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To improve the behavior of building structures subjected to lateral loads, such as wind and earthquake excitations, tuned mass damper (TMD) has been used extensively theoretically and experimentally in previous researches. To increase the effectiveness of TMD mechanism, different methods have been proposed to determine the optimal values of TMD parameters including its mass, stiffness and damping. In using single TMD on the structures subjected to external vibrations, the mistuning of TMD, variation of TMD damping and changes in structural dynamic characteristics cause significant reduction in the effectiveness of TMD. Multiple tuned mass dampers (MTMDs) have been proposed to overcome the shortcomings of single TMD where each TMD has different dynamic characteristics. Based on the results of different researches, it has been concluded that the performance of MTMDs is less sensitive to uncertainty of structural dynamic parameters than that of a single TMD. In the previous researches, for designing MTMDs on the linear structures subjected to various external excitations, several methods have been proposed based on different kinds of design criteria. In most of the proposed methods, to simplify the design procedure of MTMDs, some limitations such as identical masses and damping ratios for TMDs or uniform distribution for the frequency or damping of TMDs have been considered. Also these methods require extensive numerical analysis. To generalize the design problem of MTMDs, in this paper, an effective method has been proposed for optimal design of MTMDs on the multi-degree-offreedom linear structures subjected to any desired excitation. In this method, an optimization problem is defined for designing the optimal MTMDs. The minimization of the maximum displacement of structure is considered as objective function and the parameters of TMDs are considered as variables. Since the design problem includes a large number of variables, hence, in this paper, it has been decided to use Genetic Algorithm (GA) for solving the optimization problem. To illustrate the procedure of the proposed method and also to assess the effectiveness of MTMDs in improving the seismic behavior of structures, a ten–storey linear shear building frame was subjected to white noise excitation and for different values of TMDs mass ratio and TMDs number, optimal MTMDs were designed for minimizing the maximum displacement of structure. To focus on the main objective of this paper and avoid the complexity of the problem, TMDs were located on the top floor in parallel configuration. The results of numerical simulations showed the capability of GA in solving complex MTMDsdesign problem with a large number of variables as well as the simplicity of the method under any desired external excitation. Also it was concluded that increasing of the mass ratio of TMDs could improve the effectiveness of MTMDs. To assess the performance of optimal MTMDs under other earthquakes, which are different in characteristics with design record, optimal structure-MTMDs was tested under near-fault and far-fault earthquakes and the results have been reported.

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Recent damaging earthquakes in Iran and around the world have induced great death and damage providing serious reminders of seismic vulnerability of existing structures. It is more crucial in Iran, where many structures have been built when seismic codes were not effective enough, specially considering the fact that construction is not perfectly consistent with design specifications and drawings Many of existing building, therefore, have inadequate strength when subjected to earthquake. To prevent such damage and tragic event, efficient ways are necessary for retrofitting these buildings. One of useful extensively used methods is passive control. By reducing seismic demand and increasing ductility, these control ways can reduce rate of seismic damage. Seismic resisting structures are expected to maintain adequate stiffness during frequent but moderate excitations on one hand, and to dissipate a large amount of energy under damaging earthquake on the other hand. The conventional framing systems, i.e., concentrically braced frames, and moment frames are not able to satisfy two aforementioned requirements instantaneously. The concentrically braced frames usually possess high stiffness, but poor ductility owing to the buckling of the compression diagonal braces. On the contrary, the steel moment frames can show acceptable ductility and energy dissipation capacity through flexural yielding in beams, while their stiffness is limited. A combination of these two systems can make a balance between requirements concerning stiffness and energy dissipation capacity. One of the most effective mechanisms available for dissipating seismic energy through inelastic deformations of metallic substances is use of shear panels. Use of yielding dampers has been increased recently for their high capability in energy dissipation. Due to recent advances in passive control methods specifically significant improvements in earthquake energy dissipation and prevention of damage in the main parts of the structures, this paper provides a new hysteretic damping system, especially beneficial to retrofitting steel structures having Eccentrically Braced Frames, EBFs. Despite eccentrically braced frames, EBFs, these pieces are not embedded in floor and can be exchanged easily with little cost after earthquakes. The basic role of shear panel system is to absorb a major portion of input seismic energy, thus reducing energy dissipation demand on structural members and minimizing probable structural damage. This research studies the seismic performance of EBFs with double shear panels. Five specimens have been evaluated using nonlinear finite element analysis under cyclic and monotonic loading. The findings present the proper performance of proposed revision for EBFs, the rise in energy dissipation and the elimination of damage to the main parts of structure (column, bracing, main beam) and its concentration in shear panels reducing displacement of horizontal link (main beam) of EBFs. The analytical results showed the shear panel shear distortion capacity of 0.08 - 0.15rad (displacement of horizontal link of 0.5 - 1.25cm). The response modification factor of this system was also obtained in the range of 8.7-9.8.

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Abstract: Recent damaging earthquakes in Iran and around the world have induced great death and damage providing serious reminders of seismic vulnerability of existing structures. It is more crucial in Iran, where many structures have been built when seismic codes were not effective enough, especially considering the fact that construction has not been perfectly consistent with design specifications and drawings. Many of existing building, therefore, have inadequate strength when subjected to earthquake. To prevent such damage and tragic event, efficient ways are necessary for seismic upgrading of these buildings. One of useful extensively used methods is passive control. By reducing seismic demand and increasing ductility, these control ways can reduce rate of seismic damage. One of the most effective mechanisms available for dissipating seismic energy through inelastic deformations of metallic substances is use of shear panels. Use of such yielding dampers has been increased recently for their high capability in energy dissipation. Despite eccentrically braced frames, EBFs, these pieces are not embedded in floor and can be exchanged easily with little cost after earthquakes. The basic role of shear panel system is to absorb a major portion of input seismic energy, thus reducing energy dissipation demand on structural members and minimizing probable structural damage. Due to recent advances in these passive control methods specifically significant improvements in earthquake energy dissipation and prevention of damage in the main parts of the structures, this paper provides a new hysteretic damping system, especially beneficial to retrofitting steel structures having Eccentrically Braced Frames, EBFs. This research studies the seismic performance of EBFs with double shear panels. Five specimens have been evaluated using nonlinear finite element analysis under cyclic and monotonic loading. The findings present the proper performance of proposed revision for EBFs, the rise in energy dissipation and the elimination of damage to the main parts of structure (column, bracing, main beam) and its concentration in shear panels reducing displacement of horizontal link (main beam) of EBFs. The analytical results showed the shear panel shear distortion capacity of 0.08 - 0.15rad (displacement of horizontal link of 0.5 - 1.25cm). The response modification factor of this system was also obtained in the range of 8.7-9.8. Based on the results obtained in this paper, using double SPS in addition to dissipating more than 70% of imposed energy and increasing the structural ductility, can reduce lateral displacements due to decreasing seismic demand. Finally, using shear panel is highly recommended as an effective and efficient way for seismic design of new steel building structure and also for seismic retrofit of existing steel buildings.  

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In recent years, development of effective devices for seismic energy dissipation in structures has become more important to keep response of structure in elastic range. Dampers are used in structures to reduce response and effect of seismic forces. Also, using secondary mass technology can help seismic energy dissipation. Among these systems one can mention tuned mass damper and tuned liquid column damper, working base on secondary inertia in structures. In this paper, hybrid system of tuned mass & liquid column dampers in series was considered with mass ratios 0.035-0.005, 0.03-0.01 and 0.02-0.02. Time history analysis using the Northridge, Tabas and Loma Prieta earthquakes for 20 story structures were modeled in Simulink Matlab software considering shearing structure and damper modeling in every blocks separately. Effect of damper to structure is determined as forces applying on corresponding story. Performance indices using software outputs such as root mean square and Maximum of displacement and acceleration of stories were calculated. Performance of single and hybrid systems has been compared due to different earthquakes. Also effect of hybrid systems in series was studied by increasing head loss coefficient. Results show that performance of hybrid systems is dependent on earthquake characteristics that improves with increasing secondary mass ratio. For example under the Northridge earthquake, hybrid system in series tuned mass & liquid column damper with mass ratios 0.035-0.005, 0.03-0.01 and 0.02-0.02 decrease root mean square of displacement of stories 45, 27 and 2 percent respectively and also by selecting optimum frequency ratio based on responses of structure. For example maximum acceleration of hybrid system of tuned mass & liquid column damper in series with mass ratio 0.035-0.005 is optimum frequency ratio in 2.9 and also by selecting this frequency ratio decrease maximum acceleration of up and down stories in 20 story structure. By comparing effects of hybrid system Tuned Mass & Liquid Column Damper in series with different mass ratios on two structures with periods of 1.5 and 2.44 second are considering where by increasing stiffness of structure, performance of hybrid system was improved leading to decrease of acceleration responses and reduction of displacement responses. For example, J1 in 20 story structure with period 1.5 second is 0.71 whereas in other structure is 0.79 that show hybrid system has better performance in structure with period 1.5 second. Hybrid system in series damper with mass ratio 0.035-0.005 have best performance to reduce displacement stories of 20 story structure with period 1.5 second as J3=0.56 means decrease 44%. Also in other structure, hybrid system with mass ratio 0.035-0.005 has best performance to reduce displacement at top floor with J4=0.56. Also performance of hybrid system to reduce maximum displacement of stories was improved by increasing head loss coefficient in tuned liquid column damper

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In this investigation, seismic response of steel structures utilizing Cylindrical Frictional Dampers (CFD) is studied. CFD is an innovative frictional damper which comprises two principal elements, the shaft and the hollow cylinder. These two elements are assembled such that one is shrink-fitted inside the other. If the damper’s axial force overcomes the static friction load, the shaft inside the cylinder will move and results in considerable mechanical energy absorption. To assess the efficacy of CFD, various steel frames are constructed and analyzed . Nonlinear time history analyses and Incremental Dynamic Analysis (IDA) are applied to the frames and clear distinction has been drawn between the frames comprising CFD and the counterparts without CFD to emphasize the effectiveness of CFD in altering seismic responses. The results show that CFD extremely improves the seismic response of the structure. Frictional devices dissipate energy through friction caused by two solid bodies sliding relative to each other. The idea of using frictional dampers was first proposed by Pall (1979). Pall and Marsh (1982) proposed frictional dampers installed at the crossing joint of the X-brace. Tension in one of the braces forces the joint to slip thus activating four links, which in turn force the joint in the other brace to slip. This device is usually called the Pall frictional damper (PFD). B. Wu et al. (2005) introduced improved Pall frictional damper (IPFD) which replicates the mechanical properties of the PFD, but offers some advantages in terms of ease of manufacture and assembly. Sumitomo friction damper (1990) utilizes a more complicated design. The pre-compressed internal spring exert a force that is converted through the action of inner and outer wedges into a normal force on the friction pads. Fluor Daniel Inc., has developed and tested other type of friction device which is called Energy Dissipating Restraint (EDR) (1994). The design of this friction damper is similar to Sumitomo friction damper since this device also includes an internal spring and wedges encased in a steel cylinder. The EDR utilizes steel and bronze friction wedges to convert the axial spring force into normal pressure on the cylinder. Constantine et al. (1990) proposed frictional dampers composed of a sliding steel shaft and two frictional pads clamped by high strength bolts. Grigorian et al. (1998) studied the energy dissipation effect of a joint with slotted holes both analytically and experimentally. Mualla and Belev (2002) proposed a friction damping device and carried out tests for assessing the friction pad material. Cho and Kwon (2004) proposed a wall-type friction damper in order to improve the seismic performance of the reinforced concrete structures. Recently Mirtaheri et.al. (2011) proposed an innovative type of frictional damper called cylindrical friction damper (CFD). In contrast with other frictional dampers the CFDs do not use high-strength bolts to induce friction between contact surfaces. This reduces construction costs, simplifies design computations and increase reliability in comparison with other types of frictional dampers.

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Turbine tip leakage flow is one of the effective factors in reducing the efficiency and performance of axial turbines, which can also destroy turbine blades. Accordingly, it is important to identify and control the tip leakage flow. In this paper, we investigate the effect of tip clearance sizes and changes in tip shape as a passive control method on tip structure and total turbine flow performance. For this purpose, the flow loss in a two-stage axial turbine is performed using the CFX software. In order to ensure the accuracy of the results, the turbine performance curves were compared with the experimental results which good consistency have been observed. Considering the four cases for tip clearance size, the turbine performance curves and resulting pressure loss have been investigated. It was found that increasing the tip clearance size leads to reduced efficiency and increased losses in the axial turbine. In the following, we examine the application of the passive control method through the change of the tip geometry. In this regard, the shape of the blade tip is somehow considered that the tip clearance size is variable from leading edge to trailing edge. The results show that in these cases, tip leakage flow and the resulting vertices are weakened, which leads to a decrease in the rotor loss coefficient. Observing the flow contours results in lower temperatures in the blade region due to the formation of a weaker tipping leak flow, which helps cool the turbine blades.

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In this work, transmitted sound power control through a doubly curved laminated shell by the aid of RL-shunt is investigated. Therefore, vibration equations of a doubly curved shell with piezoelectric layers are firstly derived utilizing Hamilton’s principle. Then, the obtained equations are verified considering the results reported by other researchers. In addition, by applying a shunt circuit, which is parallel to the piezoelectric layer, the effect of resonant shunt method in passive control of the sound transmission loss of the shell is explored. It is indicated that with applying the shunt circuit, and then tuning the circuit with the resonance frequency, the amplitude of sound transmission loss has been significantly reduced. In next step, by applying three shunt circuits, parallel with one piezoelectric layer, it is found that passive control of this doubly curved structure can decrease the sound transmitted in resonant frequency. Finally, performance of these circuits is improved by using genetic algorithm to optimize RL-shunt circuit parameters. As a prominent result, it is shown that this method has an excellent effect on improvement of sound transmission loss up to 80dB.

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Using a Tuned Mass Damper (TMD) in a structure, is a reasonable solution for absorbing its movements caused by external forces. However, when designing a TMD on the grounds of passive control, it is a challenging task as this device can be tuned once and for a specified range of frequency. Employing more than one TMD is another option; although this will lead to higher cost and might increase the base shear of the structure. In this paper, to provide a wide range of frequency and mode shapes in the analysis, nine types of steel structures are designed, having the story number of 4, 8, and 12, respectively, and then subjected to 22 acceleration records of FEMA-P695; These records, are a suitable choice for generating statistical results as they provide a wide range of magnitudes. Three of these structures are uncontrolled, and the remaining are equipped with a TMD on their roof, being of 0.5% and 1% mass ratio and considering the first mode frequency for the TMD design. The design of the TMDs is carried out via Den Hartog's formula.
Using incremental dynamic analysis (IDA), fragility curves are created with constant 0.1g steps for PGA intensity measure. In addition, for considering the uncertainties in the performance of the TMD and the structure due to the changes in frequency, a 10% error is applied for the first mode frequency in the nonlinear design of the structures. The maximum drift ratio is used as a damage measure due to its simplicity and comprehensive coverage. Multiple earthquake recordings and their statistical characteristics, such as mean, median, 16%, and 84% of the recorded amplitudes and their more robust components, are utilized to examine the IDA curves to eliminate any ambiguity about structure response. This paper presents its novelty by applying a statistical method for choosing the mass ratio of TMD, considering the possible real-world quantities for this parameter and a wide range of frequencies for the excitations; therefore, limiting the TMD stroke. Subsequently, verifying the linear and non-linear behavior of the model used in this paper is carried out by modeling a 40-story steel structure equipped with a TMD situated on its roof and tuned based on its first mode under the Kobe Earthquake. Furthermore, the displacement response of the 4, 8, and 12-story structures, being equipped with a single TMD of 0.5% and 1% mass ratio, respectively, are compared to their uncontrolled state by exposing them to the Landers earthquake.
Results show that using TMD reduces the maximum drift ratio of the structures. Considering the first 16% of the acceleration records, as expected, a 12-story steel structure equipped with a TMD of 1% mass ratio on the roof, presents the best results of maximum drift improvement ratio of 2.54%. Moreover, for reducing computational effort, another alternative is applying a limited number of earthquakes to the structure. By using the median for the duration and PGAs of all FEMA-P695 data to estimate this earthquake record, the maximum drift improvement ratio is then 1.61% for the twelve-story structure resulting in decent numbers compared to the first method. Moreover, all types of the 4, 8, and 12-story structures (uncontrolled, controlled with a TMD of 0.5% mass ratio, and controlled with a TMD of 1% mass ratio) were subjected to the Kobe earthquake, and their average roof displacements were compared. Among these three types of structures, the 12-story structure was recorded to have the highest rate of maximum roof displacement compared to its uncontrolled state, being 3.47%.