Doktora Tezleri

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Enhancing earthquake performance of civil structures via structural control
(Izmir Institute of Technology, 2021-10) Şenol, Vedat; Turan, Gürsoy; Izmir Institute of Technology
In 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.
Examination of bonding between successive lifts of concretes having different thixotropy levels by surface scanning
(Izmir Institute of Technology, 2020-12) Ersöz, Hasan Yavuz; Erdem, Tahir Kemal; Izmir Institute of Technology
Unexpected 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.
Hydrological and meteorological drought and trend analysis in Afghanistan and their implications on transboundary rivers
(01. Izmir Institute of Technology, 2022-06) 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.
Impact behavior of textile reinforced concrete slabs
(Izmir Institute of Technology, 2021-03) Batarlar, Baturay; Saatçi, Selçuk; Izmir Institute of Technology
Reinforced 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.
Investigation of liquefaction potential of sand-tire granulated rubber mixture that used around the buried pipes with shake table tests
(Izmir Institute of Technology, 2022-06) 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.
Manipulation of structural design parameters to mitigate the concentrarion of interstory drift ratios
(Izmir Institute of Technology, 2020-06) Sönmez, Egemen; Dönmez, Cemalettin
Although 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.
Modal identification of structures by using Bayesian statistics
(Izmir Institute of Technology, 2019-07) Hızal, Çağlayan; Turan, Gürsoy
Bayesian 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.
Modeling of concrete under high strain rate conditions using nonlinear finite element method
(Izmir Institute of Technology, 2017-07) Çankaya, Mehmet Alper; Saatcı, Selçuk; Taşdemirci, Alper
In this study, a comprehensive experimental and numerical study was undertaken to model concrete under high strain rate conditions. Concrete cylinder specimens, all obtained from the same batch, were tested both under ststic and high strainrate conditions. 15 eylinder specimens were tested under 3.55x10-5, 3.23x10-4, 2.97x10-3 1/s strain rates, whereas three identical specimens were tested using a Split Hopkinson Pressure Bar SHPB) tes setup under 235, 245, 260 1/s strain rates. Used SHPB setup was modified to include quartz crystal stress developed in the specimens werw directly obtained, eliminating common isssues regarding stress readings in a conventional setup. Stress-strain behavior and other material parameters that would be necessary for numerical modeling were obtained under various strain rates. Test samples were modeled using an explicit finite element program LS-DYNA, using Holmquist-Johnson-Cook model with experimentally obtained model parameters. To verify the obtained parameters further, drop tower test on concrete plates were also performed and modeled. Numerical modeling of both SHPB samples and concrete plates were successful in capturing the observed behavior. The study also provided the literature with a reliable test data with complete parameters that can be used for further studies in the area.
Probabilistic finite element model updating and damage detection of structures by using Bayesian statistics
(01. Izmir Institute of Technology, 2022-12) 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.
Properties of hybrid fiber reinforced concrete for impact loading
(Izmir Institute of Technology, 2021-07) Alami, Mohammad Musa; Saatçi, Selçuk; Erdem, Tahir Kemal; Izmir Institute of Technology
Concrete 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.
Three dimensional numerical modelling of recharce: case study: Eğri Creek sub-basin, İzmir
(01. Izmir Institute of Technology, 2022-05) Şahin, Yavuz; Tayfur, Gökmen; Baba, Alper; Izmir Institute of Technology
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.

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