Product(s):	Sacs IV
Version(s):	All
Area:	Solution

Community Question:

Could you please clarify the meaning of the warning "MAX. SIGNIFICANT DIGITS LOST = X SOLUTION ACCURATE TO X DIGITS OUT OF POSSIBLE XX"?

Answer 

Section 3.1 of the SACS IV manual describes this warning message:

When using a computer to perform a solution, there exists a finite number of digits that can be used to define any one number. During numerical procedures within the program, accuracy may be lost due to the relative size of the numbers used in the mathematical operations. SACS IV determines the accuracy lost during solution and reports it as the ‘Maximum Number of Significant Digits lost’ in the output listing file. In general, solutions with six or fewer significant digits lost are sufficiently accurate while solutions with twelve or more significant digits lost are not.

It is possible for the solution to lose sufficient accuracy such that the solution becomes trivial or the structure becomes mathematically unstable (the matrix is Non-Positive Definite). Common reasons for a structural model to lose significant accuracy or become mathematically unstable follow:

1. Very stiff element attached to a very soft element.

2. A stiff structure attached to the ground through a relatively soft spring system.

3. A structure with little stiffness attached to the ground through a relatively stiff spring system.