Phd Degree / Doktora

Permanent URI for this collectionhttps://hdl.handle.net/11147/2869

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Author: search.filters.author.Aktaş, Engin

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  • Doctoral Thesis
    Reliability Assessment Based on Structural Health Monitoring Data and Bayesian Updating of Structural Models
    (01. Izmir Institute of Technology, 2024) Uzun, Ertuğrul Türker; Aktaş, Engin; Hızal, Çağlayan
    Finite element (FE) models are commonly used in numerical modeling of structures, but their assumptions can lead to inaccuracies and uncertainties. To address this, FE model update methods have been developed, calibrating the model based on structural health monitoring (SHM) data. However, a general framework for realistic life cycle performance assessment of structures using monitored data has not yet been presented. Bayesian modeling can characterize uncertain structural parameters as random variables, but it is complex and time-consuming. Metamodeling techniques, which are effective stochastic predictors, can be used to decrease the computational burden of model updating. Adapting a Polynomial-Chaos-Kriging (PCK) metamodeling technique to Bayesian model updating in order to reduce uncertainty and circumvent computational challenges using SHM data in order to assess the reliability of structures more precisely is the objective of this research. Therefore, the effectiveness of the proposed method has been tried and demonstrated through experimental and numerical studies. An experimental study of a bridge column is used to evaluate the reliability of structures subjected to various corrosion effects. As a result, the proposed solution method reduces computational costs and enables an updated FE model to be closer to real structure measurements. The updated models are found to be more reliable in reliability evaluations, providing more accurate predictions on issues like structure safety, service life, and maintenance cost compared to non-updated models.
  • Doctoral Thesis
    Probabilistic Performance-Based Optimum Seismic Design of Reinforced Concrete Structures
    (01. Izmir Institute of Technology, 2023) Karımzada, Nisar Ahmad; Aktaş, Engin; Girgin, Sadık Can
    Traditional seismic design codes have been developed and used for decades to stipulate the rules for earthquake-resistant design of structures. They are mainly based on the Force-Based Design (FBD) approach and on some linear elastic techniques. The inelastic seismic response of the structure is not directly addressed in the traditional seismic design codes. The initial aim of the current seismic design codes is public safety. In seismic codes, some information is provided regarding the damaged states of structural components, while limited information is provided regarding the damaged states of nonstructural members. In addition, no clear information is provided regarding economic losses and business interruption. The Performance-Based Seismic Design (PBSD) approach, a reliable approach for the seismic design of structures, is capable of providing more detailed information on the performance levels of both structural and nonstructural members and content systems. Some current seismic design codes adapted concepts of the PBSD approach in a deterministic manner, considering uncertainties implicitly. In this study, efforts have been made to develop a Probabilistic Performance-Based Optimum Seismic Design (PPBOSD) methodology for Reinforced Concrete (RC) structures, considering uncertainties explicitly to provide a more practice-oriented approach. It is a powerful seismic design tool that provides structures with economical, robust, and rational design. In addition, structures designed using this approach could satisfy the target performance levels at multi-limit states. For the optimization problem, the objective function is given in terms of minimizing the expected total cost of the structure at a specific intensity level. Pacific Earthquake Engineering Research Center's Performance-Based Earthquake Engineering (PEER PBEE) methodology is used for the performance assessment of the structure. The Endurance Time method is used in the PEER PBEE methodology framework while performing optimization. After the optimum solution is obtained, the Incremental Dynamic Analysis (IDA) method is used to verify the performance levels. The proposed methodology is applied to RC frame buildings with different numbers of stories. OpenSees software is used together with codes written in python for the design and analysis purpose.
  • Doctoral Thesis
    Toughening of Carbon Fiber Based Composites With Electrospun Fabric Layers
    (Izmir Institute of Technology, 2017) Beylergil, Bertan; 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.