فهرست مطالب javad vaseghi amiri

Resistance to explosive loads is one of the most important aspects of the design of critical structures due to the wide range of threats. With the widespread use of concrete in the development of explosion-resistant structures, there is much research on the response of concrete components against dynamic loads such as explosion and impact in the technical literature. This paper experimentally and numerically investigates the simultaneous effect of increasing the density of concrete and reinforcing it with hooked-end steel fibers against blast caused by the high explosive pentolite mixture (C-4). In this regard, a total of 150 cubic and cylindrical concrete specimens containing steel fibers in different volume fractions (0, 0.5, and 1%) were made to evaluate the physical and mechanical properties of normal-weight (NW) and heavyweight (HW) concrete. After examining the NW and HW concrete mix designs in the laboratory, blast field tests were conducted by directly attaching the explosive on a set of six concrete blocks with dimensions of 50×50×10 cm fabricated with optimal mix designs of both concrete types. Subsequently, numerical modeling of the concrete behavior against blast load was performed using LS-DYNA finite element software to validate the examined parameters, and then the modeling output was compared with laboratory and field results. The results obtained from the finite element modeling indicate an acceptable agreement with field observations and laboratory results

In recent decades, construction of massive structures has increased around the world. These structures which are built for different purposes, should have a useful life-time. Therefore, composite structures can increase the construction costs and useful life of such structures. Therefore, CFDST members can practically increase the efficiency of composite structures. Since the diameter and thickness of the outer tube have the greatest impact on determining the bearing capacity of CFDST columns, this study investigated four different values of D/t for the outer tube including 86, 85.8, 45.6 and 44. Two types of concrete were used to fill CFDST samples including C10 and C20. Besides, a comprehensive formula for estimating the bearing capacity of CFDST columns using artificial intelligence methods is proposed. The results showed that CFDST columns with lower Do / to value have higher load capacity and with increasing Do / to, it is observed that the ductility decreases. As a comparison between the filled concretes in CFDST samples of C10 and C20 grades, the stress-strain curves have shown that with increasing compressive strength, the bearing capacity for CFDST columns increased by about 6%. In the modeling section, the R2 for artificial neural network (ANN), support vector machines (SVM) and model tree (M5-MT) in the testing stage were determined to be 0.95, 0.96 and 0.97, respectively. Accordingly, the M5-MT method in the testing stage, had a better performance than other proposed methods for estimating the bearing capacity of CFDST columns. Comparison of traditional equations and AI models in estimating the bearing capacity of CFDST columns show that the formula presented by M5-MT with an average value of 1.01 and a standard deviation of 0.19 It has performed better in modeling the bearing capacity of CFDST columns than other intelligent models and traditional relationships.

In engineering designs, structural analysis is generally performed assuming a rigid base. While introducing the effect of structural substrate flexibility on the response and dynamic properties of structures is important. Introducing different solutions to reduce the response of the structure against dynamic forces is another important issue in engineering designs. In this paper, the passive Pall friction damper system has been used for this purpose. In the researches that have been done so far, various optimization methods have been used for the optimal design of friction dampers, but in most of these methods, the effect of soil-structure interaction has not been considered for friction dampers, while in earthquake soil-structure interactions is important. One of the main objectives of this study is to investigate the effect of soil-structure interaction on the optimization of friction dampers. The actual forces and displacements of a structure due to free-surface seismic movements can be determined by considering the effects of soil-structure interaction. In this regard, in this paper, two-dimensional frames of 4, 8, and 12 floors equipped with dampers were analyzed in nonlinear structural analysis software under seven accelerometers using nonlinear time history method once, considering the effect of soil-structure interaction and introducing 3 Different lateral loading patterns and again without this effect. The results show that considering this issue in terms of cumulative triangular slip load pattern has increased the loss of earthquake input energy.

A convenient family member of composite columns is concrete filled double skin tube (CFDST); it contains two tubes of concentric steel and also shell concrete that there is between them. The superior or equal potential offering characteristics of CFDST columns are more than counterparts of filled steel tubes of classical concrete (CFST). The purposes of the present study are to provide experimental investigation results into the prestressed load-carrying capacity of CFDST columns. Here, an innovative and novel technique is used to confined concrete prestressing, in which the fresh concrete is compressed for a short-run duration. Sixty-four total specimens were tested with various outer thickness (to) and diameter (Do), inner thickness (ti) and diameter (Di), and also CFDST columns of concrete strength that resist axial compression. The experimental results support that the present technique prestressed confined concrete, and it demonstrates that CFDST specimens' load-carrying capacity enhanced significantly. The development of an accurate and simple formula for the short circular of CFDST columns' ultimate axial strength prediction in terms of active/passive confinement, concrete compressive strength, Di/ti, and Do/to is the other purpose of the present study.

Viscous dampers, as a special type of passive dampers, are efficient tools for seismic improvement of the buildings and eliminating the effects of seismic pounding during earthquakes. Such dampers can be installed between the adjacent buildings or inside them and also at different story levels. This paper attempts to investigate the effects of different damper placements. Accordingly, two single degree of freedom systems connected by viscous dampers were considered first and the dynamic characteristics of the system have been determined. Then, relative displacements of the buildings are compared with damping inside building for one-story models and with free undamped response for two-story models, under ideal white noise seismic input. Absolute accelerations of the one-story buildings are also compared by assigning the Kanai-Tajimi filtered white noise as the input. Results reveal that using dampers between buildings is more suitable than using them inside buildings, as the relative displacement between adjacent buildings would be reduced for lower values of uncoupled frequency ratios of structures and the absolute acceleration would also be significantly reduced for both buildings, rather than just for one of them. Furthermore, use of dampers at the top level would be more beneficial than using them throughout the height of the systems for seismic pounding mitigation.

The present paper investigates and compares the crack propagation in concrete gravity dams using two models of linear fracture mechanics and plasticity damage concrete. The first model is based on linear concrete behavior using the extended finite element method without considering the effect of strain softening on the crack tip while the second model is based on the nonlinear concrete behavior and the strain softening in tension with damage parameter. According to two different algorithms and based on two models, several benchmark examples are reviewed and the results compared with those reported in the literature. Then, path of the crack growth in Koyna gravity dam due to a seismic excitation of Koyna earthquake in 1962 has been performed by considering the dam-reservoir interaction. The results show that due to low compressive stresses during analysis of concrete gravity dams, consideration of compressive nonlinear behavior has no effect on crack initiation and almost is the same for two models. However because of crack opening and closing with tapping the crack faces together in extended finite element model, the compressive stress will be more than the allowable stress of concrete. Crack initiation at downstream and upstream face occurred at angle of 90 and zero degrees respectively, which in both models, the numerical results are in agreement with the experimental model. The crack in the extended finite element model grows faster such that the crest block of dam in this model is separated from the dam body, earlier than the concrete plastic damage model. Also the values of dam crest displacement and hydrodynamic pressure in the reservoir in extended finite element model with linear elastic fracture mechanic are more than the other model, which can be attributed to the linear and nonlinear behavior of concrete in extended finite element and concrete plastic damage model respectively. In the extended finite element model, due to using linear fracture mechanic, the maximum principal stress in the cracked elements reaches the values greater than the maximum tensile strength, but in the concrete plastic damage model as soon as the stress reaches a tension limit value, elements are damaged and the stress is reduced. In both models, the crack located at the slope change area, propagates with the downward slope from downstream dam face and connects to the crack at upstream face which is growth horizontally. Because of laboratory sample dimension and boundary condition of dam-reservoir compared with actual manner, neither of two crack profiles covered the experimental model, accurately. But it is shown that the crack profiles in the extended finite element model are more consistent with experimental results. Finally, the results show that the crack profile are slightly different in the two models because of quasi brittle behavior of the dam concrete, which can be attributed to the small fracture process zone of the crack tip in comparison with the dimension of the concrete gravity dams such that by removing strain softness part, the error in the amount of additional computation can be neglected.

The prediction of nonlinear behavior of shear walls under lateral loads requires simple and precise analytical models representing the nonlinear behavior of the shear walls as well as the experimental results. In recent decades, various analytical models have been proposed by researchers to predict the nonlinear behavior of reinforced concrete shear walls in order to consider the most important behavioral properties of the wall. The model used for analysis and design of the wall should be simple and accurate enough to predict the hysterical behavior. Using the method of fiber section elements can be considered as one of the most accurate methods in wall modeling. But due to the complexity of the modeling and high duration of the analysis, the use of other methods is suggested. Considering the recent progress in modeling nonlinear behavior of materials to provide more accurate behavior against cyclic and seismic loads, these material models can be directly used in modeling of structural systems. In this study, in addition to using the fiber section model, a method based on axial elements is also used. It is implemented by the concept of equivalent spring’s behavior and using the advanced material models in the platform of OpenSees. The results show that the use of this simplified model compared to the fiber method, in addition to reducing the computational cost in both aspects of the duration of modeling and analysis, also satisfies the desired accuracy.

The serviceability and safety of building and bridge structures may gradually degrade due to long term use, natural environmental deterioration, poor initial design and/or construction, increased design loads, extreme events such as earthquakes, and/or lack of maintenance. It is needed to use materials that are not susceptible to the same causes that trigged the deterioration in the first instance. Since the rebuilding of old infrastructure is very expensive and time consuming, cost-efficient and durable techniques of strengthening/rehabilitating are needed. There is a growing interest in the use of Fiber Reinforced Polymer (FRP) composite materials for the strengthening and retrofitting of concrete beams. Also it is too important to know the influence of effective parameters on the fractural behavior and strengthening of cracked concrete beams, so the present study is dedicated to numerical analysis of carbon fiber reinforced polymer (CFRP) sheet-strengthened notched concrete beams. The bearing capacity, CFRP debonding and crack mouth opening displacement (CMOD) of notched beams were evaluated with nonlinear finite element method. Numerical results have been compared with experimental results of other researches. After validation the numerical modeling of CFRP plated notched beams, parametric studies reveal quantitatively the effects of various factors such as; thickness of CFRP sheet, compressive strength of concrete, initial crack length and interfacial bond strength on the CMOD curve of mentioned beam, which is found to be characterized by two peak loads. The first and second peak loads increase with the increase in concrete strength, the thickness of CFRP sheet and the interfacial bond strength. It is also found that increasing the initial crack length decreases the first peak load but no exerts on the second peak load.

Tuned mass damper is a common tool in passive control, which is used in many structures. However, with all the proper features, its most important functional limitation is the weakness against broad band excitation. Various methods have been proposed to overcome this problem, among which using hysteretic damping of materials with nonlinear behavior is known effective. Among materials with nonlinear behavior, shape memory alloys have good features and large hysteresis loops. Hence, in this paper, using nonlinear stiffness and hysteretic damping of a shape memory alloy spring, linear stiffness and viscous damping of a common tuned mass damper are replaced. Then, the modified damper has been used to control responses of a single degree of freedom structure under harmonic loadings and the effect of the loading amplitude on the control of the structural responses was determined. Subsequently, the damper has been used to control seismic responses of single degree of freedom structures to compare its performance under broad band seismic loadings with the performance of conventional tuned mass dampers. Results of the analyses show that the characteristics of shape memory alloys can adequately control the impact of the loading amplitude on the performance of nonlinear mass dampers. Also, the presence of hysteretic damping can significantly improve control of seismic responses of single degree degrees of freedom structures compared to conventional tuned mass dampers, provided that dynamic properties of the nonlinear mass damper take their optimal values.

Usually during moderate and strong earthquakes, if the space between the adjacent structures is not enough, structural pounding will occur which is due to the out-of-phase vibration of the structures regarding the differences in dynamic properties of the structures. Different factors contribute in the state of structural pounding, and researchers have already studied several factors including the space between two structures, the type of pounding, force transmission means, reducing the pounding damage and etc. Record is one of the factors which can affect the structural pounding, and one of the most crucial parameters of record is strong ground motion duration which its analysis is very important in the quality of structural response.In this study, effect of strong ground motion duration on the adjacent structural pounding is investigated. To do so, steel moment resisting frames in 5, 8 and 12 floors are designed which are paired together in three forms of 5-8, 5-12, and 8-12 models for nonlinear time history analysis. In this analysis, 15 records that include three intervals for strong ground motion duration are being implemented (0 to 10 seconds, 10 to 30 seconds, 30 seconds and more). Besides, in order to obtain the pounding force between structures, linear elastic elements are used in floor levels.Results show that in structures in which the space equal to 1% of their heights are not considered, with increasing the strong ground motion duration, maximum structural pounding will also increase with a smooth gradient, and for the structures with space equal to 0.5% of their heights, this increase rate is more compared to the structures with no space.

Seismic pounding occurs as a result of lateral vibration and insufficient separation distance between two adjacent structures during earthquake excitation. This research aims to evaluate the stochastic behavior of adjacent structures with equal heights under earthquake-induced pounding. For this purpose, many stochastic analyses through comprehensive numerical simulations are carried out. About 4.65 million time-history analyses were carried out over the considered models within OpenSees software framework. Various separation distances effects are also studied. The response of considered structures is obtained by means of Hertzdamp contact element. The models have been excited using 25 earthquake records with different peak ground accelerations. The probability of collision between neighboring structures has been evaluated. An efficient combination of analytical and simulation techniques is used for the calculation of the separation distance under the assumptions of non-linear elasto-plastic behavior for the structures. The results obtained through Monte Carlo simulations show that use of the current provision’s rule may significantly overestimate or underestimate the required separation distance, depending on the natural vibration periods of adjacent buildings. Moreover, based on the results, a formula is developed for stochastic assessment of required separation distance.

Progressive collapse is usually defined as follows: the expansion of an initial local damage within structures as a chain chemical reaction that led to the partial or total collapse of the structure. Studies in relation to the failure of structural systems in recent years highlight the importance of the phenomenon of progressive collapse caused by abnormal loading such as: accident injuries, earthquake, explosion, and etc. in order to prevent or reduce the occurrence of progressive collapse, various strategies are provided for design against progressive failure in American government documents such as: GSA and UFC. In these regulations, the loss of bearing capacity of column is considered as a promising phenomenon to investigate the performance of structure against abnormal loading. In this study, three steel structures with dual side load bearing system, average moment frame, and bracing system of 5, 10, and 15 floors were designed in Etabs 2013 software; then, using the GSA2003 regulation and selecting the method of alternative route of load transformation, foregoing structures were modeled as three-dimensional in OpenSEES software; and using nonlinear static analysis and nonlinear dynamic analysis, structures has been investigated against progressive collapse; and the results of nonlinear static analysis and nonlinear dynamic analysis compared with each other. After reviewing the results of the analysis showed that for every two analysis, removing the side column is most critical state of column elimination. In all three structures, removing the column in ground floor level creates most critical state for structure towards higher levels. By increasing the height of the structure, robustness index also is increased.

This paper concentrates on the presentation of a closed form solution of dam-reservoir interaction considering reservoir viscosity and bottom absorption under harmonic excitations. For this purpose, the governing equations of dam-reservoir are solved in the frequency domain and using the obtained frequency response function, the effects of different parameters are considered on dam or hydrodynamic pressure response. The results show that consideration of reservoir viscosity creates an imaginary term and the responses depend on values of viscosity, bottom absorption, exited period and dam-reservoir period.

The determination of structural and nonstructural damage under earthquake excitations is usually considered as a key factor in performance-based seismic design (PBSD) methods is In this regard, various damage indices have been developed in recent years to quantitatively estimate structural damage. The aim of this study is to develop a simple method to evaluate performance levels of zipper-braced frame (ZBF) structures by using damage indices based on the results of nonlinear static and dynamic analyses. To this end, 5, 7, 10, 12 and 15 story zipper-braced frames (ZBF) are modeled and undergone to twenty different synthetic ground motion records and their damage values have been computed. In dynamic damage analysis procedure, the performance levels of the ZBF models have been computed based on the FEMA-356 standard. Considering the results of the nonlinear dynamic analyses, the correlation between FEMA-356 performance levels and damage indices has been investigated and some simplified formula is presented. On the other side, in static damage analysis approach, by using pushover analysis the performance points of ZBF models have been estimated based on capacity spectrum method (CSM) provided by ATC-40 standard. Then, the correlation between ATC-40 performance levels and some static damage indices has been investigated and some simple equations have been proposed. These relations can be utilized to estimate the performance levels of structures from damage indices. Finally, tables are represented for determination of the structural damage index values for assumed performance levels of the ZBF structures based on static and dynamic damage analysis.

Construction of large-scale structures has been considered as one of the human's main achievements. With their suitable view and high economical aspects, High-strength steel cables have been developed for analysis and erection of cable-stayed bridges in light of high speed development in computer technology. This type of bridges, while providing different behavior due to cable flexibility, has been recognized as one of the most practical choices for mid to large span bridges. This paper studies the non-linear dynamic behavior of cable bridges and the effect of some parameters (such as cable arrangement and shape of pylon) on them. For this purpose, CSI Bridge software with the direct integration method of dynamic analysis has been used and the behavior of structure under different earthquake components has been analyzed for various conditions of cable arrangements and pylon shapes. Results indicate that the most suitable behavior would be for cable bridges with H-shape pylons arranged in series and also with A-shape pylons in radial arrangement.

One of the most common connecting systems that is used for the manufacture of space structure, is MERO connecting system. So a perfect knowledge of the behavior of this connection is the most essential problem for the engineers and designers. The results has shown that different parameters including the degree of tightness of the bolts affect on the static and dynamic characteristics on these kinds of jointings. Distinctly in this research the effect of the bolts tightness degree and the magnitude of the vibration on the dynamic characteristics of the MERO connecting system has been experimentally evaluated. Therefore a member made by this system in the form of a cantilever beam at different degree of bolt tightness and by the free vibration method and by making initial displacement was experimented in order to determine the quantity of dynamic system parameters including the damping ratio, damping coefficient and the natural frequency. The results obtained showed that, the magnitude of bolt tightness is directly proportional to the structure vibration. Also the maximum damping ratio and damping coefficient occurs at 60 Nm bolt tightness, and if the tightness degree is more or less than this quantity, the magnitude of damping ratio and damping coefficient would be less than that. Also the results showed that there is a direct relation between the initial displacement with the damping, it means that by increasing the vibration amplitude the damping will increase and vice versa.

Effect of soil-structure interaction (SSI) and site condition on elastic and inelastic seismic demands of structure is investigated in this research. More emphasis is on Effect of soil-structure interaction and site condition is assessed by using earthquakes recorded on different site classes C, D and E (NEHRP [1]) where SSI has a significant effect on the structural demands because of noticeable difference between the stiffness of soil and the structure. In order to develop a comprehensive statistical parametric study, the structure is modeled as an elastic-plastic single degree of freedom (SDOF) system and the soil beneath the foundation is supposed as a homogeneous half space and is idealized by fundamental lumped mass parameters based on the concepts of the cone model (Wolf, 1994[17]). To consider the frequency dependency of soil’s dynamic stiffness in this model, the soil is represented with a three-DOF system that introducing an internal DOF in the soil can represent this effect. Then the whole of soil-structure model is analyzed under 45 earthquakes recorded on previously mentioned site classes. A parametric study is done for a wide range of non-dimensional parameters controlling the SSI effects on structural demands. It is concluded that SSI affects on structural seismic demands such as elastic and inelastic strength, strength reduction factor, ductility demand, and inelastic displacement ratio especially for structures located on site class E. Results exhibit remarkable differences in comparison to fixed base one. Consequently, in some cases, using parameters derived for fixed base case, lead to non-conservative results in design and assessment parameters of structures located on soft soils.

When structures are subjected to strong ground motion excitations, structural elements may be prone to yielding, and consequently experience significant levels of inelastic behavior. The effects of inelastic behavior on the distribution of peak floor loads are not explicitly accounted for in current seismic code procedures. During recent years, many studies have been conducted to develop new design procedures for different types of buildings through proposing improved design lateral load patterns. One of the most important parameters of structural damage in performance-based seismic design is to limit the extent of structural damages (maximum inter-story ductility ratio) in the system and distribute them uniformly along the height of the structures. In this paper, a practical method is developed for optimum seismic design of zipper-braced frames (ZBF) subjected to seismic excitations. More efficient seismic design is obtained by redistributing material from strong to weak parts of a structure until a state of uniform ductility ratio (damage) prevails. By applying the proposed design algorithm on 5, 10 and 15‐storey zipper-braced frames subjected to 10 synthetic seismic excitations, the efficiency of the proposed method is investigated for specific synthetic seismic excitations. The results indicate that, for a constant structural weight, the structures designed according to the proposed optimization algorithm experience up to 50% less global ductility ratio (damage) compared with code-based design structures.

This study focuses on non-linear seismic response of a concrete gravity dam subjected to translational and rotational correlated components of ground motions including dam-reservoir interaction. For this purpose rotational components of ground motion is generated using Hong Non Lee improved method based oncorresponding available translational components. The 2D seismic behavior of the dam concrete is taken into account using nonlinear fracture mechanics based on the smeared- crack concepts and the dam-reservoir system are modeled using Lagrangian Lagrangian approach in finite element method. Based on presented formulation, Pine Flat concrete gravity dam is analyzed for different cases and results show that the rotational component of ground motion can increase or decrease the maximum horizontal and vertical displacements of dam crest. These results are dependent on the frequency of dam reservoir system and predominant frequencies of translational and rotational components of ground motion.

Agreat number of studies on the vibration of plates subjected to moving loads are available which are gained by the moving force and moving mass modelling frameworks. As a result, evaluating the reliability of approximate simulation of a moving oscillator problem through moving force/mass would be of interest in engineering application. Therefore, in this article, transverse vibration of a thin rectangular plate under a traveling mass-spring-damper system is revealed by eigenfunction expansion method. Both moving force and moving mass modelling approaches are compared with the moving oscillator and various plate fixity cases and load trajectories are involved to present benchmark solutions. The spring stiffness range for which the plate response agrees closely with the corresponding moving force/mass analysis is recommended. The results elucidate that the moving mass can be considerably unrealistic in predicting the contact force of an undamped oscillator. Moreover, errors of orbiting force/mass simplification of the orbiting oscillator in predicting the resonant conditions of the plate vibration are not negligible.

Agreat number of studies on the vibration of plates subjected to moving loads are available which are gained by the moving force and moving mass modelling frameworks. As a result, evaluating the reliability of approximate simulation of a moving oscillator problem through moving force/mass would be of interest in engineering application. Therefore, in this article, transverse vibration of a thin rectangular plate under a traveling mass-spring-damper system is revealed by eigenfunction expansion method. Both moving force and moving mass modelling approaches are compared with the moving oscillator and various plate fixity cases and load trajectories are involved to present benchmark solutions. The spring stiffness range for which the plate response agrees closely with the corresponding moving force/mass analysis is recommended. The results elucidate that the moving mass can be considerably unrealistic in predicting the contact force of an undamped oscillator. Moreover, errors of orbiting force/mass simplification of the orbiting oscillator in predicting the resonant conditions of the plate vibration are not negligible.

Buckling-restrained braced frames (BRBFs) are used as primary lateral load resisting systems for buildings in high seismic areas. The main characteristics of buckling-restrained braces (BRBs) are enhanced energy dissipation potential, excellent ductility and nearly symmetrical hysteretic response in tension and compression. This paper presents an analytical study aimed to assess the feasibility of using BRBs as a retrofit scheme for existing steel frames. For that purpose, the seismic response of four two-dimensional frame models representative of typical steel frames was analyzed prior to and after including BRBs as a retrofit strategy. Comprehensive nonlinear static and time-history analysis were carried out for analyzing the frames. A set of seven code-compliant natural earthquake records was selected and employed to perform nonlinear time-history analysis. Frames were analyzed and designed based on 2800 Iranian Seismic Code. The evaluation was based on comparing seismic displacement demands such as target roof displacements, maximum displacements and inter-storey drifts. The results showed that the effectiveness of braced frames can vary significantly with ground motions, frame rises and retrofitting systems. It perceived that the seismic response of frames coincides well with the given design objectives.