مجله ایرانی علوم و تکنولوژی-ب
سال:1382 | دوره: | شماره: |تعداد مقالات:13

Interest in the unsaturated soils of semi-arid to arid climates has increased, especially in recent years, due to the adverse effect of industrial and municipal activities on the subsurface environment. The most important soil parameter affecting the rate at which water and dissolved chemicals move through the vadose zone is the soil water characteristic curve (SWCC), which describes the relationship between the suction, ّ, and the volumetric water content, è. In this study, laboratory soil water characteristic curves were obtained for compacted marine soils of different lithologies and genesis taken from three different locations in North Cyprus and the influence of varying physical and chemical soil properties on the SWCC were studied.

One of the most important problems in foundation engineering is determination of the ultimate bearing capacity of underlying soil. Exact solution will be obtained based on limit state theorems when upper bound and lower bound solutions are the same. A correct stress field, which does not violate yield criteria, is required to find a lower bound solution. An acceptable failure mechanism is required to find an upper bound solution. Ordinary finite element programs can obtain stress distribution in soil mass but cannot obtain the related failure mechanisms. Different types of interface elements may be used for modeling relative displacements between two surfaces, but they need a prescribed location in the region of interest. This limitation enforces the use of a time consuming adaptive mesh algorithm for finding the failure mechanism. For accurate and efficient determination of ultimate bearing capacity and the corresponding failure surface a new type of finite element has developed. Using this element, the failure surface is automatically simulated and utilized to accurately determine the ultimate bearing capacity.

In this paper, the influence of confinement on tensile forces in concrete, using fibers in the form of closed fibers (hoops), is theoretically investigated. In addition to the resistance of tension, the hoops confine concrete. Physical analysis is based on the elongation of concrete under tension. However, hoops prevent this elongation and consequently compress the concrete. Assumptions are made to simplify the complex analysis of the hoop within the concrete structure. The theoretical results indicate that tensile force and stress within the hoop are not uniform and shear force direction is not constant. Final formulations depict distribution of tensile force, normal stress and shear stress in circumference of hoop with respect to the axis of tensile force imposed on the concrete. These formulations are capable of including strength and ductility of concrete and hoop materials. Theoretical behavior of hoop and fiber within a continuum reveals that fiber is under shear and bearing stresses while a hoop is under normal tensile stress. However, due to confinement, the concrete structure reinforced with hoop possess higher strength and ductility than the concrete structure reinforced with fiber.

In this paper, we report a new mathematical programming model for the tactical planning of freight railways, which addresses the train formation problem. The problem results in a multi-commodity network flow formulation with both integer and continuous variables. The modular structure of the model allows considering special conditions of the network under study and testing various operating policies.

The behavior of compression members within a double-layer space truss has a commanding influence on the overall behavior of the structure. Using finite element method, a bar element with a piecewise linearized axial stress-strain relationship has been used to represent the ideal behavior of each member in collapse analysis of the double layer space trusses. According to static collapse analysis of double-layer space trusses, three different collapse mechanisms will be possible, namely: overall collapse of the structure, local collapse of the structure with dynamic snap-through, and local collapse of the structure without snap-through. In the case of local collapse with a snapthrough, it has been indicated that to evaluate realistic response of the structure, the static collapse analysis is not adequate by itself, and a dynamic snap-through analysis must be performed, as well. In this paper, a methodology based on energy methods is presented to determine dynamic snap-through response of truss type structures. Finally a typical double-layer grid structure has been used to describe the approach and show three different possible collapse mechanisms.

Selection of the spillway capacity is subjected to technical as well as socioeconomical considerations. Statistics of dam failure due to different causes show that overtopping and seepage (or piping) are the major causes of dam failure. The high costs of spillway construction and the enourmous loss and damage caused by dam failure make the economic sizing of the spillway a very important task in dam design. Spillway design, like many other hydraulic structures, is subjected to several types of uncertainties. This paper attempts to integrate in an optimization model for spillway design of the hydrologic uncertainties involved in estimating flood magnitude and flood routing process. As imprecise information often exists about overtopping damages, a method is presented for loss function estimation using fuzzy reasoning. Because of the non-analytical objective function, a direct search method has been used to find the optimal spillway capacity and dimensions. To study its usefulness, the model is applied to the Makou dam in Iran. Results show that the model is a promising tool in the cost-effective design of spillway.

The dynamic localization of saturated soil is investigated by considering the influence of higher strain gradient. It is shown that the strain gradient has a significant influence on the evolution of shear band in saturated soil and that the width of shear band is proportional to the square root of the strain gradient softening coefficient. The numerical simulation is processed to investigate the influences of shear strain gradient and other factors on the evolution of shear band.

In this article, the formulation of the curved beam element with six nodes for curvature is presented, where the nodal parameters are chosen to be based on curvatures. The shear and tangential strains are incorporated into the total potential energy by the force equilibrium equation in order to avoid membrane and shear locking phenomena. To ease the treatment of the general boundary conditions, a transformation matrix between nodal curvature and nodal displacement vectors is introduced. When this matrix is used to represent all the kinematical variables, the final finite element equilibrium equation is written in terms of the displacement components of only three nodes. The interesting point in this formulation is that for most problems, choosing just one element might be adequate to obtain the solution. The formulation is applied to five sample problems to verify its capabilities. The results demonstrate that the six-nodded element is capable of representing the behavior of the curved beam, with adequate accuracy and efficiency as compared to a previously developed three-nodded formulation. To avoid mathematical and manipulation errors, the Mathematica software package was employed throughout the whole process.

The ultimate bearing capacity of ring footings for different conditions such as internal to external diameter ratio, base roughness and thickness of sand layer were determined in the laboratory. The deformed shape of the soil under footing at failure was investigated, and accordingly a semi-empirical relation for determining the bearing capacity of ring footings was developed. The bearing capacity of ring footings was also determined using finite element method. The results of experiments, a semi-empirical relation and numerical methods were compared, and reasonable agreements were observed.

The exceedingly large top drift and bottom bending moment due to the weakening of shear walls in tall buildings by door, window and corridor openings is one of the most frequently encountered problems of structural engineering. An elegant way of tackling the foregoing problem is to employ stiffening beams. Authors who tried to find closed form solutions could handle only cases with up to two stiffening beams. With the approach used in the present study, a computer program which can find analytical expressions for the design of coupled shear walls with many stiffening beams has been prepared making use of the MATHEMATICA computer algebra system, rendering possible an optimization with respect to different parameters. In all of the previous works, all properties of the shear walls and the connecting beams have to be kept constant along the height of the building. The present work can treat a coupled shear wall with any number of stiffening beams and flexible connections, having any number of occasional changes in the properties of the shear walls and the connecting beams. A perfect agreement has been observed between the results of the present study and those of (SAP90) Structural Analysis Program in the examples used for the verification of the present method. Thus, the present method proves to be a perfect mean of predesign, since it gives results in an extremely short time.

A new simple shear apparatus which is capable of applying monotonic and cyclic load on soil specimens is fabricated. Cylindrical specimens are placed in a special triaxial type cell where confining pressure, pore air and pore water pressures can be controlled. The apparatus consists of an inner part and an outer portion. The inner portion that is inside the triaxial cell consists of a pedestal attached to a movable base. The movable base is mounted on a pair of horizontal rolling tables, which makes the inner portion capable of moving back and forth, horizontally. The outer portion consists of a cylindrical frame that holds the top cap and prevents its lateral movement. A high air entry ceramic disk is sealed into the pedestal using an epoxy resin. The specimen is sheared in a simple shear mode. The horizontal load required to shear the specimen, divided by the nominal area of the specimen gives the shear stress. Soil suction can be measured using the so-called axis-translation technique [1] by increasing the pore air pressure through the top low air entry disk and recording the pore water pressure through the high air entry ceramic disk. Using the fabricated apparatus it is possible to measure and control the vertical normal stress as well as the all-around confining pressure applied to the soil specimen. Hence, testing the soil under initial isotropic or anisotropic confining pressure could be easily achieved. In this paper, the main features of the apparatus are discussed via some test results carried out on unsaturated soil specimens.

Interannual hydroclimatic variability in the Middle East is explored. A review is done on studies linking local climate with large-scale teleconnections. These studies suggest that El Niٌo has a weak tendency of bringing wetter than normal conditions to the region during fall/winter. This signal is unstable in time and has considerable decadal variability. No consensus exists on the general start and end dates of various climate epochs, nor is there an explanation of the physical causes of the decadal variability, suggesting that the interannual signal of El Niٌo in the Middle East may be transient and difficult to predict. In contrast, the influence of the North Atlantic Oscillation (NAO) on temperature, precipitation and streamflow is strong, with high NAO winters favoring dry and cool conditions in the region. Recommendations are made on how to improve the ability to understand and forecast Middle Eastern interannual variability, namely, 1) improve access to instrumental data, 2) coordinate research, forecasts and user involvement through a regional forum and 3) further explore the impacts of NAO in the Middle East.