فهرست مطالب seyed mohammadreza hasani

In this article, the Jennings method has been improved by using the third-order spline interpolation function. In this research, in order to be able to compare the advantages and disadvantages of the Jennings method, which is based on exact relationships and the assumption of linear changes of the excitation, compared to the spline interpolation method, a damped one-degree-of-freedom linear system under sinusoidal harmonic loading is considered has been taken. Exact values of the deformation response coefficient of this system are available for different excitation frequencies. The approximate values of the deformation response coefficient of this system for different excitation frequencies were calculated assuming linear interpolation of excitation and also assuming interpolation with spline function and were compared with their exact corresponding values. This work was done for two, five, ten and twenty percent damping. The results of the work indicated that when the number of points by which the sine wave is approximated is small and also the amount of damping is low, the interpolation with the spline function has a significantly higher accuracy than the linear interpolation mode. Another noteworthy point is the high sensitivity of the cubic spline interpolation to the level of system damping, so that for a certain number of divisions, the error value is highly sensitive to the damping value. Thus, with the increase of damping, the sign of the error changes and its value increases strongly for the number of large divisions.  This phenomenon is not seen at all in linear interpolation.Therefore, considering that in interpolation using the spline method, the continuity of the slope and the second derivative are maintained in the internal points, it is suggested to use this method to calculate the dynamic response of linear systems and the results obtained with the results of Jennings' method should be compared and appropriate decisions should be made if there is a significant difference in the analysis results obtained from these two methods.It is important to mention that the execution time of the computer program related to the cubic spline interpolation method is longer than the Jennings method. Because in the spline interpolation method, it is necessary to solve the system of linear equations with the number of unknowns equal to four times the number of intervals, while in the excitation linear interpolation method, it is not necessary to solve the system of equations at all and simply by having the velocity and displacement at the beginning of the step and assuming that the excitation is linear during the desired time step, the coefficients A, B, C, D, A', B', C', D' were calculated and using this eight coefficient, velocity and displacement at the end of the time step are obtained. It should be noted that if the time interval between different accelerogram points is constant, it is sufficient that the coefficients A, B, C, D, A', B', C', D' are calculated only once.

In this paper, the pseudo-acceleration spectra of the three accelerograms of El Centro, Naghan and Tabas, which were obtained by the linear interpolation method of stimulation (Jennings’s method) and also by using the cubic spline function, are compared with each other. Jennings' method is based on linear interpolation of excitation and is acceptable for short periods of time. In this research, three accelerograms of El Centro, Naghan and Tabas were considered, and their pseudo-acceleration response spectrum (base shear coefficient) was calculated for different damping values using linear interpolation method and interpolation with spline function. The considered damping were zero, two, five, ten and twenty percent. In order to determine the difference between the linear interpolation method and the spline function interpolation method, the time interval between the accelerometer points was divided into 2, 5, 10, 20, 50, 100, and 200 equal parts, respectively. Then, once with the linear interpolation method and again with the spline interpolation method, the internal points between the internal acceleration points were obtained and their pseudo-acceleration spectrum was calculated using the Jennings method and the interpolation method with the spline function. The calculation results showed that the pseudo-acceleration spectrum values calculated by the spline interpolation method in very short periods and low damping have a significant difference with the corresponding values of the pseudo-acceleration spectrum calculated by the linear interpolation method. It should be noted that the initial time interval between the accelerogram points of El Centro, Naghan and Tabas is 0.02 seconds.

The main purpose of this paper is to develop a design guide for locating underground urban shelters based on the criteria that are used in the passive defense. Today, the passive defense alongside the defense is one of the security systems for residential settlements of interest to urban planners; underground shelters are one of the main examples of the passive defense among them. Underground shelters once more emphasize that sub-surface structures such as subway lines and spaces such as underground shops and shopping centers are defined as urban elements in the urban spatial structure.The multiple uses of metro stations, both in times of peace and in times of crisis, have increased their functional importance. Metro stations will be used as shelters in war, but in terrorist attacks, they will be considered as a place to kill people. In order to preserve human capital and reduce the vulnerability of people and places as much as possible, it is necessary to examine these cases and consider the effective role of passive defense in showing the subway station as a safe haven in crisis situations and creating a safe place. Therefore, by applying non-active defense in the metro station, the crisis can be better controlled to some extent and the metro station can be considered as a strategic and busy place to establish security.

Usually, by modeling the structures using the finite element method, their undamped free vibration frequencies are calculated analytically. In addition, the issue of accurate calculation of natural frequencies and the shape of vibration modes corresponding to them for bending systems that have distributed mass and elasticity and possibly a combination of several bending beams, sometimes requires solving complex mathematical equations and requires a relatively heavy mathematical work demands. Bending beams are beams whose axial deformations is insignificant compared to their bending deformations, and as a result, these members are assumed to be axially rigid. By using the conventional finite element method, the natural vibration frequencies of these beams can be obtained approximately. By increasing the number of finite elements used in the model, the calculation error of natural frequencies of vibration decreases. When the consistent-mass matrix is used, the frequency values obtained from the finite element method converge to the exact frequency values with larger values, while if the lumped-mass matrix is used, the frequency values obtained from the finite element method converge to the exact frequency values with smaller values. It should be noted that the consistent-mass matrix is non-diagonal, but the lumped-mass matrix is diagonal. The interpolation functions (shape functions) used for bending finite elements (beam elements) are polynomial functions of the 3rd degree. This bending finite element has two nodes, each node has one translational degree of freedom and one rotational degree of freedom. The new idea that came to the authors of this article is that instead of using polynomial functions, trigonometric and exponential interpolation functions are used to calculate the stiffness matrix and mass matrix of the finite element. In fact, these trigonometric and exponential functions are the solutions of the differential equation governing the free vibration of bending beams with distributed mass and elasticity. The argument of these trigonometric and exponential functions includes a parameter called beta, which is proportional to the square root of angular frequency of the bending beam. By changing this parameter in a suitable range and with a certain step, it is possible to plot the changes in the frequencies of the different modes of the studied prismatic beam in terms of beta. In this paper, three models were studied, which included a uniform cantilever beam, a uniform beam clamped at left side and simply supported at right side, and a uniform beam free at both ends. Using the conventional finite element method and using the consistent-mass matrix, these three models were analyzed and the approximate frequencies of the first few modes of these beams were calculated, which were greater than their corresponding exact values. In the innovative method presented in this article, a uniform beam was modeled with a finite element model with one translational degree of freedom and one rotational degree of freedom. The stiffness matrix and the mass matrix of this beam were calculated for different betas and having these two matrices, the first and second frequency values of this model were calculated for different beta values and its graph was drawn for different betas. The values of the maximum frequency of the first frequency are the same as the values of the minimum frequency of the second for certain betas, and by specifying these betas, the frequencies of different vibration modes can be accurately determined. The detected frequencies of different modes with this method had a very good match with their exact corresponding frequencies. For the second model investigated in this paper, one rotational degree of freedom was considered. Considering that this beam had only one rotational degree of freedom, therefore, by plotting the first frequency of this model for different betas and finding its minimum, the frequency values of different modes of this beam were obtained, which matched the exact values like the previous model very well. The third model was the same as the previous two models. The diagram of the first to fourth natural frequencies of this model was drawn for different betas. By having the approximate values of the frequencies of different modes obtained from the conventional finite element method and these diagrams, the frequencies of different modes of the model were identified, which were in good agreement with their corresponding exact values.

Green concrete can be produced as environmentally friendly concrete using recycled materials. In this study, the compressive and endurance behavior of green concrete containing waste aggregates as a replacement to coarse (gravel) and cement-replaced marble powders was evaluated experimentally. In the present research, 10 mix designs in addition to the control sample mix design were constructed and compared. The main variables included the volume percentage of waste aggregates as replacement sand with percentages of 0, 25, 50 and 100, and the volume percentage of marble powder as replacement cement with percentages of 0, 10 and 20. Compressive strength at 28 days and water absorption percentage and density tests were performed to evaluate the endurance of concrete. According to the results, gravel substitute waste stone had a negative effect and cement substitute marble powder had a positive effect on the compressive strength of concrete. By replacing gravel with 100% waste stone, approximately 49% reduction in strength was observed compared to the control sample, and by replacing cement with 10% marble powder, approximately 13.4% increase in resistance was observed compared to the control sample, demonstrating the minimum and maximum changes in compressive strength of concrete compared to the control test, respectively.

The vulnerability and improper seismic behavior of beam-column connections in reinforced concrete structures designed based on past codes without seismic considerations has been established through data obtained from experiments and past earthquakes. In this research, the use of fiber-reinforced polymer (FRP) sheets attached to the surface of the member for strengthening connections was investigated. Here, an analytic model was presented for simulating the nonlinear behavior of connections strengthened with FRP sheets. In this model, the nonlinear behavior of the core zone of the connections was simulated with two diagonal linear springs. The corresponding load-displacement relationship in the linear springs is a function of the principal tensile stress-shear deformation in the core zone of the connection. Therefore, based on the behavioral mechanism of the connections and using the experimental results, the principal tensile stress-shear deformation relationships for reinforced concrete connections with different restraints for the longitudinal rebars of the beam were developed. Comparing the results of the numerical model with those obtained from the experiments verified the ability of the proposed model in predicting the nonlinear behavior of connections strengthened with FRP sheets.

The need to use non-uniform members in the structures from the past till now had been proposed although the subjects and research references in this case are insufficient. Therefore the use of non-uniform members is inevitable and should be carefully examined. About this members the main subject that should be investigated is buckling that if it occures it leads to member failure which may leads to collapse of the local or entire structure. So the exact solution of buckling load of non-uniform columns is available only for simple cases. In this paper a method based on finite element method is used for studing the problem of elastic buckling load of non-uniform columns with general cases of cross-section and axial loading. In first the column is devided to arbitrary numbers of finite elements. Then the stiffness matrix and geometric stiffness matrix is obtained based on existing relationship for each element and then for overall column. After applying boundary conditions using the eigenvalue equation to obtain the critical buckling load .It is clear that the effects of changes in cross-section of a column appear in stiffness matrix and changes in axial loading in geometric stiffness matrix. The proposed method in addition to high accuracy is general and could be used by th Matlab codes. Results obtained from this method are compared to ones that presented by Serna et al. The procedure in this paper is rating as a exact solutions but Serna’s method is a closed form method that is from approximated solutions. The results are presented for uniform columns under non-uniform axial loading and non-uniform columns under non-uniform axial loading. Different non-uniform columns contain single web-tapered columns, double web-tapered columns and web-tapered & flange-tapered columns. Also results are presented for various end boundary conditions that contain hinged-hinged, clamped-clamped, hinged-clamped and clamped-free

According to increase of population in cities, tall buildings and especially framed tube structures have been become interesting for structural engineers. Framed structure act primarily like cantilevered box beams and since they generally have much larger lateral dimensions than the internal shear wall cores, they are more effective in resisting the overturning moments of the lateral loads. However, due to flexural and shear flexibilities of the frame members, the basic beam bending actions of the framed tubes are complicated by the occurrence of shear lag, which could significantly affect the stress distributions in the frame panels and reduce the lateral stiffnesses of the structures. In this paper, a method has been adapted to study the shear lag coefficients for web and flange panels and for point, uniform distributed and triangular distributed loadings and then, the shear lag coefficients for the web and the flange panels are obtained for the parabolic shape lateral loads. Finally, by analyzing a 40-story framed tube structure under parabolic Shape lateral loading using this approximation method and comparing it with computer analysis, the error value in stress and displacement is estimated. The results of the research indicate that this method is suitable for the initial stages of the analysis and design of these types of structures.

The available analytical and finite element methods set the values of elastic modulus of nanocomposites more than the values obtained from the test. In this thesis, a new linear and continuous analysis method is used to determine the best and most similar value of elastic modulus for polymer nanocomposites to laboratory value is provided. For this purpose, the governing elasticity equations in polar coordinates have been solved for nanocomposite representative volume element (RVE) with shear-lag model by assuming perfect bond condition between CNT and matrix. Then, using the finite element modeling with Ansys software, elastic modulus of connectivity states of almost complete and half break between the two phases are calculated and finally the proposed new model that is modeling with spring in the states of almost complete connectivity and half break between the two phases are provided and elastic modulus is calculated. In this method, shear stress is also offering a new relationship between CNT and matrix areas that have been. To ensure the validity of the results of presented model to determine the elastic modulus, the obtained results are compared with laboratory method results of other researchers that in all cases the proposed elastic modulus is much closer to laboratory sample and finite element models and good agreement

In This research, the subject of buckling of columns on elastic foundation with different boundary conditions by energy approach has been presented. Columns modeled by two-end rigid bars where interconnected to each other by rotational springs, and elastic foundation modeled by translational springs. By preparing computer programs for five column types with different boundary conditions, the critical loads of these columns have been obtained. This five types of columns include two hinge end column, one fix end-one hinge end column, two fix end column, one fix end-one free end column and one hinge end-one free end column which are located on elastic foundation. At first, the stiffness of translational springs is set equal zero, and by knowing the analytical solutions, the stiffness coefficients of rotational springs have been gathered. Then, by defining a dimensionless parameter which showing the relative stiffness of elastic foundation, the critical load for a wide range of foundation stiffness has been obtained and results are presented in form of some graphs. There is analytical solution for two hinge end column on elastic foundation, therefore, at this stage, verification has been done for this kind of column and a very good consistency is observed between analytical buckling load and finite element buckling load for different relative stiffness of foundation. In fact, there is a linear relation between buckling load of two hinge end column and relative stiffness coefficient of foundation. But, for other four type columns, this relation may not be linear. Therefore, by means of fitting curve method, for other four type columns, some mathematical relations are proposed, which by these mathematical relations, the buckling load of above mentioned columns are computed in according to relative stiffness of foundation by acceptable errors.

By aging of steel and concrete structures in the world, structural retrofitting and strengthening has become among the major issues of engineering. several methods of strengthening have been developed by researchers for bonding the FRP sheet to the concrete surface until now, including the Externally Bonded Reinforcement , Near Surface Mounted for non-prestressed or prestressed FRP rebars or strips, and also the new developed grooving method with two techniques of Externally Bonded Reinforcement on Grooves (EBROG) and Externally Bonded Reinforcement in Grooves (EBRIG). The results of the researchs on EBROG method showed that by choosing specified width and height for grooves, this method can considerably postpone the debonding and in some cases may completely eliminate it. This study has tried, to evaluate the factors affecting EBROG method, function of this method has been examined by using finite element model in Abaqus software. The results show the high accuracy of numerical modeling compared with the experimental specimens. Moreover, it examins the effect of the distance between the grooves on the function of specimens reinforced by EBROG method. The results show that the rate of increasing ultimate load in specimens compared to unreinforced specimen, reduces up to 30 percent with increasing the distance between the grooves. So that in each group, specimen with the largest distance between the grooves, has the lowest increase its ultimate load. Also, it is observed that the ultimate load and energy absorption in the samples have been raised by increasing the number of grooves in this method.