Phd Degree / Doktora

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Department: search.filters.department.Thesis (Doctoral)--İzmir Institute of Technology, Civil Engineering

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Now showing 1 - 10 of 17
  • Doctoral Thesis
    Effects of mix design parameters on physical properties, strength, and impact behavior of hempcrete
    (01. Izmir Institute of Technology, 2024) Taher, Abubaker Rafea Taher; Saatcı, Selçuk; Erdem, Tahir Kemal
    Yapı ve inşaat sektörü, karbon emisyonlarını düşürmek için giderek daha fazla sürdürülebilir malzeme kullanmaya başlamaktadır. Bu malzemelerden biri de kenevir betonudur. Ancak, kenevir betonun farklı uygulamalar için kullanılabilirliğini artıracak performans özelliklerinin optimize edilmesi hala bir zorluk teşkil etmektedir. Bu araştırma, sönmüş kireç yerine doğal hidrolik kireç, uçucu kül ve öğütülmüş granüle yüksek fırını cürufu gibi çeşitli bağlayıcı malzemelerin kenevir betonun yoğunluğu, kenevir kıtığı yönelimine paralel ve dik yöndeki, basınç dayanımı, ısıl iletkenlik ve kapiler su emilimi üzerindeki etkilerini değerlendirmeyi amaçlamaktadır. Ayrıca, araştırma, hempcrete'in enerji emme ve dağıtma kapasitesini bir darbe testi ile incelemektedir. Araştırma, farklı bağlayıcı bileşimlerini, kenevir kıtığı parçacık boyutlarını (kaba, orta ve ince), kıtık : bağlayıcı oranlarını, ve kıtık : su oranlarını deneyerek bu parametrelerin kenevir betonu performansı üzerindeki etkilerini analiz etmektedir. Sonuçlar, bu parametrelerin kenevir betonun yoğunluğu, basınç dayanımı, ısıl iletkenlik ve kapiler su emilimi üzerinde etkili olduğunu, bazı parametrelerin ise diğerlerinden daha belirgin bir etki sağladığını göstermektedir. Ayrıca, kıtık yönelimine dik yöndeki basıncın, belirli bir birim deformasyon oranı için numunelerin performansını arttırdığı gözlemlenmiştir. Darbe testi sonuçları, kenevir betonun darbe kuvvetlerini ve kiriş yer sehimlerini önemli ölçüde azalttığını göstermektedir. Daha düşük bağlayıcı içeriği, şok emme etkisini artırarak daha iyi enerji dağılımı sağladı; ancak, bağlayıcı içeriğe daha düşük olan numuneler test sonrası kırılmaya daha yatkındı. Bu bulgular, kenevir betonun darbeye maruz betonarme yapıları koruma potansiyelini öne çıkarmıştır. Test edilen özellikler ve bulguların, önümüzdeki yıllarda kenevir betonun çeşitli uygulamalarda kullanımını genişletmesi beklenmektedir.
  • 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
    Soil Erosion and Sediment Transport Modelling for River Basins in Afghanistan Using Gis-Based Models
    (01. Izmir Institute of Technology, 2023) Ansarı, Ahmad; Tayfur, Gökmen
    Soil erosion is a critical environmental issue with significant impacts on agricultural productivity, water quality, and land degradation. Afghanistan has five major river basins; Kabul River Basin (KBL), Helmand River Basin (HRB), Harri Rod-Murghab River Basin (HMRB), Northern River Basin (NRB), and Amu Darya River Basin (AMRB). Afghanistan is prone to soil erosion due to various natural and anthropogenic factors. This study is the first-ever assessment at the country scale, estimating the rainfall erosivity from measured and satellite rainfall products, calculating the soil erodibility factor based on two known databases (HWSD and ISRIC), the slope length and steepness factor (LS), the crop management factor (C), and the support practice factor (P). Finally, the average annual soil erosion potential is computed by multiplying the developed raster data from each Revised Universal Soil Loss Equation (RUSLE) analysis using the GIS and R-Studio software. Furthermore, the sediment delivery ratio for the Upper-Helmand and Arghandab Catchments are also estimated. The RUSLE model estimates showed that the entire watersheds in Afghanistan produce an average soil loss rate of 13.8 t ha-1 yr-1, of which the most severe classes of soil losses occurred in the southeast and northeast regions of the study area. The soil classes of very high, severe, and very severe cover an area of 30.4% (19,258,056 ha) of the entire study region with 273,763,211 t yr-1 of soil loss. Approximately 30.4% of the total area should be placed under urgent soil conservation measures, specifically the regions located in the KRB and AMRB watersheds. In addition, the SDR is computed for Upper-Helmand and Arghandab Catchments, and it is found to be 23.5% and 24.2%, respectively. The rainfall erodibility, slope-length gradient, and crop management factors are identified as the most crucial elements associated with soil erosion in the study area, and the rate of soil erosion gradually increases with the increasing soil erodibility and slope gradient. Six erosivity maps are generated using the TRMM-3B42 precipitation, Local rain-gauges stations (using MFI and mean annual precipitation), TRMM3B43-monthly product, ERA5 monthly product (spanning 1990 to 2020), and Global Rainfall Erosivity map. The correlation between the R-factor derived from the TRMM-3B42 and the R-factor estimated from measured rainfall are compatible, implying the suitability of the employed methods. This study revealed that satellite products of the Tropical Rainfall Measuring Mission (TRMM-3B42), Harmonized World Soil Database (HWSD), International Soil Reference and Information Centre (ISRIC) of soil data, the Digital Elevation Model (DEM) of the Shuttle Radar Topographic Mission (SRTM), and the Moderate Resolution Imaging Spectroradiometer (MODIS-13Q1) can be used as an alternative input while estimating RUSLE factors. This study provides valuable soil, water conservation, and management insights for the entire watersheds in Afghanistan. This research also establishes a technical foundation for utilizing the RUSLE model to estimate soil erosion in the region.
  • Doctoral Thesis
    Material Model Calibration of Fiber Reinforced Concrete Using Deep Neural Network
    (01. Izmir Institute of Technology, 2023) Yaşayanlar, Yonca; Saatcı, Selçuk; Erdem, Tahir Kemal; Saatcı, Selçuk; Erdem, Tahir Kemal
    The numerical modeling of fiber reinforced concrete (FRC) structures is quite challenging due to the material's heterogeneous and anisotropic nature. The majority of material models that are suitable for regular concrete are not able to account for the FRC material's increased tensile capacity and ductility. In this study, a calibration method is proposed that is simple and effective for modeling FRC structures using a selected concrete material model. The Karagozian and Case (K&C) material model in LS-DYNA is capable of representing the ductile nature of FRC, and its parameters related to tensile behavior were calibrated to reflect the tensile-softening behavior. The calibration process was executed using the uniaxial direct tension test results of two different FRC mixtures. In addition, single element numerical models were constructed using LS-DYNA under uniaxial tension. The tensile parameters of K&C were varied over a wide range using single elements to form a database. Then, a Deep Neural Network (DNN) was constructed to pass the database through and find the K&C parameters that best matched the experimental uniaxial test results. The proposed methodology was tested under static and high-strain rate loading conditions, and the results were compared to the experimental findings. The performance of the DNN-achieved parameters was found to be satisfactory. The results showed that the DNN-calibrated parameters were able to accurately predict the behavior of FRC structures under static and dynamic loading conditions.
  • Doctoral Thesis
    Continuum Damage Mechanics Based Modelling of Laminated Fiber Reinforced Composites
    (01. Izmir Institute of Technology, 2023) Yaşayanlar, Süleyman; Özdemir, İzzet
    Multiscale modeling, which merges the worlds of macro- and micromechanics, is establishing itself as a viable alternative to experimental procedures in the characterization of the mechanical behavior of complex materials. Advanced composite materials are a perfect field for the application of such modeling concepts. This thesis focuses on failure mechanics of fiber reinforced composites and addresses the modeling of failure processes at both micro- and macro-scales. First, a novel damage-plasticity model is developed and implemented within finite element software Abaqus as a user defined element. It is verified that the model gives mesh objective results, and the model is calibrated with experimental stress-strain curves from the literature. Representative volume elements (RVEs) based micro-mechanical models are constructed where damage-plasticity model and cohesive surfaces are employed to capture failure in matrix and matrix-fiber interface, respectively. A sufficiently large number of RVE analysis results are used to generate discrete failure envelopes. These failure envelopes are compared with continuous ones resulting from Puck's criteria. Furthermore, the influence of microstructural imperfections is investigated systematically, and an extended version of Puck's criteria is assessed from a micro-mechanical perspective as well. In the last part of the thesis, a macroscopic model is proposed which combines Puck's criteria with localizing implicit gradient damage model. It is shown that the model provides consistent results such that the failure angle obtained at material point and the orientation of the emerging macroscopic damage band match provided that sufficiently small internal length scale parameter is used.
  • Doctoral Thesis
    Probabilistic Finite Element Model Updating and Damage Detection of Structures by Using Bayesian Statistics
    (01. Izmir Institute of Technology, 2022) Ceylan, Hasan; Turan, Gürsoy
    Finite element (FE) model updating is extensively employed in many applications of various engineering branches for damage detection purposes. An FE model is expected to reflect the properties of actual structures. However, it is almost impossible for an FE model to carry the properties of the real-life structure. As a result, differences exist between analytical models and actual structures. The aim of model updating is to minimize these differences as much as possible. In model updating procedures, there are inevitable uncertainties due to inevitable measurement noise and modelling errors. Therefore, model updating and damage detection process should be performed in a probabilistic way instead of a deterministic one. To this end, Bayesian model updating methods have gained much attention in the literature to account for the uncertainties of the parameters to be updated. Among these methods, those that use the concept of system modes have gained much more attention since it enables researchers to account for modelling error uncertainties and to avoid mode matching problem. For those methods, discrepancies between system modes and measured modes are considered and system modes are updated to obtain those that best fit the measured modes. Further, system modes are connected to the FE model via eigenvalue equations. In this study, a two-stage Bayesian model updating method which utilizes the concept of system modes has been firstly reformulated to compare three different assumptions on the modelling error variance of eigenvalue equations. Results reveal that the Bayesian model updating formulations that use the system modes concept give unreasonably too small posterior uncertainties for the updated parameters. This makes the probabilistic approach questionable since getting such small uncertainties may almost be equivalent to a deterministic approach. To increase the posterior uncertainty levels to more reasonable levels, a two-stage sensitivity-based Bayesian model updating methodology is proposed in this study. The results show that the proposed method successfully improves the updating results and increases the posterior uncertainties to more realistic levels.
  • Doctoral Thesis
    Hydrological and Meteorological Drought and Trend Analysis in Afghanistan and Their Implications on Transboundary Rivers
    (01. Izmir Institute of Technology, 2022) Hayat, Ehsanullah; Tayfur, Gökmen
    Afghanistan as a landlocked country located within central and southwestern Asia has an arid to semi-arid climate. Most of the people are involved in agricultural activities and major part of the country's gross domestic product depends on agriculture, but the country has the lowest water storage capacity. Consecutive periods of droughts and rapid snowmelt due to climate change have made it more challenging for convenient water resources management practices. Other major concern is transboundary water that flows to downstream countries without any sharing agreement rather than one on the Helmand River. Therefore, this study first aims to investigate the historical hydrological and meteorological drought characteristics across the whole country using Standardized Precipitation Index (SPI) and Reconnaissance Drought Index (RDI) for meteorological drought analysis for a period 1979-2019, and Streamflow Drought Index (SDI) is used for hydrological drought analysis using two different time series (1960-1979 and 2008-2020) from 55 hydro-meteorological stations. Missing streamflow data in four stations across four transboundary River Basins (RBs) of the country is modeled using Remote Sensing (RS) techniques via Landsat satellite imagery time series data from 1988 to 2021, and which is the novelty of this study. Afterwards, trends in streamflow, precipitation and temperature are examined using Mann-Kendall's and Sen's slope statistical tests. A four decadal countrywide drought maps are also generated. Finally, the effects of droughts and trends on transboundary rivers in the country are investigated. Data from (55) ground stations is used for this purpose in this study.
  • Doctoral Thesis
    Investigation of Liquefaction Potential of Sand-Tire Granulated Rubber Mixture That Used Around the Buried Pipes With Shake Table Tests
    (Izmir Institute of Technology, 2022) Karaman, Mustafa; Ecemiş Zeren, Nurhan
    Liquefaction causes major deformations in infrastructures. The rapidly increasing of scrap car tires causes to find new areas to recycle them. It has been seen that granular rubbers to be an effective filling material with their density, permeability, compressibility and also damping characteristics for liquefaction remedation. Firstly, this study aims to explain the effect of rubbers mixed with sand on the liquefaction potential of the mixture during and after earthquakes. For this reason, one-dimensional shaking table experiments were carried out with granular rubbers-sand backfills in a large scale laminar box with varying rubber diameters and varying ratios of rubber. Secondly, the study aims to explain the effect of these mixtures on pipeline performance when used as filling around buried pipelines. Lastly, this study focuses on the possibility of rubber to contaminate groundwater with inorganics. For this purpose, a series of batch tests and column leaching tests were performed. Consequently, mixing rubbers with sand is effective in liquefaction remedation. They reduce the pore water pressure thanks to the high permeability, affect the consolidation characteristics with its permeability and compressibility, also reduce the earthquake loads with their damping facilities. In order to prevent the buried pipelines from uplifting during an earthquake, a limit criteria is suggested to design of the pipe diameter, burial depth and filling conditions with a predicted seismic load. Rubbers aren’t hazardous for contaminating the inorganics into groundwater. If granular rubbers are used in environmentally sensitive areas, it is recommended to use them after a prewash process or soaking in water for a day.
  • Doctoral Thesis
    Three Dimensional Numerical Modelling of Recharce: Case Study: Eğri Creek Sub-Basin, İzmir
    (01. Izmir Institute of Technology, 2022) Şahin, Yavuz; Tayfur, Gökmen; Baba, Alper
    Although the science of water management has experienced significant improvements over the past century, many issues still require the attention of the scientific community. Global change, growing population and increasing pressure on existing water supplies have intensified the need for further improvement of water resources management practice. The purpose of this special issue is to present some of the latest research carried out in the area of water resources management under uncertain and changing conditions. Study in this issue highlight recent consuming in this basin covering all the surface & groundwater of the hydrologic cycle. The large demand for drinking, irrigation and industrial water in the region of K. Menderes Basin. The main objective of the study is to emerge capacity of surface and groundwater. Also, notice that decreasing groundwater level in basin. This river basin agricultural dominant has fertile land and range of harvest diversity in all season. In dry periods, Groundwater level has been facing decreases for past 30 years. Every private farm has private wells that were drilled without permission. These cause depletion of groundwater and restraining the usage of groundwater. Another subject is industrial usage of groundwater and increasing population in area. For this purpose, surface artificial recharge methods in conjunction with underground dam construction were investigated in Egri Creek sub-basin. Thus, their contributions to the groundwater levels were investigated with the help of a numerical model.