Scientia Iranica
Volume:19 Issue: 5, 2012

Based on the concept of independent stress state variables to consider the impact of unsaturated conditions, an elasto-plastic critical state constitutive model for saturated and unsaturated interfaces is introduced in this paper. The proposed model is capable of predicting many characteristics of unsaturated interface behavior, such as the dependence of initial tangent modulus, peak shear stress, dilatancy, and ultimate strength on matric suction, net normal stress, and the interface state measured with respect to the critical state line. To this aim, two distinct yield mechanisms are employed in the model. While change in stress ratio generates plastic deformation in the first mechanism, plastic deformations are due to an increase in net normal stress, decrease in matric suction, or both, in the second mechanism. The presence of appropriate state dependent ingredients enables the model to provide realistic predictions over a wide range of variations of density, net normal stress, and matric suction. By direct comparison of the model predictions with experimental data, the predictive capacity of the proposed model is evaluated.

This paper proposes an effective algorithm based on the Level Set Method (LSM) to solve the problem of topology optimization. The Hamilton–Jacobi Partial Differential Equation (H-J PDE), level set equation, is modified to increase the performance. We combine the topological derivative with nonlinear LSM to create a remedy against premature convergence and strong dependency of the optimal topology on the initial design. The magnitude of the gradient in the LS equation was replaced by several Delta functions and the results were explored. Instead of the explicit scheme, which is commonly used in conventional LSM, a semi-implicit additive operator splitting scheme was carried out in our study to solve the LS equation. A truncation strategy was implemented to limit maximum and minimum values in the design domain. Finally, several numerical examples were provided to confirm the validity of the method and show its accuracy, as well as convergence speed.

In this work a double step algorithm is presented for the integration of equations governing the behavior of von Mises material in plastic limit. The isotropic and kinematic hardenings employed are of general type and the evolution of back stress follows the Armstrong–Frederick rule. Theoretical and numerical aspects are discussed in detail and a comparison is made with the classical backward Euler method.

In the Endurance Time (ET) method, structures are subjected to a calibrated intensifying accelerogram and their performance is assessed based on their response at various equivalent intensity levels. Application of the ET method in performance-based design of structures has been studied by introducing a continuous performance target curve, which expresses the limit of the proper seismic performance of a structure along various times of the ET accelerogram. The correlation between time in the ET method and the return period for different structural periods is investigated. The procedure is based on the coincidence of response spectra obtained from the ET accelerogram at different times and response spectra defined by ASCE41 at different hazard levels. The results show that substitution of the return period for time in ET analysis and performance curves increases the usefulness of these curves and can simplify application of the ET method in performance-based design.

Concrete made using geopolymer technology is environmental friendly and could be considered as part of the sustainable development. Even though aggregate constitutes major volume in geopolymer concrete, only limited study related to this parameter has been reported. This paper presents the summary of study carried out to understand the influence of aggregate content on the engineering properties of geopolymer concrete. Influence of other parameters on engineering properties of geopolymer concrete such as curing temperature, period of curing, ratio of sodium silicate to sodium hydroxide, ratio of alkali to fly ash and molarity of sodium hydroxide are also discussed in this paper. Based on the study carried out, it could be concluded that a geopolymer concrete with proper proportioning of total aggregate content and ratio of fine aggregate to total aggregate, along with the optimum values of other parameters, can have better engineering properties than the corresponding properties of ordinary cement concrete.

This paper presents a combined application of the Ritz method, the Differential Quadrature (DQ) method, and the Integral Quadrature (IQ) method to vibration problem of rectangular plates subjected to accelerated traveling masses. In this study, the Ritz method with beam eigenfunctions is first used to discretize the spatial partial derivatives with respect to a co-ordinate direction of the plate. The DQ and IQ methods are then employed to analogize the resultant system of partial differential equations. The resulting system of ordinary differential equations is then solved by using the Newmark time integration scheme. The mixed scheme combines the simplicity of the Ritz method and high accuracy and efficiency of the DQ method. The accuracy of the proposed method is demonstrated by comparing the calculated results with those of the existing literature. It is shown that highly accurate results can be obtained using a small number of Ritz terms and DQM sampling points. Finally, the effects of following parameters having something to do with the title problem are investigated: moving load speed and acceleration, and transverse inertia of the moving load. Numerical results show that all the above-mentioned parameters have significant effects on the transient response of such structures under traveling dynamic loads.

Significant quantities of steel slag are produced as by-product every year from steel industries in Iran. Although it can be used as an artificial source of aggregates, it is sent to landfills for disposal. The disposal of steel slag occupies a significant portion of landfills and causes many serious environmental problems. This study aims to investigate the feasibility of utilizing steel slag aggregates in Stone Matrix Asphalt (SMA) mixtures. The results show that the use of steel slag as the coarse portion of aggregates can enhance Marshall stability, resilient modulus, tensile strength, resistance to moisture damage and resistance to the permanent deformation of SMA mixtures.

This paper studies loading rate effect on stress–strain behaviors of high compacted rockfill materials. The large scale triaxial-dynamic equipment was used for this propose. To check the equipment capabilities, tests have been carried out on elastic spring and damping rubber specimens. The laboratory tests results indicate that the loading rate effect in elastic metal spring is negligible, while it has considerable effects on stress–strain behaviors in damping rubber, and especially in rockfill materials. The laboratory test results on rockfill materials have shown that dry specimens have higher E and D values compared to the saturated samples. Anisotropic specimens have higher Young’s modulus and less damping ratio in comparison to isotropic samples. Generally, the influence of the main parameters on E and D in rockfill specimens is initial vertical effective stress and loading frequency, respectively. The high compacted rockfill specimen exhibits a non-destructive response to the application of cyclic loading below a threshold axial strain level of <0.005%. Since the frequency effect was observed in the strain levels lower than 0.005%, and there is not meaningful build up of pore pressure in this strain level, therefore any effect of excess pore water pressure on the frequency effect is negligible.

Inadequate separation distance between neighboring buildings with out-of-phase response would increase the probability of pounding during an earthquake and may cause severe damages to the structures. A rational estimation of the maximum impact force would help us to control the extent of damages in different structures. As a practical tool, the pounding force response spectrum, which shows the value of maximum impact force as a function of the structural vibration periods, is considered in this paper. It is well-known that strong ground motion in the near-field area has different characteristics from far-field ones. In this paper, pounding force response spectra for elastic structures subjected to near-field and far-field ground motions are presented. Both of the adjacent buildings were modeled simply as Single-Degree-Of-Freedom (SDOF) systems and pounding effect has been simulated by applying the nonlinear viscoelastic model. In the analysis, the effect of different parameters, such as mass, damping ratio has been studied. The effects of gap distance on maximum pounding force due to near- and far-field earthquake ground motions were investigated comprehensively. As a result, the characteristics of earthquake ground motions along with the properties of structures should be considered in gap distance controlling between adjacent buildings.