STABILITY ANALYSIS OF WYE-BRANCH STEEL LINING UNDER EXTERNAL PRESSURE

Power stations with high heads are nowadays almost without exception designed so as to have one pipeline, or only very few pipelines, supplying the water to the generating station. the penstocks is connected to a distributor, which has the task of distributing the flow, with the lowest possible losses of energy, to the individual turbines. the function and location of this section of the plant make it understandable that its safety is regarded as a matter of outstanding importance. furthermore, well-substantiated method of computation and experimental stress and instability investigations make it possible to control stresses occurring in the complicated bifurcations. thus adequate safety and economical design of the construction methods are ensured by suitable matching of stress levels to the properties of the thin walled structure and materials used.generally, in this type of structures, external pressures can occur either during the tunnel grouting operation or on tunnel dewatering after a prolonged period of use, and very often it is the external rather than the internal pressure that determines thickness of the steel liner. in cases where a substantial increase in liner thickness would be required to prevent buckling due to external pressure, it may be more economical to provide external stiffener ribs.a 4-node quasi-conforming stress/strain resultant shell element, 4-node assumed strain shell elements are developed and implemented for the nonlinear static solution of stability problems of stiffened plates and shells. the geometric non-linear shell elements, which are based on the updated lagrangian method. the formulation of these elements uses mindlin-reissner theory, which the formulation of the linear and geometrical stiffness is exactly defined by incorporating membrane forces, bending moment and transverse shear resultant forces. in order to remove rigid body rotations, the co-rotational method is used. the material is assumed to be isotropic and elastic material. by using six degrees of freedom per node, the present element can model stiffened plate and shell structures. linear analysis and large deformation analysis of the elastic shell using 4-node quasi-conforming shell element are carried out.