Civil Engineering Infrastructures Journal
Year:2011 | Volume:44 | Issue:5|Papers Count:16

Monitoring the behavior of earth dams during the stages of construction, impoundment and operation is of a great significance. The results obtained from monitoring can be used to check the validity of the design parameters and to improve the design of future dams. Maroon reservoir dam with the height of approximately 170 m is one of the highest earth dams in Iran and is located in Khuzestan province in the southwest of Iran. In this dam, various types of instruments such as total pressure cells, piezometers, inclinometers and settlement plates have been installed within five sections to monitor the behavior of the dam. This dam had two phases of impoundments. In the first phase, the water level reached an elevation of 426 m, but the reservoir was emptied due to some construction concerns. In the second phase of impoundment which was the main impoundment, the maximum water level reached an elevation of 485 m. In this article, the results of three dimensional analyses of Maroon dam during the second impoundment using FLAC3D program are presented and are compared with the results obtained from the instrumentation data. This software involves finite difference method to carry out the computations. The reason for using three dimensional analyses was that the dam is constructed in a relatively narrow valley. Mohr-Coulomb constitutive model was used in the analyses in which the main input parameters consist of cohesion, angle of internal friction, bulk modulus and shear modulus of soil. The dam was modeled in 17 layers each with a thickness of 10 m. Considering the actual time of construction, average rate of construction was calculated as 17 cm per day. Therefore, for each layer of 10m thickness, 60 days was allocated for consolidation. It is concluded that a rather good consistency exists between the computational results and monitoring results. It is also shown that arching has taken place both along the dam axis and along the upstream-downstream direction; however, the magnitude of the arching has not been to an extent that could cause cracking or piping of the dam.

Evaluation of strong ground motions and effect of them on performance of structures are the important subjects that nowadays are considered in structural and earthquake engineering. It is necessary to evaluate of the near-field earthquake characteristics and effect of them on response of buildings. In this paper, the performance of steel Moment-Resisting Frames (MRFs) in tall buildings in near fault areas has been studied. Six steel MRFs 15- and 20-story with three different ductility; ordinary, intermediate and special moment-resisting frames have been designed according to the Iranian seismic code 2800 (3th Edition) and Iranian national building code-steel structures (Part 10). To assign the performance parameters of frames and to present the effects of near-field earthquakes on steel MRFs, pushover and non-linear dynamic time history analyses have been performed and conducted for chosen structural models. The ensemble of used earthquakes contains both far-field and near-field earthquake records. It is worth mentioning that the selected earthquake records contain some strong ground motions from the 1978 Tabas, 1979 Imperial Valley, 1992 Landers and 1994 Northridge Earthquakes. According to the analytical results, concerning near-field earthquake records which emerge velocity pulses in their time history, the maximum values of story displacement, story drift, roof displacement and base shear are relatively higher than those which are obtained from near-field records that do not display velocity pulse and also those ones obtained from far-field strong ground motions. Based on using near-field earthquake records specially those ones that display velocity pulses, the calculated members stress and deformation resultants are higher than those ones that are recommended by the Iranian Seismic Code. Also, by more ductility in steel special moment-resisting frames the structural damping in non-linear domains can be considerably higher than the value of this parameter in other structural systems. This subject should be confirmed by comparing the calculated values of seismic base shear in analyzed models. On the other hand, in special moment-resisting frames, under the effect of near-field earthquake records which emerge velocity pulses, the maximum values of storey displacement and storey drift is much higher than allowable values which are recommended by the Iranian seismic code. Hence, it is advised to increase the capacity of member sections and connections, in order to achieve the code-defined limited values of displacement and drift demands in steel special moment-resisting frames.

Concrete filled double skin steel tubular (CFDST) members can be recognized as a new kind of CFT construction. They consist of two concentric steel tubes with concrete sandwiched between them, the steel tubes can be circular hollow sections (CHS) or square hollow sections (SHS) where both inner and outer tubes are circular hollow sections. CFDSTs were firstly introduced for vessels to resist external pressure. Strong potential of using CFDST in offshore construction, highways and high-rise bridge piers. All of analysis have been undertaken using ANSYS which is a general purpose finite element programe designed especially for advanced structural analysis. In order to verify the finite element model, a comparison between the experimental results and finite element results was carried out. Generally, good agreement between the experimental and numerical curves is achieved. The effective parameters are the bond between the steel and internal concrete, local buckling, strength of steel tube and concrete, loading conditions of columns.The result of this study clearly exhibit the high strength and ductility of the CFDST columns under axial loading, and they have lighter weight, than their CFT counterparts.

In this article, it is tried to overcome one of the weaknesses of the current assumptions used in the ordinary pushover analysis. This includes the assumption in which it is assumed that “the first mode controls the response of the structure and this mode remains unchanged after it undergoes to inelastic range”, this assumption is inadequate especially in tall buildings with long natural period of vibration. It seems that the invariant lateral loads used in this procedure cannot depict these effects. Here, by presenting the modal pushover analysis (MPA) and considering three lateral load patterns related to the three first modes of the structural deformation, it is shown that how the higher modes affects the accuracy of seismic response of MRF compared to ordinary method introduced in FEMA273 and non-linear Response History Analysis.

Over the present experimental research, a comprehensive study has been carried out to investigate the effect of the structure's primary slope on the amount of overtopping the irregular waves of the sea from the reshaping rubble-mound seawalls.The experiments of physical modeling of this research have been done in the flume of the Soil Conservation and Watershed Management Research Institute (SCWMRI). The waves applied to the structure model are irregular and the energy spectrum of the applied waves is JONSWAP. To study the slope effect of seawalls, the structure slopes of 1:2.5, 1:2 and 1:1.25 have been tested. The material of armor layer have been regarded with the scale of 1:20 model and grading class of D85/D15=1.82. A number of 1000 waves were applied on the structure over the research in general and some 57 experiments have been totally accomplished. The achieved outcomes of the experiments after drawing the figures were analyzed and the effect of the structure's primary slope on the amount of waves overtopping were investigated. The outcomes indicate that the primary slope enjoys no considerable effect on the amount of overtopping from the reshaping rubble-mound seawalls, although following the fall of the structure's primary slope, the mean overtopping discharge faces with a least decrease.

This article presents the results of static and dynamic numerical analyses of an existing embankment dam with an inclined clay core (Bidvaz Dam) in Iran. The analyses are repeated for the same dam, but with the assumption that the core is vertical. The results consist of displacements and pore water pressures within the dam core. The Mohr Coulomb elasto plastic model and an elastic model are considered for the dam and foundation materials, respectively. For the earthquake loading, nonlinear dynamic analyses are carried out. The seismic stability of the earth dams is evaluated using nonlinear displacement analyses, considering the accelerograph obtained by ground response analysis. The scaled Tabbas earthquake time history (1978) is employed as the input motion to the bedrock. The analyses for the earthquake loading are performed using the total strength parameters of the materials. The results of the analyses for both of the dams are carefully evaluated and compared. The results are summarized as follows: for the pre-earthquake (end of construction) loading, the deformations developed in the dam body are comparatively larger in the dam with the inclined core, while the excess pore water pressures in the core are comparatively higher in the dam with the vertical core; for the earthquake loading, the deformations developed in the dam body are comparatively larger for the dam with the inclined core; sensitivity analyses with various peak ground accelerations indicate that the dam with the inclined core is more susceptible to instability during earthquakes.

In this work the general aspect and the behavior of a simplified theoretical model for studying mass isolated systems are investigated. The model is in the form of a dual mass model in a single storey arrangement. To find damping ratio and natural frequency of the system, Non-classical Eigen analysis has been performed on the model. Later, time integration analyses subjected to different earthquake records were carried out on the same model. In time integration analyses the model was equipped with linear, nonlinear and controllable viscous energy dissipaters. The results of Eigen analyses on the model show a limit point on the level of maximum achievable damping ratio in the system. According to the results of time integration analyses, using nonlinear viscous devices in the system (instead of the linear one) reduces the sensitivity of the model to its design parameters (including damping constant) and provides the system with a practical optimal design solution. The results also show that, using viscous devices with controllable features in the model lead us to a system with higher level of structural performances.

Two-way slab structures, owing to their economical and architectural advantages, are used extensively in civil engineering. In addition, these systems not only resist gravity loads and transfer them to the columns, but also sometimes in combination with other structural members make a lateral resisting system. Elastic analysis of equivalent frame method is a precise and comprehensive method which is widely used for analysis of two-way slab structures. Because of non-prismatic sections of slab-beams and columns, carry over and stiffness factors and fixed end moments of members with constant moment of inertia could not be used. Some tables have presented these values for different types of the slab-beams, columns and torsional members. Due to low precision of these tables and complicated and time-consuming procedure of equivalent frame method, precise analysis of two-way slab systems using equivalent frame method including stiffness degradation in all members, using slope-deflection equations, in the form of a prepared software is pursued in this research. Two analyses of example structures were performed and the efficiency and accuracy of the program were verified by comparing the hand-calculated results, results of a finite element approach and the developed computer program's results, which shows some differences in the support moments.

The reinforced soil walls have shown an acceptable seismic behavior in most earthquakes. However, they can show a considerable deformation which has been studied by many researchers. The presented research aims to study different parameters which affect the seismic deformation of reinforced soil walls. In the research, five different series of tests conducted on reinforced soil wall models on a shaking table and the effects of soil density, embedded depth, length of geogrids, stiffness of geogrids and friction angle between soil and geogrids were investigated. The behavior of the models was monitored using four accelerometers and four LVDTs in each test.The methodology of the presented research could be summarized as follows: 1. The increase of geogrids length from 0.7 H to 0.9 H could decrease horizontal deformations up to 50%. It also significantly reduces the amplification of the acceleration. 2. Embedded depth has an important effect on reducing the vertical deformation around the developed failure zone.3. When stiff reinforcements with relatively high tensile strength are used, the pullout mode of failure happens rather than yielding of reinforcements. The stiffness of geogrids, in this case, does not have any effect on seismic deformation.4. The friction angle between the soil and the geogrids had a significant effect on seismic deformations.5. Two modes of deformation, named bulging and tilting, have been investigated in the study. These modes are related to the overall stiffness of the system and the dynamic wave characteristics. The overall stiffness of the reinforced soil walls is mainly related to the soil density, geometrical properties of the wall; and also the number of reinforcement layers, length and mechanical properties of geogrids.The paper concludes that it is possible to design the reinforced soil walls in a way which decreases the horizontal and the vertical deformations up to a requested performance design level for seismic loads; however it needs more research. In this research it is found that the most considerable parameters which have effect on seismic deformations are soil density, physical and mechanical characteristics of geogrids (length, tensile strength, friction angle between soil and geogrids) and embedded depth. On the other hand, the characteristics of the dynamic waves, such as the amplitude, duration and the seismic wave frequency content, may have also effect on seismic deformations which also needs more research.

To have a more clear judgment about distribution and point of application of active earth pressure behind retaining walls, simultaneous studies on both physical and numerical models are required. In this study the behavior of backfill soil under static and cyclic surcharges has been evaluated using the FLAC software while the wall has some lateral movements. To verify the numerical model the results have been compared with those measured behind the same physical model developed in soil mechanics laboratory of Amirkabir University of Technology. After that an extensive parametric studies have been carried out to investigate the influences of different factors such as amplitude, position and type of surcharges as well as backfill soil properties and the amount of the wall movements. The results of these studies have been explained and discussed in different section of the present paper.

One of the most important factors in the design of retaining walls is to determine the earth pressure acting on such walls. In this paper in order to study the at-rest earth pressure to rigid retaining walls, a fixed retaining wall with a cohessionless dry backfill is modeled with the computer program FLAC 2D. The geometrical and mechanical properties of the numerical model are based on the physical model developed in the laboratory. To verify the model, the results of the numerical model are compared to the results of the experimental model. Then cyclic surcharges with different amplitudes and frequencies are applied in different distances from the wall and the pressure distribution on the wall, the resultant force and its point of application are investigated. The effect of soil, wall and loading parameters on the cyclic pressure are also evaluated. The results of the model show that the pressure distribution to the retaining wall under cyclic surcharge is nonlinear and there is a significant increase in the earth pressure due to cyclic loading compared to that due to static loading.

The flow around bluff bodies is a subject of much concern to several researchers. The pier shape effects on flow pattern, vorticities and shedding in the wake region and corresponding fluid forces acting to bridge piers. There are fewer studies about appropriate shape of pier to reduce dynamic forces. In this research, two dimensional water flow around different common shapes of pier was simulated using FLUENT at Reynolds number 2×105, and the effect of pier shape on the drag and lift coefficients was studied. Also drag and lift forces in the case of the inclined pier subjected to flow was studied. The main object of research is obtaining the appropriate shape for reducing cross forces. Finally the investigation showed that rectangular pier with round and sharp nose in the case of round side flow had better performance in both straight and inclined flow and less dynamic forces.

The occurrence of skin cancer is related to many parameters which are spatially distributed. GIS can be used for analysing such spatial relationships. On the other hand, the occurrence of skin cancer is a probabilistic phenomenon. In other words, even in presence of all causing factors, skin cancer does not necessarily occur. In this research, a geostatistical model is developed to simulate the spatial distribution of skin cancer. First, the data framework is designed and proper data are gathered. Then, the data are processed in GIS and prepared to enter the model. Finally, by creation of the geostatistical model, the relations between the disease and its factors are studied. As a result, the effecting factors and their importance degrees are extracted. The developed model can be used both as simulator tool and as a decision support tool. In the other words, by changing the values of one or more parameters, the probability of occurring changes in the occurrence of disease can be studied. This can help the managers in evaluating the result of any possible course of action, when challenging the disease.