Scientia Iranica
Year:2010 | Volume:17 | Issue:2 (TRANSACTION A: CIVIL ENGINEERING)|Papers Count:8

Double Concave Friction Pendulum (DCFP) bearing is a new generation of friction isolator that contains two separate concave sliding surfaces with different properties. Accommodating enhanced performance, compared to the Friction Pendulum System (FPS), is one of the most important benefits of DCFP. Herein, the seismic behavior of structures isolated by DCFP bearings is compared with the response of the same buildings using the FPS bearing. Accordingly, a series of nonlinear dynamic analyses are carried out under ensembles of ground motions at three different hazard levels (SLE, DBE and MCE). Moreover, the adaptive behavior of DCFP and its advantages in protecting secondary systems is investigated. The probability of exceedance curves of peak roof acceleration, peak inter-story drift and peak isolator displacement is compared for two types of isolation system. The result supports the advantages of DCFP isolation systems.

One of the sources of disturbance at intakes is the occurrence of free-surface vortices with an air core. The most common solution for avoiding air-entrainment is the use of anti-vortex devices and, especially, plates for large pipe or shaft intakes. If plates are used, then, the geometry and position of them should be studied experimentally. Since only general guidance for use of plates is available, a study for the more precise placement of plates is needed. Hence, a comprehensive set of experiments have been carried out using rectangular plates with different dimensions and at various positions with respect to the vertical outlet pipe intakes and two different pipe diameters (D=75 and 100mm). The results of critical submergence with respect to the dimensions and positions of the plates are presented as graphs and equations. Thus, design guides and recommendations are provided.

In nonlinear static (pushover) methods of analysis as an alternative to time history analysis, the capacity curve of the structure is established with respect to the roof displacement. Disproportionate increases in the roof displacement and even outright reversals of the higher modes can distort the capacity curve of the equivalent single degree of freedom system in these kinds of method, including MPA. To overcome this problem, recently, “Energy-Based” the Modal Pushover Analysis (Energy-Based MPA) method has been introduced. In this method, the absorbed energy and/or the external work in the pushover analysis is considered. Accordingly, the assessment of the Energy-Based MPA method is important in the seismic analysis of asymmetrical and tall buildings. In this paper, the seismic demands of concrete structures with irregularity in elevation are determined, using Energy-Based MPA. For assessment of the presented technique, the results are compared with those from the Non-Linear Time History Analysis (NL-THA). Seven examples including a 2-D simulation of a 12-story building are modeled, using the Opensees Code. For each case, different types of irregularity, such as mass, geometry and variations due to the difference in elevation are considered. Story-drifts and floor-displacements are used as the main parameters for assessment of the results. Based on a study of the structural performance of the models, it has been made clear that different types of the above-mentioned irregularity in elevation do not have any significant effect on the Energy-Based MPA method. Consequently, this method can be considered as an accurate alternative technique for NL-THA, to fairly estimate the seismic demands of structures.

Flow discharge over an oblique weir is greater than that over a straight or plain weir for the same water head due to its extra length with respect to the channel width or fully extended plain weir. In this study, a new theoretical approach is used for the hydraulics of oblique weirs. The main objective is to investigate the effect of different hydraulic and geometric properties of the flow and the weir on the flow deflection angle and discharge coefficients for free and submerged flow over oblique weirs. This approach is based on energy, momentum and continuity equations. For improving the performance of this kind of weir, one approach is to increase the flow deflection until it is perpendicular to the oblique weir for maximum use of the weir length. The submerged guide vanes have also been used and investigated theoretically. The data for calibration of the models are taken from Borghei et al. (2003). It is shown that by employing guide vanes, for some cases, the discharge coefficient can be increased up to 33%. Finally, new relations were developed for practical purposes.

Retaining walls may be constructed with an inclination angle of less than 90_from the horizontal axis. In the present study, using the horizontal slices method and limit equilibrium principles, in addition to assuming variation of the seismic coefficient with height, a new formulation is proposed to calculate the active seismic pressure on retaining walls. The general arrangement of the proposed pseudo-dynamic formulation allows analysis of inclined or vertical retaining walls in frictional, cohesive and cohesive-frictional soils. Results from the proposed method were compared with those of previous researchers under similar conditions and showed a negligible difference. The horizontal slices method was able to assess an inclined wall, determine the active earth pressure distribution at different points along the wall height and consider the angle of failure wedge as a variable in the time domain. The findings show that despite the accepted assumptions for conventional vertical walls the distribution of earth pressure on an inclined wall follows a non-linear pattern at each moment.

In this paper, a graph theoretical method is used to introduce three basic operations of configuration processing; rotation, translation and reflection. The relationships between translation and rotation, and reflection and rotation, are established. Examples are then constructed using three different procedures of configuration processing. From the comparison and discussion of the results, it is concluded that rotation can be considered as a general operation of configuration processing.

In this paper, a new computational technique is presented for the modeling of moving boundaries in large plastic deformations based on an enriched arbitrary Lagrangian-Eulerian finite element method. An Arbitrary Lagrangian-Eulerian (ALE) technique is employed to capture the advantages of both Lagrangian and Eulerian methods and alleviate the drawbacks of mesh distortion in Lagrangian formulation. An enriched finite element method is implemented based on the extended FEM technique to capture the arbitrary interfaces independent of element boundaries. The process is accomplished by performing a splitting operator to separate the material (Lagrangian) phase from the convective (Eulerian) phase, and partitioning the Lagrangian and relocated meshes with some sub-quadrilaterals whose Gauss points are used for integration of the domain of elements. In order to demonstrate the efficiency of the enriched ALE finite element model in large deformations, several numerical examples including the coining problem with horizontal and vertical moving boundaries and a tensile plate with a moving interface are presented and the results are compared with those of the standard finite element and extended finite element methods.

Corrosion of steel structures is a serious problem throughout the world. The most significant consequence of corrosion is reduction of the section size. This, in turn, leads to a reduction in the carrying capacity and structural safety. The analysis of existing structures may differ from the analysis of structures under design, especially if there is damage by corrosion. Common examples of corrosion that have been found in the petro-chemical industry include the loss of sections in flanges and holes in the web. As there is no clear provision of recommendations for such cases in the present codes, there is a need for simple assessment methods to deal with them effectively. The overall aim of this paper is to develop a simple method which can be used to make reliable estimates of the remaining moment capacity, using thickness loss information provided by visual inspection or measurements. The results of this study can be used for calculating the remaining service life, such as the moment capacity of deteriorated I-beams.