Computational Methods in Civil Engineering
Volume:3 Issue: 2, 2012

Efficient mode shape extraction of fluid-structure systems is of particular interest in engineering. An efficient modified version of unsymmetric Lanczos method is proposed in this paper. The original unsymmetric Lanczos method was applied to general form of unsymmetric matrices, while the proposed method is developed particularly for the fluid-structure matrices. The method provides us with significant capabilities of symmetric matrix storage scheme and application of real variables in calculations. The efficiency of the proposed method has been examined in mode shape extraction of several dam-water systems and is compared with pseudo subspace method which has identical capabilities. The results show significant time efficiency especially for problems with large dimensions.

In order to mobilize shallow soil to participate in the interaction of piled raft foundation sufficiently, the concept of piled raft has been modified to new type of foundation named composite piled raft. In the system of composite piled raft, the short piles made of flexible materials is used to strengthen the shallow soft soil, while the long piles made of relatively rigid materials is used to reduce the settlements and the cushion beneath the raft is used to redistribute and adjust the stress ratio of piles to subsoil. Finite element method is applied to study the behaviour of this new type of foundation subjected to seismic forces. This paper focuses on behaviour of various components of foundation system such as long pile, short piles and subsoil under seismic force (2-I-2(1995, HYOUGOKEN_South, EW)) in layered soil related to Surat city geological condition. A comparative study is done to understand the effect of cushion on axial stresses, shear stresses and shear forces along piles and soil beneath the raft.

In this paper، the linearly conforming enriched radial basis point interpolation method is implemented for the elasto-plastic analysis of discontinuous medium. The linear conformability of the method is satisfied by the application of stabilized nodal integration and the enrichment of radial basis functions is achieved by the addition of linear polynomial terms. To implement the method for the analysis of a discontinuous medium، an interface layer is assumed between different materials. Interfaces are simulated by the concept of linkage element and there is no need of node or element in the traditional sense. The stiffness of each interface layer has been taken into account by defining normal and tangential stiffness coefficients along the layer. The displacement of each point across the interface layer is tied to the displacement of surrounding material nodes. The final system of equations is derived by the combination of equations for different parts of a discontinuous medium in the global coordinate. Based on the derived equations a computer code has been developed and the results of analysis with the mesh-free method are compared with the results of the finite element analysis and experimental tests.

The isogeometric analysis is increasingly used in various engineering problems. It is based on Non-Uniform Rational B-Splines (NURBS) basis function applied for the solution field approximation and the geometry description. One of the major concerns with this method is finding an efficient approach to impose essential boundary conditions، especially for inhomogeneous boundaries. The main contribution of this study is to use the well-known Lagrange multiplier method to impose essential boundary conditions for improving the accuracy of the isogeometric solution. Moreover، Dirichlet boundary conditions on the derivatives of the solution field (which are not directly defined as independent degrees of freedom) can be treated easily. The Lagrange multipliers must be interpolated on a set of boundary points. Among the different available choices for the boundary interpolation points، the Greville abscissas are considered in this work. Several plate problems have been solved، which demonstrate significant improvement in accuracy and rate of convergence in comparison with the direct imposition of essential boundary conditions.

Flood routing has many applications in engineering projects and helps designers in understanding the flood flow characteristics in river flows. Floods are taken unsteady flows that vary by time and location. Equations governing unsteady flows in waterways are continuity and momentum equations which in case of onedimensional flow the Saint-Venant hypothesis is considered. Dynamic wave model as one of the flood routing methods is used for flooding operations because of its high accuracy. Finding the best numerical methods is the main challenge for optimal modeling of dynamic waves and predicting the flood behavior. In this study، a - km reach of Ghare-Aghaj River (between two hydrometric stations) is investigated and in addition to the unsteady flow equations (nonlinear Hyperbolic partial differential equations)، Preissman implicit scheme and Mac Cormak explicit scheme which are both based on the finite difference numerical methods have been used. Developed models have also been compared with one of the most reliable Mike series computer program. It is aimed to find the most suitable finite difference scheme by comparing the results of two above schemes with the results of Mike 11 program. The results confirmed the superiority of Preissmann implicit scheme in predicting flood wave characteristics for the studied area.

Following the 1994 Northridge and 1995 Kobe earthquakes, most of modern structures damaged seriously or devastated totally despite the seismic codes of these countries that had been expected to bear advanced criteria for seismic design of structures. After extensive research, the most probable reason of those destructions was attributed to special specifications of near-field earthquakes. In this paper, in addition to a brief review on the special characteristics of near-field earthquakes, using actual records of these ground motions and pulse like inputs simulating near-filed records, the effects of this type of earthquakes on the distribution of the maximum shear force and flexural moment demands are evaluated. Results show that the response is very sensitive to the ratio of natural period of structure to the governing period of ground shaking pulse, such that, in some cases it leads to increasing force demand ratios of structure and even transmitting its centroid to upper stories.