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Abstract Studying the response details of steel moment connections is very important due to the role of connections in moment resisting frames. The aims of this research were: i) to study the damage indices of steel material including: Pressure Index, Mises Index, Equivalent Plastic Strain Index, Triaxiality Index, and Rupture Index and ii) to compare these indices at connections of steel moment frames under earthquake loads. To achieve this, time history nonlinear dynamic analysis is performed using selected earthquake records on 2D model of special steel frame with ten storey and one bay to determine maximum rotations of connections. Then, damages indices of the selected connections under maximum rotation of records are investigated with selecting two types of moment connections. The results indicate that damage indices are dependent on type of connection, location of surveying, and rotations caused by earthquake movements. This dependency is very considerable for Equivalent Plastic Strain Index and Ruptureindices

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One of the problems that influence on seismic behavior of structures and associate designing to itself is the grouping of structural element within analyzing and design of structures. Generally this grouping is due to facilitate of performing the structures. This paper investigates these grouping effects on behavior of concrete structures with RC bending frame systems andmoderate ductility scale. Study cases that is used for assessment of these grouping effect contains buildings with 4, 8 and 12 story RC structures. Each of these buildings designed one time without grouping and several times with grouping consideration about columns of structures in the height direction of buildings.Codes that are used for design purposes are Iranian Seismic code and RC structure design code. IDARC program V7.0 is utilized for estimation of damage indices, maximum story drifts or displacements and energy dissipated by building structural systems to comparing thenonlinear seismic behavior of column grouped and non-grouped structures. Damage indices calculated by this program is based on modified Park-Ang-Wen model and represented individually by elements, stories and a total damage index. The selected structures are analyzed with nonlinear dynamic analyzing method under Tabas earthquake record using several peak ground accelerations (0.35g, 0.50g, 0.75g and 0.90g) and pushover analysis with force-control and displacement-control methods. Maximum responses such as maximum displacements, damage indices(with grouping and non-grouping design method) were used to realization result of this designing method.Comparing the result of nonlinear analysis showed increasing of damage with increasing of PGA. This is due to a better distribution of forces in the elements of structures in case of non-grouped designed structures. Analytical results showed that the effect of grouping in PGA less than 0.5g is not sensible, but in larger PGA the grouped designed structures suffer more damages. The grouping of structural elements causes to concentration of energy in elements that their demand to capacity ratio (D_E/C_E) is greater than others.This causes that these elements embroil more damage and save other element from greater damage.One of the other results of this designing method (Grouping of element) is formation of soft stories in the structure. Also the reason of this behavior is due to lumping of hysteretic energy on these stories. This subject causes to generate soft and weak story in the structure and increase the overall damage indices.Furthermore result of pushover analysis showed that grouped element structures have a more stiffness and so in a weak earthquake (a low PGA) have a less or equal damage index in comparison to non-grouped element structures. As an overall result determined when D_E/C_E for all elements is close to each other distribution of damage is uniform and vice versa.

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In this paper the behavior of framee, the process of plastic hinge formation and energy absorption of frames with two spans and one floor with three types of slab including bubble deck slab, hollow core slab and reinforced slab under three earthquake accelerations have been analyzed and compared. The results show that bubble deck slab and hollow core slab as rigid as normal reinforced slab, although bubble deck slab has higher strength and stiffness compared to other slabs. Partnering slab in analysis make period of slab reduce more over bubble deck slab and hollow core to the comparison of reinforced slab, have more effect on period reduction. Ultimate displacement of frame with reinforced slab reach to failure mechanism is more than two mentioned case, however frame with bubble deck slab reach to failure mechanism under stronger earthquake acceleration and smaller displacement than reinforced slab. Comparison base shear of three discussed case shows that maximum base shear is in bubble deck slab and minimum base shear is in normal reinforced slab. Formation of plastic hinge in frame with bubble deck slab is similar with that in frame with hollow core slab with the difference that plastic hinge in former occurs later at the top end of the middle column and two ends of middle beams. In fact, formation of plastic hinges in this frame requires higher acceleration because of the higher amount of concrete and stiffness. In all samples, plastic hinge first occur in the frame and then yielding lines occur in the tensile region of the slabs. The failure mechanism of slab and steel frame occur at the same time in frame with hollow core slab and reinforced slab; however, this is not the case in the frame with bubble deck slab and even though with occurring of yielding lines, the slab does not fail. The stress distribution due to gravity loads is symmetric across all the slabs; however, the increase rate of stress is different. This difference is particularly notable in seismic behavior of slabs in a way that the formation of plastic hinge and yielding lines in hollow core slab, because of the holes, is totally different with that of in reinforced slab. In comparison with other slabs and due to the formation of plastic hinge, reinforced slab absorb lower energy. Columns, beams and connections play different role in energy dissipation. In all frame, the contribution of connections to dissipate energy is minor and this is because yielding does not occur in connections. Contrary to the frame with reinforced slabs, because of yielding in several places of columns, columns dissipate energy more than beams in the frames with hollow core slabs. It was concluded that hollow core slab and bubble deck slab have maximum and minimum contributions to the energy dissipation, respectively.

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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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Dams shall safely retain the reservoir and any stored solids, and pass environmentally acceptable flows, as required for all loading conditions, ranging from normal to extreme loads, commensurate with the consequences of failure. The new trend for performance-based design is to consider 2 levels of seismic actions and analyze the situation where the limit of force balance is exceeded for high intensity ground motions, associated with a very rare seismic event. For the design, two basic requirements are defined: (i) Non-collapse requirement (ultimate limit states), i.e. after the occurrence of the seismic event, the structure shall retain its structural integrity, with respect to both vertical and horizontal loads, and adequate residual resistance, although in some parts considerable damage may occur, (ii) Minimization of damage (serviceability limit state) , i.e. after seismic actions with high probability of occurrence, during the design life of the structure, some parts can undergo minor damage without the need of immediate repair. This study evaluates the behavior of a typical earth dam by nonlinear seismic analyses, in two performance levels, named “Base Performance Level” and “Desired Performance Level.” The level of seismic action and related acceptance level of damage are defined for each performance level. In “Base Performance Level,” with seismic levels of OBE (0.3g) and MDE (0.5g), the structure shall be serviceable and repairable and in “Desired Performance Level”, with seismic levels of MDE (0.5g) and MCE (0.7g), the structure shall be serviceable and repairable, respectively. Also, the stability of dam has been assessed by the “Strength Reduction Analysis.” The analyses are nonlinear and the constitutive law of the materials was assumed to follow "Finn" and "Mohr-Coulomb" models, incorporated into “FLAC 2D” finite difference analysis program. The factors such as initial shear modulus, variation of shear modulus versus shear strain, generation and dissipation of pore pressure and hysteretic damping are considered in this study. In addition, using the scaling method of applying maximum acceleration, the response of dam is investigated in different maximum accelerations. The results show that the dam needs to be changed in geometrical specifications or seismically improved in “Desired Performance Level”, in contrast with “Base Performance Level.” Results are confirmed by low amount of safety factors of stability in dam, which are calculated for different seismic loads. Also, the behavior of dam is examined by sensitivity analysis for type of accelerograms, constitutive model and the standard penetration number in shell of dam. Two accelerograms, including “Friulli” and “Sakaria” are considered. Maximum acceleration and duration of both of them are equalized and frequencies more than 5Hz are filtered. Sensitivity analyses of “Friulli” and “Sakaria” accelerograms, despite the difference in response spectra and specific energy density, show approximately similar results. “Finn” model predicts the amount of excess pore water pressure to be more than "Mohr- Coulomb" up to %20, and shows the occurring of liquefaction in SPT more than 35 and acceleration more than 0.7g, in shell of upstream of dam

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Concrete buttress dams are constructed in large numbers at medium sites in many countries such as Iran because of their considerable technical and economical benefits in previous century. This type of dams is exposed to damages due to earthquakes as other structures. Some buttress dams such as Sefidroud dam in Iran, Hsinfengkiang dam in China and Honenike dam in Japan have undergone some damages due to recent earthquakes. After these incidents, some investigations have been carried out. However, these investigations have just mentioned the manner of incidents and the resulting damages. Therefore, the seismic behavior and sensitivity recognition of these dams with respect to different factors have been ignored; however the study of behavior and seismic sensitivity of this type of dams is important. In this paper, the tallest monolith of the Sefidroud concrete buttress dam is analyzed using a 3D model with massless foundation to study the seismic behavior and sensitivity of this type of dam. The interaction of the dam with the reservoir, the reservoir bottom absorption and upstream radiation of hydrodynamic waves are considered, but the cross-canyon component of earthquake is neglected. The applied accelerograms to the system are scaled according to the Sefidroud dam site DBE response spectrum. To determine the initial conditions before occurring earthquake, a series of detailed static analyses are done under the effect of dam body weight, hydrostatic pressure, uplift pressure and ambient temperature. Seismic loading due to longitudinal and vertical components of earthquake is applied and the nonlinear behavior of dam under various factors such as different seismic loading scenarios and different properties of dam body and also foundation materials is investigated. The results of analyses show that the dam body downstream kink, heel, toe and buttress web are sensitive and vulnerable zones. The results also demonstrate that the compressive stresses in the dam body are usually much less than the compressive strength of concrete. Therefore, the possibility of compressive failure is almost zero. But the conditions of tensile and shear stresses are different and large stresses may occur at the mentioned zones and considerable tensile and shear damages to the dam body are possible. According to the results of analyses, it is apparent that when the ratio of dam body modulus to that of the foundation (called softness modulus) is small, i.e. when the foundation modulus is high and near to that of dam body, the construction of concrete buttress dams at highly seismic zones may cause local failure and unfavorable situations for the tensile stresses at the kink, the heel and the toe of the dam body. Therefore, adaptation of this dam type in such sites should be carefully studied and in these circumstances, the modulus of the concrete of dam body should be kept more than usual practice. Furthermore, the shear damage at the dam-foundation contact surface is highly dependent to the applied earthquake type, but increasing the softness modulus could reduce this type of damage. The compressive strength of concrete has no effect on the shear damage at the dam-foundation contact surface.

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In this paper, a new formulation is developed for nonlinear dynamic analysis of 2-D truss structures. This formulation is based on dynamics of co-rotational 2-D truss. The idea of co-rotational approach is to separate rigid body motions from pure deformations at the local element level. Using this approach, internal force vector and tangent stiffness matrix, inertia force vector and the tangent dynamic matrix are derived. Furthermore, the inertia force vector, tangent dynamic matrix, mass matrix and gyroscopic matrix are directly derived from the derivation of current orientation matrix with respect to global displacements or orientation matrixes. Using this new formulation, nonlinear response of any 2-D truss structures can be examined. Here, for example the response of tensegrity structures under dynamic loads are investigated. Tensegrity structures are a class of structural system composed of cable (in tension) and strut (in compression) components with reticulated connections, and assembled in a self-balanced fashion. These structures have nonlinear behaviour due to pre-stress forces. And their integrity is based on a balance between compression and tension. Two numerical examples are presented to illustrate the new formulation and results show that the new formulation has more convergence rate than the existing models.

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In this article, using finite element method the effects of the preload on the nonlinear dynamic behavior of the noncircular two lobe aerodynamic journal bearing have been investigated. Assuming that the rotor is solid, the governing Rynolds equations for both the gas lubricant and rotor equation of motion in static and dynamic conditions have been derived and performance of the noncircular aerodynamic journal bearing in different conditions has been evaluated. Rung Kutta method has been used to solve the time dependent equations of motions of noncircular aerodynamic journal bearing and its gas lubricant. Using the numerical results, to investigate the motion of the center of the rotor in dynamic conditions, the graphs of frequency response, power spectrum, dynamic trajectory, Poincare map and bifurcation diagram have been plotted. The results show periodic, quasi periodic and chaotic rotor behavior for different bearing preload. It is concluded that appropriate selection of rotor parameters like its preload and suitable design and fabrication of rotor and its bearing can prevent any undesirable perturbed motions of the shaft and both the collision and wear of the rotor and bearing.

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In this work, using finite element method (FEM) the effects of preload factor on the dynamic stability of noncircular two lobe hydrodynamic micropolar lubricated journal bearing based on the linear and nonlinear analytical dynamic models are presented. Assuming that the rotor is solid, the governing Reynolds equations for incompressible lubrication of journal bearing have been modified using micropolar theory. Later, the linear and nonlinear dynamic models, including a certain harmonic disturbances and time dependent trajectory of rotor center are applied to obtain the stability performance of bearing. The 4th order Rung-Kutta method has been used to solve the time dependent equations of rotor motion. Finally, the numerical results for the critical mass parameter and whirl frequency ratio of rotor as the stability characteristics of bearing are evaluated for different values of preload factor and compared together. Results show that the stability performance of two lobe bearing enhances by increasing the amount of bearing noncircularity in terms of the critical mass parameter increase and decrease of the whirl frequency ratio. Also, by comparing two dynamic analysis methods, it is obvious that the results of linear dynamic model are more cautious in different investigated cases. The results of nonlinear dynamic analysis reveal that by increasing the value of preload factor the dynamic response of rotor center involves return to steady state equilibrium position, limit cycle periodic motions and contact between rotor and bearing's shell.

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This paper focuses on nonlinear dynamic analysis of a solar-powered free piston hot-air engine. First, dynamic and thermodynamic equations governing the free piston hot-air engine are extracted. Accordingly, by coupling the obtained relationships, nonlinear behavior of the free piston hot-air engine is simulated. Then, motion and velocity of the pistons in steady state condition are discussed using numerical solution of the nonlinear equations and using the phase plane analysis. Next, the stroke of pistons, maximum and minimum volumes and pressure as well as the produced work and power are studied corresponding to the change of temperatures in the hot and cold chambers. Since, the damping coefficient between power piston and the cylinder wall is variable due to the temperature changes and environmental conditions its effect on the stroke of pistons, maximum and minimum volumes, pressure, produced work and power is investigated. The results obtained clearly indicate that there is an optimal power for a certain value of damping coefficient. Then, stroke of the pistons, maximum and minimum volume and pressure as well as the produced work and power are studied according to the changes in engine parameters such as mass and stiffness of work and displacer pistons. The ranges of variations of engine parameters are selected so that the motions of displacer and power pistons fall into a limit cycle. Finally, sensitivity analysis of the produced power is carried out considering changes in engine parameters.

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Structure safety in the design of civil engineering projects has always been very important for engineers. One of the mechanisms that structure will fail and in recent years is much attentioned to it is progressive collapse. Progressive collapse in structures during earthquakes even in an explosion near the construction has become a major challenge and can create problems for structures and may even lead to the destruction of the entire structure. Currently the most available structures is only designing against the gravity loads and lateral loads (wind and earthquake).In fact a resistant structure against the earthquake is not resistant against the progressive collapse necessarily. Therefore designing the new and special structures against the progressive collapse is necessary. Progressive collapse is defined as extension of primary local failure from element to other element that finally collapsed all part of the structure or big part of it. Potential hazards that cause progressive collapse are fires, gas explosions, make a mistake in design of structure, accidents, bomb and even an unprincipled excavation that cause sudden removal one or more elements of structure and etc. The purpose of this paper is to investigate progressive collapse in steel structures with eccentric braced frames that also the influence of parameters such as height, bracing arrangement and type of structural system is examined In this study it is analyzed the progressive collapse due to column removal in steel eccentric braced frames that are designed seismically according to Iranians guidelines(seismic regulations of Iranian 2800 code) with using of alternate path method and nonlinear dynamic analysis. Also in the continuation of research it is analyzed the progressive collapse due to column and brace removal simultaneously in steel eccentric braced frames and analysis the progressive collapse in moment frames and comparison of it with eccentric braced frames. Also it is evaluated the influence of parameters like number of floors, location of braces and type of connections. For this intent two structures with five and ten stories with braces in middle spans, and also two structures with five and ten stories with braces in lateral spans, one structure with five story with system of moment frames and one structure with combinatorial system of moment frames with eccentric brace which is five story in ETABS program were analyzed. Then one of outside frames for analysis of progressive collapse modeled in SAP2000 program. Results showed that remove a single column only when there is not any brace beside the removed column and simultaneous removal of columns and braces only in the last floor causing progressive collapse to the structure. Results showed that the probability of progressive collapse with simultaneous removal of columns and braces will increase when the height of the frames that middle spans is braced increases and will decrease when the height of the frames that lateral spans is braced increases. Also with comparison of eccentric braced frames and moment frames is resulted that eccentric braced frames is stronger than the moment frames against the progressive collapse. But combinatorial system of moment frames and eccentric brace in comparison with the other analyzed systems is completely resistant system.

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In this research, seismic performance of Endurance Time (ET) method is considered for assessment of jacket platforms. ET is a new method that can assess the extreme response of the structure at various seismic excitation levels with very low computational costs. For this aim, artificial records have been generated that gradually increase with time. They have been named Endurance Time Acceleration Functions (ETAFs). For determination of the seismic response of the jacket platforms, various nonlinearity such as buckling of the brace members, material nonlinearity, soil structure interaction and fluid structure interaction are important and can be a challenging issue for the ET approach. In this way, a real jacket platform located in the Persian Gulf is studied. Finite element method is utilized to prepare a three dimensional model of this platform with using ANSYS software. Moreover, various nonlinearity sources are considered in this model. Fluid structure interaction is included by using Morison equation that hydrodynamic added damping and added mass are considered by nonlinear drag force and inertia force, respectively. Soil–pile–structure interaction is also considered by near and far field soil effects. Near field soil is modeled by nonlinear spring and elastic solid elements are used to model far field effects. Material nonlinearity is considered by a standard bilinear stress-strain curve with 5% strain hardening and the von Mises yield criterion. Buckling of the brace members is also modeled by the initial imperfections at the mid-span of the braces, as recommended by previous studies. A methodology is also addressed for assessment of this type of offshore structures. For considering the accuracy and the reliability of this approach, the results of the ET method are compared with the typical time history method. In this regard, seven records are selected for soil type C from FEMA 440 and FEMA 695 and scaled to the ELE event such that their spectral accelerations match the ELE spectral acceleration at the main period of the platform. For other excitation levels, scale factors change proportional to the ELE ratio. For example, in this case, the ratio of the ALE spectral acceleration to the ELE one is 1.4; therefore, the scale factors of the ALE event are 1.4 times of the ELE one. ET records are scaled such that the response acceleration spectrum of the ETAF until target time becomes compatible with the ELE spectral acceleration. Initial studies recommend that 10 s is an appropriate target time. Due to linear increase of the excitation of the ET records, each time can be in accordance with the especial level of ELE, for example, 5s and 15s indicate 0.5 and 1.5 times of the ELE event. A comparison between the results of the earthquake records and the results of the ETAFs show that the ET method can accurately estimate engineering demand parameters such as maximum deck displacement, maximum base shear, maximum axial force in the leg and maximum axial force in the brace. The ET method is a vigorous approach that can be successfully estimated the seismic excitation of the buckling initiation. Moreover, the results indicate that despite significant decrease in the computational costs of the ET method, this approach can show appropriate performance.

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Terrorist attacks and explosions in the vicinity of buildings and vital areas are happening in different countries repetitively. Most of these incidents lead to global and local failure in the main elements of the buildings and in some cases due to intensity of explosions can occur entire structure collapses. Columns are the key bearing elements in the building, and between all columns, the exterior of them are more vulnerable to terrorist attacks. Usually blast resistant design of structure is carried out by simplifying the models and considering a single column with nonlinear behavior under blast loading. Explosion is a complex phenomenon with high strain rate, which affects strongly on behavior and material property of structural elements. Operation of experimental test on structures under blast load is very expensive, difficult and dangerous. Hence, simulation of experimental models using nonlinear finite element software is very useful. In this paper, to achieve better performance of columns under blast loading, the response of steel columns with different cross-sections has been investigated. In addition, effects of blast wave incidence angle, blast distance and different boundary conditions are considered. For this purpose, wide flange steel column of experimental test has been simulated under axial force and blast load using LS-DYNA software. Numerical model is simulated using shell elements and its result has been validated with the full scale blast experimental data. In the finite element analysis the effects of high strain rate and material nonlinearity are considered. The columns with different cross sections of wide flange, cross-IPE and box sections are simulated under two angles of blast waves extensive, zero and 45 degree. Also, two support conditions of fixed-fixed end and pinned-pinned end have been considered. The results show in the both boundary conditions for blast with zero angle, the dynamic response of column with wide flange section subject to blast load has been less than the other cross sections. Also, the box section has better performance than cross-IPE. In 45 degree blast angle and fixed end boundary conditions, the displacement time history of box column is less than two other sections and it shows better performance respect to other sections. But, under pined end boundary conditions, cross-IPE section has better and stronger behavior respect to wide flange and box sections. In addition, the displacement of wide flange section (section with non-identical strong axes) in 45 degree blast angle has more than zero degree. However, in the columns of box and cross-IPE section under the same explosion situation in 45 degree blast angle, the dynamic response is less than zero degree, because they have two identical strong axes. Then for corner columns of buildings that direction of blast wave propagation may be 45 degree the best section (based on minimum deflection criteria) is column section with two strong axes such as box and cross-IPE, however for peripheral middle column of building that bending moment of explosion may be accrued about strong axis, the wide flange section with only one strong axis is better. Various distances of explosion from column cause different nonlinear behavior, therefore investigation of optimum column cross section under blast loading depends to distance of explosion from the column. Then displacement criteria may be not enough and use of additional criteria such as residual load bearing capacity can be appropriate.

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Nonlinear behavior is a common feature of all real-world systems. However, for the sake of simplicity, a linear model is often used in the controller design procedure. Nevertheless, the neglected nonlinear dynamics could degrade the performance of controller drastically. This study presents a new method of designing a model predictive controller (MPC) for a class of nonlinear systems. In the proposed method, an MPC is first designed in state space based on a linear model and then modified by using modal series to compensate for the effect of the neglected nonlinear dynamics in the linear model. Because the proposed controller adjusts a linear controller instead of designing a new one, it can be easily applied in industries to modify controllers that have been designed based on linear models. In addition, its computational burden is much less than that of nonlinear MPC methods. In this study, the proposed technique is used to control two real-world systems, and the results of its application are discussed.

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When one or more vertical elements of a structure fail due to defects in construction stages or over loading or etc., load distribution path of the structure changes and local failure arises in the damaged area. This kind of damage is not considered by engineers and can cause local collapse. The local collapse can spread vertically or horizontally to the other areas of the building if no alternate path exists to redistribute the loads. Therefore, limiting the local collapse in the damaged area is major idea to mitigate progressive collapse in the buildings.
Nowadays, analyzing the structures which are designed based on the current standards, against progressive collapse and offering ways to improve and strengthen them is leading to part of the designing stages of the special buildings. Thus, some standards and codes in this field are being produced or updated. The most common method to analyze the structure against progressive collapse is the alternate path method. In this direct design method, the critical columns be removed immediately and stability of the remaining structure is investigated. But there is no references talk about the effect of lateral resistant of the infill panels. This is one of the simplifier assumptions which are used in numerical studies of progressive collapse phenomenon in structures indicate inconsistency between the numerical and experimental full-scale results. Unlike numerical studies, experimental studies showed that the structure remain stable even if more than one column removed.
As a case study, in this research, a steel structure with 8 stories with moment resistant frame is analyzes and designed considering effect of unreinforced masonry infill panels (URM). URM infill panels in full contact with the frame elements on all four sides shall be considered as primary elements of a lateral force-resisting system. Recognizing this behavior, the stiffness contribution of the infill is represented with an equivalent compression strut connecting windward upper and leeward lower corners of the infilled frames. So, analytical macro-model based on the equivalent strut approach is used to simulate the effective infill panels. Potential of progressive collapse of the one of the peripheral frames is evaluated with the Opensees program based on the nonlinear dynamic analysis. Researchers found that linear static analysis might result in non-conservative results since it cannot reflect the dynamic effect caused by sudden removal of columns. So, time-history analysis should be applied to seek dynamical response of the structure.
Results indicate that considering effect of the infill panels increase axial force of the columns and decrease bending moment of the beams and nodes displacements. So results are closer to the experimental studies and prove stability of the structure after column removal and increase resistant of the building against progressive collapse.
As it distinct, modeling the infill panels in the analysis is complex and time-consuming, so in this research, the coefficients are proposed to apply to the load combinations instead of modeling the infill panels in order to closer the results together. The proposed coefficients are larger than one for columns forces and smaller than one for the beams forces.

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In seismic active zones, large mainshocks usually follow by numerous aftershocks. Because of the short time intervals between consecutive shocks, additional damage due to the accumulation of inelastic deformations from all sequences is increased and the structures that has been already damaged by the preceding shock collapse before any repair is possible. Moreover, despite the importance of seismic sequence phenomena on increased damage and the evidence of structural damage caused by the recent multiple earthquakes such as Nepal and Hindu-Kush (2015), most structures are designed according to the modern seismic codes which only apply a single earthquake on the structure in the analysis and design process. In this case, the structure may sustain damage in the event of the "Design earthquake", and this single seismic design philosophy does not take the effect of strong successive shocks on the accumulated damage of structures into account. For this reason, the effect of various parameters such as Peak Ground Acceleration, Magnitude, Shear Velocity Wave, Effective Peak Acceleration, Peak Ground Velocity, Epicentral distance, the time gap between first and second earthquakes, Period of reinforced concrete frames and etc, is examined on the damage of reinforced concrete frames under single and consecutive earthquakes. At first, six concrete moment resisting frames with 3, 5, 7, 10, 12 and 15 stories, are designed according to the Iranian Code of Practice for Seismic Resistant Design of Buildings (i.e. Standard No. 2800 guideline) and analyzed under three different databases with/without seismic sequences phenomena. For each database, single and consecutive earthquakes are selected according to Peak Ground Acceleration (PGA), Effective Peak Acceleration (EPA) and Peak Ground Velocity (PGV) criteria from Pacific Earthquake Engineering Research (PEER) and United States Geological Survey’s Earthquake Hazards (USGS) centers. At next step, in order to train the multilayer artificial neural networks with back-propagation learning algorithm, period of reinforced concrete fames (T) and some of earthquake features including PGA, PGV, EPA, magnitude (M), shear wave velocity in the station (Vs), epicentral distance (Epc) and time gap between consecutive earthquakes (Tg) as artificial neural network inputs and Park-Ang (1985) damage index - as the results of nonlinear dynamic analysis in OpenSees software and neural network target – are selected. For each database, 400 neural networks are designed with a different number of neurons in each hidden layer from 1 to 20 and ideal neural network is determined with the least value of Mean Square Error (MSE) and maximum value of regression (R) among all networks. Then, for considering the effect of input parameters on structural damage (Park – Ang 1985) caused by single and consecutive seismic scenario, the range and reference values for each group of input parameters – single and consecutive cases in each database – are chosen to be close to the median values and introduce to ideal neural networks and damage indexes are determined. The results show that structural damage caused by with/without seismic sequence scenario is more sensitive than other parameters to Magnitude and Acceleration for single earthquakes and the ratio of these parameters in the second shake to first for consecutive shocks.

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In this study, nonlinear behavior of an atomic force microscopes (AFM) immersed in acetone, water, carbon tetrachloride (CCl4), and 1-butanol is investigated using non-classical strain gradient theory. In this theory, the size effect of system is taking into account by means of material length scale parameter. The nonlinear behavior of the AFM is due to the nonlinearity of the AFM tip–sample interaction caused by the Van der Waals attraction/repulsion force. Behavior of micro beam immersed in liquid is completely different with its behavior in air and vacuum due to the existence of hydrodynamic force. The Resonant frequencies, mode shapes, governing nonlinear partial and ordinary differential equations (PDE and ODE) of motion, stability analysis, boundary conditions, potential function and phase-plane of the system are obtained analytically in the present study. Furthermore, the results are compared with the one obtained by the modified couple stress theory. For this purpose, the AFM and the probe at the free end of micro beam are modeled as a lumped mass. The fixed end of micro beam is excited by piezoelectric element. The nonlinear PDE of motion is derived based on Euler-Bernoulli theory by employing the Hamilton principle. The Galerkin method is utilized to gain the governing nonlinear ODE of motion and the obtained ODE is analytically solved by means of perturbation techniques.

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Dams are one of the most important structures which are built to prepare water for different usages such as drinking, agriculture, industrial, flood control and hydro power generation. Due to the importance of dams and increasing number of terrorist attack, Stability of dam structures against blast loading is important. Dam responses depend on magnitude of released energy and if the dam structure could not be able to resist and maintain its stability against this energy, irreparable consequences will happen. Explosion is a sudden release of energy which could be like gases combustion, nuclear explosion or any kind of bombs. TNT unit usually used as reference to determine the explosion power. Some of basic properties of an explosion are random location of explosion, transient loading and short time loading (up to few seconds). When blast loading happened, energy will released suddenly and this released energy include thermal radiation and wave scattering in air and earth. The waves which scattered in the air are the main factor to structural damage. These waves move faster than sound wave velocity and impact the structure. Due to reflex from structure surface, the pressure of these waves increased and also some air waves penetrate to structural elements from openings such as doors and windows. This process continues until all available parts of the structure affected by pressure waves. In this research, the effects of blast loading on Karun 4 dam are investigated. To this purpose, dynamic analysis of dam-reservoir-foundation system is performed by finite element model using ABAQUS software. Dam-reservoir-foundation modeled three dimensional in which reservoir length is three times greater than dam height. The foundation modeled as a hemi-sphere with a radius of three times greater than dam height. Non-linear material behavior also considered by using concrete damage plasticity method. The CONWEP theory is used to model blast loading. To verify the blast modeling theory and software abilities, a steel plate which investigated under blast loading in references has been modeled and the results shows same responses with the paper. The responses of dam are investigated under two different reservoir conditions include full and empty conditions. Analysis also done for three different explosion points in three different elevations. Explosion points are near base, mid height of dam and near dam crest respectively. All these points have 10 m distance from dam structure. TNT mass used in each noticed conditions, is the minimum amount of TNT which cause damage in dam body. The results indicate that the responses of the dam is very sensitive to mesh dimensions. The results also show, water level has not great effect on explosive mass which is needed for structural damage of the dam. In both full and empty reservoir conditions, when explosion happened near the dam crest, the displacement is more than other cases. It is noteworthy that when the explosion happened near the crest, the maximum displacement of the crest and the point in front of the explosion point, occur in same time but when the explosion point is in middle and also near the base, theses displacements are not in same time.

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Time history analysis, which is the most important analysis tool in performance-based seismic design, has become more and more popular worldwide. In the seismic design, seismic demand is mainly governed by three factors including the peak value of ground motion, the characteristic of earthquake spectrum and duration. An earthquake intensity index of ground motions is normally used as a scaling parameter that is critical for seismic analysis and design. A number of researchers have, from their own perspective, proposed various intensity indices. However, due to the complexity and randomness of earthquake motion, it has been a difficult task to accurately evaluate the applicability of various existing intensity indices. In addition, an objective and quantitative method is lacking in the evaluation of the applicability of such indices. This has been a challenging issue in seismic engineering research and has become a fundamental problem in performance-based seismic design. Nonlinear structural response is often highly sensitive to the scaling of input ground motions. Thus, many different ground motion scaling methods have been proposed. The “severity” of an earthquake ground motion is often quantified by an intensity measure, IM, such as peak ground acceleration, PGA, or spectral acceleration at a given period. The PGA of a record was a commonly used IM in the past. More recently, spectral response values such as spectral acceleration at the fundamental period of vibration have been used as IM. Scaling of ground motions to a given spectral level at the fundamental period of vibration significantly decreases the variability in the maximum demand observed in the structural system. However, it is widely known that for records with the same spectral acceleration at the fundamental period of vibration value, spectral shape will affect the response of multi-degree of- freedom and nonlinear structures, because spectral values at other periods affect the response of higher modes of the structure as well as nonlinear response when the structure’s effective period has lengthened. Similar attention to the influence of nonlinear behavior of a structure on the period of vibration led to an IM that accounts for period softening to reduce variability at high levels of maximum inter-story drift ratio, drift demands larger than 5%, for composite structures. Previous studies have focused on evaluation of different ground motion scaling methods in single-degree-of freedom and buildings of multi-degree-of-freedom with shear-type behavior or common steel-moment frame structures. However, over the last decade, the performance-based seismic design philosophy has emerged as a promising and efficient seismic design approach. The novel Performance-based plastic design (PBPD) approach explicitly accounts for the inelastic behavior of a structural system in the design process itself. PBSD approaches based on plastic analysis and design concepts were recently developed for different lateral load resisting systems such as steel moment resisting frames, steel braced frames, etc. In these design methods a pre-selected yield/failure mechanism and a uniform target drift (based on inelastic behavior) were considered as performance objectives. The analytical validation of these methods showed that structures designed using these methods were very effective in achieving the pre-selected performance objectives. Considering a gradual shift towards PBSD for seismic design methods in general, this study is aimed at examining the effects of six different IMs on the estimation and distribution of the maximum inter-story drift for three short, moderate, and long-period steel-moment resisting frames designed with PBPD method buildings using the concepts of efficiency and sufficiency. An ensemble of 42 far-filed earthquake ground motion without pulse characteristics were used and scaled based on two target spectrum MCE and Design Response Spectrum to conduct nonlinear dynamics analyses by using OPENSEES. Results indicate that, the cod-compliant scaling method was not reliable for nonlinear dynamic analyses of structures designed by PBPD method, and cloud be very sensitive to the ground motion characteristics. Among them, depending on the number of stories, the three scaling methods including scaling ground motions to a given PGA and those that take into account for periods of higher modes generally decrease the variability in the maximum demand observed in the structural systems.

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