پژوهشنامه حمل و نقل
سال هفتم شماره 4 (پیاپی 25، زمستان 1389)

Block-flexure toppling failure is a common failure of layered rock masses in rock slopes. In the failure, rock blocks fail due to bending stresses and overturning. In this paper, initially, a brief review of toppling failure in rock slopes is summarized. Then, a theoretical model is suggested for rock slopes with potential of block-flexure toppling failure, and consequently a new analytical approach is presented for analysis of the failure. Next, a computer code is developed for assessment of rock slopes against block-flexure toppling failure on the basis of presented method in the paper. Finally, two case studies are analyzed by the new suggested approach and the theoretical and real results are compared.

Strut and tie models have been increasingly used in recent years. Most of design procedures for concrete beams in shear are based on the truss analogy. The truss analogy proposes that the behavior of a cracked reinforced concrete beam is like a truss with parallel longitudinal chords. In prestressed concrete beams, prestressing force has two vertical and horizontal components along the beam length in which the amounts of these components depend on slope of cable in each section of the beam. In this research, internal vertical prestressing by means of suitable rods perpendicular to beam axis was used to create compressive stress in the beam web.Shear strength of bridge pre-stressed concrete beams with and without vertical prestressing is studied in this paper. First, several models of reinforced and prestressed concrete beams are selected and analyzed using the finite-element method according to continuous stress field theory. The analysis is performed using nonlinear behavior for materials. The concrete material is defined based on concrete damage plasticity by means of experimental stress-strain diagrams. The steel material is also defined according to perfect plasticity and stress-strain diagrams.In this regard the validity of modeling is evaluated based on some tests results. In the second part of the study two beam specimens are built and tested and their load-deformation curve and cracking pattern are studied. Using all numerical and experimental studies, the slope of shear cracks and shear reinforcement ratio are investigated and the effect of horizontal and vertical pre-stressing on the noted parameters is evaluated. Then the compression strut slopes derived from nonlinear analysis, the code relations are studied and compared.This paper focuses on a more realistic determination of the angle of orientation of the compression struts in prestressed concrete beams. The findings should be of interest to researchers as well as practicing engineers involved in analysis and design of prestressed concrete beams.

During the last few decades, application of geotextile in asphalt concrete before laying it for the purpose of reducing the overlay thickness has drawn the attention of many researchers, therefore determination of its structural value is considered as an important issue.Falling Wight Deflectometer (FWD) is the most popular non - destructive test for thedetermination of asphalt overlay structural value. In this paper, to determine this value, twelve sections, 150 meters each, were studied on Tehran–Qom old road. Each section was tested twice (once before and once after the overlay implementation) and each time, at least 12 FWD tests were carried out. Overlay thickness was determined using the tests results. For the first time, a model has been developed to evaluate the asphalt overlay structural value using FWD test data of 5 sections with pure asphalt overlay (4 sections to develop the model and one to verify the output). The seven remaining sections were implemented using various popular, Iranian or foreign geotextiles as interlayers, and their structural values were determined by help of a developed model. Based on the outcome of the present research, the average structural value of overall geotextiles is equivalent to a 2.67 cm thick Hot Mix Asphalt (HMA) overlay while that of the Iranian ones equals 2.29 cm.

Dynamic response of railway bridges during the passage of high speed trains depends on several parameters, e.g dynamic properties of bridge and train, pavement type and train speed. This paper presents the results of a research in which the impacts of these parameters were investigated on four simple span bridges with the 10, 15, 20 and 30m span lengths and under train speed at the range of 100-450 km/h. The first three bridges were designed in accordance to the guidelines of Iranian Management and Planning Organization. The forth bridge, was designed as a prestressed concrete bridge. To evaluate the dynamic response of bridges, two different models of the bridge and vehicle were chosen: one considering the interaction between bridge and vehicle over flexible pavements (ballast) and the other by modeling the train as a series of moving loads without considering the pavement effect. The measures that were considered for comparisons were: resonance and cancellation speeds, the impacts of stiffness variation on them, and the influence of the number of coaches. The results indicated that the acceleration magnitude in the middle of 10,15 and 20 m span bridges were lower than the 30m prestressed concrete bridge which may be attributed to their lower weight. The results indicated that whilst the resonance speed obtained via moving loads modeling was somewhat higher than other modeling methods, the bridges response in resonance conditions was conservative. During the bridge operation, as the time goes by, the changes in the mass and stiffness of the bridges cause the resonance occur at slower speeds. The results also indicated that the number of coaches would only affect on the main resonance speed.

Economic growth may result in inequality. Empirical evidence shows that in many countries economic growth generally leads to inequality. There is a need consider this topic in our country as well. The basic question in this paper is that economic growth in transportation and communication sector, which provides equal conditions and access to competitive opportunities for all segments, can lead to economic growth with the proper distribution of income? The results of this study, using data of the years 1969-1995 indicate that economic growth resulting from transportation sector, is associated with reduced income inequality. In addition, the strategic location of transportation sector in the region and also Iran's transportation network has a lot of potentials that can basically reduce inequality between urban and rural areas.

The aim of this research is to develop a pricing model for on-street parking lots to promote the investment on the construction of parking garages. The objective was to put a price on on-street parking spaces in such a way that the customer prefers to park in the adjacent parking garages. First, the most effective factors in pricing were identified by developing a questionnaire. Factors were divided into two categories as; control parameters and directly effective parameters. The hierarchical analysis process (AHP) was used to estimate the average weight of each direct parameter. Furthermore, a behavioral model based on the customer survey was developed to obtain the suitable price for a parking space. Finally, a comprehensive model integrating the AHP and behavioral models was developed in which, it would be possible to determine the desired price for a customer to prefer to park in a garage rather than on the street. Using this model it would be also possible to price an on-street parking space in such a way that the drivers tend to divert from on street parking spaces to the parking garages. In this case, investment on the development of parking garages would economically be possible.

The rail yard is responsible for receiving inbound freight, sorting it, and sending it to the next destination. Freight arrives at yards for four primary reasons, and, for each case, the processing steps differ. First, the freight may be connecting from one train to another. Second, the yard may be the final destination for arriving freight or the origin for departing freight. Third, the train upon which the freight is traveling may be simply passing through the yard in route to another destination. Finally, the train upon which the freight is arriving may require a mandatory safety inspection. In this paper, for the first time in Iran, a simulation model is developed for a flat classification yard. The proposed model is to analyze the performance of available resources in the flat classification yards. The inputs of model include: • number of receiving, classification and departure tracks, • number of switching locomotives, • probability distributions such as: of train arrival interval times, number of cars in each train, • types of incoming trains,• switching operation time, • inspection time, • stopping time, •etc.The presented model simulates the Aprin’s operations including car grouping, the train arrival, train making up and train departure processes. The main outputs of the model include: •total and average time spent in the yard, •total and average waiting time for sorting and making up trains, •utilization rate of switching locomotives,•etc.The current situation of Aprin’s operations is simulated. It is then used to validate the proposed simulation model. To analyze the performance of Aprin yard and to examine the availability of resources the number of switching locomotives and car incoming rates were changed and the treats of the system were studied. The proposed model can serve as a reliable tool to analyze and decide for the increase of facilities and equipment in the classification yard.