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    A Practical Methodology to Estimate Site Fundamental Periods Based on the KiK-net Borehole Velocity Profiles and Its Application to Istanbul
    (Seismological Soc Amer, 2022) Gullu, Ahmet; Hasanoglu, Serkan; Yuksel, Ercan
    Determination of site fundamental periods is remarkably important to classify soil deposits and to identify the resonant probability of any structures during an earthquake. Recent developments in the literature exposed that the fundamental period is a better proxy than time-averaged velocity to 30 m (VS30) or the best complementary parameter to VS30 to evaluate the soil characteristics. Because great efforts have been paid to achieve VS30 maps of many regions and countries, an approximate methodology based on this parameter and engineering bedrock depth (Zbr), in which the shear-wave velocity reaches 760 m/s (Z0.76) or 1000 m/s (Z1.0), is presented here to find out the site fundamental periods. The meth-odology is developed and verified using the Kiban Kyoshin network database. Outcomes of the proposed methodology are also compared with some of the literature equations and methods. The comparative studies resulted in a great correlation with a relatively low standard deviation. Therefore, it is conceivable to apply the proposed methodology easily to the regions where VS30 and engineering bedrock depth are already known.
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    Characterization of energy dissipative cushions made of Ni-Ti shape memory alloy
    (Iop Publishing Ltd, 2022) Gullu, Ahmet; Danquah, Josiah Owusu; Dilibal, Savas
    Earthquake-resistant design of structures requires dissipating seismic energy by deformations of structural members or additional fuse elements. Owing to its easy-to-produce, plug-and-play, high equivalent damping ratio, and large displacement capacity characteristics, energy dissipative steel cushions (SCs) were found to be an efficient candidate for this purpose. However, similar to other conventional metallic dampers, residual displacement after a strong shaking is the most notable drawback of the SCs. In this work, cushions produced from Ni-Ti shape memory alloy (SMA) are evaluated numerically by experimentally verified finite element models to assess their impact on the performance of earthquake-resistant structures. Furthermore, a reinforced concrete testing frame is retrofitted with energy dissipative steel and Ni-Ti cushions. Performance of the frames (e.g. dissipated energy by the cushions, hysteretic energy to input energy ratio, maximum drift, and residual drift) with different types of cushions are evaluated by nonlinear response history analyses. The numerical results showed that the SCs are effective to reduce peak responses, while Ni-Ti cushions are more favorable to reduce residual drifts and deformations. Hence, a hybrid system, employing the steel and SMA cushions together, is proposed to reach optimal seismic performance.
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    Closed-Form Demonstration and Shake Table Verification of Damping Effect on the Seismic Energy
    (World Scientific Publ Co Pte Ltd, 2022) Gullu, Ahmet; Yuksel, Ercan; Altinta, Elif
    This paper discusses closed-form demonstrations of the damping effect for a basic mass, spring, and damper (MSD) and a single degree of freedom (SDOF) system that are exposed to harmonic loading. Initially, the energy balance equations of the systems were solved in closed form by considering the damping ratio and loading frequency. Verification of the solutions obtained for SDOF systems is achieved by shake table tests. Based on the analytical and experimental results, it was found that the damping effect is highly related to the ratio of loading frequency to the natural vibrational frequency of the system. For the lower and higher values of the ratio, damping is found to be almost ineffective. However, the effect becomes substantial when the ratio reaches unity i.e. at the resonant frequency. Damping has a reverse relation with seismic energy at the dominant frequency. In contrast, the relation is proportional in the vicinity of resonance frequencies. Hence, considering the damping as a parameter for the energy-based design of structures is suggested.
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    Cyclic behavior of reinforced concrete cladding panels connected with energy dissipative steel cushions
    (Elsevier Sci Ltd, 2019) Karadogan, Faruk; Yuksel, Ercan; Khajehdehi, Arastoo; Ozkaynak, Hasan; Gullu, Ahmet; Senol, Erkan
    Precast concrete structures show damage after the destructive earthquakes and indicate that the connections of reinforced concrete (RC) cladding panels might be inadequate. RC cladding panels greatly increase the lateral stiffness and strength of the building when they are rigidly connected to the structural system. However, this also increases the seismic requirements. Consequently, a robust mechanical connection device with energy-dissipating capability was produced for RC cladding panels. Extensive experimental and numerical studies on an energy-dissipative steel cushion (SC) connection device were carried out in the framework of the SAFECLADDING project. Cladding panel tests were conducted with various connection configurations. The fundamental variables are the location, quantity, and thickness of SCs used in the cladding systems. The test results demonstrate that the SCs used in panel-to-panel and panel-to-support connections made large contributions to the total energy dissipation capacity. The parameters of a numerical model were also evaluated to reproduce the experimental results.
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    Damping effect on seismic input energy and its verification by shake table tests
    (Sage Publications Inc, 2021) Gullu, Ahmet; Yuksel, Ercan; Yalcin, Cem; Buyukozturk, Oral
    Seismic input energy per unit mass (E-I/m) imparted into a structure is a function of earthquake (duration, frequency content, amplitude etc.), soil (shear velocity, dominant period etc.) and the structural (vibrational periods etc.) characteristics. Generally, the damping properties of the structure is assumed negligible for seismic input energy. Most of the existing spectral equations derived for SDOF systems generally use a constant damping ratio of 5%. In this study, the damping effect on E-I/m is investigated experimentally and numerically on SDOF systems with distinct damping ratios. Experimental investigation and numerical computations proved that seismic input energy is very sensitive to variation of damping within the vicinity of fundamental frequencies. Specifically, up to 50% increment was observed in the plateau region of the input energy spectrum, where maximum E-I/m values occur, by variation of damping from 2% to 10%. Hence, a novel damping modification factor (DMF), which could be utilized for existing energy spectra, is proposed in this paper. Validation studies of the proposed DMF are achieved through the various energy spectra found in the literature.
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    Multi-objective optimal sizing of energy dissipative steel cushions for longitudinal loading
    (Springer, 2021) Gullu, Ahmet; Korpeoglu, Seda Goktepe; Selek Kilicarslan, Elif Sila
    Contemporary approach in earthquake engineering tends to dissipate some part of seismic energy by additional fuses. Owing to its easy-to-produce and plug-and-play characteristics, energy dissipative steel cushion (SC) can be a prominent candidate to serve this purpose. Since its closed form design equations are already available in the literature, seismic performance and energy dissipative characteristics of SC might be improved by optimal sizing. Hence, distinct mathematical optimization techniques, namely sequential quadratic programming, gradient-based method, and Lagrange multiplier method, are employed. Results of the optimization techniques are evaluated through experimentally verified finite element analyses. Consequently, some geometric dimension ratios are provided for the optimal sizing of SCs. Comparisons between the optimization studies yielded that the gradient-based method requires fewer function evaluations to converge while the Lagrange multiplier method with a Hessian produces more accurate results.
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    Piecewise exact solution of the seismic energy balance equation and its verification by shake table tests
    (Springernature, 2022) Gullu, Ahmet; Yuksel, Ercan
    The seismic energy-based design concept is attracting increasing attention due to its known advantages such as counting for frequency content of earthquake and duration-related cumulative damage. The concept requires the solution of a relatively complex integration namely the energy balance equation. Thus, some researchers have preferred to use equivalent parameters (e.g. spectral velocity) and prediction equations for the determination of seismic energy. In this study, a piecewise integration technique is proposed to achieve the exact solution of the energy balance equation. The proposed algorithm was validated through shake table tests conducted on the single degree of freedom (SDOF) and multi-degree of freedom (MDOF) systems in elastic and inelastic ranges, as well as analyses of the nonlinear response history of a benchmark frame. To evaluate the efficiency of the proposed solution technique, two MDOF specimens were supplemented by metallic dampers to have discrete damping properties. The seismic energy responses of all specimens with and without metallic dampers were determined satisfactorily. A maximum relative difference of 15% was obtained between the algorithm and the results of the experimental and numerical examples used for the validation.
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    Simultaneous Multi-Objective Optimal Sizing of Energy Dissipative Steel Cushions for Transversal Loading
    (World Scientific Publ Co Pte Ltd, 2022) Gullu, Ahmet; Korpeoglu, Seda Goktepe
    Current practice in structural engineering requires dissipating some part of seismic energy by sacrificial elements rather than structural members and/or joints. As an easy-to-produce and plug-and-play damper with stable hysteretic loops and large displacement capacity, steel cushion (SC) is a significant device to improve the seismic performance of the structures. It was stated in the previous studies that SC is less effective in the transversal direction. Hence, the efficiency of the damper is improved by optimal sizing through intelligent optimization techniques in this study. The complex optimization problem could be converted to a relatively simple mathematical problem since closed-form equations of the damper are exist in the literature. The optimal sizing problem was solved using two distinct methods namely the 6-constraint method and the elitist non-dominated sorting genetic algorithm (NSGA-II). The employed optimization methods were verified by each other as almost similar geometric ratios were obtained. The efficiency of the optimization is evaluated through finite element analysis (PEA). It is shown that the optimally sized SC is superior in terms of energy dissipation.