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This paper presents the effect of tilt angle on static and dynamic performance characteristics of two gas-lubricated noncircular journal bearing configurations, namely two and four lobe bearings. The linearized system approach using finite element method is used to obtain both steady state and dynamic characteristics. The results of the investigation show that tilt angle has a significant effect on static and stability characteristics. With an increase in tilt angle, power loss is decreased while stability margins are increased.

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In this paper, a linear model for vibration of electrostatically actuated annular microplate with In this paper, a linear model for symmetrical vibrations of electrostatically actuated annular microplate with thermoelastic damping is considered for calculating the quality factor of this damping. The Kirchhoff–Love plate theory is used to model the microplate which is coupled with thermal conduction equation one dimensionally. For calculating the Q-factors in each mode, two methods are compared with respect to linearization of frequency equation. Also the dependency of thermoelastic damping to electrostatic load and geometry of annular microplate is investigated with clamped-clamped and clamped-free boundaries. A silicon annular microplate is considered as an example. The results show that, there are a critical radius and thickness which make the thermoelastic damping to be maximal. Also the results show that the effect of electrostatic load on thermoelastic damping depends on the type of boundary conditions. The effect of electrostatic load on thermoelastic damping for clamped-free boundaries is more than for clamped-clamped boundaries.

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The importance of non-structural components in seismic Performance Based Design of buildings is well known nowadays. In this research calculation of absolute acceleration applied on nonstructural components located on floors of moment-resisting, eccentric braced and concentric braced frames subjected to earthquake ground motions has been studied. The results of nonlinear time-history analyses of 3, 5 and 7-story steel frames with 8 different periods and 5 reduction factors subject to 15 records of near-field earthquakes and 15 records of far-field earthquakes has been used to investigate the effects of different parameters on absolute acceleration induced in each floor of the structures. The steel frames have been designed in accordance with ASCE 2005 requirements. The effect of inelastic behavior of system, natural periods of primary and secondary systems, structural system type and near-field ground motions have been studied. To perform expanded parametric studies on various frames with different stiffness and strength, modified shear building models for these frames were constructed. The shear building models are set to have an equivalent lateral force-deformation behavior in each story to the one’s of given steel frames. Reliability of these models to estimate the maximum roof displacement and the maximum inter-story drift of steel frames has been investigated elsewhere through probabilistic analysis of the results obtained from comprehensive incremental dynamic analyses. In this study for each frame 8 period: 1/4, 1/3, 1/2, 2/3, 1, 4/3, 2, 4 seconds are considered. In order to achieve the proper period of shear-building models, keeping the stiffness and strength ratio of stories unchanged, initial stiffness of first story is adjusted proportionally. The relationships that are presented in building codes to calculate the force applied on nonstructural components has been usually expressed as a ratio of peak ground acceleration. This method of calculating of input acceleration to nonstructural elements ignores the effect of frequency content of design ground motion. The results of the present study have been used to introduce a new method for calculation the force applied on nonstructural components based on ground acceleration spectrum. In this method the input acceleration to nonstructural components has been expressed as a ratio of earthquake response spectrum (instead of the peak ground acceleration). For this ratio which is entitled as “spectral amplification factor” two different expressions have been proposed for use in structures with linear and nonlinear behavior. This approach explicitly accounts for the frequency content of design earthquake in calculation of peak floor acceleration. The results of this study show that Euro-code 8 and ASCE 7-2010 recommendations need to modify specially for the precise location of the non-structural element and inelastic behavior of the structure. It has been demonstrated that structural system type does not significantly affect the amount of induced acceleration on each floor of steel frames.

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Abstract: The importance of non-structural components in seismic Performance Based Design of buildings is well known nowadays. In this research calculation of absolute acceleration applied on non-structural components located on floors of moment-resisting, eccentric braced and concentric braced frames subjected to earthquake ground motions has been studied. The results of nonlinear time-history analyses of 3, 5 and 7-story steel frames with 8 different periods and 5 reduction factors subject to 15 records of near-field earthquakes and 15 records of far-field earthquakes has been used to investigate the effects of different parameters on absolute acceleration induced in each floor of the structures. The effect of inelastic behavior of system, natural periods of primary and secondary systems, structural system type and near-field ground motions have been studied with use of modified shear-building models of steel frames. The shear building models are set to have an equivalent lateral force-deformation behavior in each story to the one’s of given steel frames. Reliability of these models to estimate the maximum roof displacement and the maximum inter-story drift of steel frames has been investigated elsewhere through probabilistic analysis of the results obtained from comprehensive incremental dynamic analyses. The relationships that are presented in building codes to calculate the force applied on non-structural components has been usually expressed as a ratio of peak ground acceleration. This method of calculating of input acceleration to non-structural elements ignores the effect of frequency content of design ground motion. The results of the present study have been used to introduce a new method for calculation the force applied on non-structural components based on ground acceleration spectrum. In this method the input acceleration to non-structural components has been expressed as a ratio of earthquake response spectrum (instead of the peak ground acceleration). For this ratio which is entitled as “spectral amplification factor” two different expressions have been proposed for use in structures with linear and nonlinear behavior. This approach explicitly accounts for the frequency content of design earthquake in calculation of peak floor acceleration. The results of this study show that Euro-code 8 and ASCE 7-2010 recommendations need to modify specially for the precise location of the non-structural element and inelastic behavior of the structure. It has been demonstrated that structural system type does not significantly affect the amount of induced acceleration on each floor of steel frames.

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In this paper, the effect of size of an atomic force microscope (AFM) with an assembled cantilever probe (ACP) on resonant frequencies and their sensitivities are investigated using the strain gradient elasticity theory. The proposed ACP comprises a horizontal microcantilever, an extension and a tip located at the free end of the extension, which make the AFM capable of scanning the sample sidewall. First, the governing differential equation of motion and boundary conditions for dynamic analysis are obtained by a combination of the basic equations of the strain gradient elasticity theory and Hamilton principle. Afterwards, the flexural resonant frequency and sensitivity of the proposed AFM microcantilever are obtained numerically. The results of the proposed method are compared with those of modified couple stress and classical beam theories. The comparison shows that the difference between the results predicted by the strain gradient elasticity theory with those obtained by couple stress and classical beam theories become significant when the horizontal cantilever thickness comes approximately close to the material length scale parameter.

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Currently, seismic design provisions of most building codes are based on strength or force (base shear) considerations. These building codes are generally regarding the seismic effects as equivalent static forces with a height wise distribution which is consistent with the first vibration mode shape. However, the design basis is being shifted from strength to deformation in modern performance-based design codes. This paper presents a practical method for optimization of steel moment resisting frames (SMRF), based on the concept of uniform deformation theory. This theory is based on this concept that the structural weight of a lateral load resisting system with uniformly distributed ductility demand-to-capacity ratio (or any other damage index) will be minimal compared to the weight of an ordinary designed system in which deformation is not distributed uniformly and just some of structural elements have reached their ultimate states. The state of uniform deformation can be achieved by gradually shifting inefficient material from strong parts of the structure to the weak areas. In the first part of this paper, the uniform deformation theory is implemented on 3, 5 and 10 story moment resisting frames subjected to 12 earthquake records representing the design spectrum of ASCE/SEI 7-10. This includes design of an initial structure according to conventional elastic design procedures, followed by an iterative assessment process using nonlinear dynamic analyses till the state of uniform deformation is achieved. Results show that the application of uniform deformation theory leads to a structure with a rather uniform inter-story drift distribution. Subsequently, the optimum strength-distribution patterns corresponding to these excitations are determined, and compared to four other loading patterns. Since the optimized frames have uniform distribution of deformation, they undergo less damage in comparison with code-based designed structures. Also, as the shear strength of each story is in proportion to the weight of that story, the optimized structures have minimum structural weight. For further investigation, the 10 story SMRF is redesigned using four existing load patterns and subjected to 12 earthquake excitations. Then a comparison is made between maximum beam rotations of each model and those belonging to the optimized one which revealed that the optimized SMRF behaves generally better than those designed by other loading patterns. Also, it is found that for none of the conventionally designed SMRFs, beam rotation demand is distributed uniformly. In other words, for all of the considered load patterns the maximum rotation of the beams in some stories exceeds the rotation associated with the performance level. Finally, assuming that the probability distribution of maximum rotations under different excitations follows a lognormal distribution, the probability of exceeding the allowable rotation associated with the LS performance level is calculated for different load patterns and compared to each other. Based on this comparison, the efficiency of each loading pattern is evaluated and the best one is determined. Application of optimization method presented in this paper avoids the concentration of deformation and damage in just one story and makes each story deformation and damage uniform over the height of the structure.

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In this study, the nonlinear vibration behavior of a dynamic atomic force microscope (DAFM) in the tapping mode is investigated. First, the governing differential equation of motion and boundary conditions for dynamic analysis are obtained by a combination of the basic equations of the modified couple stress theory and Hamilton principle. Regarding the nonlinear dynamics of the probe, perturbation technique has been used to solve the nonlinear equations. Afterwards, closed-form expressions for nonlinear frequency and effective nonlinear damping factor are derived. The effect of connection position of the tip on the vibration behavior of the microcantilever are also analyzed. The results obtained by couple stress theory are compared with those of classical beam theory. The results show that the nonlinear frequency and effective nonlinear damping factor are size dependant. According to the results, an increase in the equilibrium separation between the tip and the surface sample reduces the overall effect of van der Waals forces on the nonlinear frequency, but its effect on the effective nonlinear damping factor is negligible. The results also indicate that the change in the distance between tip and cantilever free end has a significant effect on the accuracy of the DAFM.

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Performance-based design optimization (PBDO) is a relatively new concept in structural seismic design optimization. One of the PBDO methods which has been introduced in recent years is the optimization based on the uniform deformation theory. This method is quite different from other optimization techniques and formed based on the concept of structural performance and uniform distribution of deformation demands in the structure subjected to the seismic excitation. The aim of this method is to assign specific sections to elements such that all of the elements can reach their allowable deformation capacity during the earthquake. According to this theory, inefficient material is gradually shifted from the strong to weak areas leads to a uniform deformation (ductility) state at the end of a repetitive process. Although the base of this theory and proposed algorithm is to attain a uniform state of deformation in the whole structure, but the allowable limit of deformation values defined in PBD codes is not constant for all of structural elements. Additionally, in these codes, some actions of structural elements may be controlled by deformation and some controlled by force. Therefore, by considering the acceptance criteria of PBD codes, it is not possible to reach a uniform deformation state in the whole structure. Hence, in this paper uniform distribution of demand capacity ratio (DCR) is considered instead of uniform state of deformation. Historical review of applying this methodology shows that researchers mostly have used it to the optimum design of the structures under the earthquake records separately. Since earthquakes are random by nature, it is unlikely that the same earthquake ground motion will be repeated at some future time. This reveals that design based only one earthquake is insufficient and it is necessary to consider several earthquakes in checking the dynamic responses of a building. This paper presents an algorithm to PBDO of steel moment frames under set of ground motion records using the basic concepts of the uniform deformation theory. The proposed method consists of two phases. In the first phase of the search, to enhance the convergence rate, the search space of design variables is assumed to be continuous. Additionally in this phase of the search, only the deformation-controlled elements may vary. In the Second phase of the search, first for each structural element groups, the nearest discrete section to the imaginary section achieved in the first phase is identified and selected and then the structure is analyzed again and the DCRs are controlled. In this phase, acceptance criteria for both deformation and forced-controlled elements are supposed to be satisfied. Efficiency of the proposed algorithm is demonstrated in the optimum design of two baseline steel moment frames under a set of ground motion records. Results indicate that the proposed algorithm has a high speed to reach the optimum solution. The results are also compared with the optimum designs obtained by pushover analysis. It is shown that the optimization based on the pushover analysis results higher frame weight than time history analysis.

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These days the accurate estimation of seismic demand and capacity of structures are truly significant in the field of performance based earthquake engineering. Several methods exist to determine these parameters such as non-linear time history analysis and Incremental dynamic analysis (IDA). Because the history of seismic accelerogram records refers to the current century, in some areas there still exists no appropriate seismic record to perform the analyses; therefore in these cases we need to generate artificial accelerograms. In this paper a new combinational method is introduced to generate far-field artificial accelerograms using artificial neural network and wavelet packet transform (WPT) methods. In this method according to the geoseismic characteristics of the site and non-linear characteristics of the equivalent single degree of freedom (SDOF) system, several artificial accelerograms are generated. In order to consider the non-linear parameters to generate the accelerograms, IDA method is used. The values of intensity measure (IM) for all IDA curves are determined at specific levels of damage measure (DM) and are considered as the input data of the multilayer feed forward (MLFF) neural network. Accelerograms which are selected according to the geoseismic characteristics of the site are changed to standard forms and then decomposed using wavelet packet transform. The effective wavelet packet coefficients are selected according to an appropriate desired effective variance ratio of wavelet packet coefficient. Then, effective coefficient of each packet is considered as the output of a neural network. In order to enhance the efficiency of the network, principal components analysis (PCA) is used to reduce the number of the input data dimensions. In this paper neural network is trained by backpropagation algorithm as repetitive. After training the MLFF neural network, we should test the network for accelerograms not included in the training set. For this purpose we should use the IDA curve of each accelerogram out of the training set as the input of the neural network to generate the effective WPT coefficients. When a neural network is trained properly, we can now generate artificial accelerograms using a 50% fractile IDA curve as the input of the neural network. Adding a Gaussian random number to the output of each neuron in the neural network layers, we are able to generate several accelerograms according to 50% fractile IDA curve. In order to improve the condition of generated accelerograms according to 50% fractile IDA curve, a correction factor is used repeatedly for detail coefficients of discrete wavelet transform in jth level of generated accelerogram. Finally a SDOF system with perfectly elasto-plastic initial loading curve is used to show the efficiency of the proposed method to generate artificial accelerogram. The accuracy of this method depends on the accuracy of the trained neural networks. If the neural networks are trained appropriately with IDA curve, the generated accelerogram can estimate the IDA parameters of the SDOF system more properly. Also it is shown that suggested method can generate artificial accelerograms with frequency content almost close to the initial earthquake records.

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Additional dampers TADAS are a kind of passive control systems which can be used in seismic design or retrofit of structures. In this study, behavior of TADAS dampers in large deformation has been examined and some of the possible errors in its design are expressed. It is shown that how the lack of attention can result in damage to the structure and reduce the ability of energy dissipation in the damper. To investigate this issue, TADAS damper with all its details was simulated in ABAQUS finite element software. TADAS damper made up of several components, these components include the upper plate, the lower plate, triangular plates, rods rollers (pins) and connection plates. Damper modeling in ABAQUS determined that in a large deformation, the damper stiffness strongly and suddenly increases. It is examined that this sudden changes in damper characteristics is mainly due to the collision of the damper pin and the upper wall of its slot. This sharp increase could lead to adverse responses and even help to the destruction of the structures. In this paper, two suggestions are presented to prevent this situation. These suggestions include increasing the slot height and putting pins in the lowest point than slot bottom during damper installation. Assuming uniform curvature over the damper plates, a relationship has been proposed to predict the amount of the large displacement corresponding to the high stiffness of the damper. Using this relationship can get awareness of the occurrence or non-occurrence of increasing stiffness of the damper in the various classes of structures. It can also be used as a design tool for selecting the proper height of the damper slots.
Also, a frame equipped with TADAS damper is constructed and get under cyclic loading to large deformation. This frame was simulated in ABAQUS and its behavior was compared with laboratory sample. This comparison indicates that there is a good agreement between laboratory and software results. From laboratory and software models it became clear that the frame equipped with TADAS damper even in large deformation has stable and acceptable behavior, but two very important defects are observed in the frame. One of these defects is buckling of braces despite their design based on the toleration of the maximum capacity of damper. This buckling has occurred due to the rotation of beam-to-column connections. To prevent damper from degradation, it must be considered in the design process as far as the large deformations is concerned. As per the design codes, damper’s retainer system TADAS (Chevron braces) should not be damaged and/or buckled. The second defect is related to the looseness of damper’s pins and the looseness of damper’s connection bolts inside their slots. It will be shown that how this looseness causes a delay in the performance of damper and will increase the possibility that the damper plays a lesser role during earthquake. Therefore, the looseness in pins and bolts must be properly prevented. In this study, 10 bolts with 24 mm diameter were used to the connecting damper to floor-beams and Chevron braces.

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Inimitable properties of graphene sheets enable a variety of applications such as axially moving nanodevices. Axial velocity affects dynamical response of systems. In this study linear vibration of an axially moving two-layer graphene nonoribbon with interlayer shear effect is proposed using nonlocal elasticity theory. Based on this theory stress at a point is a function of strain at all other points of the body. Euler-Bernoulli theory is used to model the system due to nanoribbon thickness and length. It is assumed that the layers have the same transverse displacement and curvature and there is no transverse separation between layers surfaces. A shear modulus is imported in the potential energy expression in order to consider the interlayer shear effect due to weak Van der Waals forces. Governing equations are obtained using Hamilton’s principle and are solved by Galerkin approach. Results for clamped-free boundary conditions are presented and compared to other available studies. Results for pinned-pinned boundary conditions are presented and it is observed that increasing axial velocity causes divergence and flutter instabilities in the system. Effects of different shear modulus and nonlocal parameter on critical speeds are also proposed.