Question
When modeling using compression-only springs for underground structures, displacement errors occur.
Answer
When analyzing underground structures, the 'Displacement Error' message that appears on the Message Window is not an actual error.
The parts of the underground structure contacting with the soil can be represented using compression-only springs, which are nonlinear elements.
In static analysis using nonlinear elements, the following nonlinear elements can be used: tension-only truss elements, hook elements, cable elements, compression-only truss elements, gap elements, and tension / compression-only spring elements. To solve the static equilibrium equations of the structural system using nonlinear elements, the stiffness of the nonlinear elements based on displacement or member force is reconstructed and iterative analysis should be performed until the equilibrium equations converge.
When modeling and analyzing underground structures using the compression-only spring, which is a nonlinear element, the 'DISPLACEMENT ERROR NORM' that appears during analysis on the Message Window indicates that iterative analysis is being performed to achieve convergence of the equilibrium equations. Therefore, it is not an error due to incorrect modeling.