Analysis capabilities

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Analysis capabilities |

The general objective of GFAS is to provide analytical tools for deformation and stress assessment of plane structures in direct support of geotechnical analysis.

The program provides three basic analysis tools:

The Linear Eleastic Analysis tool can be used to perform a finite element analysis of a membrane of any general geometry subjected to plane stress, plane strain or axisymmetric stress and strains. The conditions of plane stressand plane strainare two similar two-dimensional states of stress. When forces are applied to a thin two-dimensional plate in its own plane, the state of stress and deformation in the plate is called plane stress. A typical example would be a shear wall that, due to it being a thin plate, will experience mainly in-plane stresses. No restraint is provided for out-of-plane deformation. On the other hand, a prismatic solid subjected to a constant loading normal to its axis can be analyzed as an infinite length of two-dimensional slices of unit thickness experiencing plane strain. A dam wall, for example, would typically be subjected to hydrostatic and soil pressures normal to its surface. A slice is taken from the wall will be restrained from deforming out-of-plane.

The Bearing Capacity Analysistool can be used to compute the response to loading of a nonlinear material. Plane strain conditions are enforced and in order to monitor the elasto-plastic behavior, the loads are applied incrementally. The method uses constant stiffness iterations, thus the relatively time consuming stiffness matrix factorization process is called just once, while the backward substitution phase is called at each iteration. Several failure criteria have been implemented for representing the strength of soils as engineering materials. For soils with both frictional and cohesive components of shear strength Mohr-Coulomb failure criteria is appropriate. For undrained clays, which behave in a "frictionless" manner, Von-Misses failure criteria may be used.

The Slope Stability Analysistool can be usedto carry out the slope stability analysis of a given structure. During the analysis the program gradually reduces the basic strength characteristics of the soil mass until failure occurs. The factor of safety (FS) is to be assessed, and this quantity is defined as the proportion by which tan Φ (friction angle) and c (cohesion) must be reduced in order to cause failure with the gravity loading kept constant. This is in contrast to the bearing capacity analysis in which failure is induced by increasing the loads with the material properties remaining constant. The program can give information about the deformations at working stress levels and is able to monitor progressive failure including overall shear failure. The present release can be applied only for two-dimensional plain-strain problems. Either the Mohr-Coulomb or Von-Misses constitutive models can be used to describe the soil or rock material properties. Gravity loads are generated automatically and applied to the slope in a single increment. A trial strength reduction factor loop gradually weakness the soil parameters until the algorithm fails to converge. The factored soil strength parameters that go into the elasto-plastic analysis are obtained from:

where SRF is strength reduction factor. Several increasing values of the SRF factor are attempted until the algorithm fails to converge, at which point SRF is then interpreted as the factor of safety FS. This actually means that no stress distribution can be achieved to satisfy the failure criterion and global equilibrium. Non-convergence within a user-specified number of iterations in finite element program is taken as a suitable indicator of slope failure and is joined by an increase in the displacements. Usually the value of the maximum nodal displacement just after slope failure has a big jump compared to the one before failure.

The Staged Analysis option can be used to carry out staged construction analysis. During the Staged construction analysis, the loads are increased from 0 to 1, for each stage of construction. As soon as the load parameter reaches the value of 1.0, the constructions stage is completed and the analysis of the current phase is completed, and go the the next phase of the construction. If a staged construction calculations finishes while the load factor is smaller than 1.0, the program will stop the analysis. The most likely reason for not finishing a construction stage is that a failure mechanism has occurred.

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