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This page is devoted containsto *frequently asked questions* (FAQ) related to *[numerical problems|kb:numericalNumerical problems]*.

*On this page:*

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h1. Numerical Problems

h2. What is the difference between numerical problems encountered during equation assembly and numerical problems encountered during equation solution?

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h1. Lost Accuracy\\

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h2. What does the lostLost accuracyAccuracy warning message meanindicate, and how is canit Iresolved?
get
rid of it?

_See the_ _[lost accuracy|lost accuracy]_ _page._*Answer:* For response, please see the [Lost accuracy|kb:Lost accuracy] article.


h2. What does the warningLost message "lost 6.4 digitsDigits of accuracy" mean Accuracy warning message indicate?

SAP2000 equation*Answer:* Equation solutions are performed in double precision arithmetic, whichto allowsprovide for 15 significant figures. If you have global or local instability (e.g.occurs, a node thatfor example when a [restraint|kb:Constraint] is not restrained),assigned the solution will suffer to a losssupport, of[lost accuracy. The program issues a warning if the|kb:Lost accuracy] will occur. If this loss is estimated to be in aboveexcess of 6six digits, a warning message will be issued, and numerical formulation will cease processingto ifprocess thewhen detectedin lossexcess isof above 11 digits.


h2. How exactly does the program estimatesis the number of lost digits whenof accuracy is lost?

Lostestimated?

*Answer:* The number of lost digits areis determined during the reduction of the stiffness matrix bythrough comparison comparingof the order of magnitude of numbersthe thatvalues are being subtracted. The orderoperation of operationssequence is based on the re-orderedreorder equations, and sotherefore it is not always possible to predictcompute the exact number that will be reported by the program or where it will be located. This algorithm does not check for the loss of accuracy that can occur 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 two different stiffnesses are added together, rather.
it
checks the accuracy of the solution of the assembled equations
h2. To h2.what What does the lost accuracy apply to?
Displacements? Forces? Or both?

It
*Answer:* Lost accuracy applies to the termsstiffness ofmatrix theand stiffnessits matrixvalues during thenumerical solution. ofSince thestiffness equation. Please see governs the aboverelationship FAQbetween itemforce forand detailsdisplacement, onlost howaccuracy thewill programaffect determinesboth theof digitsthese ofresponse lost accuracy.measures.
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h2. I received a warning message which forreports lost accuracy for body constraints. How areis lost digits of accuracy calculated for body constraints?

this calculated?

*Answer:* [Body|kb:Constraint] and other [rigid|kb:Rigid behavior] constraints create an internal [master nodes|master node] thatwhich becomegoverns partthe ofnodes theassigned systemto ofthe equationsconstraint. TheThese constrainedmaster nodes are [slaves|slave node] and are not become part of the system of equations. So when the loss of accuracy occurs at the master node while the constrained nodes do not. When [lost accuracy|kb:Lost accuracy] occurs within the constraint, it is reported to be for the master constraintnode, ratherand than for any of not the individual slavecomponents.
nodes.

h2. {hidden-content}
*What iscauses the causenumerical ofproblem andDiagonal the< solution0, forand numericalhow problemis "Diagonal < 0"?

it resolved?*
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*Draft replyReply:*
* For nonlinearlinear analysis, negative terms on diagonal can be caused by P-delta effects when the structure suddenly goes beyond buckling.
* For linear analysis, this could be caused by edge constraint, when the constrained points are not exactly on the area edge, but this is a rare issue. [See 9/2/2010 email from fs.|$3705068]
* See also Incident 27490 for example of "Diagonal < 0"Diagonal < 0 could result from an edge [constraint|kb:Constraint], where constrained points are not exactly on the [area|kb:Shell] edge. [See 9/2/2010 email from fs.|$3705068]

* For [nonlinear|kb:Nonlinear] analysis, when [P-Delta|kb:Geometric nonlinearity] effect is considered, negative terms may occur on the diagonal of the stiffness matrix at the onset of [buckling|kb:Buckling].

* *See Also:* Incident 27490 - example of Diagonal < 0 numerical problem.
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*What is the difference between equation-assembly and equation-solution numerical problems?*
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