B31.3 paragraph A319.1.2(a) states "No specific stress-limiting criteria
or methods of stress analysis are presented since stress–strain behavior of most nonmetals differs considerably from that of metals covered by para. 319 and is less well defined for mathematical analysis."
But the code goes on to say (in A319.4.2)"If substantially elastic behavior can be demonstrated for the piping system [see para. A319.2.2(a)], methods outlined in para. 319.4 may be applicable."
So, if you wish to a flexibility analysis of nonmetallic materials using B31.3 in CAESAR II, you will have to provide Sc & Sh. Your values HDS & HDBS would be used to set pressure thickness and the Code points you to specific ASTM and AWWA standards for evaluating external loads.