Ferdowsi Civil Engineering
Year:2009 | Volume:21 | Issue:1|Papers Count:10

Cracking increases the flexibility and displacement of structures. This paper introduces a triangular element with internal crack considering the increase in flexibility of the element. First, Relationship between strain energy release rate and stress intensity factor by fracture mechanic approach is used to calculated energy released by the crack. Then, additional flexibility matrix is obtained in the independent system. This matrix is added to the compliance matrix of a non-cracked element. Finally, Stiffness matrix is calculated by compliance matrix and transformation matrix for the plane problem.  Stiffness matrix obtained with this approach includes the decreasing effect of stiffness due to cracking.

The main purpose of this research is to identify air transit potential in the over-flight sector and to estimate the demand diversion from Iran's air corridors to other substitute paths. After determining the study area and its zoning, three special information zones were formed for statistical studies. The collected statistical samples in the databases mainly relate to the time interval of 20th to 27th, of May 2002. The main reason to select the 1-week time interval is the period of repetition of regular flights throughout the world. Eleven different zones have been identified as air transit potential of the country. The results indicate that, despite the shortness of air paths of the country among the identified effective air transit zones, less than 40% of the transit flights have entered these corridors and the remainders are absorbed in parallel paths. Different approaches have been adopted in the analysis of Iran's air transit potentials. In this study, after forming classified crossing tables, the roles of various airlines have been evaluated on the basis of the portion of the evident potentials in the total potential flights. The categorical data according to logarithmic-linear models have also been statistically analyzed in the research and the zero hypotheses of the combined mutual effects have been studied by using these models. The results of fitting these models into classified crossing tables clearly show the effective role of the classified variable of different airlines in the air transit market. In other hand, with applying the one way t-test, different airlines have been categorized, by rate of loosing revenue. The international corridors have also been prioritized in this approach in terms of volume of transit and time distribution of the present demand.  In addition, considering the present evaluated difference between the visible potentials in the country's sky and the present potential, including evident and hidden potentials, some strategies have been presented for converting the latent demand to the evident level.

Estimating the sediment being transported by river flow is one of the important aspects in water resources engineering. Erosion and sediment transport phenomena in watersheds and rivers are complex hydrodynamic problems. Due to large number of obscure parameters involved in these phenomena, the theoretical governing equations may not be of much advantage in gaining knowledge of the overall process. Researchers have developed practical techniques that do not require much theory, algorithm, or rule development, and thus, reduce the complexities of the problem. One such technique is known as Artificial Neural Networks (ANN). In this paper, Auto-Regressive ANN was utilized to estimate suspended sediment lood in rivers. Various network topology, data partitioning and parameters were examined to find the best network with the best results. For increasing the efficiency of the models, Early Stopping technique has been used. Results of these networks were compared to the conventional sediment rating curves method and it was shown that ANN presented better results especially in peak flow discharges. Trained networks were able to model the sediment transport phenomena in rivers successfully, presumably because of the superior capability of ANN in nonlinear mapping, without any extra information from governing equation.

Winkler hypothesis is a well-known model in foundation engineering and pavement design. Today, despite development of advanced soil models, it is still the most common method in foundation engineering practice. However, the main problem with this method is evaluation of the coefficient of subgrade reaction, ks, and since it depends on numerous parameters, its estimation is one of the most sophisticated and misunderstood concepts in geotechnical engineering. Various relations have been proposed by researchers since generation of this model. In this paper different methods proposed for determination of ks are compared and evaluated for their suitability and accuracy. The geotechnical parameters of a site on Tabriz Marl were selected as the base and settlement analysis results with these methods are compared with that of obtained from analysis with advanced soil models using Safe and Plaxis software. It was disclosed that of Tabriz Marl, soft soil model is the best governing model and Vesic relation among the methods for determining of ks leads to negligible errors in comparison to soft soil model. Also, to get more accurate results from these methods, it is proposed to use mean elasticity modulus which takes into consideration the effect of geometric and mechanical properties of sub-layers.

Concrete with high strength and quality as an economical and effective material has been developed considerably in construction industry. Components of concrete and interaction between them is significant in mechanical characteristics of it, so by a good understanding of this relationship, it is possible to select the type and amount of materials in order to have a concrete with an efficient behavior against external loads. One of the effective methods for determining the precise behavior of reinforced concrete beams starting from first step of loading and ending at failure stage and also calculating the cracking, yielding and ultimate loads is nonlinear finite elements. The advantage of this method is to determined, the behavior of beam in all loading stages and also in all points in a beam. In this method, the geometry of a beam is introduced by 4-node elements and strain, stress and deflection in various points of beam can be determined using nonlinear analysis. In this paper the behavior of reinforced concrete beams with high strength concrete which had been tested, is investigated applying finite elements and the effective parameters are determined. Considering the parametric studies, the nonlinear results show a good agreement with available experimental data.

Localized edge loading or patch loading of bridge girders, is frequently encountered in practice. The behavior of girders under patch loading represents complex stability and elastoplastic problems. In situation where the location of the patch load is fixed, transverse web stiffeners can be used to provide increased resistance, but for economic reasons should be avoided wherever possible. For moving load in some structures such as bridges, it is not possible to provide transverse stiffeners at all critical locations. One option is to use longitudinal stiffeners in these situations. In this article, ANSYS finite element software was used for structural analysis to investigate the ultimate resistance of plate girders subjected to patch loading. The finite element model was validated with experimental results taken from literature and found to be more accurate. Extensive parametric study was also performed to investigate the ultimate resistance of longitudinally stiffened plate girders. Furthermore, the ultimate resistance and the optimum position of the longitudinal stiffeners were presented in closed-form which showed satisfactory correlation with the theoretical results.

The most common method for determining soil shear strength parameters is using laboratory tests and Mohr-Coulomb criteria. These methods have some limitations such as preparation of undisturbed samples, samples size, modeling of real in-situ stresses in laboratory condition, discontinuity of data. According to the repeatability and continuous measurements in CPT and CPTu, these tests results used for determination of soil strength parameters in geotechnical engineering. Most studies in fine soils focused on determination of undrained shear strength while in geotechnical design long-term bearing capacity is considered. Thus, in this paper a new method has been suggested for determination of c and j in drained conditions by using all data (qc,fs,u) of CPT and CPTu in various soil types.The Proposed analytical model composed of bearing capacity theory and shear stress equation at rupture surface. Instead of total bearing capacity (qt), effective bearing capacity (qE) has been used and the shear stresses have been related to sleeve resistance (fs). Assessment and comparison of results of suggested method with current analytical approaches and laboratory tests obtained from five sites shows that results have a good agreement with measurements and predictions. Therefore, by using of all cone penetration test data, the prediction of soil shear strength parameters can be done with better accuracy.

Transportation networks are significantly vulnerable to natural disasters. The disasters cause disruption or reduction of capacity of important elements of the transportation networks such as bridges or tunnels particularly those for which alternative routes are not provided. The disruption or capacity reduction of the transportation networks results in decrease of an evaluation index of performance of the transportation networks. The evaluation index is highly dependent on stability levels of network components. Because of budget limitations, human resources, and time on one hand and having considerable exposed cost on the transportation networks due to disruption or capacity reduction of the network elements on the other hand, developing an investment prioritization scheme for the transportation networks is of importance. The stability of the network components are defined herein using appropriate probability distribution functions. Therefore, optimum resource allocation on the network elements is carried out by applying risk analysis techniques (using a simulation technique and dynamic programming) in order to enhance the performance of the whole transportation networks and reduce instabilities risk of the transportation networks. A hypothetical transportation network is applied to illustrate the capability of the proposed technique in investment prioritization and network performance enhancement.

Many formulas have been proposed over the last century for the evaluation of Manning’s n in composite material both for single and compound channels. However, when stage is plotted as a function of discharge, many of them show discontinuity at the junction of the main channel and flood plain. These discontinuities have not been observed in other hydraulic parameters such as specific energies, and mean velocities. The discontinuity arises from two major sources, firstly Manning’s n and secondly. Investigations of 26 formulas for equivalent Manning’s in composite channels show that many of them cannot be applied for a two-stage channel because of the discontinuity. It is observed that only three of those formulas can be used to meet this requirement. In order to solve the problem of the lack of continuity for the second term a new technique is proposed. Using this technique a number of geometrical parameters of a trapezoidal channel such as the ratio of the flood plain to main channel as well as the side slope of the main channel and flood plain are altered to graphically investigate their behaviour on the stage-discharge diagrams. The results show that as the ratio of flood plain to the main channel area is increased, the differences between the hydraulic variables will be increased. Also the increment of the side slope of the main channel and flood plain do not show a significant change in the dimensionless discharge especially when the water level is above the flood plain stage.

Since lime- stabilization is effective on different soil size ranging from sand and finer, the influence of lime and clay content on CBR strength of these soils is investigated. Lime combines with the clay part of these soils and provides cemented materials, which is similar to concrete. Hence, the variation in clay and lime content will affect the amount of cemented materials and consequently the strength. Providing some lime- stabilized clayey sand samples, the effect of clay and lime content on CBR strength is evaluated. The results show that in a particular range of clay content, the mixes have the maximum strength variations and, the optimum lime content is about 7 to 8 percent.