Structural Analysis of Post-Tensioned Concrete Containment Building Repair using 3-d Finite Elements

The most efficient method of replacement of major internal components such as steam generators in nuclear power plants may require the creation of a construction opening in the side of the containment building. For repairing post-tensioned concrete containment buildings, a major design challenge is to develop the most efficient scheme for tendon detensioning and subsequent re-tensioning. Stresses and displacements must be monitored in the containment wall and liner throughout the repair sequence. Nonlinear finite element analysis using the ANSYS general analysis package can be used to simulate the entire construction process and thus assure an adequate design margin of safety at all stages. Recent developments in computer CPU speed and RAM advancements have made it possible to perform complex nonlinear Finite Element Analysis (FEA) on an entire containment building overnight on a desktop machine. The nonlinear analysis technique discussed in this paper includes explicit modeling of the tendons and concrete including the tendon-concrete load interaction. Tendon tensioning and de-tensioning is modeled using an initial strain approach where link elements are coupled to the containment building wall. The wall is modeled with 3-D brick elements. Element birth and death is used to simulate the process of cutting the construction opening and subsequent patching of the wall in a stress-free state. This modeling method is critical to capture the local bending response in the patch that is often neglected in simplified models. This paper presents the details while illustrating the importance of explicitly modeling the construction and hole-patch-wall interaction.