Nonlinear analysis of prestressed concrete reactor pressure vessels

A computational model for circular symmetric reinforced concrete shell problems is described. The model is based on the Finite Element Method. Non-linear stress-strain constitutive relations are used for the concrete, the reinforcement and for the liner. The reinforcement layers may be of different steel qualities. Each layer may be given a specified prestressing. This can be done at the beginning of the computations or the specific reinforcement layer can be considered inactive until a specified level of loading is reached. Thus, the prestressing procedure may also be analyzed in detail. Bond-slip effects are not accounted for. However, no bond may be assumed for prestressing cables by inserting special reinforcement elements. Cracking of concrete is accounted for according to a principal strain criterion. Further, creep is analyzed using the 'rate of creep' or the 'linear superposition' method. Thus, the model is capable of covering the whole range of loading and the load history. 'Three-dimensional' axisymmetric elements are used for very thick shells. For more slender structures, thin shell elements are available. A combination is also possible. The three-dimensional elements may be used in regions where shear effects are important, and the thin shell elements in the other parts of the structure. The equilibrium and the incremental equations are established by a summation of the contributions from the liner, the concrete and each reinforcement layer. These contributions are achieved by using a numerical integration scheme. The structure may in the analysis be subjected to several independent load vectors. The solution is performed in several load steps with equilibrium iterations for each step