جستجوی مقالات مرتبط با کلیدواژه "روش المان محدود" در نشریات گروه "عمران"

As the use of mechanized tunnel boring machines (TBMs) increases, the performance of these machines has become increasingly important for researchers to work on. Disc cutters, as one of the main components of the cutter head, are responsible for transmitting the thrust force to the rock and breaking it. Studying the rock-cutting process by TBM disc cutters is vital to mechanized tunnel excavation. For this reason, this issue has been widely studied by researchers in recent decades. The shape and layout design of the cutter head, disc cutters spacing, and the size and shape of cutters are the adjustable factors in improving the efficiency of rock cutting and TBM excavation. It is important to determine the forces acting on the disc cutters in mechanized tunnel boring machines because these forces are used in the design. A review of the history of studies shows that lateral forces have been studied much less than other forces. Therefore, in this article, a numerical study of the side forces on the disc cutters and their relationship with the installation radius and the tip width has been studied. Furthermore, the information of lot 2 Kani Sib water conveyance tunnel has been used to validate the results. For this, numerical simulations have been conducted with three types of 17-inch disc cutters in installation radii of 0.3 m, 1 m, and 2 m with tip widths of 18.5 mm, 19.6 mm, and 20.8 mm. The results of the numerical models show that the side forces acting on the central disc cutters with low installation radius are much higher than others. Additionally, these forces are mainly asymmetrical and become symmetrical as the installation radius increases. Central disc cutters are subjected to unbalanced and asymmetric forces because of the excessive side force. This will cause the disc cutter bearing system to malfunction and increase costs significantly. A decrease in the balance of the cutter head and increased vibrations are other consequences. Increasing the tip width aggravates this condition. The damage pattern of the disc cutters in lot 2 Kani Sib tunnel confirms this conclusion. According to this project, more than 74% of central cutters' failures with low installation radius are related to bearing problems. This type of failure is caused by large asymmetric side forces, while only 26% of the face disc cutters with a large installation radius failed in this way, which confirms the results of the numerical models.

In this study, the linear and nonlinear seismic behavior of the Koyna dam has been investigated for the condition of the dam with unsaturated concrete and the dam with a combination of saturated and unsaturated environment. For this purpose, using Abaqus software the boundary line between the saturated and unsaturated environment has been determined. In the following, various parameters of saturated concrete have been evaluated. Finally, linear and nonlinear analyzes have been performed under the effect of the Koyna earthquake. To model the nonlinear behavior of concrete, the CDP behavior model has been used. The results show that the saturation of the dam has reduced the maximum values of displacements in linear and non-linear mode. In the linear analysis, the amount of reduction of the maximum horizontal displacement in the saturated state compared to the unsaturated state for the reservoir up to the height level of 36, 66.5 and 103 meters is equal to 17.34%, 24.00% and 17.30%, respectively. Also, in the non-linear analysis, the reduction of the maximum horizontal displacement in the saturated state compared to the unsaturated state for the reservoir up to the height level of 36, 66.5 and 103 meters is equal to 31.58%, 30.72% and 22.38%, respectively. For the condition of the dam with a saturation elevation lower than the downstream slope change elevation, the maximum values of the first and third principal stresses in the linear analysis and the amount of damage in the nonlinear analysis have decreased. For a case of the full reservoir where the downstream slope change is saturated, the amount of cracking in the entire structure has decreased, but it has increased in the downstream and upstream slope change location. Finally, Results show that it is necessary to evaluate the behavior of the dam with saturated and unsaturated environment simultaneously.

With the increasing cost of petrol for Iranian customers, the interest in using dual fuel cars is increasing widely. It is common to use CNG vessels in Iranian dual-fuel cars. Nowadays there are millions of CNG vessels with more than 10 years of Commissioning time in Iran. In this study, the burst CNG car tank performance of steel reinforced concrete (RC) columns was numerically investigated with Abaqus software; and using Phast software for modeling equivalent TNT force exertion. Recently, the number of explosions of CNG tanks is increasing which has caused a lot of casualties and financial losses. In this study, the effect of CNG tank explosion on the Reinforced concrete column was investigated by considering four variables, including the distance between the CNG tank and column, reduction of the resistance of the car body, strength of concrete, and dimensions of the column’s section. Investigations showed that the most effective parameters according to the changes of the examined parameters are 1) distance between CNG tank and column and reduction of the resistance of the car body 2) section area of a column and 3) strength of concrete as, by increasing distance from 50 to 130-centimeter the maximum displacement decreased 63 %, by decrease the resistance of the automobile body to 80% the maximum displacement decreased 64%, by change strength of concrete from 20 to 40 Mpa the maximum displacement decreased 15% and by increase section from 35×35 to 50×50 cm2 the maximum displacement decreased 45%.

For the first time in January 1991, the issue of studies and construction of urban trains was proposed by the esteemed representatives of the city of Tabriz in the Islamic Consultative Assembly. Due to the increasing growth of intercity travel in Tabriz and the need to use a 2-rail transportation system in order to reduce traffic problems, studies of Tabriz city train lines were conducted and Tabriz city train network including 4 routes, a total of 60km and 63 stations were designed.The analysis of underground structures is very complex due to the interaction with the surrounding environment in a dynamic state. For this reason, less research has been done. One of the first people who investigated the effect of earthquakes on underground structures is Newmark, in 1968 he presented an easy method to calculate the relative strains of underground pipe structures and long tunnels (Bozorgnia et al., 1995). In 1969, Kuesel established a simple method for calculating earthquake forces on linear tunnels, which in most cases is the basic of later works. Douglas and Warshaw in 1971 presented the results of tunnel design analysis under the influence of earthquakes. Lee and Trifunac in 1979 investigated the response of the lined tunnel under the shear wave effect. In 1981, Owen and Scholl's studied three types of deformations, including compressive-tensile, longitudinal bending, and rhomboidal and elliptical deformations as the result of seismic vibrations in underground structures.

Tubular structures are made of hollow steel members with circular cross-sections and connecting them is one of the major challenges in their design. So far, some techniques to improve the performance of tubular connections have been proposed. Most of these methods can only be used for structures during the fabrication, but there are only a few techniques that can be applied during both fabrication and operation. Due to the successful experience of using composite materials for the repair and retrofitting of structures, special attention has now been paid to examining the efficacy of these materials in offshore structures.In this research, the effect of CFRP and GFRP wraps on the ultimate strength and deformation of tubular X-joints under compressive load is investigated. For this purpose, the results of the finite element (FE) models were validated with several available experimental tests. After that, the nonlinear static behavior of tubular X-joints strengthened with FRP was studied. In these FE models, the contact between the FRP layers and the steel members (chord, weld, and braces) was considered. Also, the weld profile in the brace/chord intersection was modeled.

Steel moment frames have been known as one of the most reliable structural systems for many years. These structures were being designed and used as an alternative to masonry structures. It was assumed that these structures are very robust and will not collapse. However, some historic events such as the 1994 Northridge earthquake and subsequent tests proved otherwise. As a result, researchers tried to find ways to improve the seismic behavior of these structures. One of these methods is creating an opening in the beam web to move the plastic hinge to a region far from the connection welds. The purpose of this research is to analyze and investigate the dynamic behavior of steel moment frames with openings in the beam web and secondly to compare their behavior with the conventional moment frames. To achieve this goal, two four-story special moment frames with and without beam web openings have been modeled in OpenSees software, and nonlinear dynamic analysis has been performed on them. Finally, IDA curves, fragility curves and drift distribution in the height of structures are presented and compared. The results showed that the creation of beam web opening, increases the seismic collapse capacity of the steel moment frames and reduces their probability of failure during a seismic event. On average, the collapse capacity of a moment frame with beam web openings is almost 25% higher, and its collapse probability at different acceleration levels is 25% to 30% lower than the frame beam web openings.

This article compares the characteristics of the improved bogie used in the long-edge mineral freight car to bear the axial load of 22.5 tons with the original H665 bogie. The H665 bogie is being used in the Middle East and specifically in the cargo fleet of Iran. Static and vibration analysis of individual parts and the infrastructure of railway vehicles is an integral part of designing and improving the performance of railway vehicles. Therefore, static and vibration analysis of the bogie frame was performed in our model using the finite element method. The Abaqus software package performed numerical calculations based on the FEM approach. In this research, theoretical results related to structural stress values , natural frequencies, and shape of vibration modes of the improved bogie and the original bogie were extracted. While comparing the results, the amount of structural stress reduction and the feasibility of increasing this axial load were investigated.

Recently, the growing needs of the engineering community for smart structures have led to targeted material technology gradually replacing conventional homogeneous and laminated composite materials. Among the sandwich-shaped structures with targeted core, the thin walled closed beam of sandwich box (with functionally graded core), due to having desirable properties such as high flexural stiffness, low density, desirable static and dynamic properties likewise high strength against mechanical loads and stresses have received much attention.
In some specific applications, components of modern structures, such as thin walled functionally graded beams with a closed box geometry, are subjected to axial or concentrated or wide transverse loads. Under these conditions, the choice of loading parameters may lead to lateral or axial buckling, which results in irrecoverable damage to the structure. Buckling of an axially loaded thin-walled functionally Graded sandwich box beam in presence of structural non-homogeneity is studied.
In this study, taking into account the effects of the structural heterogeneity (via power law distribution), porosity and effect of the cross-sections warping by means of Vlasov’s beam assumptions and applying the Hamilton’s principal the governing equations of the Buckling instability of the thin-walled functionally graded sandwich box porous beam were derived. Then the finite element method was developed to formulate the problem. In following, for the case of various boundary conditions, the related governing eigenvalue problem was derived and solved numerically. The validity of the results was accomplished by comparing the results with those of the literature. In following beside by providing the characteristic buckling curves, the effect of different parameters of the problem, including the beam aspect ratio, mixing volume fraction index, porosity index, core component modulus ratio and ceramic layer thickness on the buckling load were investigated comprehensively. It was observed that different parameters, especially the mixing index and structural porosity index has a considerable influence in determining the buckling boundary.

Non-ductile reinforced concrete frames are commonly found in older buildings in many parts of the world. These structures designed for gravity loads, have limited lateral strength and ductility, are prone to excessive one-way lateral movement and soft-story mechanism. This paper focuses on the retrofit of an existing reinforced concrete frame, using steel X-braces by indirect internal connection method. The main purpose is the analytical study of general behavior and response of large scale vulnerable frames. An experimental study was used to validate the numerical modeling performed in ABAQUS. Next the base samples were retrofitted with X-braces and four proposed indirect internal connection methods. Furthermore, in a separate parametric studies, the effect steel frame profile type, connection type and use of high strength grout in the boundary zones between steel and reinforced concrete frames were investigated. The results indicated that the stiffness and bearing capacity of the reinforced concrete frame by using steel X-braces increases up to 3.7 and 2.8 times, respectively. It also reduces the final displacement and drift by 50%. Moreover, bracing acts like the first defense system against lateral loads, such as structural fuse with its yield, increases the amount of energy dissipation. It also removes the plastic hinges by reducing the ultimate displacement and stress of lateral load in the panel zone.

Composite steel sheet shear wall (CSPSW) is a relatively new structural system that has been proposed to improve the performance of steel sheet shear walls by adding one or two layers of concrete wall to the filler plate. In addition, the buckling of the filler steel sheet has a significant negative effect on the shear strength and energy dissipation capacity of general systems. Accordingly, in this study, using the finite element method (FE), the performance and behavior of composite steel shear walls using T-shaped hardeners to prevent buckling of the filler steel sheet and increase the capacity of the CSPSW system have been investigated. In this paper, after modeling the shear walls of composite steel sheets with finite element methods and calibrating the models with experimental results, the effect of parameters such as the type of hardener including vertical, horizontal, diagonal and a combination of T-shaped stiffeners and its effect On the final capacity, the buckling of the sheet plates, the von Mises stress and the failure states have been investigated. The results show that the arrangement of vertical and horizontal stiffeners does not have a significant effect on the capacity and performance of CSPSW. Also, the use of vertical or horizontal stiffeners does not significantly affect the capacity and performance of CSPSW. On the other hand, the use of diagonal hardeners potentially affects the performance of CSPSW and increases the capacity of steel shear walls by up to 25%.

Today, reinforced concrete structures have a special place in the construction industry. To retrofitting some existing concrete buildings, steel braces are used with direct connection to concrete frame. The important issue here is what the details of the connection of the brace to the concrete structure should be. The connections in concrete structures always have the most damage during an earthquake. Therefore, the performance of the connections and the type of steel braces connection to the concrete frames will affect the seismic behavior of them. For this purpose, in this study, three methods of steel brace connection to the concrete frame have been investigated. Models are created as one-story single-span and two-story single-span frames using the finite element method in the Abaqus software. The results indicate that the change in the type of connection causes changes in energy absorption, stress distribution and maximum lateral displacement of the frame. Meanwhile, by applying bilateral loads, first the bolt planting model and then the metal jacket method are considered as selected models. Concerning the performance, which is represented by plasticity and damage of elements, the maximum difference between connection methods is 10 percent and all of them are considered acceptable.

Today, the structural health monitoring system based on various non-destructive analysis and tests has been greatly expanded in order to better performance and prevent further damage to the structure especially in areas with high seismicity. Given that, various structural health monitoring systems have been introduced. On the other hand, the discussion of maintenance costs with these methods can be helpful in choosing the most optimal method for this purpose. To this end, five new systems for health monitoring of structures, including the method of examining changes in basic information; Finite element model upgrade method, acoustic and ultrasonic method, magnetic field method and thermal field method were selected in 5 categories: management, technical, economic, human source and safety along with 20 sub-criteria in the field of maintenance. Then, through the pairwise comparison questionnaire of analytic hierarchy process method and receiving the opinions of experts in this field, different structural health monitoring systems were evaluated and prioritized to select the optimal system based on the most important criteria of building maintenance. The results showed that the structural health monitoring system by finite element model method has gained the first rank in all 5 main factors of management, technical, economic, human source and safety.

Reinforced concrete structures with standard steel rebar are vulnerable to corrosion and harsh environmental conditions, hence RC structures reinforced with fiber-reinforced polymer (FRP) rebar were commonly used these days. Du to FRP rebar’s better performance such as high strength, low self-weight, electromagnetic transparency and, as mentioned, non-corrodibility nature, using them as reinforcing bar is very widespread now. Because of financial matters, between different kinds of FRPs, GFRP is a better choice. Considering GFRP’s high strength and elastic behavior until failure, Although a large amount of reinforcement ratio is needed in composite beam components, the flexural stiffness of GFRP rebar reinforced beams is relatively lower compared to steel-RC, and more deflection and cracking are allowed in the serviceability design of these beams. Recently, shear and flexural behavior of continuous concrete beams reinforced with GFRP bars has been well investigated. Because of linear elastic behavior of GFRP materials until failure, considering moment redistribution in analysis and design of these beams is not allowed in almost all of cods and guidelines. Although many experimental and numerical researches investigated the moment redistribution in FRP-RC continuous beams with rectangular section, the behavior of these beams with T-section is almost unknown. This paper is a numerical investigation of existence and variety of moment redistribution in concrete continuous T-section beams reinforced with GFRP bars using finite element method with ABAQUS software. The verification of numerical models was done with some experimental beams, so the simulation can be used for further researches. The considering variables included the longitudinal reinforcement percentage, the number of main bars with constant bar ratio, transverse reinforcement ratio, stirrup space with constant ratio and constant bar size. For investigating mentioned parameters, 35 beams were modeled in software according to Canadian design and construction of building structures with FRP code, so 5 groups of beams were made which one beam is constant in each group. T-section beams were modeled assuming which failure happens because of concrete crashing not rebar failure. Deflection and serviceability were not interested, so bond-slippage behavior of GFRP rebar with concrete is not considered in modeling. Problem is indeterminate, so the percentage of moment redistribution was determined by comparing the reactions resulted from numerical and elastic analysis. Load-deflection and load-moment redistribution curves were used to discuss. The results show, as there is in steel-RC structures, moment redistribution exist in GFRP-RC continuous beams with T-section; however the amount of it is lower. Amount of bars between 2.5 times of balance reinforcement ratio and 3.5 times of it, in top and bottom of beam, shows the highest flexibility load and moment redistribution capacity. Increasing the number of main bars with constant reinforcement ratio and increasing the stirrup space with constant transverse reinforcement ratio reduce the moment redistribution capacity. It seems that the minimum amount of transverse reinforcement considered in Canadian code is not enough for preventing shear failure in these beams. So, with considering some points, the moment redistribution can be taken to account in analysis and design of GFRP-RC continuous beams with T-section.

Pervasive construction of elevated liquid tanks as the most important sources for urban hygienic water, and their failure under earthquake, highlighs the necessity for research toward new passive control devices. Design and use of base-isolation systems for elevated liquid tanks is propounded as a novel seismic engineering technology. To date, no special examination has been discussed in relation with the workability of such systems in reducing the seismic response of elevated liquid tanks considering tank wall flexibility. In this study, the seismic performance of a steel elevated water tank with/without isolator was analyzed and outcomes were evaluated using lead-rubber and elastomeric bearings by performing time-history analyses. To this end, finite shell elements for tank wall and internal finite elements for liquid, and bilinear hysteretic elements for the base-isolation systems are elected, and the seismic analysis of flexible liquid tank is performed in the time domain under real earthquake. The results show that the lateral control device despite excessive structural displacement could offer a substantial benefit for reducing the system’s seismic response which depends on the earthquake characterestics.

Liquid storage tanks are very important structures for storing various types of fluids in urban and industrial areas. Therefore, designing, manufacturing, and evaluating their performance for different conditions is very important. In this study, the behavior of steel tanks with changes in the ratio of height to diameter of the structure under the effects of far and near field earthquakes has been analyzed and studied. For this purpose and for better analysis of these structures, 4 models of steel tanks with different height to diameter ratios and in empty and 90% full states have been subjected to seismic analysis using modeled ANSYS finite element software. To better investigate the trend of seismic effects on steel tanks, selected far and near-field earthquakes have been scaled to the same amount of 0.5g. The results showed that in empty tank conditions, by increasing the ratio of height to diameter to more than 1, the maximum values of displacement in the far and near-field earthquake are close to each other. The results also show the extraordinary importance of adding water to the response of steel tanks due to far-field earthquakes for a height to diameter ratio of more than 1. In most cases, by increasing the ratio of height to diameter of the structure and adding the fluid to the analysis, the values of the first main stress and the maximum reaction of the horizontal abutment in the two modes of analysis under the effects of the far and near-field earthquakes get closer to each other. The results show that in order to analyze and investigate steel tanks affected by earthquakes, in addition to near field earthquakes, far-field earthquakes should be studied and analyzed.

The settlement and lateral spreading of the building due to the occurrence of liquefaction phenomenon in previous earthquakes have caused significant damage to structures and their infrastructure. Numerous studies have been performed to evaluate the settlement of shallow foundations located on the liquefiable soils as a horizontal model. In fact, in most cases, there is a mild slope in the layers that can affect the settlement and lateral spreading of the foundation. In this research, the displacements of the structure and shallow foundation/ground surface on the sloping liquefiable three-layers soil with different relative densities have been investigated parametrically, applying three-dimensional finite element (FE) simulations using OpenseesSP. The purpose of this study is to investigate the effect of parameters including the slope of soil layers, the density of liquefiable layer, groundwater level, foundation contact pressure, and length to width ratio of the foundation on the settlement and lateral spreading of the liquefiable sloping model. The results are shown that increasing the slope of the ground increases the difference between the settlement of the two sides of the foundation and increases the lateral displacements. Decreasing the relative density of the liquefiable layer increases the excess pore water pressure and the settlement of the shallow foundations. The results also are shown that the lower the groundwater level is increased the effective stress and reduces the vertical and horizontal displacements. Besides, increasing the contact pressure is amplified by the foundation of static and dynamic volumetric strains and increases the settlements.

Excavating tunnels in soft urban land has its own limitations. One of the most important limitations is the control of surface settlements to prevent damage to surface structures. Accordingly, choosing the best method for tunnel excavation is very significant. Among the existing methods for digging urban tunnels, step drilling methods are very widely used. Typically, these methods are not comparable to the mechanized drilling method for long tunnels. However, step drilling methods are used to drill short tunnels, large sections such as subway stations, non-circular sections, and complex structures such as intersections. Among the most important executive parts of step drilling are the length of the drilling step and the distance between the work fronts. Although these two parameters are very influential factors on the technical and economic aspects of the step drilling method, but there is no coherent process to determine these two parameters. In this research, the drilling stages of the tunnel of line A of Qom metro have been investigated in order to determine the length of the drilling step and the distance between the work fronts. The method of excavating this tunnel is pre support method. Using three-dimensional finite element modeling (Praxis 3D Tunnel), tunnel drilling is simulated in different progress modes. Eight progressive modes are provided for step length and six modes for spacing between fronts. The length of the drilling step and the distance between the work fronts are selected by examining the effect of different forward modes on the transverse diagrams of surface settlement, settlement values and soil displacement around the tunnel. Studies show that the same drilling step of 1 meter for the upper and lower parts with a distance of 25 meters between the fronts (a distance of more than one diameter of the tunnel), has the best performance for tunneling.

Repairing and strengthening of structures are unavoidable. The main reasons for repairing and strengthening concrete structures are improper design or construction, weakness of the structural elements, changing the usage of the building and damages that happen in structural elements. There are various ways for strengthening the vulnerable structures. In recent years, usage of high strength concrete reinforced with textile meshes (Textile Reinforced Concrete (TRC)) for strengthening the concrete structures has become a proper alternative for retrofitting the buildings. Using reinforced concrete with TRC is one of the new methods for strengthening the concrete structures. This method has become popular in recent years due to its light weight, high strength, no changes in the dimensions of reinforced concrete elements, and ease of use. The aim of this study was to evaluate the efficiency of this method in strengthening concrete columns with a low reinforcement ratio that need reinforcement. In this investigation, after verifying two models simulated by finite element software (ABAQUS) with experimental results, eight models were studied numerically. The models consisted of columns and supporting beams and they were categorized into four groups by different parameters which are effective in strengthening the concrete columns. In this paper, various parameters such as geometry of cross-section, mortar strength, and number of textile layers were investigated. The columns and their connections to supporting beams were strengthened with TRC. Models strengthening were carried out using vertical layers plus horizontal layers of textile meshes in cement mortar. Lateral force capacity, crack, and stress pattern on concrete and mortar surfaces and yielding of rebars in reinforced concrete columns were compared and evaluated. The cyclic behaviors of all TRC strengthened models were improved compared to non-strengthened ones.

The use of multilayer elastomeric bearings in bridges has showed that application of this bearings improved seismic behavior of this types of infrastructures during recent earthquakes. Applications of the elastomeric isolation bearings in long-spans or tall pier bridges can experience rotation due to flexure of the deck or the piers, that it can be effective on their seismic performance. In current study, a two-dimensional mechanical model of elastomeric bearings, under the simultaneous effect of pressure, shear and end rotation is investigated. This mechanical model consists of three layers of axial springs with nonlinear hysteresis behavior that play the role of bending and compressive deformation distribution. The model can take into account end rotations of the bearing, and the overall rotational stiffness includes the effect of the variation of vertical load on the bearing. Comparison of the results of nonlinear static bending tests of this model with the results of laboratory studies shows very well agreement. The results indicate that rotational stiffness increases with higher values of vertical pressure, however decreases with increasing shear deformation. As conclusion, end rotation does not affect the critical displacement appreciably, but it significantly influences on the critical shear force.

Due to population growth in large cities and the growth of cities vertically, in order to facilitate serviceability and provide low-cost and easier access to the needed resources, engineers are forced to build high-rise buildings on less desirable soils. Given the seismic conditions of our dear country Iran and the variety in thickness of soil in different places, it is important to know the structure suffers the most damage under which conditions. In this study, a nonlinear three-dimensional model of soil-structure interaction in tall buildings with finite element method by Abacus software in the time domain is investigated to evaluate the effect of soil-structure interaction. the dynamic implicit used to calculate the problem. Thickness effect with considering soil-structure interaction for 4 different thicknesses by Drucker-Prager modeling method which also considers soil hardening properties under the effect of Lomaprita earthquake which is considered as an earthquake with low frequency content ( Earthquakes with low frequency content have more destructive effects), has been calculated. infinite boundaries used to absorb the reflect of waves. the boundaries The research results show that changes in thickness have significant effects on the analysis results. In this study, for each model, lateral displacement , acceleration, subsidence, acceleration spectrum and stress contours are investigated.