Abstract in English:Abstract This paper presents results of the experimental study on the behavior of plain and fiber reinforced cement mortars with different fibers under static and impact compressive load. Glass, polypropylene and carbon fibers were used in equal dosage by mass. The impact test was conducted using an impact tower with drop hammer, which represented the modification of the split-Hopkinson pressure bar system, with strain rates ranging from approximately 35 to 60 s-1. The results of the static test and impact test with two different drop heights were compared and discussed. Among other, it has been concluded that the tested fiber reinforced mortars had no greater static and impact strength compared to the plain mortar. Only their ductility was increased at both static and impact failure. Strengths and ductility of all composite specimens were similar, i.e. without the effect of fiber type. With the increase of strain rate, compressive strength is increased and ductility is decreased for all tested specimens.
Abstract in English:Abstract This study investigates the application of operational modal analysis along with bees optimization algorithm for updating the finite element model of structures. Bees algorithm applies instinctive behavior of honeybees as they look for nectar of flowers. The parameters that needed to be updated are uncertain parameters such as geometry and material properties of the structure. To determine these uncertain parameters, local and global sensitivity analyses have been performed. An objective function is defined based on the sum of the squared errors between the natural frequencies obtained from operational modal analysis and finite element method. The natural frequencies of physical structure are determined by stochastic subspace identification method which is considered as a strong and efficient method in operational modal analysis. To verify the accuracy of this method, the proposed algorithm is implemented on a three-story structure to update parameters of its finite element model. Moreover, to study the efficiency of bees algorithm, its results are compared with those of the particle swarm optimization, and Nelder and Mead methods. The comparison indicates that this algorithm leads more accurate results with faster convergence.
Abstract in English:Abstract A 2D formulation for incorporating material discontinuities into the meshless finite volume method is proposed. In the proposed formulation, the moving least squares approximation space is enriched by local continuous functions that contain discontinuity in the first derivative at the location of the material interfaces. The formulation utilizes space-filling Voronoi-shaped finite volumes in order to more intelligently model irregular geometries. Numerical experiments for elastostatic problems in heterogeneous media are presented. The results are compared with the corresponding solutions obtained using the standard meshless finite volume method and element free Galerkin method in order to highlight the improvements achieved by the proposed formulation. It is demonstrated that the enriched meshless finite volume method could alleviate the expecting oscillations in derivative fields around the material discontinuities. The results have revealed the potential of the proposed method in studying the mechanics of heterogeneous media with complex micro-structures.
Abstract in English:Abstract A nodal averaging technique which was earlier used for plane strain and three-dimensional problems is extended to include the axisymmetric one. Based on the virtual work principle, an expression for nodal force is found. In turn, a nodal force variation yields a stiffness matrix that proves to be non-symmetrical. But, cumbersome non-symmetrical terms can be rejected without the loss of Newton-Raphson iterations convergence. An approximate formula of volume for a ring of triangular profile is exploited in order to simplify program codes and also to accelerate calculations. The proposed finite element is intended primarily for quasistatic problems and large irreversible strain i.e. for metal forming analysis. As a test problem, deep rolling of a steel rod is studied.
Abstract in English:Abstract An improved dynamic contact model for mass-spring and finite element systems is proposed in this paper. The proposed model avoids the numerical troubles of spurious high-frequency oscillations for mass-spring and finite element systems in dynamic contact problems by using the parametric quadratic programming technique. The iterative process for determination of contact states are not required for each time step in the proposed method, as the contact states are transformed into the base exchanges in the solution of a standard quadratic programming problem. The proposed methodology improves stability and has good convergence behavior for dynamic contact problems. Numerical results demonstrate the validity of the proposed method.
Abstract in English:Abstract In blast loading, out-of-plane behavior of infill wall is activated initially. Contrary to other types of infill wall such as brick or concrete wall, infill steel plate wall exhibits more ductility. Since blast impulsive loading suddenly exerts a large amount of kinematic energy to infill wall, energy absorption characteristic of the infill wall should be taken into account, especially for protection of vulnerable buildings. Out-of-plane ductility of infill steel plate reduces the transmitted impulsive loading to the structure. In present study, out-of-plane behavior of infill steel panel has been studied as a sacrificial element. In-plane behavior of infill steel panel is also investigated as a lateral bearing system. In-plane action should satisfy both resistance and performance criteria. In this research, finite element analysis, including geometric and material nonlinearities is used for optimization of the steel plate thickness and stiffeners arrangement to obtain more efficient design for out-of-plane and in-plane actions. The results of analyses show that for out-of-plane action, the plate thickness and stiffeners arrangement can be determined such that on one hand, the impulse transmission can be minimized, and on the other hand, the maximum residual deformation can be limited to the predefined damage level. Additionally, the effect of stiffener arrangement on the performance of in-plane are studied and some practical rules have been derived for designing the infill steel panel against blast.
Abstract in English:Abstract This paper presents the development of a formulation, based on Positional Finite Element Method, to describe the viscoelastic mechanical behavior of space trusses. The numerical method used was chosen due to its efficiency in the applications concerning nonlinear numerical analyses. The formulation describes the positional variation over time under constant stress state (creep). The objective is to provide a way to quantify the creep behavior for space truss structures and thus contribute to the encouragement of GFRP usage in such structural components. Time-dependent behavior of such materials is one the most important factors for their use in design of structures, demanding studies about the deformations expected within the operational life of the structural systems. To perform this study, the proposed methodology considers a standard solid rheological model to describe stress-strain time-dependent law. This model is implemented in the formulation for quantify the total strain energy. The effects of the model parameters in the mechanical response of the structure with accentuated geometric nonlinearity were presented. In this analysis, it was possible to identify the influence of the elastic and the viscous moduli on the creep response. Model calibration was performed using test data obtained from literature and a GFRP transmission line tower cross-arm was simulated to predict the evolution of displacements under real operational loads. From the results, it was possible to observe a fast evolution of displacements due to the creep effect in the first 7,500 h. This increase was close to 0.6% in relation to the displacement obtained in the elastic behavior. The presented methodology provided a simple and efficient way to quantify the creep phenomenon in viscoelastic GFRP composites truss structures, as can be seen in the developed analyses.
Abstract in English:Abstract Post-Northridge welded connections are widely used in engineering projects as lateral-force-resisting systems. In this study, we aim to assess the cyclic performance of the post-Northridge welded connections to compare the connections with each other. For this purpose, we focus on Welded Unreinforced Flange – Welded Web (WUF-W), Welded Flange Plate (WFP) and Reduced Beam Section (RBS) connections. ANSYS Finite Element Analysis (FEA) software is used to do this investigation and evaluate the adequacy of the numerical analysis through an experimental specimen from the literature. Welded connections are designed according to ANSI 358-10 and FEMA-350 using the same material properties, beam spans, and steel profiles so on to make comparison possible. Material and geometry nonlinearities are adequately considered and also hexahedral solid elements are used in three-dimensional FEA. Based on the analyses subjected to cyclic loading, the performance of the connections are compared in terms of failure modes, plastic hinge locations, hysteretic curves, energy dissipation capacities, hysteretic equivalent viscous damping ratio and initial rotational stiffness values. The results showed that using WFP provides high stiffness in a connection while using RBS provides high safety in a connection.
Abstract in English:Abstract The use of numerical methods such as the finite elements method for solving structural analysis problems is becoming ever more efficient. An interface element capable of associating flat plate/shell element in three different combinations has been developed. In a structural analysis, when only the flat plate/shell’s finite elements are used in the discretization, some problems concerning dimensional variation of the transversal section, as well as overlapping of areas can occur. . The aforementioned problems can be solved by the developed interface elements that can also simulate a possible deformable connection that is existent in the association of materials with different characteristics, such as the composite steel-concrete elements. One of the applications of the finite elements developed is for the numerical simulation of composite steel-concrete elements, such as the composite beam formed by a reinforced concrete slab attached to a steel beam using a deformable connection. In this case, the concrete slab and the steel beam are discretized by the flat plate/shell element, and the deformable connection done by the interface elements. In the validation of the implemented elements, we used numerical and experimental results found in the literature, and analytical solutions considering the classical plate theory.
Abstract in English:Abstract The present work aims to study the nonlinear behavior of reinforced concrete structures via Refined Plastic Hinge Method (RPHM). Pseudo-springs are used at the finite element ends, where the gradual loss of stiffness is determined by the combination of the normal force and bending moment (NM) in the cross section. The limiting of the uncracked, elastic and plastic regimes is done in the NM diagram. The concrete cracking is explicitly simulated with two approaches to calculate the effective moment of inertia of the cross section. The displacement-based formulation is referenced to the co-rotational system and coupled with continuation strategies to allow to overcome the possible critical points in the equilibrium paths. For validation of the numerical simulations, the results found with the proposed formulation are confronted with experimental and numerical data present in literature.