Abstract in English:Abstract Reinforced concrete (RC) Pile cap, a thick reinforced concrete block, is constructed to provide a connection between a structure and multiple single piles. In the existing design methods, the behavior of RC pile cap is usually considered as a rigid body and resulting forces or reactions on piles are assumed equal. However, in actual conditions, there is a possibility of bending of pile cap and un-equal reactions or forces on piles. This study represents a detailed finite element analysis of RC eight-pile cap by using a computer program ATENA. ATENA serve as rational tools to explain the behavior of RC structures. Research parameters included different types of loading conditions, analysis types such as linear and non-linear, support conditions, length of pile, strength of concrete and thickness of pile cap. The finite element analysis results indicate that the behavior of RC pile cap is not a rigid body and resulting forces or reactions are unequal. Also, it was observed that, support conditions, analysis type and thickness of RC pile cap had a significant effect on the global behavior of RC pile caps.
Abstract in English:Abstract The importance of knowing critical loads and post-critical behavior of thin-walled structures motivates the development of several scientific and practical studies. Most references are concerned with stability analysis for small displacements (first order approach), or with second order stability analyzes, a less precise geometric nonlinear strategy. However, very flexible structures or ones that present small loss of stiffness after the first critical load need more careful analysis. Here we present a shell numerical formulation capable of carrying out stability analysis of thin-walled structures developing large displacements. This formulation uses generalized unconstrained vectors as nodal parameters instead of rotations. To make possible a complete stability analysis using unconstrained vectors, we present an original strategy that imposes a Conjugate Modal Force at the vicinity of critical points, allowing an accurate choice of post-critical paths. Non-conservative forces are also considered and results are compared with literature benchmarks, demonstrating the accuracy and capacity of the proposed formulation.
Abstract in English:Abstract The strain energy function of hyperelastic material models must fulfill some mathematical conditions to satisfy requirements such as numerical stability and physically plausible mechanical behavior. In the framework of computer simulations with Newton-type methods, numerical stability is assured by the positive definiteness of the tangent operator. The Baker-Ericksen inequalities, on the other hand, are sufficient and necessary conditions in order to guarantee that the material behaves in a physically plausible way, although they are rarely taken into account during parameter identification. The present work proposes a modification in the strain energy function of a previously developed model for isotropic rubber-like materials. The new expression for W allows the satisfaction of both of the aforementioned requirements. The complete constitutive framework for its implementation in a Finite Element code is provided. Representative examples are analyzed and to show the superior performance when compared to well-known models found in the specialized literature both for homogeneous and non-homogeneous cases of deformation.
Abstract in English:Abstract Twenty-four rectangular RC column specimens, constructed at 1/3 scale, were tested under axial loading to investigate the use of ASWM (one, two, three, four, and five layers) exposed to elevated temperatures. Six of the column specimens were subjected to laboratory temperature (control or un-damaged specimens) of 23oC. Eighteen of the column specimens were subjected to elevated temperatures (six columns at 250oC, six columns at 500oC, and six columns at 750oC) for 2 hr. The obtained results clearly showed that each axial load resistance, toughness, and stiffness were all negatively affected by escalated temperature degrees. Also, the confinement effectiveness in terms of the ultimate load was decreased with the increase in concrete compressive strength (un-damaged). The number of ASWM layers significantly influenced the ductility, energy absorption, and ultimate load improvement percentage. The number of the ASWM layers considerably enhanced the ductility up to a certain number of erected layers; however, adding more layers had no apparent effect.
Abstract in English:Abstract The paper presents the theoretical background and four applications examples of the new method for the estimation of support stiffness coefficients of complex structures modelled discretely (e.g. with the use of the Finite Element Model (FEM) method based on the modified Particle Swarm Optimization (PSO) algorithm. In real-life cases, exact values of the supports’ stiffness coefficients may change for various reasons (e.g. order of fastening, state of the contact surfaces, environment changes, etc.). Because of the unknown coefficients, reliable simulations of fixed structure (i.e. mounted, assembled, not in a free-free state) are difficult to perform. The method serves as a tool to obtain good correlation between the FEM of a structure and the experimental data. Simple modal tests are required to estimate the first few modes of the fixed system. The FEM of the structure is considered in a free-free state and the support stiffness coefficients of the FEM are estimated by the proposed method. Further simulations with test-tuned and correlated complete FEM could be performed in time or frequency domain.
Abstract in English:Abstract This work is meant to investigate and control the nonlinear dynamical behaviours of a nonlinear asymmetric rotating shaft system. The studied system is modeled as a nonlinear dynamical system having both quadratic and cubic nonlinearities and excited at the primary resonance. A linear proportional-derivative controller is introduced to eliminate the nonlinear behaviours and to suppress the lateral vibrations of the rotating shaft. The influences of different control parameters on the oscillatory behaviours of the considered system are explored. The main obtained analytical results showed that the uncontrolled shaft may respond with forward and backward whirling motion. However, the proposed controller illustrated its feasibility in eliminating the nonlinear behaviours of the studied system and forcing it to behave like the linear systems. Moreover, the obtained results confirmed that the proportional control gain plays a dominant role in destabilizing the studied rotor system. Finally, the safe operation of the studied system with avoiding the rub-impact force occurrence is discussed.
Abstract in English:Abstract Damage detection in structures and systems is essential for monitoring parameters that can affect their integrity. This paper evaluates the efficiency of four damage indices (DIs) commonly used with temporal wave signals. The root mean square deviation, mean absolute percentage deviation, covariance, and correlation coefficient deviation DIs are presented, and a normalization is then proposed. An Euler-Bernoulli beam is used as a guided wave modelled with the spectral element method and excited by a toneburst signal. It includes the theoretical background of the throw-off beam, undamaged and cracked beam spectral elements. The DIs for a single crack position and a map varying crack depth and positions are calculated with deterministic and random temporal signal responses derived from noise addition. Results showed that DIs could identify and quantify the damage conditioned to the pulse location point and the influence of noise in the estimation, which leads in an analysis comparable to practical applications.
Abstract in English:Abstract The aim of this study is to compare the seismic performances of strengthening techniques applied to reinforced concrete (RC) non-ductile frames tested under the effects of seismic loads. In the experimental part of the study, one-third scale, one-bay, two-storey non-ductile hollow brick infilled RC frames were strengthened with five different techniques and tested under reversed cyclic lateral loads. One being reference, five of the frames were infilled with hollow bricks and plastered on both sides, one was strengthened with plain mortar, one was strengthened with steel fiber reinforced mortar, two were strengthened with precast RC plates. The last frame was strengthened with RC infill wall. Strengthening techniques increased strength and stiffnesses of the frames in the ranges from 57% to 189% and from 186% to 486% respectively. In the numerical analysis part, an existing RC non-ductile building located in Istanbul, strengthened with the techniques, were analyzed using a computer program. Numerical results were evaluated in terms of interstorey drift ratio together with the overall seismic performance of the building.
Abstract in English:Abstract Yielding shear plates have been widely used in metallic dampers as the primary source of energy dissipation. This study investigated the behavior of yielding shear plates with different aspect ratios when subjected to a gradually increasing lateral load. Five shear links with the thickness of 1mm and aspect ratios ranging from 1.0 to 1.5 were constructed and tested. The force-displacement relationships of the shear links were idealized and represented by bilinear curves. The obtained results indicated that the square shear links had a larger displacement ductility ratio, initial and effective stiffness when compared with rectangular shear links. It was also observed that the initial, effective, and post-yield stiffness of square shear links were increased as their size was increased. However, the displacement ductility ratio of shear links with square diaphragm was decreased as the size of the shear link was increased.
Abstract in English:Abstract Asymptotic Homogenization (AH) and the Extended Multiscale Finite Element Method (EMsFEM) are both procedures that allow working on a structural macroscale that incorporates the effect of averaged microscopic heterogeneities, thus resulting in computationally efficient strategies. EMsFEM works directly on coupled finite micro and macroscales using numerically built discrete interpolation functions. Periodic Truss Metamaterials (PTMMs) are cellular materials formed by the periodic repetition of a truss-like unit cell and engineeringly tailored to show a given macroscopic response. In this work we analyze the numerical behavior of selected PTMMs that were designed for extreme Poisson ratios using AH theory. As a first issue, we study macroscopic structures made of finite unit cells and verify how close their average behavior coincides with the material properties predicted by AH. For comparison, we solve the macroscopic plane stress associate problems that employ the elastic constitutive tensor obtained by AH. The second issue is concerned with the ability of EMsFEM to reproduce the structural behavior of the full macro-micro model. We employ two versions of the EMsFEM, adopting linear (LBC) and periodic (PBC) boundary conditions to build the numerical interpolation functions. The third and most important aspect discussed in this research concerns evaluation of the EMsFEM downscaled displacement fields. We observe that according to the layout of the AH designed unit cell, to the use of LBC or PBC and, depending on the boundary conditions present in the macroscopic problem, spurious downscaled displacements might occur. Such spurious displacements are due to excessive compliance of the corresponding unit cell and can be detected when building the numerical interpolation functions. We conclude that the layout optimization of PTMM using AH must be carefully interpreted and that EMsFEM is a good tool to detect a macroscopic excessively compliant response at an early design stage.