On a general note there are two aspects to backward compatibility :
1) Syntax changes in input:
Every new version of STAAD.Pro comes with a set of new features that are described in the Software Release Report for that version. These are features which were not present in the previous versions. So, if your model contains commands associated with those new features, those commands will not be recognized by older versions of the program.
Existing features too may have undergone changes because of which the syntax of those commands may have changed. Commands specified using this new syntax will be flagged as errors when the file is opened in the older version of the software.
So, as long as your STAAD.Pro V8i model doesn't contain commands associated with these two conditions, the input file is compatible with STAAD.Pro 2006.
2) Changes in the output format:
When you run the analysis and design, several result files are produced to store information such as joint displacements, member forces, support reactions, plate and solid element stresses.
In rare cases, the format in which data is stored in these files is changed, so that a specific result type can be displayed in an enhanced way or to enable a new method of presenting that result in the post-processing and report generation pages. However, the older version of STAAD.Pro may not recognize this modified format, due to which it may fail to display that result.
Specifically in your model, there needs a few modification in order to me it compatible with STAAD.Pro 2006.
1. In the material definition used for steel, there are two commands:
TYPE STEEL
STRENGTH
FY 253200 FU 407800 RY 1.5 RT 1.2
These commands are introduced in STAAD.Pro V8i and are used in RAM Connection interface from STAAD.Pro v8i. You need to inactivate these commands.
2. You have used the following commands for Response Spectrum load:
SPECTRUM CQC 1893 X 0.036 ACC DAMP 0.05 MIS 0 ZPA 0 DOMINANT 0
Dominant method used to provide signed Response Spectrum results is not available in STAAD.Pro 2006. Thus, you need to use the following commands:
SPECTRUM CQC 1893 X 0.036 ACC DAMP 0.05 MIS 0 ZPA 0
The model compatible with STAAD.Pro 2006 has been attached.