nonlinear settlement analysis with self weight was conducted by use of concrete material model of LUSAS. The existing cracks' pattern of the structure was approximately simulated by the 1st principal stress contours. The concrete material model of LUSAS is suitable for analysis of masonry structures.
Identification of Vezirağa Aqueduct in Izmir as a historical monument
(Izmir Institute of Technology, 2016-07) Mamaklı, Fatma Sezgi; Aktaş, Engin; Turan, Mine
During 17th century, the developing commercial activities in İzmir took the attentions of senior people from the government to the city. Köprülü Fazıl Ahmet Paşa realized the lack of water supply in İzmir and ordered to construct an aqueduct on Melez valley and fountains to the city center. The system was constructed in 1674 with an aqueduct named Vezirağa Aqueduct, ten old restored fountains and seventy-three new fountains in the city center. This study aims to identify historical, architectural and structural characteristics of Vezirağa Aqueduct so that its heritage values and conservations problems can be understood. The geographical and historical characteristics of Vezirağa Aqueduct are described by taking the effects of site and the socio-cultural situation of city into consideration. The architectural characteristics of the aqueduct are prepared by using the site survey datas to reveal the current condition and find out the original state. The original state of the structure explains its historical value and the importance during the construction period. The resistance assesment of the structure is defined by using equivalent static analysis and dynamic analysis by using finite element modeling and time-history analysis. Finally, the historical, architectural and structural characteristics of Vezirağa Aqueduct prove its historical, documentary and aesthetic values and the structural problems that can damage the aqueduct and affect its values.
Performance-Based Seismic Design of reinforced concrete frame buildings: A direct displacement-based approach
(Izmir Institute of Technology, 2015-07) Karimzada, Nisar Ahmad; Aktaş, Engin
Structures are designed using current seismic design codes which are mostly based on Force-Based Design approach. The initial aim of the current codes is the public safety. However, no clear information are provided regarding economic losses and business interruptions or downtime. Some information about damage states of structural components are provided, but very limited information is given for the damage states of non-structural members and content systems. Performance-Based Seismic Design (PBSD), which is a new concept in seismic design of structures, is a reliable approach capable of providing more detailed information on the performance levels of both structural and non-structural elements. Direct Displacement-Based Design (DDBD) approach, which is one of the available PBSD procedures, is implemented in this study. This approach has been utilized on four reinforced concrete irregular frames which are different in terms of number of stories. Story drift ratios were chosen as deformation limits to define the performance levels for specific earthquake hazard levels. DDBD approach in this study has been utilized in compliance with Turkish Seismic Design Code 2007. Furthermore, capacity design principles were adopted to make sure that plastic hinges occur in beams rather than in columns. After obtaining design internal forces (design moments, shear forces and axial forces), TS500 2003, is utilized to design members. Damage states for each member was determined, and non-linear pushover and time history analysis were carried out using SAP2000 v17.10 to check if story drift ratios meet the ones chosen. The base shear forces and the top displacements, in addition, for each frame were also checked with the ones obtained through DDBD approach.
POLİAMİD 6/6 (PA 66) MİKROFİBERLER İLE TOKLAŞTIRILMIŞ TABAKALI KOMPOZİTLERİN MOD-I DELAMİNASYON DİRENCİNİN İSTATİKSEL ANALİZİ
(2019) Beylergi̇l, Bertan; Aktaş, Engin; Tanoğlu, Metin; Tanoğlu, Metin
Fiber takviyeli kompozitler, ağırlıklarına oranla yüksek mukavemet ve rijitlikleri nedeniyle uzay ve otomotivyapısal parçalarında yaygın olarak kullanılmaktadır. Delaminasyon, bu kompozitlerde görülen en yaygın vekritik hasar modudur. Bu kompozitlerin delaminasyon direncini arttırmak amacıyla, nanokatkılar ile epoksitoklaştırması, dikişleme, z-pimler ve arayüzeyde mikro/nanofiber kullanılması gibi pek çok sayıda teknikgeliştirilmiştir. Bu çalışmada, poliamid 6/6 (PA 66) mikrofiberler ile toklaştırılmış karbon fiber/epoksikompozitlerin Mode-I delaminasyon direnci Weibull dağılımı kullanılarak istatiksel olarak analiz edilmiştir. Testverilerinin istatiksel olarak değerlendirilmesi sonucunda PA 66 mikrofiberlerin kullanılmasının %90 güvenseviyesinde kırılma tokluğunu yaklaşık %445 mertebesinde arttırdığı gözlemlenmiştir.
A practical finite element model of tsadwa type semi-rigid connections for push-over analysis of steel frames in SAP2000
(01. Izmir Institute of Technology, 2021-12) Şeker, Öncel; Aktaş, Engin; 01. Izmir Institute of Technology
In steel structural analyses, beam to column connections is traditionally fully pinned or fully rigid. In the analysis assumption, moment (rigid) connections are assumed not to undergo rotation, and shear (pinned) connections can't transfer moment. These classifications do not represent actual connection behavior. Moment (rigid) connections have some relative flexibilities to rotate, and shear (pinned) connections have some capacity to transfer moment. In the light of these actual connection capabilities, another type of connection called partially restrained (PR) or semi-rigid connection is introduced. This study focuses on the behavior of bolted top, and seat angle with double web angle (TSADWA) connection using finite element method (FEM) software (ANSYS) to obtain moment-rotation curves, including geometrical and mechanical properties of the connection and adaptation of the PR connection behavior into the planar frame is performed for performance analysis through SAP2000 software by considering current codes and literature. Azizinamini's (Azizinamini, 1985) experimental and Frye and Morris's (Frye & Morris, 1975) mathematical moment-rotation curves are used to compare with the curves produced in the ANSYS to validate the feasibility of the finite element based PR connection models. A portal frame model with an 8S10 PR connection in the SAP2000 for performance analysis is also compared with the ANSYS portal frame model to demonstrate the correlation between the push-over analysesto reduce the margin of error. Although there is a lack of experimental information on the TSADWA connected frame models, results show that semi-rigid connection models and their usage on the steel frames can propose a reliable and practical methodology for the analysis process.
Project plannig and management using Building Information Modeling (BIM)
(Izmir Institute of Technology, 2018-12) Politi, Ruti Ruth; Aktaş, Engin; İlal, Mustafa Emre
Today, in construction practice, implementation of effective construction management techniques and tools is becoming essential, especially as the scale of the projects increases. Large scale projects pose a challenge to industry; as the number of tasks, parameters and constraints to be considered rises, interaction of these tasks and parameters increases the complexity, as well. While the search for techniques and tools for better coping with complexity will never end, Building Information Modeling (BIM) has recently emerged as an approach that achieves considerable improvement in efficiently handling complexity. BIM is able to provide a digital visualization of a building or structure in 3D. The necessary geometric data is modeled but more importantly all semantic data associated with the project (function of the element, material properties, construction details, schedules, etc.) is stored with the model. The intent is to have a single model of the project for all stakeholders over all phases of the project. In this study, the main challenges within the current practice of project management are analyzed and what BIM-based project management has to offer to those challenges have been discussed. The thesis explores how BIM method can be applied to project management and how information can be collected to build a BIM model. The advantages of BIM-based scheduling (4D), cost estimation (5D), sustainability (6D), facility management (7D) and structural analysis in the scope of project management is discussed. A case study is given to validate the use of the computer aided programs for nD BIM during the construction phase.
Reliability based seismic assessment of unanchored circular steel storage tanks
(Izmir Institute of Technology, 2020-07) Bektaş, Nurullah; Aktaş, Engin
Since liquid storage tanks are important systems for the safety of industrial facilities, the tightness of these structures and their serviceability are key issues. Therefore, the ability of large liquid storage tanks to withstand damage during earthquakes is of great relevance not only to the engineering profession but also to the safety of society in general. This is because these structures often form an important part of a community's lifeline and therefore should remain in use in emergency situations. In addition, since the content stored in some facilities could be hazardous, in this case, necessary measures must be taken against accidental infiltration. Due to the importance of storage tanks for society, there are many studies and standards in the literature on the behavior of liquid storage tanks. The most detailed method is the incremental dynamic analysis, which considers the time history analysis within a finite element model for the seismic analysis of unanchored circular steel liquid storage tanks to assess performance. In this study, a performance-based study was conducted on the dynamic behavior of an unanchored circular steel liquid storage tank in order to consider possible improvements in the design phase of the model to withstand earthquakes. It is assumed that the tanks stand on a rigid foundation and are exposed to one-way horizontal ground motion. The main purpose of the study is to determine the performance of the structure for certain failure criteria by using the engineering program ABAQUS to perform reliability-based analyzes of the model.
Seismic vulnerability assessment of Bergama Bedesten
(Izmir Institute of Technology, 2022-07) Demir, Hatice Ayşegül; Turan, Mine; Aktaş, Engin
This study aims to document Bergama Bedesten with emphasis on its construction technique and material usage; and assess its structural condition for future seismic action through numerical analysis. The monument (15th -16th century) is unique among the historical trade buildings at the center of Bergama, İzmir. It is revealed that the structure is a rare example by the use of plane triangles as transition elements, the oculi placement in domes, and the thickening application at parts where load-bearing arches integrate into the walls and at the corners. In order to investigate the vulnerability of the building, numerical analysis using the finite element method was conducted in ANSYS software. According to the Turkey Building Earthquake Code (2018), single-mode equivalent earthquake analysis considering the horizontal elastic design spectrum of the site was carried out for x and y directions, separately. The maximum principal stress concentration locations obtained through analyses point vulnerable zones. These were specifically observed on load-bearing arches, on some plane triangles, on the transition zones, at dome parts close to the transition zone and peak level, at the parts where arches integrate to the walls, around top windows, on the west entrance door arch, and on a secondary door arch in the north wall. The method comparing the structural failure mapping and visual structural alterations analysis of architectural restoration with finite element model results, emphasizes that some damaged or altered portions in the current state are in seismically vulnerable spots. Some precautions, further studies and interventions were suggested for these locations.
Structural analyses and assessment of historical Çardak Caravanserai in Denizli
(Izmir Institute of Technology, 2019-07) Şahin, Esra; Aktaş, Engin; Turan, Mine
The assessment of historical masonry structures are significant in terms of transferring our cultural heritage to future generations. The conservation of these masonry structures is possible through investigating problems and their causes with tools such as structural modeling and analyses. In this study, the structural modeling and analyses of a historical masonry structure was aimed. Çardak caravanserai in Denizli was chosen as the case study since it has preserved its authenticity and integrity. In this scope, the software used for modeling and structural analyses was SAP2000 v20.2 commercial finite element software. For modeling, frame elements were used in the columns and shell elements were used in the other structural elements. Selfweight analysis, modal analysis and response spectrum analysis were conducted in terms of the assessment. The self-weight analysis was made under dead load. The analysis indicated the current state of the structure. In modal analysis, free vibration periods, mode shapes and mass participation ratios of the structure were determined. In response spectrum analysis, according to Turkish Siesmic Design Code 2018, mode integration method was used as linear calculation method. In this analysis, the khan was evaluated under the dead load and earthquake loads in the x and y directions. The structural elements that exceed the compressive, tensile and shear strength were identified as hot spots and recommendations on the further investigations on these critical points of the structure were presented.
Structural conservation of Cevher Paşa Bath in Kale, Denizli
(Izmir Institute of Technology, 2018-12) Durmuşlar, Feyza; Turan, Mine; Aktaş, Engin
The study has focused on structural conservation of a historical monument in an archeological site. Cevher Paşa Bath, which is located in Kale (ancient Tabae) archeological site in Denizli and dated to the 15th century, is selected as the case study. The aim of the study is to propose a framework for structural conservation of the ruins of Cevher Paşa Bath so that conservation work regarding similar masonry ruins in archeological sites belonging to Turkish period can be guided. First, the bath was documented by using 3D Laser Scanner. Then, historical and theoretical framework including development of baths, loads of masonry buildings and evaluation of interventions in similar cases was constituted. Characteristics of the case study was defined in terms of location, landform, history, site and mass characteristics, architectural features, structural characteristics, structural calculations and alteration analysis. Condition report of the bath was prepared in form of tables. To identify original state of the bath, restitution was carried out. Nevertheless, the restoration approach is not reintegration, but only consolidation and presentation in order to sustain integrity with the archaeological site whole. Therefore, to prevent further damages, emergency interventions such as temporary shoring are proposed. Structural calculations have revealed that strength of the walls corresponds to stresses and overturning moment. So, the restoration work includes only supporting of arch remains and the weakest corner of the bath, which was determined by calculations. Some walls of the ruin are weaker than other parts. These parts need further analysis by civil engineers, and if necessary, consolidation can be carried out. The monument management plan points out the necessity of collaboration of Kale Municipality, RT Ministry of Culture and Tourism, Aydın Regional Council for Conservation of Cultural Assets, Directorate of Excavations in General Directorate of Cultural Assets and Museums, Aydın Conservation Implementation and Inspection Office, Department of Architectural Restoration and Department of Civil Engineering of IZTECH and History of Art Department of Ege University, and monitoring of the changes the bath four times in every year.
The structural conservation problems of traditional houses in Eryeri, Mardin, Artuklu
(01. Izmir Institute of Technology, 2023-06) Ercan Gündüz, Yasemin; Turan, Mine; Aktaş, Engin
The aim of this study is to document the construction techniques, material usage, and structural failures of the traditional houses in a village of Mardin, Eryeri, in order to guide a conservation aimed development plan that will be prepared in the future and enhance the related rehabilitation implementations. Eight houses that have preserved their authenticity were selected. Field survey focusing on the structural elements such as foundations, walls, floors, and roofs was conducted with the conventional techniques of architectural restoration. Both failures and also alterations of the buildings were documented. The changes and alterations of the houses are documented. Visual analysis of the constructional qualities was made with Archicad23 tools. Comparison with the traditional houses in the vicinity was made with reference to data gathered via field survey, literature review and archive research. As a result, the traditional houses are integrated with the rocky terrain of the region and in masonry system. There is an intense use of local stone, Yellow Limestone. Vaults are common superstructure elements, but timber floor is also possible. The walls are stone masonry in all. The common structural problems are demolition of structural elements such as roofs. The causes of severe failures are neglect and natural aging. The reinforced concrete additions flanking to the authentic structures also cause failure. Within the scope of the conservation plan, it is important to prevent reinforced concrete annexes and to intervene the annexes that cause excessive loads urgently.
Structural model besed root cause investigation of damage in Urla Helvacılar Mosque and a proposal for strengthening
(Izmir Institute of Technology, 2019-07) Narin Güzel, Ceren; Kul Özdemir, Fatma Nurşen; Aktaş, Engin
The period when small independent beyliks (principalities) were founded in Anatolia in the 13th century is called the principalities period. The process of creating a new architectural style in Western Anatolia, as well as the works-built marks an important milestone in this period, which is defined as a transition period between the Seljuk architecture and the Ottoman architecture. The Aydın Dynasty, one of the important principalities of western Anatolia, had its capital at Birgi from 1308 to 1426. The dynasty is known for with its economic and political power and the important works it left behind. Helvacılar Mosque is one of the important works of the Aydın Dynasty, which was built in the 15th century and survives today. It is also an outstanding example to understand the single-domed mosque typology in western Anatolia. Helvacılar Mosque, located in the Kuşcular Neighborhood of the Urla District, is today derelict and abandoned. Heavy structural damage in the structure poses a grave risk for the structure to survive. This study compared and discovered the similarities and differences between Helvacılar Mosque and the similar-period structures situated in Urla and identified the architectural and structural characteristics of the structure with the aid of measured drawing projects. This study used a combination of architectural restoration and structural engineering. For Helvacılar Mosque, the study carried out the self-weight analysis, modal analysis, settlement analysis, response spectrum analysis, time history analysis using three real earthquake records in a finite element model. The aim of engineering research is to identify the present causes of damages in the structure. With the help of finite element analysis, the study put forward the essential repair and strengthening methods for Helvacılar Mosque to repair the damages observed in the structure, strengthen the structure, and obviate the causes of the damages based on the intervention methods offered in the guideline titled Earthquake Risk Management of Historical Structures issued by the General Directorate of Foundations.
Toughening of carbon fiber based composites with electrospun fabric layers
(Izmir Institute of Technology, 2017-11) Beylergil, Bertan; Tanoğlu, Metin; Tanoğlu, Metin; Aktaş, Engin
The objective of this PhD thesis is to investigate interlaminar Mode-I fracture toughness of carbon fiber/epoxy composites interleaved by micro and/or nano scaled PA66 nonwoven veils. Also, the effects of electrospun PVA nanofibers on the mechanical performance of these composites were investigated. Additionally, this thesis also deals with the effects of aramid nonwoven veils on the mechanical properties of CF/EP composites. The produced nanofibers produced by electrospinning were directly deposited on carbon fiber fabrics. Then, reference and nano-modified laminates were manufactured by vacuum infusion method. A series of mechanical tests such as tensile, compression, three point bending, Charpy-impact, interlaminar shear strength and open hole tensile tests (OHT) were carried out on the prepared specimens. Double cantilever beam (DCB) tests were conducted on reference and interleaf-modified laminates. The effect of PA 66 nanofiber areal weight density was also evaluated with varying electrospinning time. Scanning electron microscopy (SEM) was used to investigate the fiber morphology and to understand the toughening mechanisms. Dynamic mechanic analysis (DMA) was used to investigate the thermo-mechanical behavior of reference and interleaf-modified composite specimens. Differential scanning calorimetry (DSC) was used to determine the thermal properties of micro and electrospun PA66 nonwoven veils. Comparing the mechanical test results, the most effective nonwoven interleaving system was determined in terms of higher delamination resistance and in-plane mechanical properties. Finite element method (FEM) was used to evaluate the effects of electrospun PA66 nonwoven veils on the CF/EP composites. Numerical simulations of Mode-I fracture toughness tests were carried out using ANSYS Workbench. The results showed that the most effective material was electrospun PA66 nonwovens considering the higher delamination resistance. Additionally, the electrospun PA 66 nonwovens also improved Charpy-impact and interlaminar shear strength of the reference CF/EP composites. Numerical results showed good agreement with the experimental ones.