Phd Degree / Doktora
Permanent URI for this collectionhttps://hdl.handle.net/11147/2869
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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 KemalThe 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, İzzetMultiscale 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 Hydrological and Meteorological Drought and Trend Analysis in Afghanistan and Their Implications on Transboundary Rivers(01. Izmir Institute of Technology, 2022) Hayat, Ehsanullah; Tayfur, GökmenAfghanistan 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 Three Dimensional Numerical Modelling of Recharce: Case Study: Eğri Creek Sub-Basin, İzmir(01. Izmir Institute of Technology, 2022) Şahin, Yavuz; Tayfur, Gökmen; Baba, AlperAlthough 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.Doctoral Thesis Enhancing Earthquake Performance of Civil Structures Via Structural Control(Izmir Institute of Technology, 2021) Şenol, Vedat; Turan, GürsoyIn this study, two different benchmark buildings (3 and 20-story) are employed to attenuate structural responses under seismic disturbances. As control devices, active (actuators), semi-active (Magneto-rheological dampers), passive (Tuned mass dampers and Friction Pendulum Bearings), and hybrid controllers are utilized. The 3-story structure is modeled linearly and employed to apply to different control strategies. Some control algorithms: LQR, PDD-state-feedback, pole-placement, $H_{\infty}$, $ H_2 $, are used with active and semi-active control devices. As passive devices, TMDs and FPBSs are utilized on the nominal-linear model. Thereafter, hybrid controllers are employed: one composed of a TMD and actuator/MRD and one composed of an FPBS and actuator/MRD. A robust controller, $\mu$-synthesis, is employed to control the same linear structure having uncertainties in mass, stiffness, and damping matrices within reasonable ranges. A nonlinearly-modeled 20-story benchmark structure is employed to implement passive and hybrid control strategies. As passive devices, STMD and MTMD setups are employed. Further, a robust control algorithm is used through an actuator serially connected to the STMD. Subsequently, variations caused by nonlinearities are determined. These variations are regarded as uncertainties, and the $\mu$-synthesis is utilized in the design of a robust controller on a truncated linear model. Then, the designed robust control is employed to control the 20-story benchmark structure modeled nonlinearly. The structural responses in both frequency and time domains are discussed. Matlab, Python, and OpenSees framework (Tcl/Tk) were employed to realize all linear and nonlinear simulations throughout the study.Doctoral Thesis Properties of Hybrid Fiber Reinforced Concrete for Impact Loading(Izmir Institute of Technology, 2021) Alami, Mohammad Musa; Saatçi, Selçuk; Erdem, Tahir KemalConcrete is a brittle material and does not have significant energy absorption capacity before its fracture. Adding fibers to a concrete mix increases its ductility. Recently, there is significant development in the concrete technology to produce a concrete that can exhibit deflection hardening and show high energy absorption capacity. In this thesis, two kinds of cement based composites with high energy absorption capacity were studied: 1. Engineered Cementitious Composites (ECC). This material can exhibit deflection hardening under bending and it is produced only with synthetic fibers and fine aggregate, 2. Hybrid Fiber Reinforced Concrete (HyFRC). This material can exhibit deflection hardening under bending. It was produced with fine and coarse aggregates and hybrid fibers (both steel and synthetic fibers). The experimental program of this study consists of two main stages. The first stage is to design these composites and test their basic properties in fresh and hardened states, such as compressive strength, flexural behavior, freezing-thawing resistance, chloride ion permeability and sorptivity. In the second stage, dynamic tests (drop tests on small size specimens and pendulum impact tests on real size new generation road concrete barriers with a selected HyFRC mixture) were carried out to determine their energy absorption capacities. Based on the ECC results, fly ash/cement ratio of 1.2 and 20% perlite replacement of sand were selected for HyFRC mixtures. According to the mechanical behavior and durability test results of HyFRC, ST3,0.75_P0.25_D16 mixture (steel fiber type= ST3, steel fiber volume=0.75%, PVA volume=0.25%, Dmax=16mm) was found to have the best performance, and accordingly, this composite was selected for the real-size barrier pendulum test. The same mixture without fibers was also tested under pendulum test as control normal concrete since the present road barriers in the market do not employ fibers. As a result of this study, the HyFRC barrier was found to perform higher impact resistance.Doctoral Thesis Impact Behavior of Textile Reinforced Concrete Slabs(01. Izmir Institute of Technology, 2021) Batarlar, Baturay; Saatçi, SelçukReinforced concrete (RC) technology is still the most preferable and common method to build civil engineering structures. In accordance with design purposes and needs, these structures are built to resist various loading scenarios. Throughout the lifespan of RC structures, they may be subjected to high rate loading scenarios due to either expected or unexpected reasons such as impacts caused by vehicular collisions, debris generated by typhoons, tsunami or floods, rock or object falls to protective shelters. Therefore, understanding of impact behavior of RC members plays a vital role not only for design stages but also retrofitting and strengthening purposes thereafter. For this purpose, an experimental program was carried out to reveal the impact behavior of RC slabs strengthened with carbon textile reinforcements. In this program, four slabs specimens, two unstrengthened and two strengthened with two different carbon textile reinforcements, having dimensions of 1.5 m × 1.5 m × 0.2 m were tested by using an advanced impact testing facility at Otto-Mohr Laboratiorum of Technische Universität Dresden. In these tests, all slabs were tested under repeated impact loads by using the same steel striker with a 200 mm - diameter flat contact surface in the velocity range of 25.2 to 30.2 m/s. The results obtained from these tests are presented in terms of midpoint-displacement histories, reaction force histories, slab accelerations, and strain histories of steel reinforcements for each impact. As a result of the test program, it is shown that carbon textile reinforcements have significant effects on enhancing impact capacity as well as limiting maximum and residual midpoint displacements. By using the data obtained from tests, a finite element (FE) modeling study was performed by using the LS-DYNA software tool. In this study, two FE models with different mesh sizes were created and compared with each other to obtain efficient modeling conditions. In the light of the tests and validated models, a parametric study was performed to figure out efficient impact conditions and parameters for carbon textile reinforcements. It is shown that carbon textile reinforcements are more effective for limiting damage levels under low-velocity impacts.Doctoral Thesis Examination of Bonding Between Successive Lifts of Concretes Having Different Thixotropy Levels by Surface Scanning(Izmir Institute of Technology, 2020) Ersöz, Hasan Yavuz; Erdem, Tahir KemalUnexpected delays in casting can result in strength losses and should be examined considering the thixotropy of the concrete. The effects of thixotropy and delays in casting (0, 45, and 90 min) on concrete bonding strength and on the surface properties were examined. Firstly, 15 self-consolidating concretes (SCC) having different compositions with three different water/cement ratios (0.36, 0.41 and 0.46) three different fine/all aggregate ratios (0.45, 0.50 and 0.55) and two slump flow diameters (60 cm and 70 cm) were produced. Slump flow, visual stability index, T50, V-funnel, static sieve segregation, rheology, and thixotropy tests were carried out on fresh concrete. Considering the results obtained so far, three mixtures were selected to have high, low, and moderate thixotropy for subsequent tests on hardened concrete specimens. Five different roughening patterns were applied on the concrete specimens' interlayer zones by using wooden frames. Slant shear, bi-surface shear, and pull-off tests were conducted on the cylindrical, cubic and drilled specimens to assess bond strength, respectively. Slant shear test gave the highest bonding test results. The lowest results were obtained for the pull-off test. Lateral surfaces of the specimens were scanned with two different lenses. 2D and 3D scanning procedures were adapted with those lenses. Pattern and thixotropy effect was investigated. Lowest void area was obtained for high thixotropy. Highest compressive strengths were obtained for mid thixotropy and high thixotropy for slant shear and pull-off tests respectively. Image J and Matlab were used to the analyze images of the surfaces. Ultrasonic pulse velocity test was conducted on all specimens, moderate thixotropy gave the highest results for Ultrasonic Pulse Velocity test.Doctoral Thesis Manipulation of Structural Design Parameters To Mitigate the Concentrarion of Interstory Drift Ratios(Izmir Institute of Technology, 2020) Sönmez, Egemen; Dönmez, Cemalettin; Dönmez, CemalettinAlthough the interstory drift ratio is used as a limiting factor for specific performance levels by the structural engineering profession, its distribution among the stories is generally disregarded. Observations and analytical studies have shown that even the structures are designed to conform seismic design codes, interstory drifts tend to concentrate at certain regions of the frame structures. In other words, the seismic demand the earthquake imposes is attempted to be provided from a limited part of the structure. As a result, the damage concentrates, and the stiffness of the corresponding stories decreases significantly. Locally, the story drifts become larger. Soft-story mechanisms and abrupt failures may occur under such conditions. In this study, a seismic design method was developed to control the distribution of interstory drifts in a frame structure during the nonlinear seismic response. This method is based on two observations: (i) in the inelastic range, the drift distribution is highly dependent on the yield strengths of the members; (ii) there is a strong correlation between the interstory drift distribution and the plastic rotation distribution at member-ends. Thus, an iterative design procedure is developed to control the distribution of the interstory drifts by adjusting the member yield strengths. Plastic rotations are used as a tool for estimating the required yield strengths. The efficiency of the proposed method was tested using nonlinear time-history analyses. The results demonstrated that the frames designed using the proposed method had well-distributed interstory drift and story damage patterns compared to those of the conventionally designed frames. Furthermore, the overall damage of frames was reduced remarkably.Doctoral Thesis Modal Identification of Structures by Using Bayesian Statistics(Izmir Institute of Technology, 2019) Hızal, Çağlayan; Hızal, Çağlayan; Turan, Gürsoy; Turan, GürsoyBayesian Probabilistic approaches in the health monitoring of civil engineering structures has gained remarkable interest during past decades. When compared to the available Operational Modal Analysis (OMA) methods, Bayesian Operational Modal Analysis (BAYOMA) determines a probabilistic range with a most probable value and uncertainty instead of a certain result. For this reason, the most important difference of BAYOMA lies in its capability of uncertainty quantification. Therefore, the modal parameters of a measured structure can be determined based on a probabilistic logic according to various cases (for example single measurement setup, well separated and/or closely spaced modes, multiple measurement setups). Further, the finite element model of the investigated structure can also be updated by a Bayesian approach incorporated with modal identification procedure. Some efficient BAYOMA methods such as Bayesian Spectral Density Approach (BSDA) and Bayesian Fast Fourier Transform Approach (BFFTA) have been presented by various researchers during the past two decades. Despite their efficient and fast solution procedure, the available methods have some critical issues that need to be solved. Most of these problems especially lie in the quantification of posterior uncertainties and some special cases arise in closely spaced modes and/or multiple setup measurement cases. In the literature, solutions for the aforementioned problems have been also presented by various researchers. In the light of the accumulated knowledge in the literature, this study presents a computational framework for BAYOMA and Bayesian Model Updating (BMU). In addition to some improvements to the available methods, new and alternative approaches are presented for BAYOMA and BMU. According to the results, it is seen that the quality of identified modal parameters and updated finite element models increases significantly by the proposed computational procedure.