LEE- Yapı Mühendisliği-Doktora

Bu koleksiyon için kalıcı URI


Son Başvurular

Şimdi gösteriliyor 1 - 3 / 3
  • Öge
    Mechanical behavior of the bi-directional beams
    (Lisansüstü Eğitim Enstitüsü, 2022) Çelik, Murat ; Artan, Reha ; 693155 ; Yapı Mühendisliği
    Nanotechnology, which is one of the most developed areas of today's technology, has a widespread usage area because it can be easily integrated into many areas of engineering. The design and production of very small-scale functional materials in fields such as aerospace and aviation industry, medicine, energy and civil engineering, and their effective use in related fields make significant contributions to engineering science. The fact that these very small-scale materials used in many areas of technology have spread to almost every field of engineering has revealed the need to determine their internal structure and mechanical properties. Materials exhibit some mechanical behavior such as bending, torsion and buckling under loads. It is an important issue for the stability of the structure to be able to detect such mechanical effects in the most accurate way. In this sense, the dimensions of nanoscale structures, which are comparable to the distance between atoms that make up that material, have shown that the classical elasticity theory, which is currently widely used, is not sufficient in determining the mechanical behavior of such structures. Since the size effect is an important factor in nanoscale structures, defining a more general continuum model that includes such parameters increases the accuracy of the solution of the related problems. In this thesis, the bending and buckling behavior of beams is investigated within the framework of the strain gradient theory. In determining the mechanical behavior of nanostructures, the non-local elasticity theory, the couple stress theory and the strain gradient theory, which are proposed by respectable scientists such as Eringen and Mindlin, are widely used. The calculation load of the problem may increase depending on the loading, boundary conditions and geometric properties of the analyzed structure. Thus, the Initial Values Method is used to solve the problem in the thesis. The transport matrix approach is used for the static bending analysis of the beam within the scope of Initial Values Method. In the first part of the study, the bending behavior of the Euler-Bernoulli nano beam whose material properties change in two directions (bi-directional) is investigated. Therefore, it is assumed that the modulus of elasticity is variable along the axis and thickness of the beam. In the literature, the modulus of elasticity for a functionally graded material (FGM) is expressed in terms of arbitrary functions and the bending behavior is investigated for the first time within the scope of the gradient elasticity theory. In this context, the basic equations and boundary conditions (simply supported and fixed at both ends) are obtained with the help of Hamilton's principle for the beam under uniformly distributed load. While 4 end conditions can be written depending on the boundary conditions in the classical elasticity approach, 6 end conditions are obtained by using the gradient elasticity theory. While each loading case is required for the solution of the 6th order differential equation in the classical solution, the vertical displacements for two different types of loading cases are calculated by solving 3 linear equation systems based on 3 unknowns using the initial values method. As a result of the study, it was observed that the size effect decreased depending on the increase of nano beam length. In other words, the difference between local and non-local theory becomes important in small scales. However, it was observed that there is a lower vertical displacement in the bi-directional Euler-Bernoulli nano beam due to the increase in the inhomogeneous material constant (β). The accuracy and importance of the study for both boundary conditions are demonstrated with the help of graphics. In the second part of the study, the buckling behavior of the Euler-Bernoulli nano beam (FGM) whose material properties change in two directions (bi-directional) is investigated. Basic equations and boundary conditions are derived with the help of Hamilton's principle. Since the transport matrix cannot be calculated analytically for buckling analysis, the approximate transport matrix (Matricant) is used in the solution. Critical buckling loads are calculated for the classical theory of elasticity and the gradient elasticity theory depending on the number of intervals. In the results of study; It is observed that the first two terms of the transport matrix reflect the exact result if the number of intervals is chosen large enough. Remarkably, it is concluded that the first and second type of boundary conditions in the buckling calculation may depend on the type of material used. The accuracy of such a proposition can only be demonstrated more clearly by experimental studies. In addition, it is observed that the buckling resistance of the beam increases depending on the increase in the material characteristic length (γ). The accuracy and scientific contribution of the study is expressed with the help of the diagrams. The greatest contribution of the subject investigated herein to science is that it can guide the analysis and design of nanostructures used in many areas of technology. The importance of parameters such as the size effect and inhomogeneous material coefficient in the analysis of very small-scale structures has once again been demonstrated with strong propositions and results. Thus, it is supported by previous studies that theories such as the non-local elasticity theory and the gradient elasticity theory give more realistic results compare to the classical theory. With the design of micro and nano electro-mechanical systems (MEMS and NEMS), which are frequently encountered in the electronic device industry, as functional graded materials, it has become more important to determine the mechanical properties of such small-scale structures. In this sense, it is aimed that the relevant thesis study and the international publications published by us will make significant contributions to the literature.
  • Öge
    Assessment of seismic performance of RC members after fire exposure through large-scale testing
    ( 2020) Demir, Uğur ; İlki, Alper ; Green, F. Mark ; 648701 ; İnşaat Mühendisliği Ana Bilim Dalı
    Quantifying the seismic resistance of reinforced concrete (RC) buildings after fire is currently difficult because of the lack of information regarding their strength and ductility under earthquake loads. This thesis presents the results of an experimental study, which was carried out to investigate the post-fire seismic behavior of reinforced concrete columns. The thesis is mainly comprised of three papers accepted by high quality journals. In the first chapter, the factors affecting the post-fire seismic behavior of RC columns are analyzed with a particular focus on the behavior of concrete and steel under elevated temperatures and after cooling. In the second chapter, post-fire seismic behavior of cast-in-situ RC columns are investigated. Five cast-in-place RC columns were tested to failure under constant axial load and reversed cyclic lateral displacements after being exposed to ISO-834 standard fire for 30, 60 or 90 minutes. All of the columns are full-scale and designed to behave in flexure-controlled manner complying with major design codes (e.g. ACI 318-14). Other than the effects of fire exposure durations, the effects of thickness of concrete cover (25 and 40 mm) on structural performance was also investigated for the short fire exposure duration (30 minutes). The responses of the columns are analyzed in terms of lateral load-displacement relationships, ductility, stiffness, energy dissipation capacities and residual displacements. The test results indicated that fire exposure reduced the lateral load capacity of the columns whereas the deformability capabilities were found to be satisfactory in terms of structural response. It was also seen that the thickness of the concrete cover has only a slight influence on the post-fire seismic behavior of the columns which is attributed to the fact that lower concrete cover thickness results in higher effective depth which in turn leads to higher bending moment capacity and thereby higher lateral load capacity. Furthermore, a theoretical study was conducted to predict the load-displacement response of the fire exposed columns. The comparison of the experimentally and theoretically obtained load-displacement relationships indicated that the principals of structural mechanics usually applied to conventional columns are also valid for the columns exposed to fire in case the proposed algorithm is followed. In the third chapter, a similar approach as stated for the cast-in-situ columns is followed for precast RC columns as well. The precast columns had the same cross-section and reinforcement configuration and had been exposed to the same fire scenarios with the cast-in-situ columns. These columns were inserted into a socket foundation and had a lower axial load ratio (i.e. 10%) in order to represent the condition in common industrial buildings. The findings indicated that the repair mortar between the columns and foundation behaved in a satisfactory manner and therefore, similar post-fire seismic behavior was observed for cast-in-situ and precast columns. In the fourth chapter of the thesis, an experimental study is presented to examine the impact of time after fire on (i) post-fire behavior of small-scale specimens (cubes and cylinders), (ii) seismic behavior of full-scale reinforced concrete columns. The post-fire seismic response of the columns are analyzed 30, 60 and 360 days after fire exposure. Impact of time after fire exposure on residual lateral load capacity and ductility of the columns was found to be limited while the column subjected to seismic test 30 days after fire exposure, exhibited less stiff behavior with respect to the columns tested later. Furthermore, an analytical study is conducted for the prediction of seismic behavior of reinforced concrete columns after fire exposure considering the variations in residual properties of concrete by time, and the proposed model is found to be in good agreement with the test results.
  • Öge
    Alkali ile aktive edilmiş harç ve atık çelik tel donatılı betonların (SIFCON) fiziksel, mekanik ve dayanıklılık özelliklerinin araştırılması
    (Lisansüstü Eğitim Enstitüsü, 2020-12-11) Gök, Saadet Gökçe ; Şengül, Özkan ; 501132005 ; Yapı Mühendisliği ; Structural Engineering
    Geleneksel beton üretimi, günden güne büyümekte olan büyük ölçekli bir endüstridir. Çimento üretimi sırasında açığa çıkan yüksek miktarlardaki karbondioksit emisyonu ve üretim için ihtiyaç duyulan büyük enerji ihtiyacı, bu süreci maliyetli, kirletici ve çevreye zararlı hale getirmektedir. Bu durum, uygun işlenebilirlik, dayanım ve dayanıklılık özelliklerine sahip olmanın yanı sıra ekonomik de olan alternatif yapı malzemeleri arayışına ihtiyaç doğurmaktadır. Sürdürülebilir bir beton üretimi için, sürdürülebilirliğin temel ayakları olan çevresel, ekonomik ve sosyal açıdan iyileştirme gerekmektedir. Farklı sanayilerden elde edilen yan ürünlerin veya atık ürünlerin betonda yeniden değerlendirilmesi, söz konusu sürece dikkate değer bir katkı sağlayacaktır. Alkali ile aktifleştirilmiş malzemeler, sürdürülebilir olmaları ve çimento içermeyişleriyle öne çıkan birtakım olumlu özelliklere sahip doğa dostu malzemelerdir. Bu araştırmada, atık ya da yan ürünlerin kullanımıyla bir malzeme üretmek amaçlanmış olup öğütülmüş yüksek fırın cürufu; sodyum hidroksit (NaOH) çözeltisi ve sıvı sodyum silikat kullanılarak aktive edilmiştir. Sodyum silikat yerine, silikat kaynağı olarak öğütülmüş atık camın kullanılabilirliği araştırılmıştır. Bu amaçla, sodyum silikat, öğütülmüş atık cam ile ikame edilerek değişen molaritelerde (8-14 M) NaOH içeren harç numuneler üretilmiştir. Alkali ile aktifleştirilmiş harçların mekanik ve dürabilite özellikleri, su kürü, hava kürü ve ısıl işlem (24 saat 60˚C) olmak üzere üç farklı kür koşulunda incelenmiştir. Referans karışımlarda, CEM I 42,5 R Portland çimentosu, su ve standart kum kullanılmıştır. Üretilen numunelerin 3, 7, 28 ve 90 günlük basınç dayanımları ve eğilmede çekme dayanımları ölçülmüştür. Bununla birlikte harçlar üzerinde, sülfat direnci, donma-çözülme, kılcal su emme, toplam su emme miktarının belirlenmesi, hızlı klor geçirimliliği ve elektriksel özdirenç deneyleri gerçekleştirilmiştir. Çalışmanın sonunda, atık camın alkali ile aktifleştirilmiş harçlarda alternatif silikat kaynağı olarak kullanılabileceği sonucuna varılmıştır. Çalışmanın diğer aşamasında alkali ile aktive edilmiş beton üretimi gerçekleştirilmiş ve alkali ile aktifleştirilmiş beton numunelerin geçirimliliğini incelemek amacıyla kılcal su emme, toplam su emme miktarı, hızlı klor geçirimliliği ve elektriksel özdirenç deneyleri gerçekleştirilmiştir. Çalışmanın son aşamasında çimento bulamacı emdirilmiş lifli beton üretimi gerçekleştirilmiştir. Burada, Portland çimentosu ile yapılan üretimin yanı sıra, 14 M sodyum hidroksit çözeltisi ve sıvı sodyum silikat, 8 M sodyum hidroksit çözeltisi ve sıvı sodyum silikat, 14 M sodyum hidroksit çözeltisi ve öğütülmüş atık cam, 8 M sodyum hidroksit çözeltisi ve öğütülmüş atık cam aktivatör olarak kullanılarak yüksek fırın cürufu ve ince kum ile üretim gerçekleştirilmiştir. Kullanılmış araç lastiklerinin geri dönüştürülmesiyle elde edilen hurda çelik teller, çelik lif olarak kullanılmıştır. Yanak teli (sürekli lif) ve kırılmış tel (süreksiz lif) olmak üzere iki farklı lif tipinin etkisi incelenmiştir. Çelik tel olarak atık lastiklerin içerisinde bulunan çeliğin seçilmesindeki sebep, bu tellerin yüksek çekme dayanımına sahip olması ve büyük miktarlarda kullanılması gerektiğinde, temininde bir zorlukla karşılaşılmayacak olmasıdır. Kullanılamaz duruma gelen, otomobil ve kamyon/tır lastiklerinin miktarı göz önünde bulundurulduğunda, bu lastikleri depolamak ve bertaraf etmek bir sorun haline gelmektedir. Türkiye'de, bu telleri ayrıştırmada ve malzemenin geri kazanımını sağlamada, piroliz ve geri dönüşüm hizmeti veren çok sayıda firma mevcuttur. Çalışmada, farklı geri dönüşüm firmalarından temin edilen hurda çelik tellere ait boy/çap dağılımları histogram olarak verilmiştir. Üretilen lifli betonlarda fiber hacim oranı %0, 1, 2, 3, 4 ve 5 olarak değişmektedir. Üretimi gerçekleştirilen numunelerde deplasman kontrollü üç noktalı eğilme deneyi yapılmış ve yük-sehim grafikleri elde edilmiş; numunelerin eğilmede çekme, yarmada çekme ve basınç dayanımları belirlenmiş, tokluk ve kırılma enerjisi değerleri hesaplanmıştır. Atık çelik liflerin kullanımı, eğilme dayanımlarını ve toklukları arttırmıştır. Kullanılan atık çelik lifler, uygulamada kullanılan ticari liflere benzer sonuçlar vermiştir.