جستجوی مقالات مرتبط با کلیدواژه "nonlinear finite element" در نشریات گروه "عمران"

In this research, a numerical study has been performed on the seismic behavior of the deficient RC beam-column joints (with non-seismic detailing), and the seismic rehabilitation of these joints by using FRP composite laminates. At first, based on previous experimental studies, a series of RC joint specimen were considered to verification the proposed numerical model. These series of specimens include six RC interior joint specimens with non-seismic detailing and retrofitted with FRP laminates under cyclic loading. Comparison between the load-displacement curves obtained from numerical model with corresponding experimental data show that the proposed model is capable to high accurately predict the response of RC joints under cyclic loading. Then, based on the verified model, the performance of two dimensional (2-D) deficient and strengthened exterior RC joint, three dimensional (3-D) interior and exterior RC joints with considering the effect of slab and beam perpendicular to the plane of frame and also strengthening of (3-D) RC joints with focusing on the behavior of the FRP to concrete interface is evaluated. It was observed that the failure mode of the retrofitted RC exterior specimens, unlike the deficient specimens, is the formation of the plastic hinge in the beam section. In addition, it is seen that the slab and the lateral beam have a significant effect on the performance of deficient joints, which can increase the resistance of the 3-D specimens by more than 20%; also, in 3D strengthened joints, the possibility debonding of FRP laminates from concrete is higher than the 2-D model.

Due to the fact that common structures are designed only against gravity and seismic loads, the performance of these structures must be investigated against impact loading, in order to protect against this damage if necessary. In this paper, the effect of the arrangement of different shape of transverse bars and their spacing on the performance of beam-column element against blast loading has been studied. In this study, 13 model of concrete beam-column element with different transverse bar arrangement has been explored to examine their performance against explosions with 25 kg TNT. The finite element modeling was performed in Autodyn software and the parameters of explosion modeling, behavioral models of concrete elements and rebar, air meshing dimensions and concrete elements were determined with the results of the field test of explosion in previous studies. In this research, five type of transverse bars shape were modeled proportional to the three spacing of transverse bars. The degree of deformation and the mode of failure of the elements were compared as a measure of the performance of different states of each other. The results showed that for constant area of rebar, the element with one square tie and one link has best performance comparing to other arrangements. The maximum deformation rate in the A-1 (with one square tie ) model was 65 mm and in the D-2 model (one square tie and one link), it was 26 mm, showing a decrease of 60%.

In the past few years, with the advances made in the production of geosynthetic products, the use of these products in the pavement reinforcement has increased dramatically. Among the geosynthetic products, geocell improves the performance of the pavement due to the proper 3D confining of aggregates materials inside its cells. In order to investigate the effect of base layer reinforced with the geocell, six different geometries with reinforced and unreinforced base layer were simulated in ABAQUS finite element software. The behaviour of aggregate base and subbase layers were consirdered nonlinear which is implemented in ABAQUS by a FORTRAN subroutine. In the next stage, the results of the ABAQUS program as well as FORTRAN subroutine were validated using the results of KENLAYER and MICHPAVE programs as well as a full scale test. The results showed that the results of the ABAQUS program are consistent with results of other programs and also with the results of the fieled test. According to this research, by reinforcement of base layer using geocell, the surface deflection responses decreases betwen 5 to 10.28%, the tensile strain responses at the bottom of the asphalt decreases between 7.15 to 12.24% and the vertical compression strain on the top of subgrade decreases between 5.1to 9.19%. Also, the damage analysis was conducted based on the Asphalt Institute relations and results showed that the reinforcement of base layer increase asphalt fatigue life by 21.68-34.94% and subgrade rutting life by 20.89-35%.

Reinforced concrete squat walls with an aspect ratio less than 2 are prevalent in low-rise construction and at lower levels of tall buildings or nuclear power plants. They show a significant amount of shear deformation as compared to bending deformation. Shear strength governs the design of such walls. Prediction of seismic behavior, and followed by the design of these systems need an efficient modeling method. this study calibrates the effective modeling parameters of squat shear walls in small and large scales based on the test results. The modeling to be accomplished initially with non-linear finite element software on small scale (Vector2) and then with software by large scaleability (OpenSees), and the results were calibrated with test results. The study results show nonlinear finite element software has high accuracy in squat shear wall modeling. However, if a large-scale squat shear wall modeling along with a three-dimensional structure is required, on condition of a suitable aspect ratio, this paper is recommended to use OpenSees software.

With the advent and expansion of new technologies in recent years, masonry infill walls are gradually being replaced by other materials such as sandwich composite panels. Ignoring the effect of concrete sandwich panel (CSP) as infill on the behaviour of framed buildings in the analysis and design process, especially in seismic regions, is not always on the safe side. In the present study, the effect of presence of CSP, as infill, with opening on the in-plane behaviour of steel frames is numerically investigated by using finite element modelling. First, the proposed model is validated with existing experimental data. Next, a parametric study is conducted to evaluate the influence of effective factors e.g. size, type (window and door) and location of the openings (eccentric and central). The results show that the presence of openings within an infill panel modifies the lateral behaviour of the infilled frame by changing the distribution of stresses and formation of compression diagonal struts and leads to considerable change in the lateral stiffness and strength of the infilled frames.

Earth as a dynamic system is constantly undergoing the Change and variation. The changes causing factors include a wide range of dynamic processes, which consist of tidal effects, the loading on the crust and tectonic activity of tectonic plates. Accumulation of strain energy at the boundary between the plates and the surrounding area of faults are the most important consequences of movement and deformation of the tectonic plates. This energy may causes tension forces in the fracture sits, and if these forces can overcome the rocks resistance, an earthquake will occur. To measure strain energy, some mechanical tools contain strain gauges and turnkey have been installed at faults location which measure the deformations in situ. Due to the high cost of purchasing and maintenance of such these devices, based on geodetic observation, some alternative methods have been developed for study of the earth crust in local scales. Today, time measurement of the earth's crust deformations has been recognized as one of the notable branches of geodesy.
Measuring the Earth's crust deformation by geodetic methods has significant impacts on geological studies and provides a deep insight into understanding the mechanism of tectonic activities such as earthquakes and volcanoes.
Over time, measurement methods in geodesy significantly expande and space measurement methods have been replaced classic geodetic observations. The Global Positioning System (GPS) playes an important role in the development of observational methods in geodesy. By using GPS observations, determining of three-dimensional positioning become possible in a Earth-fixed coordinate system. Therefore, by comparing the obtained positions in different epochs, displacement vector can be calculated. Displacement vectors are not suitable quantities for deformation geometric interpretation, because they must be determined in comparison with a constant basic coordinate system.
On the other hand, strain tensor can be calculated by using displacement vectors. This tensor contains invariant parameters of coordinate system and has a direct association with geometric interpretation of deformation.
Since Iran is a seismically active and has been located within the convergence zone of two rigid plates, the Arabian and Eurasian plates, It is known as a natural laboratory for studying the kinematics and dynamics of tectonic interactions. Considering these reasons, researchers aim to study the geodynamic network of Iran. The neotectonics of the Iranian plateau is complicated due to various tectonic processes, (Zagros, Alborz, Kopeh Dagh, and Talesh), subduction of the oceanic crust (Makran), and a transition zone between the Zagros fold-and-thrust belt and the subduction zone of Makran.
In this paper, using observations of GPS velocity vectors at the different station of Geodynamic network of IRAN, two-dimensional deformation of the earth crust was estimated by 2-D strain analysis within 2009 -2013. And then, in order to deformation interpretation, invariant geometric criteria like dilatation and maximum shear were evaluated.
In this regard, as the first step, coordinates of the all GPS stations was calculated in two observational epochs in global coordinate system. In addition, considering the proper map projection, the coordinates were transfered into the projection plane. In the next step, the apparent displacement vector was determined for each GPS station. Finally, using displacement vectors, strain tensor components as well as dilatation and maximum shear invariant parameters was computed for the whole Geodynamic network.
Strain tensor calculates by using displacement vector derivatives but geodetic observations, only provide the discrete values of the displacement in GPS stations. Using the data interpolation techniques such as finite element method is an appropriate way to solve this problem. In this method, the domian of the data point partitions into some smaller sub-domains and in each one, a smooth function fits in that domain by considering the specified constraints on the existing data point. In order to create a smooth surface through the data points there must be a smooth transition from one patch or element to another across all shared edges. Interpolating functions in each sub-domain must apply in some continuity conditions such as continuity of the function itself, the first or higher order derivative when passing between two adjacent elements.
Due to the nature of the Earth's crust and two-dimensional strain analysis; triangular element is the simplest geometric element in this study. In addition to determine the mathematical form of displacement function using discrete values of displacement, nonlinear finite element method was known as one of the most important point of this study. In this method, after an area meshing by Delaunay Triangular Element, C1 continuous interpolation was adopted.
The interpolating function (displacement vector) on each triangle was a rational function gained by blending three cubic polynomials that are known as the Cubic Bezier Triangular Patches. Thus, unlike the previous methods, in this case, the displacement vector in common boundary of the two adjacent Triangular elements had C1 continuity, so a smooth interpolation having C1 continuity in the entire GPS network (vertices of the triangle domain) is achievable.
Since we are working with triangles, we will utilize barycentric coordinates rather than Cartesian coordinates. So the Bezier Triangular patches (displacement vector) or Bezier triangles were defined by Bezier ordinates or control points in barycentric coordinates form.
For our interpolating problem, only numerical values of displacement vector at the three vertices of the domain triangle was provided. So the inner control points of the control mesh (unknown Bezier ordinates) must be determined. This can be done if information about the gradient or normal on the control points is given. Least Squares Minimization Method based on Finite Difference Techniques was helpful for prescribing the first order derivatives of displacement function at the vertices of each triangle for creating a C1 surface. The number of vertices around the data point is chosen by the minimum distance technique.
According to the conducted analysis, the maximum amount of shear quantity equals to 5.58×10-5 unit/year value in the southern parts of Iran. The maximum amount of dilatation quantity equals to the -2.89×10-4 unit/year value in the southeast of the country, which represents a net contraction in these areas. This contraction demonstrates the collision of the Arabian plate from the southwest to Eurasian from the Northeast, and confirms the reverse and strike-slip faults in the Iranian plateau. Also the result of the computation and the evaluations by comparison with the seismic map of the region show the success and usefulness of the presented method for deformation study of the curst.

In recent decades, significant progress has been made on the use of Fiber Reinforced Polymers (FRP) in civil infrastructures. These materials have been used widely for the repair of concrete members, but their application to steel structures has been so far limited. Offshore structures, especially offshore platforms are very important and expensive ones, due to rising demand of energy. These structures may require repair and strengthening due to damages they may suffer in service. Searching for methods which are faster, more reliable, and less expensive led to application of FRP in repairing of offshore structures. Durability and high resistance against fatigue as well as high ratio of strength to weight enable these material superior to other conventional materials for this purpose. In addition high resistance of these materials against corrosion is an advantage for their use in marine environment. Composite materials are being increasingly used for the strengthening and repair of offshore structures. More recently, carbon fiber reinforced laminates have been used to upgrade fire walls in Mobil's Beryl Bravo platform to enable them to withstand blast loading. Also a number of corroded conductors and caissons have been repaired by composites on several Gulf of Mexico and North Sea platforms.
In current research, the behavior of T-shaped tubular joints reinforced with FRP material under compressive axial load is studied to evaluate the efficiency of these materials in strengthening the connections. For this purpose and in order to examine the effect of different variables, a numerical study was carried out using the non-linear finite element program. An elastic-perfectly plastic stress-strain curve was used for steel and glass/epoxy composite was used as the FRP. A four-node quadrilateral shell element was used to model the tubular members and composite. A perfect bond between steel and composite was considered. For considering different modes of FRP failure the criteria proposed by Hashin is used. The numerical model was verified using the data available for a T- joint which was tested earlier. The model showed acceptable accuracy especially up to the level of maximum strength of the joint. Using the verified model a number of joints with different strengthening scheme ware analysed under a monotonically increasing axial compression loads. Material and geometric nonlinearities were considered in the analyses. The analysis method was modified RIKS algorithm. The effect of number of FRP layers and the fibers direction on the ultimate capacity of reinforced tubular joints was studied. The results of numerical analysis showed improvements in the capacity of reinforced joints which were further enhanced by increasing the number of layers. Comparing FRP failure initiation load with ultimate capacity of joint, it was found FRP can bear a considerable amount of ultimate load without breaking. In addition, by comparing the Von-Misses stress and the vertical and horizontal displacement (ovalization) of chord in reinforced joints were observed that a substantial reduction in mentioned factors led to hindering the yielding of steel and increased the stiffness and ultimately the strength of connections.

Determination of optimal steel reinforcement layout for shear wall with opening is an important topic in designing this structural member.Strut-and-Tie Method (STM) is one way to design such members, where steel reinforcement is arranged based on the selected model.In this study, a strategy was introduced to determine optimal Strut-and-Tie model by selecting a good volume fraction in continuum topology optimization problem.The elastic strain energy was selected as the objective function to achieve the optimal pattern, and the modified Solid Isotropic Material with Penalization (SIMP) with continuous penalty function was used to prevent local minimum solution. Finally, By using Nonlinear Finite Element and defining qualitative and quantitative conditions for optimal STM, the resulting models, obtained with topology optimization for three shear wall with various opening, were compared with previous models and models with manufacturing constraint.The results show that, topology optimization models have the best ultimate load to steel weight ratio and get the optimal steel reinforcement layout for shear wall with opening.