جستجوی مقالات مرتبط با کلیدواژه « مطالعه عددی » در نشریات گروه « عمران »

Hydraulic jump is a fast and irreversible variable flow that occurs downstream of hydraulic structures and the result of which is the rapid transformation of supercritical flow into subcritical flow, which increases the depth of the flow and causes a significant loss of energy. By examining the previous studies, it is clear that the roughness bed or the divergence plays an important role in hydraulic jump characteristics.  FLOW-3D is one of the suitable software in hydraulic Phenomenon modeling. In order to ensure the appropriate capability of FLOW-3D software in simulating hydraulic jump, first, a research related to this issue which has been investigated in the past in a laboratory, is simulated in the software. Then, by comparing the results of the numerical simulations with the laboratory data and ensuring the proper functioning of the software, new simulations are made. The experiments related to the physical model used were carried out in a laboratory flume with walls and floors made of transparent plexiglass, 5 meters long, 0.3 meters wide, 0.45 meters high, and zero longitudinal slope. To create a supercritical flow, a steel valve with a height of 0.65 meters and a thickness of 3 mm and an opening height of 1.7 cm was used for a non-prismatic channel with a divergence ratio of 0.33. In order to create symmetrical opening ratios of 0.33, glass boxes with a length of 0.5, a height of 0.2 and a width of 0.1 meters were placed on both sides of the flume. After the simulation and by checking the R2, MAE and RMSE parameters, the k-ɛ model and the mesh with the number of 687600 cells were selected as the optimal mesh. In this research, according to the selection of four types of bed (smooth bed, rough bed with hemispherical roughness and diameter of 3, 4 and 5 cm) and three divergence angles (7, 14 and 90 degrees) and five Froude numbers (Froude number: 4.34, 5.71, 6.95, 8.17, and 9.37) in total, Simulated for 60 different experiments.  The results showed that for the maximum discharge for the sudden divergent channel, the roughness of the bed with a diameter of 5 cm causes the amount of the flow depth to decrease by 19.77% compared to the smooth bed. Also, the sudden divergence of the bed reduces the flow level by 23.75%. In all cases, the value of y2/y1 increases with increasing Froude number. With the increase of the Froude number from 4.34 to 9.37, the value of y2/y1 on the flat bed increases by 15.54%. With the increase of the Froude number from 4.34 to 9.37, the value of y2/y1 on the rough bed with a diameter of 3 cm decreases by 5.67% on average compared to the smooth bed. As the roughness diameter increases from 3 to 5 cm, the value of (Lj/y1) decreases by 15.58% on average. The results show that the (EL/E1) ratio increases with the application of bed roughness. So that by applying a roughness with a diameter of 5 cm, the value of (EL/E1) compared to a smooth bed increases by about 5% on average, and by applying a sudden divergence, the value of (EL/E1) compared to a bed diverging under an angle of 7 degrees The average increases by 4.58%. FLOW-3D is a good software to prediction of hydraulic jump characteristics in divergent channel with smooth bed and rough bed and The k-ɛ turbulence model was selected as the optimal turbulence model. Increasing the bed roughness size decreases the secondary depth for all values ​​of 𝐹𝑟1. The sudden expanding of the rough bed with a roughness of 4 cm reduces the y2/y1 by 12.98% compared to the bed with a divergence under 7 degrees. Increasing the size of the roughness decreases the length of the hydraulic jump. According to the results, the length of the hydraulic jump in the rough bed with a diameter of 5 cm compared to the smooth bed decreased by 26.12%. Also, by increasing the roughness diameter from 3 to 5 cm, the value of (Lj/y1) decreases by 15.58% on average. By applying roughness with a diameter of 5 cm, the value of (EL/E1) increases by about 5% on average compared to the smooth bed. Results show that the divergence angle of the bed is effective on streamlines. But, increasing the roughness of the bed, there is no noticeable change in the streamlines. However, increasing the bed roughness size, increases the amount of disturbance energy in the section.

Due to the large number of fire and the resulting financial and human costs, it is necessary to prevent the roof from collapsing as one of the key members of the building during a fire. Because concrete has an inherent weakness in tensile stress, cracking in the tensile area of slabs is inevitable. On the other hand, in a system of prestressed concrete joist slab system, the tensile area of the concrete is usually completely eliminated or effectively reduced, and the performance of the floor can be expected to improve under fire conditions. The critical temperature was obtained and compared in 9 samples that differed in the amount of prestressing force, strength and concrete cover as well as the span length. The thermal load was applied to the studied samples according to the ISO 834-1 standard protocol. The temperature distribution across the slab was applied uniformly and incrementally and the failure states in the model were predicted and verified with laboratory results. The results showed that the prestressing force has the greatest effect and the span length has the least effect on decreasing or increasing the critical temperature index. Changing the prestressing parameter from zero to 600 and 1120 MPa, increases the critical temperature index by 20 and 43% and changing the beam length parameter from the initial value of 4.5 m to 6 and 7.5 m, decreasing the critical temperature index by 1 and 2%, respectively. Changing the parameter of concrete cover from 20 mm to 10 and 30 mm has caused a decrease and increase of critical temperature index by 11 and 9%, respectively. Also, in all samples, the same failure mode occurred in the middle of the joist in a bending manner.

During the past earthquakes, the formation of plastic hinges in columns has given rise to global structural damage in moment resisting frame structures. If the plastic hinges are formed in the beams, the most suitable energy dissipating mechanism in structure will happen. The kinked rebar has locally curved regions (usually near the inflection points in beams) which can be gradually straightened under tension. Due to lower initial yielding flexural capacity compared with that of a cross section reinforced with traditional straight bars, the beam section reinforced with kinked rebars will yield first when the RC frame is subjected to seismic loading, and thus, the strong column-weak beam hierarchy can be realized. In this study, first the load-deflection behavior of a reinforced concrete beam was numerically simulated by ABAQUS software, and the reliability of the finite element model was verified by comparing with the experimental results of other researchers. Then, the load-deflection response of the RC beams with kinked bars which has two-step behavior were investigated and described in steps by using finite element modeling results. Finally, the control beam was used for further analyses to investigate the effect of important parameters including: concrete compressive strength, reinforcement ratio, and the ratio of beam span to depth on load-deflection curve of beams. Results showed that the ultimate bearing capacity of RC beam increases as the reinforcement ratio increases, however, it doesn’t have an important effect on the first yielding point. The increase of concrete compressive strength also contributes to greater initial yielding. In addition, the increase of beam span can have negative effects on the flexural behavior of RC beams with kinked bars, and reduce initial strength and stiffness of beams.Finally, the behavior of an internal connection including a beam with kinked rebar to the column was evaluated.

Time-dependent behavior in soils causes mechanical, chemical, and geo-mechanical changes. This behavior in soils considers as a major challenge in geotechnical engineering, and yet need more attentions. The impact of time on geo-materials and their behavior such as rock and soils are undeniable. Based on researches previously performed on this issue, the importance of time-dependent behavior emerged. Creep (strain deformation under constant stress level), stress relaxation (stress deformation under constant strain level), and loading pace are among time-dependent special features, which are seen in experimental models. Creep divided in three categories: primary creep, in which rate of deformation reduced with time. This creep starts immediately after loading procedure. Secondary creep in which rate of deformation is constant. Finally, tertiary creep in which rate of deformation increased and at last material failed. In this research, the effect of time on the stability of the saturated sand has been investigated by building a small-scale experimental model and using finite element numerical method. Preparation of the sand slope was using sand downfall and the saturation process of the experimental models are carried out artificially with a flow rate of 1 liter per minute. Due to the friction angle of the sand, two physical models were created with slope angle of 35 degrees and 37 degrees. The results of physical modeling showed that drainage in sandy slopes, despite stability in non-drainage conditions, gradually increases the deformation in the slope so that over time it will cause failure in the slope. The results of physical modeling showed that in the first model, the maximum horizontal and vertical displacements are 10.5 and 9 cm, respectively. For the second model, these values ​​are 10.5 and 7 cm, in order. At the end of this research, statistical analyzes have been used to provide relationships for predicting vertical settlement in saturated sand slopes. Using numerical models with dimension analysis rules (make the dimensions 100 times in numerical models), in first model in order to reach 8.81 meter displacements, 3780 minutes of raining needed. While in second model, for reaching 7.20 meter displacement, 2280 minutes needed. Results indicate that by increasing cohesion, vertical displacement decrease. As friction angle increase, vertical displacement also decreases. While by increasing slope angle, vertical displacement increase. At the end of this study, statistical analysis has been used to provide relationships to predict the amount of vertical displacement in the slope crown, which is the maximum amount of displacement in the slope.

In recent years, the use of Cold-formed steel (CFS) as load bearing structural members in residential and commercial buildings has become widespread. There are a few researches about fire performance of CFS columns. In particular, CFS columns with a built-up section, which are more common in construction, have been less studied. Also, the existing standards for the design of cold-formed steel structures do not provide sufficient design criteria for designing against fire. In this research project, a numerical study will be conducted to investigate the buckling behavior of CFS columns with built-up cross sections, in fire conditions. Numerical analysis method using ABAQUS finite element software has been used to evaluate the models. After numerical modeling and validation using the results of previous experiments, parametric studies have been performed to investigate the effect of various parameters such as cross-sectional thickness, type of steel, etc. on the buckling behavior of CFS columns at elevated temperatures.The load bearing capacity of CFS columns is extracted from load-displacement curve obtained from numerical analysis. Also, using the direct strength method (DSM), the axial resistance of the CFS columns is computed and compared with the results of numerical analysis. Failure modes of CFS columns have also been obtained and discussed. The results showed that the DSM method estimates the ultimate load capacity of the columns very close to the values obtained from the numerical analysis method. Also, the use of high-strength steel has increased the load-bearing capacity of the columns at different temperatures, although the amount of increase is not very significant.

Tabriz, one of the metropolises of Iran, has a particular geotechnical condition. Recently, it has been reported that some of the new structures are tilted in Freshteh town located in the northwest of Tabriz. Different thicknesses of the first subsurface layer, the high level of ground water, and the presence of sand layers between the clay layers are the main reasons for uneven consolidation settlement of buildings in the mentioned area. Residents usually do not feel safe to live in these tilted buildings. However, demolishing and reconstructing tilted structures is a waste of money, unless there is no other solution. Moreover, the demolishing of valuable historical buildings is out of choice. Thus, restoration processes for tilted structures should be considered and developed. Various methods are used to rectify the tilted structures including compaction grouting, underpinning of the foundation, chemical grouting, jacking, soil extraction technique, etc. Among different methods for rectifying uneven settlement of structures, in this study, because of some advantages, the jacking technique has been chosen to rectify the Tilted Reinforced Concrete (RC) buildings. This method can be used in dense urban areas and the residents can stay in their apartment or office in much of the rectifying process. Geotechnical finite element software of Midas GTS NX was used for numerical modeling and analysis of this direct soil-structure interaction rectifying project. The results were verified based on the data reported by Ma et al. (2008). The force applied by the jacks to rectify the uneven settlement should continue until the two sides of the foundation would reach the equal value of vertical displacement, or the uneven settlement of the foundation is reduced to the minimum possible amount. Also, the structural elements of the building should be able to withstand the force applied by the jack and do not crack or collapse during the rectification stages.

One of the efficient ways in order to reinforce the earth slopes is Geogrid Encased Stone Column (GESC). This technique can increase bearing capacity and decrease settlement rate of slopes. The aim of this study is to perform a three dimensional finite difference method (FLAC3D) for GESC in the stabilization of sand slopes. According to the results of 3D numerical analysis, the existence of geogrid-encased stone column, increases stability to an ideal level compared with ordinary stone column-reinforced slopes. Different parameters including stone column diameter, the coupling spring stiffness, coupling spring cohesion, coupling spring friction, S/D ratio, and the layout of encasements evaluated and discussed in this paper. The influential parameter in geogrid in order to reinforce stone column, is coupling spring cohesion. By increasing the parameter, slope bearing capacity of reinforced slope increases linearly. This indicate that use of geogrid reinforced stone column, based on type of geogrid characteristics, can enhance earth slope bearing capacity up to 1.8 times of slope reinforced by ordinary stone column. The maximum bearing capacity with a row of stone columns was obtained for S/D=2. However, the reduction of S/D does not necessarily imply the increase of bearing capacity of slopes.