Çelik, Cam FRP ve Hibrit Donatılı Betonarme Kirişlerin Eğilme Performansının İncelenmesi

Elyaf takviyeli polimer (FRP) donatılar korozyona karşı direnç, ağırlıkça yüksek mukavemet oranı, iyi yorulma özellikleri ve kullanım kolaylığı gibi avantajlar nedeniyle, alternatif olarak tercih edilmekte ve kullanımı giderek artmaktadır. İnşaat mühendisliği uygulamalarında düşük maliyetlerinden dolayı en yaygın olarak Cam-FRP (GFRP) donatılar kullanılmaktadır. Son zamanlarda, kiriş ve kolonlarda kullanılan çelik donatılar yerine Cam FRP donatılar ve çelik ile Cam FRP donatının birlikte kullanıldığı “Hibrit” donatı düzeni üzerinde birçok araştırma yapılmıştır. Bu çalışmada; Cam FRP ve Hibrit donatıların betonarme kirişlerde geleneksel çelik donatılara göre avantajları araştırılmıştır. Çelik çubuklardan ve farklı sayıda cam çubuklardan oluşan betonarme kirişler üretilmiştir. Üretilen betonarme kirişlere dört nokta eğilme testleri yapılmıştır. Deney sonucu her üç numune için yaklaşık eşit maksimum yük taşıma kapasitesi elde edilmiştir. Bununla birlikte, çelik donatılı ve hibrit donatılı kiriş, gevrek bir davranış sergilerken, GFRP donatılı kiriş sünek bir davranış sergilemiştir. Tamamen GFRP donatılı numunenin enerji emme kapasitesi diğer numunelere göre ciddi oranda artış gösterdi. Numunelerde oluşan hasarları açıklamak için hasar analizi sunuldu

Crack sliding model for non-shear FRP-reinforced slender concrete elements under shear

Fiber-reinforced polymer (FRP)-reinforced concrete (RC) elements fail under one-way shear in a devastating and complicated manner with no adequate warning. In recent decades, there has been pioneering research in this area; however, there is no agreement among researchers regarding mechanically-based models. Thus, in this current study, a plasticity-based model is developed for FRP-RC elements under shear. A selected model was firstly assessed for its accuracy, consistency, and safety against an extensive experimental database. Secondly, a plasticity-based model (i.e., crack shear sliding model) was adapted, refined, and proposed for FRP-RC elements under one-way shear. The two proposed models were found to be reliable and more accurate with respect to selected existing methods. Modeling of FRP’s axial rigidity is more consistent only under Young’s modulus with respect to the experimental database. Several concluding remarks on the selected existing models are outlined and discussed to assist the future development of these models and design codes

Analytical review on potential use of waste engine oil in asphalt and pavement engineering

This article provides a comprehensive overview of the research utilising waste engine oil (WEO) in asphalt binders for multiple application purposes and the economic and environmental implications. It covers the various types and sources of WEO for information on their characteristics and the process of preparing WEO-asphalt binders. The study collects the effects of WEO in different applications, including asphalt modification, aged asphalt rejuvenation, self-healing asphalt agents, and WEO composites. It also discusses works on the economic and environmental appraisal associated with a wide WEO utilisation. WEO exhibits both positive and negative influences on asphalt properties. Generally, it improves the performance of asphalt at low temperatures, specifically in terms of reducing issues like thermal cracking and enhancing fatigue resistance. However, it may have a negative impact on the performance of modified asphalt binders at high temperatures. WEO, due to its high concentration of light components, improves the overall performance of aged asphalt. The integration of WEO and reclaimed asphalt binders can enhance the crack resistance, which however highly relies on the added WEO quantity. Future research should be prioritised to understand the comprehensive impact of WEO on reclaimed asphalt binders for the compatibility between the rejuvenator compound and reclaimed aged asphalt, and the effect of WEO on the durability of modified asphalt mixes. In addition, field investigations and analyses are required for a bigger, inclusive, and more detailed picture of the economic and environmental impacts

Use of calcium carbonate nanoparticles in production of nano-engineered foamed concrete

Researchers have shown significant interest in the incorporation of nanoscale components into concrete, primarily driven by the unique properties exhibited by these nanoelements. A nanoparticle comprises numerous atoms arranged in a cluster ranging from 10 nm to 100 nm in size. The brittleness of foamed concrete (FC) can be effectively mitigated by incorporating nanoparticles, thereby improving its overall properties. The objective of this investigation is to analyze the effects of incorporating calcium carbonate nanoparticles (CCNPs) into FC on its mechanical and durability properties. FC had a 750 kg/m3 density, which was achieved using a binder-filler ratio of 1:1.5 and a water-to-binder ratio of 0.45. The CCNPs material exhibited a purity level of 99.5% and possessed a fixed grain size of 40 nm. A total of seven mixes were prepared, incorporating CCNPs in FC mixes at the specific weight fractions of 0% (control), 1%, 2%, 3%, 4%, 5%, and 6%. The properties that were assessed included the slump, bulk density, flexural strength, splitting tensile strength, compressive strength, permeable porosity, water absorption, drying shrinkage, softening coefficient, and microstructural characterization. The results suggested that incorporating CCNPs into FC enhanced its mechanical and durability properties, with the most optimal improvement observed at the CCNPs addition of 4%. In comparison to the control specimen, it was witnessed that specimens containing 4% CCNPs demonstrated remarkably higher capacities in the compressive, splitting tensile, and flexural tests, with the increases of 66%, 52%, and 59%, respectively. The addition of CCNPs resulted in an improvement in the FC porosity and water absorption. However, it also led to a decrease in the workability of the mixtures. Furthermore, the study provided the correlations between the compressive strength and splitting tensile strength, as well as the correlations between the compressive strength and flexural strength. In addition, an artificial neural network approach was employed, utilizing k-fold cross-validation, to predict the compressive strength. The confirmation of the property enhancement was made through the utilization of a scanning electron microscope

A comparative study on yield line mechanisms for four bolted extended end-plated connection

Extended end-plated connections are preferred in moment resisting frames due to their advantages such as no required in-situ welding, accurate fabrication and economic feasibility compared to flange welded moment connections. The capacity of the extended end-plated connections depends on bolt configurations, end-plate thickness, bolt diameter and their material properties excluding column part. The thickness of end-plate can be computed using yield line mechanisms. Different yield line patterns are available in the literature and some of these are adopted in seismic codes to estimate the thickness of end-plate. In this study, the accuracy of different yield line patterns is compared using collected experimental data and numerical analysis. A parametric numerical analysis was conducted utilizing the finite element tool, ABAQUS. The results of experimental data and parametric study were evaluated for both unstiffened and stiffened four bolted extended end-plated connections. The results revealed that the capacity of the end-plate connections significantly depends on the yield line mechanism. Therefore, selecting an accurate yield line mechanism is essential in order not to overestimate the thickness of the end-plate. More importantly is that these yield line mechanisms can be directly implemented to AISC 358 and Turkish Building Earthquake Code 2018 (TBEC-2018)

The Effects of Steel Core Imperfection, Gap Size and Friction Coefficient on the Behavior of All-Steel Buckling Restrained Braces

Buckling restrained braces (BRBs) which are generally composed of a steel core and a encasing(buckling restrainers) are utilized to resist lateral forces in high seismic regions since BRBs exhibit high energy dissipation capacity, ductility and stiffness. The steel core carries both compressive and tensile forces. During the compression, the core starts buckling and the encasing tries to prevent this buckling. However, due to the unbonding layer/gap between the encasing and steel core, the steel core eventually buckles and contacts with the encasing. Buckling phenomenon is also associated with the initial imperfection and gap size. In this study, the effects of the initial imperfectionof steel core, gap size (1-5 mm)and friction coefficient (0.01-0.5)between the encasing and steel core on the behavior of BRBs are investigated. Pursuant to this goal, numerical analyses using a finite element tool ABAQUS were conducted. A total of 19 numerical models were developed and monotonically loaded. Initial imperfection was implemented to the models using buckling mode shapes.The results revealed that increasing gap size leads to a reduction in load-carrying capacity. It is recommended to keep gap sizes between 1 and 2 mm. On the other hand, initial imperfection does not significantly affect load-carrying capacity and global behavior. However, it was also observed that the fluctuations in load increase as the amplitude of the mode shape and gap size increase. Moreover, the friction coefficient should be kept between 0.01 and 0.05; otherwise, undesired behaviors can be observed

Burkulması önlenmiş çelik çaprazlı çerçevelerin sismik performans faktörlerinin değerlendirilmesi.

Buckling restrained braces (BRBs) have been used recently in seismic zones due to their high energy dissipation capacity. Buckling restrained braces can yield under both tension and compression without buckling and they have balanced hysteresis loops. In this thesis, seismic performance factors given in ASCE 7-10 Minimum Design Loads for Buildings and Other Structures for buckling restrained braced frames (BRBFs) were evaluated. Pursuant to this aim, twenty four archetypes composed of single diagonal and chevron BRBFs were designed according to the United States provisions. The archetypes were subjected to Maximum Considered Earthquake (MCE) ground motions using 22 sets of Far Field records defined in FEMA P695. Elastic and inelastic time history analysis of these archetypes were conducted by using Opensees software. Cumulative brace axial strains, brace axial strains and interstory drift ratios were obtained from numerical analysis and compared with the requirements of AISC 341-10. The results show that the current codified value of deflection amplification factor under-estimates the deflection of lower stories and over-estimates the deflection of upper stories. To address this issue, deflection amplification factors that vary over the height of the structure was proposed. Archetypes were redesigned using proposed modifications and reevaluated using FEMA P695 Methodology. The results indicate that proposed modification was adequate to satisfy the requirements of AISC 341-10.  M.S. - Master of Scienc

Dış merkez çelik çaprazlı perdelerde değiştirilebilir bağ kirişleri için deneysel ve numerik çalışmalar.

Eccentrically braced frames (EBFs) are extensively used as a steel lateral load resisting systems in high seismic regions since EBFs simulate ductility and high energy absorption capacity of moment resisting frames (MRFs) and high stiffness of concentrically braced frames (CBFs). High stiffness and high ductility of EBFs are obtained from diagonal braces and yielding of link element, respectively. This thesis reports findings of a three phase experimental and numerical research program on replaceable links for EBFs. The first research program was conducted to investigate four-bolt extended end-plate connections for replaceable shear links. Extended end-plate moment connections are used in a number of applications including the beam-to-column connections in seismic moment resisting frames (MRFs) and replaceable link-to-frame connections in eccentrically braced frames (EBFs). While the extended end-plate connections have been extensively studied for MRF applications, little is known about their performance in EBFs. The loading conditions and the acceptance criterion are different for the same connection when used in MRFs or EBFs. An experimental and numerical study has been undertaken to investigate the performance of four-bolt extended unstiffened and stiffened end-plate connections used for replaceable shear links. Pursuant to this goal, 10 nearly full-scale EBF tests were conducted where the thickness, width and stiffening of the end-plate were considered as the variables. The results showed that the design recommendations given in AISC guidelines and Eurocode provisions provide conservative estimates of the end-plate thickness. Finite element simulations were conducted to investigate the bending strains for different plate thicknesses and to determine the sources of conservatism in the capacities determined using the design guidelines. Modifications to the AISC design guidelines were proposed to more accurately determine the required end-plate thickness. The second research program was performed to develop novel detachable links for eccentrically braced frames. Post-earthquake replacement of links enables the use of eccentrically braced frames (EBFs) after a seismic event. Recent years have witnessed the development of numerous replaceable links. The extended end-plated replaceable links are the most efficient among the developed details. The use of these links enables to minimize the size and the weight of the part to be replaced. In addition, the performance of end-plated links is similar to the conventional links. Research reported to date showed that these links have disadvantages in terms of removal and replacement. Large axial forces can develop within the link member which may require using hydraulic jacks for the removal operation. More importantly is the difficulties associated with the link replacement under residual frame drifts. A novel detachable replaceable link is proposed in this study which employs a splice connection at the mid-length of the link. The splice connection consists of saw cut I-sections welded to both parts of the replaceable link. The detail provides an erection tolerance which facilitates easy removal and enables replacement under residual frame drifts. Proof-of-concept testing of the proposed links was performed on 3 specimens where the type of force transfer in the splice connection was considered as the prime variable. All specimens failed at link rotation angles that are significantly higher than the link rotation angle required by AISC341 and demonstrated the potential of the proposed link concept. Complementary finite element parametric studies were conducted to validate the design procedure developed for the proposed replaceable link concept. The third research program was carried out to enhance replaceable links by introducing frictional dampers. The main target was to extend the low-cycle fatigue life of the replaceable links by dissipating energy through the link yielding and frictional resistance provided faying surfaces of the side plates. Pursuant to this goal, brass shims were placed between saw cut I-sections and side plates which were drilled with slotted holes, and bearing type connection was utilized to promote bolt slippage. The proposed frictional mid-spliced connection was validated through a comprehensive experimental study. A total of eight specimens with proposed connection were tested considering slot size, the number of bolts and the number of slotted holes as the primary parameters. Moreover, three specimens with extended end plated connection were tested to compare low-cycle fatigue life with the link having the proposed mid-spliced connection. The link with proposed frictional mid-spliced connections exhibited significantly higher rotation capacity that the required link rotation. Furthermore, the link with the proposed connection excessively increased low-cycle fatigue life and energy dissipation capacity of links.Thesis (Ph.D.) -- Graduate School of Natural and Applied Sciences. Civil Engineering

Buckling resistance of the cylindrical shells with two secondary stiffening rings under external pressure

The common way to amplify strength and stiffness of the tank wall is to use the stiffening rings. These stiffening rings can be classified as the primary and secondary stiffening rings (PSRs and SSRs). PSR is placed around the top of the tank shell and it assists to avoid ovalization at the top when the open-top tank is exposed to the external pressure. PSR having small dimensions may be used for fixed roof tanks since the roof system attached to the top of the cylindrical shell provides a natural restraint at this location. On the other hand, one or more SSRs may be required to preclude local buckling in both fixed and open-top cylindrical steel tanks (CSTs) which are exposed to external pressure. The requirements of single secondary stiffening ring have been investigated in detail in previous studies. However, in same cases, a single SSR does not provide sufficient resistance to maintain stability over the entire shell height. Thus, buckling capacity of the CSTs with two identical secondary stiffening rings have been explored in this present work. Pursuant to this aim, the requirements for stiffness of the SSRs which are given in terms of minimum second moment of area (SMA) were evaluated by performing Linear Elastic Bifurcation Analysis (LBA) of CSTs under uniform external pressure. Analysis results show that minimum SMA expression proposed by Blackler underestimates critical buckling value for the tanks, having especially low height-to-diameter ratios and low radius-to-thickness ratios. Furthermore, to trace strength reduction in the tank due to geometrical imperfections, Geometrically Nonlinear Analysis including Imperfections (GNIA) was performed. Analysis findings affirm that thin-walled structures are very sensitive to geometrical imperfections

The plastic and the ultimate resistance of four-bolt extended end-plate connections

Widely used design guidelines and specifications provide methods to calculate plastic resistance of end-plates considering yield line mechanisms. Experiments reported to date showed that the ultimate resistance of end-plates can be considerably larger than its plastic resistance due to strain hardening. Using the ultimate strength in design can be a viable and cost-effective option; however, limited studies exist on quantifying the ultimate strength. In this paper, a database of four-bolt extended end-plate moment connections was developed. The capacities of these connections were calculated using AISC Design Guidelines and EN1993-1-8. The results showed that the plastic resistances of connections are on average 29% greater than the capacities calculated using the AISC Design Guidelines. On the other hand, the test-to-predicted ratios were found to have an average of 1.08 for EN1993-1-8. The ratios of the ultimate resistance to the calculated plastic resistance are 2.07 and 1.73 for AISC and European approaches, respectively. A numerical study was undertaken to propose modified design expressions for the plastic resistance which are compatible with AISC Guidelines. Furthermore, the ultimate resistance of end-plates was quantified. Stiffened and unstiffened T-stub models were analyzed using the finite element method. Expressions based on yield line mechanisms were developed to calculate the plastic and ultimate resistance. The evaluations showed that the averages of the test-to-predicted ratios are 1.06 and 1.17 for the plastic and ultimate resistance respectively when the proposed expressions are used