جستجوی مقالات مرتبط با کلیدواژه « Finite Element Model » در نشریات گروه « عمران »

Air temperature variations due to daily, seasonal, and annual changes can affect the bridges. The deformation and stress caused under solar radiation should not neglected in bridge design and their effects can be compared with dead and live loads. Thus, the components of bridges such as bearings, dampers and so on, are seriously affected by the combination of external loads and thermal stress. Different countries have provided temperature gradients in their codes. Almost in all of the codes, the vertical temperature gradient is specified, but unfortunately in none of them, the lateral temperature gradient is presented. Furthermore, in the vertical gradient, there are numerous lacks exist in different codes. For an example, most of the codes do not involve either temperature variations due to annual changes, or not considered the longitude or latitude of the location of the designed bridges. These problems lead the engineers to do the precise study beside the codes provided by each country, for temperature effects on the bridge structures. This paper investigates the effect of vertical and lateral thermal gradient loads for concrete box girder designed based on Iranian Standard Loads for Bridge (ISLB) code, using experimental test and three-dimensional finite element analysis. The ISLB code has two main problems in the field of thermal gradients. Firstly, the vertical temperature gradient provided in ISLB code, cannot used for all bridges in Iran, because each bridge has its unique geographical environment, latitude, longitude, and axis of orientation. Secondly, it does not contain any models for the lateral temperature gradient. To handle these problems, the experimental test is done and thermocouples are installed in different parts of the segment to get the thermal gradients and investigate their effects. In the case of predicting vertical gradient, the recorded data show that, the maximum temperature difference occurs in 1430 hrs with the value of 8.8 °C , while by considering the lateral gradient, the maximum temperature difference occurs at 1130 hrs with the value of 2.9 °C . In this paper, comparison between vertical and lateral gradients leads to consider only vertical gradients in further investigations. Moreover, by applying vertical gradient in the finite element model of the Jenah bridge, maximum thermal stress is occurred in the intersection of the web and top flange with the value of 1.96 MPa and maximum deflection of 4.36 mm in the midspan of the bridge. As a solution for mitigating the negative effects of the thermal gradients, using polyurethane insulation is proposed and modeled in the FE model. Results of simulation reveal that utilizing insolation can reduce the top slab temperatures to 30.5 °C , 29.16 °C , 27.5 °C  and 26.4 °C  from 33.6 °C  in the case of using 2, 3, 5 and 6mm polyurethane insulations, respectively, which results in stress reduction from 1.96 MPa to 1.65, 1.35, 0.63 and 0.38 MPa in the case of using 2, 3, 5 and 6 mm polyurethane insulations, respectively. Furthermore, using insulation can reduce the deflection of the bridge, which in this study, the maximum deflection of the 48 m span is reduced from 4.36mm to 3.57, 2.86, 1.63 and 1.07 mm, by utilizing 2, 3, 5 and 6 mm polyurethane insulations, respectively.

The applications of steel pipelines for oil and gas transportation have grown over the past three decades. Due to the biological, service life performance, economical issue and human safety, it is important to keep a safe performance for oil/gas pipeline and to evaluate their serviceability under uncertainties including environmental issues (i.e. corroded defects) and applied loads (i.e. internal pressure). The accurate estimation of structural performance as load capacity-based burst pressure of steel pipes under corroded defects can be provided a robust design in service life. Therefore, predicting the residual strength of pipelines with corrosion defects is an important problem in these kinds of industrial structures. A one of oriented design relation of corroded burst pressure models is the ASME B31G which was presented by the American national standard Institution (ANSI). In the term of PCORRC criterion, Stephens and leis (2000) presented a mathematical model based on exponential nonlinear function using experimental results for low and moderate-strength steels. Generally, the shape of corrosion was considered as a rectangular form using DNV RP F101 (2004) for low and moderate-strength steels. Zhu and Leis (2005) applied the material hardening behavior in the mathematical model for prediction of corroded burst pressure. The presented model is extracted from X80 steel grade while it may be not covered the vast categories of steel pipes. (Ma et al., 2013) applied the Ramberg-Osgood relationship for material in finite element models (FEM) of steel pipes under single corrosion. However, they did not discuss the different yield strains of materials. The FEM have been used for computing the strength capacity of corroded pies by (Mechri et al., 2016), (Hieu et al., 2017) and (Shuai et al., 2017), but the Ramberg-Osgood nonlinear martial model has not considered with different yield strains.

Abrupt changing in track vertical stiffness on railway bridges transition zone, increased dynamic loads and asymmetric deformations. Using elastic pads on the deck such as under sleeper pads and rails reduce the track vertical stiffness on deck. So a finite element model of the ballasted track and short span bridge were created and validated.The results show that at method used under sleeper pads on deck , state use the under sleeper pads with variable stiffness for all sleepers on the deck is the best performance and nearly 20, 5 and 7 percent improvement in behavior in terms of static rail vertical displacement, ballast layer vertical displacement and normal stress in subgrade layer respectively and based on dynamic analysis rail dynamic vertical displacement and rail vertical acceleration improved 9 and 16 percent respectively. About combinatorial transition method include improved embankment in transition zone with resilient pads under the rail for rail on deck improved track behavior based on the criteria the rail vertical displacement and ballasted layer vertical displacement about 31 percent, also decreased the normal stress in the middle of the subgrade layer approximately 24 percent and based on dynamic analysis, rail dynamic vertical displacement and rail vertical acceleration is improved by about 27, 34 percent respectively.

Wood is one of the oldest materials that has always been known as a construction material. Despite its widespread application and unique properties, it has weaknesses such as relatively brittle behavior, especially in bending. In order to improve its performance and boost its features or strengthen wooden structural members, one solution is to employ FRP component, which is widely used as a rehabilitation tool. The lightness and ease of application for different materials such as concrete, wood, and steel have made it possible to increase the strength and ductility of structural elements. In this study, the few models that were available for the wood-FRP system were collected, and the uncertainty of six variables, as the most effective parameters in the strength of this system, was selected for reliability analysis. Also, during the steps, three live to dead load ratios equal to 0.75, 1, and 1.25 were assumed as the design loads. What was achieved during this research was that during the design process of wood-FRP, uncertainties had a significant impact on the reliability of existing models. Some models could be reliable enough to be applied for design purposes; however, uncertainties seriously affect the reliability of some models. Accordingly, the average reliability index of all six available models decreased from 4.88 in case one fails to consider uncertainties to 2.71 with uncertainties of the models. Among the five existing models presented in previous studies, the Palizi and Toufigh model with an average reliability index of 3.61 was the best-proposed model, which is in good agreement with the acceptable index of existing structural design criteria. It is worth noting that various variables affect the reliability of this system. However, in this study, only the uncertainty of the six variables of modulus of elasticity, thickness, length, and width of FRP, along with wood width and its compressive strength, was considered. Other sources of uncertainties may be considered while analyzing this system in future studies.

Prying action in bolted beam-to-column connections may cause brittle failure due to increasing the bolt force to a value greater than its design strength. In the most common design methods, the prying force is generally determined using analytical models. Existing analytical models are based on the research for the bolted connections with rolled T-stubs. In the end plate connections, the T-stub is fabricated by the complete joint penetration welding of steel plates. In this research, an analytical model is proposed for determining the prying force in built-up T-stubs. For this purpose, three specimens were tested under monotonic loading to determine the load-displacement relationship and the load-carrying capacity of T-stubs. An advanced non-linear finite element model was established to evaluate the load-carrying capacity of the bolted T-stubs, and it was validated using the experimental data. A parametric study was performed to evaluate the effects of geometric and mechanical properties of the connections on the magnitude and location of the prying force resultant, the eccentricity of the bolt force, and the location of plastic hinges. Consequently, a formula was proposed to determine the location of the prying force resultant. It was shown that a plastic hinge is formed at a distance of about 10 mm from the T-stub web. Also, it was shown that the eccentricity of the bolt force reduces the nominal capacity of the bolt by about 35%. With these data, a simplified analytical model was developed to predict the T-stub load-carrying capacity assuming the location determined for the bolt force, prying force, and plastic hinge. The magnitude of prying force and T-stub load-carrying capacity were determined using the proposed model. The results showed that by using the proposed model, an average of 10.6% improvement in the accuracy of the bolted T-stub load-carrying capacity determination is achieved.

Abrupt and non-uniform changes in track vertical stiffness in railway bridges transition zone, increase dynamic loads, asymmetric deformations and track components damage and as a result, it increases maintenance costs. In this paper, the most widely used methods for stiffness transition are investigated, including concrete approach slab, hot mixed asphalt layer, stabilization and improvement of transition zone embankment. Then, the innovative combinatorial methods including improvement of transition zone backfill (embankment) materials with resilient pads under sleepers and rails on the deck based on specific criteria according to the finite element model. The results show that despeite various stiffness transition methods, the track according to the rail vertical displacement, ballasted layer vertical displacement and stress in the middle of the subgrade layer states has more desirable behaviour by about 14 to 22 percent, 3 to 29 and 4 to 36 percent respectively, rather than there is no stiffness transition in transition zone. Based on dynamic analysis, rail dynamic vertical displacement, rail dynamic vertical acceleration and force in subgrade layer is decreased by about 11 to 17, 32 to 40 and 1 to 22 percent respectively. Finally it appears that a combinatorial transition method include improved embankment in transition zone with resilient pads under the rail for rail on deck improved track behavior based on the criteria the rail vertical displacement and ballasted layer vertical displacement about %31, also decreased the normal stress in the middle of the subgrade layer approximately %28 and based on dynamic analysis, rail dynamic vertical displacement, rail dynamic vertical acceleration and force in subgrade layer is decreased by about 21, 34 and 23 percent, respectively. This method can provide the best impact and performance between different methods for concrete short span bridge and the relative displacement reduced in transition zone from 0.55 mm to 0.11 mm.

Although steel plate shear walls have the inherent advantages of formability and increasing the energy absorption of the lateral load system, there are some shortcomings such as buckling in the early stages of loading and high demand for shear force of the column. In order to address these cases, a new moment connection using steel trapezoidal plate in the form of numerical models of steel shear wall system under monotonic loading and cycles is presented. The main aim of this study is to provide a system of low-thickness steel shear wall that has the same capacity and energy absorption in accordance with the direct connection sample, while at the same time concentrate the plastic joint area on the connection plate. The connection plate acts like a fuse. In spite of conventional steel shear wall systems, the connection of the steel wall plate to the column has not been established and vertical lateral stiffeners have been used to improve its performance. Also, three different thicknesses used for the connection plate. The finite element numerical model was validated with eight test specimens and a good accuracy was established in terms of load-bearing capacity, elastic stiffness, and failure mode. The results of nonlinear static analyses on the developed models showed that the use of the connection plate was able to act as an energy-absorbing member and prevent the boundary elements from entering the plastic status. Therefore, the use of lower cross-section of column elements is possible. There is also no need to follow a weak beam- strong column rule and to use continuous or doubler plates in the panel zone of the column. Finally, it was found that the relationships presented in the AISC regulations are less than 7% to 25% for estimating the angle of the tensile field and the shear capacity.

Due to good ductility and stiffness, the eccentrically braced frame (EBF) has attracted the interest of seismic design regulations. Recently, the replaceable link beams (RLBs) were proposed as a new approach to solve the problems in designing the EBF system. This approach allows to easily repair or replace the damaged link beam by uncoupling the yielding element (link beam) from the main floor beam. One of the most common types of RLB is the double-channel link beam with the web-bolted connection. The identification of factors involved in the behavior of this type of link beam is less commonly found in the studies. In this paper, therefore, using the ABAQUS software, the numerical modeling was first validated to a laboratory model. Then, by changing the validated link components (including the number and thickness of plates, number and spacing of bolts, number and steel grade of link beam section), a total of 16 new numerical models were generated and evaluated under cyclic loading. The results limited to the assumptions of this paper show that in the web-connected horizontal replaceable shear link, the full shear yielding occurred, achieving the full functionality of ductile seismic fuse. In addition, the bolted, web-connected RLB demonstrates excellent seismic performance and plastic rotation capacity. Also, the smart selection of link components increases the shear strength capacity, stiffness, ductility and energy dissipation. However, the effect of increasing the size of beam cross-section is more noticeable. In addition, in order to ensure the full shear yielding behavior for RLB, it is necessary to add the upper and lower flange stiffeners near the connection.

One of the most important challenges in development projects beside designing and performing them is to control immunity and stability of the several parts during different stages of construction, impoundment, and operation.  The   main purpose of instrumentation in dams is to evaluate operation and reaction of these structures under different load conditions, developing a better understanding of their behaviors, and checking on the design concepts. Geotechnical engineers must work with natural geologic materials, while the properties of geologic materials are generally diverse, unpredictable or even unknown.  However, data derived from instrumentation which is usually being done via back analysis, would help them to estimate design parameters more accurately.  In this research, the datasets obtained from instrumentation on the Cheragh veis Dam (62meter height, 10meter crest width and274 meter crest length) constructed in Iran, will be monitored. Out of raw original data, valid ones were selected based on compatibility terms and engineering judgment.  Eventually, reliable ones were compared with the results of back analysis applying finite element model. Based on the alleged comparison, having done some iterative analysis, modified properties of materials would be obtained.

During many decades,maintenance and behavior evaluation of civil structures are drastically become of more importance problem of urban planning. Many researchers have tried to present various efficient and effective methods to evaluate more reliable and much precisely structural damages especially in the beginning of the damage formation. Among different proposed methods, model updating method is truly known as an accurate and reliable method for detecting location and severity of structural damages. Model updating method is a properly substitution for the traditional methods. In this paper, a methodology for model updating of complex andor irregular two degrees of freedom moment-resisting steel structures and braced steel structures is presented. In order to increase accuracy and performance of this methodology, model updating is designed to carry out in two stages. Damages may be much dispersed and their severities may be so small or much intense. This methodology is powered by an iterative sensitivity analysis using nonlinear constraint optimization, which is worked based on the Newton trust region algorithm. In order to shown capability of the proposed methodology, different damaged steel frames are examined. Model updating parameters are defined as stiffness reduction coefficients of members or a member is divided into some smaller parts. Updating is obtained during reducing differences between measured and analytical modal parameters of the structure. These parameters may be the values of frequencies and modal shapes of the real damaged structure and its related analytical model. Extensive results obtained from analysis of different models with various dispersed damages which were so small or very influenced in severity, shown that the proposed methodology has able to detect three levels of the structural health monitoring stages, accurately and more precisely. These three levels are as follows: (1) detection that the considered structure is healthy or damaged, (2) detection of damage locations (along whole or part of the members of the considered structure), (3) detection of the severity of the damaged member, which may be so small or so intense.

In this study, damage detection methods of cable stayed bridges were investigated. Cable stayed bridges are flexible structures; meanwhile they are sensitive to vibrations due to their complicated and multiple vibrations modes; therefore, damage detection methods based on vibration data in cable-stayed bridges has become a challenging issue. In the present study, finite element model of Bill Emerson, Missouri cable stayed bridge was simulated in order to achieve a precise finite element model to simulate the damage scenarios in bridge and the study of them. General process includes four damage detection indices based on the modal data (mode shapes and natural frequencies) achieved by modelling structure and simulated damages and in each case the results of damage detection were presented by indices. These methods are: Enhanced Coordinate Modal Assurance Criterion (ECOMAC), Mode Shape Curvature (MSC), Modal Flexibility Index (MFI), Damage Index (DI). Some of the methods were applied in damage detection of the pervious structures and bridges. In this paper, correlative study of these methods were performed based on different damage scenarios as well as study of challenges such as different levels of random noise in the input data, incomplete modal data and low damage intensity in detection of damage in cable-stayed bridge and then, performance of the methods were assessed.

Hybrid masonry system is a new structural system, in which the masonry panels link to the frame with steel plate connections. Hybrid masonry uses masonry infill for lateral stiffness and strength within frames in addition to supporting out-of-plane (flexural) loads. There are many primary reasons for the development of hybrid masonry systems. Most important ones are to simplify the construction of framed buildings with masonry infill and to provide structural redundancy, which can be utilized for limiting progressive collapse.There are three hybrid wall types including, ordinary hybrid shear wall, intermediate hybrid shear wall and special hybrid shear wall. Hybrid masonry systems can be applied to either concrete or steel-framed structures. This research focuses on steel frames. In this paper, seismic behavior of ordinary, intermediate and special hybrid shear walls are investigated using nonlinear static analysis. To this end, 15 steel frames including, bare frames, infill frames, ordinary hybrid shear wall, intermediate hybrid shear wall and special hybrid shear wall are modeled using ABAQUS software. The stress contours and capacity curves of models are estimated using nonlinear static analysis. Results indicate that failure capacity, and ductility of special hybrid shear walls are more than ordinary and intermediate ones. Moreover, the increase in moment of inertia of frame sections, increases the stiffness and energy dissipation capacity for all three hybrid shear walls

One of the methods for improving the seismic behavior of the moment resisting frames is the using of reduced beam section (RBS) dog-bone connections. In this paper the finite element models of RBS doge-bone connections is made and calibrated to experimental results, the collapse mechanism and stress distribution of these connections is discussed. Moreover, Fema recommendation is considered. This study has been done through the detailed numerical analyses of several samples of these systems. For expressing the advantages of this system, the seismic parameters of RBS dogbone frames have been compared with those of special moment resisting frames, which are obtained based on several analyses for some common buildings with both connections. Based on the obtained results, the behavior factor of structure is a function of its first fundamental period and increases with period. The increasing rate of the behavior factor of the dog-bone connection system in terms of period is more than that of the special moment resisting frames. The behavior factor of the flexible structures with dog-bone connections is about 20% larger than that of these structures without dog-bone connections. The investigation of ordinary beam to column connections in steel structures shows that non steady stress distribution and stress centralization in welded zone causes the connection fail in strange earthquakes. In RBS dog bone connections the proportion of maximum stress in beam flange to yield stress is about 0.8. The usage of RBS system occasions stress redistribution in beam flange, increasing the effect beam flange width and increasing the connection ductility. In corporation between ordinary and RBS dog bone connections shows the increasing the period of structure about 3 percent. In elastic step, tendency of RBS and ordinary connection system is same but in plastic step the resistance reduction coefficient of RBS system is more about 23 percent than ordinary connection system. This shows the structures that made by RBS systems, have more energy attraction capacity.

The ability of elastomers to withstand very large strains (of beyond 500%) without breakage or permanent deformation makes them an ideal material for many applications, including, but not limited to, aerospace, medical, and automobile industries, bridge bearings, seismic isolation, and supplemental dampers. It is essential for design purposes to simulate accurately the response behavior of elastomers under various loading conditions. Given the nonlinear stress-strain relationship in an elastomeric material, a hyperelastic model, instead of Hooke's law, must be employed in stress analysis of the material. The literature includes a variety of constitutive hyperelastic models for elastomeric materials. However, choosing a suitable constitutive model that simulates the elastomer stress-strain behavior under different loading conditions is challenging. This paper examines the effectiveness of various hyperelastic models of which the constant model parameters have been evaluated using the results of a standard uniaxial tensile test conducted at different strain values. The model parameters are evaluated through a curve fitting technique performed by MSC-MARC, a commercial finite element software program. A thorough examination of the efficiency and accuracy of various hyperelastic constitutive models at different strain ranges shows that the Neo-Hooken and Arruda-Boyce are suitable models for the range of small deformations, and the Ogden and Yeoh models are suitable for a wide range of deformations.