Numerical problems FAQ

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What does the Lost Accuracy warning message indicate, and how is it resolved?

Answer: For response, please see the Lost accuracy article.

What does the Lost 6.4 Digits of Accuracy warning message indicate?

Answer: Equation solutions are performed in double-precision arithmetic to provide for 15 significant figures. If global or local instability occurs, for example when a restraint is not assigned to a support, lost accuracy will occur. If this loss is estimated to be in excess of six digits, a warning message will be issued, and when in excess of 11 digits, numerical formulation will cease to process.

How is the number of lost digits of accuracy estimated?

Answer: The number of lost digits is determined during the reduction of the stiffness matrix through comparison of the order of magnitude of the values being subtracted. The operation sequence is based on the reorder equations, therefore it is not always possible to compute the exact number of digits lost, and the location of the occurrence. As a result, quantity and location may be estimated. Note that accuracy is checked during solution of the assembled equations, and not during assembly, when stiffnesses are added together.

To what does lost accuracy apply?

Answer: Lost accuracy applies to the stiffness matrix and its values during numerical solution. Since stiffness governs the relationship between force and displacement, lost accuracy will affect both of these response measures.

I received a body-constraint lost-accuracy warning message. How is this calculated?

Answer: Body and other rigid constraints create an internal master node which governs the nodes assigned to the constraint. These master nodes become part of the system of equations while the constrained nodes do not. When lost accuracy occurs within the constraint, it is reported for the master node, and not the individual components.