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    Evaluation of seismic response factors for BRBFs using FEMA P695 methodology
    (Elsevier Sci Ltd, 2018) Ozkilic, Yasin Onuraip; Bozkurt, Mehmet Bakir; Topkaya, Cem
    This paper reports the details of a numerical study undertaken to evaluate seismic response factors for steel buckling-restrained braced frames (BRBFs) using the FEMA P695 methodology. In the United States, BRBFs are designed according to Minimum Design Loads for Buildings and Other Structures (ASCE 7) and the Seismic Provisions for Structural Steel Buildings (AISC 341). Twenty-four archetypes were designed according to the U.S. specifications and their behavior was assessed by making use of non-simulated collapse models. The interstory drift, brace axial strain and cumulative brace axial strain demands under collapse level ground motions were determined. The results obtained indicate that the current seismic response factors are adequate in terms of interstory drift and cumulative axial strain demands. On the other hand, large differences between the design level and collapse level axial strains were reported, which can result in undesirable brace behavior. Modified approaches were developed to estimate the axial strains for collapse level ground motions. These indude a modification to the deflection amplification factor and a modification to the AISC 341 requirements for expected brace deformations. The archetypes were redesigned using the proposed modifications and reevaluated using the FEMA P695 methodology. The results indicate that the proposed modifications result in axial strain demands that are in close agreement with the calculated demands. (C) 2018 Elsevier Ltd. All rights reserved.
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    Extended end-plate connections for replaceable shear links
    (Elsevier Sci Ltd, 2021) Ozkilic, Yasin Onuralp; Topkaya, Cem
    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 these connections when they are used in MRFs or in EBFs. This paper presents an experimental and numerical study undertaken to investigate the performance of extended unstiffened and stiffened end-plate connections used in 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 end-plates designed according to the AISC guidelines or Eurocode provisions show acceptable performance in terms of the target link rotation angle. Due to strain hardening effects, thinner plates than the ones suggested by the codes were also found to show satisfactory performance. Finite element simulations were conducted to investigate the bending strains for different plate thicknesses and to study the levels of axial forces developed in the links. Modifications to the AISC design guidelines have been proposed to determine the plastic resistance of end-plated connections more accurately.
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    Frictional mid-spliced shear links for eccentrically braced frames
    (Wiley, 2023) Ozkilic, Yasin Onuralp; Un, Elif Muge; Topkaya, Cem
    According to the AISC Seismic Provisions for Structural Steel Buildings (AISC341-16) and EC8, the inelastic rotation demand at the design story drift is limited to 0.08 rad for I-shape shear links in eccentrically braced frames (EBFs). Numerical studies on EBF archetypes show that the single-sided inelastic rotation demands can be much higher than the limiting value. In addition, these links can fail due to low-cycle fatigue (LCF) which depends on the loading history. A mid-spliced end-plated detachable replaceable link has recently been developed to promote easy replacement of end-plated links. In this paper, a frictional mid-spliced shear link is developed to increase the inelastic link rotation capacity and LCF life of shear links. The proposed link utilizes a splice connection at the mid-length, where frictional faying surfaces are introduced to dissipate energy. Slip at the mid-splice connection causes a relative vertical displacement between the link ends which eventually reduces the rotation demands on the I-shape members. Experimental and numerical studies were conducted to study the proposed link concept. Three conventional and eight frictional mid-spliced links were tested using a nearly full-scale test setup. The results showed that the proposed links have a pinched link shear versus link rotation response. The links were able to sustain a link rotation demand of 0.23 rad together with a significant increase in their LCF life. Numerical studies were conducted to investigate the link rotation, interstory drift, and residual interstory drift of EBF archetypes equipped with the proposed frictional link.
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    Mid-spliced end-plated replaceable links for eccentrically braced frames
    (Elsevier Sci Ltd, 2021) Ozkilic, Yasin Onuralp; Bozkurt, Mehmet Bakur; Topkaya, Cem
    Eccentrically braced frames (EBFs) can be re-used after a major seismic event by replacing the link members. Recent years have witnessed the development of numerous replaceable links. Among various details developed for this purpose, extended end-plated replaceable links are found to be the most efficient. The use of these links enables the engineer to minimize the size and the weight of the part to be replaced. In addition, the performance of end-plated links is similar to that of conventional links. Research reported to date has shown that end-plated links have disadvantages in terms of removal and replacement. Large axial forces can develop within the link member. Removal and replacement operations may require the use of hydraulic jacks to push the ends of the collector beams and the flame cutting of the links to gradually release the residual stresses and deformations. More important are the difficulties associated with 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 channel sections welded to both parts of the replaceable link. The detail employed 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 were 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.
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    The plastic and the ultimate resistance of four-bolt extended end-plate connections
    (Elsevier Sci Ltd, 2021) Ozkilic, Yasin Onuralp; Topkaya, Cem
    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 endplates 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. (c) 2021 Elsevier Ltd. All rights reserved.
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    Stability of laterally unsupported shear links in eccentrically braced frames
    (Wiley, 2022) Ozkilic, Yasin Onuralp; Zeybek, Ozer; Topkaya, Cem
    I-shaped links in eccentrically braced frames (EBFs) are susceptible to lateral or lateral torsional buckling when subjected to cyclic link rotations. Lateral bracing should be provided at the ends of the I-shaped links in order to prevent these failures. Requirements for such braces are available in widely used design specifications such as the AISC Seismic Provisions for Structural Steel Buildings (AISC341-16) and EC8. These requirements limit the use of I-shaped links in bridge piers and elevator shafts. A combined experimental and numerical study was undertaken to investigate the behavior of laterally unsupported I-shaped links under cyclic loading. The experimental study consisted of testing of six nearly full-scale EBFs where the link length, link length ratio, and presence of lateral supports were considered as the prime variables. The test results demonstrated that short links with link length ratios less than 1.22 can experience inelastic link rotations greater than the codified limit of 0.08 rad even without lateral bracing. A numerical parametric study was conducted to develop more generalized design recommendations for laterally unsupported I-shaped shear links. Stability of one-story one-bay EBFs was studied using geometrically and materially nonlinear finite element analysis including imperfections. The numerical results showed that I-shaped links without lateral bracing can provide a stable response when the link length ratio is less than 1.15. In addition to this limit, the ratio of the elastic critical buckling capacity to the plastic shear capacity should be greater than 3.5 and 2.5 for links with and without axial force, respectively.