TR.38 Change Specification
This command is used to reset the stiffness matrix. Typically, this command is used when multiple analyses are required in the same run.
This command indicates that input, which will change the stiffness matrix, will follow. This command should only be used when an analysis has already been performed. The CHANGE command does or requires the following:
sets the stiffness matrix to zero
makes members active if they had been made inactive by a previous INACTIVE command
allows the re-specification of the supports with another SUPPORT command that causes the old supports to be ignored. The SUPPORT specification must be such that the number of joint directions that are free to move (DOF or "releases") before the CHANGE must be greater than or equal to the number of "releases" after the CHANGE.
the supports must be specified in the same order before and after the CHANGE command. To accomplish this when some cases have more supports than others do, you can enter unrestrained joints into the SUPPORT command list using FIXED BUT FX FY FZ MX MY MZ. It is best to put every joint that will be supported in any case into every SUPPORT list.
CHANGE, if used, should be after PERFORM ANALYSIS and before the next set of SUPPORT or LOADS.
Only active cases are solved after the CHANGE command.
Analysis and CHANGE are required between primary cases for PERFORM CABLE ANALYSIS.
Analysis and CHANGE are required after UBC cases if the case is subsequently referred to in a Repeat Load command or if the UBC case will be re-solved after a Select command or after a Multiple analysis.
Before CHANGE is specified:
1 PINNED 2 FIXED BUT FX MY MZ 3 FIXED BUT FX MX MY MZ
After CHANGEis specified:
1 PINNED 2 FIXED 3 FIXED BUT FX MZ
The CHANGE command is not necessary when only member properties are revised to perform a new analysis. This is typically the case in which the user has asked for a member selection and then uses the PERFORM ANALYSIS command to reanalyze the structure based on the new member properties.
If new load cases are specified after the CHANGE command such as in a structure where the INACTIVE MEMBER command is used, the user needs to define the total number of primary load cases using the SET NL option (see TR.5 Set Command Specification and Example 4).
Multiple Analyses using the CHANGE command should not be performed if the input file contains load cases involving dynamic analysis or Moving Load Generation.
Section forces and moments, stress and other results for postprocessing will use the last entered data for supports and member properties regardless of what was used to compute the displacements, end forces and reactions. So beware of changing member properties and releases after a CHANGE command.