نشریه مهندسی عمران مدرس
سال بیست و سوم شماره 3 (امرداد و شهریور 1402)

The health of structures, provision of safety, and the sense of security are among constant requirements and perpetual challenges of engineering and managers in the field of crisis management. Erosion and occurrence of minor local damage to structures and structural members in the early stages of construction or during operation, especially in critical structures such as power plants, tall buildings, stairs, dams, airports, and hospitals, among others, have always been among major problems. In case the damage sites are not identified timely and decisions are not made appropriately, substantial irreparable damage is expectable. Structures are always affected by various natural or unnatural factors such as earthquakes, explosions, and unprincipled excavations, which can aggravate the local damage in them and lead to their destruction, hence substantial human and financial losses. Therefore, it is highly crucial to monitor the health of structures and structural members. Therefore, health monitoring in structures and structural members is highly important. The column is one of the most significant members of engineering structures, especially in building structures and bridges, so that the instability of one of these members can lead to instability and destruction of the structure. Hence, design engineers expect columns to be the last members of structures to be damaged. In this paper, the health monitoring of the column as a structural member was performed by considering the effect of axial load on modal dynamic responses (i.e., natural frequencies and mode shapes). The results showed that the natural frequencies of all modes in both healthy and damaged states decreased with increasing axial load in proportions of the base critical load (the worst-case limit load). Also, at the same loads, the frequency of the healthy sample was always higher than that of the damaged sample so that the frequency difference between healthy and damaged states increased with greater severity of the damage. By introducing a Damage Detection Index (DDI) based on the wavelet coefficients obtained from the details of wavelet analyses of damaged and undamaged modes, the damage sites could be identified with a simple check and high accuracy by observing vibrations in DDI. Also, studies have shown that the DDIs of different damaged sites are independent of each other and are only affected by the severity of the damage and that the effects of axial load on DDI are very small and negligible. The independence of the DDIs of different damaged sites indicates the effectiveness of the proposed method in identifying damaged sites. Otherwise, failure to identify one damaged site may affect the identification of other damaged sites. The damage detection capability using the proposed DDI was investigated in columns with different support sections and conditions, and successful troubleshooting results were obtained. Moreover, investigations were performed with other wavelet functions, and the damage site was successfully identified. The proposed damage detection indicator is an efficient index in the column structures under the effect of axial load with axial buckling-prone support conditions and is proposed as a reliable method in identifying column damage sites in practical health monitoring of structures.

Nowadays, non-linear analysis of structures has become an attractive matter and appears necessary for most structural engineering applications, whereas tendency for more accurate structural analysis has increased by many engineers. In general, nonlinear analysis is divided into two main parts: geometrical and material nonlinearity. In some cases, both nonlinear types are used simultaneously in the analysis process. In this paper, a new Incremental-iterative method for the analysis of truss structures, including both geometric and material nonlinearity behavior, is proposed. Nonlinear equilibrium equations are solved using an Incremental-iterative method based on the displacement control process. The basis of geometric nonlinear process is based on rotational formulation and material nonlinearity is based on tracing the stress-strain relationship for post-buckling behavior of the truss members. In this proposed method, it is assumed that the magnitude of the displacement increment vector to the size of the total displacement vector at the beginning of each load increment, is a fixed value. Based on this idea and thinking, the corresponding equations are written and a new formulation has been developed based on the displacement control scheme, as, by employing a specified displacement, the corresponding load will be obtained. Using fixed incremental displacement algorithm, this paper, proposed a novel method that has a possibility of passing the limit points in the case of highly nonlinear behavior state. The proposed method, is able to pass the limit points including snap-through and snap-back. Some examples are provided for the proposed method and by solving them, the efficiency of the proposed algorithm is examined. A limit point refers to the turning point for the equilibrium path of a structure, which can be further considered as the transition point from stable to unstable equilibrium states or vice versa. In analysis of such structures, the simple incremental iterative methods unable to pass this limit points. The simple incremental iterative methods couldn’t trace the equilibrium path after the limit points. For resolving such disadvantages, advanced analysis methods have been developed numerical examples demonstrate the feasibility and accuracy of the proposed algorithm, to be highly suitable in predicting nonlinear response of structures with multiple limit points and snap-back points, and trace the equilibrium path accurately. The results show that the method developed in this paper, traces the equilibrium curve as well as the modified arc-length method with a small difference. In spite of the fact that the procedure herein explained is only implemented for truss structures, it is possible to generate the proposed method for other structures. This paper is organized as follows. In the first section, we will have a brief review of nonlinear analysis of trusses, including both geometric and material nonlinearity. A literature review is done and a number of available methods in this field is describe. In the second section we explained the basic concepts in nonlinear analysis. The third section geometric and material nonlinear behavior of trusses are reviewed. Section four is allocated to implementation of proposed method for nonlinear analysis. In section five, the validity of the proposed method is illustrated with some examples.

Transportation issues are categorized into three strategic, tactical, and operational levels, each of which has a different level of influence, required budget, decision makers, and time period. The issue of developing the rail transportation network is one of the key issues at the strategic level. In short, network design deals with the solution of allocating a limited budget to a feasible subset of the set of projects, in such a way that specific goals: such as minimizing the total travel time in the network, the developing costs of the network, maximizing revenue from freight transportation, or maximizing the attraction of freight demand to the rail mode should be taken into account. In this issue, two stakeholders are considered. On one side, the operators make the macro decisions to meet the criteria; such as maximization of benefit, maximization of travel coverage, minimization of development costs, minimization of casualties and minimization of total travel time. On the other side users who try to maximize their benefits such as finding the shortest route through the network.The general form of the network design problem is a two-level problem in the category of NP-hard problems, which is difficult to solve in even small scales. To solve this problem, the solution algorithms are classified into two general categories: exact and approximate. The exact solution algorithm give the best global solution among the possible solutions, they are so-called intractable in terms of memory usage and solution time with the increase in the size of the problem. Therefore, the second category of so-called approximate algorithms was presented to solve network design problem. Greedy algorithms are classified in the category of approximate algorithms. In the greedy algorithm, reaching the goal in each step is independent of the previous step. That is, at each step to reach the solution, regardless of what choices was made in the previous stages.In this article, the greedy algorithm is presented to solve the problem of network design trying to reduce network development costs. The proposed algorithm is designed to develop the blocks with priority of the lowest cost, and this process continues until the entire level of incoming demand can be transferred through the network. This algorithm is implemented with Java language and the railway of Iran is used as a case study. Considering the nature of two objectives in the problem, freight demand passing through and development cost in the network, "pseudo-pareto" solutions with different percentages of the importance of two mentioned objectives are discussed. The analysis has shown that with the increasing importance of the development cost, fewer blocks are developed and as a result, less demand is passed through the network. Also, with the increasing importance of freight demand, the algorithm leads to solutions that have caused extensive development in the network. The proposed greedy algorithm has a light computational load, and it achieves its solutions in less than 1 hour. Also, the algorithm is implemented for two demand levels of 70 million tons per year and 110 million tons per year and the results are analyzed.

The evaluation of the fragility functions is an analytical approach that allows different ground motions to be used at varying intensity levels and represent various characteristics of low-intensity and high-intensity shakings. The fragility curves demonstrate the structure’s probability of collapse, or other limit states, as a function of some ground motion intensity measures (IM). The intensity measure is often quantified by spectral acceleration (Sa) or peak ground acceleration (PGA). Based on the statistical procedures, the parameters of the fragility functions are computed by assessing the results of nonlinear dynamic time history analyses. Therefore, the probability of failure associated with a prescribed criterion (e.g. the maximum inter-story drift) is estimated based on the probabilistic distribution relations.This paper evaluates the effects of internal flexural frames on the seismic performance of diagrid structures based on fragility curves. This evaluation is achieved by designing a group of 24-story studied diagrid models with various diagonal angles of 49, 67, and 74 according to the Iranian Standard No. 2800 (4th edition) and the Iranian National Building Code (Steel Structures-Issue 10). Then, some specific interior gravity frames of the studied diagrid models are replaced with bending frames. The seismic vulnerability of the studied diagrid structures with and without internal bending frames is assessed using nonlinear time history and incremental dynamic analyses (IDA) under near-field earthquake records containing different directivity effects. Finally, the fragility curves for the studied structures were obtained based on the lognormal probabilistic distribution function for the seismic performance limit states including IO, LS, CP, and global instability (GI). Moreover, the seismic performance levels of the studied structures were determined based on the FEMA 356.The results of performed nonlinear time history analyses indicate that the application of internal bending frames in diagrid structures would reduce the value of inter-story drift in upper floor levels, especially when the angles of exterior diagonal members are large. The results also show that the global instability of diagrid structures without internal bending frames can occur at a faster rate than the skeletal models with internal bents. Also, the contribution of the internal bending frames in improving the nonlinear behavior of diagrid structures depends on the perimeter triangular patterns. Due to this dependency, the increase in the angle of the inclined members in skeletal geometric configuration can increase the effectiveness of the internal bending frames in preventing the occurrence of global dynamic instability. The fragility curves of the studied diagrid structures illustrate that the internal bending frames reduce potentially excessive seismic performance levels. Furthermore, the internal bending frames amplify the seismic energy dissipation capability of the diagrid structures.

There are different ways to design energy dissipators. According to the type of performance of these elements, it is better to adopt the design method according to the consumed energy in order to determine what load should be considered as the design load for the energy-dissipating elements so that these elements have the best seismic performance. One of these methods is the use of energy-based performance index. This method is usually used to design pall friction dampers. In this research, it has been tried to use this method for a 12-story reinforced concrete structure of friction dampers, viscous, and seismic isolators of the lead rubber support type, and their performance is investigated. Based on this, for the earthquake records, the energy-based performance index has been calculated and the optimal design load has been calculated for these three energy wasters. Structures with optimal design load are subjected to dynamic loads and the contribution of elements in energy loss and their fragility curve have been evaluated. The results show that in a constant spectral acceleration (Sa), the probability of exceeding the specified performance level of a structure with a friction damper that is designed on this basis is lower than other structures. Also, in the acceleration of the design, the seismic isolator has wasted more energy than the other two structures.

The moment-resisting steel frame building is highly used due to their advantages such as, high speed construction coupled with appropriate strength and ductility. The main advantage of this system is related to architectural considerations and the possibility of creating openings within all spans. Connections play an outstanding role in the seismic responses of this structural system. The connections are generally assumed to have a rigid behavior in analyzing and designing of the moment-resisting steel buildings. Studying of the previous investigations indicates that the assumption of rigid behavior for the beam-to-column connections is not always correct and can bring about a significant error in the responses. In this study, behavior factor of moment-resisting steel frames considering joint flexibility is evaluated. To do so, some intermediate moment-resisting steel frames with various number of stories and bays including 1-bay, 1- and 2-story frames, 2-bay, 2-, 4-, 6-, 8-, and 10-story frames, 3-bay, 3-, 6-, 9-, and 12-story frames and 4-story, 2-, 6-, 10-, and 14-story frames are designed regarding Iranian seismic code and Iranian national building code for designing steel structures. After that, the capacity curves of these frames are achieved using pushover analysis once considering rigid connections and again taking joint flexibility into consideration using OpenSees software. To model the nonlinear behavior of connections, one zero-length rotational spring is assigned to each end of beam members. Then, the behavior factor of each frame is calculated using the recommended procedure of FEMA-P695. The outcomes show that for the frames with rigid connections, the acquired behavior factors are almost close to 5 (which is the prescribed behavior factor in Iranian seismic code for the intermediate moment-resisting steel frames). Furthermore, for the frames with semi-rigid connections (60%), the behavior factors are close to 5 as well. For 10-story 2-bay, 12-story 3-bay, and 14-story 4-bay frames the prescribed behavior factor in Iranian seismic code does not meet. For these frames, the ratio of height to total-span that is known as the slenderness coefficient of the frame is higher than others, so these frames fall into slender frames. Results show that for the frames with semi-rigid connections (60%), despite of decreasing the over-strength factors in some cases, their ductility increased, therefore, the behavior factors are achieved higher than those of the frames with rigid connections. All in all, it is observed that the nonlinear behavior of connections can significantly affect the seismic behavior of the moment-resisting steel frames. Comparing the behavior factors calculated in this investigation with the prescribed value of this factor in code 2800 showed that for the frames with rigid connections, 80% of the obtained behavior factors are higher than 5. For frames with semi-rigid connections (80% and 60%), 0%, and 66% of the behavior factors meet the proposed value of code 2800, respectively. Regarding the observations, it is recommended that the influence of joint flexibility be considered in assigning a value of behavior factor to design the moment-resisting steel frames.

Today, the use of functionally graded materials is increasing. In these materials, the mechanical properties change as a continuous function throughout the problem domain. Due to these continuous changes, the problems of non-adhesion of materials, delamination and stress concentration at the joint, which can be problematic in composite structures, do not arise. Numerical methods such as the finite element method can be used to analyze functionally  graded materials, but due to the limitations of this method, we will face many problems. The most important of these problems are the lack of a suitable element for the analysis of problems that can accommodate changes in the properties of materials, or the inability to accurately model the edges of shapes that have complex geometry, so in this research, the isogeometric method is used in which these weaknesses are eliminated. Also, since the error is an inseparable part of any numerical analysis and the reliability of the results has always been the main concern of the researchers, and in general, there is no exact answer to many problems, finding a solution to estimate the error in the calculations is of special importance. Therefore, in this article, for the first time, the isogeometric method has been developed in the analysis of problems with functionally graded materials with the approach of improving the stress field and estimating the error in it. This error estimator is in the category of error estimation methods based on stress recovery, and the goal is to increase the impact index of the error estimator and more adapt the error distribution method obtained from the proposed error estimator with the exact error estimator in solving problems. In this method, by using superconvergent points, where the order of convergence of the gradient of a function is one order higher than the value expected from the approximation of the shape function related to the approximate solution, a hypothetical surface is made for each stress value. To define this surface, we use the same shape functions used in the isogeometric method to approximate unknown functions. This hypothetical level is created when the coordinates x, y and z of its control points are specified. The x and y coordinates of each control point are used to model the geometric shape. The z component of the control points is calculated by minimizing the distance between this hypothetical level and the stress level obtained from isogeometric solution at the gauss-elements points of each region using the minimum square sum method. From the comparison of the exact error norm and the approximate error norm for sample problems, it can be seen that the proposed error estimation has a suitable efficiency for estimating the error in the analysis of problems with functionally graded materials by isogeometric method, and it can be used as a solution to error estimation and calculate the improved stress field level in solving functionally graded problems by isogeometric method. It is also possible to identify areas of the isogeometric solution domain that have a large error with the help of the proposed error estimator method and achieve local improvement of the network in those areas and increase the accuracy of the isogeometric solution.

Seismic waves of structural vibrations propagating through the soil and transmitting to other structures, and the effect this has on seismic performance, have recently come up due to the result of recent ground movements originating in soft soil zones like Mexico City. In regions with densely built structures, this vibration may have a significant impact on structural responses. The purpose of this research is to evaluate the seismic performance of a single structure on soil (Soil-Structure Interaction, or SSI) vs. that of a pair of similar structures with differing soil conditions (Structure-Soil-Structure Interaction, or SSSI). Recent research suggests that damage risks may increase due to the SSSI impacts. The studied structure is a three-dimensional, six-story steel building with a foundationally sound moment and braced frames lateral force resisting system. To account for the non-linear behavior shown by SSSI and SSI models, a three-dimensional steel structure is presented in OpenSEES. For simulating the soil easily under the foundations and between structures, the nonlinear Beam-on-Nonlinear-Winkler-Foundation (BNWF) model is employed. There is a meter of space between structures. Therefore impact between buildings is prohibited. The SSSI and SSI systems are examined using 11 horizontal components. Ground motion magnitudes ranges from Mw = 5.0 to Mw = 8.5, soil shear velocity varies from Vs30=185 m/s to Vs30=365 m/s, and distance from faults goes from 10 km to 50 km. The two orthogonal horizontal components of selected seismic ground motion stimulate the system. Inter-story drift ratio, roof displacement, and plastic hinge rotations of structural elements are among the reactions of importance. In the SSSI and SSI models, the Park-Ang damage index is utilized to calculate the local and global damage index. This damage indicator is divided into two categories: deformation and energy-based indices. The current study's findings show that the SSSI model increases the roof displacement response by up to 58%. When the SSI and SSSI cases are compared, it is discovered that the SSSI case increases the inter-story drift ratio by 118% in the moment frame and by 53% in the braced frame. In addition to this, it is shown that,  in general, a second structure may have a significant impact on the frequency amplitude of a system that is adjacent to it. According to the data, the amplitude of the power spectrum density in the SSSI model is more than 44.6% higher than that which is found in the SSI model. According to the findings, the damage index predicted by SSSI models is 32% greater than that predicted by SSI models. It is important to keep in mind that constructing a second building next to an existing one is often counterproductive and raises the possibility of damage occurring in both of the structures. As a result of the findings, it is clear that more study into SSSI phenomena and their influence on structural seismic risk is necessary. This is because it has been shown that adjacent buildings may significantly increase a structure's vulnerability to earthquakes.

Wharf is an engineering structure which is constructed generally for loading or unloading of goods. The structure may be constructed on the weak layers like gravel and sand with respect to the bank conditions. In case of incorrect design of this type of structure and its failure, the wharf activities may be stopped for a long time due to damage to adjacent facilities. For this reason, investigating the behavior of coastal structures against failure factors such as earthquake and the liquefaction due to it, is of great importance. Analysis of wharf performance against liquefaction is done generally using the numerical methods. In this article using the Flac2D software which has the capability of nonlinear analysis of effective stress and generation of excess pore water pressure in the soil continuum, the liquefaction phenomenon in the soil surrounding the wharf is simulated using the behavioral model Finn. In continuation, the impact of different earthquake parameters on the wharf behavior is investigated. Finally, the results of the excess pore water pressure, horizontal displacement, soil settlement and bending moment of piles are presented. Then, the correlation between these parameters and different earthquake parameters is investigated. As the earthquake intensity criteria have great importance in terms of statistical assessment of seismic demand of various types of structures, therefore, in this study in order to investigate the quality of earthquake intensity criteria, the determining indices such as being optimal and applicable, efficiency index, sufficiency with respect to the earthquake magnitude and distance to the center of earthquake propagation are investigated. The results show that compatibility between the earthquake parameters and soil settlement is generally better with respect to other parameters.

Hybrid simulation is a relatively new and efficient tool that uses the advantages of both numerical and experimental methods to evaluate the performance of structures under different loadings. In this paper, a hybrid simulation framework has been developed, in which OpenSees was considered as finite element software for modeling numerical part, OpenFresco as middleware for data exchange and LabVIEW as data collector and actuator movement controller. Utilizing OpenFresco in the developed framework, would be facilated conducting geographically distributed hybrid simulations. As the connection between OpenFresco and LabVIEW is hand shaking, the processing speed is very high and the delay between sending displacement and receiving force is only due to the dynamics and movement of the engine and the bandwidth of the sensor, so there is no interruption during this process and in this case, there is no need to define the predictor-corrector algorithm to keep the actuator movement continuous. To validate the accuracy and efficiency of the developed framework, an improved widened flange beam-column connection was considered in a one story one bay steel moment frame, and the mentioned frame was divided into two numerical and experimental substructures in such a way that half of the frame was modeled two-dimensionally in OpenSees and the other half was constructed in the laboratory. The horizontal acceleration of the Tabas earthquake was subjected to the frame. The accuracy of the control system which was used in the developed framework was investigated by comparing the measured and the command displacements, and the small value of HSEM indicated the proper performance of the control process. To evaluate the performance of hybrid simulation, a coupled numerical simulation (virtual hybrid simulation) was considered as a reference model. In this fully numerical simulation, ABAQUS and OpenSees were used as finite element software and OpenFresco as middleware. Results of coupled numerical simulation and hybrid simulation were compared with each other and the obtained accuracy index (εrms) indicated the accuracy and appropriate performance of the developed hybrid simulation framework.

Silos are widely used in many industrial and engineering processes, so discharge grain from the silos is considered as one of the most important issues. Due to the high cost of laboratory studies on different materials and conditions, computational methods are used as a substitute approach for much less cost. Because of the ability of nonlinear Lagrangian methods to model large deformations and discontinuities, this study develops and evaluates a mesh-free Lagrangian model based on a weakly-compressible MPS formulation to simulate of the discharge of the granular silo. In Lagrangian methods, unlike the Eulerian method, instead of networking the solution field and breaking the equations on the nodes, the solution field is divided into a number of particles and the broken equations are solved on these particles. In fact, the governing equations are transformed into particle interaction equations using different operators. In the meantime, the particles that are closer to the particle under study will have a greater effect on that particle. In such a way that the effect of relatively distant particles can be ignored in comparison with closer particles and the interaction between particles can be limited to a specific domain called the radius of effect. The effect of each particle on the calculated particle is measured by a weight function. In the WC-MPS method, the system is considered as a system with weakly compressibility and calculates the pressure of each particle using the state equation. In this study, the Tait's state equation is used, which is used for high pressure water flow. The MPS method uses particle density to track the free surface. Because there are no particles outside the free surface, the density of the particles on the free surface decreases sharply. A particle is known as a free surface particle whose density is somewhat lower than the standard particle density. The value of this limit may be selected from 80% to 99% depending on the problem Therefore, the pressure of this particle on the free surface will be set to zero in each time step and in the MPS method, and there is no need to apply any additional condition for the free surface. For solid (impermeable) boundaries, such as walls or beds, this boundary condition is applied. In the vicinity of solid boundaries, the particle density decreases, which can lead to computational disturbances. Therefore, a number of ghost particles are located outside the boundaries to prevent this density reduction.  In this method the granular material is considered as a non-Newtonian visco-plastic fluid and an exponentially Herschel-Balky (H-B) rheological model in combination with pressure-dependent yield criteria model is employed to model non-cohesive grain behavior. The ability of the developed numerical method to discharge grain from silos has been evaluated and compared with the experimental results and the DEM method. Comparison of the results of the developed numerical method for the discharge of grains from the silo with the available experimental measurements and the DEM method shows the capabilities of the proposed model to accurately predict the surface and velocity profiles in this sample problem.

Different methods of seismic analysis of structures generally include Incremental Dynamic Analysis (IDA), Time History Analysis (THA), and Static Pushover Analysis (SPA), which in the same order have a decreasing trend in the computational effort and the estimation accuracy of seismic demands. In the structural engineering problems involving probabilistic seismic analyses such as seismic fragility assessment, and seismic risk based design optimization due to the consideration of a wide range, respectively, for uncertain parameters and the decision-making variables, many repetitions of the time-consumed seismic analyses on the finite element models has been required. This issue is considered as a burden for computational efficiency due to the significant increase in the computational cost, especially in the case of large-scale structural systems such as Cable Stayed Bridges (CSBs). Therefore, in this paper, an attempt will be made to develop and adapt the approach of the static pushover method to the seismic behavior of the CSB in both longitudinal and transverse direction. This approach includes producing the structure's capacity curve through the proposed static pushover approach and intersecting it with the corresponding record demand spectrum, while considering all the important linear and nonlinear behavior modes of the structure in order to calculate the performance point of the CSB. Then the desired seismic responses of the CSB has been recorded at the performance point of structure. The intended seismic demands of the CSB include pylon head displacement, critical pylon section curvature, cable tension, and bearing device displacement. It is worth mentioning that according to the purpose of this research, which includes the probabilistic form of seismic analysis, the seismic analyses consist of the applying the various seismic records on the samples produced by the uniform design sampling method in such a way that the uncertainty in the structural and seismic parameters is taken into account at the same time. In this way, the mentioned approach as well as other more accurate methods including IDA and THA are used in order to estimate the probability distribution of aforementioned seismic demands in three case studies of existing CSBs in Iran. Then, in relation to the above-mentioned proposed approach entitled Developed Nonlinear Static Pushover (DNSP) method, the complete justification of the relatively small errors in the outputs estimation of this method will be performed by explaining the sufficient reasons and details to clarify its effective computational efficiency in the seismic assessment of the CSBs. In the next step, the seismic fragility curves related to the various components of the case studies are generated as the final result of probabilistic seismic analysis of structures. For the comprehensiveness of validation and in line with the recommendation of the DNSP method for the CSBs, the relationship between the accuracy of methods in estimating seismic responses and seismic fragility is also will be discussed. In the end, after comparing the seismic outputs and the computational cost of different methods, this study concluded that the proposed DNSP approach in estimating the demand and seismic fragility of the CSBs has an appropriate accuracy and at the same time leads to that the computational workload has been significantly reduced compared to existing methods.

Regarding the dependence of ductility response in structural components on their ability to keep stability after yielding of the material, in this paper, the influence of change in tangential modulus of work-hardening part of stress-strain response, was observed on the load carrying capacity and plastic buckling response of stainless-steel tubular columns. To keep cost-effectivity in the research, the objective of the study was followed by FE modeling, which was verified by simulation of plastic buckling in an experimental specimen with D/t=60, made of duplex stainless-steel. For all components of the models, S4R elements were used and both material and geometrical nonlinearity were included in the models. To conduct deformation of the columns according to the experimental observations, an initial imperfection equal to t/100 to combination of first three mode shapes of the columns was imported. The material stress-strain response after yield point was determined for the model by a multilinear curve according to the tensile stress-strain curve, obtained experimentally. The main parameters for comparison of the FE model and experimental observation were force-displacement curves. The FE study was extended by modeling of stainless-steel columns with various D/t ratios in range of D/t=30-120. Two main parameters comprised of energy absorption capacity and deformation of the column related to yield of the section and collapse threshold (e/g) were compared to the columns with various D/t ratios.According to force-deformation curves, by decrease of D/t ratio, the energy absorption capacity increased considerably for the columns, for example the energy absorption capacity and e/g ratio increased by 12% and 12%, respectively for comparison of the columns with D/t=60 and D/t=30, however, e/g ratio for the columns of D/t=120 and 100 were less than two, categorized as a force-controlled column. Two models with D/t=30, 60 were selected to follow the objective of the study. The work-hardening response of the material was approximated by two linear segments, the first by a tangential modulus equal to E=7.9 GPa and the second was by the tangential modulus equal to E=2.4 GPa. The influence of change in tangential modulus through a range between 0-200% was observed on the structural parameters related to ductility, comprised of energy absorption capacity up to collapse threshold and deformation of column at the collapse threshold. The results showed different reaction of the columns with different D/t ratios, increase of tangential modulus at the first work-hardening part was more significant than increase of the influence by the later part of the work-hardening response for the column of D/t=60, however an inverse effect was observed for the column of D/t=30, i.e. the influence of tangential modulus at the later part was more significant for this column. By doubling the tangential modulus of earlier part of the work-hardening response, the energy absorption capacity and e/g ratio for the column of D/t=60 increased by 26% and 46%, respectively. By doubling the tangential modulus of later part of work-hardening response, the energy absorption capacity and e/g ratio for the column of D/t=30 increased by 111% and 71%, respectively. The results showed significant parts of work-hardening response of duplex stainless-steel, to be exploited for development of ductility in the tubular columns.