Department of Structural Engineering (2007 - Present)
Structural Engineering
Civil Engineering, Sharif University of Technology, Tehran, Iran
Structural Engineering
Civil Engieering, Sharif University of Technology, Tehran, Iran
Civil Engineering
Civil Engineering, Sharif University of Technology, Tehran, Iran
EPS concrete is a relatively new lightweight building material with a combination of favorable properties. Available experimental results have shown that, for low volume contents of EPS, the presence of lightweight beads reduces strength more than density. This fact has led a few researchers to try to reinforce EPS concrete with fibers to partially compensate the strength loss. In this regard, the current study examines the simultaneous effects of EPS beads and polypropylenee fibers on tensile splitting, uniaxial compressive, and confined behavior of concrete by running 160 tests on cylindrical specimens. The results of different tests consistently showed that the strengthening effect of fibers is noticeably improved at higher EPS volume co
The partial replacement of aggregates with expanded polystyrene (EPS) beads is a common technique to produce lightweight concretes. Much research have been already conducted on the compressive strength of EPS concretes containing high volumes of EPS beads. In the current study, however, specimens with low EPS volume contents are also examined. The results show that all the strength versus EPS volume content graphs initiate with a nearly plateau region. The length of this plateau region is found to be an increasing function of W/C ratio. It is also observed that the replacement of redundant water with the EPS beads initially improves strength but the trend is reversed beyond a critical volume content. Finally, the new empirical findings are
The micropillar compression test is a novel experiment to study the mechanical properties of materials at small length scales of micro and nano. The results of the micropillar compression experiments show that the strength of the material depends on the pillar diameter, which is commonly termed as size effects. In the current work, first, the experimental observations and theoretical models of size effects during micropillar compression tests are reviewed in the case of crystalline metals. In the next step, the recent computer simulations using molecular dynamics are reviewed as a powerful tool to investigate the micropillar compression experiment and its governing mechanisms of size effects. View Full-Text
Comb-teeth damper (CTD), is a new type of metallic yielding damper, which is made of steel plates and includes a number of teeth that dissipate energy through in-plane flexural yielding. The behavior of individual samples of CTD have been previously studied numerically and experimentally and it has been shown that this damper has excellent energy dissipating capacity and large ductility ratio. In this paper, application of this type of damper to steel frames is studied. Sample steel frames are constructed and equipped with CTDs and tested under cyclic loading. The results show that these dampers can serve their intended duties and dissipate considerable amount of energy. Numerical modelling of the frames confirms the experimental results an
With a special attention to the different stages of a typical loading path travelled in a fluid confined concrete test, this paper introduces a physically consistent model for the stress-strain curve of actively confined normal-strength concrete in the axial direction. The model comprises of the five elements of:(1) a criterion for the peak or failure strength,(2) an equation for the peak strain,(3) a backbone hydrostatic curve,(4) a transient hardening curve linking the point of departure from the hydrostatic curve to the failure point, and finally (5) a set of formulas for the post-peak region. Alongside, relevant details and shortcomings of existing models will be discussed in each part. Finally, the accuracy and efficiency of the propos
In this article, first some published evidence for yielding of aluminum and polymeric replicated foams and a numerical study on regular porous structures are revisited in which a kind of twist in the shape of yield surface along the hydrostatic axis was reported. Next, motivated by this observation, some existing appropriate yield functions and a new extended generalized failure criterion capable of capturing this effect are presented. Finally, using the experimentally measured yield clouds of aluminum replicated foams from the literature, the efficiency, inherent features, and limitations of these criteria are thoroughly assessed, compared, and discussed.
In this paper, a new type of metallic yielding damper called comb-teeth damper, CTD, is introduced. CTD is made of steel plates and includes a number of teeth that dissipate energy through in-plane flexural yielding. An optimum geometry of teeth is suggested, which assures uniform distribution of stress along them and prevents strain localization. Finite element modeling is used to verify the design of proposed damper and to study nonlinear behavior of the damper subjected to monotonic as well as cyclic loading. Three full scale specimens have also been made and tested under cyclic loading. In order to restrict out-ofplane buckling of damper teeth, a special clamp has been designed. A numerical study has elaborated the effect
Rigid Body-Spring Models (RBSMs) are a kind of discrete models which are developed mainly for the simulation of quasi-brittle materials ranging from ceramic, concrete, and masonry, to rock and soil. In this approach, material domain is discretized to a set of rigid cells interconnected through a set of translational and rotational springs located at cell interfaces. These cells are constructed over a set of points (seeds) distributed regularly or randomly over the domain. When it comes to heterogeneous materials, the seeds may be located in accord to the geometry and distribution of inclusions. For two-dimensional problems, each rigid cell has normally two translational and one rotational degrees of freedom (DOFs). The springs may be distri
In this paper, Lattice-Discrete Particle Model (LDPM) of concrete has been extended in 2-D to account for the effect of non-circular aggregates. To this end, the flexible equation of super-ellipse is employed for generating aggregates in order to add the simulation possibility of a greater spectrum of aggregate samples in 2-D to lattice-Discrete particle Model.
The special concentrically braced frame (SCBF) is a common type of steel structural system for resisting lateral loading. This type of system provides the required stiffness and strength at relatively low cost. Design codes, based on past research, have recommended some details for connections and braces, in order to ensure sufficient ductility and energy dissipation capacity in this system for their application in seismic design. In this paper, the behavior of four SCBF specimens with different details of connections is studied using experimental and numerical models. They are single story single bay sub-assemblages of frames with a single brace that are subjected to lateral cyclic loads. In the first specimen, a 2tp straight line clearanc
This paper proposes a numerical framework for realistic simulation of replicated foams. The method is inspired by the actual processes used in the manufacturing of this class of cellular materials. Accordingly, an assembly of space‐holders is generated first, then the physical process of cold isostatic pressing is simulated and, finally, the structure resulting from the leaching process is modeled using a voxel‐like finite element model and an efficient inverting algorithm. A simple yet effective solution for the simulation of the influence of infiltration pressure on the microstructure of foam is also suggested. Next, the numerical approach is verified using the experimental results of a set of replicated aluminum foams whose preforms
This study first re-examines key existing calibration procedures for the Chaboche decomposed nonlinear kinematic hardening rule. Special attention has been paid to the extent to which the predictions of calibrated models represent the physics of phenomena observed in strain- and stress-controlled cyclic tests. It has been shown and discussed that newer methods suffer from some weaknesses and may yield nonphysical results. Second, with the aim of improving the accuracy of its ratcheting strain predictions in unsymmetric stress-controlled tests, a modified calibration procedure and a new Chaboche hardening rule with an evolving material parameter is introduced. The former improves the accuracy of predicted ratcheting strains at the initial cy
Metal foams as a new class of materials with interesting properties such as high stiffness and strength to density ratios, capacity to absorb impact energy, and reproducibility, are rapidly growing their share in engineering applications such as aerospace, automotive industry, lightweight structures, and energy absorbers. Different numerical approaches have been already developed for the simulation of this class of materials from which the two-scale microplane model has been focused in this research. First a simple algorithm has been proposed for the numerical implementation of microplane model to simulate the mechanical behavior of closed-cell metal foams. The structure of foam is assumed to be an assembly of firmly bonded spherical shells
Experimental studies have confirmed very stable hysteretic behavior of CTDs under cyclic loading with large displacement amplitudes. For example, a specimen was loaded under twenty fully reversed cycles at amplitude of 40 mm and even after these cycles, hysteretic curves were quite stable and thus displacement amplitude was increased to 60 mm. It should be noted that 40 and 60 mm in amplitude respectively correspond to more than 20 and 30 times the yield displacement of outer fiber of links.After numerical and experimental investigation of the behavior of CTD specimens, the behavior of three simple steel frames equipped with such dampers is studied under cyclic loading. In this regard, CTDs are installed between beams and Chevron bracing. T
This work incorporates newly introduced Lattice Discrete Particle Model (LDPM) to assess the failure mechanism and strength of hollow concrete blocks. Alongside, a method for the graphical representation of cracked surfaces in the LDPM is outlined. A slightly modified calibration procedure is also suggested and used to estimate required model parameters for a tested concrete sample. Next, the model is verified for a compressively loaded hollow block made of the very same concrete. Finally, four geometries commonly used in the production of hollow concrete blocks are selected, numerically simulated, and their failure properties are explored under concentric and eccentric compressions.
Metal foams as a new class of materials with interesting properties such as high stiffness and strength to density ratios, capacity to absorb impact energy, and reproducibility, are rapidly growing their share in advanced materials market. However, due to their porous microstructure, experimental investigations of their properties are not trivial and normally need rigorous procedures and high end equipments. Accordingly, there is a growing research interest towards the numerical modeling of their cellular structure in which the following three kinds of models have been commonly employed:(1) structures based on a unit cell or a building block,(2) random Voronoi diagrams, and (3) CAD data provided by X-ray micro-computed tomography. In the cu
Experimental studies have confirmed very stable hysteretic behavior of CTDs under cyclic loading with large displacement amplitudes. For example, a specimen was loaded under twenty fully reversed cycles at amplitude of 40 mm and even after these cycles, hysteretic curves were quite stable and thus displacement amplitude was increased to 60 mm. It should be noted that 40 and 60 mm in amplitude respectively correspond to more than 20 and 30 times the yield displacement of outer fiber of links.After numerical and experimental investigation of the behavior of CTD specimens, the behavior of three simple steel frames equipped with such dampers is studied under cyclic loading. In this regard, CTDs are installed between beams and Chevron bracing. T
This paper proposes a new method of simulating ductile fracture in steel structures under large amplitude cyclic straining experienced in earthquakes. The method is developed based on an existing micromechanical model originally proposed for predicting crack initiation in ultra-low cycle fatigue, ULCF. It involves a step-by-step simulation of material degradation within the framework of conventional nonlinear FEM. The method is validated through simulating fracture in a structural detail (column-to-base plate connection) for which several cyclic tests has been previously conducted. It is found that the method can successfully predict the cracking site, its propagation path, the number of cycles corresponding to crack initiation, and also fi
HIS chapter is dedicated in its first section to the conventional plas-ticity modeling of metal foams. It begins with an introductory part discussing briefly the structure of the models and the definitions of their basic constituents; ie, the yield/failure criterion, hardening rule, hardening variables, flow rule, and consistency condition. It continues with a part fully devoted to the issue of failure in this class of materials. Two distinct physical and mathematical strategies toward the identification of yield/failure criteria are explored and examples are given. The success of both approaches relies on the accuracy of the data provided by experiments, or nowadays by analytical or numerical mesostructural analyses. The related important
Accepted in Revised Form: April 30, 2013) Abstract: Hollow concrete blocks are one of the widely used building elements of masonry structures in which they are normally loaded under combined action of shear and compression. Accordingly and due to their structural importance, the present study intends to numerically search for an optimum shape of such blocks. The optimality index is selected to be the ratio of block’s failure strength to its weight, a non-dimensional parameter, which needs to be maximized. The nonlinear analysis has been done using a homemade code written based on the recently developed Lattice Discrete Particle Model (LDPM) for the meso-scale simulation of concrete. This numerical approach accounts for the different aspec
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