The CAESAR II Expansion Joint Modeler uses "vendor supplied" databases to acquire specific data (lengths, stiffnesses, etc) about the various joints. Not all of these databases include "large" pipe sizes.
You can determine this by viewing the "expjt.hed" file found in the current data directory. This is a text file you can open with NotePad. Line 23 of this file lists the nominal diameters (in inches) that the database addresses. If your diameter is outside of these values, you'll see that message.
In this case, contact the specific expansion joint vendor and ask for advice.
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Regards,
Richard Ay - Consultant