Scientia Iranica
Volume:20 Issue: 6, 2013

The aim of this study is to investigate the mechanical properties of clayey soil with dierent moisture content. Two groups of clayey soil with dierent moisture content were prepared as samples. One group of samples was subjected to eleven freeze-thaw cyclesunder a constant freezing apparatus temperature in an enclosed apparatus. The othergroup was subjected to one freeze-thaw cycle exposed to a decreasing freezing pparatustemperature in an enclosed apparatus. A series of triaxial compression experiments wereconducted on both groups of samples. The rst experimental results indicated that themechanical properties of the clayey soil changed after the freeze-thaw cycles under constant freezing apparatus temperature, and cohesion decreased with an increased number of freeze-thaw cycles and increased moisture content. In addition, the internal friction angle increased with the number of freeze-thaw cycles and decreased with increased moisture content. However, the moisture content of the clayey soil had a greater in uence on cohesion than the internal friction angle. Additional experimental group results indicated that the internal friction angle of the clayey soil diminished with a decrease in the freezing apparatus temperature and increased moisture content. Finally, cohesion increased with a decrease in either the freezing apparatus temperature or moisture content.

One of the methods in limiting tensile stresses in arch dams and removal stress concentrations at dam-foundation interface is setting the dam body on a concrete saddle called PULVINO. In the present study, effect of PULVINO and peripheral joint on the static behavior and seismic performance of arch dams are investigated. DEZ dam with 203m height was selected as case study and all contraction joints of the dam body were modeled using discrete crack approach based on as-built drawings. Also, surrounding rock was modeled as a mass-less medium tied to PULVINO. The dam-reservoir-foundation system was analyzed under static loads accounting for stage construction effects, hydrostatic and thermal loads and then the provided numerical model was excited using near- and far-field earthquake ground motions. It was found that modeling peripheral joint between saddle and the main dam body changes the direction of principal stresses and their distribution patterns and safety of the system is improved. In addition, over stressed surfaces on faces of the dam body decreased in comparison with the model without peripheral joint.

The Newton–Raphson method, which is based on the Taylor series expansion, and uses the tangent stiffness matrix, has been extensively used to solve nonlinear problems. This traditional method, especially for the large-scale, is time consuming. Consequently iterative algorithms cannot be effective for analyzing of the process. In the incremental-iterative analysis of elastic nonlinear structures, great saving in computation can be achieved if distinction is made between the predictor and corrector phases. This paper shows how a simple assumption can improve the computational efficiency of the nonlinear analysis of structures. It is shown that very high computational efficiency may be obtained by assuming the pursuit of each degree of freedom (DOF) by a quadratic curve. Through examples it is demonstrated how this efficiency significantly, decreases the computing time of analysis compared with time taken to deploy the Newton-Raphson, modified Newton-Raphson and conjugate gradient (CG) methods.

This paper presents an effective theory for safe controlof air cushion surge chambers (ACSCs) in hydropower systems. On the basis of the emergency pressures and heights of air in chambers, the acceptable limits of the monitoring parameters for three kinds of control modes are derived theoretically to control the air compressor. The prerequisites for each control mode are determined with considering the air leakage and solution. By analyzing these prerequisites, the selection criteria are established to determine the appropriate control mode for the practical hydropower system with the ACSC. According to the presented control scheme, the air compressor need not be actuated for any variation of the air temperature, and the operating conditions of the hydropower system need not be considered for safe control of ACSCs. Those are favorable for safety and economy of the hydropower plant. To detail and validate the derived limits and selection criteria for different control modes, the safe control theory is applied to the practical hydropower plant, and the graphic analysis is conducted on the basis of the perfect gas law and the relationship of the initial steady-state pressure and height of the enclosed air in chambers.

An accelerated firefly algorithm (AFA) for fast size optimization of truss structures is proposed in this paper. Metaheuristic firefly algorithm has been recently developed and its effectiveness in solving practical problems such as sizing optimization of truss structures has not been thoroughly explored. The numerical experiments show that although the standard firefly algorithm (FA) is a powerful approach for truss optimization, it suffers from slow rate of convergence, and hence it should be modified to solve real-life problems. The proposed AFA imposes some improvements on the searching procedure by both reduction of randomness and scaling the random term in fireflies'' motion. The effectiveness and robustness of the algorithm are investigated by solving some benchmark problems. The results revealed that the proposed AFA remarkably enhances the rate of convergence and stability of standard firefly algorithm.

Mechanical properties of slab tracks on a foundation with nonlinear stiffness are accounted for. At first, the cracking stages were inspected in FEM models, and it was gathered that slab tracks have one-way flexural behavior. Secondly, experimental full-scale models were made, and the accuracy of analyses was verified by comparing theFEM load-deflection curves with those of previous studies and validating the cracking and ultimate loads with those obtained from experiments. Finally, the effects of several parameters on the cracking and ultimate loads and the energy absorption of steel fiber-reinforced slab tracks were investigated by examining the real behavior of slab tracks on elastic foundations before and after cracking. Steel fibers increased the compressive and flexural strengths as well as ductility and energy absorption. The 2.5 m width was the optimal width and the fracture pattern changed at this width. Finally, based on the obtained fracture loads, design curves were plotted for AASHTO’s factored loads.

The dependence of undrained behavior of silty sand on initial state of stress and direction of principal stresses with respect to vertical (α) is assessed under generalized loading paths using hollow cylinder apparatus. During applying shear load, value of intermediate principal stress parameter (b) is held constant and α value is increased from zero to the aimed value and held constant. Specimens are consolidated both isotropically and anisotropically to evaluate the effect anisotropic consolidation on the behavior of these soils. Wet tamping method was selected to prepare specimen. Shear loading was carried out under strain-controlled condition to capture post-peak strain-softening response. The results of this study reveal that the principal stress direction and initial anisotropy in stress condition have considerable effects on the behavior of silty sands. By increasing α, susceptibility of silty sand to instability increases. In addition, increase in silt content, silty sand mixtures present higher tendency to flow compared to pure sand. For specimens with high silt content, it is observed that stress-strain response has a sudden reduction in shear strength after a peak. This study reveals that shear strength, steady state friction angle of the silty sands are affected by α and the amount of fines present in the sand.

Modified bitumen enhances the performance properties of asphalt mixtures. A number of studies in the last decade have evaluated the performance properties of modified binders at different service temperatures. It is however possible to evaluate some modified binder time-temperature characteristics and performance properties through their rheological master curves. In this research, styrene butadiene styrene, ethylene vinyl acetate, polyphosphoric acid and crumb rubber were used to modify a PG58-22 base binder. Rheological master curves for |G*| and phase angles of these binders were generated using data from the dynamic shear rheometerand bending beam rheometerat temperatures ranging from -30℃to about 88℃. The rheological interpretation of the results indicated improved high and low temperature properties and reduced temperature susceptibility for most of the modified binders. A comparison of the modified binders was done based on the visual inspection of master curves for different temperature ranges.

In this paper, a novel procedure is introduced and employed to transform the partial differential steady saturated flow equation in porous media into a system of linear equations using Pore Network Models (PNMs). At first, a simple Square Pore Network Model (SPNM) is introduced and then this model is improved by increasing node connectivity (Square-Diagonal PNM; S-DPNM) and modification in handling impermeable boundaries by introducing imaginary nodes and pipes (S-DPNMi). Finally, a generalized formulation for unstructured discretization (Unstructured PNM; UPNM) of the domain is given and the effect of handling impermeable boundaries (UPNMi) on model accuracy is investigated. To explore the capabilities of these models as numerical tools, three examples are solved. Application of these models without modifications for impermeable boundaries (SPNM, S-DPNM, and UPNM) yields comparable results to those of traditional Finite Difference (FD) and Finite Element (FE) methods. Modification in handling impermeable boundary nodes not only yields results that are everywhere more accurate than FD and FE, but also benefits the simplicity of formulations.

Dam failure can result in a catastrophic break followed by a flood wave often with considerable loss of life or property. One of the main causes of dam failure is loss of shear strength and existence of discontinuity within foundation. Dynamic analysis of concrete dams usually considered concrete behavior to be nonlinear and foundation rock is assumed to be linear. In this study, seismic analysis of concrete gravity dam was conducted to investigate effect of foundation on the nonlinear response. A finite element model of dam-reservoir-foundation was considered to properly model the foundation as well as dam body nonlinear behaviors. An elasto-plastic formulation was used to model the foundation. Mohr-Coulomb model was utilized for the yield and potential functions of the foundation. The dam body modeled by using smeared crack model. After modeling the dam-reservoir foundation, horizontal recorded ground acceleration of Kobe 1995 earthquake was applied to the model and results were studied. It was found that cracks form at the crest and hill of the dam.Using elastoplastic model for the foundation is more realistic and under different boundary conditions a significant amount of energy will be dissipated in the foundation.

During earthquake seismic waves propagate vibrations that carry energy from the source of the shaking outwards. Seismic waves can be distinguished by the velocity and shape of propagation. The velocity of waves depends on the elastic properties and density of the soil layers through which the waves pass. Probabilistic analysis of earthquake waves can be used as an effective tool to evaluate inherent uncertainty in the soil properties and the resulting uncertainty in site classification. In this research the jointly distributed random variables method is used for probabilistic analysis and reliability assessment of shear wave velocityrelationship. The selected stochastic parameters are density, elastic modules and Poisson''s ratio which are modeled using truncated normal probability distribution functions. The results are compared with the Monte Carlo simulation, point estimated method and first order second moment method. Comparison of the results indicates very good performance of the proposed approach for assessment of reliability. It is shown that this method can correctly predict the influence of stochastic input parameters and capture the expected probability distribution of shear wave velocity correctly. It is also shown that the modulus of elasticity is the most effective parameter in shear wave velocity.

Double-I built-up sections were extensively used in Iran according to the previous versions of Iranian building codes. Despite the recent developments in steel moment connections, based on studies for I-beam to H-shaped column connections, there are not enough documents available for I-beam to double-I built-up column connections. Recent studies have revealed that connections with double-I built-up columns exhibit different behaviors comparing to the connections with H-shaped columns. Due to excessive out-of-plane deformations of the cover plate in the non-retrofitted moment connections, they have semi-rigid behavior. The objective of this study is to build on previous studies and to investigate strain distribution pattern and load transfer mechanism in the retrofitted I-beam to double-I built-up column connections using through plates and T-stiffeners. In this paper, both horizontal and vertical elements of the T-stiffeners were modified and studied. Analytical models were designed and analyzed under cyclic and monotonic loadings. Three different indices were computed in the connection region in order to assess the local failure potential; and the moment-rotation hysteretic curves were employed to evaluate the seismic performance of the connections. With the proposed configurations, strain is uniformly distributed within the critical elements. Moreover, excessive deformations of the column cover plate are eliminated.